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Development of a regenerative braking control strategy for hybridized solar vehicle
Advances in Automotive Control Advances in Automotive Control June 19-23, 2016. Sweden Preprints, 8th IFACNorrköping, International Symposium on June 19-23, 2016. Sweden Preprints, IFACNorrköping, International ononline at www.sciencedirect.com Available Advances 8th in Automotive Control Symposium Advances Automotive Control June 19-23,in2016. Norrköping, Sweden June 19-23, 2016. Norrköping, Sweden
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Development of control 49-11 (2016) braking 497–504 DevelopmentIFAC-PapersOnLine of a a regenerative regenerative braking control strategy strategy for hybridized solar vehicle Development of regenerative braking control strategy fora solar vehicle Development of a hybridized regenerative braking control strategy for hybridized solar vehicle M. Grandone, M. Naddeo, D. Marra, G. for hybridized solar vehicle M. Grandone, M. Naddeo, D. Marra, G. Rizzo Rizzo M. Grandone, M. Naddeo, D. Marra, G. Rizzo M. Grandone, M. Naddeo, D. Marra, G. Rizzo Department of Industrial Engineering, University Department of Industrial Engineering, University of of Salerno, Salerno, Via Giovani Paolo II 132, 84084 Fisciano (SA) -- Italy Via Giovani Paolo II 132, 84084 Fisciano (SA) Italy Department of Industrial Engineering, University of Salerno, (Tel: +39 089964239; e-mail: [email protected]) Department of Industrial Engineering, University of Salerno, (Tel: +39 089964239; e-mail: [email protected]) Via Giovani Paolo II 132, 84084 Fisciano (SA) - Italy Via(Tel: Giovani II 132, e-mail: 84084 Fisciano (SA) - Italy +39Paolo 089964239; [email protected]) (Tel: +39 089964239; e-mail: [email protected]) Abstract: This This paper paper focuses focuses on on the the development development of of aa braking braking control control strategy strategy that that allows allows the the best best tradetradeAbstract: off between mechanical and regenerative braking on a hybridized vehicle. The research work is part of aa off between mechanical and regenerative braking of onaa braking hybridized vehicle. The that research work parttradeof Abstract: This paper focuses onanthe development control strategy allows the is best project for the development of automotive hybridization kit aimed at converting conventional cars into Abstract: paper focuses onanthe development of aa braking control that allows the is best project forThis the development of automotive hybridization kit aimed atstrategy converting conventional cars into off between mechanical and regenerative braking onaspect hybridized vehicle. The research work parttradeof a Through The Road hybrid solar vehicles. The main of the project is the integration of state-of-theoff between mechanical and regenerative braking a hybridized vehicle. research work is cars part of Through Road hybrid solar The mainonaspect of project is The the integration of state-of-theproject forThe the development of anvehicles. automotive hybridization kitthe aimed at converting conventional intoa art components (i.e. in-wheel motors, photovoltaic panels, batteries) with the development of an optimal project for theRoad development ofmotors, anvehicles. automotive hybridization kitthe aimed at converting conventional into art components (i.e. in-wheel photovoltaic panels, batteries) with ancars optimal Through solar mainhybrid aspectstructure of project isthe thedevelopment integration ofofstate-of-thecontrollerThe for Road powerhybrid management. A mild mildThe parallel is obtained obtained by substituting/integrating substituting/integrating Through The hybrid solar vehicles. The main aspect of the project is the integration of state-of-thecontroller for power management. A parallel hybrid structure is by art motors, photovoltaic batteries)panels with the of an optimal the components rear wheels wheels(i.e. within-wheel in-wheel motors and adding addingpanels, photovoltaic anddevelopment lithium-ion battery. A art components (i.e. in-wheel motors, photovoltaic panels, batteries)is with the development of an optimal the rear with in-wheel motors and photovoltaic panels and aa substituting/integrating lithium-ion battery. A controller for power management. A mild parallel hybrid structure obtained by hybridizing equipment prototype, patented by the University of Salerno, is installed on a FIAT Grande controller forequipment power A mildand parallel structure is obtained by substituting/integrating hybridizing prototype,motors patented byadding the hybrid University of Salerno, isand installed on a FIAT Grande the rearA wheels withmanagement. in-wheel panels afrom lithium-ion battery. A Punto. model useful useful for real-time braking control hasphotovoltaic been developed, developed, starting vehicle longitudinal longitudinal the rear wheels with in-wheel motors and adding photovoltaic panels and a lithium-ion battery. A Punto. A model for real-time braking control has been starting from vehicle hybridizing equipment prototype, patented by the University of Salerno, is installed on a FIAT Grande model and and considering considering dynamic weight weight distribution in front front of andSalerno, rear axles axles and related related wheel slipping hybridizing equipment prototype, patented by the University is installed on a FIAT Grande model dynamic distribution in and rear and wheel slipping Punto. model useful for real-time control hasinvestigated, been developed, starting from vehicle effects.A Different braking strategiesbraking have then then been in order order to maximize maximize thelongitudinal benefits of of Punto. A model useful for real-time braking control hasinvestigated, starting from vehicle longitudinal effects. Different braking strategies have been in to the benefits model and considering dynamic weight distribution inbeen frontdeveloped, and rear axles and related wheel slipping regenerative braking. model and considering dynamic weight in front and rear axlestoand related the wheel slipping regenerative braking. effects. Different braking strategies havedistribution then been investigated, in order maximize benefits of Keywords: automotive hybridization kit; hybrid solar vehicles (HSV); vehicle electrification; HSV effects. Different braking strategies have then been investigated, in order to maximize the benefits of © 2016, IFAC (International Federation of Control) Hosting(HSV); by Elsevier Ltd. All rights reserved. regenerative braking. Keywords: automotive hybridization kit;Automatic hybrid solar vehicles vehicle electrification; HSV prototype; control strategy; regenerative braking; in-wheel motors. regenerative braking. prototype; control strategy; regenerativekit; braking; motors. (HSV); vehicle electrification; HSV Keywords: automotive hybridization hybridin-wheel solar vehicles Keywords: automotive hybridization kit; hybrid solar vehicles prototype; control strategy; regenerative braking; in-wheel motors. (HSV); vehicle electrification; HSV prototype; control strategy; regenerative braking; in-wheel motors.relevant number of braking events representing a because because of of relevant number of braking events representing a 1. 1. INTRODUCTION INTRODUCTION big energy loss with aa great potential. big energy with number great saving saving potential. because of loss relevant of braking events representing a Obviously, in order to be cost effective, in regenerative In recent times, hybrid vehicles (HEVs) have received an 1. of loss relevant number of braking events Obviously, in order togreat be saving cost effective, in aarepresenting regenerativea In INTRODUCTION recent times, hybrid vehicles (HEVs) have received an because big energy with a potential. 1. INTRODUCTION braking system the primary energy saved over a specified increasing attention as an alternative to traditional vehicles energy loss with potential. braking system the aprimary energy saved inover a specified increasing attention as anvehicles alternative to traditional vehicles Obviously, inoffset order togreat be saving cost effective, a regenerative In recentbytimes, hybrid (HEVs) have received an big lifetime must the initial cost, size and weight penalties powered internal combustion engines using fossil fuels. Obviously, in order to be cost effective, in a regenerative lifetime must offset the initial cost, size and weight penalties In recent times, hybrid vehicles (HEVs) have received an powered by internal combustion engines using fossil fuels. braking system The the primary energy unit savedmust overbea compact, specified increasing attention as an alternative to traditionaland vehicles of the system. energy storage Considering recent advances in battery technology motor braking system the primary energy saved over a specified of the system. The energy storage unit must be compact, increasing attention as an alternative to traditional vehicles Considering recent advances in battery technology and motor lifetime mustcapable offset the initial cost, size andlevels weightefficiently, penalties powered by HEVs internalare combustion engines using way fossiltofuels. durable and of handling high power efficiency, aa valid and feasible reduce lifetime mustcapable offset the initialstorage cost, and weight penalties durable and ofenergy handling highsize power levels efficiently, powered by HEVs internal combustion engines using way fossiland efficiency, are valid and feasible tofuels. reduce of the system. The unit must be compact, Considering recent advances in battery technology motor and any auxiliary energy transfer or energy conversion emissions and fuel consumption, and represent an effective of the system. The energy storage unit must be compact, and any auxiliary energy transfer or energy conversion Considering recent advances in battery technology and motor emissions and fuel consumption, and represent an effective andmust capable of handling high power levels efficiently, efficiency, HEVs are athe valid and electrification. feasible way to reduce durable be compact and of reasonable cost. bridge toward vehicle Recently, durable andauxiliary capable of handling high power levels efficiently, equipment must be efficient, efficient, compact and of reasonable cost. efficiency, HEVs are athe valid andand feasible wayanto reduce equipment bridge solution solution toward vehicle electrification. Recently, and any energy transfer or energy conversion emissions and fuel consumption, represent effective However, mechanical braking is still required in HEVs to keep some solutions for converting conventional vehicles into and any auxiliary energy transfer or energy conversion However, mechanical braking is still required in HEVs to keep emissions and fuel consumption, and represent an effective some solutions for converting conventional vehicles into equipment mustmore be efficient, compact and oftoreasonable cost. bridge solution have toward thebeen vehicle electrification. Recently, braking phases safe and, for example, avoid problems hybrid vehicles also proposed (Arsie et al., 2013; equipment must be efficient, compact and of reasonable cost. braking phases more safe and, for example, to avoid problems bridge solution toward the vehicle electrification. Recently, hybrid vehicles have also been proposed (Arsie et al., 2013; However, mechanical braking is still required in HEVs to keep some solutions for converting conventional vehicles into in case of electrical failure. Marano et Hall et 2016), optimal energy mechanical braking is still requiredtoinavoid HEVs to keep in case of electrical failure. some for converting conventional vehicles into However, Maranosolutions et al., al., 2013, 2013, Hall been et al., al.,proposed 2016), and and optimal energy hybrid vehicles have also (Arsie et al., 2013; braking phases more safe and, for example, problems Several studies on braking control strategies are management strategies suitable for such of vehicles have phases more safe and, for example, to avoid problems Several studies on regenerative regenerative braking control strategies are hybrid vehicles have also been proposed (Arsie et al.,energy 2013; management strategies suitable for such kind kind ofoptimal vehicles have braking Marano et al., 2013, Hall et al., 2016), and in case of electrical failure. available in literature (Rizzoni, et al., 2009; Maia, et al., 2015; been studied (Pisanti et al, 2014). in case of electrical failure. available in literature (Rizzoni, et al., 2009; Maia, et al., 2015; Marano et al., 2013, Hall et al., 2016), and optimal energy been studied (Pisanti et al, 2014). Several et studies on regenerative braking control strategies are management strategies suitable for such kind of vehicles have Kumar, al., 2016). They are carried out on classical An important advantage of HEVs compared to conventional Several et studies on regenerative braking control are al., 2016). They are mostly mostly carried outstrategies onal., classical management strategies suitable for such kind of have Kumar, An important advantage of2014). HEVs compared to vehicles conventional available in literature (Rizzoni, et al., 2009; Maia, et 2015; been studied (Pisanti et al, HEV’s or EV’s, and cannot be simply transposed to hybridized vehicles is the possibility to save energy during braking available in literature (Rizzoni, et al., 2009; Maia, et al., 2015; HEV’s or EV’s, and cannot be simply transposed to hybridized been studied (Pisanti et al, 2014). vehicles is the possibility to save energy during braking Kumar, etwhere al., 2016). They are are mostly carried outwheels on classical An important of HEVs compared tobrakes, conventional vehicles, wheel motors added in in phases. When aaadvantage conventional vehicle applies its kinetic Kumar,oretwhere al., 2016). They aresimply mostly carried outto on classical wheel motors are added in rear rear wheels in an an An important advantage of HEVs compared conventional phases. When conventional vehicle applies itsto brakes, kinetic vehicles, HEV’s EV’s, and cannot be transposed hybridized vehicles is the possibility to save energy during braking existing conventional vehicle. Comparing to classical native energy is converted to heat due to friction between the brake HEV’s or EV’s, and cannot be simply transposed to hybridized existing conventional vehicle. Comparing to classical native vehicles is the possibility to save energy during braking energy is converted to heat due to friction between the brake vehicles, hybridized where wheelvehicles motors are added in rear wheels in the an phases. When a conventional vehicle away applies its brakes, kinetic HEVs, are characterized by pads and wheels. This is carried in the airstream and vehicles, where wheelvehicles motors are intorear wheels in the an hybridized areadded characterized bynative phases. When a conventional vehicle applies its pads andis wheels. Thistoheat heat isdue carried away inbetween thebrakes, airstream and HEVs, existing conventional vehicle. Comparing classical energy converted heatThe to friction thekinetic brake impossibility to modify/adapt the original engine control kinetic energy is wasted. total amount of energy lost in existing conventional vehicle. Comparing to classical native impossibility to modify/adapt the original engine control energy is converted to heat due to friction between the brake kinetic is This wasted. total amount energy lostand in HEVs, hybridized vehicles are characterized by the padsway andenergy wheels. heatThe is carried away in of the airstream strategy (i.e. original ECU) order to for the this depends on how how hard how long hybridized vehicles characterized by strategy (i.e. to original ECU) in inare order to account account for the pads andenergy wheels. This heatoften, is carried awayand in of the airstream and this way depends on how often, how amount hard and how long brakes brakes impossibility modify/adapt the original engineelectrical control kinetic is wasted. The total energy lost ina HEVs, electrical parts and their control (i.e., battery pack, are applied. A regenerative brake is a system that allows impossibility to modify/adapt the original engine control electrical parts and their control (i.e., battery pack, electrical kinetic energy is wasted. The total amount of energy lost in are applied. A regenerative brake is a system that allows a strategy (i.e.solar original ECU) in order to account for the this waytodepends on how often, how hard anddissipated how long brakes engine and panels). Moreover, for the specific vehicle vehicle recover part of the kinetic energy during (i.e. original ECU) in(i.e., order to for the engine and solar panels). Moreover, for the account specific vehicle this waytodepends on how howishard anddissipated how long brakes vehicle recover part ofoften, the brake kinetic energy duringa strategy electrical parts and their control battery pack, electrical are applied. A regenerative a system that allows developed, all the information needed to control the vehicle are braking phases. Recovered energy can be stored in aa battery in electrical parts and their control (i.e., battery pack, electrical developed, all the information needed to control the vehicle are are applied. A regenerative brake is a system that allows a braking phases. Recovered energy can be stored in battery in and solar panels). Moreover, for themeasured specific vehicle vehicle to recover and partused of the kinetic energy dissipatedphases, during engine derived from a limited number of variables by the form of electricity during vehicle acceleration engine and solar panels). Moreover, for the specific vehicle derived from a limited number of variables measured by the vehicle to recover part of the kinetic energy dissipated during form of electricity and used during vehicle acceleration phases, developed, all theet information needed to control the vehicle are braking phases. Recovered energy can be stored in a battery in OBD port (Arsie al., Marano et al., estimating increasing efficiency and fuel economy. developed, all athe information needed to the vehicle are port (Arsie et al., 2013; 2013; Marano et control al., 2013), 2013), estimating braking phases. Recovered energy can be stored in a battery in OBD increasing efficiency fuel economy. derived from limited number of variables measured by the form of electricity andand used during vehicle acceleration phases, the missing data with suitable mathematical models (Naddeo The energy available for storage depends on drive train derived from a limited number of variables measured by the the missing data with suitable mathematical models (Naddeo form of electricity and used during vehicle acceleration phases, The energy available for storage depends on drive train OBD port (Arsie et al., 2013; Marano et al., 2013), estimating increasing efficiency and and fuel economy. et al., 2014). Such models been implemented in efficiency, drive cycle vehicle weight (Guzzella and port (Arsie et al., 2013; have Marano et al., 2013), et al., 2014). Such models have been implemented in the the increasing efficiency andfor fuelstorage economy. efficiency, drive cycle and vehicledepends weight on (Guzzella and OBD The energy available drive train the missing data with suitable mathematical modelsestimating (Naddeo braking control strategy presented in this work. Sciarretta, 2013). The impact of regenerative braking on the missing data with suitable mathematical models (Naddeo braking control strategy presented in this work. The energy available for storage depends on drive train Sciarretta, 2013). The impact of regenerative braking on al., 2014). Such models ahave been implemented in the efficiency, driveis cycle and vehicle weight (Guzzella and et In the following chapters, study on energy energy much more significant in urban driving et al., Such models been implemented in the In the 2014). following chapters, ahave study on the the energy recovery recovery efficiency, driveis cycle and weight and energy saving saving much morevehicle significant in (Guzzella urban driving Sciarretta, 2013). The impact of regenerative braking on potentialities braking control strategy presented in this work. by means of regenerative braking in aa hybridized braking control strategy presented in this work. potentialities by means of regenerative braking in hybridized Sciarretta, 2013). The impact of regenerative braking on energy saving is much more significant in urban driving In the following chapters, a study on the energy recovery In the followingmeans chapters, a study onbraking the energy recovery energy saving is much more significant in urban driving potentialities of regenerative in a hybridized Copyright © 2016 IFAC 507 potentialities by by means of regenerative braking in a hybridized Copyright © 2016 IFAC 507 2405-8963 © 2016, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Copyright 2016 responsibility IFAC 507Control. Peer review©under of International Federation of Automatic Copyright © 2016 IFAC 507 10.1016/j.ifacol.2016.08.073
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vehicle is presented, and control strategy are described. Moreover, the case with mechanical braking on rear axle coupled with regenerative one is also considered with the objective to define the best distribution of mechanical braking torque between front and rear axle to maximize regenerative effect, and guarantee a suitable and safe braking control.
Li-Ion Battery Voltage [V]
96
Li-Ion Battery Mass [kg]
45
In-wheel motors power [kW]
14
Tab. 1. Vehicle and Hybridization Kit Characteristics
2. A KIT FOR SOLAR HYBRIDIZATION A solar-hybridized vehicle has been developed at the University of Salerno through the installation of an additional battery (Lithium-Ion) and two electrically driven in-wheel motors on the rear wheels on a conventional vehicle. Moreover, solar cells on vehicle bonnet and roof have been installed so that it is possible to charge battery pack taking advantage of solar energy. In that way, the vehicle can operate in pure electric mode (when ICE is switched off or disconnected by the front wheels) or in hybrid mode (when the ICE drives the front wheels and the rear in-wheel motors operate in traction mode or in generation mode, corresponding to a positive or negative torque). The battery can be recharged by both rear wheels, when operating in generation mode, and photovoltaic panels. Furthermore, it is possible to charge battery from electric grid. A Vehicle Management Unit (VMU) receives data from OBD gate, from battery (SOC estimation) and drives in-wheel motors by properly acting on the electric node EN. A display on the dashboard may advice the driver about the actual operation of the system (Fig. 1).
Fig. 2. Hybrid Solar Vehicle by University of Salerno
A study on the technical and economic feasibility of the hybridization kit confirms that significant reduction of fuel consumption and CO2 emissions (18- 22%), comparable with HEVs benefits, but at lower investment cost, can be achieved. The results show that driving distance and type (urban vs. highway) and the availability of charging infrastructure play an important role in fuel economy, CO2 emissions savings and pay-back time. Moreover, in spite of high cost of the flexible PV panels, the solution with PV panels result in lower payback time with respect to solutions with in-wheel motors only. 3. IN-WHEEL MOTORS One of the most interesting aspects of the solar hybridization kit is represented by the partial recovery of kinetic energy allowed by wheel motors during braking. The optimization and the control of such feature represent the object of this paper. Use of in wheel motors (IWM) allows to electrify the vehicle with no impact on mechanical transmission. A significant amount of space is therefore saved in the vehicle, compared to other hybrid solutions where electric motor is placed out of vehicle chassis. A hub motor typically may be designed in three main configurations. The least practical is an axial-flux motor, where stator windings are typically sandwiched between sets of magnets. The other two configurations are both radial designed with the motor magnets bonded to the rotor. In one of them, the rotor sits inside the stator, as in a conventional motor while in the other one, the rotor sits outside the stator and rotates around it. A typical IWM scheme is shown Fig. 3.
Fig. 1. Scheme of a system to upgrade a conventional car into a Mild Hybrid Solar Vehicle
The kit has been installed on a FIAT Punto, at the laboratories of the University of Salerno (Fig. 2). Nominal ICE power [kW]
75
Fuel
Diesel
Coefficient of drag (Cd)
0.325
Frontal area [m2]
2.05
Rolling radius [m]
0.295
Rolling resistance coefficient [/]
0.02
Base vehicle mass [kg]
1105
Driver mass [kg]
70
PV installed power [kW]
0.280
PV mass [kg]
4.7
Li-Ion Battery Capacity [kWh]
4.2
508
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499
Fig. 3. In-Wheel Motor Scheme
The application of hub motors in cars is still evolving, and no configuration has become standard. In wheel motors main problem is their high weight that may affect comfort and safety. The so-called "non-suspended" masses (i.e. in wheel motors and brakes) must be as light as possible compared to "suspended masses" that, actually, consist in the entire body of the car (Anderson and Harty, 2010).
Fig. 5. In-Wheel Motors maps
Max Torque [Nm]
200
The addiction of IWM makes the normal vehicle a 4x4 traction one. Vehicle safety and stability can be enhanced by implementing an accurate electronic control of driving and braking torque, separately for each wheel. Moreover, a significant improvement in acceleration capability is achieved, thanks to the higher torque delivery at low rotation speed typical of electric motors (Fig. 6).
150 100 50 0 0
In order to develop the hybridization kit patented by University of Salerno and carry out the regenerative braking control strategy described in this paper, IWM produced by Kelly Controllers have been purchased on Chinese market. Their features and characteristic maps are shown in Fig. 4, Fig. 5, Fig. 6 and Tab. 2.
200
400 600 800 Wheel rotation speed [rpm]
1000
Fig. 6. IWM max Torque curve
Pick Power [kW]
7
Pick Torque [Nm]
150
Diameter-Width [mm]
303-107
Max speed [rpm]
1500
Weight [kg]
20
Tab. 2. In-Wheel Motors Characteristics
4. LONGITUDINAL DYNAMIC MODEL In order to develop the regenerative braking control strategies, whose results are presented in the following section, a longitudinal vehicle model has been developed under the following hypotheses (Guzzella and Sciarretta, 2013):
the drag force is considered acting on vehicle center of gravity;
vehicle inertia accounts for both vehicle mass (MHSV) and rotational inertia of ICE, EM/EG and wheels, through the term Meff;
the effects of elasticity transmission are neglected.
Fig. 4. In-Wheel Motor installed on HSV Prototype
in
the
mechanical
According to these assumptions, the power at wheels to the road load (Pwheel) has been modeled as follows:
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𝑃𝑃𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒 = 0.5𝜌𝜌𝐶𝐶𝑥𝑥 𝐴𝐴𝑣𝑣 3 + 𝑀𝑀𝐻𝐻𝐻𝐻𝐻𝐻 𝑔𝑔𝑔𝑔[𝐶𝐶𝑟𝑟 cos(𝛼𝛼) + sin(𝛼𝛼)] 𝑑𝑑𝑑𝑑 + 𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 𝑣𝑣 𝑑𝑑𝑑𝑑
starting from different target speed. During deceleration tests there was neither torque from engine nor from brakes, thus Equation 3 can be simplified setting Tcomb=0 and Tbrake=0, Tdrive = 0; furthermore there was no road slope so Tmg=0.
(1)
In these conditions, Equation 3 becomes:
where and v are the road grade and vehicle speed, respectively.
𝑇𝑇𝑑𝑑𝑑𝑑 = −(𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 ∗
Equation 1 can be rewritten as: 𝑃𝑃𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑃𝑃𝑚𝑚𝑚𝑚 + 𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 Pwheel is the power at wheels 𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 = 0.5𝜌𝜌𝐶𝐶𝑥𝑥 𝐴𝐴𝑣𝑣3 is the aerodynamic power
𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑀𝑀𝐻𝐻𝐻𝐻𝐻𝐻 𝑔𝑔𝑔𝑔𝐶𝐶𝑟𝑟 cos(𝛼𝛼) is the rolling power
𝑃𝑃𝑚𝑚𝑚𝑚 = 𝑀𝑀𝐻𝐻𝐻𝐻𝐻𝐻 𝑔𝑔𝑔𝑔sin(𝛼𝛼) is the power due to vehicle weight 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑
𝑣𝑣 is the inertial forces power.
In Fig. 8 the driveline braking torque computed for different gear ratios is shown. It is worth remarking that the driveline torque increases by decreasing the gear ratio. This is mainly due to the engine passive torque that increases with engine speed, which is higher for lower gear ratio (once the car speed is fixed).
The aerodynamic drag coefficient Cx is a parameter provided by vehicle manufacturer. (Anderson and Harty, 2010) 5. DRIVELINE TORQUE EFFECT In order to estimate the braking torque value and how it is split between rear and front axles the power balance expressed by Equation 2 has been written in terms of torque dividing per wheels rotational speed:
-100
Driveline braking torque [Nm]
𝑇𝑇𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒
𝑑𝑑𝜔𝜔 = 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 𝑑𝑑𝑑𝑑
(3)
Where: 𝑇𝑇𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑇𝑇𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 − 𝑇𝑇𝑑𝑑𝑑𝑑 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑇𝑇𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑇𝑇𝑚𝑚𝑚𝑚
(6)
For each deceleration test, Equation 6 describes driveline braking torque profile that can be expressed as function of velocity once gear is defined. This dependence is expressed for each gear as a polynomial function of vehicle speed. The parameters have been identified by minimizing the error between the driveline torque computed by using experimental data (i.e. velocity profile) according to the right hand side of Equation 6 and the value provided by the polynomial model. Fig. 7 shows the comparison between the driveline torque estimated by the polynomial regression and the experimental data for the third gear.
(2)
Where:
𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 = 𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒
𝑑𝑑𝜔𝜔 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ) 𝑑𝑑𝑑𝑑
(4)
Speed-Driveline braking torque (Third gear) Estimated from Experimental Data Polynomial Approximation
-150 -200 -250 -300 -350 40
Twheel is the torque at wheels
50
60 Speed [km/h]
70
80
Taero is the aerodynamic torque Fig. 7. Data-Polynomial approximation Comparison
Troll is the rolling torque Tmg is the torque due to vehicle weight
Driveline Braking Torque [Nm]
0
Tcomb is the propulsive part of engine torque Tbrake is the brakes torque Tdl is the engine/transmission passive torque which will be indicated as driveline braking torque hereafter. It is important noting that Tdl accounts for IWM passive torque due to internal friction too. During braking phases there is no propulsive torque from engine, so: 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = − (𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 ∗
𝑑𝑑𝜔𝜔 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ) − 𝑇𝑇𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑
-200
-400
-600
-800 0
20
40 60 Speed [Km/h]
80
Gear 1 Gear 2 Gear 3 Gear 4 Gear 5 100
(5) Fig. 8 – Driveline torque computed for different gear ratio.
In order to quantify the driveline braking effect an experimental tests campaign was carried out: the vehicle reached a target speed and the gas pedal was released, with gear engaged. These tests were carried out for each gear
Using these polynomial functions to identify driveline braking torque for each gear, it is possible to calculate braking torque profile (Fig. 10) regarding to a real driving cycle through
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Equation 5. Speed profile of this driving cycle is shown in Fig. 9.
X1 e X2 are longitudinal forces on front and rear axles respectively;
Z1 e Z2 are vertical forces on front and rear axles respectively;
u is the vehicle speed;
l is the distance between front and rear wheels;
a e b are distances between vehicle center of gravity and front and rear axles respectively;
h is the vehicle center of gravity height;
W = mg is the vehicle weight;
G is the vehicle center of gravity position;
X0-Z0 is the street reference system;
X-Z is the vehicle reference system.
20
speed [m/s]
15
10
5
0 0
50
100 time [s]
150
200
Fig. 9. Driving Cycle Speed Profile
0
The proposed model is based on the following assumptions: 1. constant deceleration;
-500
Torque[Nm]
501
2. flat road;
-1000
3. absence of lateral forces; -1500
4. equal adhesion conditions for the wheels on the same axle.
-2000 0
If these assumptions are satisfied, then it is possible to write the relative dynamic equations: 50
100 time [s]
150
200
𝑚𝑚𝑢𝑢̇ = −(𝑋𝑋1 + 𝑋𝑋2 ) 0 = 𝑍𝑍1 + 𝑍𝑍2 − 𝑚𝑚𝑚𝑚 0 = (𝑋𝑋1 + 𝑋𝑋2 )ℎ − 𝑍𝑍1 ∗ 𝑎𝑎 + 𝑍𝑍2 ∗ 𝑏𝑏
Fig. 10. Braking Torque Profile
(7)
6. REGENERATIVE BRAKING STRATEGY 6.1 Safety braking region definition
At constant cruise velocity, W1 and W2 are static loads for both axles and can be defined as: 𝑏𝑏 𝑊𝑊1 = 𝑚𝑚𝑚𝑚 𝑙𝑙 (8) 𝑎𝑎 𝑊𝑊2 = 𝑚𝑚𝑚𝑚 𝑙𝑙
In the previous chapter, a model to estimate braking torque for a real driving cycle has been developed. The objective of this one is to analyze how braking torque can be split between front and rear axles. In order to reach that goal, it is necessary to refer to a simplified model of the vehicle that defines a safety braking region.
During deceleration, load on front axle becomes greater than the rear one according to: 𝑚𝑚ℎ 𝑢𝑢̇ 𝑙𝑙 𝑚𝑚ℎ 𝑍𝑍2 = 𝑊𝑊2 − ∆𝑍𝑍 = 𝑊𝑊2 + 𝑢𝑢̇ 𝑙𝑙
𝑍𝑍1 = 𝑊𝑊1 + ∆𝑍𝑍 = 𝑊𝑊1 −
(9)
The maximum possible deceleration is obtained when both axles are at grip limit: 𝑋𝑋1 = 𝜇𝜇𝑍𝑍1 𝑋𝑋2 = 𝜇𝜇𝑍𝑍2
(10)
Substituting these values in the equations of the dynamics of the system (Equation 7): Fig. 11. Forces acting on the vehicle during braking
According to the scheme of Fig. 11:
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|𝑢𝑢̇ |𝑚𝑚𝑚𝑚𝑚𝑚 = 𝜇𝜇𝜇𝜇)
(11)
Combining results, the maximum braking force applied on both axles without slip are: ℎ 𝑋𝑋1𝑝𝑝 = 𝜇𝜇(𝑊𝑊1 + 𝑚𝑚 𝜇𝜇𝜇𝜇) 𝑙𝑙 ℎ 𝑋𝑋2𝑝𝑝 = 𝜇𝜇(𝑊𝑊2 − 𝑚𝑚 𝜇𝜇𝜇𝜇) 𝑙𝑙
(12)
Assigning this maximum value of in wheels motor control signal if the working point is inside the feasible region.
(13) 2500
Front Braking Torque
𝑋𝑋20
𝜇𝜇𝑊𝑊1
ℎ 1 − 𝜇𝜇 𝑙𝑙 𝜇𝜇𝑊𝑊2 = ℎ 1 + 𝜇𝜇 𝑙𝑙
Identifying feasible braking region and the position of the working point corresponding to the condition listed above;
In case this condition is not verified, the controller will assign the maximum value of in wheels motor control signal to set the working point on the right side of the feasible region edge line. Since the total braking torque required by driver must be kept constant, mechanical braking torque will increase and the working point will move upper as shown in Fig. 13.
In case braking force is applied only on one axle: 𝑋𝑋10 =
In this way it is possible to identify the feasible braking region on plane X2-X1, once defined grip coefficient and car specification.
2000
Feasible Braking Region Feasible Braking Region Working point position before controller correction Working point position after controller correction
1500 1000 500 0 0
500
1000 1500 Rear Braking Torque
2000
2500
Fig. 13. Controller Braking Modulation Effect
The approach explained above is consistent either in case on rear axle there is an electric brake only or mechanical and electric brakes are both installed. In case of electric brake only, considering maximum in wheels motor control signal, x coordinate of the point is calculated using motor maps only while y coordinate is obtained per difference as already explained above. On the other hand, considering mechanical braking torque too on rear axle, x coordinate of the point is obtained summing, to maximum electric braking torque from maps, a percentage of the remaining one depending on how mechanical braking is departed (usually 65% front axle, 35% rear axle).
Fig. 12. Feasible Braking Region
The feasible braking region identifies an area where the vehicle brakes without grip loosing; on the right side of this zone the rear wheels slip, above front wheels lose grip.
Y coordinate of the point is calculated per difference. This means that the working point in the second scenario is positioned lower and further on the right side compared to the first one since the whole braking torque is constant.
6.2 Braking control implementation Once feasible braking region is identified, it is important to develop a control strategy that avoids grip loosing and maximize regeneration. In order to get this result, a regenerative brake controller has been developed; the operating scheme of the controller can be described through the following steps:
Identifying a braking phase from negative value of Twheels;
Considering the maximum braking torque of in wheel motor corresponding to operating speed in order to maximize regenerative effect;
Calculating mechanical braking torque subtracting electric torque from Tbrake;
The second case is less stressful for front axle brakes since the whole system works on bottom right, but minimizes regenerative braking effect; on the other hand, the first one allows to maximize recovered energy. Unfortunately, when the controller modulates regenerative braking to move to the edge line in the first case scenario, this operation stresses front axle only with the possibility to cross over the top of feasible braking region. However, if that happens, the ABS will operate as in a conventional vehicle in order to avoid front wheels slipping. 7. SIMULATION RESULTS
by
The results of some tests performed on the Hybridized Prototype developed at the University of Salerno are presented 512
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in the following. Only electric regenerative brakes are installed on rear axle of this prototype due to design constraints. The impact of regenerative braking has been evaluated using a real driving cycle whose characteristics are reported in Fig. 9 and considering wet conditions.
Braking Torque[Nm]
50
100 time [s]
150
200
Braking Torque[Nm]
RTTR [%]
80 60 40 20 0 0
Total Braking Torque Rear Axle Braking Torque Conventional rear axle Braking Torque
50
100 time [s]
150
200
Fig. 15. RTTR and Braking Torque compared to a conventional distribution between front and rear axles (relative to the second case)
In this case, braking torque is well balanced between axles in all conditions and wheels never slip during driving. On the other hand, regenerative braking controller does not need to module electric braking torque so the energy recovered during driving shows an increment of 76% compared to the first case.
500
50
100 time [s]
150
200
In order to have the whole picture of the situation, a configuration that takes into account mechanical brakes on rear axle coupled with electric brakes is considered. The case analyzed is the one with a conventional distribution of mechanical braking torque (65% front, 35% rear).
Rear Axle Braking Torque/Braking Torque Conventional rear axle Braking Torque/Braking Torque 100
Results are shown in Fig. 16. In this case the working point is always inside the feasible braking region due to the action of the regenerative braking control system (17 times operating). Anyway, compared to the first case, recovered energy shows only a little decrease (3%). This happens because, in the most of case, load transfer to front axle during braking phases is low and it is possible to completely exploit the regenerative brakes on rear axle reaching a really high RTTR values without causing wheels slip.
80
RTTR [%]
500
100
1000
60 40 20 0 0
1000
Rear Axle Braking Torque/Braking Torque Conventional rear axle Braking Torque/Braking Torque
1500
0 0
1500
0 0
This configuration allows the maximum regenerative effect (28.5% of the electric energy used for traction is recovered), however, it leads to a low RTTR during high magnitude braking phases due to limited power of in wheel motors. This means the working point on the feasible braking region moves above the upper edge line during the driving cycle and front wheels ABS operates.
2000
Total Braking Torque Rear Axle Braking Torque Conventional rear axle Braking Torque
2000
Considering the percentage ratio between rear axle braking torque and total braking torque (Rear-Total Torque Ratio, RTTR) for the mentioned driving cycle, it is possible to notice its profile in Fig. 14 (bottom) and the comparison with the standard braking distribution (red line). In this case, most of braking phases are completely covered by electric brakes because of their low magnitude. This means that RTTR is often 100% or around this value. At the same time, in Fig. 14 (top) total braking torque, hybridized vehicle rear torque and conventional vehicle one profiles are shown.
503
50
100 time [s]
150
200
Fig. 14. RTTR and Braking Torque compared to a conventional distribution between front and rear axles (relative to the first case)
To avoid this kind of problems, a configuration with double IWM torque values has been considered and simulation results in terms of braking torque and RTTR are shown in Fig. Fig. 15.
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Guzzella, L. and, Sciarretta, A. Vehicle propulsion systems (Vol. 1). Springer-Verlag Berlin Heidelberg.
Total Braking Torque Rear Axle Braking Torque Conventional rear axle Braking Torque
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Hall, J., Bassett, M., Borman, S., Lucas, T., Whitehead, A. (2016). Through-the-Road Parallel Hybrid with InWheel Motors (No. 2016-01-1160). SAE Technical Paper.
1500 1000
Kumar N. and Subramanian S. (2016). Cooperative control of regenerative braking and friction braking for a hybrid electric vehicle. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 230(1), 103-116.
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50
100 time [s]
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Maia R., Silva M., Araùjo R., Nunes U. (2015). Electrical vehicle modeling : a Fuzzy logic model for regenerative braking. Expert Systems with Applications. Volume 42, Issue 22.
Rear Axle Braking Torque/Braking Torque Conventional rear axle Braking Torque/Braking Torque 100
Marano, V., Medina, H., Sorrentino, M., Rizzo, G. (2013). A model to assess the benefits of an after-market hybridization kit based on realistic driving habits and charging infrastructure. SAE International Journal of Alternative Powertrains, 2(3), 471-481.
RTTR [%]
80 60 40 20 0 0
50
100 time [s]
150
Naddeo, M., D'Agostino, M., Rizzo, G. (2014). Development and validation of a model to detect active gear via OBD data for a Through-The-Road Hybrid Electric Vehicle. IFAC World Congress. Cape Town (South Africa), In World Congress (Vol. 19, No. 1, pp. 66186623).
200
Fig. 16. RTTR and Braking Torque compared to a conventional distribution between front and rear axles (relative to the third case)
Pisanti C., Rizzo G., Marano V. (2014). Energy Management of Through-The-Road Parallel Hybrid Vehicles. 19th IFAC World Congress, August 24-29, 2013, Cape Town (S.Africa), Vol. 19, No. 1, pp. 2118-2124.
8. CONCLUSIONS A model able to estimate the vehicle braking torque, considering aerodynamics, vehicle friction and engine passive losses in different gears, has been developed and identified over road tests, for a vehicle hybridized with wheel motors on rear wheels. The model has been used to analyze different braking strategies on real road tests, with and without mechanical brakes on rear wheels, in order to maximize the recovered energy by wheel motors and to prevent slipping conditions. The preliminary results show that the model is a useful tool to design real-time braking strategies, if properly combined with estimation of slipping coefficient and use of ABS systems.
Rajamani R., (2011). Vehicle dynamics and control. Springer Science & Business Media. Rizzoni, G., Marano, V., Tulpule, P., Stockar, S. (2009, December). Comparative study of different control strategies for plug-in hybrid electric vehicles. In 9th International Conference on Engines and Vehicles, ICE 2009. White and Korst. (1972). The determination of vehicle drag contributions from coast-down tests (No. 720099). SAE Technical Paper.
9. REFERENCES Anderson, M. and Harty, D. (2010, August). Unsprung mass with in-wheel motors-myths and realities. In 10th International Symposium on Advanced Vehicle Control, Loughborough, UK. Arsie, I., Naddeo, M., D'Agostino, M., Rizzo, G., Sorrentino, M. (2013, September). Toward the Development of a Through-The-Road Solar Hybridized Vehicle. In Advances in Automotive Control (Vol. 7, No. 1, pp. 806-811). Christensen, L. (2014). Designing In-Hub Brushless Motors. Machine Design. Guiggiani, M. Dinamica del Veicolo. CittàStudiEdizioni, 1998. ISBN 88-251-7248-6. 514
ScienceDirect
Development of control 49-11 (2016) braking 497–504 DevelopmentIFAC-PapersOnLine of a a regenerative regenerative braking control strategy strategy for hybridized solar vehicle Development of regenerative braking control strategy fora solar vehicle Development of a hybridized regenerative braking control strategy for hybridized solar vehicle M. Grandone, M. Naddeo, D. Marra, G. for hybridized solar vehicle M. Grandone, M. Naddeo, D. Marra, G. Rizzo Rizzo M. Grandone, M. Naddeo, D. Marra, G. Rizzo M. Grandone, M. Naddeo, D. Marra, G. Rizzo Department of Industrial Engineering, University Department of Industrial Engineering, University of of Salerno, Salerno, Via Giovani Paolo II 132, 84084 Fisciano (SA) -- Italy Via Giovani Paolo II 132, 84084 Fisciano (SA) Italy Department of Industrial Engineering, University of Salerno, (Tel: +39 089964239; e-mail: [email protected]) Department of Industrial Engineering, University of Salerno, (Tel: +39 089964239; e-mail: [email protected]) Via Giovani Paolo II 132, 84084 Fisciano (SA) - Italy Via(Tel: Giovani II 132, e-mail: 84084 Fisciano (SA) - Italy +39Paolo 089964239; [email protected]) (Tel: +39 089964239; e-mail: [email protected]) Abstract: This This paper paper focuses focuses on on the the development development of of aa braking braking control control strategy strategy that that allows allows the the best best tradetradeAbstract: off between mechanical and regenerative braking on a hybridized vehicle. The research work is part of aa off between mechanical and regenerative braking of onaa braking hybridized vehicle. The that research work parttradeof Abstract: This paper focuses onanthe development control strategy allows the is best project for the development of automotive hybridization kit aimed at converting conventional cars into Abstract: paper focuses onanthe development of aa braking control that allows the is best project forThis the development of automotive hybridization kit aimed atstrategy converting conventional cars into off between mechanical and regenerative braking onaspect hybridized vehicle. The research work parttradeof a Through The Road hybrid solar vehicles. The main of the project is the integration of state-of-theoff between mechanical and regenerative braking a hybridized vehicle. research work is cars part of Through Road hybrid solar The mainonaspect of project is The the integration of state-of-theproject forThe the development of anvehicles. automotive hybridization kitthe aimed at converting conventional intoa art components (i.e. in-wheel motors, photovoltaic panels, batteries) with the development of an optimal project for theRoad development ofmotors, anvehicles. automotive hybridization kitthe aimed at converting conventional into art components (i.e. in-wheel photovoltaic panels, batteries) with ancars optimal Through solar mainhybrid aspectstructure of project isthe thedevelopment integration ofofstate-of-thecontrollerThe for Road powerhybrid management. A mild mildThe parallel is obtained obtained by substituting/integrating substituting/integrating Through The hybrid solar vehicles. The main aspect of the project is the integration of state-of-thecontroller for power management. A parallel hybrid structure is by art motors, photovoltaic batteries)panels with the of an optimal the components rear wheels wheels(i.e. within-wheel in-wheel motors and adding addingpanels, photovoltaic anddevelopment lithium-ion battery. A art components (i.e. in-wheel motors, photovoltaic panels, batteries)is with the development of an optimal the rear with in-wheel motors and photovoltaic panels and aa substituting/integrating lithium-ion battery. A controller for power management. A mild parallel hybrid structure obtained by hybridizing equipment prototype, patented by the University of Salerno, is installed on a FIAT Grande controller forequipment power A mildand parallel structure is obtained by substituting/integrating hybridizing prototype,motors patented byadding the hybrid University of Salerno, isand installed on a FIAT Grande the rearA wheels withmanagement. in-wheel panels afrom lithium-ion battery. A Punto. model useful useful for real-time braking control hasphotovoltaic been developed, developed, starting vehicle longitudinal longitudinal the rear wheels with in-wheel motors and adding photovoltaic panels and a lithium-ion battery. A Punto. A model for real-time braking control has been starting from vehicle hybridizing equipment prototype, patented by the University of Salerno, is installed on a FIAT Grande model and and considering considering dynamic weight weight distribution in front front of andSalerno, rear axles axles and related related wheel slipping hybridizing equipment prototype, patented by the University is installed on a FIAT Grande model dynamic distribution in and rear and wheel slipping Punto. model useful for real-time control hasinvestigated, been developed, starting from vehicle effects.A Different braking strategiesbraking have then then been in order order to maximize maximize thelongitudinal benefits of of Punto. A model useful for real-time braking control hasinvestigated, starting from vehicle longitudinal effects. Different braking strategies have been in to the benefits model and considering dynamic weight distribution inbeen frontdeveloped, and rear axles and related wheel slipping regenerative braking. model and considering dynamic weight in front and rear axlestoand related the wheel slipping regenerative braking. effects. Different braking strategies havedistribution then been investigated, in order maximize benefits of Keywords: automotive hybridization kit; hybrid solar vehicles (HSV); vehicle electrification; HSV effects. Different braking strategies have then been investigated, in order to maximize the benefits of © 2016, IFAC (International Federation of Control) Hosting(HSV); by Elsevier Ltd. All rights reserved. regenerative braking. Keywords: automotive hybridization kit;Automatic hybrid solar vehicles vehicle electrification; HSV prototype; control strategy; regenerative braking; in-wheel motors. regenerative braking. prototype; control strategy; regenerativekit; braking; motors. (HSV); vehicle electrification; HSV Keywords: automotive hybridization hybridin-wheel solar vehicles Keywords: automotive hybridization kit; hybrid solar vehicles prototype; control strategy; regenerative braking; in-wheel motors. (HSV); vehicle electrification; HSV prototype; control strategy; regenerative braking; in-wheel motors.relevant number of braking events representing a because because of of relevant number of braking events representing a 1. 1. INTRODUCTION INTRODUCTION big energy loss with aa great potential. big energy with number great saving saving potential. because of loss relevant of braking events representing a Obviously, in order to be cost effective, in regenerative In recent times, hybrid vehicles (HEVs) have received an 1. of loss relevant number of braking events Obviously, in order togreat be saving cost effective, in aarepresenting regenerativea In INTRODUCTION recent times, hybrid vehicles (HEVs) have received an because big energy with a potential. 1. INTRODUCTION braking system the primary energy saved over a specified increasing attention as an alternative to traditional vehicles energy loss with potential. braking system the aprimary energy saved inover a specified increasing attention as anvehicles alternative to traditional vehicles Obviously, inoffset order togreat be saving cost effective, a regenerative In recentbytimes, hybrid (HEVs) have received an big lifetime must the initial cost, size and weight penalties powered internal combustion engines using fossil fuels. Obviously, in order to be cost effective, in a regenerative lifetime must offset the initial cost, size and weight penalties In recent times, hybrid vehicles (HEVs) have received an powered by internal combustion engines using fossil fuels. braking system The the primary energy unit savedmust overbea compact, specified increasing attention as an alternative to traditionaland vehicles of the system. energy storage Considering recent advances in battery technology motor braking system the primary energy saved over a specified of the system. The energy storage unit must be compact, increasing attention as an alternative to traditional vehicles Considering recent advances in battery technology and motor lifetime mustcapable offset the initial cost, size andlevels weightefficiently, penalties powered by HEVs internalare combustion engines using way fossiltofuels. durable and of handling high power efficiency, aa valid and feasible reduce lifetime mustcapable offset the initialstorage cost, and weight penalties durable and ofenergy handling highsize power levels efficiently, powered by HEVs internal combustion engines using way fossiland efficiency, are valid and feasible tofuels. reduce of the system. The unit must be compact, Considering recent advances in battery technology motor and any auxiliary energy transfer or energy conversion emissions and fuel consumption, and represent an effective of the system. The energy storage unit must be compact, and any auxiliary energy transfer or energy conversion Considering recent advances in battery technology and motor emissions and fuel consumption, and represent an effective andmust capable of handling high power levels efficiently, efficiency, HEVs are athe valid and electrification. feasible way to reduce durable be compact and of reasonable cost. bridge toward vehicle Recently, durable andauxiliary capable of handling high power levels efficiently, equipment must be efficient, efficient, compact and of reasonable cost. efficiency, HEVs are athe valid andand feasible wayanto reduce equipment bridge solution solution toward vehicle electrification. Recently, and any energy transfer or energy conversion emissions and fuel consumption, represent effective However, mechanical braking is still required in HEVs to keep some solutions for converting conventional vehicles into and any auxiliary energy transfer or energy conversion However, mechanical braking is still required in HEVs to keep emissions and fuel consumption, and represent an effective some solutions for converting conventional vehicles into equipment mustmore be efficient, compact and oftoreasonable cost. bridge solution have toward thebeen vehicle electrification. Recently, braking phases safe and, for example, avoid problems hybrid vehicles also proposed (Arsie et al., 2013; equipment must be efficient, compact and of reasonable cost. braking phases more safe and, for example, to avoid problems bridge solution toward the vehicle electrification. Recently, hybrid vehicles have also been proposed (Arsie et al., 2013; However, mechanical braking is still required in HEVs to keep some solutions for converting conventional vehicles into in case of electrical failure. Marano et Hall et 2016), optimal energy mechanical braking is still requiredtoinavoid HEVs to keep in case of electrical failure. some for converting conventional vehicles into However, Maranosolutions et al., al., 2013, 2013, Hall been et al., al.,proposed 2016), and and optimal energy hybrid vehicles have also (Arsie et al., 2013; braking phases more safe and, for example, problems Several studies on braking control strategies are management strategies suitable for such of vehicles have phases more safe and, for example, to avoid problems Several studies on regenerative regenerative braking control strategies are hybrid vehicles have also been proposed (Arsie et al.,energy 2013; management strategies suitable for such kind kind ofoptimal vehicles have braking Marano et al., 2013, Hall et al., 2016), and in case of electrical failure. available in literature (Rizzoni, et al., 2009; Maia, et al., 2015; been studied (Pisanti et al, 2014). in case of electrical failure. available in literature (Rizzoni, et al., 2009; Maia, et al., 2015; Marano et al., 2013, Hall et al., 2016), and optimal energy been studied (Pisanti et al, 2014). Several et studies on regenerative braking control strategies are management strategies suitable for such kind of vehicles have Kumar, al., 2016). They are carried out on classical An important advantage of HEVs compared to conventional Several et studies on regenerative braking control are al., 2016). They are mostly mostly carried outstrategies onal., classical management strategies suitable for such kind of have Kumar, An important advantage of2014). HEVs compared to vehicles conventional available in literature (Rizzoni, et al., 2009; Maia, et 2015; been studied (Pisanti et al, HEV’s or EV’s, and cannot be simply transposed to hybridized vehicles is the possibility to save energy during braking available in literature (Rizzoni, et al., 2009; Maia, et al., 2015; HEV’s or EV’s, and cannot be simply transposed to hybridized been studied (Pisanti et al, 2014). vehicles is the possibility to save energy during braking Kumar, etwhere al., 2016). They are are mostly carried outwheels on classical An important of HEVs compared tobrakes, conventional vehicles, wheel motors added in in phases. When aaadvantage conventional vehicle applies its kinetic Kumar,oretwhere al., 2016). They aresimply mostly carried outto on classical wheel motors are added in rear rear wheels in an an An important advantage of HEVs compared conventional phases. When conventional vehicle applies itsto brakes, kinetic vehicles, HEV’s EV’s, and cannot be transposed hybridized vehicles is the possibility to save energy during braking existing conventional vehicle. Comparing to classical native energy is converted to heat due to friction between the brake HEV’s or EV’s, and cannot be simply transposed to hybridized existing conventional vehicle. Comparing to classical native vehicles is the possibility to save energy during braking energy is converted to heat due to friction between the brake vehicles, hybridized where wheelvehicles motors are added in rear wheels in the an phases. When a conventional vehicle away applies its brakes, kinetic HEVs, are characterized by pads and wheels. This is carried in the airstream and vehicles, where wheelvehicles motors are intorear wheels in the an hybridized areadded characterized bynative phases. When a conventional vehicle applies its pads andis wheels. Thistoheat heat isdue carried away inbetween thebrakes, airstream and HEVs, existing conventional vehicle. Comparing classical energy converted heatThe to friction thekinetic brake impossibility to modify/adapt the original engine control kinetic energy is wasted. total amount of energy lost in existing conventional vehicle. Comparing to classical native impossibility to modify/adapt the original engine control energy is converted to heat due to friction between the brake kinetic is This wasted. total amount energy lostand in HEVs, hybridized vehicles are characterized by the padsway andenergy wheels. heatThe is carried away in of the airstream strategy (i.e. original ECU) order to for the this depends on how how hard how long hybridized vehicles characterized by strategy (i.e. to original ECU) in inare order to account account for the pads andenergy wheels. This heatoften, is carried awayand in of the airstream and this way depends on how often, how amount hard and how long brakes brakes impossibility modify/adapt the original engineelectrical control kinetic is wasted. The total energy lost ina HEVs, electrical parts and their control (i.e., battery pack, are applied. A regenerative brake is a system that allows impossibility to modify/adapt the original engine control electrical parts and their control (i.e., battery pack, electrical kinetic energy is wasted. The total amount of energy lost in are applied. A regenerative brake is a system that allows a strategy (i.e.solar original ECU) in order to account for the this waytodepends on how often, how hard anddissipated how long brakes engine and panels). Moreover, for the specific vehicle vehicle recover part of the kinetic energy during (i.e. original ECU) in(i.e., order to for the engine and solar panels). Moreover, for the account specific vehicle this waytodepends on how howishard anddissipated how long brakes vehicle recover part ofoften, the brake kinetic energy duringa strategy electrical parts and their control battery pack, electrical are applied. A regenerative a system that allows developed, all the information needed to control the vehicle are braking phases. Recovered energy can be stored in aa battery in electrical parts and their control (i.e., battery pack, electrical developed, all the information needed to control the vehicle are are applied. A regenerative brake is a system that allows a braking phases. Recovered energy can be stored in battery in and solar panels). Moreover, for themeasured specific vehicle vehicle to recover and partused of the kinetic energy dissipatedphases, during engine derived from a limited number of variables by the form of electricity during vehicle acceleration engine and solar panels). Moreover, for the specific vehicle derived from a limited number of variables measured by the vehicle to recover part of the kinetic energy dissipated during form of electricity and used during vehicle acceleration phases, developed, all theet information needed to control the vehicle are braking phases. Recovered energy can be stored in a battery in OBD port (Arsie al., Marano et al., estimating increasing efficiency and fuel economy. developed, all athe information needed to the vehicle are port (Arsie et al., 2013; 2013; Marano et control al., 2013), 2013), estimating braking phases. Recovered energy can be stored in a battery in OBD increasing efficiency fuel economy. derived from limited number of variables measured by the form of electricity andand used during vehicle acceleration phases, the missing data with suitable mathematical models (Naddeo The energy available for storage depends on drive train derived from a limited number of variables measured by the the missing data with suitable mathematical models (Naddeo form of electricity and used during vehicle acceleration phases, The energy available for storage depends on drive train OBD port (Arsie et al., 2013; Marano et al., 2013), estimating increasing efficiency and and fuel economy. et al., 2014). Such models been implemented in efficiency, drive cycle vehicle weight (Guzzella and port (Arsie et al., 2013; have Marano et al., 2013), et al., 2014). Such models have been implemented in the the increasing efficiency andfor fuelstorage economy. efficiency, drive cycle and vehicledepends weight on (Guzzella and OBD The energy available drive train the missing data with suitable mathematical modelsestimating (Naddeo braking control strategy presented in this work. Sciarretta, 2013). The impact of regenerative braking on the missing data with suitable mathematical models (Naddeo braking control strategy presented in this work. The energy available for storage depends on drive train Sciarretta, 2013). The impact of regenerative braking on al., 2014). Such models ahave been implemented in the efficiency, driveis cycle and vehicle weight (Guzzella and et In the following chapters, study on energy energy much more significant in urban driving et al., Such models been implemented in the In the 2014). following chapters, ahave study on the the energy recovery recovery efficiency, driveis cycle and weight and energy saving saving much morevehicle significant in (Guzzella urban driving Sciarretta, 2013). The impact of regenerative braking on potentialities braking control strategy presented in this work. by means of regenerative braking in aa hybridized braking control strategy presented in this work. potentialities by means of regenerative braking in hybridized Sciarretta, 2013). The impact of regenerative braking on energy saving is much more significant in urban driving In the following chapters, a study on the energy recovery In the followingmeans chapters, a study onbraking the energy recovery energy saving is much more significant in urban driving potentialities of regenerative in a hybridized Copyright © 2016 IFAC 507 potentialities by by means of regenerative braking in a hybridized Copyright © 2016 IFAC 507 2405-8963 © 2016, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Copyright 2016 responsibility IFAC 507Control. Peer review©under of International Federation of Automatic Copyright © 2016 IFAC 507 10.1016/j.ifacol.2016.08.073
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vehicle is presented, and control strategy are described. Moreover, the case with mechanical braking on rear axle coupled with regenerative one is also considered with the objective to define the best distribution of mechanical braking torque between front and rear axle to maximize regenerative effect, and guarantee a suitable and safe braking control.
Li-Ion Battery Voltage [V]
96
Li-Ion Battery Mass [kg]
45
In-wheel motors power [kW]
14
Tab. 1. Vehicle and Hybridization Kit Characteristics
2. A KIT FOR SOLAR HYBRIDIZATION A solar-hybridized vehicle has been developed at the University of Salerno through the installation of an additional battery (Lithium-Ion) and two electrically driven in-wheel motors on the rear wheels on a conventional vehicle. Moreover, solar cells on vehicle bonnet and roof have been installed so that it is possible to charge battery pack taking advantage of solar energy. In that way, the vehicle can operate in pure electric mode (when ICE is switched off or disconnected by the front wheels) or in hybrid mode (when the ICE drives the front wheels and the rear in-wheel motors operate in traction mode or in generation mode, corresponding to a positive or negative torque). The battery can be recharged by both rear wheels, when operating in generation mode, and photovoltaic panels. Furthermore, it is possible to charge battery from electric grid. A Vehicle Management Unit (VMU) receives data from OBD gate, from battery (SOC estimation) and drives in-wheel motors by properly acting on the electric node EN. A display on the dashboard may advice the driver about the actual operation of the system (Fig. 1).
Fig. 2. Hybrid Solar Vehicle by University of Salerno
A study on the technical and economic feasibility of the hybridization kit confirms that significant reduction of fuel consumption and CO2 emissions (18- 22%), comparable with HEVs benefits, but at lower investment cost, can be achieved. The results show that driving distance and type (urban vs. highway) and the availability of charging infrastructure play an important role in fuel economy, CO2 emissions savings and pay-back time. Moreover, in spite of high cost of the flexible PV panels, the solution with PV panels result in lower payback time with respect to solutions with in-wheel motors only. 3. IN-WHEEL MOTORS One of the most interesting aspects of the solar hybridization kit is represented by the partial recovery of kinetic energy allowed by wheel motors during braking. The optimization and the control of such feature represent the object of this paper. Use of in wheel motors (IWM) allows to electrify the vehicle with no impact on mechanical transmission. A significant amount of space is therefore saved in the vehicle, compared to other hybrid solutions where electric motor is placed out of vehicle chassis. A hub motor typically may be designed in three main configurations. The least practical is an axial-flux motor, where stator windings are typically sandwiched between sets of magnets. The other two configurations are both radial designed with the motor magnets bonded to the rotor. In one of them, the rotor sits inside the stator, as in a conventional motor while in the other one, the rotor sits outside the stator and rotates around it. A typical IWM scheme is shown Fig. 3.
Fig. 1. Scheme of a system to upgrade a conventional car into a Mild Hybrid Solar Vehicle
The kit has been installed on a FIAT Punto, at the laboratories of the University of Salerno (Fig. 2). Nominal ICE power [kW]
75
Fuel
Diesel
Coefficient of drag (Cd)
0.325
Frontal area [m2]
2.05
Rolling radius [m]
0.295
Rolling resistance coefficient [/]
0.02
Base vehicle mass [kg]
1105
Driver mass [kg]
70
PV installed power [kW]
0.280
PV mass [kg]
4.7
Li-Ion Battery Capacity [kWh]
4.2
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499
Fig. 3. In-Wheel Motor Scheme
The application of hub motors in cars is still evolving, and no configuration has become standard. In wheel motors main problem is their high weight that may affect comfort and safety. The so-called "non-suspended" masses (i.e. in wheel motors and brakes) must be as light as possible compared to "suspended masses" that, actually, consist in the entire body of the car (Anderson and Harty, 2010).
Fig. 5. In-Wheel Motors maps
Max Torque [Nm]
200
The addiction of IWM makes the normal vehicle a 4x4 traction one. Vehicle safety and stability can be enhanced by implementing an accurate electronic control of driving and braking torque, separately for each wheel. Moreover, a significant improvement in acceleration capability is achieved, thanks to the higher torque delivery at low rotation speed typical of electric motors (Fig. 6).
150 100 50 0 0
In order to develop the hybridization kit patented by University of Salerno and carry out the regenerative braking control strategy described in this paper, IWM produced by Kelly Controllers have been purchased on Chinese market. Their features and characteristic maps are shown in Fig. 4, Fig. 5, Fig. 6 and Tab. 2.
200
400 600 800 Wheel rotation speed [rpm]
1000
Fig. 6. IWM max Torque curve
Pick Power [kW]
7
Pick Torque [Nm]
150
Diameter-Width [mm]
303-107
Max speed [rpm]
1500
Weight [kg]
20
Tab. 2. In-Wheel Motors Characteristics
4. LONGITUDINAL DYNAMIC MODEL In order to develop the regenerative braking control strategies, whose results are presented in the following section, a longitudinal vehicle model has been developed under the following hypotheses (Guzzella and Sciarretta, 2013):
the drag force is considered acting on vehicle center of gravity;
vehicle inertia accounts for both vehicle mass (MHSV) and rotational inertia of ICE, EM/EG and wheels, through the term Meff;
the effects of elasticity transmission are neglected.
Fig. 4. In-Wheel Motor installed on HSV Prototype
in
the
mechanical
According to these assumptions, the power at wheels to the road load (Pwheel) has been modeled as follows:
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𝑃𝑃𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒 = 0.5𝜌𝜌𝐶𝐶𝑥𝑥 𝐴𝐴𝑣𝑣 3 + 𝑀𝑀𝐻𝐻𝐻𝐻𝐻𝐻 𝑔𝑔𝑔𝑔[𝐶𝐶𝑟𝑟 cos(𝛼𝛼) + sin(𝛼𝛼)] 𝑑𝑑𝑑𝑑 + 𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 𝑣𝑣 𝑑𝑑𝑑𝑑
starting from different target speed. During deceleration tests there was neither torque from engine nor from brakes, thus Equation 3 can be simplified setting Tcomb=0 and Tbrake=0, Tdrive = 0; furthermore there was no road slope so Tmg=0.
(1)
In these conditions, Equation 3 becomes:
where and v are the road grade and vehicle speed, respectively.
𝑇𝑇𝑑𝑑𝑑𝑑 = −(𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 ∗
Equation 1 can be rewritten as: 𝑃𝑃𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑃𝑃𝑚𝑚𝑚𝑚 + 𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 Pwheel is the power at wheels 𝑃𝑃𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 = 0.5𝜌𝜌𝐶𝐶𝑥𝑥 𝐴𝐴𝑣𝑣3 is the aerodynamic power
𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑀𝑀𝐻𝐻𝐻𝐻𝐻𝐻 𝑔𝑔𝑔𝑔𝐶𝐶𝑟𝑟 cos(𝛼𝛼) is the rolling power
𝑃𝑃𝑚𝑚𝑚𝑚 = 𝑀𝑀𝐻𝐻𝐻𝐻𝐻𝐻 𝑔𝑔𝑔𝑔sin(𝛼𝛼) is the power due to vehicle weight 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑
𝑣𝑣 is the inertial forces power.
In Fig. 8 the driveline braking torque computed for different gear ratios is shown. It is worth remarking that the driveline torque increases by decreasing the gear ratio. This is mainly due to the engine passive torque that increases with engine speed, which is higher for lower gear ratio (once the car speed is fixed).
The aerodynamic drag coefficient Cx is a parameter provided by vehicle manufacturer. (Anderson and Harty, 2010) 5. DRIVELINE TORQUE EFFECT In order to estimate the braking torque value and how it is split between rear and front axles the power balance expressed by Equation 2 has been written in terms of torque dividing per wheels rotational speed:
-100
Driveline braking torque [Nm]
𝑇𝑇𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒
𝑑𝑑𝜔𝜔 = 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 𝑑𝑑𝑑𝑑
(3)
Where: 𝑇𝑇𝑤𝑤ℎ𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑇𝑇𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 − 𝑇𝑇𝑑𝑑𝑑𝑑 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑇𝑇𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑇𝑇𝑚𝑚𝑚𝑚
(6)
For each deceleration test, Equation 6 describes driveline braking torque profile that can be expressed as function of velocity once gear is defined. This dependence is expressed for each gear as a polynomial function of vehicle speed. The parameters have been identified by minimizing the error between the driveline torque computed by using experimental data (i.e. velocity profile) according to the right hand side of Equation 6 and the value provided by the polynomial model. Fig. 7 shows the comparison between the driveline torque estimated by the polynomial regression and the experimental data for the third gear.
(2)
Where:
𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 = 𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒
𝑑𝑑𝜔𝜔 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ) 𝑑𝑑𝑑𝑑
(4)
Speed-Driveline braking torque (Third gear) Estimated from Experimental Data Polynomial Approximation
-150 -200 -250 -300 -350 40
Twheel is the torque at wheels
50
60 Speed [km/h]
70
80
Taero is the aerodynamic torque Fig. 7. Data-Polynomial approximation Comparison
Troll is the rolling torque Tmg is the torque due to vehicle weight
Driveline Braking Torque [Nm]
0
Tcomb is the propulsive part of engine torque Tbrake is the brakes torque Tdl is the engine/transmission passive torque which will be indicated as driveline braking torque hereafter. It is important noting that Tdl accounts for IWM passive torque due to internal friction too. During braking phases there is no propulsive torque from engine, so: 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = − (𝑀𝑀𝑒𝑒𝑒𝑒𝑒𝑒 ∗
𝑑𝑑𝜔𝜔 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ) − 𝑇𝑇𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑
-200
-400
-600
-800 0
20
40 60 Speed [Km/h]
80
Gear 1 Gear 2 Gear 3 Gear 4 Gear 5 100
(5) Fig. 8 – Driveline torque computed for different gear ratio.
In order to quantify the driveline braking effect an experimental tests campaign was carried out: the vehicle reached a target speed and the gas pedal was released, with gear engaged. These tests were carried out for each gear
Using these polynomial functions to identify driveline braking torque for each gear, it is possible to calculate braking torque profile (Fig. 10) regarding to a real driving cycle through
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Equation 5. Speed profile of this driving cycle is shown in Fig. 9.
X1 e X2 are longitudinal forces on front and rear axles respectively;
Z1 e Z2 are vertical forces on front and rear axles respectively;
u is the vehicle speed;
l is the distance between front and rear wheels;
a e b are distances between vehicle center of gravity and front and rear axles respectively;
h is the vehicle center of gravity height;
W = mg is the vehicle weight;
G is the vehicle center of gravity position;
X0-Z0 is the street reference system;
X-Z is the vehicle reference system.
20
speed [m/s]
15
10
5
0 0
50
100 time [s]
150
200
Fig. 9. Driving Cycle Speed Profile
0
The proposed model is based on the following assumptions: 1. constant deceleration;
-500
Torque[Nm]
501
2. flat road;
-1000
3. absence of lateral forces; -1500
4. equal adhesion conditions for the wheels on the same axle.
-2000 0
If these assumptions are satisfied, then it is possible to write the relative dynamic equations: 50
100 time [s]
150
200
𝑚𝑚𝑢𝑢̇ = −(𝑋𝑋1 + 𝑋𝑋2 ) 0 = 𝑍𝑍1 + 𝑍𝑍2 − 𝑚𝑚𝑚𝑚 0 = (𝑋𝑋1 + 𝑋𝑋2 )ℎ − 𝑍𝑍1 ∗ 𝑎𝑎 + 𝑍𝑍2 ∗ 𝑏𝑏
Fig. 10. Braking Torque Profile
(7)
6. REGENERATIVE BRAKING STRATEGY 6.1 Safety braking region definition
At constant cruise velocity, W1 and W2 are static loads for both axles and can be defined as: 𝑏𝑏 𝑊𝑊1 = 𝑚𝑚𝑚𝑚 𝑙𝑙 (8) 𝑎𝑎 𝑊𝑊2 = 𝑚𝑚𝑚𝑚 𝑙𝑙
In the previous chapter, a model to estimate braking torque for a real driving cycle has been developed. The objective of this one is to analyze how braking torque can be split between front and rear axles. In order to reach that goal, it is necessary to refer to a simplified model of the vehicle that defines a safety braking region.
During deceleration, load on front axle becomes greater than the rear one according to: 𝑚𝑚ℎ 𝑢𝑢̇ 𝑙𝑙 𝑚𝑚ℎ 𝑍𝑍2 = 𝑊𝑊2 − ∆𝑍𝑍 = 𝑊𝑊2 + 𝑢𝑢̇ 𝑙𝑙
𝑍𝑍1 = 𝑊𝑊1 + ∆𝑍𝑍 = 𝑊𝑊1 −
(9)
The maximum possible deceleration is obtained when both axles are at grip limit: 𝑋𝑋1 = 𝜇𝜇𝑍𝑍1 𝑋𝑋2 = 𝜇𝜇𝑍𝑍2
(10)
Substituting these values in the equations of the dynamics of the system (Equation 7): Fig. 11. Forces acting on the vehicle during braking
According to the scheme of Fig. 11:
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|𝑢𝑢̇ |𝑚𝑚𝑚𝑚𝑚𝑚 = 𝜇𝜇𝜇𝜇)
(11)
Combining results, the maximum braking force applied on both axles without slip are: ℎ 𝑋𝑋1𝑝𝑝 = 𝜇𝜇(𝑊𝑊1 + 𝑚𝑚 𝜇𝜇𝜇𝜇) 𝑙𝑙 ℎ 𝑋𝑋2𝑝𝑝 = 𝜇𝜇(𝑊𝑊2 − 𝑚𝑚 𝜇𝜇𝜇𝜇) 𝑙𝑙
(12)
Assigning this maximum value of in wheels motor control signal if the working point is inside the feasible region.
(13) 2500
Front Braking Torque
𝑋𝑋20
𝜇𝜇𝑊𝑊1
ℎ 1 − 𝜇𝜇 𝑙𝑙 𝜇𝜇𝑊𝑊2 = ℎ 1 + 𝜇𝜇 𝑙𝑙
Identifying feasible braking region and the position of the working point corresponding to the condition listed above;
In case this condition is not verified, the controller will assign the maximum value of in wheels motor control signal to set the working point on the right side of the feasible region edge line. Since the total braking torque required by driver must be kept constant, mechanical braking torque will increase and the working point will move upper as shown in Fig. 13.
In case braking force is applied only on one axle: 𝑋𝑋10 =
In this way it is possible to identify the feasible braking region on plane X2-X1, once defined grip coefficient and car specification.
2000
Feasible Braking Region Feasible Braking Region Working point position before controller correction Working point position after controller correction
1500 1000 500 0 0
500
1000 1500 Rear Braking Torque
2000
2500
Fig. 13. Controller Braking Modulation Effect
The approach explained above is consistent either in case on rear axle there is an electric brake only or mechanical and electric brakes are both installed. In case of electric brake only, considering maximum in wheels motor control signal, x coordinate of the point is calculated using motor maps only while y coordinate is obtained per difference as already explained above. On the other hand, considering mechanical braking torque too on rear axle, x coordinate of the point is obtained summing, to maximum electric braking torque from maps, a percentage of the remaining one depending on how mechanical braking is departed (usually 65% front axle, 35% rear axle).
Fig. 12. Feasible Braking Region
The feasible braking region identifies an area where the vehicle brakes without grip loosing; on the right side of this zone the rear wheels slip, above front wheels lose grip.
Y coordinate of the point is calculated per difference. This means that the working point in the second scenario is positioned lower and further on the right side compared to the first one since the whole braking torque is constant.
6.2 Braking control implementation Once feasible braking region is identified, it is important to develop a control strategy that avoids grip loosing and maximize regeneration. In order to get this result, a regenerative brake controller has been developed; the operating scheme of the controller can be described through the following steps:
Identifying a braking phase from negative value of Twheels;
Considering the maximum braking torque of in wheel motor corresponding to operating speed in order to maximize regenerative effect;
Calculating mechanical braking torque subtracting electric torque from Tbrake;
The second case is less stressful for front axle brakes since the whole system works on bottom right, but minimizes regenerative braking effect; on the other hand, the first one allows to maximize recovered energy. Unfortunately, when the controller modulates regenerative braking to move to the edge line in the first case scenario, this operation stresses front axle only with the possibility to cross over the top of feasible braking region. However, if that happens, the ABS will operate as in a conventional vehicle in order to avoid front wheels slipping. 7. SIMULATION RESULTS
by
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in the following. Only electric regenerative brakes are installed on rear axle of this prototype due to design constraints. The impact of regenerative braking has been evaluated using a real driving cycle whose characteristics are reported in Fig. 9 and considering wet conditions.
Braking Torque[Nm]
50
100 time [s]
150
200
Braking Torque[Nm]
RTTR [%]
80 60 40 20 0 0
Total Braking Torque Rear Axle Braking Torque Conventional rear axle Braking Torque
50
100 time [s]
150
200
Fig. 15. RTTR and Braking Torque compared to a conventional distribution between front and rear axles (relative to the second case)
In this case, braking torque is well balanced between axles in all conditions and wheels never slip during driving. On the other hand, regenerative braking controller does not need to module electric braking torque so the energy recovered during driving shows an increment of 76% compared to the first case.
500
50
100 time [s]
150
200
In order to have the whole picture of the situation, a configuration that takes into account mechanical brakes on rear axle coupled with electric brakes is considered. The case analyzed is the one with a conventional distribution of mechanical braking torque (65% front, 35% rear).
Rear Axle Braking Torque/Braking Torque Conventional rear axle Braking Torque/Braking Torque 100
Results are shown in Fig. 16. In this case the working point is always inside the feasible braking region due to the action of the regenerative braking control system (17 times operating). Anyway, compared to the first case, recovered energy shows only a little decrease (3%). This happens because, in the most of case, load transfer to front axle during braking phases is low and it is possible to completely exploit the regenerative brakes on rear axle reaching a really high RTTR values without causing wheels slip.
80
RTTR [%]
500
100
1000
60 40 20 0 0
1000
Rear Axle Braking Torque/Braking Torque Conventional rear axle Braking Torque/Braking Torque
1500
0 0
1500
0 0
This configuration allows the maximum regenerative effect (28.5% of the electric energy used for traction is recovered), however, it leads to a low RTTR during high magnitude braking phases due to limited power of in wheel motors. This means the working point on the feasible braking region moves above the upper edge line during the driving cycle and front wheels ABS operates.
2000
Total Braking Torque Rear Axle Braking Torque Conventional rear axle Braking Torque
2000
Considering the percentage ratio between rear axle braking torque and total braking torque (Rear-Total Torque Ratio, RTTR) for the mentioned driving cycle, it is possible to notice its profile in Fig. 14 (bottom) and the comparison with the standard braking distribution (red line). In this case, most of braking phases are completely covered by electric brakes because of their low magnitude. This means that RTTR is often 100% or around this value. At the same time, in Fig. 14 (top) total braking torque, hybridized vehicle rear torque and conventional vehicle one profiles are shown.
503
50
100 time [s]
150
200
Fig. 14. RTTR and Braking Torque compared to a conventional distribution between front and rear axles (relative to the first case)
To avoid this kind of problems, a configuration with double IWM torque values has been considered and simulation results in terms of braking torque and RTTR are shown in Fig. Fig. 15.
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Total Braking Torque Rear Axle Braking Torque Conventional rear axle Braking Torque
2000
Braking Torque[Nm]
M. Grandone et al. / IFAC-PapersOnLine 49-11 (2016) 497–504
Hall, J., Bassett, M., Borman, S., Lucas, T., Whitehead, A. (2016). Through-the-Road Parallel Hybrid with InWheel Motors (No. 2016-01-1160). SAE Technical Paper.
1500 1000
Kumar N. and Subramanian S. (2016). Cooperative control of regenerative braking and friction braking for a hybrid electric vehicle. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 230(1), 103-116.
500 0 0
50
100 time [s]
150
200
Maia R., Silva M., Araùjo R., Nunes U. (2015). Electrical vehicle modeling : a Fuzzy logic model for regenerative braking. Expert Systems with Applications. Volume 42, Issue 22.
Rear Axle Braking Torque/Braking Torque Conventional rear axle Braking Torque/Braking Torque 100
Marano, V., Medina, H., Sorrentino, M., Rizzo, G. (2013). A model to assess the benefits of an after-market hybridization kit based on realistic driving habits and charging infrastructure. SAE International Journal of Alternative Powertrains, 2(3), 471-481.
RTTR [%]
80 60 40 20 0 0
50
100 time [s]
150
Naddeo, M., D'Agostino, M., Rizzo, G. (2014). Development and validation of a model to detect active gear via OBD data for a Through-The-Road Hybrid Electric Vehicle. IFAC World Congress. Cape Town (South Africa), In World Congress (Vol. 19, No. 1, pp. 66186623).
200
Fig. 16. RTTR and Braking Torque compared to a conventional distribution between front and rear axles (relative to the third case)
Pisanti C., Rizzo G., Marano V. (2014). Energy Management of Through-The-Road Parallel Hybrid Vehicles. 19th IFAC World Congress, August 24-29, 2013, Cape Town (S.Africa), Vol. 19, No. 1, pp. 2118-2124.
8. CONCLUSIONS A model able to estimate the vehicle braking torque, considering aerodynamics, vehicle friction and engine passive losses in different gears, has been developed and identified over road tests, for a vehicle hybridized with wheel motors on rear wheels. The model has been used to analyze different braking strategies on real road tests, with and without mechanical brakes on rear wheels, in order to maximize the recovered energy by wheel motors and to prevent slipping conditions. The preliminary results show that the model is a useful tool to design real-time braking strategies, if properly combined with estimation of slipping coefficient and use of ABS systems.
Rajamani R., (2011). Vehicle dynamics and control. Springer Science & Business Media. Rizzoni, G., Marano, V., Tulpule, P., Stockar, S. (2009, December). Comparative study of different control strategies for plug-in hybrid electric vehicles. In 9th International Conference on Engines and Vehicles, ICE 2009. White and Korst. (1972). The determination of vehicle drag contributions from coast-down tests (No. 720099). SAE Technical Paper.
9. REFERENCES Anderson, M. and Harty, D. (2010, August). Unsprung mass with in-wheel motors-myths and realities. In 10th International Symposium on Advanced Vehicle Control, Loughborough, UK. Arsie, I., Naddeo, M., D'Agostino, M., Rizzo, G., Sorrentino, M. (2013, September). Toward the Development of a Through-The-Road Solar Hybridized Vehicle. In Advances in Automotive Control (Vol. 7, No. 1, pp. 806-811). Christensen, L. (2014). Designing In-Hub Brushless Motors. Machine Design. Guiggiani, M. Dinamica del Veicolo. CittàStudiEdizioni, 1998. ISBN 88-251-7248-6. 514