- Email: [email protected]
Development of a Brushless AC Servo Drive for the Tube Locator Module for Steam Generator Tube Inspection Device
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 86 (2014) 511 – 519
1st International Conference on Structural Integrity, ICONS-2014
Development of a Brushless AC Servo Drive for the Tube Locator Module for Steam Generator Tube Inspection Device Sree Ranjini K.Sa,*, Joel Joseb, T. Anuradhaa, Vasan Prabhua and S. Sakthivelb b
a Anand Institute of Higher Technology, Chennai-603103, India Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India * E-mail ID: [email protected]
Abstract The fundamental issue in the area of remote handling and robotics is the reliability of power electronics system. Detailed analysis and simulation studies are very much relevant as the prebuilt drive solutions are costly and performance of drive system depends upon the reliability of control algorithms incorporated in the drive. The modulation techniques also play an important role in the dc voltage utilization. This paper presents a comparative simulation study for the analysis of complete BLAC drive using PI controller using two types of modulation techniques namely Sinusoidal Pulse Width (SPWM) and Space Vector Modulation (SVPWM). © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research Keywords: SCARA, Field Oriented Control, Proportional Integral Controller, BLAC, Space Vector Modulation
1. Introduction Selective Compliance Articulated Robot Arm (SCARA) finds wide application in industries where planar reach is required. The remote inspection of steam generator tubes is no exemption due to the precise reach in the planar task space of the remotely deployed device over the tube sheet connecting all 547 tubes in Prototype Fast Breeder Reactor (PFBR). SCARA is a jointed two-link arm similar to our human arms rigid in the Z-axis and has good compliance in the XY-Cartesian space. The actuation of the two joints has to be provided by two independent servomotors, which have better dynamic performance, smaller size, high instantaneous torque and little torque ripple. BLAC motor is selected to be suited for this payload, which is handling the Eddy Current Testing (ECT) probe along with its cable. A high performance servo control system is required and control algorithm has to properly chosen. As the prebuilt control and driver modules are quite big and expensive to suit the application and less flexible to integrate the complete user routines, the drives were preferred to be built and the control routines
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research doi:10.1016/j.proeng.2014.11.075
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programmed. Hence, simulation studies are carried out to investigate the appropriate control algorithm and modulation technique in order to have maximum voltage utilization of DC bus. Previously, efforts has been taken to compare the performance of various pulse width modulation techniques implemented for BLAC drive based on FOC using SVPWM, SPWM and Third Harmonic Injection PWM [2]. The mathematical model of BLAC drive system based on the rotor field oriented control with PI controller; the entire BLAC driver control system is analyzed and static and dynamic characteristics are determined [3]. Also direct position detection of BLAC is tested using SIMULINK model and static and dynamic characteristics are analyzed [4]. The mathematical model of BLAC machine is framed from the governing equations in dq plane for the verification of control algorithms and modulation techniques. Simulation is carried out with the powerful modeling capabilities of MATLAB/Simulink, for checking the effectiveness of modulation technique that can be incorporated into the drive system. The speed PI controller gain value was tuned and set according to Symmetric Optimum (SO) criterion and current PI controller gains were tuned and set by Absolute Value Optimum (AVO) criterion. The simulation results show that speed, current, torque follow the given value without much disturbance, precise and accurate control of motor can be achieved. The closed loop stability of the system is also being ensured by bode plot analysis. A methodology is proposed for hardware implementation of BLAC drive. This paper deals with the complete simulation of the drive system which gives confidence in building the drive for the SCARA device for Steam Generator tube inspection and simulation results are presented in detail. 2. Basic Theory 2.1. Brushless Alternating Current Machine (BLAC) The selection of machine for robotic arm actuation was the prime problem to be solved. Stepper motor because of rotation in steps and requirement of large step angle was rejected. Hence next move was towards Brushless Excited machines. Brushless excited machines have become the focal point in recent days due to their higher reliability, lack of maintenance and enhanced performance. The nature of the output waveform determines their classification into Brushless AC machine (BLAC) and Brushless DC machine (BLDC). BLDC machine has improved control characteristics than the BLAC machines. However torque per weight ratio of BLAC machines is very much higher than the BLDC machine which makes it suitable for the precise control of robotic arm. 2.2. Control Strategy of BLAC The various control strategies of controlling an AC motor scalar control and vector control. The scalar control allows the control of AC motor by considering only the magnitude of voltages and currents and adjusting the frequency to run the motor at synchronous speed. This method is simple to implement however gives poor dynamic performance and unstable operation of drive system. Field oriented control (FOC) or vector control is the method of control, which makes ac drives equivalent to dc drives. In this method, ac motor can be controlled like a separately excited dc motor. FOC is based on transformation from a three phase system (abc coordinates) to two phase system (dq) coordinates. There are two control loops for FOC control of BLAC. They are outer speed control and inner current control. FOC control is implemented with direct axes current equal to zero since the magnetizing current is provided by permanent magnets on the rotor structure. The quadrature axes of current controls the torque component there by making the coupling characteristics of BLAC decoupled one. FOC algorithm is preferred for the control of BLAC drive as it provides an easy way of obtaining reference current and direct control of torque, which is primarily required for robotic arm control. 2.3. Mathematical Modeling of BLAC The mathematical model of BLAC machine is used in simulation studies since it provides better dynamic and steady state performance [5]. The mathematical model is framed in rotor reference frame neglecting damper winding, saturation effects, iron losses, and considering back emf as sinusoidal. The mathematical model uses dq
K.S. Sree Ranjini et al. / Procedia Engineering 86 (2014) 511 – 519
equations (1)-(3) which during steady state are dc quantities as a result control is precise and have less error. ݅ௗୀ ݅ ൌ
െ
െ
ோ ݅ ௗ
ோ ݅
െ
ܲ߱ ݅
ܲ߱ ݅ௗ െ
(1)
ఒ ು ఠೝ
(2)
ܶ ൌ ͳǤͷൣߣ݅ ൫ܮௗ െ ܮ ൯݅ௗ ݅ ൧
(3)
where p denotes differential operator, R is the armature winding resistance, Vd and Vq denotes voltage in dq axis, Ld and Lq denotes inductance in dq axis, Te is the electromagnetic torque, P is number of poles is the rotor resistance, wr is the angular velocity. With the consideration of mechanical load, dynamic equation of the BLAC machine can be defined as Jmpwr + Bmwr = Te -TL
(4)
where Tl denotes the load torque, Jm denotes the coefficient of inertia, Bm denotes the frictional coefficient. The current control loop of BLAC drive with sinusoidal pulse width modulation was built from the basic blocks of PI controller for speed, SPWM block, voltage measurement block and mathematical model of BLAC. The current control loop of BLAC with SVPWM was built using two controllers one for d-axis current and other for q-axis current co-ordinate transformation block, inverter circuit and mathematical model of BLAC. 2.4. Functional Block Diagram The block level functional diagram of FOC control of BLAC drive using SPWM and SVPWM techniques is shown in the Fig.1.
Speed Contr oller
Speed Controlle r
Fig 1: Functional block diagram of BLAC Drive with SPWM and SVPWM
3. Simulation ǤͷǤ Three-phase two level inverter consists of two switches in each of the three legs. The switches may be BJT, GTO, or MOSFET. Three of the switches must be always ON and switches of same leg should not conduct at a time. Two types of modulation techniques – SPWM and SVPWM are discussed, which produce gate pulse required
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for conduction of switches.
3.1.1. Sinusoidal Pulse Width Modulation WM technique where carrier used is high frequency trian ngular signal. SPWM is carrier comparison based PW Three phase modulating signals are compared with w a carrier wave to generate PWM pulses for every threee-phase. By varying modulation index, magnitude frequenncy of fundamental component can be controlled. Sinu usoidal pulse width modulation is simple modulation techniqque and is linear between 0 – 78.5% of six step voltage value which result in poor voltage utilization and increassed harmonic content. The frequency of carrier wavefo orm used in simulation of sinusoidal pulse width modulationn is 6 kHz.
Fig 2: Generatiion of pulses using SPWM technique 3.1.2. Space Vector Pulse Width Modulation d cycle for each leg in the inverter circuit to generatee a reference Space vector modulation determines duty voltage phasor by selecting nearest of two volltage vectors at each sample time. SVPWM tries to produce a three phase balanced sinusoidal output having redduced harmonic components. According to switching sequence of MOSFET switches there are six non zero voltagge space vectors which are denoted by V1, V2, V3, V4, V5, V6 and two zero space vectors denoted by V0, V7. Placeement and order of non-zero space vectors are key facctors, which determine the performance of this control strateegy. Sector judgment is done according to formula N = A + 2B + 4C where A, B and C are determined fromܸఈ ܸ ܸఉ .
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q axis V3 (010) (-1/3, 1/¥3)
3
V2 (110) (1/3, 1/¥3)
2 Vref
T3
1 V1
V4 (011)
T4 6
4
(100) d axis (2/3, 0)
5 (-1/3, -1/¥3)V5 (001)
V6 (101)
Fig 3:: Space Vector Modulation In Fig.3, the reference voltage vector lies in secctor 3. The magnitude of the reference voltage and the tim me period of operation of adjacent vectors and the operatingg time for the switches of 3 phases were determined and gate pulses were produced after comparison with the carrierr signal.
Fig 4: Simulink Model for Space Vector Modulation 3.2. Speed Controller PI controller is one of the most widelyy used in speed control of drives. To obtain an optimal solution for hardware the simulation of FOC of BLAC machine m was verified by using traditional PI controller. Derivative control is not used in this case due to the introoduction of high amount of noise in the measurement off torque and speed. The reference speed is compared withh sensed speed and error is given to speed PI controlller. The dq projections of stator phase current was comparred with their reference values Iq(ref) and Id (ref) which is set to be zero and output is controlled by means of traaditional PI controller. The Speed loop PI controller is i tuned by Symmetric Optimum (SO) method and currentt loop PI controller was tuned by Absolute Value Optim mum (AVO) method [6]. AVO and SO methods depend on the closed loop transfer function of the system. These tw wo methods he system’s employ an approximate transfer function of thee system to be controlled. The AVO method assumes th transfer function in open loop as eq.(5). ͳ ܩሺݏሻ ൌ (5) ʹ߬ఀ ݏሺͳ ߬ఀ ݏሻ SO method considers the system’s open loop traansfer function as eq.(6)
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ܩሺݏሻ ൌ
ሺͳ Ͷ߬ఀ ݏሻ ͺ߬ఀ ଶ ݏଶ ሺͳ ߬ఀ ݏሻ
(6)
d to the digital implementation of the control (which h implies the where IJȈ is sum of all small delays in loop due sampling of signals), the use of filters, the processing of the control algorithm and the use of Pulse P Width ng eq.(7) and Modulators (PWM). The ki and kp values of currrent controller is calculated as per AVO criterion by usin eq.(8). ܮ ߬ ൌ Ǣ ߬ఀ ൌ ʹ߬௦ (7) ܴ௦
݇ ൌ
ܮ
Ǣ ݇ ൌ
݇
(8) ʹ߬ఀ ߬ Similarly ki and kp values of current controller is i calculated as per SO criterion using eq.(9) and eq.(10) ͵ ఠೝ ߬௦ ఠ ೝ (9) ఠ ఠೝ ߬ఀ ൌ ߬௦ ʹ߬ఀ െ Ǣ ߬ ൌ Ͷ߬ఀ ೝ ʹ ʹ ఠ (10) ݇ ೝ ܬ ఠ ఠೝ Ǣ ݇ ൌ ݇ ೝ ൌ ఠ ఠ ͵߰ܲଶ ߬ఀ ೝ ߬ ೝ The sampling time for current controller is choosen to be 10 μs while sampling time for speed controllerr was chosen to be 1ms. In addition, Fig. 5 shows Open Loopp Bode plot which ensures the stability of BLAC drive with SVPWM and PI controller for entire response time.
Figg 5: Open Loop Bode Plot At a response time of 0.063s the gainn margin was found to be 19.8 db and phase margin was found to be 80.1degrees. 4. Simulation Results and Discussion The computer simulation for control of BLAC drive is implemented with MATLAB/Simuliink software with SPWM and SVPWM modulation techniquues using (Table: I).The effectiveness of both modulation n techniques are verified using PI controller and the results are a compared.
K.S. Sree Ranjini et al. / Procedia Engineering 86 (2014) 511 – 519
Fig 6: Torque and Speed response of BLAC drive with SPWM and SVPWM TABLE I.
MOTOR PARAMETERS
Stator Resistance (Rs ) Direct axis reactance (Ld) Quadrature axis reactance(Lq) Moment of inertia(J) Rotor flux(Ȝ) Viscous coefficient(B)
1.4 0.014mH 0.014mH .0001051Kgm2 0.1848 Wb 4047e-0.005 Nm
The simulation study reveals that SVPWM gives 15% enhanced fundamental output with better quality i.e. lesser Total Harmonic Distortion (THD) compared to SPWM, which proves the effectiveness of SVPWM over SPWM. 5. Hardware Implementation The hardware implementation of BLAC drive with PI controller can be realized with FPGA [7]. In an alternative approach of speed estimation of brushless AC servomotors, a network structure representing the electrical and mechanical properties of the servomotor is built via Artificial Neural Network (ANN) and trained with the results of mathematically modelled driver system [11]. Single chip solutions are also available for the current as well as velocity control of BLAC such as one provided by International Rectifiers which does not require any programming to complete AC servo control algorithm and has flexible configuration ability [8]. IRMCK201 is a one-chip solution for complete closed loop current control and velocity control for a high performance servo drive system. Compared with the traditional control chip, it includes not only the motion control peripheral functions but also magnetic field oriented control (FOC) algorithm and closed loop speed control realized in hardware, and simplifies hardware structure and shortens the development cycle of the system.
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DC Input
Processor/ Controller
Switching Circuit IRMCK201
Isolation Current sensor
BL AC Encoder
Fig 7: Schematic diagram of BLAC drive using IRMCK201 IRMCK201 is a core control chip used for complete current and velocity control of PMSM machine. An external current sensor and Encoder is used for feedback. A processor communicates with IRMCK201 reads and writes the control word to the memory of IRMCK201. The entire systems avoids the software programming of Field orientated control algorithm, and greatly reduces the development cycle, and shortens the system response time, and make speed smoother. Through four analog output interfaces, the current and voltage in the main circuit can be easily shown by using signal detection equipment. To flexibility in the application of the Field orientated control algorithm to BLAC there are up to 126 bytes write registers and 44 bytes Read registers in the IRMCK201.By reading and writing the registers of IRMCK201, closed loop current control and velocity control can be achieved. The IRMCK201 also provides direct digital interface pins for linear current sensing IC IR2175. In order to deal with current signals conveniently, the current signal is transformed from the servo motor driving circuit to control circuit through the linear current sensing IC IR2175. The communication is realized between C8051 and IRMCK201 via high-speed SPI0 host interface. SPI0 is an enhanced serial peripheral interface. When configured as a master, SPI0 can operate in one of three different modes: multi-master mode, 3-wire single master mode, and 4-wire singlemaster mode. 6. Conclusion This paper gives the detailed analysis and simulation studies of BLAC servo drive with PI controller for SPWM and SVPWM modulation that has been carried out as pre-requisite to determine an efficient hardware for SCARA application. Different hardware implementation techniques of BLAC drive with PI control has been discussed and Single chip solution using IRMCK201 is selected for optimum performance. A typical implementation BLAC drive using IRMCK201 is also proposed. References [1] Thirunavukkarasu, S., B. P. C. Rao, T. Jayakumar, and B. Raj, Annals of Nuclear Energy, vol. 38, No. 4, p817
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[2] P.Ramana, B. Santhosh Kumar, Dr. K. Alice Mary, Dr.M.Surya Kalavathi, International journal of advanced research in electrical, electronics and instrumentation engineering, vol.2, issue 7(Jul.2013) p. 2928, [3] Ting-ting Liu, Yu Tan, Gang Wu, Shu-mao Wang, ICMTMA (2009) , vol. 2, p.343 [4] Zhonghui Zhang, Jiao Shu, 3rd IEEE International Conference on Computer Science and Information Technology (ICCSIT), (2010) , vol 9 , p. 539 [5] Pragasan Pillai, R.Krishnan, IEEE Transactions on Industrial Electronics, Vol. 35, Issue: 4, p 537 [6] B. Zigmund, A. Terlizzi1, X. T. Garcia, R. Pavlanin, L. Salvatore, Advances in Electrical and Electronic Engineering, vol. 5, (2011) p.114 [7] Marufuzzaman M, Reaz M.B.I, Ali, M.A.M., International Conference on Computer Applications and Industrial Electronics (ICCAIE), 2010, (Dec. 2010), p. 602 [8] http://www.irf.com/product-info/datasheets/data/irmck201.pdf accessed on 29/10/2013 [9] K. Vinoth Kumar, Prawin Angel Michael, Joseph P. John and Dr. S. Suresh Kumar, ARPN Journal of Engineering and Applied Sciences, vol. 5 (July 2010), p.61 [10] Fábio Roberto Garcia de Lima, Wânderson de Oliveira Assis, Alessandra Dutra Coelho, ABCM Symposium Series in Mechatronic, vol. 4, p.100 [11] Sibel PARTAL ˙Ibrahim S¸ ENOL, Ahmet Faruk BAKAN amuran Nur BEK˙IRO˘GLU, Turk J Elec Eng & Comp Sci, Vol.19, No.3, (2011)
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ScienceDirect Procedia Engineering 86 (2014) 511 – 519
1st International Conference on Structural Integrity, ICONS-2014
Development of a Brushless AC Servo Drive for the Tube Locator Module for Steam Generator Tube Inspection Device Sree Ranjini K.Sa,*, Joel Joseb, T. Anuradhaa, Vasan Prabhua and S. Sakthivelb b
a Anand Institute of Higher Technology, Chennai-603103, India Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India * E-mail ID: [email protected]
Abstract The fundamental issue in the area of remote handling and robotics is the reliability of power electronics system. Detailed analysis and simulation studies are very much relevant as the prebuilt drive solutions are costly and performance of drive system depends upon the reliability of control algorithms incorporated in the drive. The modulation techniques also play an important role in the dc voltage utilization. This paper presents a comparative simulation study for the analysis of complete BLAC drive using PI controller using two types of modulation techniques namely Sinusoidal Pulse Width (SPWM) and Space Vector Modulation (SVPWM). © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research Keywords: SCARA, Field Oriented Control, Proportional Integral Controller, BLAC, Space Vector Modulation
1. Introduction Selective Compliance Articulated Robot Arm (SCARA) finds wide application in industries where planar reach is required. The remote inspection of steam generator tubes is no exemption due to the precise reach in the planar task space of the remotely deployed device over the tube sheet connecting all 547 tubes in Prototype Fast Breeder Reactor (PFBR). SCARA is a jointed two-link arm similar to our human arms rigid in the Z-axis and has good compliance in the XY-Cartesian space. The actuation of the two joints has to be provided by two independent servomotors, which have better dynamic performance, smaller size, high instantaneous torque and little torque ripple. BLAC motor is selected to be suited for this payload, which is handling the Eddy Current Testing (ECT) probe along with its cable. A high performance servo control system is required and control algorithm has to properly chosen. As the prebuilt control and driver modules are quite big and expensive to suit the application and less flexible to integrate the complete user routines, the drives were preferred to be built and the control routines
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research doi:10.1016/j.proeng.2014.11.075
512
K.S. Sree Ranjini et al. / Procedia Engineering 86 (2014) 511 – 519
programmed. Hence, simulation studies are carried out to investigate the appropriate control algorithm and modulation technique in order to have maximum voltage utilization of DC bus. Previously, efforts has been taken to compare the performance of various pulse width modulation techniques implemented for BLAC drive based on FOC using SVPWM, SPWM and Third Harmonic Injection PWM [2]. The mathematical model of BLAC drive system based on the rotor field oriented control with PI controller; the entire BLAC driver control system is analyzed and static and dynamic characteristics are determined [3]. Also direct position detection of BLAC is tested using SIMULINK model and static and dynamic characteristics are analyzed [4]. The mathematical model of BLAC machine is framed from the governing equations in dq plane for the verification of control algorithms and modulation techniques. Simulation is carried out with the powerful modeling capabilities of MATLAB/Simulink, for checking the effectiveness of modulation technique that can be incorporated into the drive system. The speed PI controller gain value was tuned and set according to Symmetric Optimum (SO) criterion and current PI controller gains were tuned and set by Absolute Value Optimum (AVO) criterion. The simulation results show that speed, current, torque follow the given value without much disturbance, precise and accurate control of motor can be achieved. The closed loop stability of the system is also being ensured by bode plot analysis. A methodology is proposed for hardware implementation of BLAC drive. This paper deals with the complete simulation of the drive system which gives confidence in building the drive for the SCARA device for Steam Generator tube inspection and simulation results are presented in detail. 2. Basic Theory 2.1. Brushless Alternating Current Machine (BLAC) The selection of machine for robotic arm actuation was the prime problem to be solved. Stepper motor because of rotation in steps and requirement of large step angle was rejected. Hence next move was towards Brushless Excited machines. Brushless excited machines have become the focal point in recent days due to their higher reliability, lack of maintenance and enhanced performance. The nature of the output waveform determines their classification into Brushless AC machine (BLAC) and Brushless DC machine (BLDC). BLDC machine has improved control characteristics than the BLAC machines. However torque per weight ratio of BLAC machines is very much higher than the BLDC machine which makes it suitable for the precise control of robotic arm. 2.2. Control Strategy of BLAC The various control strategies of controlling an AC motor scalar control and vector control. The scalar control allows the control of AC motor by considering only the magnitude of voltages and currents and adjusting the frequency to run the motor at synchronous speed. This method is simple to implement however gives poor dynamic performance and unstable operation of drive system. Field oriented control (FOC) or vector control is the method of control, which makes ac drives equivalent to dc drives. In this method, ac motor can be controlled like a separately excited dc motor. FOC is based on transformation from a three phase system (abc coordinates) to two phase system (dq) coordinates. There are two control loops for FOC control of BLAC. They are outer speed control and inner current control. FOC control is implemented with direct axes current equal to zero since the magnetizing current is provided by permanent magnets on the rotor structure. The quadrature axes of current controls the torque component there by making the coupling characteristics of BLAC decoupled one. FOC algorithm is preferred for the control of BLAC drive as it provides an easy way of obtaining reference current and direct control of torque, which is primarily required for robotic arm control. 2.3. Mathematical Modeling of BLAC The mathematical model of BLAC machine is used in simulation studies since it provides better dynamic and steady state performance [5]. The mathematical model is framed in rotor reference frame neglecting damper winding, saturation effects, iron losses, and considering back emf as sinusoidal. The mathematical model uses dq
K.S. Sree Ranjini et al. / Procedia Engineering 86 (2014) 511 – 519
equations (1)-(3) which during steady state are dc quantities as a result control is precise and have less error. ݅ௗୀ ݅ ൌ
െ
െ
ோ ݅ ௗ
ோ ݅
െ
ܲ߱ ݅
ܲ߱ ݅ௗ െ
(1)
ఒ ು ఠೝ
(2)
ܶ ൌ ͳǤͷൣߣ݅ ൫ܮௗ െ ܮ ൯݅ௗ ݅ ൧
(3)
where p denotes differential operator, R is the armature winding resistance, Vd and Vq denotes voltage in dq axis, Ld and Lq denotes inductance in dq axis, Te is the electromagnetic torque, P is number of poles is the rotor resistance, wr is the angular velocity. With the consideration of mechanical load, dynamic equation of the BLAC machine can be defined as Jmpwr + Bmwr = Te -TL
(4)
where Tl denotes the load torque, Jm denotes the coefficient of inertia, Bm denotes the frictional coefficient. The current control loop of BLAC drive with sinusoidal pulse width modulation was built from the basic blocks of PI controller for speed, SPWM block, voltage measurement block and mathematical model of BLAC. The current control loop of BLAC with SVPWM was built using two controllers one for d-axis current and other for q-axis current co-ordinate transformation block, inverter circuit and mathematical model of BLAC. 2.4. Functional Block Diagram The block level functional diagram of FOC control of BLAC drive using SPWM and SVPWM techniques is shown in the Fig.1.
Speed Contr oller
Speed Controlle r
Fig 1: Functional block diagram of BLAC Drive with SPWM and SVPWM
3. Simulation ǤͷǤ Three-phase two level inverter consists of two switches in each of the three legs. The switches may be BJT, GTO, or MOSFET. Three of the switches must be always ON and switches of same leg should not conduct at a time. Two types of modulation techniques – SPWM and SVPWM are discussed, which produce gate pulse required
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for conduction of switches.
3.1.1. Sinusoidal Pulse Width Modulation WM technique where carrier used is high frequency trian ngular signal. SPWM is carrier comparison based PW Three phase modulating signals are compared with w a carrier wave to generate PWM pulses for every threee-phase. By varying modulation index, magnitude frequenncy of fundamental component can be controlled. Sinu usoidal pulse width modulation is simple modulation techniqque and is linear between 0 – 78.5% of six step voltage value which result in poor voltage utilization and increassed harmonic content. The frequency of carrier wavefo orm used in simulation of sinusoidal pulse width modulationn is 6 kHz.
Fig 2: Generatiion of pulses using SPWM technique 3.1.2. Space Vector Pulse Width Modulation d cycle for each leg in the inverter circuit to generatee a reference Space vector modulation determines duty voltage phasor by selecting nearest of two volltage vectors at each sample time. SVPWM tries to produce a three phase balanced sinusoidal output having redduced harmonic components. According to switching sequence of MOSFET switches there are six non zero voltagge space vectors which are denoted by V1, V2, V3, V4, V5, V6 and two zero space vectors denoted by V0, V7. Placeement and order of non-zero space vectors are key facctors, which determine the performance of this control strateegy. Sector judgment is done according to formula N = A + 2B + 4C where A, B and C are determined fromܸఈ ܸ ܸఉ .
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q axis V3 (010) (-1/3, 1/¥3)
3
V2 (110) (1/3, 1/¥3)
2 Vref
T3
1 V1
V4 (011)
T4 6
4
(100) d axis (2/3, 0)
5 (-1/3, -1/¥3)V5 (001)
V6 (101)
Fig 3:: Space Vector Modulation In Fig.3, the reference voltage vector lies in secctor 3. The magnitude of the reference voltage and the tim me period of operation of adjacent vectors and the operatingg time for the switches of 3 phases were determined and gate pulses were produced after comparison with the carrierr signal.
Fig 4: Simulink Model for Space Vector Modulation 3.2. Speed Controller PI controller is one of the most widelyy used in speed control of drives. To obtain an optimal solution for hardware the simulation of FOC of BLAC machine m was verified by using traditional PI controller. Derivative control is not used in this case due to the introoduction of high amount of noise in the measurement off torque and speed. The reference speed is compared withh sensed speed and error is given to speed PI controlller. The dq projections of stator phase current was comparred with their reference values Iq(ref) and Id (ref) which is set to be zero and output is controlled by means of traaditional PI controller. The Speed loop PI controller is i tuned by Symmetric Optimum (SO) method and currentt loop PI controller was tuned by Absolute Value Optim mum (AVO) method [6]. AVO and SO methods depend on the closed loop transfer function of the system. These tw wo methods he system’s employ an approximate transfer function of thee system to be controlled. The AVO method assumes th transfer function in open loop as eq.(5). ͳ ܩሺݏሻ ൌ (5) ʹ߬ఀ ݏሺͳ ߬ఀ ݏሻ SO method considers the system’s open loop traansfer function as eq.(6)
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ܩሺݏሻ ൌ
ሺͳ Ͷ߬ఀ ݏሻ ͺ߬ఀ ଶ ݏଶ ሺͳ ߬ఀ ݏሻ
(6)
d to the digital implementation of the control (which h implies the where IJȈ is sum of all small delays in loop due sampling of signals), the use of filters, the processing of the control algorithm and the use of Pulse P Width ng eq.(7) and Modulators (PWM). The ki and kp values of currrent controller is calculated as per AVO criterion by usin eq.(8). ܮ ߬ ൌ Ǣ ߬ఀ ൌ ʹ߬௦ (7) ܴ௦
݇ ൌ
ܮ
Ǣ ݇ ൌ
݇
(8) ʹ߬ఀ ߬ Similarly ki and kp values of current controller is i calculated as per SO criterion using eq.(9) and eq.(10) ͵ ఠೝ ߬௦ ఠ ೝ (9) ఠ ఠೝ ߬ఀ ൌ ߬௦ ʹ߬ఀ െ Ǣ ߬ ൌ Ͷ߬ఀ ೝ ʹ ʹ ఠ (10) ݇ ೝ ܬ ఠ ఠೝ Ǣ ݇ ൌ ݇ ೝ ൌ ఠ ఠ ͵߰ܲଶ ߬ఀ ೝ ߬ ೝ The sampling time for current controller is choosen to be 10 μs while sampling time for speed controllerr was chosen to be 1ms. In addition, Fig. 5 shows Open Loopp Bode plot which ensures the stability of BLAC drive with SVPWM and PI controller for entire response time.
Figg 5: Open Loop Bode Plot At a response time of 0.063s the gainn margin was found to be 19.8 db and phase margin was found to be 80.1degrees. 4. Simulation Results and Discussion The computer simulation for control of BLAC drive is implemented with MATLAB/Simuliink software with SPWM and SVPWM modulation techniquues using (Table: I).The effectiveness of both modulation n techniques are verified using PI controller and the results are a compared.
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Fig 6: Torque and Speed response of BLAC drive with SPWM and SVPWM TABLE I.
MOTOR PARAMETERS
Stator Resistance (Rs ) Direct axis reactance (Ld) Quadrature axis reactance(Lq) Moment of inertia(J) Rotor flux(Ȝ) Viscous coefficient(B)
1.4 0.014mH 0.014mH .0001051Kgm2 0.1848 Wb 4047e-0.005 Nm
The simulation study reveals that SVPWM gives 15% enhanced fundamental output with better quality i.e. lesser Total Harmonic Distortion (THD) compared to SPWM, which proves the effectiveness of SVPWM over SPWM. 5. Hardware Implementation The hardware implementation of BLAC drive with PI controller can be realized with FPGA [7]. In an alternative approach of speed estimation of brushless AC servomotors, a network structure representing the electrical and mechanical properties of the servomotor is built via Artificial Neural Network (ANN) and trained with the results of mathematically modelled driver system [11]. Single chip solutions are also available for the current as well as velocity control of BLAC such as one provided by International Rectifiers which does not require any programming to complete AC servo control algorithm and has flexible configuration ability [8]. IRMCK201 is a one-chip solution for complete closed loop current control and velocity control for a high performance servo drive system. Compared with the traditional control chip, it includes not only the motion control peripheral functions but also magnetic field oriented control (FOC) algorithm and closed loop speed control realized in hardware, and simplifies hardware structure and shortens the development cycle of the system.
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DC Input
Processor/ Controller
Switching Circuit IRMCK201
Isolation Current sensor
BL AC Encoder
Fig 7: Schematic diagram of BLAC drive using IRMCK201 IRMCK201 is a core control chip used for complete current and velocity control of PMSM machine. An external current sensor and Encoder is used for feedback. A processor communicates with IRMCK201 reads and writes the control word to the memory of IRMCK201. The entire systems avoids the software programming of Field orientated control algorithm, and greatly reduces the development cycle, and shortens the system response time, and make speed smoother. Through four analog output interfaces, the current and voltage in the main circuit can be easily shown by using signal detection equipment. To flexibility in the application of the Field orientated control algorithm to BLAC there are up to 126 bytes write registers and 44 bytes Read registers in the IRMCK201.By reading and writing the registers of IRMCK201, closed loop current control and velocity control can be achieved. The IRMCK201 also provides direct digital interface pins for linear current sensing IC IR2175. In order to deal with current signals conveniently, the current signal is transformed from the servo motor driving circuit to control circuit through the linear current sensing IC IR2175. The communication is realized between C8051 and IRMCK201 via high-speed SPI0 host interface. SPI0 is an enhanced serial peripheral interface. When configured as a master, SPI0 can operate in one of three different modes: multi-master mode, 3-wire single master mode, and 4-wire singlemaster mode. 6. Conclusion This paper gives the detailed analysis and simulation studies of BLAC servo drive with PI controller for SPWM and SVPWM modulation that has been carried out as pre-requisite to determine an efficient hardware for SCARA application. Different hardware implementation techniques of BLAC drive with PI control has been discussed and Single chip solution using IRMCK201 is selected for optimum performance. A typical implementation BLAC drive using IRMCK201 is also proposed. References [1] Thirunavukkarasu, S., B. P. C. Rao, T. Jayakumar, and B. Raj, Annals of Nuclear Energy, vol. 38, No. 4, p817
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[2] P.Ramana, B. Santhosh Kumar, Dr. K. Alice Mary, Dr.M.Surya Kalavathi, International journal of advanced research in electrical, electronics and instrumentation engineering, vol.2, issue 7(Jul.2013) p. 2928, [3] Ting-ting Liu, Yu Tan, Gang Wu, Shu-mao Wang, ICMTMA (2009) , vol. 2, p.343 [4] Zhonghui Zhang, Jiao Shu, 3rd IEEE International Conference on Computer Science and Information Technology (ICCSIT), (2010) , vol 9 , p. 539 [5] Pragasan Pillai, R.Krishnan, IEEE Transactions on Industrial Electronics, Vol. 35, Issue: 4, p 537 [6] B. Zigmund, A. Terlizzi1, X. T. Garcia, R. Pavlanin, L. Salvatore, Advances in Electrical and Electronic Engineering, vol. 5, (2011) p.114 [7] Marufuzzaman M, Reaz M.B.I, Ali, M.A.M., International Conference on Computer Applications and Industrial Electronics (ICCAIE), 2010, (Dec. 2010), p. 602 [8] http://www.irf.com/product-info/datasheets/data/irmck201.pdf accessed on 29/10/2013 [9] K. Vinoth Kumar, Prawin Angel Michael, Joseph P. John and Dr. S. Suresh Kumar, ARPN Journal of Engineering and Applied Sciences, vol. 5 (July 2010), p.61 [10] Fábio Roberto Garcia de Lima, Wânderson de Oliveira Assis, Alessandra Dutra Coelho, ABCM Symposium Series in Mechatronic, vol. 4, p.100 [11] Sibel PARTAL ˙Ibrahim S¸ ENOL, Ahmet Faruk BAKAN amuran Nur BEK˙IRO˘GLU, Turk J Elec Eng & Comp Sci, Vol.19, No.3, (2011)
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