Electric Tractor Motor Drive Control Based on FPGA

Agriculture 5th IFAC Conference on Sensing, Control and Automation for August 2016. Seattle, Washington, USA 5th IFAC14-17, Conference on Sensing, Control and Automation for Available online at www.sciencedirect.com Agriculture Agriculture August 14-17, 2016. Seattle, Washington, USA August 14-17, 2016. Seattle, Washington, USA

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IFAC-PapersOnLine (2016)Control 271–276 Electric Tractor Motor 49-16 Drive Based on FPGA Electric Tractor Motor Drive Control Based on FPGA Electric Tractor Motor Control Based on FPGA Yanni Chen*.Drive Bin Xie**. Enrong Mao***

Yanni Chen*. Bin Xie**. Enrong Mao*** Yanni Chen*. Bin Xie**. Enrong Mao***  * College of Engineering, China Agricultural University, Beijing, China (e-mail: [email protected]). * College of Engineering, China Agricultural University, Beijing, China * College China Agricultural ** Collegeof ofEngineering, Engineering, China AgriculturalUniversity, University,Beijing, Beijing,China China (e-mail: [email protected]). (e-mail: [email protected]). (e-mail: [email protected]) ** College of Engineering, China Agricultural University, Beijing, China ** College China Agricultural *** Collegeof ofEngineering, Engineering, China AgriculturalUniversity, University,Beijing, Beijing,China China (e-mail: [email protected]) (e-mail: [email protected]) (e-mail: [email protected]) *** College of Engineering, China Agricultural University, Beijing, China *** College of Engineering, China Agricultural University, Beijing, China (e-mail: [email protected]) (e-mail: [email protected]) Abstract: The electric tractor represents the development trend of sustainable and modern agriculture. With the higher energy tractor efficiency and lower electric research and is significant to modern Abstract: The electric represents the pollution, development trendtractor of sustainable modern agriculture. Abstract: The tractor represents thetechnology, development trend of sustainable and modern agriculture. agriculture. As electric the electric tractor’s critical motor drive control directly affects the With the higher energy efficiency and lower pollution, electric tractor research is significant to stability modern With the higher energy and lower pollution, tractor research is significant to stability modern of tractor speed and the efficiency accuracy field work. So thiselectric paper drive presented an directly electric tractor the motor drive agriculture. As the electric tractor’sof critical technology, motor control affects agriculture. As the electric tractor’s critical technology, motor drive control directly affects the stability control system which was suitable for tractor work characteristic based on Brushless DC (BLDC) motor. of tractor speed and the accuracy of field work. So this paper presented an electric tractor motor drive of tractor speedwhich and PID the of thisinner paper presented electric DC tractor motorcontrol drive Double closed loop control outerwork. speed current loop combined with(BLDC) PWM control system wasaccuracy suitablewith forfield tractor workSoand characteristic based onan Brushless motor. control system which was suitable for tractor work characteristic based on Brushless DC (BLDC) motor. was implemented as the motor drive control strategy. The motor controller was designed based on FPGA Double closed loop PID control with outer speed and inner current loop combined with PWM control Double closed loop control withcontrol outer speed and inner current loop ARM combined PWM technique, so hardware NIdrive myRIO as control core which integrated with with Xilinx FPGA, and was implemented as PID the used motor strategy. The motor controller was designed based oncontrol FPGA was implemented as the motor drive control strategy. The motor controller was designed based on FPGA control program used LabVIEW as development environment. Indoor experiment was conducted to technique, so hardware used NI myRIO as control core which integrated ARM with Xilinx FPGA, and technique, hardware used NI myRIO as control core which integrated ARM with Xilinx FPGA, and validate program thesoelectric tractor motor drive control design. control used LabVIEW as development environment. Indoor experiment was conducted to control program used LabVIEW as development environment. Indoor experiment was conducted to validate the electric tractor motor drive design. © 2016, IFAC (International Federation ofcontrol Automatic Control) Hosting Elsevier Ltd. All rights reserved. Keywords: tractor, BLDC motor drive, design. FPGA, Double closedbyloop, PWM. validate theElectric electric tractor motor drive control

Keywords: Electric tractor, BLDC motor drive, FPGA, Double closed loop, PWM.  Keywords: Electric tractor, BLDC motor drive, FPGA, Double closed loop, PWM. needs less maintenance cost. These advantages meet the  1. INTRODUCTION  technical electric drive. FPGA needs lessrequirements maintenance ofcost. Thesetractor’s advantages meet the 1. INTRODUCTION needs lessrequirements maintenance cost. These advantages meet the (field-programmable gate array) is integrated circuits which technical of Due to the current 1.situation of fossil fuel exhaustion and electric tractor’s drive. FPGA INTRODUCTION electric tractor’s drive. FPGA technical requirements of are designed to be programed a hardware description environment pollution, payingof more to electric gate array)using is integrated circuits which Due to the current situation fossil attention fuel exhaustion and (field-programmable (field-programmable gate array) is integrated circuits which language (HDL). Compared with conventional motor Due to the current situation of fossil fuel exhaustion and agricultural machinery is a significant and sustainable are designed to be programed using a hardware description environment pollution, paying more attention to electric are designed to be programed using a hardware description controller with the core of microcontroller, FPGA has faster environment pollution, paying more attention to electric developing with conventional tractor, language (HDL). Compared with conventional motor agricultural direction. machineryCompared is a significant and sustainable language (HDL). Compared with conventional motor agricultural machinery is a significant and sustainable processing speed with the inherent parallel architectures, and electric tractor has higher energywith efficiency and lower air controller with the core of microcontroller, FPGA has faster developing direction. Compared conventional tractor, controller with the core ofFPGA microcontroller, FPGA has faster developing direction. Compared with conventional tractor, easier programmability. can validate the designed pollution,tractor additionally, it can alsoefficiency reduce noise speed with the inherent parallel architectures, and electric has higher energy and pollution, lower air processing processing speed with theFPGA inherent parallel and algorithm by using simulation toolsarchitectures, before electric has higher energy efficiency and pollution, lower Till air control easier programmability. can validate the hardware designed thereforetractor electric tractor research is useful andnoise necessary. pollution, additionally, it can also reduce easier programmability. FPGA can validate the designed the design cost tools and risk arehardware reduced pollution, additionally, it can on also reduce noise pollution, control algorithmthus by using simulation before now the electric worldwide research tractor had made therefore tractor research iselectric useful and necessary. Till implementation, control algorithm by using simulation tools before hardware therefore electric tractor research is useful and necessary. Till (Tashakori, A. et al., 2015). In addition, controller based on progress. General Electric Company (America) designed implementation, thus the design cost and risk are now the worldwide research on electric tractor had made implementation, thus the design cost and risk are reduced reduced now the worldwide research on electric tractor had made FPGA has high reliability due to the HDL programing. NI Elec-Trak series electric tractors, which were driven by (Tashakori, A. et al., 2015). In addition, controller based progress. General Electric Company (America) designed (Tashakori, A. et al., 2015). In addition, controller based on on myRIOhascontroller designed Instruments progress. General Electric Company (America) designed brushless DC motor and tractors, mainly used forwere mowing lawn. high reliability due tobythe National HDL programing. NI Elec-Trak series electric which driven by FPGA FPGA has high reliability due to the HDL programing. NI Company integrated dual-core ARM Cortex-A9 real-time Elec-Trak series electric tractors, which were driven by Electric Company (Canada) Electric 0x myRIO controller designed by National Instruments brushlessTractor DC motor and mainly useddesigned for mowing lawn. myRIO controller designedXilinx by FPGA. National Instruments brushless DC tractors, motor and mainly for mowing lawn. with customizable It can guarantee series electric which have used dualdesigned motor drive system Company integrated dual-core ARM Cortex-A9 real-time Electric Tractor Company (Canada) Electric 0x processing Company integrated dual-core ARM Cortex-A9 real-time Electric Tractor Company (Canada) designed Electric 0x control accuracy and reliability, and its portability makes it and canelectric not only mow lawn buthave alsodual hitchmotor furrowdrive plowsystem (Gao, processing with customizable Xilinx FPGA. It can guarantee series tractors, which processing with customizable Xilinx FPGA. It can guarantee possible to be applied to electric tractor. series electric tractors, which have dual motor drive system control accuracy and reliability, and its portability it 2007). and can not only mow lawn but also hitch furrow plow (Gao, control accuracy and reliability, and its portability makes makes it and can not only mow lawn but also hitch furrow plow (Gao, possible to be applied to electric tractor. 2007). paper presented anelectric electrictractor. tractor BLDC motor drive possible to be applied to Motor 2007). drive control is a critical technology of electric tractor, This control design based on FPGA. BLDC motor drive principle which directly affects the stability of tractor speed and the This paper presented an electric BLDC motor drive Motor drive control is a critical technology of electric tractor, This paper presented an electric tractor tractor among BLDC motor drive was the foundation of controlling, which the Motor drive control is a critical technology of electric tractor, accuracy of field work, so it’s necessary to research motor control design based on FPGA. BLDC motor drive principle which directly affects the stability of tractor speed and the control design based on FPGA. BLDC motor drive principle which directly affects the stability of tractor speed and the electronic commutation was the core point. Motor drive drive control system which is suitable for electric tractor was the foundation of controlling, among which accuracy of field work, so it’s necessary to research motor was the foundation of controlling, among which the the accuracy of field work, so it’s to research motor control strategy combinedwas double closedpoint. loop Motor PID control work characteristics. As electric tractor generally works in electronic commutation the core drive drive control system which is necessary suitable for electric tractor electronic commutation was the core point. Motor drive drive control system which is suitable for electric tractor with PWM control to implement speed and current optimal the fields and suffers environment, theworks control strategy combined double closed loop PID control work characteristics. As adverse electric tractor generally in control control strategy combined double closed loophardware PID control regulation. design included and work characteristics. As adverse electric tractor works in with system hardware, including motor andgenerally motor the controller, PWM Motor controlcontroller to implement speed and current optimal the fields and suffers environment, control with PWM control to implement speed and current optimal were both based on FPGA technique. the fields and suffers adverse environment, the control control program that regulation. Motor controller design included hardware and should have high reliability. DC motor (BLDC)controller, motor is system hardware, includingBrushless motor and regulation. Motor controller design included hardware and This study also provided a platform for future electric tractor system hardware, including motor and motor controller, control program that were both based on FPGA technique. widely used in the area of Vehicle Engineering, which has should have high reliability. Brushless DC (BLDC) motor is control program that were both based on FPGA technique. development andprovided research.a platform for future electric tractor should have high Brushless DC (BLDC) motor is This advantages efficiency, highEngineering, power density, strong study also widely usedof in high thereliability. area of Vehicle which has This study also provided a platform for future electric tractor widely used in the area of Vehicle Engineering, which has and research. overload capability and high starting torque (Miller, T.J.E., development advantages of high efficiency, high power density, strong development 2. ELECTRIC TRACTOR CONTROL SYSTEM and research. advantages of high efficiency, high power density, strong 1989). BLDC motorand is designed according conventional overload capability high starting torque to(Miller, T.J.E., 2. ELECTRIC TRACTOR CONTROL SYSTEM overload capability highelectronic starting torque T.J.E., Electric DC motor’s principle commutation instead control system was shown in SYSTEM Fig. 1. Electric 1989). BLDC motorand isusing designed according to(Miller, conventional 2. tractor ELECTRIC TRACTOR CONTROL 1989). BLDC motor is designed according to conventional of commutation, so it has commutation simple structure and tractor was driven by BLDC motor. The driving was DCmechanical motor’s principle using electronic instead Electric tractor control system was shown in Fig. power 1. Electric DCmechanical motor’s principle using electronic instead Electricwas tractor control system was shown in Fig. 1. Electric of commutation, so it has commutation simple structure and tractor driven by BLDC motor. The driving power was of mechanical commutation, so it has simple structure and tractor was driven by BLDC motor. The driving power was Copyright © 2016 IFAC 276 2405-8963 © 2016, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. 2016 responsibility IFAC 276Control. Copyright Peer review©under of International Federation of Automatic Copyright © 2016 IFAC 276 10.1016/j.ifacol.2016.10.050

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transmitted to driving wheels and PTO through transmission mechanism. Electric tractor controller took charge of the running, controlling and monitoring of the tractor. On one hand, electric tractor controller identified the driver intention through accelerator pedal, brake pedal and work condition selecting, and then electric tractor controller sent motor speed control signals to BLDC motor controller according to control strategy. Simultaneously, torque and rotational speed sensor mounted on driving wheel axle sent feedback signals of torque and speed to electric tractor controller. BLDC motor controller received speed control signals to control the motor to run at the set speed. On the other hand, electric tractor controller received battery management system signals, including battery’s SOC, discharging current, terminal voltage and temperature, thus electric tractor controller implemented monitoring and protecting functions. Monitor system took charge of monitoring the current, voltage and temperature of the motor and other electric equipment, which simultaneously sent these signals to electric tractor controller to judge the security of electric equipment.

T1

T3

T5

iA iB

VDC

iC

T2

T4

R

L-M

eA

R

L-M

eB

R

L-M

eC

N

T6

Fig. 2. Schematic diagram of BLDC motor drive system. Back EMF Phase current

Phase A Winding

θr

Phase B Winding

θr

Phase C Winding

θr

Driving Wheel Battery Management System

Other Controllers Accelerator Pedal Brake Pedal

Battery

nM

IM BLDC Motor Controller

Electric Tractor Controller

Work Condition

nW

TW

Monitor System Electrical Connection Mechanical Connection

BLDC Motor

Transmission Mechanism

0° 30°

PTO

90°

150°

210°

270°

330° 360°

Fig. 3. Typical waveform of three-phase BLDC motor back EMF and phase current.

Torque and Rotational Speed Sensor

Driving Wheel

BLDC motor was energized with 2 switches on every instant, one in a high side and the other in a low side. Therefore, two phase windings were connected in series across the DC bus (Sathyan, A. et al., 2009). Every motor commutation changed one switch’s state, thus every switch keeps on for 120 electrical degrees. The switch-on sequence in a 360 electrical degrees cycle was: T1T6-T3T6-T3T2-T5T2-T5T4-T1T4T1T6, referring to Fig. 2. For commutating correctly, the rotor position must be detected accurately. Three Hall sensors were used to detect rotor position. They were installed in 120 degrees phase difference. Each sensor kept high level for 180 electrical degrees in a cycle, as shown in Fig. 4, whose Xaxis represented rotor rotation angle. Every Hall sensor sent a binary number out simultaneously, thus there would be six states in a 360 electrical degrees cycle. The Hall sensors state and corresponding active switches were as shown in Table 1.

Fig. 1. Electric tractor control system. 3. BLDC MOTOR DRIVE PRINCIPLE AND CONTROL STRATEGY 3.1 BLDC Motor Drive Principle BLDC motor ran based on electronic commutation. As shown in Fig. 2, motor stator with three-phase windings connected three-phase voltage source inverter with six power switches. Hall sensors were used to detect motor rotor position and send position signals out. Inverter drove switches on or off by rule, thus the three-phase windings of motor stator were energized in sequence and produce rotating magnetic field to drag motor rotor moving. Typical waveforms of back EMF and phase current of the three-phase BLDC motor were shown in Fig. 3, whose Xaxis represented motor rotor rotation angle. Approximately, the back EMF per phase was constant for 120 electrical degrees, forward and backward 60 electrical degrees of which were linearly with rotor angle. In order to output stable speed and torque, the phase windings could only carry current during the flat part of the back EMF. Therefore the motor commutateds among phase windings every 60 electrical degrees.

HA

HB

HC 0°

60°

120°

180°

240°

300°

360°

Fig. 4. Hall sensors output signals in a 360 electrical degrees cycle. 277

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1) Speed Loop

Table 1. Hall sensors state and corresponding active switches Hall Sensors HA

HB

HC

1 1 1 0 0 0

0 0 1 1 1 0

1 0 0 0 1 1

Active Switches High Low Side Side T1 T4 T1 T6 T3 T6 T3 T2 T5 T2 T5 T4

Speed loop controlled the motor speed to track the setting speed value all the way. Speed loop input was the error of speed which was the difference between setting speed value and the actual speed value. Actual speed was measured by speed sensor, whose output signal was voltage value. Speed calculation module converted the voltage value to actual speed value. Speed loop output was reference phase current, which was sent to current loop as part of the input. Speed loop used discrete PID algorithm to implement controlling.

Motor Phase Windings Current Direction Phase Phase Phase A B C + off + off off + + off off + off +

2) Current Loop Current loop controlled the motor phase current to track the setting phase current value, which was the output of speed loop. Under the condition of load fluctuation, current loop could limit the allowed maximum current to motor, resist disturb of voltage leap, produce optimal torque and sped up dynamic process. Current loop input was the error of phase current which was the difference between setting phase current value and the actual phase current value. Actual phase A and B currents were measured by current sensing circuit, and according to the phase current characteristic of BLDC 0 , thus phase C current could be motor, iA  iB  iC 

3.2 Electric Tractor Motor Drive Control Strategy Electric tractor motor drive control system should have two primary functions. First, as tractor speed stability affected the quality of field work relatively, motor drive control system must keep tractor speed always tracking set speed value. Second, motor drive control system must regulate motor current and improve motor dynamic response because tractor suffered relatively large load fluctuation, which would lead to speed fluctuation, thus it could result in motor voltage leap and even damage motor. Electric tractor BLDC motor drive control system used double closed loop control strategy, outer speed loop and inner current loop.

calculated. As only two phase windings of BLDC motor were energized every instant, dividing the sum of 3 phases current absolute values by two is the average phase current, which was equivalent to actual phase current value. Current loop output was motor controlling voltage, which was sent to PWM control module. Current loop used discrete PID algorithm to implement controlling.

PWM is one of the most widely used techniques in BLDC motor controlling (Paul, A. R. et al., 2011). PWM uses digital pulse train with the same period to control analog circuit. By changing PWM pulse duty cycle could modulate the voltage across analog circuit. According to the characteristic of BLDC motor, regulating motor phase voltage can change motor speed, thus modulating high level pulse width can achieve this goal.

4. ELECTRIC TRACTOR MOTOR DRIVE CONTROLLER 4.1 Hardware Electric tractor motor drive controller design used NI myRIO as control core, and the hardware design mainly included power switch and driver circuit, current sensing circuit and protection circuit.

Therefore, electric tractor motor drive control strategy was as shown in Fig. 5.

A B C

VDC

MOSFET was selected as power switch for the advantage of faster switching speed and lower driving power compared with IGBT (Hemanand, T. et al., 2007). As PWM control signals couldn’t directly drive three-phase voltage source inverter with MOSFET, driver circuit must be built to amplify drive power. In this paper, BLDC motor rated voltage was 72V and rated power was 7.5kW, so MOSFET chose IR Company IRFP4468, and driver circuit chip chose IR Company IR2110.

Speed Sensor

A B C

Power Switches Control Module

iB

iA

Current Sensing Circuit

iC iA iB

PWM Control Module

1) Power switch and driver circuit

BLDC Motor Hall Sensors

iA

iB

Speed Calculation Module

iC

  

2) Current sensing circuit

2

Current PID Controller

eI  t 

I PHA ‒ + I REF Speed PID Controller

273

en  t 

Sampling resistance method and current sensor are the most common ways to sense current. Conventional current sensing method is measuring DC bus current of inverter, which is the average current after LC filtering and couldn’t reflect phase instantaneous current effectively (Tan, B. et al., 2011). Therefore this paper used sampling resistance method to measure phase A and B current directly. Sampling resistances

nACT ‒ + n SET

Fig. 5. Electric tractor motor drive control strategy.

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were connected between the midpoint of inverter bridge arm and the ground, and the current was converted to voltage signal sent to myRIO controller.

switch of upper bridge arm to implement motor voltage control. PWM duty cycle was controlled by ARM program algorithm and was transmitted to FPGA program through read/write control function. On the other hand, speed sensor data and current sensing circuit data were sent to FPGA program through FPGA analog I/O.

3) Protection circuits Protection circuits included overcurrent protection circuit and overheat protection circuit. Overcurrent protection circuit measured DC bus current to examine if overcurrent occurred by Hall current sensor. Circuit output high-level signal to myRIO controller if overcurrent occurred and the motor controller stopped power switching immediately. Overheat protection circuit measured motor temperature and power switches temperature by temperature sensors. Circuit outputs high-level signal to myRIO controller if overheat occurred and the motor controller stops motor running immediately. 4.2 Control program based on LabVIEW Control program design should implement four major functions, including speed PID control, current PID control, PWM control and power switches control. As previously mentioned, myRIO controller integrates dual-core ARM realtime processing with Xilinx FPGA customizable I/O. Therefore power switches drive control and PWM control program was established based on FPGA, for utilizing I/O interfaces to send and receive data to connect and control hardware system.

Fig. 6. Control program based on FPGA. 2) Control program based on ARM Control program based on ARM was the main program of motor drive control that controlled motor system running, referenced FPGA program, processed signals and interacted with programmer. ARM program mainly implemented speed PID control, current PID control, PWM duty cycle calculating and sensor data process, as shown in Fig. 7. Program with flat sequence of 3 frames ran following the order from left to right. The first frame referenced FPGA program to start the control program running. The second frame implemented the main functions of ARM program. Data communicated through read/write control function. Input data included speed sensor and current sensing circuit data, and output data included PWM duty cycle and frequency. Input data were converted through speed conversion subprogram and current conversion subprogram. Speed PID control and current PID control were implemented through PID subprogram. PWM duty cycle equalled to the result of current PID output divided by motor rated voltage. PWM frequency would be a definite value after program debugging. The third frame stopped the entire program running and reports errors.

LabVIEW was selected as the programing software, which is also developed by National Instruments Company (America). LabVIEW is a development environment using high-level data flow graphical programing language to visualize, create and code engineering systems. It is suitable to develop applications that are interactive, executing in parallel and multi-core. Programing NI myRIO requires LabVIEW RealTime Module, LabVIEW FPGA Module and LabVIEW myRIO Toolkit. 1) Control program based on FPGA LabVIEW FPGA Module extends the LabVIEW graphical development platform to target FPGAs on NI myRIO. FPGA program can run in parallel, so different processing operations are assigned to a dedicated section of hardware FPGA chip, and can function autonomously without any influence from other logic blocks. LabVIEW FPGA default clock frequency is 40 MHz, which can guarantee precise timing and increase running speed. Control program based on FPGA was as shown in Fig. 6. FPGA program mainly implemented power switches drive control and PWM control. Program with flat sequence of 2 frames ran following the order from left to right. The first frame initialized 6 switches to low level. Protection circuits sent high-level or low-level signals to true/false case structure. Only when the two signals both turned low level, could the program conduct the following motor control process. Hall sensors output position signals through FPGA digital I/O, and three binary numbers were converted to a decimal number which represented one case of case structure with six cases in total. Case structure implemented power switches on-off control. Simultaneously, PWM pulse was sent to high-level 279

Switch drive control was implemented on FPGA and control algorithm was implemented on ARM separately because of their different running rate. The advantage of running separately was that it could reduce the real-time processor load and improve the accuracy and reliability of control system effectively (Xie, B. et al., 2015). In addition, control program based on LabVIEW supported online debugging, which made programing procedure more efficient and convenient. What was shown in Fig. 8 is the front panel of motor online debugging program. It was in favour of debugging PID parameters and PWM frequency.

IFAC AGRICONTROL 2016 August 14-17, 2016. Seattle, Washington, USA Yanni Chen et al. / IFAC-PapersOnLine 49-16 (2016) 271–276

275

Fig. 7. Control program based on ARM.

1. Battery 2. NI myRIO controller 3. Power driver board 4. DC/DC converter 5. Reduction box 6. BLDC motor 7. Chain 8. Transmission mechanism 9. PTO 10. Driving wheel Fig. 9. Electric tractor prototype. Set the motor starting speed value as 1500 r/min, and keep uniform speed running for about 9s, motor starting current is as shown in Fig. 11. Since motor controller conducted current limit control, the starting current was limited around rated current value, and motor controller garanteed the safety of motor. BLDC motor current under no-load condition was usually 10% to 55% rated current value, the actual current was around 63A in experiment, which met the motor theory.

Fig. 8. Front panel of online debugging program based on ARM.

5.2 Experiment under load condition Considering electric tractor working in the fields, the soil surface was rough which made the drive challenging. Therefore, for simulating electric tractor working condition, indoor experiment was set a series of random obstacles to model the soil force. Set the motor speed value as rated speed 1500 r/min, and drive electric tractor running over obstacles at uniform speed, experiment result was as shown in Fig. 12. Due to the change of resistant load, motor speed had larger fluctuation compared with no-load condition. Motor speed suddenly change led to motor current increase or decrease sharply. Motor controller limited current within a highefficiency and safe range.

5. EXPERIMENTAL RESULTS This paper used the laboratory’s existing electric tractor prototype as experimental subject, as shown in Fig. 9, whose driving motor was the same model as the one mentioned in this paper. Power driver board integrated with power switch driver circuit, current sensing circuit and protection circuits. Based on above, an indoor experiment under no-load and load condition was conducted. 5.1 Experiment under no-load condition Experiment under no-load condition mainly aimed at testing the starting performance and acceleration performance of electric tractor motor. Set the motor starting speed value as 200 r/min, and keep uniform speed running for about 4s. Then set the speed value as 1500 r/min, which was the motor rated speed, and keep uniform speed running for about 5s. Speed experiment result was as shown in Fig. 10. Motor starting time and acceleration time were both less than 0.8s, which resulted in the good speed tracking performance of motor controller. There existed overshoot within a certain range when motor accelerated to high speed. However, due to the motor control algorithm, speed was reduced to reasonable range of set speed error within 1s. Due to the speed sensor precision problem and the vibrate of electric tractor transmission components during running, motor actual speed kept fluctuating.

Fig. 10. Motor speed experiment under no-load condition. 280

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Doctoral Students on Engineering Program.

Agricultural

and

Mechanical

REFERENCES Gao, H. S., Zhu, S. H., and Lv, Z. B. (2007). Development of electric tractor and key techniques. Tractor and Farm Transporter, 34(6), 4-7. Hemanand, T., and Rajesh, T. (2007). Speed control of brushless DC motor drive employing hard chopping PWM technique using DSP. Power Electronics, 2006. IICPE 2006. India International Conference on (pp.393 396). Miller, T.J.E. (1989). Brushless Permanent-Magnet and Reluctance Motor Drives. Clarendon Press, Oxford. Paul, A. R., and George, M. (2011). Brushless DC motor control using digital PWM techniques. Signal Processing, Communication, Computing and Networking Technologies (ICSCCN), 2011 International Conference on (pp.733-738). Sathyan, A., Milivojevic, N., Lee, Y. J., Krishnamurthy, M., and Emadi, A. (2009). An FPGA-based novel digital pwm control scheme for BLDC motor drives. IEEE Transactions on Industrial Electronics, 56(8), 3040-3049. Tan, B., Liu, W., Ruiqing, M. A., and Zhao, J. (2011). Current close-loop research on permanent magnet brushless motor of aviation based on phase current. Micromotors, 44(3), 63-67. Tashakori, A., Hassanudeen, M., and Ektesabi, M. (2015). FPGA based controller drive of BLDC motor using digital PWM technique. The 11th IEEE International Conference on Power Electronics and Drive Systems (PEDS 2015), 658-662. Xie, B., Zhang, C., Mao, E. R., and Chen, Y. N. (2015). Motor controller design and indoor experiment for electric tractor based on myRIO. Transactions of the Chinese Society of Agricultural Engineering, 31(18), 5562.

Fig. 11. Motor current experiment under no-load condition.

Fig. 12. Motor speed and current experiment under load condition. 6. CONCLUSIONS In this paper, an electric tractor BLDC motor drive controller based on FPGA was presented. The control strategy combined double closed loop control with PWM control to implement speed and current optimal regulation. Double closed loop consisted of outer speed loop and inner current loop, both of which adopted PID algorithm. PWM implemented motor speed control by regulating motor phase voltage through changing PWM duty cycle. The combined control strategy was easy to realize and had high reliability. Motor drive controller hardware was based on FPGA, which had fast processing speed due to the inherent parallel architectures and easy programmability. So that it was not only suitable for tractor adverse work environment but also could improve controller performance, reliability and accuracy. Indoor experiment was conducted to validate the design of motor drive controller. ACKNOWLEDGEMENT The authors would like to thank for the support by the Twelfth Five-Year National Science and Technology Support Program 2014BAD08B04 and International Communication and Scientific and Technological Innovation Research of

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