Motor controller design and indoor experiment for electric tractor based on myRIO

Abstract: It is well known that off-road vehicles contribute a huge amount of pollution to air, and studies have shown that renewable electric vehicles (EVs) decrease petroleum consumption and consequently reduce criteria emissions under nearly all circumstances. Diesel tractors exhaust CO, NOx PM10 and pollutants that are harmful to human body. As a consequence, diesel tractors are not allowed to work in some limited and closed environments such as greenhouse. It is imperative that a kind of small electric tractor with less noise, zero emission and more comfort should be developed with the least delay. A brushless direct current (BLDC) motor drive is characterized by higher efficiency and lower maintenance. Therefore, it is necessary to have a low-cost but effective BLDC motor controller. This research develops a new low-cost method for the design of BLDC motor controller and designs a BLDC motor controller for a small electric tractor. The BLDC motor controller includes cybernetics core and controller peripheral circuits, and modular design idea is proposed. The advanced intelligent controller uses the myRIO as a cybernetics core, and the myRIO uses the latest Zynq technology from Xilinx, which features an FPGA integrated with a processor running a real-time OS that can meet the requirements of intelligent control for electric tractor. The myRIO is coupled with an onboard accelerometer, programmable LEDs (light emitting diodes) and analog and digital I/O, and USB port helps modular design ideas come to life. The controller peripheral circuits include power amplifier MOSFET gate drive, current protection, low voltage protection and temperature protection, which meet the tractor's transportation and working demands under different power conditions. Programs are developed in real-time processor ARM and I/O driver software in FPGA to the control flow in graphical programming environment. Main control and protection algorithms run in real-time OS; at the same time the driving timing sequence algorithms run in FPGA, and the algorithms with 2 different execution rates are isolating. In this way, it not only reduces the real-time processor's load, but also improves the accuracy and reliability of the controller. The software includes starter, driving, low-voltage protection, overcurrent protection, thermal protection and other sub-VI programs. The electric tractor prototype tests show that BLDC motor controller works properly with maximum starting current 120 A under zero load condition and 164 A under loaded condition with overcurrent detection. Under starting condition, after performing several experiments, when duty cycle of the PWM signal is set as 60% and the frequency is set as 10 kHz in initial stage, the better motor starting torque can be obtained, and amplitude of starting current is also limited. The sequence of motor drive voltages meets the requirement of the BLDC motor excellently with the help of hall position sensors. It is concluded that the modular design idea of developing a BLDC motor controller that combines cybernetics core and controller peripheral circuits is practicable. In the whole testing process, the disadvantage of the impact force does not exist, and less vibration and lower noise make the tractor driver more comfortable when it is working. The design proposal combining myRIO as control core and peripheral circuit solves the difficult problems of developing drive controller, it promotes the development of the intelligent electric tractor.