Design of Dual Motor Synchronous Control Platform Based on TMS320F28335

For a long time, motors have been widely used in various fields as a conversion device for mechanical energy and electrical energy. Brushless DC motors combine the advantages of DC motors and AC motors. They have the characteristics of simple structure, reliable operation and convenient maintenance of AC motors, as well as the advantages of high operating efficiency and good speed regulation performance of DC motors. It is these advantages that make brushless DC motors widely used in many fields of today's national economy. Brushless DC motors use electronic commutation devices. According to the position signal detected by the position sensor, a certain logic control PWM waveform is generated by DSP (digital signal processor) to drive the motor to achieve the smooth operation of the brushless DC motor. In recent years, with the rapid development of industry, the requirements for product performance are also increasing year by year. For some modern products, controlling only one motor can no longer meet the needs. It is necessary to control multiple motors at the same time to work in a coordinated and orderly manner to meet the functional needs.
The design uses TI's TMS320F28335 as the processor, and uses driver chips and MOSFETs to drive two brushless DC motors. On the basis of completing the hardware design, according to different control methods designed by the software, the motors can be driven to operate synchronously or according to a certain rule.

1 Dual-motor synchronous control system
The control object is two three-phase brushless DC motors with a rated power of 3kW and a rated speed of 1,500 r/min. They are mainly used in situations where synchronous movement is required to control the synchronous movement of two motors.
The control system of the brushless DC motor is mainly composed of the control part, the drive and inverter circuit part, the rotor position detection and current sampling circuit. Among them, the control part with TMS320F28335 as the core is responsible for control operations, analog sampling and other tasks; the drive circuit amplifies the weak current signal output by the control circuit, and outputs a strong current signal with a certain driving ability to control the switch tube of the inverter circuit, so as to realize the inverter conversion of DC power to the motor and achieve the purpose of controlling the motor; the position detection part detects the motor rotor signal and sends it to the control part for processing; the current sampling part completes the detection of the DC power bus current. The entire system has few peripheral devices, which reduces the design difficulty, and uses high-performance sensor detection to improve the accuracy of the system.

2 Control system hardware design
2.1 System power supply design
Different peripherals of TMS320F28335 require different voltages, the core voltage is 1.8 V, and the I/O voltage is 3.3 V; the power-on sequence also requires that the I/O voltage precedes the core voltage, so it is necessary to design a power supply that meets the control system requirements. TI's TPS767D318 is selected as the power chip to convert the input 5 V voltage into 3.3 V and 1.8 V as the power input of the DSP; while the voltage of the brushless DC motor is 24 V, the corresponding driver chip voltage is +15 V or -15 V, so the DC/DC module is used to convert the 5 V voltage into +15 V or -15 V as the power supply of the driver chip. In this way, the entire system only needs to supply 5 V and 24 V voltage to meet the needs. The peripheral circuit of TPS767D318 is shown in Figure 1.

The DC/DC module is shown in Figure 2.

2.2 Design of driver chip and peripheral circuit
The brushless DC motor uses electronic commutation instead of mechanical commutation of DC motor, and energizes different phases of the motor in a certain pattern to drive the motor to rotate. From the perspective of performance and cost comparison, the more commonly used method is the three-phase star-shaped fully controlled bridge circuit. By obtaining the phase position signals of the Hall sensor of the brushless DC motor itself, the on-off state of each phase of the brushless DC motor at each moment is determined. The DSP chip generates a PWM waveform signal with a certain pattern according to the settings. The driver chip amplifies the PWM signal output by the DSP and outputs a signal with a certain driving capability to control the operation of the switch tube in the inverter circuit. The inverter circuit is composed of a power semiconductor device MOSFET, which outputs the control logic signal required by the motor to drive the motor to rotate. The driver chip IR2136 and the peripheral circuit of the MOSFET tube are shown in Figure 3, where PWM1~6 are waveform signals from the DSP, and A, B, and C are connected to different phases of the motor respectively.

2.3 Design of detection circuit of control system
The PWM signal output by driver chip IR2136 controls the switch circuit to switch on and off in a certain pattern, so that different phases of brushless DC motor are energized at different times. In order to understand the parameters of the motor in real time during operation and ensure that the motor works under normal conditions, the running state of the motor must be detected. The signals that need to be detected are the position signals of each phase, current signals, and voltage signals. The following circuits are designed for various signals that need to be detected. 2.3.1 Position signal detection
There are 3 Hall sensors on the shaft of the brushless DC motor. Each sensor will generate an output signal with a pulse width of 180° to indicate the position of the motor at each moment. The output signals of the 3 sensors have a phase difference of 120°. The position signal of the brushless DC motor is shown in Figure 4. In this way, 6 rising edges or falling edges will be generated in each mechanical rotation, which just corresponds to 6 commutation moments. The CAP function (set to double-edge trigger) of the EV module of TMS320F28335 is used to obtain each edge that needs to be commutated, thereby controlling the commutation of the motor. The corresponding function can be realized by connecting the output position signal to the CAP pin port.

2.3.2 Current signal detection
During the operation of the motor, only two phases are energized at a time (one phase is energized in the forward direction and the other phase is energized in the reverse direction). Therefore, only one current needs to be controlled at a time. By placing the resistor between the power supply and the ground, current feedback can be realized and real-time supervision can be performed. The output of the current feedback is filtered and amplified and sent to the ADC port of the DSP for processing. The current is sampled in each PWM cycle and the speed (PWM duty cycle) is controlled. Here, the linear isolation amplifier HCNR200 is selected to process the output waveform. The current signal detection circuit is shown in Figure 5.

2.3.3 Voltage signal detection
During the operation of the motor, the DC bus voltage of the motor needs to be detected to ensure that it is within the rated voltage range of the motor. The overvoltage or undervoltage state of the motor can be understood through the A/D sampling of the DSP. The voltage signal detection circuit is shown in Figure 6.

2.4 Design of other peripheral circuits
In order to enable the entire control system to operate, other peripheral circuits need to be designed, such as the DSP clock circuit, reset circuit, JTAG circuit, RS232 circuit and DSP function port expansion design. Indicator lights need to be added in some important places to facilitate the understanding of the control system operation process. Due to the high-frequency characteristics of the DSP system, electromagnetic compatibility and other issues need to be considered during the design to ensure the normal operation of the entire system.
3 System software design
The final realization of the control task in the control system is completed by software. Therefore, on the basis of completing the hardware design, the software must be designed. The quality of the application program directly determines the quality and efficiency of the entire control system. Motor control is generally a fast process that requires a series of software processing processes to be completed within a certain period of time. For example, the feedback signals of the motor controlled parameters (speed, current, voltage, etc.) are sampled, calculated, judged and processed accordingly. In order to meet the real-time requirements of the system, the control system needs to monitor the input and output with strong real-time performance in an interrupt mode. The software design makes full use of the interrupt processing capability of TMS320F28335 to complete tasks such as current sampling, position capture and PWM waveform generation. ADC completes current and voltage sampling, and CAP completes position signal capture and commutation logic determination. The software tasks mainly include the main program and each interrupt subroutine, and its flow is shown in Figure 7.

According to the control platform hardware and software design, after debugging, the motor runs relatively smoothly. The phase voltage of a certain phase during operation is shown in Figure 8.

4 Conclusion
This paper proposes a general dual-motor control platform hardware design scheme, using TI's TMS320F28335 as the main processing chip, and the highly integrated peripheral circuit design makes the circuit simple; the rich peripherals of TMS320F28335 make the system control performance better; because TMS320F28335 has two EV modules with the same function, one controller can control two motors at the same time, saving costs. Based on this control platform, the control system is combined with the actual control strategy to achieve different control functions and methods, and then applied to different occasions.