Design of brushless DC motor servo system based on DSP

Abstract: A brushless DC motor servo control system with TMS320F2812 DSP chip as the core is designed . The three closed-loop control of current loop, speed loop and position loop is adopted. The integral separation PID algorithm is used for the position loop to reduce the influence of integral correction on the dynamic performance of the system during the operation of the motor. In order to speed up the system response speed and reduce the burden on DSP, the current loop is implemented by analog method. Practical application shows that the system has the advantages of high precision, fast response and good stability. Keywords: servo system; TMS320F2812DSP; three closed-loop control; PID algorithm

O Introduction
Brushless DC motor (BLDCM for short) is a new type of DC motor that uses an electronic commutator to replace mechanical brushes and mechanical commutators. It has the advantages of simple structure, good speed regulation, and high efficiency. It has been widely used. TMS320F2812 digital signal processor is the latest 32-bit fixed-point DSP controller launched by TI. The device integrates a variety of advanced peripherals and has flexible and reliable control and communication modules. It can fully realize the control and communication functions of the motor system and provide a good platform for the realization of the motor servo system. This paper designs a brushless DC motor servo control system with high-performance TMS-320F2812DSP chip as the core.

1 Servo control system hardware composition and working principle
The system hardware block diagram is shown in Figure 1.

1.1 Control Circuit
The control circuit is based on F2812, and also includes functional modules such as position encoding, data acquisition, data communication, and some peripheral circuits and data interfaces. Its main function is to realize the acquisition and processing of the position information of the controlled object, the reception and processing of speed feedback information, and the closed-loop control of position and speed. F2812 has a 12-bit AD converter on the chip, but in order to improve the accuracy of the servo system movement, two 6-channel 16-bit AD conversion chips are expanded on the DSP periphery to collect feedback signals and input motion command signals.
The system design uses both DSP and CPLD to improve the feasibility of the circuit. The role of DSP is mainly to output a PWM wave corresponding to the motor movement through the digital I/O port based on the feedback position and speed signals, combined with the motor's movement direction and speed, using the motor control dedicated peripheral EVA on the F2812 chip. CPLD outputs a phase voltage of corresponding size and timing according to the commutation timing information composed of the input PWM signal, control signal and digital signal, thereby driving the motor to make corresponding movements.
1.2 Signal acquisition and conditioning circuit
This circuit collects and processes various sensor signals and current and voltage signals. Including sampled current and voltage feedback signals, given control signals and other analog signals, as well as switch signals such as the output of the Hall sensor. After being processed by the conditioning circuit, their amplitude and level can meet the requirements of the DSP controller.
This system uses AD7656 to perform analog-to-digital conversion on the collected analog signals. The GPIOA0 port of F2812 is connected to the enable end of 74ACl6373 to enable the latch, and GPIOAl is connected to CONVSTX to start the simultaneous conversion of 6 A/D channels. GPIOA2 is connected to the BUSY signal. After the conversion of AD7656 is completed, the BUSY signal becomes low, and the DSP receives AD data in a query mode. 74ACl6373 is used to latch the 16-bit data after AD conversion, and 74LSl38 is used to connect the DSP address line decoding to the chip select signal of AD7656.

1.3 Drive circuit
The drive circuit of the motor is composed of the drive chip IR2130 and the three-phase full inverter circuit. The power drive circuit is powered by +15 V. The drive chip IR2130 has a built-in dead time of 2.5 μs to prevent the upper and lower MOSFETs of the same bridge arm from being turned on at the same time. When the system is undervoltage or overcurrent, IR2130 starts the built-in protection circuit to lock the subsequent PWM output to protect the system circuit. The input signal of IR2-130 is a 6-channel PWM wave obtained by CPLD solution, which is sent to IR2130 after optical coupling isolation, and the output signal is sent to MOSFET to drive the brushless DC motor. In the three-phase inverter circuit, six power devices act as winding switches, using two-two power-on, three-phase six-state mode. Two power tubes are turned on at each moment, and the phase is changed every 1/6 cycle (60° electrical angle). One power tube is changed each time, and each power tube is turned on for 120° electrical angle at a time.

2 Control strategy of servo system
This system realizes system control through a three-loop structure of current, speed and position, in which the current loop and speed loop are inner loops and the position loop is the outer loop.

Figure 2 is a block diagram of the brushless DC motor control system. The speed PI regulator and the current PI regulator are set in the system to adjust the motor speed and current respectively, and the two are connected in series. The given position signal U and the feedback position signal position are adjusted by the position PID to obtain the speed reference value SDref. The speed speed of the motor can be calculated according to the time of two captures. This speed is used as the feedback of the speed reference value. After the speed PI adjustment, the reference current Iref can be obtained. The current feedback I can be obtained through the current detection circuit. After the current PI adjustment, the final adjustment amount is used to control the duty cycle of PWM, that is, the output of the speed regulator is used as the input of the current regulator, and the output of the current regulator is used to control the PWM device.
2.1 Current loop control
The current loop is to linearly control the armature current of the motor through current feedback control, which can achieve linear control of the motor output torque, and make its dynamic range respond quickly and improve safety.
In practical applications, in order to speed up the system response speed and reduce the burden on DSP, an analog implementation method is adopted. The resistor is connected in series to the armature circuit, and at the same time plays the role of overcurrent protection of a power conversion circuit. Through current feedback control, the motor armature current is linearly controlled, which can achieve linear control of the motor output torque, and make its dynamic range respond quickly and improve safety.
In the current loop design, the current regulator uses a PI regulator; the limiter can be made together with the current regulator, and the limit value is determined by the input range of the PWM amplifier; the PWM amplifier uses a dedicated integrated circuit; the filter protection network uses an LC network to improve the EMC level, and the diode network protects the PWM amplifier; the current regulator uses a PI regulator; the sampling resistor uses 0.1Ω. If the selected PWM amplifier has a current measurement terminal, the motor current value can also be read directly.

Figure 3 is the current loop control block diagram, R-motor armature resistance, Tm-motor time constant. Ks-power amplifier voltage amplification factor. Current loop design parameters: PI regulator, first order without static error; maximum output current ≥ 0.63 A, feedback coefficient is 15.873; bandwidth ≥ 30 Hz; τi is selected as the motor equivalent time constant.
2.2 Speed ​​loop control The
speed loop is an important inner loop of the position loop. The speed closed loop can improve the linearity of the control object, improve the speed control accuracy, improve the influence of the grid voltage on the motor speed, improve the anti-interference ability, and improve the system performance.
During the time when the rotor rotates one circle, the Hall sensor outputs 3 180° overlapping signals. The motor has a phase change every 60° rotation. The speed of the motor can be calculated by detecting the time interval between two phase changes.

2.3 Position loop control
The position loop is a closed-loop control loop realized by a potentiometer installed on the motor shaft. The control objects of the position loop are the current loop and the transmission mechanism. The voltage signal measured by the potentiometer is demodulated and converted into a position feedback signal through AD conversion. Due to the large uncertainty of the position loop, the nonlinearity of the controlled object and the time-varying nature of the system parameters, in order to reduce the impact of the integral correction of the motor on the dynamic performance of the system during operation, this system uses an integral separation PID algorithm for the position loop. As shown in Figure 4, the integral separation method does not perform integration when the error is large, and only after the error reaches a certain value, the integral accumulation is added to the calculation of the control quantity. The algorithm is:

3 System software implementation
The software of the servo control system adopts modular design, which makes the software organization flexible and orderly, and is easy to adjust, modify and transplant. The DSP program mainly consists of the main program, signal acquisition and output program, PID algorithm program, serial communication program, filter program, etc. The main program first initializes the DSP, including setting the system clock, timer, system status register, and setting the IO port. Then initialize the interrupt setting, determine the interrupt type and interrupt subroutine required by the system, and then set the event manager to generate PWM waves. Figure 5 is a flow chart of the PID program with integral separation. The improved algorithm with integral separation has better effect and simple program.

4 Conclusion
This paper designs a brushless DC motor servo control system based on TMS320F2812 DSP. It adopts the integral separation PID control algorithm. According to the deviation, different PID controls are performed for different situations. The hardware design and control algorithm of the system are studied. The test results show that the system has fast response and stable performance, which can better meet the control performance requirements of the servo system.