Application of DSP in brushless DC motor control

1 Overview


This article uses the 240XDSP launched by TI as the full digital control core of the brushless DC motor. The servo system consists of only a few system components. TMS320F240X is a high-performance 16-bit digital signal processor (DSP) launched by TI in the United States. It is specially designed for digital control of motors. This DSP includes a fixed-point DSP core and a series of microcontroller peripheral circuits. It integrates the computing power of digital signal processing with the efficient control capability for motors. It can replace analog devices with software, easily modify control strategies, correct control parameters, and has the functions of fault detection, self-diagnosis and communication with the host computer. 2 Hardware Introduction The core of TMS320LF2407A is TMS320C2XX (Figure 1). It adopts Harvard structure and pipeline operation. At a clock frequency of 20MHz, the instruction cycle is only 50ns, and most instructions can be completed within one instruction cycle. The central arithmetic logic part includes a 32-bit central arithmetic logic unit (CALU), a 32-bit accumulator, a CALU input/output data scaling shifter, a 16-bit × 16-bit multiplier, a product scaling shifter, a data address generation logic (including 8 auxiliary registers and 1 auxiliary register arithmetic unit (ARAU), and a program address generation logic. When the processor works continuously, it can also simultaneously execute: a. Data reception and transmission via the serial port; b. Internal timer subtraction; c. Generation of three-phase pulse width modulation (PWM) waveforms; d. Collection of 4 analog signals; e. Watchdog timer subtraction It also includes dual 10-bit analog-to-digital converters and PWM control-based managers (6 comparison units, 12 PWM outputs, and 2-channel photoelectric encoder interface encoding units) that are not available in other series of DSP chips. Its PWM waveform generation unit includes programmable dead zone control and can output asymmetric PWM waveforms, symmetric PWM waveforms, and space vector PWM waveforms. LF2407 is the only DSP in the x240x series that can expand external memory. It is also the model with the strongest control function and the most complete on-chip facilities. It is widely used in code development, system simulation, and actual systems.

Figure 1 Overall structure of TMS320Lx240x series DSP controller

3 DSP control flow

Figure 2 is the control and drive circuit for the speed regulation of three-phase brushless DC motor using TMS320LF2407A. This design uses TMS320LF2407A microcontroller as the core of the system controller and power MOSFET field effect tube as the power conversion element. At any time, only two phases of the motor are turned on to control the commutation element, and PWM is used to control the torque and speed of the motor. Here, the three Hall sensors H1, H2, and H3 with a distribution interval of 120° are connected to the three capture pins CAP1, CAP2, and CAP3 of TMS320LF2407A respectively after the shaping isolation circuit, and the commutation moment is given by generating a capture interrupt, and the position information is given at the same time.

Figure 2 Using TMS320LF2407A to control and drive a three-phase brushless DC motor

Theoretically, the speed and torque control of brushless DC motors are mainly based on the following torque and back electromotive force engineering calculation equations

Where N is the number of coils per phase of the brushless DC motor stator, is the length of the rotor, r is the inner diameter of the rotor, B is the magnetic flux density of the rotor, is the angular velocity of the motor, i is the phase current, L is the phase inductance, is the position of the rotor, and R is the phase impedance.

From the equation, it can be seen that the back electromotive force is proportional to the speed of the motor, and the torque is almost proportional to the phase current. According to these characteristics, the control strategy shown in Figure 3 is adopted in the scheme. The given speed forms a deviation with the speed feedback, and the current reference is generated after speed regulation. The deviation between it and the current feedback is formed by current regulation to form the control amount of the PWM duty cycle, so as to realize the speed control of the motor. The current feedback is realized by detecting the voltage drop on the resistor. The speed feedback is obtained by calculating the position output by the Hall sensor. The position output by the position sensor is also used to control the commutation.

Figure 3 Speed ​​and current control of three-phase brushless DC motor

4 Software Control

The system adopts PWM control algorithm. The motor inputs DC current and only two power tubes are turned on at each moment. The PWM control signal from 2407 is directly connected to the driver, and the output of the driver is connected to the control electrode of the power MOSFET tube. The CPU clock frequency of 2407 is 20MHz and the PWM frequency is 20kHz.

4.1 Phase current detection

After TMS320LF2407A receives the voltage drop signal amplified on the resistor, it obtains the current signal through A/D conversion. At the end of the conversion, the A/D module sends an interrupt request signal to the CPU, waiting for CPU processing. Every 50us, the DSP controller samples the phase current to realize a 20kHz current regulation loop. According to the current error, the PID controller adjusts the duty cycle of the PWM pulse at the beginning of each PWM cycle.


4.2 Rotor position and speed detection

Mastering the appropriate phase change moment can reduce torque fluctuations. Position detection is not only used for phase change control, but also for generating speed control quantity.

The position signal is obtained through 3 Hall sensors. Their output signals differ by 1200. Each mechanical rotation has 6 phase changes. By setting the DSP to the double-edge triggered capture interrupt function, the correct phase change moment can be obtained. By setting the capture ports CAP1~CAP3 of the DSP as I/O ports and detecting the level state of the port, a specific capture interrupt can be obtained.

The position signal can also be used to generate speed control. As long as the time interval between two phase changes is measured, the average angular velocity of the two phase changes can be calculated according to the following formula.

The time interval between two phase changes can be obtained by reading the value of the T2CNT register of timer 2 when the capture interrupt occurs. 4.3 Current and speed regulation The phase current can be regulated by adjusting the pulse width of the PWM signal with a carrier frequency of 20kHz. Ierror="Iref" - Imea cyclenew=cycleold+IerrorK If cyclenew>=Timer_period, then cyclenew=Timer_period If cyclenew>Timer_period, then cyclenew＝0 Where Iref—the reference current desired by the user; Imea——actually measured phase current; Ierror——phase current error to be adjusted; speed regulation uses PI algorithm to obtain the best dynamic effect. The calculation formula is as follows: Where Iref - speed regulation output; ek - kth speed deviation; Kp - speed proportional coefficient; Ki - speed integral coefficient; T - speed regulation cycle; Experiments have shown that it can generate a good three-phase PWM control waveform. Figure 5 shows the generated PWM waveform.

Figure 5 PWM waveform generated by controlling a three-phase brushless DC motor using DSP

The software modules of system initialization, position signal detection, PWM signal output, etc. described above can realize a basic three-phase DC brushless motor speed control system with position sensor. However, in order to build a more complete system, some functional modules need to be added, such as the control module for adjusting the motor speed, the data recording module for saving the system operation data, etc. The communication between TMS320LF2407 and PC adopts RS-485 half-duplex interface circuit. Since the PC provides RS233 interface, it is necessary to convert the interface between RS-32 and RS-85. 5 Conclusion The innovation of this article: The PI algorithm commonly used in industrial control is realized on DSP to output PWM waveform. Due to the limitation of the performance of the single-chip microcomputer itself, it is difficult to meet the requirements of high-speed and high-precision motor control. However, DSP can well realize the PWM waveform output for DC brushless motor control.