32-bit microcontroller realizes advanced control technology

　　According to statistics from the U.S. Department of Energy, about half of the world's energy is consumed by motors, so how to improve the energy consumption of motor control systems has become an important issue. To reduce the energy consumption of motors, in addition to the motors moving from AC motors to brushless DC motors (BLDC) and the energy efficiency design of the motor body moving from IE1 to IE3, the most important thing is to have a cost-effective, high-performance, and dedicated microcontroller that is completely designed for motor control.

　　Advanced control algorithms can be implemented with a dedicated high-performance microcontroller for motor control. In addition to saving energy, the introduction of advanced control algorithms also allows the entire control system to respond quickly and smoothly to load changes without sensors. The configuration of sensors will increase component and manufacturing costs, and sensors cannot be placed in many situations, such as when there are chemicals in the compressor and some products cannot be placed due to small space. This article uses a 32-bit microcontroller to implement advanced field oriented control (FOC), high-frequency voltage injection technology and space vector PWM (SVPWM) control.

　　FOC

　　FOC is also known as vector control. The original intention of inventing FOC is to convert the control method of AC motors into that of DC motors. The control of DC motors is relatively simple. By controlling the excitation current and torque current separately, the electromagnetic torque of the motor can be simply and accurately controlled.

　　Decoupling of magnetic field and torque of induction motor: When the AC asynchronous motor is frequency modulated, the voltage remains unchanged, but the magnetic field will change. When the voltage is modulated without frequency modulation, the magnetic field will also change. Therefore, V/F is only a very rough way to control the magnetic field, and it is impossible to achieve accurate control of the magnetic field. FOC can achieve relatively more accurate magnetic field control, but FOC requires a microcontroller with higher computing power. Figure 1 is a FOC system diagram based on a 32-bit microcontroller of Weltrend Electronics. For each ADC interrupt of the inner loop, the following actions are performed:

　　Clarke transformation is used to transform the phase current from static three-phase to static two-phase current.

　　· Use Park transformation to convert the static two-phase current into dynamic two-phase current (rotating coordinate system).

　　· Use the Sliding Mode controller to calculate the speed and position of the motor.

　　Use PI controller to control speed and current.

　　· Use Park's inverse transformation to transform the dynamic two-phase current into a static two-phase current (stationary coordinate system).

　　· Use Clarke inverse transformation to transform the phase current from static two-phase to static three-phase current.

　　Update PWM output duty cycle.

　　ADC interrupt ends.

　　High frequency voltage injection estimation

　　The start of the motor is an important part of PMSM control. The FOC system of PMSM ensures the smooth start of the motor by applying a torque current perpendicular to the rotor magnetic field, but this requires knowing the initial position of the motor. Most position sensorless controls cannot predict the initial position of the rotor, and generally use open-loop start or position the motor to a predetermined position for start. Open-loop start-up often causes reverse bias, jamming, etc. due to the different angles of rotation, and the motor pre-positioning requirement is not applicable in many products.

　　For PMSM zero speed/low speed sensorless control (Figure 1), in order to solve the problem of inaccurate rotor position and speed estimation at low speed, the high-frequency signal injection method first proposed by Professors M. Corley and R. Lorenz of the University of Wisconsin in 1996 is generally used. The high-frequency voltage injection method is currently the most studied. This method is based on the salient pole characteristics of the motor, injects a high-frequency voltage signal into the motor stator, and obtains the rotor position information by performing specific signal processing (filtering, angle estimator) on the high-frequency current response.

　　Figure 1: Architecture of a field-oriented/sensorless control system based on WT58F032

　　According to the different voltage injection methods, high-frequency voltage injection methods can be divided into the following two categories: (1) Rotating high-frequency voltage injection method - Injecting a rotating high-frequency voltage signal into the stator coordinate system, the negative sequence component of the high-frequency current response contains the rotor position information; the motor rotor position is obtained by demodulating the signal. (2) Pulsating high-frequency voltage injection method - Injecting a pulsating high-frequency voltage signal into the estimated rotating coordinate system, the high-frequency component of the current response will contain the position estimation error; by processing the high-frequency current signal, the estimated position converges to the actual position.

　　Space Vector Pulse Width Modulation

　　The working principle of space vector PWM (SVPWM) is to use three sets of half-bridge inverters to synthesize the motor stator current through PWM modulation voltage vector. The stator flux vector generated by this synthesized current on the stator coil interacts with the rotor flux to generate torque, causing the motor to rotate. SVPWM is named space vector pulse width modulation because it determines the switching timing of the three sets of half-bridge inverters by synthesizing the stator flux vector. This modulation method controls the voltage vector so that the trajectory of the motor air gap rotating flux vector approaches an ideal circle, and has the smallest flux fluctuation. Its torque ripple (Torque Ripple) is the lowest, so under open-loop control, the motor speed fluctuation is also the smallest. Table 1 shows the three sets of half-bridge inverter power switch devices in the motor drive circuit. Because the space vector pulse width modulation switch control does not have the definition of upper and lower switches being turned on at the same time, it can actually be regarded as two state switch timings (upper switch OFF, lower switch ON, or upper switch ON, lower switch OFF). Therefore, the three sets of power switch devices can produce a total of eight switch state combinations. 　　Table 1: SVPWM control system

　　The SVPWM control system based on the Weltrend Electronics WT58F032 microcontroller has the following control flow:

　　(1) Main Routine

　　① WT58F032 reset;

　　② WT58F032 chip initialization settings;

　　③ The motor stops running;

　　④ Check whether the start signal is "true" - if it is true, enter the initialization motor configuration and enable interrupt; if it is "false", return to the motor stop state.

　　(2)Interrupt Service Routine

　　① The interrupt service routine starts;

　　② Input capture unit/rotor interval calculation;

　　③ The result of the calculation in "②" above is output to the speed calculation unit and the motor phase calculation unit;

　　④ The speed calculated in the above “③” is output to the motor phase calculation unit and PID controller;

　　⑤ The motor phase and PID calculated in "④" above are output to the sine wave generator to generate a sine wave.

　　Figure 2 shows the output waveform of SVPWM control based on WT58F032. Output waveform (M shape). As can be seen from the figure, the line voltage utilization rate of SVPWM is higher than that of general PWM, so it can achieve energy saving effect.

　　Figure 2: SVPWM control output waveform based on WT58F032. Relationship between rate switch switching state, line voltage, phase voltage and space vector

　　Summarize

　　The cost-effective 32-bit microcontroller proposed by Weltrend Electronics not only has a high-computing 32-bit RISC CPU and a built-in fast multiplier that can complete 32b×32b multiplication in one instruction cycle, but also integrates peripheral circuits designed for motor control, including high-speed ADC, high-speed and multi-mode operation PWM, PWM trigger ADC, high-speed comparator, QEI, etc. In addition, considering the needs of industrial control, this chip supports wide voltage operation (can operate at 2.0V~5.5V) and has excellent noise resistance, which is very suitable for use in high-performance motor control systems.