introduction

1. Overview

ST72141 is a single chip microcomputer specially used for synchronous motor control by ST company, which is especially suitable for the control of 3-Phase Brushless DC motor. Brushless DC motor can be used in industrial control, automotive electronic products, refrigerators, air conditioners, compressors, fans and other products. Brushless DC motor has the advantages of high efficiency, low working noise, small volume, good reliability and long service life.

ST72141 is a member of the ST7 microcontroller family. It includes a / D conversion and SPI interface. There are internal and external devices specially used for brushless DC motor control. The mode with sensor and mode without sensor can be selected.

The motor control circuit in ST7 chip can be regarded as a pulse width modulation multiplexer. It has 6 outputs and a back EMF zero detection circuit for brushless DC motor without sensor control.

The motor control peripheral of ST72141 has four main parts:

◇ detection circuit of demagnetization end and zero point of back electromotive force;

◇ delay management circuit;

◇ PWM management circuit (PWM signal is required to drive the motor);

◇ channel management circuit.

The typical application of ST72141 in Brushless DC motor is shown in Figure 1.

2. Basic principle of Brushless DC motor

Brushless DC motor consists of two coaxial magnetic armatures: external armature, i.e. fixed stator; Internal armature, movable rotor. The stator is the guide part of the motor; The rotor is the induction part of the motor. The rotor of the internal armature of Brushless DC motor is a permanent magnet. The armature is powered by a constant current source. The stator can have multiple phases (take 3-phase as an example here). The motor is a synchronous motor. Brushless permanent magnet DC motor is a synchronous motor. The magnetic field rotation speed of stator is the same as that of rotor.

Back EMF is the basis of driving brushless DC motor in sensorless mode using ST72141. The back EMF is directly proportional to the rotor speed, the magnetic flux flowing through the rotor and the number of rotors in the corresponding winding.

The torque generated by the winding is directly proportional to the current and magnetic flux.

ST72141 provides two control modes: voltage mode and current mode. Under current mode, the torque can be adjusted directly in proportion; In the voltage mode, the speed can be adjusted and the torque limit threshold (i.e. the current threshold) can be set.

3. ST72141 is used for brushless DC motor control

Figure 2 is the motor control schematic diagram with 6 steps.

The motor control in ST72141 is based on the standard three half bridge six step control principle.

T1, T3 and T5 are the upper end transistors of winding phases a, B and C of the motor.

T2, T4 and T6 are the lower end transistors of winding phases a, B and C of the motor.

At step 1, phase A is forward biased, so the current in this winding is forward; Phase B is reverse bias, so the current in this phase winding is negative. At this time, no power is applied to phase C winding.

In brushless mode, using ST72141 to control the motor, the back electromotive force of the phase winding without power supply can be read (step 1 starting with winding phase C here). By reading this back EMF, the actual position of the rotor can be determined.

As shown in Figure 3, the efficiency is the best when the back EMF and the current of the phase winding are in the same direction.

ST72141 can have two different driving modes: voltage mode and current mode. In current mode, the torque is changed by changing the reference current of the motor (because the torque is directly proportional to the current). The current control is adjusted by PWM. In voltage mode, the speed is changed by changing the reference voltage of the motor. This mode does not directly control the current, but sets the maximum limit of the current, that is, the maximum value of the torque. The voltage control is also realized by changing the PWM cycle.

The adjustment of motor speed is realized by closed loop. ST72141 has two speed adjustment circuits inside. The first circuit is the efficiency adjustment circuit during automatic commutation. This loop makes the back EMF and the current signal of the phase winding in the same direction. The second circuit is the speed adjustment circuit, which can maintain the motor at the set speed.

ST72141 handles the motor control based on three events: back EMF zero crossing event (Z event), commutation (C event) and end of demagnetization to the winding (d event), as shown in Figure 4.

The end of demagnetization and the zero crossing of back EMF are physical events, but the commutation event is calculated through ST72141, that is, the delay time between the zero crossing event and the next commutation is calculated. If the speed increases, the zero crossing event will occur earlier, and the delay must be reduced to make the back EMF and the current of the phase winding in the same direction.

The motor control peripheral of ST72141 always processes these three events in the same order: the Z event generates the C event after the calculated delay, and then waits for the D event. When the motor is started, it enters the automatic commutation mode according to the detection of certain continuous Z events.

In ST72141, the detection of Z event (zero crossing) and D event (demagnetization end) is processed by the same peripheral part. These signals are input through mcia, mcib and MCIC pins of ST72141. The principle of zero crossing event (Z event) detection is shown in Figure 5.

Figure 5 shows two states of motor control. In the left part of Figure 5, winding C has been demagnetized. At about 20 μ After s, the window for reading the back EMF opens. When T1 is closed, the current flows through the freewheeling diode and point a is ground. It is assumed that the back electromotive force of phase a winding is ea, the back electromotive force of phase B winding is EB, and the back electromotive force of phase C winding is EC. When EC crosses zero, there is ea = – EB, so n is zero potential. This means that the required information of back EMF can be obtained without virtual. The zero crossing event of back EMF is obtained through the output comparator. In sensorless mode, PWM signal of a certain frequency is added to T1. The voltage of C is clamped at + 5V / 0.6V by the clamping diode (and the zero crossing point needs to be paid attention to). The analysis here is also applicable to the triangular connection of motor winding.

One input of the comparator is the voltage signal of phase C winding, and the other input is a threshold voltage (0.2, 0.6, 1.2 and 2.5V can be selected by software). ST72141 waits for the back EMF of phase C winding to reach the selected threshold voltage. PWM signal is applied to T1. When T1 is closed, the voltage of phase C winding is ground. Therefore, ST72141 only needs to read the back electromotive force to detect the time point reaching this threshold.

The method of detecting demagnetization end event is the same as that of zero crossing event, and the same peripherals are used. The motor control processes the three events in a fixed sequence. After a period of delay after the Z event, a C event is generated, and then wait for a D event.

After commutation, the phase winding begins to accelerate demagnetization. In order to avoid early detection of demagnetization end events, there are 20 minutes after commutation μ S, as shown in Figure 6. In order to avoid detecting the demagnetization end event too late, the same comparator is used for the detection of demagnetization end, but the sampling frequency is 800KHz.

In sensorless mode, the output sampling frequency of the comparator is PWM signal in zero crossing event and 800KHz in demagnetization end event detection.

4. Examples of motor starting and control

Here, take the starting of two pole pairs of motors as an example. The target speed after the motor is started is 1400R / min. Before starting the motor, the position must be fixed in advance. At the beginning of startup, the back EMF signal is too weak to be read. Before reading the back EMF signal, the current must be supplied (load torque + friction torque + inertia load torque of the motor). Therefore, when starting, the PWM duty cycle of ST72141 timer a must be higher than the value required under normal operation.

After a certain step, in order to detect the zero crossing event, a special method is needed to start the motor, which is called synchronous (forced commutation) mode, or the process that the motor accelerates according to the accelerometer.

After a certain step length, a continuously increasing step length time and current are applied to the motor to accelerate the motor, and a zero crossing event can be detected. After the specified number of continuous Z events are detected during the acceleration of the motor, the adjustment is started to make the motor run efficiently, that is, the motor enters the automatic commutation mode. If the motor cannot enter the automatic commutation mode after the accelerometer is removed, the motor will stop. Figure 7 shows the starting process of the motor in closed-loop mode.

The starting process in the open-loop mode is the same, but the current or voltage can be changed by the user after the motor enters the automatic commutation mode. In closed-loop mode, the limit value of current or voltage is forcibly applied and fixed by the user until the motor enters speed adjustment. After entering the speed adjustment, the current is no longer controlled by the user (ST72141 automatic adjustment). In the closed-loop control mode, no matter which control mode (current or voltage), the speed adjustment loop starts. Under the control of single chip microcomputer, the motor runs at the speed determined by the speedometer.

Responsible editor: GT

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