Achieved with the ST72141 BLDC Motor Control

Introduction

1 Overview

ST ST72141 is a company dedicated to synchronous motor control of a single chip, especially for 3-phase brushless DC motor control. Brushless DC motor can be used for industrial control, automotive electronics, refrigerators, air conditioners, compressors and fans and other products. Brushless DC motor has the advantage of high efficiency, low noise, small size, reliability and long life.

ST72141 is the ST7 microcontroller family of products in one. It includes A / D converter and SPI interface are dedicated to brushless DC motor control chip peripherals, optional sensor model with and without sensor model.

Achieved with the ST72141 BLDC Motor Control

ST7-chip motor control circuit as a pulse width modulation multiplexer. It has six outputs and one used in brushless DC motor without sensor control of the back-EMF zero detection circuit.

ST72141 motor control peripheral has four main components:

◇ to the end and Back-EMF zero magnetic detection circuit;

◇ delay management circuit;

◇ PWM management circuitry (required PWM signal to drive motor);

◇ channel management circuit.

ST72141 in Brushless DC Motor Typical Applications shown in Figure 1.

Achieved with the ST72141 BLDC Motor Control

Figure 26 step 120-degree-driven model

2, the basic principle of brushless DC motor

Brushless DC motor includes two coaxial magnetic armatures: external armature, the stator is fixed; internal armature, the rotor can move. The stator is part of the motor lead; rotor is part of the induction motor. Internal armature brushless DC motor rotor is a permanent magnet. The armature by a constant current power supply. How can the stator phase (here an example of 3-phase). Motor is synchronous motor. Permanent magnet brushless DC motor is a synchronous motor, the stator magnetic field rotation speed and the mechanical rotation speed of the same rotor.

EMF is to use the ST72141 in non-sensor mode, the basis for driving brushless DC motors. EMF and the rotor speed, flow through the rotor windings of the rotor flux and the corresponding proportional to the number.

Winding torque generated is proportional to the size of relations with current and magnetic flux.

ST72141 provides two kinds of control modes: voltage mode and current mode. Current mode can be adjusted directly in proportion to torque; voltage mode, can adjust the speed, torque limit set threshold (ie a current threshold value).

Achieved with the ST72141 BLDC Motor Control

3 ST72141 for brushless DC motor control

Figure 2 for the use of six steps of the motor control schematic.

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

T1, T3, T5 is the motor A, B and C phase windings of the upper transistor.

T2, T4, T6 is the motor A, B and C phase windings of the lower end of the transistor.

In step 1, phase A is a positive bias, so the winding current is positive; phase B for the reverse bias, so the phase winding current is negative. Then C does not exert the power phase windings.

Brushless mode, using the ST72141 motor control, you can read this without the power to impose the phase winding back-EMF (here for phase C winding step for the beginning 1). By reading the back EMF, can determine the actual position of the rotor.

Achieved with the ST72141 BLDC Motor Control

Figure 4 Timing diagram of the incident

Shown in Figure 3, back-EMF and phase winding current in the same direction, the efficiency of the best.

ST72141 can have two kinds of different drive modes: voltage mode and current mode. Current mode, by changing the reference current of the motor torque to change the size (because torque and current is proportional to). By PWM current control is to adjust the. Voltage mode, by changing the reference voltage to change the motor speed. This model is not directly control the current, but set the maximum current limit, that is up to the maximum torque. Voltage control is achieved by changing the PWM cycle.

Use of closed-loop motor speed adjustment to achieve. ST72141 internal two speed adjustment circuit. No. 1 circuit is automatically adjusted for the efficiency of the loop when. This circuit allows winding back EMF and phase current signals in the same direction. 2nd loop the loop is the speed of adjustment, can motor to maintain the set speed.

ST72141 motor control processing based on three events: Back-EMF zero crossing event (Z event), reversing (C events), to the winding to the magnetic end of the (D case), shown in Figure 4.

To the magnetic end and back-EMF zero crossing is a physical event, but for the event is calculated by ST72141 come, which is calculated for zero events, and next to the delay between the time. If the speed is accelerated, the event will occur sooner than zero, the delay must be reduced in order to winding back-EMF and phase current in the same direction.

ST72141 motor control peripheral is always in the same order of handling three events: Z events produced in the calculation of the delay after the C event, and then wait for the D event. Motor starts, according to detect certain events of continuous Z mode into the automatic reversing.

Achieved with the ST72141 BLDC Motor Control

Figure 5, zero-crossing event detection principle

ST72141 in, Z event (zero crossing) and the D event (to the magnetic end) is detected by the same peripheral parts. These signals ST72141 the MCIA, MCIB and MCIC three pin. Zero-crossing event (Z event) detection principle in Figure 5.

Figure 5 shows the motor control of the two states. In Figure 5, left, winding C has to magnetic. About 20μs after the read-EMF of the window. In the T1 is closed, current flows through the freewheeling diode, A point of land. Suppose A phase winding back-EMF for the Ea, B-phase winding back-EMF for the Eb, C anti-phase winding emf Ec. When Ec than zero, there Ea =- Eb, so N Department of zero potential. This means you can virtually do not need to get back EMF information. EMF zero-crossing event received by the comparator output, sensorless mode, certain frequency PWM signal on T1 added. C diode clamp voltage was clamped at +5 V/0.6V (which requires attention is the zero-crossing). The analysis here also adapted for the triangle connecting the motor winding.

Comparator input is the C-phase winding of a voltage signal, another input is a threshold voltage (software selectable 0.2,0.6,1.2 and 2.5V). ST72141 wait for the C-phase winding back-EMF voltage reaches the threshold selection. PWM signal applied to the T1, when T1 off, C-phase voltage to winding. Therefore, ST72141 only need to read back-EMF can be detected to reach the threshold point in time.

The end of the event to the magnetic detection methods and the same zero-crossing events, and use the same peripherals. Motor control in order to deal with this in accordance with a fixed three events, Z after a delay after the incident, resulting in a C event, and then wait for a D event.

Achieved with the ST72141 BLDC Motor Control

Figure 6 to the end of the incident magnetic

In reversing, the start winding to the magnetic acceleration. In order to avoid premature detection of the end of demagnetization event, for the time after a 20μs filter, shown in Figure 6. In order to avoid detection to the magnetic end of the event too late to use the same magnetic detection of the end of the comparator, but the sampling frequency is 800kHz.

Sensorless mode, the comparator output sampling frequency of zero events in the past is the PWM signal to the magnetic end of the event detected at the time of 800kHz.

4 For example, motor starting and control

Here, with two pole pairs motor start as an example. Target speed after the motor start is 1400r/min. Before start the motor, must be pre-fixed position. Just started, back-EMF signal is too weak, can not read. Read the back-EMF signal from the process, the current must be provided "(load torque + friction torque + motor load inertia moment). It starts, ST72141 Timer A, PWM duty cycle during the startup process is running under needs to be higher than the value.

After a certain step, in order to detect zero-crossing event, the method requires a special start motor, known as synchronous (forced commutation) mode, or known as the motor speed according to speed up the process table.

Achieved with the ST72141 BLDC Motor Control

Figure 7, the boot process

After a certain step, continuous incremental steps to impose a long time and current to the motor, the motor speed, and can detect a zero-crossing event. Motor speed up the process of detecting a specified number of consecutive Z events, began to adjust, making high-efficiency motor is running, the motor automatically change to the mode of entry. After taking the table if the acceleration, the motor can not automatically change the mode to enter the motor will stop. Figure 7 shows the closed loop mode, the motor during startup.

Open-loop mode, the startup process is the same, but current or voltage in the motor into the automatic change to the model can then be changed by the user. Closed-loop mode, the current or voltage limits imposed by the user and the fixed force until the motor into the speed of adjustment. Into the speed of adjustment, the current is no longer controlled by the user (ST72141 automatic adjustment). Closed loop control mode, regardless of what kind of control mode (current or voltage), speed adjustment loop start. Motor under the control of the microcontroller, running at the speed of a fixed speed of the decision table.