The principle of BLDC square wave sensorless control based on ADC sampling and integration

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The principle of BLDC square wave sensorless control based on ADC sampling and integration [Copy link]

Author: Hardy Zhou

This article mainly refers to the TIDA-010031 reference design and analyzes the principle of ADC sampling integral square wave non-sensing control to help everyone better complete similar solution designs.

1. Below is a typical three-phase BLDC motor control block diagram.

Three half-bridges drive BLDC brushless motors and detect low-side bus current

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2. Typical BLDC motor phase current and back EMF waveform analysis

Judging from the waveform, every 60 degrees electrical angle, only two half-bridges have driving voltage output, and the upper and lower tubes of the other half-bridge are all turned off, and the phase voltage is in a suspended state.

After the BLDC motor runs, all phase lines have back electromotive force.

The motor back electromotive force comes from the change of magnetic flux caused by the rotation of the motor rotor, and the change of magnetic flux will generate an induced voltage on the stator winding.

For the same motor, the peak value of the back EMF is almost in a fixed ratio to the motor speed.

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3. Integral of the back EMF voltage and time from the back EMF zero point to the peak value

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According to the above description that the back electromotive force peak value is almost in a fixed ratio with the speed (electrical frequency), Vm is set as the back electromotive force peak value. For the same motor, we can assume that Kv is almost unchanged.

Taking the blue area on the left side of the above figure as an example, the electrical angle of this area (the area between the moment when the back electromotive force crosses zero and the moment when the next commutation point occurs) is 30 degrees, which is 1/12 of the electrical angle (360 degrees).

Assume the current motor electrical frequency is f, in Hz.

The peak voltage of the back EMF is , in volts.

Assume that the time for the back electromotive force to reach the maximum value from the center tap value is t1, and .

Then the integral value of the blue integral region is equal to the area of the blue triangle:

It can be seen that the integral result is the Kv value , so the integral result is almost unchanged.

Therefore, we can determine the switching point based on the comparison of the integral value with the fixed threshold .

4. How does ADC sample back EMF?

The ADC sampling integral control of square wave sensorless BLDC has a three-phase phase voltage ADC sampling circuit. In every 60-degree electrical angle interval, the two phases of the motor have input voltage and inductor current in the half-bridge. It is difficult to obtain the back electromotive force voltage without adding additional circuits. Since the suspended phase has no half-bridge voltage input and inductor current, the actual back electromotive force voltage of the phase can be deduced from the detection of the phase terminal voltage. For details, see the following derivation. The derivation process refers to TI's reference design TIDA-010031 http://www.ti.com/lit/ug/tiduej4/tiduej4.pdf?ts=1588819919326

So if you are interested in this motor control method, you can view the software and hardware development materials of the reference design on the TI official website.

The design uses the strategy of the lower tube being always on and the upper tube being PWM to drive the motor. Considering that the PWM duty of the motor drive is variable, the sampling strategy can be determined based on the length of time the upper tube of the half-bridge is on. If the upper tube is on for too short a time, in order to avoid the influence of the MOSFET on and off, the time left for sampling becomes very short, which is not conducive to the accuracy of sampling.

• When the upper tube is on for a long time, the back electromotive force voltage of the suspended phase is detected away from the MOSFET switch when the upper tube is on (generally, sampling is performed in the middle of the ON time).

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The driving logic at this time is that the upper tube of phase A is turned on and the lower tube of phase B is turned on, so there is .

Ea, Eb, Ec are the three-phase back electromotive force voltages of the motor, va, vb, vc are the mid-point voltages of the three-phase half-bridge, that is, the three-phase input voltage of the motor.

La, Lb, Lc are the three-phase inductance of the motor, ia, ib, ic are the three-phase input current of the motor, Ra, Rb, Rc are the three-phase input resistance of the motor (considering that the three-phase resistance is equal), and vn is the three-phase midpoint voltage of the motor.

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It can be obtained that when Ec=0, that is, when the back electromotive force crosses zero, when the ADC detects it , it means that this moment is the back electromotive force of phase C crossing zero. Then theoretically, after another 1/12 electrical cycle time, the motor needs to commutate.

• When the upper tube is on for a short time, the back electromotive force voltage of the suspended phase is detected when the upper tube is off (generally, sampling is performed in the middle of the off time).

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The driving logic at this time is that the upper tube of phase A is turned off, and the lower tube of phase B is still turned on. At this time, the body diode of the lower tube MOSFET of phase A is continuing current, so there is .

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It is found that when Ec=0, that is, when the back electromotive force crosses zero, vc=0. That is to say, when the ADC detects vc=0, it means that this moment is the back electromotive force of phase C crossing zero. Then theoretically, after another 1/12 electrical cycle time, the motor needs to commutate.

From the above analysis, we can see that when using ADC sampling integration method for sensorless BLDC control, the following two points need to be noted in the design:

1. The integral threshold is related to the motor's back EMF peak value and the speed ratio, which may vary from motor to motor and needs to be adjusted for the motor.
2. When the upper tube is designed as PWM driver, different ADC sampling strategies can be used for large duty and small duty situations. At the same time, the judgment of the zero crossing point of the back EMF also needs to be adjusted.
3. When the motor is running at high speed, the electrical frequency is relatively high, and the ADC integral sampling is based on the PWM switching cycle sampling. Therefore, to obtain a more accurate commutation point, a relatively high switching frequency is required. If the switching frequency is relatively low, it means that the sampling rate is slow, which may cause a large commutation delay, thereby affecting the normal control of the motor.