A control method for low speed operation of a position sensorless brushless DC motor

Abstract

The present invention discloses a control method for low-speed operation of a brushless DC motor without a position sensor. A reverse pulse is inserted in each PWM cycle to generate a reverse torque, which hinders the operation of the motor and reduces the operating speed. The forward torque is greater than the reverse torque; the magnitude of the torque is determined by the conduction time and the current, that is, under the same voltage, the forward conduction time is greater than the reverse conduction time; the magnitude of the composite torque is determined by the power-on time of the two, and the motor speed can be adjusted by changing the magnitude of the composite torque. In addition to ensuring sufficient time for position detection, the method of the present invention also realizes low-speed operation, and the low-speed speed is adjustable. The present invention can realize ultra-low-speed operation without a position sensor, and has the low-speed operation characteristics of brushed motors or sensored brushless DC motor products, and can be a perfect replacement, such as power tools and electric drills.

Description

Technical Field

[0001] The invention relates to the field of motor control, and in particular to a method for controlling a position sensorless brushless DC motor running at low speed. Background Art

[0002] Unlike brushless DC motors without position sensors, motors with Hall position sensors can achieve lower operating speeds by simply turning on a smaller PWM duty cycle. Sensorless motors need to detect the motor position when turning on the power tube. Before detection, the position signal needs to be delayed and filtered. In addition to the detection time, a larger PWM duty cycle needs to be turned on to meet the requirements. However, an excessively large PWM duty cycle causes the motor to rotate too fast and cannot achieve the required low-speed operation. Summary of the invention

[0003] The purpose of the present invention is to provide a control method for low-speed operation of a position sensorless brushless DC motor, which can achieve ultra-low-speed operation without a position sensor and has the low-speed operation characteristics of a brushed motor or a sensored brushless DC motor product.

[0004] The technical solution of the present invention is:

[0005] A control method for low-speed operation of a position sensorless brushless DC motor, wherein a reverse pulse is generated in each PWM cycle to generate a reverse torque to hinder the operation of the motor; square waves are used to control the conduction of the three phases A, B, and C of the motor in pairs, and the direction of the torque generated by the forward conduction is 180 degrees different from the direction of the torque generated by the reverse pulse, and the direction of the synthetic torque in the PWM cycle is the difference between the two;

[0006] The forward torque is greater than the reverse torque; the torque size is determined by the conduction time and the current size, that is, under the same voltage, the forward conduction time is greater than the reverse conduction time; the size of the combined torque is determined by the power-on time of both. By changing the size of the combined torque, the motor speed can be adjusted.

[0007] Preferably, pulse insertion is performed at a low PWM duty cycle while ensuring that the position signal can be detected.

[0008] Preferably, the control method comprises the following steps:

[0009] S1. Use square wave algorithm and any PWM modulation method to implement two-by-two conduction control of the three phases A, B, and C; set A+ to indicate that the upper bridge power tube of phase A is turned on, A- to indicate that the lower bridge power tube of phase A is turned on, and the same applies to the other phases B and C;

[0010] S2, with phase A as PWM output, the conduction phase of A+B- conducts the three-phase winding;

[0011] S3, when the PWM output is turned off for T1 time, the A phase output changes from A+ to A-, and the conduction phase is AB-; then delay T2 time, switch the B phase output from B- to B+, that is, the output is A-B+, and then delay T3 time, the B phase output switches from B+ back to B-; after the next PWM cycle, the hardware automatically turns on A+B-, and the cycle repeats. A reverse conduction pulse is inserted in each cycle. The time and width of the inserted pulse must be completed within one PWM cycle;

[0012] S4, turn on the three-phase winding with the conduction phase of A+C-, phase A is PWM output, repeat step S3, and change phase B to phase C;

[0013] S5: When phase B and phase C are used as PWM output, it is similar to steps S3 and S4.

[0014] The advantages of the present invention are:

[0015] The control method of the present invention inserts a reverse pulse in each PWM cycle to generate a reverse torque, hinder the motor operation, and reduce the running speed. This method not only ensures sufficient time for position detection, but also realizes low-speed operation, and the low-speed speed is adjustable. The present invention can realize ultra-low-speed operation without a position sensor, and has the low-speed operation characteristics of brushed motors or sensored brushless DC motor products, and can be a perfect replacement, such as power tool electric drill products. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0017] Figure 1 is the control logic waveform in the embodiment;

[0018] Figure 2 This is a schematic diagram of current flow when A+B- is turned on in the embodiment;

[0019] Figure 3 This is a schematic diagram of current flow when A-B+ is turned on in the embodiment;

[0020] Figure 4 The six synthetic magnetomotive force state diagrams of the two-by-two conduction mode of the square wave control in the embodiment;

[0021] Figure 5 : is the phase voltage waveform in the embodiment. DETAILED DESCRIPTION

[0022] In order to solve the problems in the prior art, the present invention provides a control method for low-speed operation of a position sensorless brushless DC motor. A reverse pulse is generated in each PWM cycle to generate a reverse torque to hinder the operation of the motor. A square wave is used to control the three phases A, B, and C of the motor to be turned on in pairs. The direction of the torque generated by the forward conduction is 180 degrees different from the direction of the torque generated by the reverse pulse. The direction of the synthetic torque in the PWM cycle is the difference between the two.

[0023] The direction of the resultant torque is the difference between the two. To ensure the normal operation of the motor, the forward torque must be greater than the reverse torque. The torque is determined by the conduction time and the current. That is, at the same voltage, the forward conduction time is greater than the reverse conduction time. The magnitude of the resultant torque is determined by the power-on time of the two. Changing the magnitude of the resultant torque can achieve motor speed regulation.

[0024] Under the premise of ensuring the detection of position signals, pulse insertion is performed at a low PWM duty cycle.

[0025] The control method comprises the following steps:

[0026] S1. Use square wave algorithm and any PWM modulation method to implement two-by-two conduction control of the three phases A, B, and C; set A+ to indicate that the upper bridge power tube of phase A is turned on, A- to indicate that the lower bridge power tube of phase A is turned on, and the same applies to the other phases B and C;

[0027] S2, with phase A as PWM output, the conduction phase of A+B- conducts the three-phase winding;

[0028] S3, when the PWM output is turned off for T1 time, the A phase output changes from A+ to A-, and the conduction phase is AB-; then delay T2 time, switch the B phase output from B- to B+, that is, the output is A-B+, and then delay T3 time, the B phase output switches from B+ back to B-; after the next PWM cycle, the hardware automatically turns on A+B-, and the cycle repeats. A reverse conduction pulse is inserted in each cycle. The time and width of the inserted pulse must be completed within one PWM cycle;

[0029] S4, turn on the three-phase winding with the conduction phase of A+C-, phase A is PWM output, repeat step S3, and change phase B to phase C;

[0030] S5: When phase B and phase C are used as PWM output, it is similar to steps S3 and S4.

[0031] The control method of the present invention for controlling the low speed operation of the position sensorless brushless DC motor has a control logic waveform as shown in FIG. Figure 1 As shown; when A+B- is turned on, the current flows to ia as shown Figure 2 As shown; when the reverse pulse A-B+ is turned on, the current flows as follows Figure 3 shown.

[0032] There are six states of synthetic magnetomotive force in the square wave control two-by-two conduction mode of the present invention, such as Figure 4 When A+B- is turned on, the magnitude and direction of the generated magnetomotive force is FA+B-; when the reverse pulse A-B+ is turned on, the magnitude and direction of the generated magnetomotive force is FA-B+, and the two are 180 degrees apart. The synthetic magnetomotive force (F) in the PWM cycle is:

[0033] F = FA+B- + (-FA-B+);

[0034] So: F = FA+B- -FA-B+;

[0035] When FA+B- > FA-B+, the motor can keep running, and the F value affects the motor running speed.

[0036] like Figure 5 As shown, the phase voltage waveform of the control method of the present invention presents a fish ripple shape, so it is also called "fish ripple control technology".

[0037] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any modifications made according to the spirit of the main technical solution of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for controlling a position sensorless brushless DC motor at low speed, characterized in that: A reverse pulse is generated in each PWM cycle, generating reverse torque and hindering the operation of the motor. The A, B, and C phases of the motor are turned on in pairs using square waves. The direction of the torque generated by the forward conduction is 180 degrees different from the direction of the torque generated by the reverse pulse. The direction of the synthetic torque in the PWM cycle is the difference between the two. The forward torque is greater than the reverse torque; the torque is determined by the conduction time and the current, that is, at the same voltage, the forward conduction time is greater than the reverse conduction time; The size of the resultant torque is determined by the power-on time of the two. Changing the size of the resultant torque can achieve motor speed regulation. The control method comprises the following steps: S1. Use square wave algorithm and any PWM modulation method to implement two-by-two conduction control of the three phases A, B, and C; set A+ to indicate that the upper bridge power tube of phase A is turned on, A- to indicate that the lower bridge power tube of phase A is turned on, and the same applies to the other phases B and C; S2, with phase A as PWM output, the conduction phase of A+B- conducts the three-phase winding; S3, when the PWM output T1 time is turned off, the A phase output changes from A+ to A-, and the conduction phase is AB-; then delay T2 time, switch the B phase output from B- to B+, that is, the output is A-B+, and then delay T3 time, the B phase output switches from B+ back to B-; after the next PWM cycle, the hardware automatically turns on A+B-, and the cycle is repeated in sequence. A reverse conduction pulse is inserted in each cycle. The time and width of the inserted pulse must be completed within one PWM cycle; S4, turn on the three-phase winding with the conduction phase of A+C-, phase A is PWM output, repeat step S3, and change phase B to phase C; S5: When phase B and phase C are used as PWM output, it is similar to steps S3 and S4.

2. The control method for low-speed operation of a position sensorless brushless DC motor according to claim 1, characterized in that: Under the premise of ensuring the detection of position signals, pulse insertion is performed at a low PWM duty cycle.

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