CN102820840A - Method for sampling current of brushless direct current motor - Google Patents

Abstract

The invention discloses a current sampling method of a DC brushless motor, which uses an incremental code disc to determine the spatial position of the motor. After the motor is powered on and started, wait for the incremental photoelectric code disc to find its zero position, use the photoelectric code disc to determine the position information of the motor, and at the same time stop using the Hall sensor. Due to the two-phase conducting three-phase six-state method, at any time, the phase current of the non-conducting one-phase winding is zero, and the phase currents of the conducting two-phase windings are equal in magnitude and opposite in direction. In this way, when the motor commutates, the current of another conducting phase can be sampled to avoid the current fluctuation at the commutation point of the motor, so as to ensure the accuracy of motor current sampling in real time and the accuracy of motor control.

Description

A Method for Sampling Current of Brushless DC Motor

technical field

The patent of the invention relates to a current sampling method of a DC brushless motor, which is simple and has high precision. the

Background technique

Compared with brushed DC motors, brushless DC motors cancel the sliding contact between brushes and commutators, so they have simple structure, good speed regulation characteristics, no commutation sparks, high efficiency, long life, reliable operation, and easy maintenance And other advantages, its application is more and more extensive. the

The main components of the DC brushless motor are composed of three parts: the motor body, the position sensor and the electronic switch. In order to drive the rotor to rotate, it is necessary to rely on the rotor position sensor to detect the position signal of the rotor, and drive the switching on and off of each power switching device connected to the armature winding through the commutation drive circuit, so as to control the electrification of the stator winding. generate a rotating magnetic field. As the rotor rotates, the position sensor continuously sends out signals to change the energized state of the armature, so that the direction of the current in the conductor under the same magnetic pole remains unchanged, and the motor can rotate with a constant torque. the

In order to improve the voltage utilization rate, the DC brushless motor usually adopts a three-phase full-control circuit, as shown in Figure 1. Six power switching devices control the turn-on and turn-off of each phase winding. Usually two-to-two conduction is adopted, and two power switching devices are conducting at each instant, and the phase is commutated every 1/6 cycle (60° electrical angle), and one power switching device is commutated each time, and each power switch The device conducts 120° electrical angle. When the motor rotates clockwise, the conduction sequence of each power switch device is V1V2→V2V3→V3V4→V4V5→V5V6→V6V1...; when the motor rotates counterclockwise, the conduction sequence of each power switch device is V3V4→V4V5 →V5V6→V6V1→V1V2→V2V3.... the

Usually the position sensor of the brushless motor adopts the Hall sensor, which has strong adaptability to the environment and low cost, but the accuracy is not high. In order to detect the speed or position information of the motor, an incremental photoelectric encoder is usually used as a position sensor. However, since the incremental photoelectric code disc cannot output the absolute position of the motor, the zero position cannot be found when the power is turned on, and the initial position of the rotor cannot be determined. Therefore, it is necessary to use the Hall sensor to start the motor, and use the Hall signal and the incremental code The disc signal is used to determine the spatial position of the rotor poles of the motor. the

Usually the closed-loop control of the motor adopts three closed-loop control of position loop, speed loop and current loop. The output of the position loop is used as the reference value of the speed loop, and the output of the speed loop is used as the reference value of the current loop, and then the reference value is compared with the current sampling value. , use the current PI adjustment to determine the duty cycle of the power switching device, and finally determine the control logic of the brushless DC motor stator winding according to the position signal and current command value, and output the corresponding PWM waveform to make the rotor run according to the control requirements. When sampling the current of the motor, the voltage dividing resistor R located between the low voltage terminal of the bridge inverter and the ground is usually used to detect the current on the main circuit or three resistors are used to sample the current of the three-phase winding. However, due to the influence of winding inductance and other factors during the commutation process, the current fluctuates during commutation. If the current is sampled to control the motor at this time, the motor will vibrate, and the closed-loop control accuracy is low. Therefore, it is necessary to avoid the current fluctuation caused by the commutation of the motor during current sampling, but for the motor using the Hall sensor as the position detection element, it is impossible to determine the position information of the motor in any space, so it is impossible to grasp the current sampling method. Timing, motor control precision is low. the

Contents of the invention

In view of the above-mentioned defects of the prior art, embodiments of the present invention provide a simple and accurate current sampling method to solve the problems of the prior art. the

A current sampling method of a DC brushless motor, the closed-loop control of the DC brushless motor adopts three closed-loop control of position loop, speed loop and current loop, the output of the position loop is used as the reference value of the speed loop, and the output of the speed loop is used as the current Then the reference value is compared with the current sampling value, the current PI adjustment is used to determine the duty cycle of the power switching device, and finally the brushless DC motor stator winding control logic is determined according to the position signal and current command value , to output the corresponding PWM waveform, so that the rotor operates according to the requirements of the control; the method includes the following steps:

The motor is powered on and started;

Use the Hall sensor and the incremental code disc to detect the spatial position of the motor rotor magnetic pole, and then stop using the Hall sensor;

The incremental code disc is used to judge the position information when the motor commutates, and the current sampling is performed according to the position information, so as to avoid the current fluctuation caused by the motor commutation during the current sampling. the

Preferably, the DC brushless motor adopts two-phase conduction, three-phase six-state mode, at any time, the phase current of the one-phase winding that is not conducting is zero, and the phase currents of the two-phase windings that are conducting are equal in magnitude and opposite in direction; In this way, when the motor commutates, the current of another conduction phase is sampled, and the fluctuation of the current at the commutation point of the motor is avoided. the

Preferably, the current sampling method adopts the method of bus current sampling, that is, the magnitude of the phase current is determined by detecting the magnitude of the bus current to realize closed-loop control. the

Preferably, it also includes adopting different conduction sequences of the power switching devices according to the positive or negative of the given bus current command value to realize forward and reverse rotation of the motor. the

Preferably, different phase currents are sampled in different intervals to obtain the positive or negative of the bus current feedback value, and then compared with the current command value, and then the PWM duty ratio can be calculated, and finally the forward and reverse rotation of the motor can be realized. the

Preferably, when the motor is running under light load or no load, current sampling is performed by sampling a plurality of current values and taking an average value. the

Preferably, the frequency of current sampling when the motor is running at high speed is higher than the frequency of current sampling when the motor is running at low speed. the

Description of drawings

Fig. 1 is the control system schematic diagram of the brushless DC motor used in the present invention:

Figure 2 is a block diagram of a three-closed-loop control system for brushless DC motors;

Fig. 3 is a flow chart of phase current sampling of brushless DC motor;

Figure 4 is a schematic diagram of the three-phase current when the DC brushless motor rotates clockwise;

Figure 5 is a schematic diagram of the three-phase current when the DC brushless motor rotates counterclockwise;

Figure 6 is the phase current of the brushless DC motor at no load or light load;

Figure 7 is the no-load phase current of the DC brushless motor after mean value processing. the

The meanings of the reference marks in the figure are as follows

M is a brushless DC motor, V1, V2, V3, V4, V5, and V6 are six power switching devices that control the commutation of the motor, and VCC is a DC power supply. the

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but not as a limitation of the present invention. the

Figure 1 is the overall block diagram of the DC brushless motor control system. The position signal of the DC brushless motor M is detected by the Hall sensor and the incremental code disc, and converted into a drive signal for driving 6 MOS tubes to drive the motor to rotate. the

Figure 3 is a flow chart of the current sampling of the DC brushless motor. After the brushless motor is powered on, the Hall signal is used to start the motor. After the incremental code disc finds the Z pulse, the Hall sensor and the incremental code disc can be used to determine the six working areas of the motor. As shown in Figure 4, after the code wheel finds the Z pulse, the pulse number of the code wheel is cleared. Assume that the motor used is a pair of poles, and the code disc is an N line. When the motor turns to point a, the electrical angle is 30°. Record the number of pulses at this time as M ₁ . When the motor turns to point b, the electrical angle is 150°, write down the pulse number M ₂ at this time, and so on, so that the motor can be determined in six working areas in the 360° space, and then the motor is positioned by the incremental code disc, and the Hall Sensors can be decommissioned.

Due to the influence of winding inductance and other factors, during the commutation process of the motor at point c, the current is not an ideal square wave, but there are certain fluctuations as shown in Figure 4. Therefore, when the current is sampled around point c, the sampling current will be inaccurate, and the motor control precision will be low. The present invention adopts the method for sampling the commutation current at the commutation point. At first, an interval is artificially delimited by an incremental code disc, such as the (b, d) interval in Fig. 4, and the number of pulses at point b is N ₁ , the number of pulses at point d is N ₂ . Before point b and after point d, the current is smooth and smooth, and at this time, the current of phase A is sampled as the sampling current; while in the interval (b, c), the two phases A and B are turned on at this time, and the currents of the two phases are equal. The direction is opposite, but the current of phase B is smoother, so the current of phase B is sampled as the sampling current in this interval; similarly, in the interval (c, d), phases A and C are turned on, and the two-phase currents are also equal in magnitude. The direction is opposite, so the C-phase current is sampled in this interval as the sampling current. In this way, the influence caused by the current fluctuation during the commutation process is avoided, the sampled current is smooth, and the control precision is high.

Since the motor adopts the two-phase conduction method, as shown in Figure 1, when the A and B phases are conducted, the current generated by the DC power supply passes through the power switching device V1, the A-phase winding, the B-phase winding, the power switching device V6 and The sampling resistor R flows through, and it can be seen that the magnitude of the bus current is equal to the magnitude of the phase current, and the conduction of other phases is the same. Therefore, in order to avoid the commutation point, the method of bus current sampling can also be used to control the motor. In order to control the positive and negative rotation of the motor by controlling the positive and negative of the bus current command value, it is stipulated that: to make the motor rotate clockwise, the bus current command value I _dc ^* >0; to make the motor rotate counterclockwise, the bus current command value is I _dc ^* <0. However, when the motor rotates in the reverse direction, there is a transition process in the middle. In this process, the conduction sequence of the power switching devices is changed, and the motor cannot rotate in the reverse direction. Therefore, the present invention determines the current operation of the motor by detecting the positive and negative of the phase current state. The detection method is as follows: As shown in Figure 4 and Figure 5, two motors are turned on, and each phase is turned on at an electrical angle of 120°. Therefore, the phase A current is detected within the range of 30°-150° electrical angle. If I _A >0 , the motor rotates clockwise, and if I _dc ^* <0, the feedback value of the bus current is I=-I _dc ; if I _A <0, the motor rotates counterclockwise, and if I _dc ^* >0, the bus current The current feedback value is I=I _dc (wherein I _dc is the bus current sampling value). Similarly, the phase B current is detected within the electrical angle range of 150°-270°, and the phase C current is detected within the electrical angle range of 270°-390°. When the state change of the motor is detected, the conduction sequence of the power switching device can be changed to realize the reverse rotation of the motor.

When the motor is running under light load or no load, the current fluctuates greatly. The method used is: in the 120° interval when the power tube is turned on, the average value is calculated by sampling multiple current values. When the winding is turned on, as shown in Figure 6, start from point a, sequentially sample a certain number of current values, store them in the array Pha_Cur[Num], and end at the winding turn-off point b, calculate the 120° interval The average value Pha_Cur_Ave of the Num sampling currents is used as the phase current in this interval:

Pha_Cur_Ave＝(Pha_Cur[1]+Pha_Cur[2]+...+Pha_Cur[Num])/Num

The effect after simulation is shown in Figure 7. It can be seen that the processed current is smooth, and the motor will run smoothly by sampling this current to control the motor. In addition, when the motor is at high speed and low speed, the current changes differently. At low speed, the switching frequency of the power tube is low, and the current change cycle is long. Conversely, at high speed, the switching frequency of the power tube is high, and the current cycle is short. Therefore, at high and low speeds, different sampling frequencies should be adopted. At high speeds, the sampling frequency should be high. At low speeds, the sampling frequency can be low. The two change in the same direction, namely:

Ferequency_of_Sampling＝K*Speed_of_Motor

K is a proportional coefficient. By setting different K, the current sampling frequency of the motor at high and low speeds can be changed, so that the current after averaging processing can truly reflect the current of the motor. the

The above descriptions are only several specific embodiments of the present invention, and the above embodiments are only used to illustrate the technical solutions and inventive concepts of the present invention and not limit the scope of the claims of the present invention. All other technical solutions that can be obtained by those skilled in the art on the basis of the inventive concept of this patent combined with the existing technology through logical analysis, reasoning or limited experiments should also be considered to fall within the scope of protection of the claims of the present invention . the

Claims (7)

1. the current sample method of a dc brushless motor; Position ring, speed ring and electric current loop three closed-loop controls are adopted in the closed-loop control of said dc brushless motor; The output of position ring is as the reference value of speed ring, and the output of speed ring is as the reference value of electric current loop, and this reference value and current sampling data compare then; Utilize current PI to regulate the duty ratio of confirming the device for power switching conducting; Confirm dc brushless motor stator winding control logic according to position signalling and current instruction value at last, export corresponding PWM waveform, make rotor move according to the requirement of control; Said method comprising the steps of:

The motor electrifying startup;

Utilize Hall element and increment code wheel to detect the locus of rotor magnetic pole, Hall element is stopped using subsequently;

Positional information when utilizing increment code wheel to judge the motor commutation is according to the opportunity of said positional information Control current sampling, so that avoid the current fluctuation that the motor commutation causes during current sample.

2. method according to claim 1 is characterized in that, said dc brushless motor adopts two three-phase six condition modes that are conducted, and at any time, a phase winding phase current of not conducting is zero, and two phase winding phase current equal and opposite in directions of conducting are in the opposite direction; Said current sample is through when the motor commutation, the electric current of another conducting phase of sampling, and avoid the fluctuation of the electric current at motor commutation point place.

3. method according to claim 1 is characterized in that, said current sample is the method that adopts the bus current sampling, promptly confirms the size of phase current through the size that detects bus current, realizes closed-loop control.

4. method according to claim 3 is characterized in that, also comprises positive and negative according to given bus current command value, adopts the different power switches break-over of device to realize the rotating of motor in proper order.

5. method according to claim 4; It is characterized in that, utilize the different phase current of different interval samplings to draw the positive and negative of bus current value of feedback, compare with current instruction value again; And then the duty ratio that can calculate PWM is big or small, finally realizes the rotating of motor.

6. according to the described method of one of claim 1-5, it is characterized in that,, average through a plurality of current values of sampling and carry out current sample when motor operates under underloading or the no-load condition.

7. according to the described method of one of claim 1-5, it is characterized in that the frequency of current sample when the frequency of current sample is higher than the motor low cruise during motor high-speed cruising.

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