Stepping motor current control system applied to voltage driving

Abstract

The invention relates to an H-bridge driving circuit, in particular to a stepping motor current control system applied to voltage driving, which comprises a digital control unit, a one-phase coil driven by a stepping motor, an H bridge and a first comparator, the first comparator is used for comparing an output current value of the H bridge with a preset current value, and when the output current value is greater than the preset current value, the first comparator is used for outputting a current value; the comparison result is sent to the digital control unit, the digital control unit obtains a driving signal for controlling the stepping motor through calculation, the system does not need an additional sensor, and the system cost and the installation complexity are reduced; the feedback regulation circuit is irrelevant to the resistance, inductance and back electromotive force of the motor, and is also irrelevant to the external load and rotating speed of the motor, so that the current can be accurately regulated, and the reliability and control performance of the system are improved.

Description

Technical Field

[0001] The invention relates to an H-bridge driving circuit, and more specifically to a stepping motor current control system applied to voltage driving. Background Art

[0002] Stepper motor is a widely used type of motor with the characteristics of simple control and fast response speed. Usually, stepper motor does not need a feedback system, and accurate stepper control can be achieved by using open-loop control. Under the condition of not overloading, the control accuracy is high, which is especially suitable for application in low-speed and high-torque scenarios. Common stepper motors are two-phase four-wire hybrids, and there are usually two schemes for drive control: current-type drive and voltage-type drive. Among them, the current-type drive mode is to input current into the coil of the stepper motor, and the current passes through the sampling resistor to obtain the sampling voltage. The sampling voltage is fed back to the comparator to compare with the reference voltage. When the current is too large, the comparator flips and turns off the output switch tube. At this time, the output current decays. After a fixed decay time, the output switch tube is reopened, and the current increases until the comparator flips. The chip keeps turning on and off the switch tube to form PWM pulse width modulation control; by changing the reference voltage of the comparator, the current change can be controlled; the disadvantages of this method are obvious heating, large noise caused by vibration, and low stepping accuracy. The voltage drive type means that when controlling, a certain voltage duty cycle is usually given to the two coils, generating sine and cosine voltage modulation waves with a phase difference of 90 degrees. The two-phase modulation waves are compared with the triangular carrier respectively to generate the corresponding PWM drive signal to act on the H bridge, thereby controlling the movement of the stepper motor. Since the voltage drive has no measurement and comparison links, the vibration and noise are very small compared with the current drive, so it is also called silent drive. However, although the voltage drive method is simple, due to the different motor parameters and their operating conditions, including resistance, inductance, back electromotive force constant, motor speed and external load, the generated current size is unknown, and the uncertain current size will cause many problems. When the current is too large, the vibration and heat of the motor are serious, which is not conducive to the mechanical system. When the current is too small, the motor is prone to stall, especially in situations where the load is large or changes drastically; on the other hand, when the motor speed changes, due to the influence of back electromotive force, the current will also change, which is manifested as large current at low speed and small current at high speed. Summary of the invention

[0003] In order to overcome the above problems, the present invention provides a current control system for a voltage-driven stepper motor, which is independent of motor parameters and operating conditions and can accurately control the current without the aid of any sensor.

[0004] The technical solution is as follows: a stepper motor current control system for voltage drive, comprising a digital control unit, a phase coil and an H-bridge driven by a stepper motor, and a first comparator, wherein the first comparator is used to compare the output current value of the H-bridge with a preset current value, and when the current value is greater than the preset current value, the comparison result is sent to the digital control unit; the digital control unit comprises an error calculation module, a measurement module, a control and regulation module, and a drive circuit, wherein the measurement module is used to measure the duration ΔT when the output current exceeds the target current, and the period of the output current is set to T, and the measurement module feeds back the ratio of ΔT to T / 2 to the error calculation module. Module, the error calculation module calculates the difference between the set expected ratio and the feedback ratio and outputs it to the control and regulation module. The control and regulation module performs PI operation and outputs the duty cycle value to the drive circuit. The drive circuit generates sine and cosine voltage modulation waves with a phase difference of 90 degrees. The two-phase modulation waves are compared with the triangular carrier respectively to generate corresponding PWM drive signals to drive the H-bridge and act on the stepper motor to generate current. The measurement module continues to measure and feed back to the error calculation module for further adjustment, forming a current regulation feedback system, thereby accurately controlling the current amplitude of the motor to reach the target current.

[0005] Its further feature is that the preset current value can be preset by the digital control unit through a digital-to-analog converter, or can be given by an external reference voltage; The output current value of the H-bridge is measured by the tail current sensing resistor connected to the H-bridge; The digital control unit is also connected to a second comparator, and the second comparator is used to detect a zero-crossing signal of the output current value.

[0006] After adopting the present invention, no additional sensors are required, thus reducing system cost and installation complexity; the feedback regulation circuit is independent of the motor resistance, inductance and back electromotive force, and is also independent of the motor external load and rotation speed, and can accurately regulate the current, thereby improving the reliability and control performance of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 It is the principle diagram of the present invention; Figure 2 is the voltage waveform diagram; Figure 3 This is a schematic diagram of the internal structure of the digital control unit. DETAILED DESCRIPTION

[0008] See Figure 1As shown, a stepper motor current control system for voltage drive includes a digital control unit 100, a one-phase coil and an H-bridge 110 driven by a stepper motor (common stepper motors are bipolar, and for the sake of simplicity, only one coil and an H-bridge are drawn in the figure), a first comparator 102, and a second comparator 101. The H-bridge 110 generally includes a tail current sensing resistor 111. Similarly, the tail current sensing resistor may include an auxiliary filter device, which is not drawn here for the sake of simplicity. The first comparator 102 is used to compare the tail current (i.e., the output current of the H-bridge) given by the tail current sensing resistor 111 with the preset current value given by the digital control unit 100 through the digital-to-analog converter 103. When the tail current is greater than the preset current value, the comparison result is sent to the digital control unit 100. The second comparator 101 is used to detect when the tail current flowing through the tail current sensing resistor 111 passes through zero, and send a zero-crossing signal to the digital control unit 100. The digital signal provided by the digital control unit 100 to the digital-to-analog converter 103 will vary with its internal state, so that the analog first comparator 102 can complete the comparison of multiple level values; since the MOS switch will have high-frequency switching noise, it will have a greater impact on the current at the moment of the MOS switch. If the current is processed at this time, it is easy to misjudge. Therefore, there will be a shortest working time after the MOS tube is turned on. During this period of time, the analog comparator does not process the current. This period of time is called the blanking time Tblank. During the Tblank time, the digital control unit 100 does not respond to the output result of the first comparator 102.

[0009] When a certain duty cycle is given, the voltage waveform is as follows: Figure 2 As shown in 201, due to the characteristics of the motor, the current in the motor coil will lag behind the voltage 201, and the final current waveform is shown in Figure 2 In 202, when the current crosses zero in the positive direction ( Figure 2 When the current continues to increase to the target current 203, the first comparator 102 detects the flip signal, and the timer stops timing, and the timing time output is Δt. As the motor runs, the current will continue to show a sinusoidal change along with the voltage waveform.

[0010] The total time that the current waveform exceeds the target current 203 is recorded as ΔT. Figure 2It can be calculated that the time can be expressed as T / 2-2Δt, where T is the current period; when the given duty cycle is large, the current amplitude in the coil will also be large, causing the current waveform to exceed the target current 203 more, resulting in a larger ΔT; when the duty cycle is small, the current amplitude in the coil will also become smaller, and the current waveform will exceed the target current 203. The smaller the portion, the smaller the ΔT; therefore, ΔT is an indirect measure of the current size. When ΔT decreases to near 0, the current amplitude output by the motor can be considered to be the target value 203 preset by the digital control unit 100. By adjusting the circuit to control the duty cycle, the output current can be close to the target current, thereby achieving precise control of the current size.

[0011] The specific implementation principle of the digital control unit 100 is shown in Figure 3 , where 305 is a measurement module, that is, the time ΔT when the output current exceeds the target current 203 is measured according to the above method, and the ratio of ΔT to T / 2 is used as the output value of the measurement module 305; 301 is the set expected ratio, which is usually set to 10%~15%; 302 is an error calculation module, which calculates the difference between the expected wall value 301 and the feedback ratio; 303 is a control and adjustment module, which performs PI operation on the input error value and outputs a duty cycle duty value, which further acts on the drive circuit 304 to generate sine and cosine voltage modulation waves with a phase difference of 90 degrees. The two-phase modulation waves are compared with the triangular carrier respectively to generate corresponding PWM drive signals to drive the H bridge and act on the motor to generate current. The measurement module 305 continues to measure and feed back to 302 for further adjustment. In this way, each circuit module forms a current regulation feedback system, thereby accurately controlling the current amplitude of the motor to reach the target current 203.

[0012] When the current passes through Figure 2 When point B reaches the negative half cycle, the above method can also be used for measurement, feedback and adjustment. Therefore, the two-phase stepper motor can measure ΔT 4 times in one cycle, that is, ΔT is updated once every 1 / 4 cycle, and the system can control and adjust the current more quickly.

[0013] In addition to being preset by the digital control unit through the digital-to-analog converter 103, the target current value 203 can also be given by an external reference voltage; the second comparator 101 can also be omitted, and the first comparator 102 is used to control the comparison value in a time-sharing manner, that is, if a zero-crossing signal is to be detected, the digital control unit 100 presets the current of the digital-to-analog converter 103 to 0, and when the current is detected to be zero, the digital control part 100 presets the target current of the digital-to-analog converter 103 to the target current 203. In addition, the detection resistor 111 for detecting the direction of the current can be omitted, and a current mirror solution with MOS internal resistance can be used instead, which can also measure the current to achieve the purpose of the present invention.

[0014] Compared with using only one first comparator to detect ΔT, the present invention can avoid the problem of being unable to accurately compare currents due to blanking time, and at the same time overcome the problem of unstable comparison signal output when the current exceeds the threshold current during low-speed or low-current driving, thereby greatly improving the reliability and stability of the system.

Claims

1. A current control system for a voltage-driven stepper motor, characterized in that: It includes a digital control unit, a phase coil and an H-bridge driven by a stepper motor, and a first comparator. The first comparator is used to compare the output current value of the H-bridge with a preset current value. When the current value is greater than the preset current value, the comparison result is sent to the digital control unit. The digital control unit includes an error calculation module, a measurement module, a control and adjustment module, and a drive circuit. The measurement module is used to measure the duration ΔT of the output current exceeding the target current. The period of the output current is set to T. The measurement module feeds back the ratio of ΔT to T / 2 to the error calculation module. The error calculation module calculates the difference between the set expected ratio and the feedback ratio and outputs it to the control and adjustment module. The control and adjustment module performs PI operation and outputs the duty cycle duty value to the drive circuit. The drive circuit generates sine and cosine voltage modulation waves with a phase difference of 90 degrees. The two-phase modulation waves are compared with the triangular carrier respectively to generate corresponding PWM drive signals to drive the H-bridge and act on the stepper motor to generate current. The measurement module continues to measure and feeds back to the error calculation module for further adjustment to form a current regulation feedback system, thereby accurately controlling the current amplitude of the motor to reach the target current.

2. A stepper motor current control system for voltage drive according to claim 1, characterized in that: The preset current value can be preset by the digital control unit through a digital-to-analog converter, or can be given by an external reference voltage.

3. The stepping motor current control system for voltage drive according to claim 1, characterized in that: The output current value of the H-bridge is measured by the tail current sensing resistor connected to the H-bridge.

4. The current control system for a voltage-driven stepping motor according to claim 1, characterized in that: The digital control unit is also connected to a second comparator, and the second comparator is used to detect a zero-crossing signal of the output current value.

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