A high-precision stepping motor micro-step subdivision driving system

Abstract

The application relates to the technical field of stepping motor driving, in particular to a high-precision stepping motor micro-step subdivision driving system which can ensure that a loop current waveform is not distorted, and the system comprises an H-bridge circuit, characterized in that the lower two MOS tubes of the H-bridge circuit are respectively connected with tail current sensing resistors and input ends of a comparator module, the input ends of the comparator module are also connected with digital waveform output ends of a digital controller, and output ends of the comparator module are connected with input ends of the digital controller; the comparator module receives a digital waveform signal output by the digital controller and a tail current signal of the H-bridge circuit, compares the two signals to obtain a comparator signal and sends the comparator signal to the digital controller, the digital controller processes the comparator signal to obtain a back electromotive force correction signal, and the digital controller outputs an H-bridge control signal with the back electromotive force correction signal to the H-bridge circuit.

Description

Technical Field

[0001] This invention relates to the field of stepper motor drive technology, specifically a high-precision stepper motor microstepping drive system. Background Technology

[0002] H-bridge driver circuits are widely used in motor drive applications. They output current (power) through the same (N-type) or different (P-type + N-type) MOSFETs on the upper and lower bridge arms, thereby efficiently driving power peripherals such as motors. For some types of motors that require precise control, such as stepper motors, precise control of the H-bridge output current is necessary to obtain smaller torque fluctuations and noise.

[0003] In the H-bridge drive circuit, such as Figure 1 As shown, four MOSFETs (101-104) form an H-bridge to drive the motor coil. The H-bridge drive voltage (the gate voltage applied to the four MOSFETs 101-104) uses pulse width modulation (also known as chopping). The drive voltage consists of a fixed-frequency pulse width modulation period. When the H-bridge is working, if MOSFETs 101 and 104 are simultaneously turned on, the current direction is as shown by the solid arrow 107 in the figure. When the H-bridge drive reaches the required current, it will turn off MOSFETs 101 and 104. Since the motor load is a winding coil, it exhibits inductive characteristics. After the drive is turned off, the coil current will continue to flow in its original direction (freewheeling characteristic).

[0004] A common freewheeling method involves turning off the upper MOSFETs 101 and 103 and turning on the lower MOSFETs 102 and 104. The current direction in this case is shown by the dashed arrow 106. Due to its operating characteristics, this freewheeling method produces a relatively small current, resulting in a slower voltage drop across the load, a phenomenon known as slow decay.

[0005] Slow decay usually results in a smoother current waveform and a more stable motor operation, but due to the influence of back electromotive force, it is prone to distortion when the frequency of the current waveform increases.

[0006] In microstepping systems, to ensure that the current flowing through the motor coils is a sinusoidal wave, precise current control via an H-bridge is required. This control is typically implemented using either open-loop or closed-loop methods.

[0007] Open-loop control simply defines the current at each point in the waveform by the voltage duty cycle. Its advantages include simple algorithm and less susceptibility to system measurement errors; however, its disadvantage is that as the motor speed increases, the back electromotive force inside the coil can distort the coil current waveform. Closed-loop control is more complex. It obtains a more accurate current waveform by measuring the current flowing through the coil and controlling the voltage of the H-bridge to change the duty cycle. Its advantage is more precise current control, but its disadvantages include more complex current and digital algorithms, and it is prone to control difficulties due to problems with the measurement circuit. Summary of the Invention

[0008] To address the problem of coil current waveform distortion caused by increased motor speed in existing open-loop control methods, this invention provides a high-precision stepper motor microstepping drive system that ensures undistorted coil current waveforms.

[0009] The technical solution is as follows: A high-precision stepper motor microstepping drive system includes an H-bridge circuit. The lower two MOS transistors of the H-bridge circuit are respectively connected to a tail current sensing resistor and the input terminal of a comparator module. The input terminal of the comparator module is also connected to the digital waveform output terminal of a digital controller, and the output terminal of the comparator module is connected to the input terminal of the digital controller. The comparator module receives the digital waveform signal output by the digital controller and compares it with the tail current signal of the H-bridge circuit to obtain a comparator signal, which is then sent to the digital controller. The digital controller processes the comparator signal to obtain a back EMF correction signal, and outputs an H-bridge control signal with the back EMF correction signal to the H-bridge circuit.

[0010] A further feature is that the comparator module includes a dual-channel analog comparator and two digital-to-analog converters, and the digital waveform output terminal of the digital controller is connected to the dual-channel analog comparator through the digital-to-analog converters;

[0011] The digital controller includes a back EMF measurement module and a sinusoidal PWM waveform correction module. The back EMF measurement module times the comparator signal to obtain the current drive time tdrive, the current decay time tdecay, and the complete step cycle. Its waveform consists of a series of steady-state current steps, each step's period being a micro-step subdivision period. A single micro-step subdivision period can be further subdivided into several normal operating cycles consisting of current charging and slow decay processes. Several back EMF detection cycles are inserted into these cycles, including charging and decay durations. Timing begins when the charging duration starts, and the decay cycle begins when the current reaches a preset target current value. The decay cycle ends when the current decays to a preset target current decay value, obtaining information on its continuous change. The sinusoidal PWM waveform correction module corrects the coil current target value and the detected back EMF value to obtain a control waveform. The control waveform compensates for the influence of the back EMF on the coil current, thus ultimately obtaining a coil current waveform close to the control target.

[0012] By using this invention, the digital waveform signal output by the digital controller and the tail current signal of the H-bridge circuit are compared using a digital controller. After processing by the digital controller, a back electromotive force correction signal is obtained, and finally, an undistorted coil current waveform is obtained. Attached Figure Description

[0013] Figure 1 Here is the schematic diagram of an existing H-bridge circuit;

[0014] Figure 2 This is a schematic diagram of the invention;

[0015] Figure 3 This is the schematic diagram of the comparator module;

[0016] Figure 4 Schematic diagrams of standard sinusoidal current, back electromotive force waveforms, and actual distorted waveforms;

[0017] Figure 5 This is the schematic diagram of the timing module;

[0018] Figure 6 This is a schematic diagram of the back electromotive force detection process;

[0019] Figure 7 This is a schematic diagram of waveform correction. Detailed Implementation

[0020] See Figure 2As shown, a high-precision stepper motor microstepping drive system includes an H-bridge circuit. The two lower MOSFETs of the H-bridge circuit are respectively connected to the tail current sensing resistor and the input terminal of the comparator module. The input terminal of the comparator module is also connected to the digital waveform output terminal of the digital controller, and the output terminal of the comparator module is connected to the input terminal of the digital controller. The comparator module receives the digital waveform signal output by the digital controller and compares it with the tail current signal of the H-bridge circuit to obtain a comparator signal, which is then sent to the digital controller. The digital controller processes the comparator signal to obtain a back EMF correction signal, and outputs an H-bridge control signal with the back EMF correction signal to the H-bridge circuit.

[0021] See Figure 3 As shown, 301 and 302 are digital-to-analog converters (DACs), and 303 is a dual-channel comparator. The digital waveforms from 301 and 302 originate from the digital controller 200. After digital-to-analog conversion by DACs 301 and 302, they are compared with the tail current of the H-bridge via the dual-channel comparator 303, and the comparison result is sent to the digital controller 200. The reason for using DACs and analog comparators, instead of analog-to-digital converters (ADCs) for digital comparison, is that among converters of the same precision, DACs are generally faster than ADCs. In this invention, timely comparison results are crucial for the final control accuracy.

[0022] See Figure 4 As shown, the digital control system 200 converts the sinusoidal drive voltage (waveform 401) into a PWM waveform and drives the MOSFETs 204 to 207 on the H-bridge. The purpose of this drive is to ensure that the phase current flowing through the load on the H-bridge exhibits sinusoidal characteristics, i.e., a sinusoidal current corresponding to the drive voltage 401. However, as the motor 208 begins to rotate, its stator coils cut magnetic lines of force, leading to the generation of back electromotive force, such as: Figure 2 The back electromotive force equivalent power source shown in Figure 209 has the following waveform: Figure 4 The dashed waveform 402 is shown in the figure. It should be noted that the direction and amplitude of this back electromotive force 209 are constantly changing as the magnetic lines of force inside the motor rotate (waveform 402); because of the existence of the back electromotive force 209 (waveform 402), the voltage across the load on the H-bridge is not actually a sinusoidal voltage as shown by waveform 401, and therefore the current 403 flowing through the stepper motor coil does not strictly correspond to the sinusoidal current of waveform 401, but is a distorted waveform affected by the back electromotive force; the precise current control strategy in this invention is to sense the back electromotive force in the motor coil in real time ( Figure 2 (209), and then compensated in the H-bridge drive signal, thereby obtaining a more accurate sinusoidal current under the condition of the back electromotive force of the rotating motor. Its specific implementation is... Figure 2 The 200 digital control section receives... Figure 3The current comparison signal output by the 303 comparator.

[0023] like Figure 5 As shown, 501 times the comparator signal to obtain the current drive time tdrive and the current decay time tdecay. It's important to note that the H-bridge decays slowly, resulting in a relatively stable decay rate. Since directly measuring the back EMF is difficult, indirectly measuring it using the current drive time and current decay time becomes a more ideal choice. Referring to the next paragraph, due to the presence of the back EMF, under the same external conditions, the current decay time exhibits different patterns: as the back EMF increases / decreases, the decay time also increases or decreases, with the polarity of the increase or decrease related to the direction of the back EMF and the current decay. 502 calculates the back EMF detected by the system and the current current waveform to produce a current signal with a back EMF correction signal, thus obtaining a current waveform unaffected by the current back EMF.

[0024] The back electromotive force detection process of this invention is as follows: Figure 6 As shown, 600 represents a complete step cycle. Its waveform consists of a series of steady-state current steps, and the period 601 of these steps is the microstep subdivision cycle, while the number of steps is the microstep subdivision number. Taking a single microstep subdivision cycle 602 as an example, it can be subdivided into a normal operating cycle consisting of several current charging processes 610 and slow decay processes 611. In this invention, the digital control system inserts several back EMF detection cycles into these cycles. This detection cycle consists of 612 and 613. Unlike the normal operating cycle mentioned above, the charging duration 612 and decay duration 613 of this detection cycle are not predefined by the system, but are measured: timing begins at the start of 612, the decay cycle begins when the current reaches the preset current target value 603, and the decay cycle ends when the current decays to the preset current decay target value 604. Without the influence of back EMF, the... Figure 1 As can be seen, the current decay rate is basically the same, but due to the presence of back electromotive force (EMF), and because of the difference between back EMF 620 and 621, the current decay rate in 613 and 611 will change with the magnitude and direction of the back EMF. Since the detection system described in this invention detects the back EMF in each microstep subdivision cycle, it can obtain information on its continuous change. Figure 4 The waveform of 402.

[0025] After obtaining accurate information about the back electromotive force (EMF), the microstepping control system described in this invention will correct the predefined sinusoidal PWM waveform according to the strength of the current back EMF to counteract its influence; this function is performed in 502. Figure 7As shown, 701 is the target value of the coil current controlled by the system (standard sine waveform), while 702 is the detected back electromotive force (EMF) value. If the control waveform is not corrected, the final coil current value will be a distorted current value. However, by correcting 701 according to 702, the control waveform 710 is obtained. This waveform effectively compensates for the influence of the back EMF 702 on the coil current, thus ultimately obtaining a coil current waveform that is close to the control target 701.

Claims

1. A high-precision stepper motor microstepping drive system, comprising an H-bridge circuit, characterized in that, The two lower MOSFETs of the H-bridge circuit are connected to the tail current sensing resistor and the input of the comparator module, respectively. The input of the comparator module is also connected to the digital waveform output of the digital controller, and the output of the comparator module is connected to the input of the digital controller. The comparator module receives the digital waveform signal output by the digital controller and compares it with the tail current signal of the H-bridge circuit to obtain a comparator signal, which is then sent to the digital controller. The digital controller processes the comparator signal to obtain a back EMF correction signal, and outputs an H-bridge control signal with the back EMF correction signal to the H-bridge circuit. The comparator module includes a dual-channel analog comparator and two digital-to-analog converters. The digital waveform output of the digital controller is connected to the dual-channel analog comparator through the digital-to-analog converters. The digital controller includes a back EMF measurement module and a sinusoidal PWM waveform correction module. The back EMF measurement module... The comparator signal is used for timing to obtain the current drive time tdrive, the current decay time tdecay, and the complete step cycle. The waveform is composed of a series of steady-state current steps, and the period of each step is a micro-step subdivision period. A single micro-step subdivision period can be further subdivided into several normal operating cycles consisting of current charging processes and slow decay processes. Several back EMF detection cycles are inserted into these cycles. The detection cycle includes the charging duration and the decay duration. Timing starts when the charging duration begins, and the decay cycle begins when the current reaches the preset current target value. The decay cycle ends when the current decays to the preset current decay target value, thus obtaining information on its continuous change. The sinusoidal PWM waveform correction module corrects the coil current target value controlled by the system and the detected back EMF value to obtain the control waveform. The control waveform compensates for the influence of back EMF on the coil current, thereby finally obtaining a coil current waveform close to the control target.

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