Research on Key Technologies of stepper motor driver

Research on Key Technologies of stepper motor driver

[Date:]

Source: single-chip microcomputer and Embedded System Application Author: Guangdong University of Technology Dong Xiaoqing Huang Jie Xian Zhang Shunyang

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Introduction

Stepper motor is an execution mechanism that converts an electrical pulse signal into an angle displacement. Its main advantage is its high positioning accuracy and no location accumulation error. Its Special Open-loop operation mechanism reduces the system cost and improves the reliability compared with the closed-loop control system, it has been widely used in the field of numerical control. However, the stepping motor has high vibration and noise during low-speed operation. It is prone to resonance when the natural oscillation frequency of the stepping motor is near the operating frequency, and the output torque decreases as the speed of the Stepping Motor increases, these shortcomings limit the application scope of stepper motors. The performance of the stepper motor depends largely on the driver used to improve the performance of the drive, which can significantly improve the performance of the stepper motor, therefore, the development of high-performance stepper motor drives is a common topic.

1 Overview of stepper motor drive control system
Generally, the stepper motor drive system consists of three parts:
① Control circuit. It is used to generate pulses and control the speed and steering of the motor.
② Drive circuit. The research in this paper is composed of the Pulse Signal Distribution and power driving circuit shown in Figure 1. Based on the pulse and direction signals input by the Controller, the correct power-on sequence of each winding of the stepper motor and the high voltage and high current required by the motor are provided. Various protection measures, such as over-current and over-temperature, are also provided.
③ Stepper motor. After the control signal is amplified by the drive, the stepper motor is driven to drive the load.

2 Comparison of stepper motor driving methods
2.1 constant voltage driving mode
2.1.1 single voltage drive
A single-Voltage Drive means that only one direction of voltage is used to power the winding during the motor winding operation. As shown in 2, L is the motor winding, and VCC is the power supply. When the input signal In is high, the base current is large enough to make the transistor T saturated. If the saturation voltage drop is ignored, the power supply voltage applies to the motor winding. When In is low, the transistor ends and the winding does not pass through the current.

In order to quickly reach the preset current when the winding current is powered on, the Rc is stringed into the resistor. To prevent the winding current change rate from too large when the T is turned off, a large back potential is generated to penetrate T, A Diode D and resistance Rd are connected in parallel at both ends of the winding to provide a discharge loop for the winding current, also known as a "continued flow loop ".
The advantages of a single-voltage power driving circuit are simple circuit structure, few components, low cost, and high reliability. However, because the power consumption increases after the string-in resistance, the efficiency of the entire power driving circuit is low, and it is only suitable for driving small-power stepper motors.
2.1.2 High and Low Voltage Drive
In order to enable the winding to quickly reach the set current during power-on, the winding current quickly degrades to zero during power-off, and at the same time has a high efficiency, there is a high and low voltage drive mode.
As shown in figure 3, Th and T1 are high-voltage and low-pressure tubes respectively. The supplies are high-voltage and V1 respectively, and the Ih and I1 are high-voltage and low-end pulse signals respectively. High voltage power supply is used at the conduction front to increase the current rising rate at the front end, while low voltage is used to maintain the current of the winding after the front end. High-voltage and low-voltage drive can obtain good high-frequency characteristics. However, due to the constant conduction time of the high-voltage tube, the winding gets too much energy at low frequency, which may lead to oscillation. The low-frequency oscillation problem can be solved by changing the conduction time of the high-voltage tube. However, the control circuit is more complex than that of a single voltage and the reliability is reduced. Once the high-voltage tube is out of control, the motor will be damaged due to the large current.
2.2 Constant Current Chopper drive mode
2.2.1 self-excited Constant Current chopper driver
Figure 4 shows the self-activated Constant Current chopper driver. Converts the winding current value of the stepping motor to A certain percentage of voltage, and compares it with the preset value output by the D/A converter to control the switch of the power tube, so as to control the phase current of the winding. Theoretically, the self-excited Constant Current Chopper drive can control the current of the motor winding to a constant value. However, because the chopper frequency is variable, the winding will trigger a high surge voltage, which will cause great interference to the control circuit, which is prone to oscillation and greatly reduce the reliability.

2.2.2 Constant Current chopper driver
A fixed frequency clock can be added to the D trigger to solve the surge voltage problem caused by a variable self-excited chopper frequency. This basically solves the oscillation problem, but there are still some problems. For example, when the on-going pulse output by the comparator is between two rising edges of the D trigger, the control signal will be lost. Generally, the clock frequency of the D trigger can be increased.
2.3 subdivided driving methods
This is the focus of this article and the driving method used by the system. The main advantage of subdivided driving is that the step angle is smaller, the resolution is improved, and the positioning accuracy, startup performance and high-frequency output torque of the motor are improved. Secondly, the low-frequency vibration of the stepping motor is weakened or eliminated, this reduces the chance of stepping motor working in the resonance zone. It can be said that the subdivided driving technology is a leap in the driving and Control Technology of stepping motor.
In each pulse switching, a subdivision drive not only transfers or removes all the current of the winding, but only changes part of the current in the corresponding winding, the electric motor's synthetic magnetic potential only rotates part of the step angle. When subdivided driving, the winding current is not a square wave but a step wave, and the rated current is a step type input or removal. For example, if the current is divided into n steps, it takes n times for the rotor to turn a step angle, that is, n subdivision, as shown in Figure 5.

The general subdivision method only changes the current of one phase, and the current of the other phase remains unchanged. 5, at O ° ~ 45 °, Ia remains unchanged, Ib increases step by step from O; 45 ° ~ 90 °, Ib remains unchanged, Ia from the rating level to 0. The advantage of this method is that the control is relatively simple and easy to implement on the hardware. However, according to the current vector synthesis diagram shown in figure 6, the amplitude of the synthesized vector is constantly changing, the output torque also changes constantly, causing constant changes in the lag angle. When the sub-score is large and the sub-step gap is very small, the difference between the lag angle changes is greater than the sub-step gap, which makes the Sub-Division actually meaningless.
This is the defect of common subdivision methods. Is there a way to keep the amplitude unchanged when the vector angle changes? The above analysis shows that it is impossible to change only the single phase current. What about changing the two phase current at the same time? That is, Ia and Ib change at the same time based on a certain mathematical relationship to ensure that the amplitude of the synthesized vector remains unchanged during the change process. Based on this, this paper establishes a Driving Method of "adjustable rated current equi-angle constant torque subdivision" to eliminate the lag angle caused by constant force distance changes. As shown in figure 7, the amplitude of A and B is always the radius of the circle with the variation of the synthesis vector angle of Ia and Ib.

The following describes the mathematical model for the synthesis of vector amplitude unchanged: When Ia = Im · cosx, Ib = Im · sinx (in formula Im is the current rating, Ia, Ib is the actual phase current, x is determined by the fine score), and its synthetic vector is always the radius of the circle, that is, the constant force distance.
The other angle means that the angle of each rotation of the synthetic arm is the same. Adjustable rated current refers to meeting the requirements of various series motors. For example, the rated current of the 86 series motor is 6 ~ 8 A, and the 57 series motor generally does not exceed 6 A, the drive has A variety of Gear current available for choice. Subdivided into rated current segments.
In order to realize the "adjustable rated current equi-angle constant force distance", theoretically only the phase current can meet the above mathematical model. This requires that the current control accuracy is very high. Otherwise, the vector angles synthesized by Ia and Ib will be deviated, that is, the step-by-step distances and subdivisions will also lose their meaning. The following describes the Driver Design Scheme Based on the driver method.

3 Overall Design Scheme of two-phase stepper motor driver
3.1 system design diagram
As shown in figure 8, the signal of the control panel is isolated by optical coupling and connected to the MCU Interrupt Port.

The single-chip microcomputer allocates the Pulse Signal Based on the received pulse signal, determines the electrical sequence of each connection, and connects it with the D trigger in the CPLD; at the same time, according to the user's set current value and fine score through the SPI port and D/A converter AD5623 communication, get the set current value (actually the current corresponding voltage value ).
The output value of AD5623 is the voltage value corresponding to the expected current. It must be compared with the voltage value corresponding to the current detected by the power module, the comparison result is connected to the D Trigger CLR pin in the CPLD.
The CPLD is connected to the dial switch for current and subdivision settings, and the obtained value is passed to the microcontroller through the SPI port. The control logic with the D trigger as the core, determine the switch of each power tube based on the comparison results of the single-chip microcomputer's interconnected electrical sequence and the comparison result of the comparator MAX907.
The power drive module is directly connected to the motor to drive the motor. Two H-bridge bipolar driving circuits are constructed using eight MOS IRF740 tubes. The IRF740 can withstand A maximum of 400 V voltage and 10 A current. The switch conversion time does not exceed 51 ns, and the value range of the tube's on-voltage Vgs is 4 ~ 20 V.
3.2 key technical solution segments
The Driving Method of "adjustable rated current and constant torque subdivision" is constant current control. The key is precise current control, which must meet the following conditions:

① The current value output by the D/A converter must be close to the expected value and the conversion speed must be fast. The system adopts AD5623 and 12-bit precision of ADI, which is divided into 4 096 levels to meet the high precision requirements of 200 segments. 2 D/A outputs meet the requirements of two phases; the SPI port can communicate with each other at a frequency of up to 50 MHz, providing fast setup time and single-voltage power supply. The connection is simple.
② The detected current must correctly reflect the current at this time. Because the phase current of the motor is usually very large and the voltage is very high, the detection is difficult. Common detection methods include external standard small resistance, simple circuit, but relatively large interference, poor accuracy; Hall sensor detection accuracy, small interference, the connection is not complex, therefore, the drive uses Hall sensors.
③ The comparator must have a high resolution and fast conversion speed. The establishment time of MAX907 is only 12 ns, and the comparison of voltage can be detected as long as the difference is 2 mV (the maximum is no more than 4 mV). The reaction is very sensitive.
④ The logic circuit used to control the power tube switch must have a high real-time performance, so as to ensure that the phase current fluctuates very little in the set current, so as not to cause a surge or interfere with the control circuit.
This article uses Xilinx's CPLD chip XC9572. The control circuit with D trigger as the core is all completed by CPLD, which replaces various discrete components with simple structure and convenient connection. Figure 9 shows the logic of the control circuit.

9. When the comparison result is low (the detected current is greater than the set current), the output of the D trigger is 1, or the output of the door is high, the pipe is closed, and the current is reduced; when the current is detected to be less than the set current, the pipe is turned on to ensure that the phase current fluctuates very little in the set current.

Knot
In this paper, the driving method of "adjustable rated current and constant torque subdivision" is established, and the two-phase Hybrid Stepping Motor Driver is designed and implemented based on this method. The maximum subdivision is 200, the driving current is adjustable from O.5 A/phase to 8 A/phase. It can drive 24 series to 86 series stepper motors. The practical application proves that this method basically overcomes the disadvantages of low-speed vibration and high noise of the traditional stepping motor. the torque of the motor remains constant within a large speed range, which improves the control precision, this reduces the probability of resonance, provides good stability, reliability, versatility, and simple structure.