Key Technology Analysis of Stepper Motor Driver

A stepper motor is an actuator that converts an electrical pulse signal into an angular displacement. The main advantages are high positioning accuracy and no position accumulation error. The unique open-loop operation mechanism reduces system cost and improves reliability compared with the closed-loop control system, and has been widely used in the field of numerical control. However, the stepping motor has large vibration and noise at low speed operation, and it is easy to generate resonance when running near the natural oscillation frequency of the stepping motor, and the output torque decreases as the rotation speed of the stepping motor increases. These disadvantages limit The application range of stepper motors. The performance of the stepper motor depends to a large extent on the driver used, improves the performance of the driver, and can significantly improve the performance of the stepper motor. Therefore, the development of high-performance stepper motor driver is a common concern.

1 Stepper motor drive control system overview

Normally, the stepper motor drive system consists of three parts:

1 control circuit. Used to generate pulses that control the speed and steering of the motor.

2 drive circuit. That is, the research content of this paper consists of the pulse signal distribution and power drive circuit shown in Figure 1. According to the pulse and direction signals input by the controller, it provides the correct energization sequence for each winding of the stepping motor, and the high voltage and large current required by the motor. At the same time, various protection measures such as overcurrent and overheating are provided.

3 stepper motor. The control signal is amplified by the driver to drive the stepper motor to drive the load.

Research on Key Technology of Stepper Motor Driver

2 Comparison of stepper motor drive methods

2.1 Constant voltage drive mode

2.1.1 Single voltage drive

Single voltage drive means that the winding is powered by only one direction voltage during the operation of the motor winding. As shown in Figure 2, L is the motor winding and VCC is the power supply. When the input signal In is at a high level, a sufficiently large base current is supplied to make the transistor T in a saturated state. If the saturation voltage drop is ignored, the power supply voltage is all applied to the motor winding. When In is low, the transistor is turned off and no current flows through the winding.

Research on Key Technology of Stepper Motor Driver

In order to make the winding current reach the preset current quickly when energized, the resistor Rc is serially connected; in order to prevent the winding current change rate from being too large when the T is turned off, a large back electromotive force is generated to break T, and a diode is connected in parallel at both ends of the winding. D and resistor Rd provide a bleeder loop for the winding current, also known as a "freewheeling loop."

The advantage of the single voltage power driving circuit is that the circuit structure is simple, the components are small, the cost is low, and the reliability is high. However, since the power consumption is increased after the resistor is connected in series, the efficiency of the entire power driving circuit is low, and it is only suitable for driving a small power stepping motor.

2.1.2 High and low voltage drive

In order to make the winding reach the set current quickly when the power is turned on, the winding current is rapidly attenuated to zero when the power is turned off, and at the same time, the efficiency is high, and the high and low voltage driving modes appear.

As shown in Fig. 3, Th and T1 are respectively a high voltage tube and a low voltage tube, and Vh and V1 are respectively high and low voltage power sources, and Ih and I1 are high and low end pulse signals, respectively. A high voltage supply is used at the leading edge to increase the leading edge rise rate of the current, and a low voltage is used to maintain the winding current after the leading edge. High-low voltage driving can obtain better high-frequency characteristics, but because the conduction time of the high-pressure tube is constant, at low frequencies, the winding obtains too much energy and easily causes oscillation. The low-frequency oscillation problem can be solved by changing the on-time of the high-voltage tube. However, the control circuit is more complicated than the single voltage, and the reliability is lowered. Once the high-voltage tube is out of control, the motor will be damaged due to the current too much.

2.2 Constant current chopper driving method

2.2.1 Self-excited constant current chopper drive

Figure 4 is a block diagram of a self-excited constant current chopper drive. The stepper motor winding current value is converted into a certain proportion of voltage, compared with the preset value of the D/A converter output, and the power tube switch is controlled to achieve the purpose of controlling the winding phase current. In theory, the self-excited constant current chopper drive can control the current of the motor winding to a certain constant value. However, since the chopping frequency is variable, the winding will cause a high surge voltage, which causes great interference to the control circuit, is prone to oscillation, and the reliability is greatly reduced.

Research on Key Technology of Stepper Motor Driver

2.2.2 Its excited constant current chopper drive

In order to solve the surge voltage problem caused by the self-excited chopping frequency variable, a fixed frequency clock can be added to the D flip-flop. This basically solves the oscillation problem, but there are still some problems. For example, when the on-pulse of the comparator output is just between the rising edges of the two clocks of the D flip-flop, the control signal will be lost, which can generally be solved by increasing the clock frequency of the D flip-flop.

2.3 Subdivision driving method

This is the focus of this article and the driving method used by the system. The main advantage of the subdivision drive is that the step angle is smaller, the resolution is improved, and the positioning accuracy, starting performance and high-frequency output torque of the motor are improved. Secondly, the low-frequency vibration of the stepping motor is reduced or eliminated, and the frequency is reduced. The probability that the stepper motor will work in the resonance zone. It can be said that subdivision drive technology is a leap in stepper motor drive and control technology.

The subdivision drive means that each pulse switching does not turn all the currents of the windings into or out, but only a part of the current in the corresponding windings, and the resultant magnetic potential of the motor only rotates a part of the step angle. When subdivided, the winding current is not a square wave but a step wave, and the rated current is stepped input or cut. For example, the current is divided into n steps, and the rotor needs n times to turn through a step angle, that is, n subdivision, as shown in FIG.

Research on Key Technology of Stepper Motor Driver

The general subdivision method only changes the current of one phase, and the other phase remains unchanged. As shown in Fig. 5, at 0° to 45°, Ia remains unchanged, Ib is gradually increased from O stepwise; at 45° to 90°, Ib remains unchanged, and Ia is gradually changed from 0 to 0. The advantage of this method is that the control is relatively simple and easy to implement in hardware; but from the current vector synthesis diagram shown in Fig. 6, the synthesized vector amplitude is constantly changing, and the output torque is also constantly changing, thereby causing the lag angle. Constantly changing. When the number of subdivisions is large and the microstep angle is very small, the difference in the change in the lag angle is greater than the microstep angle of the required subdivision, so that the subdivision actually loses its meaning.

This is the defect of the currently used subdivision method. Is there a way to change the vector angle while keeping the amplitude constant? From the above analysis, it is impossible to change only the single phase current, then change the two phases at the same time. The current? That is, Ia and Ib change simultaneously in a certain mathematical relationship, and the amplitude of the composite vector is always constant during the change process. Based on this, this paper establishes a driving method of "equal angle constant torque subdivision" with adjustable rated current to eliminate the problem of lag angle caused by constant change of force distance. As shown in Fig. 7, as the combined vector angles of the A and B phase currents Ia, Ib constantly change, the amplitude is always the radius of the circle.

Research on Key Technology of Stepper Motor Driver

The following is a mathematical model for the composite vector amplitude to remain unchanged: when Ia=Im·cosx, Ib=Im·sinx (where Im is the current rating, Ia, Ib is the actual phase current, and x is determined by the number of subdivisions) ), its composite vector is always the radius of the circle, that is, the constant force distance.

The equal angle means that the resultant force arm rotates at the same angle every time. The rated current can be adjusted to meet the requirements of various series of motors. For example, the 86 series motors are rated at 6 to 8 A, while the 57 series motors are typically not more than 6 A. The drive has various gear currents to choose from. Subdivided into subdivisions of rated current.

In order to achieve "equal angle constant force distance with adjustable rated current", in theory, as long as the phase currents of each phase can satisfy 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 angles of the steps are not equal, and the subdivision also loses its meaning. The design of the driver based on the driving method is given below.

3 The overall design of the two-phase stepper motor driver

3.1 System Design Block Diagram

As shown in Figure 8, the control board signal is connected to the microcontroller interrupt via optocoupler isolation.

Research on Key Technology of Stepper Motor Driver

The MCU performs pulse signal distribution according to the received pulse signal, determines the power-on sequence of each phase, and connects with the D flip-flop in the CPLD. At the same time, according to the current value and subdivision number set by the user, the SPI port and the D/A converter AD5623 are used. Communication, the set current value (actually the voltage value corresponding to the current) is obtained.

The output value of the AD5623 is the voltage value corresponding to the desired current. It must be compared with the voltage value corresponding to the current detected by the power module, and the comparison result is connected to the D flip-flop CLR pin in the CPLD.

The CPLD is connected to the dip switch of the current and subdivision setting, and the obtained value is transmitted to the single-chip microcomputer through the SPI port; the control logic with the D flip-flop as the core determines each according to the power-on sequence of each phase of the single-chip microcomputer and the comparison result of the comparator MAX907 Power tube switch.

The power drive module is directly connected to the motor to drive the motor. Two H-bridge bipolar drive circuits are formed by using eight MOS transistors IRF740. The IRF740 can withstand up to 400 V and 10 A, the switching transition time does not exceed 51 ns, and the tube turn-on voltage Vgs ranges from 4 to 20 V.

3.2 Segmentation of key technical solutions

The essence of the "equal angle constant torque subdivision" driving method is constant current control. The key is accurate current control, which must meet the following conditions:

The current value output by the 1D/A converter must be fairly close to the expected value, and the conversion speed is fast. The system uses ADI's AD5623, 12-bit precision, divided into 4 096 levels, meeting the high-precision requirements of 200 subdivisions; 2 D/A outputs meet the requirements of two phases; SPI port communication, frequency up to 50 MHz, established The time is fast, while the single voltage is supplied, and the connection is simple.

2 The detected current must be able to correctly reflect the phase current at this time. Since the phase current of the motor is usually large and the voltage is high, the detection is difficult. Commonly used detection methods are external standard small resistance, the circuit is simple, but the interference is relatively large, the accuracy is relatively poor; Hall sensor detection is accurate, the interference is small, the connection is not complicated, so the driver uses Hall sensor.

3 comparator resolution is high, conversion speed is fast. The setup time of the MAX907 is only 12 ns, and the comparison voltage can be detected with a difference of 2 mV (maximum of 4 mV), and the response is very sensitive.


4 The logic circuit of the control power tube switch should have high real-time performance to ensure that the phase current makes small fluctuations above and below the set current, so as to avoid surge and interfere with the control circuit.

This article uses Xilinx CPLD chip XC9572. The control circuit with D flip-flop as the core is all completed by CPLD. CPLD replaces various discrete components, and the structure is simple and the connection is convenient. Figure 9 is a logic diagram of the control circuit.

Research on Key Technology of Stepper Motor Driver

As shown in Figure 9, when the comparison result is low (the detected current is greater than the set current), the D flip-flop output is 1, or the gate outputs a high level, the tube is turned off, the current becomes smaller; when the current is detected When the current is less than the set current, the tube is turned on to ensure that the phase current fluctuates slightly above and below the set current.

Conclusion

In this paper, the "equal angle constant torque subdivision" driving method is established, and based on this method, a two-phase hybrid stepping motor driver is designed and implemented up to 200 subdivisions, and the driving current is from O. 5 A/phase to 8 A/phase adjustable, can drive 24 series to 86 series stepper motors. The practical application proves that the method basically overcomes the shortcomings of the traditional stepping motor with low speed vibration and large noise. The torque of the motor is kept constant in a large speed range, which improves the control precision and reduces the probability of resonance. Stability, reliability and versatility, and a simple structure.

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