Introduction A stepper motor is an actuator that converts electrical pulse signals into angular displacement. Its main advantages are high positioning accuracy and no cumulative position error; its unique open-loop operation mechanism reduces system cost and improves reliability compared to closed-loop control systems, leading to its widespread application in CNC (Computer Numerical Control) systems. However, stepper motors exhibit significant vibration and noise at low speeds, are prone to resonance near their natural oscillation frequency, and their output torque decreases as the motor speed increases. These drawbacks limit the application range of stepper motors. The performance of a stepper motor largely depends on the driver used; improving the driver's performance can significantly enhance the stepper motor's performance. Therefore, developing high-performance stepper motor drivers is a widely discussed topic. 1. Overview of Stepper Motor Drive Control System Generally, a stepper motor drive system consists of three parts: ① Control circuit. Used to generate pulses and control the motor's speed and direction. ② Drive circuit. This is the focus of this paper, consisting of the pulse signal distribution and power drive circuit shown in Figure 1. Based on the pulse and direction signals input by the controller, the correct energizing sequence is provided for each winding of the stepper motor, as well as the high voltage and high current required by the motor; at the same time, various protection measures are provided, such as overcurrent and overheating. ③ Stepper motor. The control signal is amplified by the driver to drive the stepper motor and drive the load. 2 Comparison of stepper motor driving methods 2.1 Constant voltage driving method 2.1.1 Single voltage driving Single voltage driving refers to using only one directional voltage to power the winding 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 high, a sufficiently large base current is provided to keep the transistor T in saturation. If its saturation voltage drop is ignored, the entire power supply voltage is applied to the motor winding. When In is low, the transistor is cut off, and no current flows through the winding. To ensure the winding current quickly reaches the preset current when energized, a resistor Rc is connected in series. To prevent the winding current from changing too rapidly when T is turned off, which could generate a large back EMF and break down T, a diode D and a resistor Rd are connected in parallel across the winding to provide a discharge path for the winding current, also known as a "freewheeling circuit". The advantages of a single-voltage power drive circuit are its simple circuit structure, fewer components, low cost, and high reliability. However, due to the increased power consumption after adding the resistor, the overall efficiency of the power drive circuit is relatively low, making it only suitable for driving small-power stepper motors. 2.1.2 High and Low Voltage Drive To enable the winding to quickly reach the set current when energized and to rapidly decay to zero when turned off, while maintaining high efficiency, a high and low voltage drive method has emerged. As shown in Figure 3, Th and T1 are the high-voltage and low-voltage transistors, respectively, Vh and V1 are the high and low voltage power supplies, respectively, and Ih and I1 are the high and low voltage pulse signals, respectively. A high voltage is used at the leading edge of conduction to increase the rate of rise of the current leading edge, while a low voltage is used after the leading edge to maintain the winding current. High- and low-voltage drives can achieve better high-frequency characteristics, but because the conduction time of the high-voltage transistor remains constant, the windings receive excessive energy at low frequencies, easily causing oscillations. The low-frequency oscillation problem can be solved by changing the conduction time of the high-voltage transistor; however, its control circuit is more complex than that of a single-voltage drive, reducing reliability. If the high-voltage transistor malfunctions, the motor will be damaged due to excessive current. 2.2 Constant Current Chopper Drive Method 2.2.1 Self-Excited Constant Current Chopper Drive Figure 4 shows the block diagram of a self-excited constant current chopper drive. The stepper motor winding current value is converted into a voltage of a certain proportion, compared with the preset value output by the D/A converter, and the switching of the power transistor is controlled to achieve the purpose of controlling the winding phase current. Theoretically, a self-excited constant current chopper drive can control the motor winding current at a constant value. However, because the chopping frequency is variable, it can induce very high surge voltages in the windings, thus causing significant interference to the control circuit, easily leading to oscillations, and greatly reducing reliability. 2.2.2 Self-Excited Constant Current Chopper Drive To address the surge voltage problem caused by the variable frequency of the self-excited chopper, a fixed-frequency clock can be added to the D flip-flop. This largely solves the oscillation problem, but some issues remain. For example, when the comparator output's turn-on pulse falls between the two rising edges of the D flip-flop's clock, the control signal will be lost. This can generally be resolved by increasing the D flip-flop's clock frequency. 2.3 Microstepping Drive This is the focus of this paper and the driving method used in this system. The main advantages of microstepping drive are a smaller step angle, improved resolution, and enhanced motor positioning accuracy, starting performance, and high-frequency output torque. Secondly, it reduces or eliminates low-frequency vibrations in the stepper motor, lowering the probability of the stepper motor operating in the resonance region. Microstepping drive technology can be considered a leap forward in stepper motor drive and control technology. Microstepping drive refers to changing only a portion of the current in the corresponding winding during each pulse switch, rather than fully switching the current in the winding. The motor's composite magnetomotive force also rotates only a portion of the step angle. In microstepping, the winding current is not a square wave but a stepped wave, and the rated current is switched on or off in a stepped manner. For example, if the current is divided into n steps, the rotor needs to rotate n times to complete one step angle, i.e., n microstepping, as shown in Figure 5. A typical microstepping method only changes the current in one phase while keeping the current in the other phase constant. As shown in Figure 5, in the range of 0° to 45°, Ia remains constant, while Ib gradually increases from 0; in the range of 45° to 90°, Ib remains constant, while Ia gradually decreases from its rated value to 0. The advantage of this method is its relatively simple control and ease of hardware implementation; however, as shown in the current vector synthesis diagram in Figure 6, the amplitude of the synthesized vector is constantly changing, and the output torque also changes continuously, causing the lag angle to change continuously. When the microstepping number is very large and the microstep angle is very small, the difference in lag angle changes exceeds the required microstep angle for microstepping, rendering the microstepping practically meaningless. This is the shortcoming of commonly used subdivision methods. Is there a way to keep the amplitude constant while the vector angle changes? As analyzed above, it's impossible to change only a single-phase current. What about changing two-phase currents simultaneously? That is, Ia and Ib change simultaneously with a certain mathematical relationship, ensuring that the magnitude of the composite vector remains constant throughout the change. Based on this, this paper establishes a "rated current adjustable constant torque subdivision with equal angle" drive method to eliminate the lag angle problem caused by the continuous change of torque. As shown in Figure 7, as the angle of the composite vector of the phase currents Ia and Ib of phases A and B changes continuously, its amplitude remains the radius of the circle. The mathematical model for keeping the composite vector amplitude constant is introduced below: When Ia = Im·cosx and Ib = Im·sinx (where Im is the rated current, Ia and Ib are the actual phase currents, and x is determined by the subdivision number), the composite vector is always the radius of the circle, i.e., constant torque. "Equal angle" means that the angle of the composite lever arm is the same each time it rotates. "Rated current adjustable" means that it can meet the requirements of various series of motors. For example, the rated current of the 86 series motor is 6-8 A, while the 57 series motor generally does not exceed 6 A. The driver has various current settings to choose from. Subdivision is a subdivision of the rated current. To achieve "adjustable rated current with constant torque at equal angles", theoretically, it is sufficient as long as the phase current of each phase can meet the above mathematical model. This requires very high current control accuracy; otherwise, the vector angle synthesized by Ia and Ib will deviate, that is, the step angles of each step will be unequal, and the subdivision will lose its meaning. The design scheme of the driver based on this driving method is given below. 3 Overall Design Scheme of 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 port through optocoupler isolation. The microcontroller performs pulse signal distribution according to the received pulse signal, determines the energizing sequence of each phase, and connects to the D flip-flop in the CPLD; at the same time, according to the current value and subdivision number set by the user, it communicates with the D/A converter AD5623 through the SPI port to obtain the set current value (actually the voltage value corresponding to the current). The AD5623 outputs the voltage value corresponding to the desired current. This value must be compared with the voltage value corresponding to the current detected by the power module, and the comparison result is connected to the CLR pin of the D flip-flop in the CPLD. The CPLD is connected to the DIP switches for current and microstepping settings, transmitting the obtained values ​​to the microcontroller via the SPI interface. The control logic, based on the D flip-flops, determines the switching of each power transistor according to the power-on sequence of each phase in the microcontroller and the comparison result of the MAX907 comparator. The power drive module is directly connected to the motor to drive it. Eight IRF740 MOSFETs are used to construct two H-bridge bipolar drive circuits. The IRF740 can withstand a maximum voltage of 400 V and a current of 10 A, with a switching time not exceeding 51 ns, and a turn-on voltage (Vgs) ranging from 4 to 20 V. 3.2 Key Technical Solutions for Subdivision The essence of the "Rated Current Adjustable Constant Torque Subdivision" drive method is constant current control, and the key is precise current control, which must simultaneously meet the following conditions: ① The current value output by the D/A converter must be very close to the expected value, and the conversion speed must be fast. This system uses the AD5623 from ADI, with 12-bit precision, divided into 4096 levels, meeting the high precision requirement of 200 subdivisions; two D/A outputs meet the requirements of two phases; SPI communication, with a frequency of up to 50 MHz, fast setup time, and single-voltage power supply, is simple to connect. ② The detected current must accurately reflect the phase current at this time. Since the phase current of the motor is usually very large and the voltage is very high, detection is somewhat difficult. Commonly used detection methods include external standard small resistors, which have simple circuits but relatively large interference and poor accuracy; Hall sensors have accurate detection, low interference, and simple connection, so this driver uses Hall sensors. ③ The comparator must have high resolution and fast conversion speed. The MAX907 has a setup time of only 12 ns and can detect voltage differences of only 2 mV (maximum not exceeding 4 mV), making it extremely sensitive. ④ The logic circuit controlling the power transistor switch must have high real-time performance to ensure that the phase current fluctuates only slightly above and below the set current, preventing surges and interference with the control circuit. This paper uses the Xilinx CPLD chip XC9572. The entire control circuit, based on the D flip-flop, is implemented by the CPLD, which replaces various discrete components, resulting in a simple structure and convenient connection. Figure 9 shows the logic diagram of the control circuit. As shown in Figure 9, when the comparison result is low (the detected current is greater than the set current), the D flip-flop outputs 1, the OR gate outputs high, turning off the transistor and reducing the current; when the detected current is less than the set current, the transistor turns on, thus ensuring that the phase current fluctuates only slightly above and below the set current. Conclusion This paper establishes a "rated current adjustable constant torque microstepping" driving method and designs and implements a two-phase hybrid stepper motor driver based on this method, achieving a maximum microstepping of 200, with a driving current from 0... Adjustable from 5 A/phase to 8 A/phase, it can drive stepper motors from the 24 series to the 86 series. Practical applications have proven that this method basically overcomes the shortcomings of traditional stepper motors, such as large vibration and noise at low speeds. The motor torque remains constant over a wide speed range, improving control accuracy, reducing the probability of resonance, and exhibiting excellent stability, reliability, and versatility, while also possessing a simple structure.