1. Introduction A stepper motor is an actuator in an open-loop servo motion system, controlled by pulses to output angular displacement. Compared to AC and DC servo motors, its outstanding advantages are low cost and no accumulated error. However, stepper motors have many shortcomings, such as low-frequency oscillation, high noise, and low resolution, which severely limit their application range. The operating performance of a stepper motor is closely related to its driver, and these shortcomings can be overcome by improving driver technology. Compared to other driving methods, microstepping not only reduces the step angle and improves resolution, but also reduces or eliminates low-frequency vibration, making the motor run more smoothly and evenly. Overall, microstepping offers the best control effect. Because commonly used low-end stepper motor servo systems lack encoder feedback, the internal control current decreases accordingly as the motor speed increases, resulting in missed steps. Therefore, it is widely used in fields where speed and accuracy requirements are not high. Because three-phase hybrid stepper motors offer better low-speed stability and output torque than two-phase stepper motors, they have a promising future in application. Traditional three-phase hybrid stepper motor control methods rely on hardware comparators. This article primarily discusses using DSP and the Space Vector Pulse Width Modulation (SVPWM) algorithm to implement three-phase hybrid stepper motor control. 2. Microstepping Principle Microstepping control of a stepper motor essentially involves controlling the current in the stator windings to change the resulting magnetic field within the stepper motor according to certain requirements, thereby achieving microstepping of the step angle. The optimal microstepping method is constant torque with equal step angles. Generally, the amplitude of the resulting magnetic field vector determines the magnitude of the motor's rotational torque, and the angle between two adjacent resulting magnetic field vectors determines the step angle. To generate a nearly uniform circular rotating magnetic field within the motor, the resulting magnetic field vector of each phase winding—that is, the resulting vector of the current in each phase winding—should rotate in space with a constant amplitude. This requires a sinusoidal current flowing through each phase winding. The working principle of a three-phase hybrid stepper motor is very similar to that of an AC permanent magnet synchronous servo motor. The permanent magnets used on its rotor are also rare-earth permanent magnet materials with high magnetic density, so the influence of the induced current generated on the rotor's magnetic field is negligible. Structurally, it is equivalent to a multi-pole AC permanent magnet synchronous motor. Since the input is a three-phase sinusoidal current, the generated spatial magnetic field is circularly distributed, and the torque characteristics of the three-phase hybrid stepper motor can be analyzed using the structural model of a permanent magnet synchronous motor (Figure 1). For ease of analysis, the following assumptions can be made: a. The three-phase windings of the motor stator are completely symmetrical; b. Magnetic saturation, eddy currents, and core losses are negligible; c. There is no dynamic response process for the excitation current. Figure 1: Simple structural model of a three-phase permanent magnet synchronous motor. U, V, and W are three coil windings on the stator, with their axes at 120°. When a single-phase winding of the motor is energized, the steady-state torque can be expressed as: T = f(i, theta). Where i is the current flowing through the winding; theta is the angle by which the motor rotor deviates from the reference point. Since the magnetic saturation effect can be ignored, and the rotor structure is circular, its torque-angle characteristic is strictly sinusoidal, i.e.: T = k * I * sin(theta), where k is the torque constant. If an ideal current source supplies the motor windings with three-phase balanced currents iU, iV, and iW of constant amplitude I, i.e.: iU = I * sin(wt) iV = I * sin(wt + 2 * PI/3) iW = I * sin(wt + 4 * PI/3) Then the steady-state torque generated by the current in each phase of the motor is: TU = k * I * sin(wt) * sin(theta) TV = k * I * sin(wt + 2 * PI/3) * sin(theta + 2 * PI/3) TW = k * I * sin(wt + 4 * PI/3) * sin(theta + 4 * PI/3) During steady-state operation, theta = wt, then the resultant torque generated by the three-phase windings is: T = TU + TV + TW = 3/2 * k * I * sin(PI/2 - wt + theta) = 3/2 * k * I. The above analysis shows that for a three-phase permanent magnet synchronous motor, when sinusoidal currents with a 120° phase difference are input to the three-phase windings, the output torque of the motor is constant due to the internal circular rotating magnetic field. Therefore, applying the AC servo control principle to a three-phase hybrid stepper motor drive system, the input 220V AC is rectified into DC, and then pulse width modulation technology converts it into three stepped sinusoidal waveform currents. These currents flow through the three windings in a fixed sequence, with each step corresponding to one rotation of the motor. The motor speed is changed by altering the frequency of the sinusoidal current output by the driver. The number of steps in the output determines the angle rotated per step. A smaller angle results in a higher number of steps, or greater microstepping. Theoretically, this angle can be set sufficiently small, allowing for a large microstepping value. The angle per step in AC servo control is highly dependent on the accuracy of the feedback encoder, typically using 2500 lines, resulting in an angle of only 0.144 degrees per step. However, a stepper motor controlled using this method, for example, with a microstepping value of 10000, would rotate an angle of 0.036 degrees per step, significantly higher than typical servo control. Of course, as the stepper motor rotates, the inductance of each phase winding generates a back electromotive force (EMF), which increases with higher frequency. Under its influence, the phase current of the motor decreases as the frequency (or speed) increases, resulting in a decrease in torque. Constant current can maintain the same phase current at both low and high frequencies, thus improving the high-frequency torque characteristics. However, this only applies at low speeds. Therefore, its overall performance (high and low speed noise, high-speed torque, high-speed stability, etc.) is difficult to surpass that of an AC servo control system. Figure 2 shows a three-phase stepped sinusoidal current three-phase hybrid stepper motor with a 120° phase difference. Generally, the three-phase windings are connected in a star or delta configuration. According to the fundamental theorem of circuits, the sum of the three-phase currents is zero, i.e., IU + IV + IW = 0. Therefore, usually only the given signals for two phase windings need to be generated; the given signal for the third phase winding can be obtained from the other two phases. Similarly, only the actual current of the corresponding two phase windings needs to be sampled; the actual current of the third phase winding can be obtained according to the formula. 3. System Composition of the Three-Phase Hybrid Stepper Motor Driver The overall scheme of the driver is shown in Figure 3, mainly consisting of a microcontroller circuit, a current-tracking SPWM circuit, and a power drive circuit. Figure 3 Overall block diagram of the driver 3.1 DSP Module Design Here, we chose TI's DSP as the CPU chip. DSP (Digital Signal Processor) is actually a type of microcontroller, integrating the central processing unit, control unit, and peripheral devices onto a single chip. However, it has its own distinct characteristics—due to the use of multiple bus technologies to achieve parallel operation, it greatly improves the computing speed, has stronger computing power, and better real-time performance. The DSP (TMS320LF2407A) selected in this paper is a dedicated motor control chip with 144 pins and abundant I/O resources. It contains four general-purpose timers, two dedicated PWM generators for controlling three-phase motors (capable of generating six PWM signals), and dedicated I/O ports for receiving external pulses and direction signals, thus simplifying circuit design and program development. DSP input signals include step pulse signal CP, direction control signal, offline signal, and overcurrent protection signal. These signals are all connected to the DSP pins through high-speed optocouplers, and there are also microstepping step count and current selection signals. When the offline signal is valid, the current output from the driver to the motor is cut off, and the motor rotor is in a free state (offline state). The feedback current is sampled by the DSP's built-in 10-bit analog-to-digital converter (AD). After passing through a certain algorithm, the feedback current is output by the DSP's built-in PWM port to control the motor. 3.2 Current Tracking Loop This transmission method uses the amplitude of analog voltage to represent the magnitude of the sampled current or voltage. It is mainly used to sample the current of phases a and b and detect the bus voltage to realize motor current control and overvoltage, undervoltage, and overcurrent protection. The driver detects the actual current of the stepper motor windings through the sampling resistor. After comparing it with the set current, it passes through a hysteresis comparator regulator. The regulator output signal is a 20KHz triangular wave carrier, forming a pulse width modulation (PWM) signal. This signal is used to control the on and off of high-power semiconductor devices through the power drive interface circuit, so that the actual current of the stepper motor windings tracks the given reference signal and changes according to a given sinusoidal law. 3.3 Power Drive Circuit The main circuit of the driver adopts an AC-DC-AC voltage-type inverter, consisting of a rectifier and filter circuit, a three-phase inverter, and a stepper motor. The rectifier and filter circuit forms a DC voltage source, converting 220V, 50Hz AC power to DC power. The inverter converts DC power to variable frequency and voltage AC power, providing the required AC current to the stator windings of the three-phase hybrid stepper motor. The inverter consists of six G30N60B3D MOS transistors manufactured by Fairchild Semiconductor, forming a three-phase inverter bridge. The driver uses two resistors to detect the instantaneous value of the stepper motor phase current. The core of the power drive circuit is the power module (MOS transistor). The MOS transistor and the current-tracking PWM output must be connected via a dedicated high-speed optocoupler. A suitable MOS transistor is selected based on its overcurrent value and the motor's peak line current; that is, the peak value of the motor's line current is less than the maximum current value of the MOS transistor. In this design, the maximum phase current of the motor is 8.1A, which is the effective value of the phase current. The peak phase current is 8.1 * sqrt(2) = 11.312A. In addition, when the motor winding is connected in delta, the line current is 3 times the phase current, so the peak line current is 19.6A. According to the G30N60B3DPDF document, its maximum current value is 30A, so it can be guaranteed to be used normally. Normal operation requires proper heat dissipation design to ensure that the internal junction temperature is always less than 150 degrees Celsius. Therefore, an external heat sink and forced air cooling are required to ensure that the MOS transistor works normally. 3.4 Parallel communication In order to avoid losses caused by power failure or other special reasons during the control process, the motor position is stored in the powered RAM to ensure that the workpiece can continue to be processed after the power is restored. The parallel RAM is faster and more reliable than the traditional E2ROM, and can more effectively record the motor running status, but it occupies more CPU I/O ports. Here, the CPU has enough resources to use. 3.5 Control Software Flowchart 4. Main Program Flowchart Figure 5. Interrupt Section Flowchart To reduce power consumption and protect the motor, an automatic half-current function is implemented, which is automatically adjusted by the hysteresis comparator. 4. Conclusion Practice has proven that the driving method described in this paper is highly adaptable and can basically adapt to all three-phase hybrid stepper motors. Especially for the three-phase winding star connection, it operates smoothly at low frequencies without oscillation, effectively suppressing oscillation and noise. In addition, the driver is internally designed with multiple protection circuits, greatly improving the overall reliability of the driver.