This paper designs a three-phase hybrid multi-microstepping stepper motor driver based on the principle of sinusoidal current microstepping drive. The system adopts current tracking and pulse width modulation technology to make the phase current of the motor a sine wave with a phase difference of 120°. This driver solves the shortcomings of traditional stepper motors, such as large low-speed vibration, resonance zone, and high noise, and improves the step angle resolution and driver reliability. 1 Introduction A stepper motor is an actuator in an open-loop servo motion system, controlled by pulses to output angular displacement. Compared with AC and DC servo motors, its outstanding advantages are low price and no accumulated error. However, stepper motors have many shortcomings, such as low-frequency oscillation, high noise, and low resolution, which seriously restricts their application range. The operating performance of a stepper motor is closely related to its driver, and its shortcomings can be overcome by improving the drive technology. Compared with other driving methods, microstepping drive can not only reduce the step angle of the stepper motor and improve the resolution, but also reduce or eliminate low-frequency vibration, making the motor run more smoothly and evenly. Overall, microstepping control offers the best performance. Because commonly used low-end stepper motor servo systems lack encoder feedback, the internal control current decreases as motor speed increases, leading to missed steps. Therefore, it is widely used in applications where speed and precision requirements are not high. Since three-phase hybrid stepper motors offer better low-speed stability and output torque than two-phase stepper motors, they have a brighter future. 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 Miniature 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 subdividing 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 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 resultant magnetic field vector of each phase winding, i.e., the resultant 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. [IMG=Simple Structural Model of a Three-Phase Permanent Magnet Synchronous Motor]/uploadpic/THESIS/2007/11/20071116164039151251I.jpg[/IMG] 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 of the motor rotor deviating 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 match that of an AC servo control system, as shown in Figure 2. [IMG=Given three-phase stepped sinusoidal current with a 120° phase difference]/uploadpic/THESIS/2007/11/2007111616441781440H.jpg[/IMG] Figure 2: Given three-phase stepped sinusoidal current with a 120° phase difference. Three-phase hybrid stepper motors typically connect the three-phase windings 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, by sampling the actual current of the corresponding two-phase windings, the actual current of the third-phase winding can be obtained according to the formula. 3 System Composition of 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. [IMG=Overall Block Diagram of the Driver]/uploadpic/THESIS/2007/11/2007111616451368515R.jpg[/IMG] 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, which also integrates the central processing unit, control unit, and peripheral devices onto a single chip. However, it has its own distinct characteristics—because it adopts a multi-bus technology 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 includes 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 the step pulse signal CP, direction control signal, offline signal, and overcurrent protection signal. These signals are all connected to the DSP pins via high-speed optocouplers. Additionally, there are microstepping 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 processing with a specific algorithm, the feedback current is output from the DSP's built-in PWM port to control the motor. 3.2 Current Tracking Circuit: 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 currents of phases a and b and detect the bus voltage to achieve motor current control and overvoltage, undervoltage, and overcurrent protection. The driver detects the actual current of the stepper motor windings through a sampling resistor, compares it with the set current, and then passes it through a hysteresis comparator regulator. The regulator output signal is a 20kHz triangular wave carrier wave, forming a pulse width modulation (PWM) signal. This PWM signal is used to control the on/off state of high-power semiconductor devices through the power drive interface circuit, making the actual current of the stepper motor windings track the given reference signal and change 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, completing the conversion from 220V, 50Hz AC power to DC power. The inverter converts DC power to AC power with variable frequency and voltage, providing the required AC current to the stator windings of the three-phase hybrid stepper motor. The inverter consists of six G30N60B3DMOS transistors manufactured by Fairchild Semiconductor, forming a three-phase inverter bridge. The driver uses two resistors to detect the instantaneous value of the phase current of the stepper motor. 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 by a dedicated high-speed optocoupler. The appropriate MOS transistor is selected based on the overcurrent value of the MOS transistor and the peak line current of the motor, i.e., the peak value of the line current of the motor 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, and the peak phase current is 8.1 * sqrt(2) = 11.312A. In addition, when the motor windings are connected in delta configuration, the line current is 3 times the phase current, so the peak value of the line current is 19.6A. According to the G30N60B3D PDF document, its maximum current is 30A, which is sufficient for normal operation. Proper heat dissipation design is required to ensure the internal junction temperature remains below 150 degrees Celsius. Therefore, an external heatsink and forced air cooling are necessary to ensure the MOSFET operates normally. 3.4 Parallel Communication: To avoid losses due to power outages or other unforeseen circumstances during control, powered RAM is used to store the motor position, ensuring the workpiece can continue processing after power is restored. Parallel RAM offers faster and more reliable data transfer than traditional E2ROM, and can more effectively record motor operating status. However, it consumes more CPU I/O ports, but the CPU has sufficient resources available. 3.5 Control Software Flowchart [IMG=Main Program Flowchart]/uploadpic/THESIS/2007/11/200711161650287038414.jpg[/IMG] Figure 4 Main Program Flowchart [IMG=Timer Interrupt Flowchart]/uploadpic/THESIS/2007/11/2007111616515429944I.jpg[/IMG] Figure 5 Timer Interrupt Flowchart Figure 4 shows the main program flowchart, and Figure 5 shows the timer interrupt flowchart. To reduce power consumption and protect the motor, an automatic half-current function is set (the current is halved when there is static shaft lock or no pulse input), which is automatically adjusted by the hysteresis comparator. 4 Performance Indicators and Current Application Status Performance indicators achieved after design: (1) Input AC voltage from 90V to 275V; (2) Input signal is opto-isolated; (3) High-efficiency chopper with IGBT as the output stage; (4) PWM (Pulse Width Modulation) constant current chopper, three-phase standard sine wave output, with a triangular carrier frequency of 20k; (5) Automatic half-current lockout; (6) Power-off protection function (automatic phase memory function after power-off); (7) Flexible current control (2.0A-8.1A), suitable for three-phase motors of various series with seat sizes of 86, 90, 110, and 130; (8) Offline (FREE) function; (9) Low noise, low heat generation, high efficiency, and good operating characteristics; (10) This driver is a single-pulse (P+D) control mode; (11) The maximum subdivision currently achieved is 16,000 p/r, and the minimum is 600 p/r, mainly depending on the user. It can be made smaller or larger. Shenzhen Zhongweixing CNC Technology Co., Ltd. was established in 1998. It is an ISO9001 certified enterprise and a high-tech enterprise. It has more than 50 senior professional technical R&D engineers, well-equipped laboratories, and a complete production assurance system. Zhongweixing CNC has always focused on the R&D and production of motion control technology products. It has independently developed the first embedded motion controller and supporting system software in China. Some of its products and technologies have obtained national patents and passed the EU CE certification. It has now formed a full range of CNC product lines, including motion control cards, motion controllers, stepper motors/drivers, and CNC application systems. Its products are widely used in national defense, scientific research, teaching, and industrial fields. The company's products are sold throughout China and exported to Italy, Australia, Singapore, Taiwan, Hong Kong and other countries and regions. The driver is now in mass production, and customers have reported that its performance is stable and reliable. 5 Conclusion Practice has proven that the driving method described in this article 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. Furthermore, the driver incorporates multiple internal protection circuits, significantly improving the overall reliability of the driver. (Proceedings of the 2nd and 3rd Servo and Motion Control Forums)