Stepper motors are widely used in control systems due to their ability to convert digital pulse signals into precise angular displacements. They can function as electromagnetic brake wheels, differentials, or angular displacement generators. Many stepper motors removed from old devices—often undamaged—can be repurposed for new applications. To do so, a custom driver is typically required. This article discusses the design of a driver specifically for a four-phase stepper motor taken from a Japanese printer. It begins by explaining the working principle of the motor and then details both the hardware and software components of the driver.
### 1. Working Principle of the Stepper Motor
A four-phase reactive stepper motor operates using a unipolar DC power supply. The rotor moves in discrete steps when the phase windings are energized in sequence. Figure 1 shows the schematic of a four-phase stepping motor. Initially, switch SB is closed while SA, SC, and SD are open. This aligns the B-phase magnetic poles with the rotor's teeth 0 and 3, while teeth 1 and 4 misalign with C and D-phase windings. As switches change states, the rotor rotates step by step, following the magnetic field alignment.
The motor can operate in three modes: single-phase, double-phase, and eight-phase. In single and double-phase modes, the step angle is the same, but the torque is lower in the single-phase mode. The eight-phase mode offers half the step angle and higher torque, improving accuracy. The timing waveforms for these modes are illustrated in Figure 2.
### 2. Circuit Design of the Stepper Motor Driver Based on AT89C2051
Figure 3 presents the circuit diagram of the driver. The AT89C2051 microcontroller generates control pulses on its P1 port, which are inverted via 74LS14 and amplified by 9014 transistors. These signals drive the photoelectric isolator, which then controls the power transistor TIP122. The motor’s phase windings are activated accordingly, enabling rotation, reversal, acceleration, and deceleration based on the input signal.
Resistors RL1–RL4 represent internal winding resistance, while the external 50Ω resistor limits current and improves the time constant. Freewheeling diodes D1–D4 protect the power transistor by dissipating back EMF. A 200μF capacitor across the 50Ω resistor enhances the current pulse rise time, improving high-frequency performance. A 200Ω resistor in series with the freewheeling diode reduces the discharge time constant, resulting in a steeper current pulse edge.
### 3. Software Design
The driver supports three operating modes controlled by DIP switches KX and KY:
- **Mode 1 (Interrupt Mode):** Uses P3.5 as the step pulse input and P3.7 for direction control. It connects to the host via only two lines.
- **Mode 2 (Serial Communication Mode):** The host sends commands to the driver, which executes them.
- **Mode 3 (Dial Switch Control):** Direct control via K1–K5.
Upon power-up or reset, the microcontroller checks the DIP switches and enters the corresponding mode. The program flow for Mode 1 includes setting flags for forward and reverse movement, ensuring smooth transitions without missed steps. The source code for Mode 1 is provided, showing how the motor steps through each position based on interrupt triggers.
### 4. Conclusion
This driver has been tested successfully with a 0.5 Nm motor and can be adjusted to drive a 1.2 Nm motor by modifying resistor, capacitor, and diode values. The circuit is simple, reliable, and compact, making it ideal for systems with limited I/O and microcontroller resources. It demonstrates a practical approach to repurposing old stepper motors with minimal cost and effort.
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