Functional Module Design

This module consists of the following three parts:

• Single-chip microcomputer system: Controls the stepper motor;

• Peripheral circuit: Interface circuit between the PIC single-chip microcomputer and the stepper motor;

• PIC program: Develops the interface program for the single-chip microcomputer to control the stepper motor and realizes the output of triangular wave signals.

(1) Interface between Stepper Motor and Single-chip Microcomputer

The single-chip microcomputer is a high-performance control processor. When controlling a stepper motor, the interface device must fulfill the following functions:① Voltage isolationThe single-chip microcomputer operates at 5 V, while the stepper motor works at several tens of volts or even higher. If high voltage from the stepper motor leaks into the single-chip microcomputer, the chip will be damaged. In addition, signals from the stepper motor may interfere with the single-chip microcomputer and cause system malfunctions. Therefore, the interface device must provide isolation.

② Signal transmissionThe interface device shall transmit control commands from the single-chip microcomputer to the stepper motor circuit and generate required control signals. Corresponding control waveforms shall be produced for different operation modes.

③ Generation of variable frequenciesTo drive the stepper motor at different speeds for various application requirements, the interface device needs to generate working signals of different frequencies.

(2) Voltage Isolation Interface

The voltage isolation interface is specially used to isolate the low-voltage single-chip microcomputer section and the high-voltage stepper motor drive circuit to ensure stable operation of both sides.

Pulse transformers or optocouplers can be adopted for voltage isolation, and optocouplers are the mainstream choice at present. Output signals from the single-chip microcomputer can pass through TTL gates or be directly sent to the base of transistors, which then drive the light-emitting diodes inside optocouplers.

Light emitted by the light-emitting diode irradiates the photosensitive transistor inside the optocoupler and is converted into an electrical signal to drive the power amplification circuit of the stepper motor. The current amplification interface serves as the preamplifier of the stepper motor power amplifier circuit. It amplifies the output current of the optocoupler to supply sufficient driving current for the power amplifier circuit.

(3) Operation Mode Interface and Frequency Generator

When a single-chip microcomputer is used to control a stepper motor, three I/O lines on the input and output interface are required. In this scheme, RA0, RA1 and RA2 of the microcontroller I/O port control the three phases of the stepper motor.

Stepper motor control is generally divided into open-loop control and closed-loop control. Among them, open-loop control is the simplest scheme. Under open-loop control, the input of control pulses for the stepper motor is independent of the rotor position; pulses are generated according to fixed rules, and the stepper motor runs relying on this set of predetermined pulses. Thanks to the inherent characteristics of stepper motors, this control method is widely applicable.Its prominent features include simple control logic, easy implementation and low cost. One key characteristic of open-loop control is that no feedback of load position is sent back to the control circuit.

Consequently, the stepper motor must respond accurately to every excitation change. If the excitation frequency is excessively high and the motor fails to move to the target position, a position error between the actual load position and the target position will occur. When the load remains relatively constant, generating control pulse sequences is straightforward. However, under conditions with large load variations, it is difficult to optimize the pulse sequence, which may lead to step loss. With the widespread application of microcomputers, microcontrollers can generate relatively complex control pulse sequences for stepper motors.