PLC Stepper Motor Control: Master the Logical Essentials

PLC Stepper Motor Control: Master the Logical Essentials

Introduction

PLCs (Programmable Logic Controllers) are industrial control computers with modular structures, flexibility, high - speed processing, and precise data handling. They excel at controlling stepper motors using high - speed pulse outputs or motion control functions.

For devices with fixed movement distances and speeds, controlling stepper motors via PLC and a stepper driver is an ideal solution.

Characteristics of Stepper Motors

Proportional Angular Displacement: The angular displacement of a stepper motor is strictly proportional to the input pulse count. After each full rotation, there's no cumulative error, ensuring good following performance.

Simple and Reliable Open - Loop Control: The open - loop digital control system composed of a stepper motor and driver circuit is simple, cost - effective, and reliable. It can also be combined with an angle feedback loop to form a high - performance closed - loop system.

Fast Dynamic Response: Stepper motors can quickly start, stop, reverse, and change speed.

Wide Speed Range: Speed can be smoothly adjusted across a broad range, with high torque output even at low speeds.

Pulse Power Requirement: Stepper motors require pulse - width - modulated power supplies and cannot operate directly on AC or DC power.

The highest stepping frequency at which the motor can respond without losing steps is the "starting frequency." The "stopping frequency" is the highest stepping frequency at which the motor can stop accurately without overshooting the target position when the control signal is suddenly cut off. The motor's starting frequency, stopping frequency, and output torque must match the load's moment of inertia. With this data, effective speed control of the stepper motor is possible.

PLC Control of Stepper Motors

When using a PLC to control a stepper motor, calculate the system's pulse equivalent, upper pulse frequency limit, and maximum pulse count using the following formulas to select the appropriate PLC and functional modules:

Pulse Equivalent = (Stepper Motor Step Angle × Lead) / (360 × Gear Ratio)

Upper Pulse Frequency Limit = (Movement Speed × Stepper Motor Microstep Setting) / Pulse Equivalent

Maximum Pulse Count = (Movement Distance × Stepper Motor Microstep Setting) / Pulse Equivalent

Coordinate System Setup for PLC Control

The PLC must first establish a coordinate system, which can be either relative or absolute. In the DM6629 word:

Bits 00 - 03 correspond to pulse output 0.

Bits 04 - 07 correspond to pulse output 1.

Setting these bits to 0 selects a relative coordinate system, while setting them to 1 selects an absolute coordinate system.

Application Example

In single - or dual - axis motion control, parameters such as movement distance, speed, and direction are set on the control panel. The PLC reads these settings, performs calculations, and generates pulse and direction signals. These signals control the stepper motor driver, achieving precise control of distance, speed, and direction. Practical tests have confirmed the reliability, feasibility, and effectiveness of this system.