FreertosHow-ToBeginner · 4 min read

How to Program Servo Motor with PLC: Step-by-Step Guide

To program a servo motor with a PLC, use motion control instructions like MOVE, SET SPEED, and START commands within your PLC program. Configure the servo parameters in the PLC software, then write ladder logic or structured text to control position, speed, and torque based on your application needs.

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Syntax

Programming a servo motor with a PLC typically involves these key commands:

SET_POSITION: Defines the target position for the servo.
SET_SPEED: Sets the speed at which the servo moves.
START_MOTION: Begins the servo movement.
STOP_MOTION: Stops the servo immediately.
READ_STATUS: Checks servo status like position reached or errors.

These commands are used in ladder logic or structured text depending on your PLC brand and programming environment.

structured text

SET_POSITION servo_axis, 1000
SET_SPEED servo_axis, 500
START_MOTION servo_axis
WAIT UNTIL servo_axis.position = 1000
STOP_MOTION servo_axis

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Example

This example shows a simple structured text program to move a servo motor to position 1000 at speed 500, then stop when the position is reached.

structured text

PROGRAM ServoControl
VAR
  servo_axis : INT := 1; // Servo axis number
  target_pos : INT := 1000;
  speed_val : INT := 500;
  current_pos : INT;
  motion_done : BOOL := FALSE;
END_VAR

// Set target position
SET_POSITION(servo_axis, target_pos);
// Set speed
SET_SPEED(servo_axis, speed_val);
// Start motion
START_MOTION(servo_axis);

// Wait until position reached
REPEAT
  current_pos := READ_POSITION(servo_axis);
  IF current_pos >= target_pos THEN
    motion_done := TRUE;
  END_IF;
UNTIL motion_done
END_REPEAT;

// Stop servo
STOP_MOTION(servo_axis);
END_PROGRAM

Output

Servo moves to position 1000 at speed 500 and stops when position is reached.

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Common Pitfalls

Common mistakes when programming servo motors with PLCs include:

Not configuring servo parameters correctly in the PLC software before programming.
Forgetting to check servo status which can cause the program to continue before motion completes.
Using incorrect axis numbers or addresses for the servo motor.
Not handling errors or alarms from the servo, which can cause unexpected stops.

Always test servo commands in a safe environment and monitor feedback signals.

structured text

(* Wrong: No status check, may stop too early or late *)
START_MOTION(servo_axis);
STOP_MOTION(servo_axis);

(* Right: Wait for position before stopping *)
START_MOTION(servo_axis);
REPEAT
  current_pos := READ_POSITION(servo_axis);
UNTIL current_pos >= target_pos
END_REPEAT;
STOP_MOTION(servo_axis);

📊

Quick Reference

Here is a quick reference for common servo motor PLC commands:

Command	Description
SET_POSITION(axis, pos)	Set target position for servo axis
SET_SPEED(axis, speed)	Set movement speed
START_MOTION(axis)	Begin servo movement
STOP_MOTION(axis)	Stop servo immediately
READ_POSITION(axis)	Get current servo position
READ_STATUS(axis)	Get servo status and errors

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Key Takeaways

Configure servo parameters in PLC software before programming.

Use motion commands like SET_POSITION, SET_SPEED, START_MOTION to control the servo.

Always check servo position or status to know when motion completes.

Handle errors and alarms to avoid unexpected stops.

Test servo programs carefully in a safe environment.