What Is an Error Signal? How Robot Controllers Correct Their Movements.

What Is an Error Signal?

An error signal is the difference between a desired value and the value measured or estimated by a control system.

In robotics, error signals help controllers determine whether a machine is performing as intended and how much correction may be needed.

For example, if a robot joint is commanded to move to 90 degrees but its encoder reports 85 degrees, the position error is five degrees.

A feedback controller can use that error to adjust the motor's command.

Key Takeaways

• Error signals describe the difference between desired and actual or estimated behavior.
• They are fundamental to feedback control systems.
• An error signal identifies a discrepancy but does not necessarily explain what caused it.

How Does an Error Signal Work?

Consider a robotic arm instructed to move its gripper to a particular position.

The controller has a target value representing the desired movement.

A sensor provides information about the actual position.

The controller calculates the error:

Error = Desired Value − Measured Value

Suppose the desired joint angle is 90 degrees and the measured angle is 85 degrees.

The error is positive five degrees.

As the joint moves toward its target, the error should decrease.

The controller repeatedly calculates updated errors and adjusts its commands according to its control algorithm.

What Are Common Types of Error Signals?

Robotic systems can calculate errors for different quantities.

Position error: Difference between commanded and measured position.

Velocity error: Difference between desired and measured speed.

Force error: Difference between a target force and the force measured during contact.

Orientation error: Difference between desired and actual orientation, represented using an appropriate rotational calculation.

Different errors may be used simultaneously within a robot's control architecture.

Error Signal vs. Fault Signal

An error signal measures a difference from a desired value.

A fault signal indicates that a system has identified an abnormal or unacceptable condition.

These are not interchangeable.

For example, a small position error during normal movement may be expected.

A persistent error exceeding specified limits could cause the controller to declare a following-error fault.

The distinction depends on the machine's operating conditions and configured limits.

Why Doesn't a Controller Always Eliminate Error Immediately?

Physical systems have inertia, friction, actuator limits, and other constraints.

A motor cannot necessarily move a heavy robotic arm to its target instantaneously.

Controllers therefore balance responsiveness with stability and acceptable mechanical behavior.

An excessively aggressive response can produce overshoot or oscillation.

Frequently Asked Questions

Is an error signal always a sign of a problem?

No. Small errors are common during normal closed-loop operation.

Can a robot operate with zero error?

Some systems can achieve negligible or zero error under specific steady-state conditions, but perfect error-free operation is not a general guarantee.

The Bottom Line

Error signals help robots compare intended behavior with measured results. They provide the information feedback controllers use to regulate movement and respond to disturbances.