What Is Position Control? How Robots Reach and Hold Target Positions.

What Is Position Control?

Position control is a control method used to move a mechanism toward a desired location or angle and regulate its position.

In robotics, position control is used to move joints, grippers, linear actuators, and other mechanical components.

For example, a robotic arm may receive a command to rotate its elbow joint to 90 degrees.

A position controller regulates the movement so the joint approaches and, when required, maintains that target.

Key Takeaways

• Position control regulates the position of a motor, joint, or mechanical component.
• Closed-loop position control uses feedback to correct differences between commanded and measured positions.
• Position accuracy depends on sensor quality, controller design, mechanical characteristics, and operating loads.

How Does Position Control Work?

Imagine a robot joint commanded to move from 30 degrees to 90 degrees.

The controller receives the target position.

An encoder reports the joint's current angle.

The system calculates the difference between its desired and measured positions.

As the motor moves the joint, the controller receives new measurements and updates its commands.

The objective is to reduce positioning error while maintaining appropriate movement behavior.

Some systems use nested control loops, with an outer position controller providing commands to faster velocity, torque, or current controllers.

The exact arrangement depends on the hardware.

Position Control vs. Velocity Control

Position control regulates where a mechanism moves.

Velocity control regulates how quickly it moves.

For example, a robotic arm may use position control to place its gripper at a specified location.

A conveyor motor may use velocity control to maintain a target speed.

Both methods can use feedback.

A robot may also use velocity control internally while following position commands from a higher-level controller.

Position Control vs. Force Control

Position control focuses on location.

Force control focuses on physical interaction forces.

Consider a robot pressing a tool against a metal surface.

A rigid position-controlled movement may apply excessive force if the surface is slightly higher than expected.

Force or compliant-control methods can help the robot accommodate these differences.

For this reason, contact-rich tasks may require more than position control alone.

What Causes Positioning Errors?

Several factors can influence performance.

Sensor errors: Incorrect or noisy position measurements.

Backlash: Lost motion within mechanical transmissions.

Mechanical flexibility: Components deform under load.

Controller settings: Excessive or insufficient gains affect the response.

External forces: Payloads and contact can disturb movement.

These factors can cause the actual position to differ from the commanded position.

Frequently Asked Questions

Does position control require an encoder?

Closed-loop position control needs position feedback or estimation, which can come from encoders or other suitable methods.

Can position control cause overshoot?

Yes. Poor tuning or mechanical dynamics can cause a mechanism to move temporarily beyond its target.

The Bottom Line

Position control allows robots to reach and maintain desired mechanical positions. It is fundamental to precise movement, but its effectiveness depends on both the controller and the physical machine.