Robot control
PID control
Definition
PID control is feedback control that combines terms proportional to the current error, the accumulated error, and the rate of change of error. These terms determine the command sent to the controlled system.
Also known as: Proportional-integral-derivative control, PID controller
Updated
Three responses to tracking error
For a joint-angle target, the proportional term reacts to the angle error now. The integral term accumulates persistent error. The derivative term reacts to how quickly the error changes and can provide damping. Modern Robotics derives these terms for a robot joint driven by torque.
A joint holding an arm against gravity illustrates their different roles. Proportional control alone may need a nonzero position error to generate the required holding torque. Under suitable conditions, integral action builds up that torque while reducing the steady-state error, as shown in the gravity example.
Common robot implementations
PD control omits the integral term. Controllers may also combine feedback with a feedforward estimate of gravity or other dynamics. The controlled quantity and output interface matter: a position loop that outputs velocity is different from a position loop that outputs torque.
More gain is not always better
Large gains can amplify sensor errors, excite unmodeled dynamics, or demand unavailable actuator effort. Excessive integral gain can also destabilize even a simplified model. Modern Robotics discusses limiting accumulated error and prioritizing stability. Tuning must therefore consider sampling rate, load, actuator limits, and the dynamics of the actual joint.
Sources
Related terms
Torque control
Torque control regulates the turning effort delivered by an actuator or robot joint. It provides an actuation interface from which motion, force, and impedance controllers can produce the joint torques their tasks require.
Force control
Force control regulates the force or wrench a robot applies to its environment. It may use a robot model, measured interaction forces, or both to produce joint commands that achieve a desired contact load.
Impedance control
Impedance control shapes the dynamic relationship between a robot’s motion and the forces it exchanges with its environment. A common goal is for the robot to respond like a chosen mass, spring, and damper at a joint or end effector.