Robot control
Computed-torque control
Definition
Computed-torque control combines a robot dynamics model with motion feedback to calculate joint forces or torques for trajectory tracking. The model supplies inverse-dynamics terms while feedback corrects tracking and modeling errors.
Also known as: Computed torque control, Computed torque method, Inverse-dynamics control
Updated
Turn a desired acceleration into torque
A computed-torque controller uses the estimated mass matrix and modeled gravity, Coriolis, and sometimes friction terms. It maps a desired acceleration plus feedback correction into a joint-torque command. With an exact model, this cancels the modeled nonlinear dynamics and leaves a chosen linear tracking-error response. MIT's manipulator-control notes also describe this model-based cancellation form under the computed-torque name.
Modern Robotics derives the method in joint space and shows the corresponding task-space form. The reference combines model-based feedforward with PID or PD feedback rather than trusting an open-loop dynamics prediction.
More than gravity compensation
Gravity compensation supplies only the modeled load due to gravity. Computed torque also uses desired acceleration and configuration-dependent inertia, and it can include velocity-dependent dynamics. It is therefore suited to following a changing trajectory rather than only supporting a static pose.
The method depends on inverse dynamics, but the terms are not identical. Inverse dynamics is the calculation of effort for a specified motion. Computed-torque control is a closed-loop control law that uses such a calculation together with measured state and tracking error.
Cancellation is only as good as the model
Payload error, flexible links, backlash, friction changes, unmodeled contacts, and delayed state estimates leave dynamics that are not canceled. Aggressive gains intended to suppress those residuals can excite flexible modes, amplify noise, or exceed actuator limits.
The Modern Robotics lesson also notes the cost of evaluating a full dynamics model in real time and warns that a poor model can hurt performance relative to simpler feedback. Tracking tests should report the trajectory, payload, update rate, torque limits, and size of model mismatch rather than treating model cancellation as exact.
Sources
Related terms
Inverse dynamics
Inverse dynamics calculates the joint forces or torques required for specified joint positions, velocities, and accelerations under a dynamics model. The result also depends on gravity and specified external loading.
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.
Gravity compensation
Gravity compensation commands forces or torques intended to balance the gravitational loading predicted by a robot model. It can make a mechanism hold a pose with less feedback error or feel lighter when a person moves it.
PID control
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.