Constant Force Control (Force Tracking)

Keeping a robot's end-effector applying a set, constant pressure against a contact surface — for example, holding 10 newtons.

Constant force control, also called force tracking, keeps the contact force between the end-effector and the environment following a set value, and is common in polishing, buffing, deburring, wiping, and ultrasonic scanning — tasks needing steady pressure. The difficulty is that the environment's position and stiffness are often not known precisely: under pure position control, a workpiece positioned just 1 millimeter off can cause a large difference in the force pressed against a stiff surface. Common approaches include: PI control directly on the force error (explicit force control); hybrid force/position control, with force controlled in the normal direction and position in the tangential direction; and wrapping an outer force loop around impedance or admittance control that adjusts the reference position based on force error — Seraji and Colbaugh (1997) achieved tracking of a desired force this way using adaptive methods even when the environment's stiffness and position were both unknown. It differs from plain impedance control in that impedance control only specifies ‘deviate this much, output that much force,’ with the actual contact force depending on the environment, whereas constant force control targets the force itself. It usually requires a 6-axis force sensor or joint torque sensors to measure force.

ExampleUniversal Robots arms' Force Mode can set the z-axis as a compliant axis in the task frame with a target force (e.g., 10 N downward), while x and y continue following the programmed position path as usual, maintaining constant pressure while moving across a surface; UR's documentation specifically notes this is not admittance control, and any direction set to force control overrides motion commands along that direction.

Also called

Force Tracking Control, Constant Force Tracking, Constant-Force Polishing

Related

Force Control · Impedance Control · Admittance Control · Hybrid Force/Position Control · Six-Axis Force/Torque Sensor · Contact-rich Manipulation

Sources

Seraji H., Colbaugh R. Force Tracking in Impedance Control. IJRR, 1997
Universal Robots ROS 2 Driver: ForceModeController 文档 (Chinese)