Force Control (Impedance / Admittance Control)
力控（阻抗控制/导纳控制）
What Is Force Control in Robotics?
Force control means a robot joint regulates its output force/torque rather than just its position, giving the robot a controllable "softness" when contacting the environment or people. Landing a step, carrying objects with two arms, and human-robot collaboration all depend on it — a position-controlled joint rams through unexpected contact, while a force-controlled joint yields.
The Three Approaches
- Torque control: command joint torque directly — the lowest-level, fastest loop; the
T_fffeed-forward torque in the MIT protocol serves exactly this; - Impedance control: command a virtual spring-damper (stiffness Kp, damping Kd) so position error maps to torque — the standard for legged robots; one MIT-protocol frame carries Kp/Kd and torque feed-forward together;
- Admittance control: measure external force, output position corrections — common on arms with wrist force sensors, bandwidth-limited by the position loop.
Do You Need a Torque Sensor?
Friction, efficiency variation, and hysteresis make current-based output-torque estimation less accurate in high-ratio joints, so precision applications often use joint or end-effector force sensors. A low-ratio quasi-direct-drive (QDD) joint can estimate output torque from phase current after calibrating motor torque constant, gearbox efficiency, and friction. This supports some proprioceptive and impedance-control tasks, but it does not replace sensing for every high-accuracy or safety-critical application. BXI 85/70/50-series joint actuators support joint control through a 19.5:1 planetary design, MIT-protocol CAN commands, and dual absolute encoders.