Torque control

Commanding effort at the joint#

A position target specifies where a joint should go; a torque target specifies its turning effort. The resulting motion also depends on inertia, gravity, friction, and external contact. Modern Robotics derives motion controllers that account for these dynamics before issuing torque commands.

For example, keeping an arm horizontal requires torque even when its position is constant. Accelerating the same arm adds an inertial requirement, and pressing against a wall changes the joint torques through the contact wrench.

How actuators implement it#

For an electric motor in its usual operating model, torque relates to motor current. Modern Robotics explains how gearing and friction affect the relationship between motor torque and joint output. Some systems use output torque sensors, while a series elastic actuator can infer torque from calibrated spring deflection.

Command accuracy has limits#

A commanded motor current is not proof that the joint delivered the requested output torque. Transmission losses, model errors, current limits, and heating can change the result. Katz's thesis separately characterizes torque accuracy and thermal behavior, illustrating why both matter. The available torque also varies with speed and operating duration, so peak torque alone is insufficient for planning sustained tasks.