Torque Constant (Kt)

How much torque a motor produces per amp of current, in newton-meters per amp (N·m/A).

The torque constant, Kt, describes the proportional relationship between a motor's current and its output torque: torque ≈ Kt × current. It's set by the motor's construction — magnets, number of winding turns — and stays roughly constant within the motor's rated range. This matters because when a robot joint does torque control, the driver is actually controlling current, and Kt is what converts a desired torque into the current to command; read the other way, current can be used to roughly estimate the force at a joint, which is the basis of sensorless force estimation. Kt is just another way of expressing the same thing as the back-EMF constant and the KV rating: a higher Kt means more torque for the same current, but a lower top speed. When a gearbox is involved, it's worth checking carefully whether a datasheet's number is measured at the motor side or the output side.

ExampleThe Unitree GO-M8010-6 joint motor's datasheet lists a torque constant of about 0.639 N·m/A.

Also called

Kt, Motor Torque Constant

Related

Torque Control · Motor Velocity Constant (KV Rating) · Torque-Speed Curve · Sensorless Force Estimation · Field-Oriented Control · Brushless DC Motor

Sources

Motor constants - Wikipedia
GO-M8010-6 Motor User Manual V1.0