Low cost torque estimator for DC servo motors

Electromagnetic torque of a DC motor can be measured directly by a torque sensor attached to the shaft. However, this is an expensive solution requiring additional power, interfacing and sometimes such a torque sensor can be too bulky to be implemented in a compact environment. Alternatively, the armature current measurement can be used to derive the electromagnetic torque of the DC motor, provided the motor torque constant is known and does not vary over the time. In addition, this method also needs an additional current sensor and its integration circuits. However, in general, DC servo motors are equipped with either absolute or incremental encoder connected to the shaft. This can be used to estimate the torque without any additional sensor.