For a useful robot arm, you need high forces/torques. Usually, this is achieved by high ratio [1] gearboxes between the motor and the joint. Those are expensive

For a useful robot arm, you need high forces/torques. Usually, this is achieved by high ratio [1] gearboxes between the motor and the joint. Those are expensive, and inefficient and make estimating the output force via the motor current almost impossible. [1] In the order of 1:100, see e.g. https://www.harmonicdrive.net/ https://www.harmonicdrive.net/