Force control of a non-backdrivable robot without a force sensor

Cooperatively controlled robots are used in many kinds of applications, including surgical robot applications where the surgeon can guide the robot end effector to a desired position. Often, a 6 degree-of-freedom (DOF) force/torque sensor is installed. However, in some cases, the sensor is only used to impose safety thresholds and to support the robot guidance task. In cases where high guidance accuracy is not required, it can be difficult to justify the added cost of a 6 DOF force sensor. One lower-cost solution is to incorporate a joystick or similar input device, but this requires additional hardware and removes the surgeon´s hands from direct interaction with the robot end-effector. This paper presents a method for achieving cooperative force control without a force sensor. The method utilizes motor current feedback and uses a calibrated current value for force estimation. The novelty of this method is that it can be applied to non-backdrivable robots. It is implemented on a 2-DOF XY stage and experiments are conducted to demonstrate accuracy and performance on this non-backdrivable robot.