A joint torque disturbance observer for robotic assembly

This paper describes an approach to achieve good trajectory tracking performance needed in printed circuit board (PCB) assembly by using a joint torque disturbance observer. By observing the difference between the reference torque and the output torque based on a prescribed model, the proposed disturbance observer immediately feeds back the discrepancy to filter off the disturbance. In this way each robot joint becomes linearized and decoupled and a simple PD with feedforward control is sufficient to achieve high accuracy trajectory tracking. A particular advantage in this observer implementation is that only position and velocity signals are required for control purposes. The observer also provides an estimation of external reaction force which is useful in deciding whether a particular threshold level has been exceeded as an indication of jam. This duality in functionality eliminates a force sensor to detect external force. Experiments are carried out on a SCARA robot to verify the accuracy of the overall controller