On the use of Torque Disturbance Observers in 2-mass systems with application to a robotic joint

Two-mass systems with elastic behaviour have long been studied to provide solutions for vibration suppression. Imperfections in the torque delivered by the commonly adopted brushless motor, as well as pulsating disturbances originating in the transmission system (e.g. in the gearbox), generate disturbances on the motor side that can compromise the overall performance of the system. Torque Disturbance Observer (TDO) is a quite effective method to estimate and then compensate for such disturbances, but the application to an elastic system can yield loss of performance on the load side, as discussed in the present paper. Making reference to a joint of the space robotic arm DEXARM, endowed with a measurement of the transmittion torque, a modified TDO system is then proposed. Experimental validation of the modified TDO scheme performed on the DEXARM joint is finally discussed.