Enhancing the force transparency of time domain passivity approach: Observer-based gradient controller

Abstract

Passivity has been the most often used constraint for the stable controller design of bilateral teleoperation systems. Especially, Time Domain Passivity Approach (TDPA) has been used in many applications since it has been known as one of the least conservative passivity-based approaches. Although TDPA were able to stabilize the system with the least conservatism, it has its own drawbacks as the cost of achieving the least conservative passivity especially when there is communication time-delay. Due to the on/off bang-bang control-like modification for instantaneous passivity recovery, it has high frequency force vibrations on the slave and especially master side. By implementing a virtual mass-spring system between the passivity controller and master device, these high frequency vibration has been eliminated. However, the gains need proper tuning as they are dependent on the teleoperation setup and application. It also tends to make the system sluggish which further distorts the transparency. We propose a new observer-based gradient controller to eliminate the force jittering on the master side. It rectifies the delayed feedback force by removing the undesired increase in force which is generated by the delay in communication channel. It does not require any system parameters and there are no gains to tune, thus it can be added to any teleoperator irrespective of its dynamics and without having any prior system information. The proposed approach was implemented to a position-force bilateral teleoperation system, and compared with TDPA with virtual mass spring with round-trip delays of up to 500 ms for hard wall contacts.