A study of operator-induced instabilities in a tactual virtual environment

One of the major outcomes of the EPSRC/British Aerospace supported project "Enabling Tactile Technology Environments", was the Reactabot, a three degree of freedom force feedback device which provides tactual/kinaesthetic feedback to an operator via a surrogate object. Systems of this nature exhibit unwanted oscillatory behaviour and eventually instability if the virtual mass is reduced to too low a value, corresponding to raising the loop gain too high. The onset of this behaviour is found to be operator dependent. This paper studies the phenomenon in detail, modelling both the open loop system and human operator in the feedback path. It is shown that the variation between operators, allied with the unwanted but inevitable phase lags in the open loop system, cause the undesirable behaviour and suggestions are made to minimise their effect.