Perception of Delayed
Stiffness
ASSAF PRESSMAN1,2,
LEAH J. WELTY3, AMIR KARNIEL2 and FERDINANDO A.
MUSSA-IVALDI1
1. Sensory Motor
Performance Program, Rehabilitation Institute of
2. Department of Biomedical
Engineering,
3. Department of
Preventive Medicine, Northwestern
The International Journal of Robotics
Research 26:1191-1203, 2007
ABSTRACT
Advanced technology has recently provided truly immersive virtual environments with teleoperated robotic devices. In order to control movements from a distance, the human sensorimotor system has to overcome the effects of delay. Currently, little is known about the mechanisms that underlie haptic estimation in delayed environments. The aim of this research is to explore the effect of a delay on perception of surfaces stiffness. We used a forced choice paradigm in which subjects were asked to identify the stiffer of two virtual spring-like surfaces based on manipulation without visual feedback. Virtual surfaces were obtained by generating an elastic force proportional to the penetration of the handle of a manipulandum inside a virtual boundary. The elastic force was either instantaneous function of the displacement, delayed at 30 or 60 milliseconds after the displacement or led the displacement (by means of Kalman predictor) by 50 milliseconds. We assume that for estimating stiffness, the brain relates the experienced interaction forces with the amount of penetration. The results of the experiment indicate a systematic dependence of the estimated stiffness upon the delay between position and force. When the force lagged the penetration, surfaces were perceived as stiffer. Conversely, when the force led the penetration, surfaces were perceived as softer. We compared the perceptual findings with different regression models. This allowed us to discard some candidate models. To further refine the analysis, we carried out a second experiment in which we introduced the delay only during part of the hand/surface interaction, either while the hand was moving into the spring-like surface or when it was moving out of it. Our findings are consistent with stiffness estimates based on dividing the maximum force with the perceived amount of penetration. Our findings are not consistent with an estimate of compliance based on the maximum position or local stiffness on the way out and with linear estimates of stiffness based on the entire force/motion history.