Sensing small interaction forces through proprioception

Abstract

Understanding the human motor control strategy during physical interaction tasks is crucial for developing future robots for physical human-robot interaction (pHRI). In physical human-human interaction (pHHI), small interaction forces are known to convey their intent between the partners for effective motor communication. The aim of this work is to investigate what affects the human's sensitivity to the externally applied interaction forces. The hypothesis is that one way the small interaction forces are sensed is through the movement of the arm and the resulting proprioceptive signals. A pHRI setup was used to provide small interaction forces to the hand of seated participants in one of four directions, while the participants were asked to identify the direction of the push while blindfolded. The result shows that participants' ability to correctly report the direction of the interaction force was lower with low interaction force as well as with high muscle contraction. The sensitivity to the interaction force direction increased with the radial displacement of the participant's hand from the initial position: the further they moved the more correct their responses were. It was also observed that the estimated stiffness of the arm varies with the level of muscle contraction and robot interaction force.

Figure 1

Percentage of correct responses varies with the level of muscle contraction where correctness was maximum (~ 100%) at high robot force with low muscle contraction (RH*ML) condition (a) experimental results (different colors represent different participants) (b) ANOVA analysis of the percentage of correct responses.

Figure 2

(a) Percentage of correct responses increases with radial displacement from the initial position and was highest during high robot interaction force with low muscle contraction (RH*ML, red) condition, during which the radial displacement was also high. Linear regression fit gives R²  =  0.228 (b) mean and standard deviation of logarithmic radial displacement of all correct response trials (c) was higher than incorrect response trials (I) (p < 0.001).

Figure 3

Norm of the arm stiffness increases with the increase of muscle contraction and robot interaction force. The average stiffness was highest during high robot interaction force with high muscle contraction (RH*MH) condition (a) experimental results (different colors represent different participants) (b) ANOVA analysis of the stiffness norm.

Figure 4

Seated human posture during the experiment with a haptic robot (a) experimental setup (b) top view of the experimental setup (c) applied robot interaction force profile for a trial of ~ 5 s.