Reflex Control of the Pisa/IIT SoftHand during Object Slippage

In this work, to guarantee the Pisa/IIT SoftHand’s

grasp robustness against slippage, three reflex control modes,

namely Current, Pose and Impedance, are implemented and

experimentally evaluated. Towards this objective, ThimbleSense

fingertip sensors are designed and integrated into the thumb

and middle fingers of the SoftHand for real-time detection and

control of the slippage. Current reflex regulates the restoring

grasp forces of the hand by modulating the motor’s current

profile according to an update law. Pose and Impedance reflex

modes instead replicate this behaviour by implementing an

impedance control scheme. The difference between the two

latter is that the stiffness gain in Impedance reflex mode is

being varied in addition to the hand pose, as a function of

the slippage on the fingertips. Experimental setup also includes

a seven degrees-of-freedom robotic arm to realize consistent

trajectories (e.g. lifting) among three control modes for the sake

of comparison. Different test objects are considered to evaluate

the efficacy of the proposed reflex modes in our experimental

setup. Results suggest that task-appropriate restoring forces

can be achieved using Impedance reflex due to its capability

in demonstrating instantaneous and rather smooth reflexive

behaviour during slippage. Preliminary experiments on five

healthy human subjects provide evidence on the similarity of the

control concepts exploited by the humans and the one realized

by the Impedance reflex, highlighting its potential in prosthetic

applications.