Matt Strong

Matt Strong

CS PhD at Stanford University

← Paper Digest

Cutaneous Force Feedback as a Sensory Subtraction Technique in Haptics

Domenico Prattichizzo, Claudio Pacchierotti, Giulio Rosati

Seminal August 2011

TL;DR Replace a haptic device's force feedback with fingertip pressure from wearable cable-driven devices ("sensory subtraction") in simulated needle insertion. With 16 users it overshoots the target less than a visual force display, and under a 50 ms delay it stays stable while full haptic feedback oscillates.

What they do

Communication delay can destabilize kinesthetic force feedback in teleoperation. This paper drops the kinesthetic part and keeps only the cutaneous part: wearable devices press on the thumb and index finger pads that hold the handle, with a force similar in intensity and location to what the handle itself would apply. The authors call this sensory subtraction rather than substitution, and test it on simulated needle insertion up to a forbidden-region virtual fixture.

How it works

  • Device. Three small motors on the back of the finger pull cables that press a contact patch onto the finger pad. Here the motors pull together, so it acts as a 1-DoF device.
  • Setup. An Omega 3 clamped to one axis moves the virtual needle. With its motors off it only tracks the hand; force toward -z is rendered on the index finger, toward +z on the thumb.
  • Simulation. A spring-damper tissue (2 N/m, 5 Ns/m) that the needle pierces above 0.1 N, and a 3000 N/m spring for the fixture.
  • Conditions. Visual force bar (VF), full haptic feedback (HF), cutaneous on the holding hand (CF) or on the other hand (CCF).

Results

  • Experiment 1 (16 subjects, 24 insertions each): average and maximum penetration past the fixture ranked HF < CF < CCF < VF, every pair significantly different. Completion time did not differ (P > 0.1).
  • Experiment 2 (32 trials): when the fixture was moved deeper without warning, HF caused significantly larger unwanted needle motion than VF, CF or CCF (P < 0.001).
  • Experiment 3 (10 subjects, 50 ms delay): only HF oscillated. Its peak penetration was significantly larger than CF (P < 0.001) and CCF (P < 0.05), and it took significantly longer than every other mode.

Limitations

Without delay, full haptic feedback still beats cutaneous-only, and realism is weaker; the authors partly attribute CF’s larger penetration to the roughly 45 ms delay of the cutaneous actuators. The task is a 1-DoF insertion into a simple simulated tissue rather than a real teleoperated robot.

Figures

From Figure 3: the operator grasps the haptic handle with thumb and index finger, each wearing a cutaneous device.
From Figure 3: the operator grasps the haptic handle with thumb and index finger, each wearing a cutaneous device.
From Figure 2: the wearable cutaneous device, which applies forces normal to the finger pad.
From Figure 2: the wearable cutaneous device, which applies forces normal to the finger pad.
From Figure 7: Experiment 1, average penetration beyond the virtual fixture (mean and SD) for visual (VF), haptic (HF...
From Figure 7: Experiment 1, average penetration beyond the virtual fixture (mean and SD) for visual (VF), haptic (HF), cutaneous (CF) and contralateral cutaneous (CCF) feedback. Zero means the target depth was hit exactly.
From Figure 13: Experiment 3, maximum penetration beyond the virtual fixture (mean and SD) for each feedback mode wit...
From Figure 13: Experiment 3, maximum penetration beyond the virtual fixture (mean and SD) for each feedback mode with a 50 ms delay in the loop.
Abstract

A novel sensory substitution technique is presented. Kinesthetic and cutaneous force feedback are substituted by cutaneous feedback (CF) only, provided by two wearable devices able to apply forces to the index finger and the thumb, while holding a handle during a teleoperation task. The force pattern, fed back to the user while using the cutaneous devices, is similar, in terms of intensity and area of application, to the cutaneous force pattern applied to the finger pad while interacting with a haptic device providing both cutaneous and kinesthetic force feedback. The pattern generated using the cutaneous devices can be thought as a subtraction between the complete haptic feedback (HF) and the kinesthetic part of it. For this reason, we refer to this approach as sensory subtraction instead of sensory substitution. A needle insertion scenario is considered to validate the approach. The haptic device is connected to a virtual environment simulating a needle insertion task. Experiments show that the perception of inserting a needle using the cutaneous-only force feedback is nearly indistinguishable from the one felt by the user while using both cutaneous and kinesthetic feedback. As most of the sensory substitution approaches, the proposed sensory subtraction technique also has the advantage of not suffering from stability issues of teleoperation systems due, for instance, to communication delays. Moreover, experiments show that the sensory subtraction technique outperforms sensory substitution with more conventional visual feedback (VF).

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