Evaluation of force and torque magnitude discrimination thresholds on the human hand-arm system

Abstract

This article reports on experiments about haptic perception aimed at measuring the force/torque differential thresholds applied to the hand-arm system. The experimental work analyzes how force is sent back to the user by means of a 6 degrees-of-freedom haptic device. Our findings on force perception indicate that the just-noticeable-difference is generally higher than previously reported in the literature and not constant along the stimulus continuum. We found evidence that the thresholds change also among the different directions. Furthermore, asymmetries in force perceptions, which were not described in previous reports, can be evinced for most of the directions. These findings support our claim that human beings perceive forces differently along different directions, thus suggesting that perception can also be enhanced by suitable signal processing, that is, with a manipulation of the force signal before it reaches the haptic device. We think that the improvement of the user perception can have a great impact in many applications and in particular we are focusing on surgical teleoperation scenarios.

Anisotropy in the Haptic Perception of Force Direction and Magnitude

“…No discrimination difference between the axes was found. A similar result was found using a range of force magnitudes in a discrimination task where subjects were holding a handle of a haptic device (Weber fraction: 15%) [10]. So, for the cardinal directions the mean Weber fraction for force magnitude seems to be 13-15%. Another parameter that can be investigated using discrimination experiments is the difference between force magnitude perception at different directions.…”

“…When the development of haptic devices began, research changed from looking only at force perception in the direction of gravity [4], [5], [6] to looking at it in three dimensions (e.g. [7], [8], [9], [10]). This also changed the term 'weight perception' to 'force perception'.…”

Force Discrimination Ability of the Human Hand near Absolute Threshold for the Design of Force Feedback Systems in Teleoperations

“…Another study fitted exponential functions to its experimental data to model the JND versus reference force in each direction (Vicentini, Galvan, Botturi, & Fiorini, 2010;Botturi, Galvan, Vicentini, & Secchi, 2009):…”

“…Nevertheless, all of the cited works and many other psychophysical studies such as Bau, Poupyrev, Israr, & Harrison (2010) estimated JNDs for forces big enough to follow Weber's law. Vicentini, Galvan, Botturi, & Fiorini, (2010) measured JNDs for different forces in a wide range and reported JND values of about 15% for forces that follow Weber's law, and JNDs of 32% and 23% for 0.5 N and 0.8 N. It was also postulated that the perception is asymmetric and varies in different directions. Another study (Hatzfeld & Werthschützky, 2012) measured JNDs of vibro-tactile forces near the absolute threshold of perception at different frequencies between 5 to 1000 Hz.…”

“…The closest studies to the current study (Höver, Luca, & Harders, 2010;Vicentini, Galvan, Botturi, & Fiorini, 2010) investigated sensitivity to force differences over a wide range of force magnitudes. These studies measured only one or two JNDs for forces below 1 N; however, this information might not be sufficient to capture all the nonlinearity inherent in the JND-stimulus relationship.…”