Friction, not texture, dictates grip forces used during object manipulation

Abstract

1. Three men and seven women, 25-40 yr of age, were asked to use the thumb and index fingers to grasp, lift, and hold the armature of a linear motor generating a 2.0-N opposing force (simulating an object weighing approximately 200 g) for 2 s. The surface in contact with the fingers was composed of smooth or polyamide plastic etched with 1.0-mm high Braille beads separated at 2.0- or 3.0-mm intervals measured from apex to apex. The surfaces were left either untreated or coated with talc, water, or sucrose films designed to change the coefficient of friction with the skin. Talc reduced the coefficient of friction, whereas water and sucrose both increased the friction against the skin. In all, 12 surface conditions were used to evaluate the effects of texture and friction on the grip force during lifting and holding. 2. For all subjects the inverse coefficient of friction was associated with proportionately scaled increases in grip force, regardless of surface texture. The peak lifting force as well as the static force used to hold the object stationary were significantly correlated with the inverse of the coefficient of friction. When coatings were applied to dissimilar surface textures to produce similar coefficients of friction, the grip force profiles were nearly identical. When strong adhesives increased the friction of the smooth surface compared with textured surfaces, grip forces decreased as friction increased. That is, although the untreated smooth surface had less friction than either of the two textured surfaces, the addition of sucrose increased the smooth surface friction to a higher level than either of the similarly treated textured surfaces. As a result, the effect of surface friction could be dissociated from the effect of either surface texture or coating. Friction appears to be a more important factor in determining the grip force than either texture or surface films at least for the range of textures and coatings examined in this study.

Maintaining rotational equilibrium during object manipulation: linear behavior of a highly non-linear system

“…In particular, G is modulated by the inertial forces acting during lifting an object (Johansson and Westling 1984), shaking and point-to-point arm movements Wing 1993, 1995;Tresilian et al 1993;Flanagan and Tresilian 1994;Kinoshita et al 1996) and during locomotion (Gysin et al 2003). It is also modulated by the weight of the object (Johansson and Westling 1984;Winstein et al 1991), abrupt load perturbations (Cole and Abbs 1988;Eliasson et al 1995, Serrien et al 1999, friction conditions (Cole and Johansson 1993;Cadoret and Smith 1996;Burstedt et al 1999), tangential torques (Kinoshita et al 1997) and gravity changes during parabolic flights (McIntyre et al 1998;Hermsdorfer et al 1999;Augurelle et al 2003). G has been assumed to be adjusted to expected L (Johansson and Westling 1984;Flanagan and Wing 1995).…”

Visual and Somatosensory Information about Object Shape Control Manipulative Fingertip Forces

“…At the same time, excessive normal (grip) forces are avoided that may cause unnecessary fatigue and may crush fragile objects or injure the hand. Accordingly, we automatically adjust the ratio between the normal and tangential forces to the frictional status at the digit-object interface such that an adequate safety margin against frictional slips is maintained during different frictional conditions (Johansson and Westling, 1984a;Westling and Johansson, 1984;Edin et al, 1992;Cole and Johansson, 1993;Forssberg et al, 1995;Cadoret and Smith, 1996). A sensorimotor memory related to the frictional experiences in previous interactions with the object determines the force balance used (Johansson and Westling, 1984a;Edin et al, 1992) according to an "anticipatory parameter control" policy (for an overview, see Johansson, 1996).…”

An MR-Compatible Haptic Interface With Seven Degrees of Freedom

“…With F max = 1.96 N, a friction coefficient of glass in a human hand μ glass = 1.1 [16], a safety margin F safety = 3 N, and a slope of s ≈ 3 [15], we obtain a grip force of F grip = 8.3 N.…”

“…From literature, we know that the correlation of the static grip force with the object weight is approximately linear [14]. Also, the correlation of the maximum grip force with the inverse coefficient of friction μ is approximately linear with a safety margin F safety [15]. Hence, the grip force can be approximated by…”