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Predicting the effect of surface texture on the qualitative form of prehension.

Flatters IJ, Otten L, Witvliet A, Henson B, Holt RJ, Culmer P, Bingham GP, Wilkie RM, Mon-Williams M - PLoS ONE (2012)

Bottom Line: Twelve participants reached to lift objects from a tabletop.A finger friction rig was used to calculate the coefficients of friction for the objects and these calculations showed that the area available for a stable grasp (the 'functional grasp surface size') increased with surface friction coefficient.Thus, knowledge of functional grasp surface size is required to predict the probability of observing a given qualitative form of grasping in human prehensile behaviour.

View Article: PubMed Central - PubMed

Affiliation: Institute of Psychological Sciences, University of Leeds, Leeds, West Yorkshire, United Kingdom.

ABSTRACT
Reach-to-grasp movements change quantitatively in a lawful (i.e. predictable) manner with changes in object properties. We explored whether altering object texture would produce qualitative changes in the form of the precontact movement patterns. Twelve participants reached to lift objects from a tabletop. Nine objects were produced, each with one of three grip surface textures (high-friction, medium-friction and low-friction) and one of three widths (50 mm, 70 mm and 90 mm). Each object was placed at three distances (100 mm, 300 mm and 500 mm), representing a total of 27 trial conditions. We observed two distinct movement patterns across all trials--participants either: (i) brought their arm to a stop, secured the object and lifted it from the tabletop; or (ii) grasped the object 'on-the-fly', so it was secured in the hand while the arm was moving. A majority of grasps were on-the-fly when the texture was high-friction and none when the object was low-friction, with medium-friction producing an intermediate proportion. Previous research has shown that the probability of on-the-fly behaviour is a function of grasp surface accuracy constraints. A finger friction rig was used to calculate the coefficients of friction for the objects and these calculations showed that the area available for a stable grasp (the 'functional grasp surface size') increased with surface friction coefficient. Thus, knowledge of functional grasp surface size is required to predict the probability of observing a given qualitative form of grasping in human prehensile behaviour.

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Object geometric properties friction-dependant functional grip area.Upper Geometric variation in stimulus sizes: Grip surface width ‘A’, the distance between the spherical surface centre-points ‘B’ and support base width ‘C’ were varied as discussed in the Method section. Lower a) Manually securing an object requires the frictional force to be greater than the tangential component of object weight at the interface between fingertip and object. A curved surface results in a normal reaction force direction (RN) unique to the point at which the object is grasped. Fearing [14] demonstrated that, for a stable grasp, the grip conditions should satisfy: tan−1/Ft//Fn<tan−1μ or μFn>/Ft/. For a stable lift, fingertip force should be applied within an angle of φs relative to the normal reaction force (RN), where: φs = tan−1μs. Extending this relationship in the direction of all tangential friction force directions generates a cone of friction of half-angle φs and cone angle ψ where: ψ = 2 φs. b) As force is applied to the curved surface at a distance dLIM from the centreline of the radius, then the force is at an angle α to the surface normal. When α = φs the force lies at the limit of the cone of friction. An increase in d results in the force lying outside the cone of friction and unstable grasp. Thus φs, and dLIM are linked to the coefficient of static friction μs such that an increase in μs extends the functional area which can be grasped to achieve a stable grasp.
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pone-0032770-g002: Object geometric properties friction-dependant functional grip area.Upper Geometric variation in stimulus sizes: Grip surface width ‘A’, the distance between the spherical surface centre-points ‘B’ and support base width ‘C’ were varied as discussed in the Method section. Lower a) Manually securing an object requires the frictional force to be greater than the tangential component of object weight at the interface between fingertip and object. A curved surface results in a normal reaction force direction (RN) unique to the point at which the object is grasped. Fearing [14] demonstrated that, for a stable grasp, the grip conditions should satisfy: tan−1/Ft//Fn<tan−1μ or μFn>/Ft/. For a stable lift, fingertip force should be applied within an angle of φs relative to the normal reaction force (RN), where: φs = tan−1μs. Extending this relationship in the direction of all tangential friction force directions generates a cone of friction of half-angle φs and cone angle ψ where: ψ = 2 φs. b) As force is applied to the curved surface at a distance dLIM from the centreline of the radius, then the force is at an angle α to the surface normal. When α = φs the force lies at the limit of the cone of friction. An increase in d results in the force lying outside the cone of friction and unstable grasp. Thus φs, and dLIM are linked to the coefficient of static friction μs such that an increase in μs extends the functional area which can be grasped to achieve a stable grasp.

Mentions: Mon-Williams and Bingham [8] have shown that two distinct movement patterns can emerge when participants are asked to reach-and-grasp an object and lift it off a tabletop (see Figure 1). In some cases, participants stop their arm moving forward before the fingers make contact with the object, adjust finger position and then grasp and lift (so-called ‘stop’ movements). In other cases, participants contact the object whilst the hand is still moving (so-called ‘on-the-fly’ movements). If the safety margins of the task decrease (e.g. by making the object wider and closer to the maximum grasp aperture) then the proportion of on-the-fly movements also decreases. This observation suggests that the probability of observing a particular movement pattern is affected by the margins of safety. On these grounds, we hypothesised that changes in an object's surface texture might alter the proportion of on-the-fly movements, because altering texture affects the safety margins (see Figure 2, Lower Panel).


Predicting the effect of surface texture on the qualitative form of prehension.

Flatters IJ, Otten L, Witvliet A, Henson B, Holt RJ, Culmer P, Bingham GP, Wilkie RM, Mon-Williams M - PLoS ONE (2012)

Object geometric properties friction-dependant functional grip area.Upper Geometric variation in stimulus sizes: Grip surface width ‘A’, the distance between the spherical surface centre-points ‘B’ and support base width ‘C’ were varied as discussed in the Method section. Lower a) Manually securing an object requires the frictional force to be greater than the tangential component of object weight at the interface between fingertip and object. A curved surface results in a normal reaction force direction (RN) unique to the point at which the object is grasped. Fearing [14] demonstrated that, for a stable grasp, the grip conditions should satisfy: tan−1/Ft//Fn<tan−1μ or μFn>/Ft/. For a stable lift, fingertip force should be applied within an angle of φs relative to the normal reaction force (RN), where: φs = tan−1μs. Extending this relationship in the direction of all tangential friction force directions generates a cone of friction of half-angle φs and cone angle ψ where: ψ = 2 φs. b) As force is applied to the curved surface at a distance dLIM from the centreline of the radius, then the force is at an angle α to the surface normal. When α = φs the force lies at the limit of the cone of friction. An increase in d results in the force lying outside the cone of friction and unstable grasp. Thus φs, and dLIM are linked to the coefficient of static friction μs such that an increase in μs extends the functional area which can be grasped to achieve a stable grasp.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3293844&req=5

pone-0032770-g002: Object geometric properties friction-dependant functional grip area.Upper Geometric variation in stimulus sizes: Grip surface width ‘A’, the distance between the spherical surface centre-points ‘B’ and support base width ‘C’ were varied as discussed in the Method section. Lower a) Manually securing an object requires the frictional force to be greater than the tangential component of object weight at the interface between fingertip and object. A curved surface results in a normal reaction force direction (RN) unique to the point at which the object is grasped. Fearing [14] demonstrated that, for a stable grasp, the grip conditions should satisfy: tan−1/Ft//Fn<tan−1μ or μFn>/Ft/. For a stable lift, fingertip force should be applied within an angle of φs relative to the normal reaction force (RN), where: φs = tan−1μs. Extending this relationship in the direction of all tangential friction force directions generates a cone of friction of half-angle φs and cone angle ψ where: ψ = 2 φs. b) As force is applied to the curved surface at a distance dLIM from the centreline of the radius, then the force is at an angle α to the surface normal. When α = φs the force lies at the limit of the cone of friction. An increase in d results in the force lying outside the cone of friction and unstable grasp. Thus φs, and dLIM are linked to the coefficient of static friction μs such that an increase in μs extends the functional area which can be grasped to achieve a stable grasp.
Mentions: Mon-Williams and Bingham [8] have shown that two distinct movement patterns can emerge when participants are asked to reach-and-grasp an object and lift it off a tabletop (see Figure 1). In some cases, participants stop their arm moving forward before the fingers make contact with the object, adjust finger position and then grasp and lift (so-called ‘stop’ movements). In other cases, participants contact the object whilst the hand is still moving (so-called ‘on-the-fly’ movements). If the safety margins of the task decrease (e.g. by making the object wider and closer to the maximum grasp aperture) then the proportion of on-the-fly movements also decreases. This observation suggests that the probability of observing a particular movement pattern is affected by the margins of safety. On these grounds, we hypothesised that changes in an object's surface texture might alter the proportion of on-the-fly movements, because altering texture affects the safety margins (see Figure 2, Lower Panel).

Bottom Line: Twelve participants reached to lift objects from a tabletop.A finger friction rig was used to calculate the coefficients of friction for the objects and these calculations showed that the area available for a stable grasp (the 'functional grasp surface size') increased with surface friction coefficient.Thus, knowledge of functional grasp surface size is required to predict the probability of observing a given qualitative form of grasping in human prehensile behaviour.

View Article: PubMed Central - PubMed

Affiliation: Institute of Psychological Sciences, University of Leeds, Leeds, West Yorkshire, United Kingdom.

ABSTRACT
Reach-to-grasp movements change quantitatively in a lawful (i.e. predictable) manner with changes in object properties. We explored whether altering object texture would produce qualitative changes in the form of the precontact movement patterns. Twelve participants reached to lift objects from a tabletop. Nine objects were produced, each with one of three grip surface textures (high-friction, medium-friction and low-friction) and one of three widths (50 mm, 70 mm and 90 mm). Each object was placed at three distances (100 mm, 300 mm and 500 mm), representing a total of 27 trial conditions. We observed two distinct movement patterns across all trials--participants either: (i) brought their arm to a stop, secured the object and lifted it from the tabletop; or (ii) grasped the object 'on-the-fly', so it was secured in the hand while the arm was moving. A majority of grasps were on-the-fly when the texture was high-friction and none when the object was low-friction, with medium-friction producing an intermediate proportion. Previous research has shown that the probability of on-the-fly behaviour is a function of grasp surface accuracy constraints. A finger friction rig was used to calculate the coefficients of friction for the objects and these calculations showed that the area available for a stable grasp (the 'functional grasp surface size') increased with surface friction coefficient. Thus, knowledge of functional grasp surface size is required to predict the probability of observing a given qualitative form of grasping in human prehensile behaviour.

Show MeSH
Related in: MedlinePlus