Tangential torque effects on the control of grip forces when holding objects with a precision grip

Area of Science:

• Biomechanics
• Human Factors Engineering
• Robotics

Background:

• Object manipulation involves complex forces on fingertips, including tangential torques and forces.
• Understanding these forces is crucial for grasp stability and preventing slips.
• Previous research has not fully elucidated the combined effects of tangential torques and forces on human grasp control.

Purpose of the Study:

• To investigate the impact of tangential torques and tangential forces on slip force and employed normal force during object manipulation.
• To determine how humans adjust grip to maintain stability under varying frictional conditions and applied torques/forces.
• To develop a predictive model for slip force based on these parameters.

Main Methods:

• Subjects held objects using index finger and thumb, with varying tangential torques (0-50 N x mm) and tangential forces (0-3.4 N).
• Grasp surfaces varied in friction (rayon, suede, sandpaper).
• Minimum normal forces to prevent slips (slip force) and employed normal forces were measured.

Main Results:

• Both slip force and employed normal force increased proportionally with tangential torque and tangential force.
• A significant interaction was observed: increased tangential force reduced the impact of tangential torque on slip force.
• A mathematical model accurately predicted slip force based on torque, force, and friction coefficient.

Conclusions:

• Tangential torque significantly influences the normal force required for stable grasp.
• Humans precisely adjust grip force, considering torque, force, their interaction, and friction.
• Safety margins against slips are generally small and consistent, highlighting efficient grasp control.