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(A) 3D contact model. (B) The sketch of 3D reaction forces of the biomimetic tarsus devices, where
, 
and 
are contact points; 
and 
are the maximum external lateral and normal forces; 
and 
are the local support forces and friction forces at the claw tips. Subscripts l and r mean left and right, respectively. θ and γ are the friction angle and the projected angle between the lines running through the claw tips to the sphere centre, respectively.
