Modeling of Robotic Hand for Dynamic Simulation

Abstract: KTHand is a robot hand designed within a doctoral thesis by Johan Tegin. KTHand is meant to be a simple construction, able to be produced for a low cost but still be functional thanks to feedback from tactile and position sensors. The mechanical construction of KTHand is based on three identical fingers, corresponding index finger, middle finger, and a thumb. Each finger consists of three movable links, called phalanges, connected by a leaf spring on the top. For the force actuation, a fishing line acting as a tendon is running through ducts in the phalanges. Each tendon is powered by a direct current motor in the hand, and this gives a finger three degrees of freedom (D.O.F.) with only one actuator, i.e. an under-actuated finger. To evaluate KTHand – simulations in the open-source software GraspIt! have been carried out. These simulations have been more or less successful, and a wish for simulation in another program has arisen. MSC ADAMS is a widely used and verified commercial program which is used for setting up advanced physical multibody simulations. ADAMS is fairly untested for the purpose of simulating robot hands and that is why it is of interest to see whether it is possible to successfully simulate the grasp dynamics of a robot hand, which is the topic for this thesis.As a starting point for the model design in ADAMS, a previously developed CAD model has been used since ADAMS provides the ability to import CAD files. To decrease the level of complexity, a number of simplifications have been done to the CAD model – unnecessary geometry such as screws and screw holes have been removed and curved lines in the geometry of the hand have been replaced by straight lines.After the simplified model was imported in ADAMS – joints, springs and contact conditions have been defined to obtain ranges of motion (R.O.M.) analogous to those of the physical prototype. To imitate the tendon which actuates a finger, a number of point forces have been defined. These point forces represent the reactive forces acting on the phalanges from the tendon. To perform a grasp simulation a number of different setups were used, each one with a different geometrical object to be grasped. Contact conditions between the hand and the objects were defined and the simulations were evaluated. The model can be used to simulate contact forces and how the hand interacts with different geometries. The conclusion of the simulations is that grasps based on shape give satisfying results while grasps based on friction force are of less accuracy. Friction conditions in ADAMS have turned out to be of low precision, and thus the grips based on friction become unrealistic.