HAND PROSTHESIS WITH VOLUNTARY OPENING
Underatuated robot hands are capable of grasping a wide variety of objects due to their ability to perfectly adapt to various shapes. These mechanisms, by definition, only require a limited number of actuators and are thus light-weight. These two advantages make under-actuated robotic hands ideal replacements for conventional hand prostheses.
Tendon-Driven Hand
The application of various principles of underactuation led to the design of an auto-adaptive tendon-driven hand. Just as the human hand, the fingers have three phalanges linked together with cylindrical joints. A tendon-driven mechanism allows the fingers to close in a natural manner. Each finger has a single actuator for three degrees of freedom (DOF). These are thus under-actuated and in this way obtain an adaptable ability. A fourth degree of freedom is added to the thumb so as to reproduce more exactly the behaviour of the human hand. A differential mechanism provides a link between the fingers and the thumb, allowing them to close under the influence of a single force. There is thus also an under-actuation between the fingers. The entire hand has a total of 17 DOFs for a single actuator. One thus obtains a light-weight mechanism which is extremely easy to use. By applying a tension on the cable motor, the hand closes on an object in a natural manner, irrespective of the shape of the object. Internal springs ensure that the fingers return to their initial position in the absence of tension in the cable.
Modelling has enabled an optimization of various design paramaters and a CAD model, presented in Figure 1, was prepared. A first prototype, built in the laboratory using rapid prototyping, is shown in Figure 2.
Selected Photos
Several photos of the tendon-driven hand are provided below. The images can be enlarged by clicking directly on the photos.
Video Clip
The tendon-driven hand can grasp objects of various shapes and textures. The following video sequence shows several of the possible grasps.
Prothesis
Following the success of the tendon-driven hand described above, the project has now been oriented towards the creation of a hand prothesis which can satisfy the needs of a person who has had an amputation. This second version is actuated by the user's body force. A dorsal harness, similar to the one used by amputees, was built in collaboration with partners at the l'Institut en réadaptation en déficience physique de Québec, IRDPQ (Institute of readaptation in physical deficiencies of the City of Quebec). This harness is composed of a loop in the shape of the number eight and enables the user to transfer to the prosthesis any movement he produces with his shoulders. A force of up to 90 Newton can be applied by a man of average strength.
In contrast to the previous version, the prothesis shown in Figure 3 has a voluntary opening. An internal spring maintains the hand in a pinched grasp. The application of an external force enables the hand to open in a natural manner. This behaviour facilitates the task of the user by allowing him to actuate the prothesis only when he wishes to open the hand. He does thus not need to exert a constant force to maintain his grasp of an object.
Selected Photos
Several photos of the prosthesis are provided below. The images can be enlarged by clicking directly on the photos.
Video Clip
The following video sequence show the operation of the prosthesis and the harness, and show several of the possible grasps.
In short...
The use of under-actuation within a prosthesis is an innovative element and introduces a host of new possibilities. This will significantly increase the performance of hand prostheses with respect to conventional prostheses which often have but one DOF and are not very flexibles. The variety of objects which can be grasped with this prosthesis and the natural movement obtained confer a definite advantage to this prosthesis.