The Minimally Invasive Manipulator (MIM) copies the movements of the hands of the surgeon to the tip of the instrument inside the operating area. The movement is mechanically copied using cables and pulleys. When the surgeon grasps and holds an object, the friction inside the MIM increases and distorts the force feedback in the remaining degrees of freedom. The increased friction is the result of the reaction forces on the cables and pulleys due to the required pinch force to hold the object.
The goal of this report is to present two designs for a bi-stable compliant grasper. The grasper will have two stable positions: open and closed. In those positions there is no need for an actuation force from the surgeon. When the grasper is closed, it will produce enough pinch force to hold an object.
The first design is a compliant grasper combined with bi-stable beam. Both the grasper and bi-stable element will be a planer design. A large scale prototype is presented and evaluated. The prototype showed bi-stable behavior over a range of 8 mm. Using finite element modeling, dimensions for a bi-stable beam suitable for the MIM are found.
The second design uses a disc spring as bi-stable element. This design is a 3D design, and is on the scale of the MIM. An ANSYS model and a database were validated. Using the model and database disc spring with suitable dimensions for the MIM scale was found. Two disc springs in parallel were needed to make a bi-stable compliant grasper.
Both the bi-stable beam and the disc spring can be used in the design for a bi-stable grasper. The bi-stable beam is more effectively creating negative stiffness inside a circular space. As a result, it should be able to produce more force and/or stroke in the same volume as a disc spring. This makes the bi-stable beam more suitable for use in a bi-stable grasper than a disc spring.