Force prediction in the cylindrical grip for a model of hand prosthesis

The aim of this paper is to present an analytical method of calculating forces acting on the thumb, index, middle finger, and metacarpal part of a hand prosthesis in a cylindrical grip. This prehension pattern represents a common operation of grabbing and manipulating everyday life objects. The design process assumed that such a prosthesis would have 5 fully operating fingers and 18 total degrees of freedom: three for each finger including the thumb, and another three for the wrist. The assumed load was 1 kg and the diameter equaled 70 mm, representing a water bottle. The method was based on analytical mechanics and as opposed to experiments or numerical methods does not require many resources. The calculations involved solving a system with seven unknown forces using an equilibrium equation for forces and moments in all three axes. The resulting equations were presented in a matrix form and solved using MATLAB software. The validation of the method with an experiment using FSR sensors and comparing it to other reports showed differences in index and middle finger involvement. However, the total sum of forces was similar, therefore it is reasoned that the grip can be performed and the prediction was accurate for the thumb and metacarpal. When using the model, the friction coefficient must be chosen with a safe margin as it influences the grip force. The presented method can be used for other models and designs by inserting their dimensions into the equations and solving them numerically to obtain forces useful in mechatronics design.