# Feasibility Study of a Gripper with Thermally Controlled Stiffness of Compliant Jaws
## 1 Introduction  
The ability to handle delicate/fragile or small-scale objects is very useful, but challenging, since such materials are either very sensitive to the gripping force or too small to easily control the tiny actuation. Creating a gripper mechanism that applies exactly the right amount of force to grip such an object without breaking it can be done in several ways: one active approach is to use force sensing technology to accurately control actuation displacement [1; 2], and another method is to use preloading displacement to generate a constant force from the jaw mechanism over a relatively large displacement range to accommodate the actuation inaccuracy [3]. The goal of this paper is to design and build a compact handheld compliant gripper that is simple enough to use and is capable of varying gripping force of the same actuation displacement (i.e., varying gripping stiffness) for grasping different objects. Devices like tweezers or tongs would be unsuitable, because they require the hand to coordinate with sensory feedback (sight and touch). In the case of objects that are very small or very brittle, sensory feedback is imperceptible [3].  
Therefore, it is constantly required to find a way to regulate the gripping force so that it does not exceed the breaking force of the object. Using the electrothermal properties of materials to actuate micro-grippers in MEMS (micro-electromechanical systems) has been reported in [4; 5; 6] for biological manipulations, where the motion of two jaws is actively controlled by the thermal deformation of the functional material (polymer with very high thermal expansion coefficient). Electrothermal control-based shape memory alloy actuators were also proposed for force tracking control of a flexible gripper in [7]. Motivated by those works, the alternative method adopted in this paper also involves the use of material thermal properties for gripper design. However, the proposed method aims to manipulate (reduce) the gripping stiffness of the gripper's jaw (rather than actively controlling the motion of jaws by thermal actuation), which allows the human user to roughly control the gripping force of the system based on the strength property of the objects. In such a way, there is no need to have precise control/sensing of the force applied on the object, but this is enough to avoid (or reduce the risk of) exceeding the breaking force.  
The design of such a desired gripper is demonstrated in Figure 1. The compliant gripper is a symmetrical design, where two jaws have rectilinear rigid-body motion towards the opposite direction to grip the object. Each half of this gripper is composed of two major compliant mechanisms: one is the parallel mechanism (actuation is imposed upon it) for guiding the jaw, so that the two jaws can only have parallel motions without producing slipping force on the object [2]; The other part is the jaw mechanism itself, which uses the symmetrical compound parallelogram mechanism [8] to change gripping stiffness. To control the stiffness, a simple heating coil is wrapped around the compliant beams of the jaw mechanism to dissipate heat for the beams. When a voltage is applied across the heating coil, the beam is heated up (without causing motion of the jaws), therefore decreasing the stiffness and allowing delicate or small-scale objects to be gripped.