# Feasibility Study of a Gripper with Thermally Controlled Stiffness of Compliant Jaws
## 3 Feasibility Study
### Prototype Fabrication and Testing  
There are three materials for the coil, jaw mechanism, and guiding mechanism, respectively. The heating coil material is selected to be annealed stainless steel wire with 0.635 mm diameter, which is able to produce a high enough temperature change. The jaw mechanism's material is polycarbonate, with a Young's modulus of 2.3 GPa, yield strength larger than 60 MPa, a relatively high coefficient of thermal expansion of 70 \(\times\) 10\({}^{-6}\) K\({}^{-1}\) for easy thermal control in changing gripping force/stiffness, and which is not conductive. The material for the guiding mechanism and other parts was aluminum alloy 1060, due to its suitability in compliant mechanisms.  
The main parts were fabricated by precise CNC (computer numerical control) milling machining, and the coils were wound by hand with 23 turns along each beam in the jaw mechanism. The final prototype is shown in Figure 2 with an overall planar dimension of 9 cm \(\times\) 15 cm, which is compact enough to be operated by hand. Note that an asymmetrical assembled prototype (with one jaw mechanism) was manufactured for simplicity, although it is not consistent with the original symmetrical and monolithic one (with two jaw mechanisms, as proposed in Figure 1). However, this does not affect the verification of the feasibility study in this paper. The beam length (\(L\)) and in-plane thickness (\(T\)) of the jaw mechanisms are 20 mm and 1 mm, respectively, with the out-of-plane thickness \(U\) = 5 mm. There was 3 mm of space between the two gripping surfaces in the original non-deformed configuration. The gripper prototype consisted of five separately fabricated components: the jaw mechanism (polycarbonate), the frame with guiding mechanism (aluminum alloy), top plate (aluminum alloy), bottom plate (aluminum alloy), and heating coil (annealed stainless steel). The jaw mechanism was arranged in the middle of the top and bottom plates, in the same plane as the frame with guiding mechanism. Bolts were used for assembly, and electrical tape was used to insulate the coil from the conductive plates.  
Changing the jaw stiffness is done by simply turning on the heating coil circuit that is powered by an 8 V battery, and then waiting for some time until steady status. For verification of the main principle of the device, it must be shown that the application of heat is actually able to effectively reduce the stiffness of the jaw mechanism. The jaw mechanism with and without heat was tested in a single-axis by a single-axis TA.HD Plus Texture Analyzer. A set of displacements were applied in the jaw mechanism, and the corresponding reaction forces were recorded.  
Figure 3 shows the comparisons of two (average) testing results with and without heat for the case study. It has been proven that the heat can effectively reduce the gripper force/stiffness of the jaw mechanism based on current/voltage control, as expected. If the breaking force of the intended object is 10 N, the maximal acceptable displacement of the jaw is about 0.65 mm without heat control, and is close to 1 mm with heat control. Therefore, the heat control can better compensate for the actuation inaccuracy. If a lower gripping stiffness/force is required, a higher voltage should be applied, as long as the beam does not have buckling. Additionally, a hysteresis loop between loading and unloading testing has been observed for the polycarbonate material, where the heat can amplify the hysteresis phenomenon.  
Figure 2: Fully assembled asymmetrical gripper shown next to a hand for reference (**above**); top plate removed, revealing the heating coil (**below**).