Improvement and application of variable-sensitivity force sensors using shape-memory polymer

This study presents a variable-sensitivity force sensor utilizing a shape-memory polymer (SMP) sheet embedded with an electric heating wire, whose stiffness fluctuates with temperature changes. Therefore, replacing the force sensor is also pointless, as both the measurement range and sensitivity can be adjusted according to the target of measurement. SMP is frequently characterized as a reversible biphasic structure including a low-temperature "glassy" hard phase and a high-temperature "rubbery" soft phase. This work illustrates a sensor that measures applied force using a strain gauge affixed to a beam of SMP. Consequently, the sensitivity and measuring range of the force can be adjusted based on temperature. In our prior investigation, we assessed an arrangement where a beam is fixed at both ends. However, due to the discrepancy between the measured and predicted values, we have enhanced the design of the force sensor considered in this study to broaden its applicability. Furthermore, we have utilized this improved version to measure the compressive elastic modulus and grip force to assess its superiority. In addition, we compared the calculation method of force with or without the consideration of the viscoelasticity for the grip force measurement. The results validate that the improved sensor can also change the range of the measured force depending on the temperature. Moreover, the results confirm that the compressive elastic modulus and the grip force can be measured by the proposed sensor. The application of a transfer function model in conjunction with a viscoelastic model demonstrated that a more precise measurement of grip force could be achieved by accounting for the viscoelasticity of the SMP.