Background and purpose

Soft actuators are characterized by high safety and shape adaptability due to their high softness. However, their low rigidity makes them unsuitable for tasks requiring high load capacity, such as grasping and manipulating heavy objects. Therefore, actuators with stiffness-adjustable materials are being developed [1][2]. They can change their own stiffness and improve their load capacity while maintaining the shape adaptability that is a strong point of soft actuators. The purpose of this study is to fabricate a stiffness-adjustable soft actuator using a normal FDM 3D printer.

Structure

The actuator to be developed consists of a conductive material, a flexible material, and SMP (Shape Memory Polymer). The actuator function is achieved by a flexible material with a bellows structure on one side, which is curved by air pressure. The conductive material is used to heat the SMP which becomes less rigid upon heating. The stiffness of the actuator is variable, allowing the actuator to be driven in a low-stiffness state and then made stiff to maintain the curvature.

Experiments

The actuator consists of a conductive material in the shape of a U, SMP in the shape of a plate, and a flexible material in the shape of a bellows actuator.　By applying electric power of 90 W to the conductive material, the thermal characteristics were investigated by using thermal sensor experimentally. Figure 1 illustrates the temperature profile of the actuator, while Figure 2 depicts the actuator's appearance in its curved posture. After 60 seconds, it became 65.1 °C from 18.6 °C which is the room temperature. It is enough high to be rubbery state of the SMP, and under this condition, pneumatic pressure was applied up to 400 kPa, resulting in a curvature of 18.3 m^-1 at maximum. Additionally, the driving characteristics were investigated in a high stiffness state without applying power. The result was 1.32 m^-1 at 400 kPa, confirming that different curving quantity can be achieved at different stiffness levels. Furthermore, the stiffness was varied after curving, and the tip force was measured. When the load cell was pushed down the actuator’s tip to be the curvature of 10.6 m^-1 from the maximum curvature while maintaining the rubbery state by application of electric power, the reaction force was 125.2mN. Conversely, when the actuator became rigid state by stopping the application of electric power and cooling naturally to the room temperature, the tip force was 140.5mN.

The developed actuator can be curved at low stiffness condition and its stiffness can be higher with maintaining the deformation state. Therefore, the actuator is expected to be applied to robot hands capable of grasping heavy objects.

Acknowledgments

This study was partly supported by JSPS KAKENHI Grant Number JP23K03644 and JKA through its promotion funds from KEIRIN RACE

References

[1] Takashima, K.; Rossiter, J.; Mukai, T.; McKibben Artificial Muscle Using Shape-memory Polymer. Sensors and Actuators A, 2010, Vol. 164, pp. 116-124.

[2] Zhang, Y.; Zhang, N.; Hingorani, H.; Ding, N.; Wang, D.; Yuan, C.; Zhang, B.; Gu, G.; Ge, Q.; Fast-Response, Stiffness-Tunable Soft Actuator by Hybrid Multimaterial 3D Printing. Advanced Functional Materials, 2019, Vol.29,1806698.