Temperature Driven Multi-modal/Single-actuated Soft Finger

TL;DR

This paper tackles the limitation of soft actuators that typically realize a single motion by introducing a single-chamber multimodal soft finger driven by pressure and temperature. By embedding a temperature-responsive Humofit variable-stiffness material into a fiber-reinforced silicone actuator, it switches among bending, twisting, and extension within about five seconds, while maintaining softness. An analytic minimum-energy model links input pressure $P$ to twist radius $R_t$, which is experimentally validated via motion capture, and the actuator’s capabilities are demonstrated in a three-actuator soft gripper capable of multiple grasp modes for objects with varying size, shape, and stiffness. The work advances multimodal soft robotics with simplified control, enhanced range of motion, and practical grasping versatility, albeit with heating-related limitations that motivate future improvements in heat delivery and closed-loop temperature control.

Abstract

Soft pneumatic fingers are of great research interest. However, their significant potential is limited as most of them can generate only one motion, mostly bending. The conventional design of soft fingers does not allow them to switch to another motion mode. In this paper, we developed a novel multi-modal and single-actuated soft finger where its motion mode is switched by changing the finger's temperature. Our soft finger is capable of switching between three distinctive motion modes: bending, twisting, and extension-in approximately five seconds. We carried out a detailed experimental study of the soft finger and evaluated its repeatability and range of motion. It exhibited repeatability of around one millimeter and a fifty percent larger range of motion than a standard bending actuator. We developed an analytical model for a fiber-reinforced soft actuator for twisting motion. This helped us relate the input pressure to the output twist radius of the twisting motion. This model was validated by experimental verification. Further, a soft robotic gripper with multiple grasp modes was developed using three actuators. This gripper can adapt to and grasp objects of a large range of size, shape, and stiffness. We showcased its grasping capabilities by successfully grasping a small berry, a large roll, and a delicate tofu cube.

Figures (9)

• Figure 1: Developed actuator and it's three motion modes (A) Actuator in resting state. (B) Bending mode. (C) Twisting mode. (D) Extension mode.
• Figure 2: Structure and construction of the actuator (A) (i)Inner components of the soft actuator (CAD image). (ii) Outer components of the soft actuator. (iii)Inner structure of the soft actuator. (iv) Outer structure of the soft actuator with water channels i.e. A complete actuator. (v) Flow of water used to heat/cool humofit. (B) Actuator’s construction process. (i)Silicon casting. (ii) Molded Silicon body. (iii) Sealing off open ends. (iv) Winding non-extensible thread. (v) Winding Humofit. (vi) Attaching water channels over the actuator.
• Figure 3: Working principle of the actuator (A)Bending Mode: String, Humofit1, and Humofit2 are stiff as they are cold(blue colour). (B)Twisting Mode: Humofit1 is hot hence stretchable (red colour) while the string and Humofit2 are cold hence stiff(blue colour). (C) Extension Mode: Humofit1 and string are stiff while Humofit2 is hot (red).
• Figure 4: Experimental setup and output of the actuator's repeatability test (A) Main components of the experiment. (B) (i) Power supply. (ii) Pressure input. (iii) Soft actuator. (iv) Hot water. (v) Cold water. (C) Origin and axes for measurement. (D) Bending Repeatability output. (E) Twisting Repeatability output. (F) Extension Repeatability output.
• Figure 5: Large range of motion exhibited by the developed actuator (A) Bending at 25° C, 20kPa. (B) Bending at 45° C, 20kPa. (C) Sweeping area of the bending actuator. (D) Volume swept by the actuator in twist mode compared to the standard bending mode presented via two views.