Wireless Micro Soft Actuator without Payloads Using 3D Helical Coils

Abstract

To receive a greater power and to demonstrate the soft bellows-shaped actuator's wireless actuation, micro inductors were built for wireless power transfer and realized in a three-dimensional helical structure, which have previously been built in two-dimensional spiral structures. Although the three-dimensional helical inductor has the advantage of acquiring more magnetic flux linkage than the two-dimensional spiral inductor, the existing microfabrication technique produces a device on a two-dimensional plane, as it has a limit to building a complete three-dimensional structure. In this study, by using a three-dimensional printed soluble mold technique, a three-dimensional heater with helical coils, which have a larger heating area than a two-dimensional heater, was fabricated with three-dimensional receiving inductors for enhanced wireless power transfer. The three-dimensional heater connected to the three-dimensional helical inductor increased the temperature of the liquid and gas inside the bellows-shaped actuator while reaching 176.1% higher temperature than the heater connected to the two-dimensional spiral inductor. Thereby it enables a stroke of the actuator up to 522% longer than when it is connected to the spiral inductor. Therefore, three-dimensional micro coils can offer a significant approach to the development of wireless micro soft robots without incurring heavy and bulky parts such as batteries.

Keywords: 3D helical inductors; liquid–gas phase changes; magnetic induction; soft actuators; wireless actuators.

Figure 1

Schematics diagram of a wireless micro soft actuator: (a) Wireless power transfer and the following expansion of the micro soft actuator by liquid–gas phase change of working fluid inside the actuator. (b) Multi-turn coils with two-dimensional spiral structure (left) and three-dimensional helical structure (right).

Figure 2

Wireless power transfer based on magnetic induction: (a) Magnetically coupled transmitting and receiving inductor. (b) Equivalent circuit containing source, mutually coupled inductors, and load (i.e., Joule heater).

Figure 3

Comparison between a two-dimensional spiral and a three-dimensional helical receiving inductor: (a) The magnetic flux density around the inductors is displayed on the surface containing the center of the inductors, and the pattern of the magnetic field is shown as the streamline. (b) Induced voltages according to the number of coil turns in the two-dimensional spiral and the three-dimensional helical inductor.

Figure 4

The fabrication process of the three-dimensional micro bellows actuator for the liquid–gas phase change mechanism: (a) The mold of the actuator is printed with the build and the support material. (b) The support material filled inside the mold is selectively removed by a dewaxing solvent. (c) Liquid PDMS is poured into the mold and cured in a convection oven. (d) The mold made of build material is dissolved with acetone, leaving the cast bellows actuator. (e) Working fluid and PDMS heater are sequentially inserted into the bellows actuator. (f) Liquid gallium is injected with a syringe into the channel of the PDMS heater to enable electrical conductivity. When the heater is heated due to the induced electromotive force by the receiving inductor, the working fluid inside the actuator evaporates and the internal pressure increases. The increased pressure causes the PDMS actuator to expand.

Figure 5

Measurement setups for the wirelessly powered bellows actuator characterization: (a) Experimental configuration measuring the operation of the soft bellows-shaped actuator with an optical microscope. (b) The soft bellows-shaped actuator has a built-in heater that is connected to the receiving inductor by wires.

Figure 6

Characteristics of the heater connected to the two-dimensional spiral and the three-dimensional helical inductor: Temperature change of the heaters (a) according to the air-gap distance between the transmitting inductor and the receiving inductor, (b) along the misaligned distance, and (c) according to winding numbers of the heaters. (d) Temperature distribution of the heaters and the receiving inductors, captured by an infrared camera.

Figure 7

Response of the bellows actuator by the liquid–gas phase change: (a) Displacement according to the bellows surface temperature. The displacement increases along the red curve during heating and decreases along the blue curve during cooling owing to the powering off. (b) IR images of the actuator at each liquid–gas phase stage. The scale bar indicates 1 mm.

Figure 8

Performance of the thermopneumatic bellows actuator connected to the two-dimensional spiral and the three-dimensional helical inductor: (a) Real-time displacement of the actuators. (b) Temperature change of the outer surface of the actuators. (c) The captured driving actuator connected to the three-dimensional helical inductor. The scale bar indicates 1 mm.