Using Voice Coils to Actuate Modular Soft Robots: Wormbot, an Example

Abstract

In this study, we present a modular worm-like robot, which utilizes voice coils as a new paradigm in soft robot actuation. Drive electronics are incorporated into the actuators, providing a significant improvement in self-sufficiency when compared with existing soft robot actuation modes such as pneumatics or hydraulics. The body plan of this robot is inspired by the phylum Annelida and consists of three-dimensional printed voice coil actuators, which are connected by flexible silicone membranes. Each electromagnetic actuator engages with its neighbor to compress or extend the membrane of each segment, and the sequence in which they are actuated results in an earthworm-inspired peristaltic motion. We find that a minimum of three segments is required for locomotion, but due to our modular design, robots of any length can be quickly and easily assembled. In addition to actuation, voice coils provide audio input and output capabilities. We demonstrate transmission of data between segments by high-frequency carrier waves and, using a similar mechanism, we note that the passing of power between coupled coils in neighboring modules-or from an external power source-is also possible. Voice coils are a convenient multifunctional alternative to existing soft robot actuators. Their self-contained nature and ability to communicate with each other are ideal for modular robotics, and the additional functionality of sound input/output and power transfer will become increasingly useful as soft robots begin the transition from early proof-of-concept systems toward fully functional and highly integrated robotic systems.

Keywords: modular robotics; multidimensional actuator; voice coil actuator.

FIG. 1.

(a) A cutaway sketch of one module from Wormbot, showing the connecting elastomeric body segments and a voice coil actuator. (b) A photograph showing Wormbot with the rearmost body segment removed to reveal the power distribution board. Color images available online at www.liebertpub.com/soro

FIG. 4.

A figure of merit (η) for self-sufficiency can be determined for any robot and serves as a metric for evaluating how integrated the system is, (a–e) show pictorial examples of how we calculate η. (a) For η = 1, all subsystems are fully embedded within the robot and take up all available space. (b) Again η = 1 as the available volume inside the robot is greater than that of the embedded components. (c) For η = 0.2, some components are not embedded into the robot despite there being enough space. (d) For η < 1, all available space in the robot is used, resulting in some required subsystems being tethered. (e) For η→0, the subsystems are too large to fit within the robot. (f) Our second-generation drive circuit board (PCB) contains a microcontroller, a voltage regulator, and coil drive circuitry. This PCB and small lithium polymer battery fit entirely within one segment, so η = 1. (g) Our first-generation power driver circuit and microcontroller sat outside the robot body, while the power distribution circuits were embedded within. Explicit calculations are provided in the Supplementary Data, and we calculate the degree of self-sufficiency as η = 0.43. PCB, printed circuit board. Color images available online at www.liebertpub.com/soro

FIG. 2.

A series of still frames from Supplementary Videos S1 and S2 (Supplementary Data are available online at www.liebertpub.com/soro) showing one cycle of sequential expansion and attraction of modules, the robot moves from left (a) to right (f). These voice coil actuators collectively propel the robot body in relation to the ground using peristaltic locomotion. We embedded indicator light-emitting diodes into the power distribution circuit board so that actuated modules light up when we drive current into the coil. The driving scheme we use is explained in Supplementary Figure S4b. Color images available online at www.liebertpub.com/soro

FIG. 3.

Block diagram, circuit, and results from our intermodule communication experiments. Each coil in the segments of Wormbot can be used for a secondary purpose: as a communication coil. (a) We use an on–off keying scheme where the presence of the carrier wave represents a digital one and the absence of the carrier signal represents a digital zero. (b) We use a microcontroller to drive a TRIAC transistor, which switches the 50 kHz carrier signal to the transmission coil (C1). If the carrier signal is applied to the transmitter coil, a current flow occurs and the alternating magnetic field causes an induced voltage in the receiver coil (C2). (c) A conditioning circuit processes the incoming signal and an ADC input on the microcontroller reads these data in. In this case, we show the transmission from one body segment to the next of the binary ASCII code (01011010) for the character, Z. Color images available online at www.liebertpub.com/soro