Jennifer C. Case

Conformable actuation and sensing with robotic fabric

Future generations of wearable robots will include systems constructed from conformable materials that do not constrain the natural motions of the wearer. Fabrics represent a class of highly conformable materials that have the potential for embedded function and are highly integrated into our daily lives. In this work, we present a robotic fabric with embedded actuation and sensing. Attaching the same robotic fabric to a soft body in different ways leads to unique motions and sensor modalities with many different applications for robotics. In one mode, the robotic fabric acts around the circumference of the body, and compression of the body is achieved. Attaching the robotic fabric in another way, along one surface of a body for example, bending is achieved. We use thread-like actuators and sensors to functionalize fabric via a standard textile manufacturing process (sewing). The actuated fabric presented herein yields a contractile force of 9.6N and changes in length by approximately 60% when unconstrained. The integrated strain sensor is evaluated and found to have an RMS error of 14.6%, and qualitatively differentiates between the compressive and bending motions demonstrated.

Variable stiffness fabrics with embedded shape memory materials for wearable applications

Materials with variable stiffness have the potential to provide a range of new functionalities, including system reconfiguration by tuning the location of rigid links and joints. In particular, wearable applications would benefit from variable stiffness materials in the context of active braces that may stiffen when necessary and soften when mobility is required. In this work, we present fibers capable of adjusting to provide variable stiffness in wearable fabrics. The variable stiffness fibers are made from shape memory materials, where shape memory alloy (SMA) is coated with a thin film of shape memory polymer (SMP). The fibers, which are fabricated via a continuous feed-through process, reduce in bending stiffness by an order of magnitude when the SMP goes through the glass transition. The transition between rubbery and glassy state is accomplished by direct joule heating of the embedded SMA wire. We employ a COMSOL model to relate the current input to the time required for the fibers to transition between stiffness states. Finally, we demonstrate how this device can be worn and act as a joint stability brace on human fingers.

Sensor enabled closed-loop bending control of soft beams

Control of soft-bodied systems is challenging, as the absence of rigidity typically implies distributed deformations and infinite degrees-of-freedom. In this paper, we demonstrate closed-loop control of three elastomer beams that vary in bending stiffness. The most stiff beam is comprised of a single prismatic structure made from a single elastomer. In the next beam, increased flexibility is introduced via an indentation in the elastomer, forming a joint. The most flexible beam uses a softer elastomer in the joint section, along with an indentation. An antagonistic pair of actuators bend the joint while a pair of liquid–metal-embedded strain sensors provide angle feedback to a control loop. We were able to achieve control of the system with a proportional–integral–derivative control algorithm. The procedure we demonstrate in this work is not dependent on actuator and sensor choice and could be applied to to other hardware systems, as well as more complex multi-joint robotic structures in the future.

Multi-Element Strain Gauge Modules for Soft Sensory Skins

In this paper, we describe the fabrication and testing of a sensory module composed of resistive strain gauges in an elastomer substrate. Each module contains three resistive gauges, providing sufficient information to reconstruct the geometry of the module. The modules are fabricated from two bonded sheets of silicone elastomer. The sensing element is a resistive strain gauge based on room-temperature liquid gallium-indium alloy contained within microchannels in the substrate. We demonstrate the functionality of the module by mechanically stretching it over a template and measuring the change in resistance of the embedded liquid metal strain gauges. Starting with known strains, we calibrate the device and fit a quadratic model. With the model and the measured error distribution, we can predict the uncertainty in the reconstructed position of the corners of the triangular modules, which we refer to as nodes.

Sensor Skins: An Overview

Sensor skins can be broadly defined as distributed sensors over a surface to provide proprioceptive, tactile, and environmental feedback. This chapter focuses on sensors and sensor networks that can achieve strains on the same order as elastomers and human skin, which makes these sensors compatible with emerging wearable technologies. A combination of material choices, processing limitations, and design must be considered in order to achieve multimodal, biocompatible sensor skins capable of operating on objects and bodies with complex geometries and dynamic functionalities. This chapter overviews the commonly used materials, fabrication techniques, structures and designs of stretchable sensor skins, and also highlights the current challenges and future opportunities of such sensors.

OmniSkins: Robotic skins that turn inanimate objects into multifunctional robots

Robotic skins are planar substrates with embedded actuation and sensing that can wrap around soft objects to turn them into robots. Robots generally excel at specific tasks in structured environments but lack the versatility and the adaptability required to interact with and locomote within the natural world. To increase versatility in robot design, we present robotic skins that can wrap around arbitrary soft bodies to induce the desired motions and deformations. Robotic skins integrate actuation and sensing into a single conformable material and may be leveraged to create a multitude of controllable soft robots with different functions or gaits to accommodate the demands of different environments. We show that attaching the same robotic skin to a soft body in different ways, or to different soft bodies, leads to distinct motions. Further, we show that combining multiple robotic skins enables complex motions and functions. We demonstrate the versatility of this soft robot design approach in a wide range of applications—including manipulation tasks, locomotion, and wearables—using the same two-dimensional (2D) robotic skins reconfigured on the surface of various 3D soft, inanimate objects.