Benjamin O'Brien

Self sensing feedback for dielectric elastomer actuators

Self sensing Dielectric Elastomer Actuator (DEA) artificial muscles will enable the creation of soft, lightweight robots with animal-like capabilities. We demonstrate a fast, accurate, and economic self sensing algorithm that enables an arbitrary voltage oscillation to be used to sense DEA capacitance during actuation in a manner that is robust to significant changes in electrode resistance and leakage current. Not only we can use this algorithm to emulate the proprioceptive feedback found in natural muscle but also we can use it for the online characterisation and analysis of DEA behavior.

Power for Robotic Artificial Muscles

Artificial muscles based on the dielectric elastomer actuator (DEA) are an attractive technology for autonomous robotic systems. We are currently exploring their use on EcoBot (Ecological roBot), an autonomous robot being developed by Bristol Robotics Lab that uses microbial fuel cells (MFCs). DEA will provide actuators for fuel cell maintenance and other goals and will increase active mission time through greater mechanical efficiency and reduced mass. Artificial muscles use high voltages and running them normally requires voltage converters to boost the voltage on delivered charge several hundred times. A dielectric elastomer generator (DEG) when used with a recently developed self-priming circuit (SPC) can supply the high-voltage power directly to artificial muscle systems. The SPC can also be started using an initial low-voltage charge from another energy harvester such as a bank of MFCs or a solar cell array. This combination could lead to a completely autonomous power source for robotic artificial muscles. We demonstrate a proof-of-concept portable self-primed DEG for harvesting wind energy from moving tree branches.

An experimentally validated model of a dielectric elastomer bending actuator

This paper presents an experimentally validated, nonlinear finite element model capable of predicting the blocked force produced by Dielectric Elastomer Minimum Energy Structure (DEMES) bending actuators. DEMES consist of pre-stretched dielectric elastomer (DE) films bonded to thin frames, the complex collapse of which can produce useful bending actuation. Key advantages of DEMES include the ability to be fabricated in-plane, and the elimination of bulky pre-stretch supports which are often found in other DE devices. Triangular DEMES with 3 different pre-stretch ratios were fabricated. Six DEMES at each stretch ratio combination were built to quantify experimental scatter which was significant due to the highly sensitive nature of the erect DEMES equilibrium point. The best actuators produced approximately 10mN blocked force at 2500V. We integrate an Arruda-Boyce model incorporating viscoelastic effects with the Proney series to describe the stress-strain response of the elastomer, and a Neo-Hookean model to describe the frame. Maxwell pressure was simulated using a constant thickness approximation and an isotropic membrane permittivity was calculated for the stress state of the DEMES membrane. Experimental data was compared with the model and gave reasonable correlation. The model tended to underestimate the blocked force due to a constant thickness assumption during the application of Maxwell stress. The spread due to dielectric constant variance is also presented and compared with the spread of experimental scatter in the results.

Closed loop control of dielectric elastomer actuators

Sensing the electrical characteristics of a Dielectric Elastomer Actuator(s) (DEA) during actuation is critical to improving their accuracy and reliability. We have created a self-sensing system for measuring the equivalent series resistance of the electrodes, leakage current through the equivalent parallel resistance of the dielectric membrane, and the capacitance of the DEA whilst it is being actuated. This system uses Pulse Width Modulation (PWM) to simultaneously generate an actuation voltage and a periodic oscillation that enables the electrical characteristics of the DEA to be sensed. This system has been specifically targeted towards low-power, portable devices. In this paper we experimentally validate the self-sensing approach, and present a simple demonstration of closed loop control of the area of an expanding dot DEA using capacitance feedback.

The need for speed

Development of dielectric elastomer actuators has been mainly targeted towards achieving giant static strain with little attention paid to their response speed, which can, depending on materials used, be as long as tens of seconds. However, most of the practical applications require actuators capable of changing shape quickly, therefore a careful choice of materials and technologies for the dielectric and electrodes must be made. Test oscillating actuators, made with a range of silicone membranes with different hardness were tested, and the compliant electrodes were made with different technologies: carbon powder, carbon grease, conductive rubber and metal ion implantation. The transient response of the actuators to a step input was measured with a high speed camera at 5000 frames per seconds for the different combinations of membrane material and electrodes. The results show that the dynamic response of the actuators is extremely dependent on the membrane material, as expected, but also on the compliant electrodes, whose impact cannot be neglected.

Pump it up

We report on the use of zipping actuation applied to dielectric elastomer actuators to microfabricate mm-sized pumps. The zipping actuators presented here use electrostatic attraction to deform an elastomeric membrane by pulling it into contact with a rigid counter electrode. We present several actuation schemes using either conventional DEA actuation, zipping, or a combination of both in order to realize microfluidic devices. A zipping design in which the electric field is applied across the elastomer membrane was explored theoretically and experimentally. Single zipping chambers and a micropump body made of a three chambers connected by an embedded channel were wet-etched into a silicon wafer and subsequently covered by a gold-implanted silicone membrane. We measured static deflections of up to 300 μm on chambers with square openings of 1.8 and 2.6 mm side, in very good agreement with our model.

An Artificial Muscle Ring Oscillator

Dielectric elastomer artificial muscles have great potential for the creation of novel pumps, motors, and circuitry. Control of these devices requires an oscillator, either as a driver or clock circuit, which is typically provided as part of bulky, rigid, and costly external electronics. Oscillator circuits based on piezo-resistive dielectric elastomer switch technology provide a way to embed oscillatory behavior into artificial muscle devices. Previous oscillator circuits were not digital, able to function without a spring mass system, able to self-start, or suitable for miniaturization. In this paper we present an artificial muscle ring oscillator that meets these needs. The oscillator can self-start, create a stable 1 Hz square wave output, and continue to function despite degradation of the switching elements. Additionally, the oscillator provides a platform against which the performance of different dielectric elastomer switch materials can be benchmarked.

A dielectric elastomer actuator thin membrane rotary motor

We describe a low profile and lightweight membrane rotary motor based on the dielectric elastomer actuator (DEA). In this motor phased actuation of electroded sectors of the motor membrane imparts orbital motion to a central gear that meshes with the rotor. Two motors were fabricated: a three phase and four phase with three electroded sectors (120°/sector) and four sectors (90°/sector) respectively. Square segments of 3M VHB4905 tape were stretched equibiaxially to 16 times their original area and each was attached to a rigid circular frame. Electroded sectors were actuated with square wave voltages up to 2.5kV. Torque/power characteristics were measured. Contactless orbiter displacements, measured with the rotor removed, were compared with simulation data calculated using a finite element model. A measured specific power of approximately 8mW/g (based on the DEA membrane weight), on one motor compares well with another motor technology. When the mass of the frame was included a peak specific power of 0.022mW/g was calculated. We expect that motor performance can be substantially improved by using a multilayer DEA configuration, enabling the delivery of direct drive high torques at low speeds for a range of applications. The motor is inherently scalable, flexible, flat, silent in operation, amenable to deposition-based manufacturing approaches, and uses relatively inexpensive materials.

Realizing the potential of dielectric elastomer generators

The global demand for renewable energy is growing, and ocean waves and wind are renewable energy sources that can provide large amounts of power. A class of variable capacitor power generators called Dielectric Elastomer Generators (DEG), show considerable promise for harvesting this energy because they can be directly coupled to large broadband motions without gearing while maintaining a high energy density, have few moving parts, and are highly flexible. At the system level DEG cannot currently realize their full potential for flexibility, simplicity and low mass because they require rigid and bulky external circuitry. This is because a typical generation cycle requires high voltage charge to be supplied or drained from the DEG as it is mechanically deformed. Recently we presented the double Integrated Self-Priming Circuit (ISPC) generator that minimized external circuitry. This was done by using the inherent capacitance of DEG to store excess energy. The DEG were electrically configured to form a pair of charge pumps. When the DEG were cyclically deformed, the charge pumps produced energy and converted it to a higher charge form. In this paper we present the single ISPC generator that contains just one charge pump. The ability of the new generator to increase its voltage through the accumulation of generated energy did not compare favourably with that of the double ISPC generator. However the single ISPC generator can operate in a wider range of operating conditions and the mass of its external circuitry is 50% that of the double ISPC generator.

Dielectric elastomer memory

Life shows us that the distribution of intelligence throughout flexible muscular networks is a highly successful solution to a wide range of challenges, for example: human hearts, octopi, or even starfish. Recreating this success in engineered systems requires soft actuator technologies with embedded sensing and intelligence. Dielectric Elastomer Actuator(s) (DEA) are promising due to their large stresses and strains, as well as quiet flexible multimodal operation. Recently dielectric elastomer devices were presented with built in sensor, driver, and logic capability enabled by a new concept called the Dielectric Elastomer Switch(es) (DES). DES use electrode piezoresistivity to control the charge on DEA and enable the distribution of intelligence throughout a DEA device. In this paper we advance the capabilities of DES further to form volatile memory elements. A set reset flip-flop with inverted reset line was developed based on DES and DEA. With a 3200V supply the flip-flop behaved appropriately and demonstrated the creation of dielectric elastomer memory capable of changing state in response to 1 second long set and reset pulses. This memory opens up applications such as oscillator, de-bounce, timing, and sequential logic circuits; all of which could be distributed throughout biomimetic actuator arrays. Future work will include miniaturisation to improve response speed, implementation into more complex circuits, and investigation of longer lasting and more sensitive switching materials.