Emilio P. Calius

Multi-functional dielectric elastomer artificial muscles for soft and smart machines

Dielectric elastomer (DE) actuators are popularly referred to as artificial muscles because their impressive actuation strain and speed, low density, compliant nature, and silent operation capture many of the desirable physical properties of muscle. Unlike conventional robots and machines, whose mechanisms and drive systems rapidly become very complex as the number of degrees of freedom increases, groups of DE artificial muscles have the potential to generate rich motions combining many translational and rotational degrees of freedom. These artificial muscle systems can mimic the agonist-antagonist approach found in nature, so that active expansion of one artificial muscle is taken up by passive contraction in the other. They can also vary their stiffness. In addition, they have the ability to produce electricity from movement. But departing from the high stiffness paradigm of electromagnetic motors and gearboxes leads to new control challenges, and for soft machines to be truly dexterous like their biolo...

Self-priming dielectric elastomer generators

Dielectric elastomer generators (DEG) in their present form are not suitable for autonomous power generation; they simply increase the amount of power that an electrical energy source can supply. They require a priming charge for each cycle, normally provided by an auxiliary power source but, due to charges being transferred to a load or depleted by system losses, the energy source will eventually need replacing. In this paper we present a self-priming DEG system that is capable of replenishing these charge losses from generated energy, meaning that the energy source no longer requires periodic replacement. We then experimentally demonstrate that this system not only can replenish charge losses, but also is capable of increasing the amount of charge in the system and the voltage across the capacitance storing the charge. For instance, the system was capable of gradually boosting its voltage from 10 V up to 3.25 kV. This is highly advantageous because it was also shown that the efficiency of DEG power generation increases monotonically with DEG voltage. Also, this system allows these higher voltages to be reached without the need for a high voltage transformer, reducing the system cost.

Soft generators using dielectric elastomers

The potential to produce light-weight, low-cost, wearable dielectric elastomer generators has been limited by the requirement for bulky rigid, and expensive external circuitry. In this letter, we present a soft dielectric elastomer generator whose stretchable circuit elements are integrated within the membrane. The soft generator achieved an energy density of 10 mJ/g at an efficiency of 12% and simply consisted of low-cost acrylic membranes and carbon grease mounted in a frame.

An integrated, self-priming dielectric elastomer generator

Dielectric elastomer generators are a form of variable capacitor electricity generator with a high energy density and high flexibility. Currently, dielectric elastomer generators require external circuitry which makes the system bulkier and less flexible. In this paper we present a system that minimizes the external circuitry to six diodes so that high energy density and flexibility is maintained at the system level. An energy density of 12.6 mJ/g was experimentally demonstrated, comparing favorably with similarly sized electromagnetic and electrostatic power generators.

The dielectric constant of 3M VHB: a parameter in dispute

Dielectric Elastomer (DE) transducers are essentially compliant capacitors fabricated from highly flexible materials that can be used as sensors, actuators and generators. The energy density of DE is proportional to their dielectric constant (εr), therefore an understanding of the dielectric constant and how it can be influenced by the stretch state of the material is required to predict or optimize DE device behavior. DE often operate in a stretched state. Wissler and Mazza, Kofod et al., and Choi et al. all measured an εr of approximately 4.7 for virgin VHB, but their results for prestretched DE showed that the dielectric constant decayed to varying degrees. Ma and Cross measured a dielectric constant of 6 for the same material with no mention of prestretch. In an attempt to resolve this discrepancy, εr measurements were performed on parallel plate capacitors consisting of virgin and stretched VHB4905 tape electroded with either gold sputtered coatings or Nyogel 756G carbon grease. For an unstretched VHB tape, an εr of 4.5 was measured with both electrode types, but the measured εr of equibiaxially stretched carbon specimens was lower by between 10 to 15%. The dielectric constant of VHB under high fields was assessed using blocked force measurements from a dielectric elastomer actuator. Dielectric constants ranging from 4.6-6 for stretched VHB were calculated using the blocked force tests. Figure of merits for DE generators and actuators that incorporate their nonlinear behavior were used to assess the sensitivity of these systems to the dielectric constant.

Leakage current as a predictor of failure in dielectric elastomer actuators

Dielectric breakdown often leads to catastrophic failure in Dielectric Elastomer Actuator(s) (DEA). The resultant damage to the dielectric membrane renders the DEA useless for future actuation, and in extreme cases the sudden discharge of energy during breakdown can present a serious fire risk. The breakdown strength of DEA however is heavily dependent on the presence of microscopic defects in the membrane giving its overall breakdown strength inherent variability. The practical consequence is that DEA normally have to be operated far below their maximum performance in order to achieve consistent reliability. Predicting when DEA are about to suffer breakdown based on feedback will enable significant increase in effective DEA performance without sacrificing reliability. It has been previously suggested that changes in the leakage current can be a harbinger of dielectric breakdown; leakage current exhibits a sharp increase during breakdown. In this paper the relationship between electric field and leakage current is investigated for simple VHB4905-based DEA. Particular emphasis is placed on the behaviour of leakage current leading up to and during breakdown conditions. For a sample size of nine expanding dot DEA, the DEA that failed at electric fields below the maximum tested exhibited noticeably higher nominal power dissipation and a higher frequency of partial discharge events than the DEA that did not breakdown during testing. This effect could easily be seen at electric fields well below that at which the worst performing DEA failed.

An adaptive control method for dielectric elastomer devices

The future of Dielectric Elastomer Actuator (DEA) technology lies in miniaturizing individual elements and utilizing them in array configurations, thereby increasing system fault tolerance and reducing operating voltages. An important direction of DEA research therefore is real-time closed loop control of arrays of DEAs, particularly where multiple degrees-of-freedom are desirable. As the number of degrees-of-freedom increases a distributed control system offers a number of advantages with respect to speed and efficiency. A low bandwidth digital control method for DEA devices is presented in this paper. Pulse Width Modulation (PWM) is used as the basis for a current controlled DEA system that allows multiple degrees-of-freedom to be controlled independently and in parallel using a single power supply set to a fixed voltage. The amplitude and the duty cycle of the PWM signal control the current flow through a high speed, high voltage opto-coupler connected in series with a DEA, enabling continuous control of both the output displacement and speed. Controlling the current in real-time results in a system approaching a stable and robust constant charge system. Closed loop control is achieved by measuring the rate of change of the voltage across the DEA in response to a step change in the current input generated by the control signal. This enables the capacitance to be calculated, which in combination with the voltage difference between the electrodes and the initial dimensions, enables the charge, strain state and Maxwell pressure to be inferred. Future developments include integrating feedback information directly with the control signal, leaving the controller to coordinate rather than control individual degrees-of-freedom.

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.

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.