Todd Gisby

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-sensing dielectric elastomer actuators in closed-loop operation

Because of their large output strain, dielectric elastomer actuators (DEAs) have been proposed for tunable optics applications such as tunable gratings. However, the inherent viscoelastic drift of these actuators is an important drawback and closed-loop operation of DEAs is a prerequisite for any accurate real-world application. In this paper, we show how capacitive self-sensing can be used to drive a DEA in closed-loop without the need for any external sensor. The method has been demonstrated on a DEA tunable grating based on a VHB acrylic and silicone membrane. The results show that the widely used VHB presents a time-dependent drift between the capacitance of the electrodes and their strain. The silicone-based grating does not exhibit such a drift, and its strain can be stabilized by regulating the capacitance of the device to a constant value. We also report on an new fabrication method for thin deformable gratings based on replication on a water-soluble master and a 27% change in the grating period has been obtained on a VHB-based device.

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.

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.

A soft and dexterous motor

We present a soft, bearing-free artificial muscle motor that cannot only turn a shaft but also grip and reposition it through a flexible gear. The bearing-free operation provides a foundation for low complexity soft machines, with multiple degree-of-freedom actuation, that can act simultaneously as motors and manipulators. The mechanism also enables an artificial muscle controlled gear change. Future work will include self-sensing feedback for precision, multidegree-of-freedom operation.

Rotating turkeys and self-commutating artificial muscle motors

Electrostatic motors—first used by Benjamin Franklin to rotisserie a turkey—are making a comeback in the form of high energy density dielectric elastomer artificial muscles. We present a self-commutated artificial muscle motor that uses dielectric elastomer switches in the place of bulky external electronics. The motor simply requires a DC input voltage to rotate a shaft (0.73 Nm/kg, 0.24 Hz) and is a step away from hard metallic electromagnetic motors towards a soft, light, and printable future.

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.

Soft Two-Degree-of-Freedom Dielectric Elastomer Position Sensor Exhibiting Linear Behavior

Soft robots could bring robotic systems to new horizons, by enabling safe human-machine interaction. For precise control, these soft structures require high-level position feedback that is not easily achieved through conventional one-degree-of-freedom (DOF) sensing apparatus. In this paper, a soft two-DOF dielectric elastomer (DE) sensor is specifically designed to provide accurate position feedback for a soft polymer robotic manipulator. The technology is exemplified on a soft robot intended for MRI-guided prostate interventions. DEs are chosen for their major advantages of softness, high strains, low cost, and embedded multiple-DOF sensing capability, providing excellent system integration. A geometrical model of the proposed DE sensor is developed and compared to experimental results in order to understand sensor mechanics. Using a differential measurement approach, a handmade prototype provided linear sensory behavior and 0.2 mm accuracy on two-DOF. This correlates to a 0.7% error over the sensors 30 mm × 30 mm planar range, demonstrating the outstanding potential of DE technology for accurate multiDOF position sensing.

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.