Benjamin M. O’Brien

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.

Standards for dielectric elastomer transducers

Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.

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.

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.

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.

A Technology for Soft and Wearable Generators

Never again needing to recharge the batteries of portable electronic devices is an exciting prospect. The uptake of portable electronics is increasing and recharging or replacing batteries is not only an inconvenience, but also contributes to an environmental hazard. Furthermore, with technologies such as GPS, pressure sensors, and cameras becoming smaller and cheaper, smart clothing such as the Adidas miCoach range are now a reality [1]. For seamless integration of smart devices into clothing, the inconvenience of battery replacement or recharging needs to be eliminated.