Iain A. Anderson

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.

Accuracy of a computer-assisted navigation system for total knee replacement

The object of this study was to develop a method to assess the accuracy of an image-free total knee replacement navigation system in legs with normal or abnormal mechanical axes. A phantom leg was constructed with simulated hip and knee joints and provided a means to locate the centre of the ankle joint. Additional joints located at the midshaft of the tibia and femur allowed deformation in the flexion/extension, varus/valgus and rotational planes. Using a digital caliper unit to measure the coordinates precisely, a software program was developed to convert these local coordinates into a determination of actual leg alignment. At specific points in the procedure, information was compared between the digital caliper measurements and the image-free navigation system. Repeated serial measurements were undertaken. In the setting of normal alignment the mean error of the system was within 0.5 degrees . In the setting of abnormal plane alignment in both the femur and the tibia, the error was within 1 degrees . This is the first study designed to assess the accuracy of a clinically-validated navigation system. It demonstrates in vitro accuracy of the image-free navigation system in both normal and abnormal leg alignment settings.

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.

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.

Subject-specific modelling of lower limb muscles in children with cerebral palsy

BACKGROUND Recent studies suggest that the architecture of spastic muscles in children with cerebral palsy is considerably altered; however, only little is known about the structural changes that occur other than in the gastrocnemius muscle. In the present study, Magnetic Resonance Imaging (MRI) and subject-specific modelling techniques were used to compare the lengths and volumes of six lower limb muscles between children with cerebral palsy and typically developing children. METHODS MRI scans of the lower limbs of two children with spastic hemiplegia cerebral palsy, four children with spastic diplegia cerebral palsy (mean age 9.6 years) and a group of typically developing children (mean age 10.2 years) were acquired. Subject-specific models of six lower limb muscles were developed from the MRI data using a technique called Face Fitting. Muscle volumes and muscle lengths were derived from the models and normalised to body mass and segmental lengths, respectively. FINDINGS Normalised muscle volumes in the children with cerebral palsy were smaller than in the control group with the difference being 22% in the calf muscles, 26% in the hamstrings and 22% in the quadriceps, respectively. Only the differences in the hamstrings and the quadriceps were statistically significant (P=0.036, P=0.038). Normalised muscle lengths in the children with cerebral palsy were significantly shorter (P<0.05), except for soleus and biceps femoris. No significant relationship was found between normalised lengths and volumes of any muscle in either group. INTERPRETATION The present results show that lower limb muscles in ambulatory children with cerebral palsy are significantly altered, suggesting an overall mechanical deficit due to predominant muscle atrophy. Further investigations of the underlying causes of the muscle atrophy are required to better define management and treatment strategies for children with cerebral palsy.

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.