Shane Xie

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.

Scalable sensing electronics towards a motion capture suit

Being able to accurately record body motion allows complex movements to be characterised and studied. This is especially important in the film or sport coaching industry. Unfortunately, the human body has over 600 skeletal muscles, giving rise to multiple degrees of freedom. In order to accurately capture motion such as hand gestures, elbow or knee flexion and extension, vast numbers of sensors are required. Dielectric elastomer (DE) sensors are an emerging class of electroactive polymer (EAP) that is soft, lightweight and compliant. These characteristics are ideal for a motion capture suit. One challenge is to design sensing electronics that can simultaneously measure multiple sensors. This paper describes a scalable capacitive sensing device that can measure up to 8 different sensors with an update rate of 20Hz.

Experimental performance and feasibility of a miniature single-degree-of-freedom rotary joint with integrated IPMC actuator

Ionic Polymer Metal Composite (IPMC) materials are bending actuators that can achieve large tip displacements at voltages less than 10V, but with low force output. Their advantages over traditional actuators include very low mass and size; flexibility; direct conversion of electricity to mechanical energy; biocompatibility; and the potential to build integrated sensing/actuation devices, using their inherent sensing properties. It therefore makes sense to pursue them as a replacement to traditional actuators where the lack of force is less significant, such as micro-robotics; bio-mimetics; medical robotics; and non-contact applications such as positioning of sensors. However, little research has been carried out on using them to drive mechanisms such as the rotary joints. This research explores the potential for applying IPMC to driving a single degree-of-freedom rotary mechanism, for a small-force robotic manipulator or positioning system. Practical issues such as adequate force output and friction are identified and tackled in the development of the mechanical apparatus, to study the feasibility of the actuator once attached to the mechanism. Rigid extensions are then applied to the tip of the IPMC, as well as doubling- and tripling the actuators in a stack to increase force output. Finally, feasibility of the entire concept is considered by comparing the maximum achievable forces and combining the actuator with the mechanism. It is concluded that while the actuator is capable of moving the mechanism, it is non-repeatable and does not achieve a level that allows feedback control to be applied.