Veiko Vunder

Ionic electroactive polymer artificial muscles in space applications

A large-scale effort was carried out to test the performance of seven types of ionic electroactive polymer (IEAP) actuators in space-hazardous environmental factors in laboratory conditions. The results substantiate that the IEAP materials are tolerant to long-term freezing and vacuum environments as well as ionizing Gamma-, X-ray, and UV radiation at the levels corresponding to low Earth orbit (LEO) conditions. The main aim of this material behaviour investigation is to understand and predict device service time for prolonged exposure to space environment.

Mechanical interpretation of back-relaxation of ionic electroactive polymer actuators

The mechanical model obtained by a non-traditional approach to the basic components of the traditional viscoelastic models—spring and damper—elucidates the back-relaxation of ionic electroactive polymer actuators. The corresponding PDE characterizes the curvature or bending moment of the actuators throughout stimulated bending forward followed by relaxation back towards the initial shape. Combining series of short entities containing the lumped electrical circuit, and the transient bending moment generated according to the PDE, results in a new model of the ionic electroactive polymer actuators. This model takes into account the back-relaxation of the actuators as well as the superposition principle. The experiments carried out with three actuators of different ionic electroactive polymer materials show excellent accordance with the model.

In situ scanning electron microscopy study of strains of ionic electroactive polymer actuators

We have developed a technique to determine the bending strain of ionic electroactive polymer actuators without the use of the macroscopic bending geometry. In situ comparisons of the scanning electron microscope micrographs from a bending ionic electroactive polymer actuator, using a digital image correction methodology, identify its bi-directional deformation field. The developed technique allows verification of the factual axial and thickness strains of any notional layer of the actuator, including the outer surfaces of the electrodes. Thus, calculation of the bending and thickness strains of the ionic electroactive polymer laminate becomes possible. Moreover, the technique allows the determination of the position of the neutral layer of bending that is an important requirement for the calculation of the second area and bending moments of the beam. The four examples presented demonstrate the potential variations of the bending schemes, in cases where the neutral layer is at the centroid and shifted away from the centroid.

Inversion-based control of ionic polymer-metal composite actuators with nanoporous carbon-based electrodes

Ionic polymer‐metal composite (IPMC) actuators are multilayer composites that change their shape and size in response to low-voltage driving signals. Because these devices possess both actuating and sensing properties, these composites have already been proposed for numerous applications. Recently, high-strain nanoporous carbon electrodes were developed with the aim of improving the performance and stability of IPMC actuators. In order to prevent damage to the device or injury, precise control of the actuator is essential. Thus far, there are no reports on the dynamic response of IPMC actuators with electrodes composed of a porous carbon material. This study fills this gap in the research by presenting the results of testing both openand closed-loop controllers of this novel actuator for position control. Inversion-based open-loop controller is first tested on the actuator to evaluate the performance of the actuator in sensorless control applications. Then, the displacement of the tip point of the actuator is used as the feedback signal for closed-loop control to compensate for the errors experienced in open-loop control.

Lifetime measurements of ionic electroactive polymer actuators

This article is focused on proposing a unified methodology for automating the measurement procedures of ionic electroactive polymer actuators. The proposed methodology and large-scale automation would make testing ionic electroactive polymer actuators less labor-intensive and allow analyzing many ionic electroactive polymer actuators simultaneously. Defining a clear framework for testing ionic electroactive polymer actuators performance and reliability would make the testing process reproducible and provide better comparison between ionic electroactive polymer actuators of either different or similar classes. Our methodology separates two types of degradation: degradation during operation and spontaneous self-degradation.

Variable-focal lens using electroactive polymer actuator

The paper describes a simple and cost-effective design and fabrication process of a liquid-filled variable-focal lens. The lens was made of soft polymer material, its shape and curvature can be controlled by hydraulic pressure. An electroactive polymer is used as an actuator. A carbon-polymer composite (CPC) was used. The device is composed of elastic membrane upon a circular lens chamber, a reservoir of liquid, and a channel between them. It was made of three layers of polydimethylsiloxane (PDMS), bonded using the partial curing technique. The channels and reservoir were filled with incompressible liquid after curing process. A CPC actuator was mechanically attached to reservoir to compress or decompress the liquid. Squeezing the liquid between the reservoir and the lens chamber will push the membrane inward or outward resulting in the change of the shape of the lens and alteration of its focal length. Depending on the pressure the lens can be plano-convex or plano-concave or even switch between the two configurations. With only a few minor modifications it is possible to fabricate bi-convex and bi-concave lenses. The lens with a 1 mm diameter and the focal length from infinity to 17 mm is reported. The 5x15mm CPC actuator with the working voltage of only up to ±2.5 V was capable to alter the focal length within the full range of the focal length in 10 seconds.

Effect of ambient humidity on ionic electroactive polymer actuators

Comparable electromechanical measurements were carried out with carbon-based ionic electroactive polymer actuators in vacuum, dry inert, and in ambient air environment. The results bring forward the effect of ambient humidity on the electrical and mechanical parameters of the laminates of this type. Presence of water decreases the Youngs modulus of the polymer and lowers the viscosity of the ionic liquid, which, in turn, is accompanied by the increase of ionic conductivity of the electrolyte. The factual bending behavior of the actuator is a result of the combined effect of these factors. A four-parameter model was developed for the quantitative estimation of the rates of forward-actuation and back-relaxation as well as the electrical parameters. An important outcome of the experiments is the observation that there is nearly no back-relaxation in vacuum and in dry inert environment.

Long-term degradation of the ionic electroactive polymer actuators

The research is focused on lifetime and degradation of ionic electroactive polymer actuators (IEAP). The lifetime measurements were carried out using identical methodology upon the different IEAP types. The experiment conducted with large number of samples shows that two types of degradation have serious effect to the IEAPs: degradation during operation and spontaneous self-degradation. Additionally, two ways of occasional damage decrease their overall reliability. In the scope of the current paper we describe degradation of two different types of IEAP actuators: with carbonaceous electrodes and with conducting polymer electrodes. Nevertheless, the common evolutionary trends, rather than the comparative data analysis or formal statistics of all particular samples, are given. Analyzing the electromechanical and electrical impedances of the samples during their whole lifetime, we have found that observing the electric current gives adequate information about the degradation level of any IEAP actuator. Moreover, tracking this electrically measurable parameter enables detecting the occasional damage of an actuator.

Long-term behavior of ionic electroactive polymer actuators in variable humidity conditions

Ionic electroactive polymers or IEAPs are considered as an attractive actuators and sensors in various applications. Many of these polymer composites are designed to be used in an ambient environment. However, the ambient conditions may significantly vary depending on the seasonal or the geographical irregularities generated by the power of nature. Taking the advantage of the fluctuating weather conditions of Estonia, different IEAP materials were continuously monitored for about 6 weeks. During this time the temperature and relative humidity of the ambient environment varied between 30-58 % and 23-29 °C respectively. The experiment was conducted in a non-air-conditioned lab facility where the parameters such as temperature, humidity, atmospheric pressure were registered. Concurrently the electromechanical impedance of 12 actuators of different types was registered. This setup brings out the degradation as well as the impact of the environment to the IEAP actuators. The analysis reveals that the performance of the actuators under research is highly correlated with the ambient relative humidity level which can increase or decrease their performance more than 2 times. Naturally, this issue needs to be addressed in characterization, modeling and control areas. In contrast, the changes of pressure and temperature appeared to have no significant influence on the performance of the actuators investigated

Force control of ionic polymer-metal composite actuators with carbon-based electrodes

To perform tasks such as hold an object with a constant force, the reliable control of an ionic electroactive polymer actuator is essential. The composite under research is an IPMC actuator with electrodes composed of nanoporous carbon and membrane made of ionic polymer. Compared to traditional platinum electrodes, these novel electrodes do not crack in clusters and have highly controllable properties which preserve even when the actuator is deformed. So far, there are no reports on the dynamic force response of this composite. We present the first attempts of testing the force dynamics of an IPMC with nanoporous carbon electrodes under open- and closed-loop controls. As many attempts have been focused on the sensorless force control of ionic electroactive polymers, we first investigate the uncompensated dynamics of the actuator, then use the direct inverse model to obtain the desired tracking performance. We also aim to identify the conditions, under which the actuator is suitable for sensorless control. Furthermore, we improve the tracking ability of the actuator using a feedback controller where the force sensor data is available and incorporate a feedforward controller into the feedback control system. Based on the experiments, the resulting effects on the tracking performance are observed.