Stephen W. John

Validation of Resonant Frequency Model for Polypyrrole Trilayer Actuators

Conducting polymers are new materials that can be used as low-voltage actuators and active flexure joints, scaling down to the microscale; however, many devices based on these actuators are limited because they can only operate when submerged in an electrolyte. Conducting polymer trilayer actuators are laminated structures with a wide range of potential applications as they are capable of operating both in air and liquid environments, but their dynamic behavior is not yet fully understood. With the view to developing a comprehensive dynamic model of trilayer actuators, this paper presents the experimentally obtained frequency response of the actuator displacement, as measured using a laser displacement sensor. A model of the resonant frequency is also presented and then comprehensively validated using actuators of various geometry and loading. This model can be used to: 1) estimate properties of the trilayer actuator from experimental measurements; 2) quantify and optimize an actuators dynamic behavior as a function of geometry; and 3) facilitate the use of these actuators as a component in practical applications, such as force and motion control systems.

Inversion-Based Feedforward Control of Polypyrrole Trilayer Bender Actuators

Conducting polymer bending actuators show potential for unique manipulation devices, particularly at the microscale, given low actuation voltages, controllable manufacture, biocompatibility, and ability to operate in either air or liquid environments; however, the impracticalities of implementing feedback in these environments and at these scales can impede positional control of the actuator. This paper presents an application of inversion-based feedforward positional control to a trilayer bender actuator, which is shown to improve the performance without the use of feedback or adjustments to the chemistry of the device. The step and dynamic displacement responses have all been improved under the feedforward control system, while the response does not change significantly under large increases in external loads. This study contributes the first implementation of inversion-based feedforward control to the emerging area of conducting polymer actuators, paving the way toward their use in functional devices, particularly where the implementation of feedback is difficult.

Towards fully optimized conducting polymer bending sensors: the effect of geometry

Conducting polymer devices have been demonstrated to generate a current or voltage in response to a displacement or force, which facilitates their use as mechanical sensors. Trilayer bending sensors are laminated conducting polymer devices that can provide self-contained operation in air, while maintaining many of the desirable conducting polymer properties including low weight and biocompatibility. This paper extends our previous characterization work by identifying the frequency response of the sensor output as the device geometry is varied and at frequencies up to 300 Hz. Current output is found to increase with the volume of conducting polymer across the sensor spectrum, while the usable sensor bandwidth is bounded by low-pass behaviour and a peak at the low frequency end of the spectrum and by a mechanical resonance at high frequencies. On the basis of these findings, suggestions are presented which can be used to optimize the current output and bandwidth of trilayer bender sensors.

Frequency response of polypyrrole trilayer actuator displacement

Conducting polymer trilayers are attractive for use in functional devices, given low actuation voltages, operation in air and potentially useful stresses and strains; however, their dynamic behavior must be understood from an engineering perspective before they can be effectively incorporated into a design. As a step towards the identification of the actuator dynamics, frequency response analysis has been performed to identify the magnitude and phase shift of displacement in response to a sinusoidal voltage input. The low damping of the trilayer operating in air and the use of a laser displacement sensor has allowed the frequency response to be continuously identified up to 100Hz, demonstrating a resonant peak at 80Hz for a 10mm long actuator. Two linear transfer function models have been fitted to the frequency response of the trilayer displacement (i) a 3rd order model to represent the dynamics below 20Hz and (ii) a higher complexity 6th order model to also include the resonant peak. In response to a random input signal, the 3rd order model coarsely follows the experimental identified displacement, while the 6th order model is able to fully simulate the real trilayer movement. Step responses have also been obtained for the 3rd and 6th order transfer functions, with both models capable of following the first 4 seconds of experimental displacement. The application of empirical transfer function models will facilitate accurate simulation and analysis of trilayer displacement, and will lead to the design of accurate positional control systems.

Towards the position control of conducting polymer trilayer bending actuators with integrated feedback sensor

Conducting polymers have a wide range of reversibly controllable properties, leading to a number of potentially useful devices for robotic applications, including actuators, sensors and batteries. Conducting polymers have the advantages of low weight, low cost, flexibility, small activation potentials (≪2V), biocompatibility and the ability to be manufactured using relatively straightforward techniques. Trilayer bending actuators which utilise the controllable change in volume of conducting polymers are potentially useful devices, but their speed and positioning ability must be improved. Significant research effort has been directed towards improving conducting polymer actuator performance through its chemistry, but the use of compensating control systems has had relatively little focus. This paper experimentally investigates three potential control systems for a trilayer bending actuator - feedforward gain control, feedforward inversion-based control and inversion-based PI control. It was found that the inversion-based PI control system provided the best performance, but the implementation utilised a large laser displacement sensor. To limit the requirement for such a large feedback sensor, a trilayer bending actuator with an integrated displacement sensor is proposed, exploiting the additional sensing capability of conducting polymers.

A practical multilayered conducting polymer actuator with scalable work output

Household assistance robots are expected to become more prominent in the future and will require inherently safe design. Conducting polymer-based artificial muscle actuators are one potential option for achieving this safety, as they are flexible, lightweight and can be driven using low input voltages, unlike electromagnetic motors; however, practical implementation also requires a scalable structure and stability in air. In this paper we propose and practically implement a multilayer conducting polymer actuator which could achieve these targets using polypyrrole film and ionic liquid-soaked separators. The practical work density of a nine-layer multilayer actuator was 1.4 kJ m−3 at 0.5 Hz, when the volumes of the electrolyte and counter electrodes were included, which approaches the performance of mammalian muscle. To achieve air stability, we analyzed the effect of air-stable ionic liquid gels on actuator displacement using finite element simulation and it was found that the majority of strain could be retained when the elastic modulus of the gel was kept below 3 kPa. As a result of this work, we have shown that multilayered conducting polymer actuators are a feasible idea for household robotics, as they provide a substantial practical work density in a compact structure and can be easily scaled as required.

Towards Micro and Nano Manipulation Systems: Behaviour of a Laminated Polypyrrole (PPy) Actuator Driving a Rigid Link

Conducting polymer actuators, such as Polypyrrole (PPy), incorporated into the structure of a manipulation system may be able to achieve micro and nanoscale precision, by avoiding effects such as backlash or friction. As a step towards this goal, laminated PPy actuators were varied in size and their behaviour investigated while constantly loaded by a rigid link. This behaviour has been evaluated in terms of the bending angle and force outputs of the actuator. It was found that the bending angles varied with length, but displayed unexpected trends due to the loading effects on the PPy. Force output of the actuators was also measured, with unloaded PPy producing greater force across all lengths than the near constant output of loaded PPy, attributable to the polymer and load interface.

Power-efficient low-temperature woven coiled fibre actuator for wearable applications

A fibre actuator that generates a large strain with high specific power represents a promising strategy to develop novel wearable devices and robotics. We propose a new coiled-fibre actuator based on highly drawn, hard linear low-density polyethylene (LLDPE) fibres. Driven by resistance heating, the actuator can be operated at temperatures as low as 60 °C and uses only 20% of the power consumed by previously coiled fibre actuators when generating 20 MPa of stress at 10% strain. In this temperature range, 1600 W kg−1 of specific work (8 times that of a skeletal muscle) at 69 MPa of tensile stress (230 times that of a skeletal muscle) with a work efficiency of 2% is achieved. The actuator generates strain as high as 23% at 90 °C. Given the low driving temperature, the actuator can be combined with common fabrics or stretchable conductive elastomers without thermal degradation, allowing for easy use in wearable systems. Nanostructural analysis implies that the lamellar crystals in drawn LLDPE fibres are weakly bridged with each other, which allows for easy deformation into compact helical shapes via twisting and the generation of large strain with high work efficiency.

Soft hip walking assist experimental system featuring variable compliance control

In this paper, we present a tethered soft assist experimental system for the hip, incorporating virtual compliant actuator model into the control system. We adjust the parameters of the virtual compliant actuator to generate assistance torques around the hip during walking. Experimental tests for level walking at 1.25 m/s shows that variable stiffness assist is capable of reducing the energy cost of walking on level ground by an average of 5.5% across 4 test subjects. This result confirms that our approach is capable of generating effective assistance.