Development of a Conducting Polymer based Flexible Position Sensor for the Control of Hand Exoskeletons

Abstract

There is an increasing demand for hand exoskeleton as they can enhance the capability of a healthy human hand beyond its natural capacity or rehabilitate a disabled hand impacted from injuries or serious illnesses such as stroke. At present, hand exoskeletons face an issue regarding to the sensors being employed to determine the finger position. These sensors utilise mechanical designs that require physical movements of rigid components. As such, these sensors are bulky and heavy. They inhibit the usability and portability of the hand exoskeleton that prevent a comfortable usage of the devices and their deployment to the patients' homes. This has been addressed by the development of a flexible position sensor. This sensor combines two types of polymer to construct the sensor. Natural rubber is employed as the substrate and chosen for its flexibility, elasticity and resilience. The role of strain sensing was assigned to a conducting polymer called polypyrrole in its thin film form. It has been chosen for its high conductivity, stability, ease of synthesis and its proven ability to measure strain. The sensor was fabricated using chemical vapour deposition to carry out the in situ polymerisation of polypyrrole on the natural rubber substrate using the vapour phase polymerisation. This produces a polypyrrole thin film with uniform, smooth and dense surface morphologies that are desired for this sensor. Prior to the deposition, the substrate was conditioned by pre-straining the natural rubber substrate to 20 % and followed by an O2 plasma treatment set to 200 W for 40 seconds to reduce the surface hydrophobicity. This helps with the adsorption of the FeCl3 oxidant and adhesion of the polypyrrole thin film to the substrate. Various deposition parameters were analysed and the optimised parameters were found to be 0.5 M FeCl3 oxidant, 0.1 M pyrrole monomer and two hours polymerisation duration. The strain sensing mechanism was established to be the elongation of the polymer chains in the polypyrrole thin film with the addition of surface micro-cracks that open up with the elongation of the polypyrrole thin film to induce changes in the electrical resistance. However, the electrical resistance of the sensor drifts upward between subsequent measurements. A calibration process was implemented by measuring the change in the electrical resistance instead of the value itself. As such, the sensor measures the relative position instead of the absolute position. The sensor has a linear relationship between the electrical resistance and the measured strain with a gauge factor in the range of 1.74 to 2.02. The error was estimated to be ±10 % when analysed from repeated strain test cycles. Meanwhile, the sensor exhibits a non-linear relationship between the electrical resistance and the angular displacement due to a non-linear strain distribution during a rotary position measurement. When implemented as the feedback in a closed loop control system, the sensor displays good performance. It was able to measure linear and angular positions with an error of 1 mm and 5° respectively. The combination of materials used in the strain sensor fabricated in the structure used in this research has not been done previously. Using a non-absorbent and yet, a soft, flexible and elastic material such as rubber means the available fabrication process has to be modified and adapted. This includes the use of acetonitrile as the solvent in the deposition process for its compatibility with natural rubber due to the similarity of the solubility constant for both materials. Utilising a vacuum assisted solution evaporation process to generate the pyrrole vapour is also crucial in maintaining the rubber substrate at its desired state and condition. Overall, the modification and adaptation of the available fabrication process has lead to a simple and controllable sensor fabrication process that is highly compatible for mass production. The sensor aims to address the issues associated with the finger position monitoring for hand exoskeletons. This has been achieved by the lightweight and low profile characteristics of the sensor. The sensor can operate discreetly and unobtrusively while adding negligible weight to the hand exoskeleton. Such a sensor can help to make the hand exoskeleton more portable and comfortable to use to enhance the rehabilitation process or strength augmentation.