Design and development of a distributed planar pressure sensor utilising electrical impedance tomography.

This thesis describes an investigation into the use of electrical impedance tomography used in conjunction with a flexible conductive sensor for the measurement of distributed pressure. The main application areas are for the constant monitoring of the pressure distribution between a patient and their support surface i.e. beds and wheel chairs, in order to reduce the formation of pressure sores and tactile sensing for robotics. A number of systems have been developed for the monitoring of patients but non-have proved suitable for constant monitoring and these are reviewed. A review of the tactile sensor techniques used in robotic grippers is presented and when the area to be monitored is relatively small (1-2 cm2) the techniques already under development can provide the resolution required. However no technique exists to measure distributed pressure over a large area. A review of both the hardware and reconstruction algorithms used in electrical impedance tomography is presented and the design of the hardware and software developed for the investigation into the sensor design is detailed. As the sensor is such that electrodes are not limited to the periphery both an experimental and computer simulated comparison of three different electrode configurations is described. The three-electrode arrangements investigated are with the electrodes placed at the periphery of both a circular and square boundary, and with electrodes evenly distributed across a square area. The results from the comparisons show that the new distributed electrode arrangement performs significantly better than when the electrodes are confined to the periphery. It also shows that the geometry of the boundary when using peripheral electrodes can also effect the performance of an EIT system. The initially investigated sensor design was based on a conductive polymer sheet and a number of samples were characterised in term of their V/I characteristics and their creep and resistance change due to applied pressure. Only one of the sample tested had a response worth investigating further but the material could not be obtained for larger area tests. Therefore an alternative sensor design was investigated. This novel sensor consisted of a conductive fluid retained beneath a flexible rubber membrane. From electrical impedance tomography images obtained from the experimental evaluation of the new sensor design it is shown that the system can image the pressure distribution across its surface. In addition, the analysis of the unprocessed data from the new sensor shows the system to have a well-defined response with a wide applied pressure range and the construction of the sensor is such that its response could be tailored to the range of pressure to be measured.