Andy Cranny

A Novel Thick-Film Piezoelectric Slip Sensor for a Prosthetic Hand

The ability to mimic the tactile feedback exhibited by the human hand in an artificial limb is considered advantageous in the automatic control of new multifunctional prosthetic hands. The role of a slip sensor in this tactile feedback is to detect object slip and thus provide information to a controller, which automatically adjusts the grip force applied to a held object to prevent it from falling. This system reduces the cognitive load experienced by the user by not having to visually assess the stability of an object, as well as giving them the confidence not to apply unnecessarily excessive grip forces. A candidate for such a sensor is a thick-film piezoelectric sensor. The method of fabricating a thick-film piezoelectric slip sensor on a prototype fingertip is described. The construction of experimental apparatus to mimic slip has been designed and analyzed to allow the coefficient of friction between the fingertip and the material in contact with the fingertip to be calculated. Finally, results show that for a coefficient of friction between the fingertip and grade P100 sandpaper of approximately 0.3, an object velocity of 0.025plusmn0.008 ms-1 was reached before a slip signal from the piezoelectric sensor was able to be used to detect slip. It is anticipated that this limiting velocity will be lowered (improved) in the intended application where the sensor electronics will be powered from a battery, connections will be appropriately screened, and if necessary a filter employed. This will remove mains interference and reduce other extraneous noise sources with the consequence of an improved signal-to-noise ratio, allowing lower threshold values to be used in the detection software

Thick film silver - silver chloride reference electrodes

The fabrication of prototype thick film silver-silver chloride electrochemical reference electrodes is described. Combinations of commercially available and proprietary thick film pastes have been used in their construction in a multi-layer planar configuration modelled upon the structure of the classic single junction silver-silver chloride reference electrode cell. Several variations in the basic electrode design were fabricated, involving combinations of one of three different paste formulations for the silver-silver chloride layer coupled with one of two combinations of paste formulation for the salt containment matrix. The relative performances of these different versions of reference electrode were evaluated in terms of their chloride ion sensitivity, hydration times required to achieve a stable potential and usable lifetime. It is shown that, depending on the processing methodology employed at certain stages in the fabrication of these devices, a large degree of variation in characteristics can be achieved and therefore exploited in the design of reference electrodes suitable for a range of specific applications.

Review on the development of truly portable and in-situ capillary electrophoresis systems

Capillary electrophoresis (CE) is a technique which uses an electric field to separate a mixed sample into its constituents. Portable CE systems enable this powerful analysis technique to be used in the field. Many of the challenges for portable systems are similar to those of autonomous in-situ analysis and therefore portable systems may be considered a stepping stone towards autonomous in-situ analysis. CE is widely used for biological and chemical analysis and example applications include: water quality analysis; drug development and quality control; proteomics and DNA analysis; counter-terrorism (explosive material identification) and corrosion monitoring. The technique is often limited to laboratory use, since it requires large electric fields, sensitive detection systems and fluidic control systems. All of these place restrictions in terms of: size, weight, cost, choice of operating solutions, choice of fabrication materials, electrical power and lifetime. In this review we bring together and critique the work by researchers addressing these issues. We emphasize the importance of a holistic approach for portable and in-situ CE systems and discuss all the aspects of the design. We identify gaps in the literature which require attention for the realization of both truly portable and in-situ CE systems.

Thick-film force, slip and temperature sensors for a prosthetic hand

Thick-film static and dynamic force sensors have been investigated for their suitability to measure the grip forces exerted upon an object held by a prosthetic hand, and to detect and correspondingly react to the possible slip of a gripped item. The static force sensors exploit the piezoresistive characteristics of commercially available thick-film pastes whilst the dynamic slip sensors utilise the piezoelectric behaviour of proprietary PZT pastes (lead zirconate titanate). The sensors are located upon stainless steel cantilever type structures that will be placed at the fingertips of each digit of the prosthetic hand. Temperature sensors are also included to provide temperature compensation for the force sensors and to prevent accidental thermal damage to the prosthesis. Results have shown that the static force sensor is capable of measuring fingertip forces in excess of 100 N, with an electrical half-bridge configuration sensitivity approaching 10 ?V/V•N (with scope for improvement) and maximum hysteresis below 4% of full scale, depending on the manner by which the cantilever sensor array is attached to the finger. Failure in the bonding mechanism that secures the PZT layer to the stainless steel cantilever meant that the proposed dynamic force sensor could not be evaluated. However, investigations using the same sensor design fabricated on an alumina substrate have shown the potential of the PZT dynamic force sensor to measure vibration and hence potentially operate as a slip sensor.

An investigation of the performance characteristics and operational lifetimes of multi-element thick film sensor arrays used in the determination of water quality parameters

An investigation of the useful lifetimes of thick film sensors suitable for the measurement of various aqueous parameters is reported. Batches of sensor arrays designed to simultaneously measure temperature, conductivity, pH, dissolved oxygen and redox potential were investigated. In particular the device to device repeatabilitys of the pH and dissolved oxygen sensor elements were studied as it was found to be the characteristics of these elements that limit the device lifetimes. Errors of less than 5% for pH sensors and 10% for dissolved oxygen sensors were observed over periods of several months continuous immersion in water. An assessment of useful continuous immersion lifetimes was made for the sensors and possible sensor failure mechanisms are postulated.

Control strategies for a multiple degree of freedom prosthetic hand

Some of the traditional methods used to control a conventional prosthetic device are described alongside the current state of new control techniques and how they may progress. The review includes implantable myoelectric sensors and describes the potential of connecting directly to the peripheral nervous system. Control methods are then deduced for each technique, where the application is a six degrees of freedom hand having integral slip, force and temperature sensors.

Application of Distributed Wireless Chloride Sensors to Environmental Monitoring: Initial Results

Over the next 30 years, it is anticipated that the world will need to source 70% more food to provide for the growing population, and it is likely that a significant amount of this will have to come from irrigated land. However, the quality of irrigation water is also important, and measuring the quality of this water will allow management decisions to be made. Soil salinity is an important parameter in crop yield, and in this paper, we describe a chloride sensor system based on a low-cost robust screen-printed chloride ion sensor, suitable for use in distributed sensor networks. Previously, this sensor has been used in controlled laboratory-based experiments, but here we provide evidence that the sensor will find application outside of the laboratory in field deployments. We report on three experiments using this sensor; one with a soil column, one using a fluvarium, and finally on an experiment in a greenhouse. All these give an insight into the movement of chloride over small distances with high temporal resolution. These initial experiments illustrate that the new sensors are viable and usable with relatively simple electronics, and although subject to ongoing development, they are currently capable of providing new scientific data at high spatial and temporal resolutions. Therefore, we conclude that such chloride sensors, coupled with a distributed wireless network, offer a new paradigm in hydrological monitoring and will enable new applications, such as irrigation using mixtures of potable and brackish water, with significant cost and resource saving.

A low cost oxygen sensor fabricated as a screen printed semiconductor device suitable for unheated operation at ambient temperatures

A novel sensor for the determination of atmospheric oxygen levels has been fabricated as a low-cost, screen-printed, conductimetric metal oxide film device. The sensor can be operated at ambient temperatures without recourse to heating of the sensing element and can employ simple detection circuits due to the relatively high level of conductance of the metal oxide films. High sensitivities to changes in oxygen concentration with good repeatability are reported. Device cross-sensitivity to the environmental parameters of temperature and humidity have also been investigated.

Sensors for Corrosion Detection: Measurement of Copper Ions in 3.5% Sodium Chloride Using Screen-Printed Platinum Electrodes

Planar screen-printed platinum electrodes developed for use in corrosion monitoring have been evaluated using cyclic differential pulse voltammetry and shown to detect cupric ions (Cu2+) over a range up to 100 mM in a background of 3.5% by weight sodium chloride solution. The reduction of Cu2+ to metallic copper is shown to proceed as two successive single-electron transfer reactions involving an intermediate chemical step where the cuprous ion (Cu+) is complexed by chloride to form the dichlorocuprous anion CuCl2-. By comparison, the complexation step during the oxidation of copper to Cu2+ can involve a number of different chlorocopper(I) complexes of the general form [CuCl(n+1)]n- depending on the chloride concentration, which can make detection via a stripping reaction difficult.

The use of pattern recognition techniques applied to signals generated by a multi-element gas sensor array as a means of compensating for poor individual element response specificity

It has been hypothesised that the lack of specificity associated with organic semiconductor gas sensors can largely be overcome with the adoption of a multi-element sensor array, thereby allowing the elimination of unwanted sensitivities through suitable signal processing. The use of a pattern recognition strategy in the realisation of such an “intelligent” chemical sensor is implicit in the assumption that individual gas species can be identified against a background of various interfering gases. It is, however, infeasible to consider the characterisation of sensor responses to individual gases and the building of a set of pattern recognition rules other than by automatic means. Thus the approach reported here was to enable the intelligent chemical sensor to learn the various response patterns associated with particular analytes and hence build a knowledge base from which future inferences may be drawn. This paper describes how a multi-element array of gas sensitive metal phthalocyanine films, constructed on a single thick-film substrate, was used as the sensing element in an intelligent chemical sensor. Since the individual sensor sites may be coated with different phthalocyanines showing varying degrees of gas sensitivity, the individual responses of each to any particular analyte will give rise to a characteristic change in the output template comprised of each of the sensor resistances. By monitoring the change in this template on exposure to specific gases of predetermined concentration and employing a suitable feature extraction algorithm, the characteristic responses to particular analytes were learnt. The success of suitable signal processing techniques to accommodate the inherent cross-sensitivities exhibited by metal-substituted phthalocyanine film gas sensors is demonstrated. The results clearly show the viability of pattern recognition methods to analyse gas mixtures through the mathematical evaluation of data from gas sensor arrays as a means of improving the selectivity and specificity.