Novel sensors show their diversity

A range of non-contact sensors, based on patented technology, is being developed by a British company. The radio frequency 'RF Sensors' - developed by Oxford University spin out firm Oxford RF Sensors, based in Weston on the Green in Oxfordshire - work with ferrous and non-ferrous metals, composites, glass, plastics and liquids. They can measure linear and rotational position, speed and displacement; fluid flow; fluid level; fluid contamination; fluid viscosity; delamination in composites; and can identify plastics. The RF sensors are immune to electrostatic contamination, are immune to dirt and dust, are very small and can be built on ASICS. They operate from DC to 1GHz and from temperatures as low as -170°C up to 1,000°C. The company's CEO Ross Walker told Eureka: "The platform sensor technology we've developed overcomes many of the operating constraints of existing technologies. Our sensors can measure higher speeds and higher resolution at higher operating temperatures. The technology works with almost any material, doesn't require magnets or phonic wheels and has very low power consumption, making it ideal for wireless applications. The sensors also work with fluids to detect water in oil, particulates and chemical changes." The RF sensor works in a unique way. According to Walker, all materials have four distinguishing electromagnetic parameters, namely the 'real' and 'imaginary' parts of the electrical permittivity (e' and e'') and the magnetic susceptibility (X' and X''). "The sensor operates by defining a sensitive volume and interrogating the values of these parameters for whatever materials invade this volume. Any or all of the parameters can be simultaneously measured by either a single sensor or a sensor pair, and the sensor deduces the nature and behaviour of the invading items from the resulting signature," he explained. The sensitive volume is defined by a radio frequency antenna, which is apparently then "tuned" to the sensor electronics. The size and nature of the antenna and the nature of the material detected, govern the optimum operating frequency of the sensor, which, said Walker, "can be a few Kilohertz for large systems or tens of Gigahertz for very small ones". An inductive antenna is used for susceptibility detection and a capacitive antenna is used for permittivity detection. The size and shape of the antenna determines the sensitive volume. And choice of construction materials of the antenna determines permissible operating temperatures. As Walker explained to Eureka: "The sensor operates by exciting the antenna at the specified operating frequency. Changes in the real part of the electromagnetic parameter being measured change the operating frequency. Changes in the imaginary parameter produce amplitude variations in the carrier. The frequency and amplitude modulations are then processed to produce the information needed, such as speed or position or composition." The technology is a cost effective alternative to existing inductive/eddy current sensors, although at present the sensors have only been developed for small-scale development projects for customers. However, RF sensors have several distinct advantages over eddy current/inductive sensors: they work with any material; have unlimited speed, a wider temperature range; and lower power consumption. Current development projects include a high resolution angular/linear position sensor; a high temperature (>300°C) speed sensor; remote temperature measurement; mass flow; fluid level detection; water-in-oil measurement; particulates-in-fluid measurement; fluid composition; and rubber conveyor belt positioning. One development project is with Oxfordshire based motorsport company Owen Developments. The TS-180 speed sensor is for specialist turbocharger companies, engine designers and motorsport teams. The rotational speed of the turbocharger is a key factor when sizing turbine and compressor wheel combinations. The best, most reliable turbocharger will be the one which stays within the manufacturer's limits, which are defined by the use of the relevant flow maps. The TS-180 sensor is able to cater for any existing or anticipated turbocharger speeds and temperatures. Unlike other sensors manufactured for this purpose, it uses a custom-developed miniature (6mm diameter) probe that radiates a high frequency electromagnetic field, which detects the passing of each turbocharger blade directly. It does not require magnetic or phonic targets, which is why it operates perfectly with aluminium, titanium, magnesium and Inconel impellers. A control unit is adjusted to the number of blades being counted. Then, as the blades pass the sensor, energy is 'pulled' from the oscillator. This energy drain is then converted to a digital pulse which can be fed to a display or data logger. For certain motorsport applications, where turbo over-speeding is a potential failure problem, the pulses can be used in conjunction with the vehicle's ECU, data logger or tachometer as a means of control. Over-speeding is controlled by a waste gate that opens to channel surplus exhaust pressure back to the car's exhaust. Inlet manifold pressure provides second order information, but cannot cope with partial or complete vacuum on the inlet impeller. The TS-180 does not require calibration, has maximum operating temperature of 200°C and maximum range of 2mm (400 micron blade width). RF sensors also look ideal for aerospace applications. Walker cited fuel flow; fuel level; hydraulic condition (water in hydraulic oil); fluid condition; particulates in fluid; oil condition; blade stretch; blade pass; remote temperature sensing; rotational speed sensing; and angular position sensing, as possible uses for the technology. The automotive recycling sector may also benefit from the technology. RF sensors can identify plastics and metals used in vehicle manufacturing. The sensors can be incorporated into an automatic conveyor recycling system, helping to sort plastic components from metal parts, reducing recycling costs. Another big application area is fluid condition monitoring. The sensors can detect in real time the number of particulates in a fluid - the concentration and size of ferrous, non-ferrous and non-metallic particulates. Soot loading can be measured in lubricating systems. The amount of water dissolved or suspended in immiscible or miscible liquids, such as oils and hydraulic fluids can also be calculated, down to concentrations as low as 0.5ppm. Physical changes of fluids can also be detected, including the formation of waxes and ice, and the precipitation of other components in fluid delivery lines. Oxford RF Sensors filed a world-wide patent application in March 2001 although new patent applications are currently in preparation. The company is seeking license partners to manufacture the sensors, hopefully in very high volumes.