Smiths Industries Plc

Smiths Industries Public Limited Company

Douglas V Luke

Peter D Baker

A tire pressure sensing system has a sensor mounted on the tire hub, the sensor having a mask that is moved on change in tire pressure. Two rows of fibres at the end of a fibre-optic cable are positioned on opposite sides of a slit in the mask. The fibres of the two rows extend along a central core and an outer ring respectively of the cable. The rear end of the cable is mounted coaxially of the tire in alignment with a stationary cable having an inner core and an outer ring which extend to a radiation source and to a detector respectively. The source supplies radiation, via the cable, to the outer ring of the cable and hence to the row of fibres. Change in pressure moves the mask, thereby changing the amount of radiation received by the other row of fibres which passes to the outer ring of the stationary cable and thereby to the detector. The detector monitors the radiation and provides an output indicative of tire pressure.

Tire pressure sensing systems

A radiation pyrometer, or other optical sensor, has a mirror assembly capable of withstanding high temperatures. The mirror assembly is a block of a steel containing between 15 and 20% chromium, 4.0 and 5.2% aluminium, 0.05 and 0.4% yttrium, 0.2 and 0.4% silicon, and 0.00 and 0.03% carbon. A reflective surface on the assembly is provided by an aluminium oxide layer. The assembly is formed by heating a body of the steel such as to produce the aluminium oxide layer which is then polished to a reflective finish. After heating, the body is machined to the desired shape of the mirror assembly.

Reflective elements and sensors including reflective elements

A fibre-optic cable has a bundle of glass-fibre or silica-fibre strands extending along its length, each end of the fibre-bundle being sealed in a metal ferrule. A sheath of woven glass-fibre or silica-fibre extends about the fibre-bundle and is secured at each end to a respective ferrule in tension, such that the sheath is constricted onto the fibre-bundle along its length. The cable has an outer convoluted metal shell which is sealed at each end to a respective ferrule. A sleeve of elastic material extends around the fibre-bundle and the sheath, within the outer shell, and serves to support and cushion the fibre-bundle along its length. The sleeve may be of a woven fibre including glass-fibre or silica-fibre, and asbestos-fibre, or the sleeve may be of an elastomeric substance. A metal braid sleeve covers the outer surface of the convoluted metal shell. The cable may be included in a radiation pyrometer to transmit radiation to a radiation sensor.

Fibre-optic cable

A fibre-optic cable includes a fibre-bundle, for transmitting light along the cable, that is sealed at either end to a metal ferrule. Each end of the fibre-bundle projects a short distance from its respective ferrule and is enclosed by a metal collar which is welded to the ferrule. The projecting ends of the fibre-bundle are sealed to the ferrules by melting a glass tube within each collar about the ends of the fibre-bundle. A thread of glass or Silastomer material extends between the two ferrules and is helically wound about the fibre-bundle so as to hold together the fibres of the fibre-bundle. Convoluted outer metal tubing extends about the fibre-bundle and is welded at each end to respective ferrules. The fibre-bundle is cushioned within the outer tubing by a sleeve of woven fibre which also extends between the two ferrules about the fibre-bundle and the helical thread. The outer tubing may be evacuated or contain an inert gas at low pressure, or the outer tubing could contain a lubricant such as powdered polytetrafluoroethylene. Alternatively, a silicone oil could be used as a lubricant and, where the cable is to have one end mounted at a hot location, the silicone oil could be chosen to be a gel at that end while being a liquid where cooler, away from that end.

A pressure transducer has a capsule having two chambers separated from one another by a diaphragm. The capsule is made entirely of a glass-ceramics material, such as, lithium alumino silicate. One of the chambers is sealed and has a lens with a curved surface adjacent the diaphragm such that, when the capsule is illuminated, a Newton's Rings pattern is produced by interference between light reflected from the surface of the lens and the diaphragm. Pressure is communicated to the other chamber so as to cause the diaphragm to be deflected into or out of the sealed chamber with a consequent shift in the interference pattern which is used to provide a measure of the pressure change. The capsule also has two further chambers which are identical to the first two chambers and which are both sealed. One of these further chambers is illuminated to produce a second interference pattern. The difference between any shift in the two patterns gives an indication of the pressure change that is independent of temperature change. The two interference patterns may be produced by light from the same source supplied to the capsule via fibre-optic cables, the light being reflected onto the capsule by a prism assembly mounted directly on the capsule. Similarly the interference patterns may be focussed by the prism assembly onto an array of the ends of fibre-optic cables used to supply light from the patterns to photocells.

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Farnham Common, United Kingdom

Douglas V Luke