This invention relates to remote sensors.
It is frequently required to detect the presence of fire, smoke or toxic or flammable gases in locations where the presence of electrical transducers would present an additional fire hazard. Various ideas have been proposed to solve this problem by the use of optical fibres to convey signals to and from the transducer.
The present invention has the object of providing a simple low- cost sensor.
According to the present invention there is provided a remote sensor for sensing remotely the change in a physical parameter, the sensor comprising a source optical fibre providing an optical or infra-red input signal, a focussing arrangement for focussing the input signal on the end of a detector fibre or back into the input fibre, and means for detecting a change in the returned signal caused by a change in the parameter and for indicating the fact.
In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which:
Figure 1 shows diagramatically one embodiment of the invention and Figure 2 shows a modification of Fig.1.
A basic form of the invention is illustrated in Figure 1. A cavity 1, open to the atmosphere, permits the ingress of the gas or smoke to be detected.
The length of the cavity will be dictated by the nature of the material to be detected and the order of sensitivity required.At one end of the cavity are provided a source fibre 2 and a detector fibre 3, located in conveniently close proximity to each other. At the remote end of the cavity is placed a concave spherical mirror 4, having a radius of curvature corresponding to the length of the cavity and a diameter sufficient to intercept the whole of the cone of light rays emerging from the source fibre 2. The spacing and angular relationship of the mirror and the two fibres is made such that the core end-face of the source fibre is imaged, in focus, upon the core end-face of the detector fibre.
The sensor so described can be placed in the hazardous area and the pair of optical fibres can be extended to a suitable safe position where the light source X, and detection circuits may be installed.
The light source, feeding into the remote end of the source fibre, might be a simple black-body radiator or a sinusoidally or pulse modulated device such as a semiconductor light-emitting diode or laser.The remote end of the detector fibre may feed any suitable photodetector Y appropriate to the wavelength of the transmitted light.
It will be apparent that the simplicity of the device leads to potentially very low production cost, and that, for appropriate applications, an extremely compact assembly may be achieved. For extremes of temperature or corrosive atmospheres, the complete assembly, including the mirror, may be constructed of a suitably resistant material, such as stainless steel, the limit being imposed only by the durability of the optical fibre material.
In the embodiment of Figure la a modification is envisaged in which the source fibre 2 and detector fibre 3 of Fig. 1 are one and the same fibre 2a (or bundle of fibres), and a beam splitter such as a half-silvered mirror 21 arranged to separate the "go" and "return" signals from the source X and to the detector Y.
Other modifications to this basic assembly are indicated, where appropriate, in describing, below, the various applications of the device.
In its simplest concept, the amount of light traversing the cavity is monitored so as to detect any obstruction in the path. The presence of smoke or dust particles may therefore be detected. It follows also that limit switching or counting functions may be performed by the deliberate insertion of an opague vane by some external mechanism to be monitored.
By making the concave mirror integral with the front face of a slug of material having a high thermal capacity, for example a stainless steel block, the device may readily sense the atmospheric conditions that give rise to condensation.A typical sudden rise in air temperature will not immediately be followed by the thermal slug, leaving the mirror surface at a lower temperature than the ambient air. In consequence, the suspended moisture will condense upon the mirror surface, scattering the reflected light rays and thereby signalling the condition by reducing the output light intensity.
Specific Gas Detector
In the foregoing two examples, the wavelength of the light is not critical, and might occupy all or any part of the transmission pass-band of the optical fibres. However, the device may equally well be used as a remote gas cell for an infra-red gas analyser, subject only to the requirement that the optical fibres shall be capable of transmitting the working length dictated by the absorption spectrum of the gas to be detected. Typically, light from a blackbody source, i.e. a wideband source, is passed through a suitable cavity into which may enter the gas to be detected, before finally falling upon a suitable photodetector. Two narrow-passband optical filters are alternately interposed in the light path at a suitable chopping frequency.The passband of one filter is chosen to be coincident with a suitable absorption peak of the gas to be detected, while the passband of the other filter is placed in a part of the optical spectrum unaffected by the gas in question or other interfering gases.
The presence of the gas to be detected will reduce the light intensity during that part of the chopping cycle during which the first filter is interposed, and is consequently signalled by an alternating output from the photodetector.ln the present application, the device here described serves as the gas cell, the light from the source being first directed through the source fibre and, after passing through the cavity, finally entering the photodetector via the detector fibre. As a typical example, the detection of hydrocarbon gases, such as propane, having an absorption band in the vicinity of 3 microns, may be achieved by the use of silica (quartz) fibres, which transmit at this wavelength.
Detection of Micromechanical Movement
It is proposed also that a slight angular change in the position of the concave mirror will cause the projected image to move away from the detecting fibre-end. By the provision of one or more additional detector fibres, with their associated photodetectors, the angular deviation of the mirror may cause the projected image to fall upon each of the fibre-ends in turn. The array of detector fibres may thus be so distributed - adjustably if required - as to signal specific increments of the angular movement of the mirror.
This is shown schematically in Fig. 2. Referring to Fig. 2 a source fibre 12 remote from a source 12a presents optical or infra-red radiation to a concave spherical mirror 14 which is pivotally mounted at 15 and moved by a lever 16 in independence upon e.g. the movement at a load cell (not shown), a temperature responsive device such a bimetal element, or some other device sensing or measuring the change in a physical parameter, at position 18.
The reflected radiation is detected by a number e.g. four detector fibres 13a, 13b, 13c, 13d, dependent upon the degree of tilting of the mirror.Each fibre would have an associated photodetector 17a to 17d, whose output could be used to give a temperature or load measurement, or measurement of some other physical parameter.
This mechanism may be made the basis of a further range of applications. As a first example, by simple leverage the mirror may respond to the compression of a load cell, enabling a succession of calibrated outputs, or an overload indication, to be signalled. A variety of similar mechanical movements might be so detected, another example being the monitoring of temperature by the expansion of a suitable rod of material coupled to the mirror level. An enhancement in this latter example might be to mount the mirror on a bi-metallic strip, so achieving a sensitive design simplification.
An important aspect of the method described is that the distribution of the detector fibres may be made such as to compensate for the "law" of the mechanical excursion. Thus, within limitations imposed by the number of detector fibres provided, a non-linear excursion might be linearised in the final output by progressively changing the separation between adjacent fibres across the width of the array.
If the projected image is not greater than a fibre diameter, dead spots will arise as the image traverses from one fibre to the next. This effect may readily be overcome by judiciously defocussing the image so as to embrace two adjacent fibre- ends.
It is also envisaged that the linear array of fibres 13a to 13d could be extended to a two-dimensional array to cooperate with pivotal movement of the mirror 14 in two directions. Thus a second lever at right angles to lever 16 could provide the other dimensional movements in response to a different physical parameter to that which controls the first lever.
The foregoing description relates to an overall objective of developing a simple, inexpensive and versative sensor unit which may be assembled, to suit a variety of applications, from a limited range of mass-produced, interchangeable piece-parts. For most applications, the device might be limited to a two- fibre system, associated with an inexpensive light-emitting diode source and a single photodetector. To bestow immunity to ambient lighting, the source might normally be amplitude- modulated at an appropriate frequency and the photodetector supplemented by a tuned or synchronouslyswitched amplifier, followed by an indicator or alarm output device. Progressive enhancement might, nevertheless, extend to the provision of a continuous ladder array of detector fibres, critically distributed as described above, in which case it would be appropriate to terminate these at a multiple CCD photodetector array such as is currently available. However, for a simple and inexpensive installation, it would be entirely practical, by using a visible light source, to group the remote ends of the detector fibres on a display panel for direct viewing by the human operator. As a final extension of the "building block" concept, for applications where the electrical hazard is not present, components might be added to the range so as to enable the fibre-end unit to be replaced by a module carrying an adjacent light source and photodetector directly in communication with the mirror cavity and conventionally hard- wired to the display or alarm position.