GB2189361A - Optical sensor system - Google Patents

An optical sensor system has a number of sensors 3,4 distributed along an optical fibre 2 which continuously transmits optical energy to them. This energy is stored in the sensors on capacitors 9,10. A selected sensor is addressed and only in response to receipt of its address does a sensor use the stored energy to transmit an output signal. <IMAGE>

SPECIFICATION A sensor system This invention relates to a sensor system and arose during a project whose aim was to produce a system in which a number of conventional electrical sensors could be powered, accessed and read via optical fibre links only.

A major problem associated with the system as described above is that of supplying adequate optical power to drive the sensors and the present invention aims to assist in overcoming this problem.

The invention provides a sensor system comprising a plurality of sensors, means for directing radiation to the sensors, and means for transmitting to the sensors an address identifying a sensorfrom which data is required, each sensor comprising means for converting the radiation into electrical energy and storing the latter, means for storing an address identifying thatsensorand means for responding to receipt of an address which matches the stored address by using the stored energy to transmit radiation representing output data on the sensor.

The "radiation" referred to is preferably electromagnetic radiation. The invention was developed in connection with wavelengths in the infra-red or visible regions but it is possible that the invention could be used for other frequencies.

It will be appreciated that each sensor can be storing energy received during periods when it is not required to produce an output in readiness forthose occasions, which may be infrequent, when it is required to produce an output. The means for transmitting addresses can address the sensors in a pre-set ordered sequence or simply as and when particular information is required in no particular sequence. The address can be transmitted as an optical signal along a fibre but an alternative possibility would beto use afocussing device to transmit a beam of radiation to the sensor without any guiding device such as a fibre. Likewise the means for directing radiation and the meansfor transmitting the output data are preferably designed to use optical fibres but could alternatively transmit beams of radiation.

The means for storing the electrical energy preferably includes a capacitor but any other storage device such as an electrolytic cell could be used as an alternative.

It is desirable to reduce as much as possible the energy required by each sensor when in a "stand-by" condition in which it is ready to respond to receipt of an address, which may or may not match its own address. To this end the means for responding to receipt ofthe address is preferably arranged to be energised via a switch which is switched on only in response to receipt of an incoming address. So each address preferably includes a flag which may be a binary digit 1. This flag serves to switch on the means for responding to receipt of the address in each sensor.

The means for directing radiation to the sensors can take the form of a pulsed laser. The resulting pulses of electricity at each sensor can then conveniently be passed through the primary of a step-up transformer enabling the capacitor or other electrical store to be charged to a suitable voltage for operating the electronic circuitry.

Each sensor preferably includes a sensing element which is connected to control or included in part of an oscillator, the arrangement being such that the frequency ofthe oscillator is a function of the quantity to be sensed. The output of the oscillator can then be used to produce an output signal.

This output signal is preferably intheform of pulses having sharp leading and trailing edges and it has been found desirable,forthe purpose of reducing the required power oftransmission, to use a duty cycle which is less than 50% and preferably less than 20%. A 10% duty cycle is considered to be about ideal.

One way in which the invention may be performed will now be described byway ofexamplewith reference to the accompanying drawing of a part of a sensor system constructed in accordance with the invention.

Referring to the drawing a laser 1 produces pulses oflightwhich ittransmits along an optical fibre 2.

The fibre 2 leads to a plurality of sensors of which onlythefirstand last,3and 4, are shown. These sensors are all identical so onlythe sensor 3 is shown in detail.

Each sensor is enclosed in a conductive box to isolate its surroundings from the electrical components within it. The fibre 2 directs light onto an optical to electrical transducer in the form of a solarcell 5. The output of the solar cell 5 is passed to the primary of a step-up transformer 6 whose output is rectified by diodes 7 and 8 and stored in capacitors 9 and 10. These capacitors have a sufficient capacitance to maintain a 2 volt potential across power rails 11 and 12 when the sensor is producing an output signal. The rails 11 and 12 are connected in a manner (not shown for simplicity of illustration) to power all the other components ofthe sensor.

An LEDl3iscontrolledbyacomputerl4to produce the address of a sensor from which an output reading is currently required. The address is preceded by a flag in the form of a single pulse of light.

The output from the LED 13 passes along a fibre 15 to all the sensors. In the sensor 3 it is received by a pin diode 16 which, in response, produces a pulse of electricity which is detected by a receiver 17. The latterthen starts a clock 18which operatesfora pro-set time and then stops. During this time the clock clocks into a shift register 19 the address which is received following the aforementionedflag. The contents of shift register 19 is compared in a series 20 of exclusive OR gates with a pre-set address held in a store 21 which address uniquely identifies the sensor. The outputs of the exclusive OR gates 20will all be zero if a match is detected between the stored and received addresses.Such a series of zeros is detected by a logic circuit 22 which then switches on an oscillator 23 and a pulse generator 24.

The oscillator 23 includes a sensing element 23A which controls its frequency. The output ofthe oscillator23 is converted to pulses having sharp leading and trailing edges and having a 10% duty cycle. These are converted into optical form by an LED 25 and the output from this LED is passed into a fibre 26 via a directional coupler 27.

The other end ofthefibre 26 is connected to a receiver 28 which includes an optical to electrical transducer which converts the received optical pulses into electrical pulses having the same frequency. These electrical pulses then pass to a frequencydetectorwhich produces a binary signal representing the required sensor measurement. This binary signal is entered into the computer 14. The computer 14 also receives data on line 1 4A in response to which it makes decisions concerning the identity of sensors to be addressed at different times.

The computer also produces an output signal on line 148 which constitutes the output of the system .

It is envisaged thatthe illustrated system will be of valueforuse in hazardous environments and in areas where high E.M.I. is expected. The petro-chemical, mining and sewage industries, for obvious reasons cannot allow electrical cables or high power electronics in their hazardous sites. The fibre optic coupled, low powersensors described with reference to the accompanying drawing could be used at little risk at such sites. The ability ofthe optical fibres to resist interference from E.M.I.

sources would makethesystem useful in the power industry, for monitoring transformer, machine and motor characteristics. Other applications include military data bus systems, e.g. on aircraft or on ships or on the battlefield.