GB2165062A

GB2165062A - Optically controlled actuator

Info

Publication number: GB2165062A
Application number: GB08424587A
Authority: GB
Inventors: David John Webb; Martin John Collier
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-09-28
Filing date: 1984-09-28
Publication date: 1986-04-03
Also published as: GB2165062B

Abstract

Compressed air to a pneumatic actuator 8 is supplied through a control device 1 having an air inlet 3 an air outlet 4 and a vent 5. Normally a smooth lamina flow stream passes between the inlet 3 and the outlet 4 and relatively little air is lost through the vent 5. The flow can be interrupted by supplying pulses of light from a source 9 along an optical fibre 11 and into the control device 1 where it strikes an absorber 6 producing acoustic energy which generates turbulence in the flow stream causing the compressed air to be vented at 5. The resulting reduction in pressure at the outlet 4 causes the pneumatic actuator 8 to produce some mechanical control function. In an alternative arrangement (not shown) the light falls on a flexible beam within the control device 1 and the resulting heating effect on the beam causes the latter to bend into the flow stream producing a similar effect. <IMAGE>

Description

SPECIFICATION An optical sensor This invention relates to an optical sensor.

Optical sensors are increasingly being considered today as alternatives to conventional electrical sensors, especially in hazardous environments, such as in the petrochemical industry, and in situations where insensitivity to electromagnetic interference may be advantageous.

It would be highly desirable to extend the advantages of optical sensors as described above to a complete control system. There are however problems in supplying down a fibre the necessary amount of optical power to produce mechanical actuation of for example switch gear, manipulators or valves.

A partial solution to this problem has been proposed in a paper entitled "The Optical Actuation of a Process Control Valve" given by Collier, McGlade and Stephens in the first international conference on optical fibre sensors 26/28th April 1983, IEE Con. Pub. No. 221. This approach converts the available optical power into a small electrical current using a p.i.n. diode, and then uses this current to actuate a conventional electro-pneumatic transducer. This "hybrid" approach provides the necessary electrical isolation between a control area and a valve to be actuated, and only uses a small electrical signal in the transducer (0-200 FA), but nevertheless, an entirely opto-pneumatic interface would be preferable and, in some circumstances, mandatory.

This invention provides an optically controlled actuator comprising means for directing visible or infra-red light onto an optical receiver which is constructed and arranged so that it induces, in response to the said light, turbulence in a fluid flow path.

The turbulence can be induced by using only a minimal amount of energy which can conveniently be provided by optical transmission along a fibre.

There are a number of ways in which the optical energy can be used to induce the turbulence. For example it could be used to produce a temperature differential resulting in the movement of an obstruction into the flow path. A preferred technique however is to modulate the light periodically e.g.

by providing it in the form of pulses and to direct it either onto an optical receiver which is mechanically resonant, the resulting vibrations causing turbulence; or, and this is the preferred technique, to direct it onto a photo acoustic element which generates sound or other pressure waves which are transmitted into the flow path to cause the turbulence.

With reference to the term "photo acoustic element" it may be useful to explain here that if intensity modulated optical radiation is incident on a solid absorber surrounded by a gas, there will be a periodic heating and cooling effect at the surface which will be communicated to the gas adjacent to the absorber. This gas will expand and contract at the frequency of the modulated light, and this piston-like behaviour will generate acoustic waves in the surrounding gas. The magnitude and phase of the waves, with respect to the incident optical power, depends on several factors, notably the optical absorption and thermal diffusion lengths of the solid absorber, the magnitude decreasing rapidly with frequency. The production of an acoustic signal by the absorption of modulated radiation is known as the photo-acoustic effect and was first observed by Alexander Graham Bell in 1880.The absorber is referred to as an "photo acoustic element." Two particular ways in which the invention may be performed will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a fluid flow controller constructed in accordance with the invention and employing a photo acoustic element; Figure 2 illustrates part of another fluid flow controller also constructed in accordance with the invention, shown as a cross section through a vertical plane; and Figure 3 illustrates a cross section through a horizontal plane of the apparatus of Figure 2.

Referring firstly to Figure 1 the illustrated apparatus comprises a tubular casing 1 having a cylindrical wall provided with a window 2 and two end walls having respectively an air inlet 3 and an air outlet 4. The inlet and outlet 3 and 4 are arranged along the axis of the casing 1 so that they are in line with each other.

Also provided on the cylindrical part of the casing 1 is a vent 5 and an opto-acoustic absorber 6 which is directly opposite the window 2. The optoacoustic absorber can be any solid material, preferably of black colour so as to absorb as much optical energy as possible. It cold simply be formed by part of the wall of the casing 1.

An air compressor 7 supplies air to the inlet 3 and the outlet 4 is connected to a pneumatic actuator 8 which performs a mechanical function in response to the receipt and/or release of pneumatic pressure in the line 4.

A light source 9 produces pulses of light under the control of a pulse generator 10. The light is transmitted down a fibre 11 and through the window 2 onto the opto-acoustic absorber 6. The resulting sound waves generated by the absorber 6 are directed into a flow path of air between the inlet 3 and outlet 4 causing turbulence which reduces the pressure at the outlet 4 causing the air to be vented at 5. The resulting change in pressure at the air outlet 4 controls the operation of the actuator 8.

The construction shown in Figures 2 and 3 is in many ways similar to that of Figure 1 and similar components are denoted by the same reference numerals suffixed by the letter A. In this construction the light shines onto a point 12 of an optoacoustic absorber in the form of a flexible beam cantilevered at one end to the casing 1A. The optical energy heats the upper surface of the beam 6A causing differential expansion between its upper and lower surfaces. This results in the beam bending downwardly to a position where it obstructs the flow path between the inlet 3A and outlet 4A causing turbulence and the resulting venting of the air at 5A and reduction in pressure at outlet 4A.

In the arrangement of Figures 2 and 3 it would be possible to use a pulsed light source as for the arrangement of Figure 1 and to tune the frequency of the pulses to the resonant frequency of the beam 6A causing vibration thereof on the flow path. This is not however necessary and a continuous light source could be used simply to move the beam 6A into a position where it obstructs the flow path. The beam 6A is preferably of heat insulating material so as to achieve as great a temperature differential as possible between its opposite surfaces. A high coefficient of thermal expansion is also obviously desirable in order to achieve the greatest bending effect for a given amount of optical energy.

Claims

1. An optically controlled actuator comprising means for directing visible or infra-red light onto an optical receiver which is constructed and arranged so that it induces, in response to the said light, turbulence in a fluid flow path.

2. An actuator according to claim 1 including control means for periodically modulating the light and in which the optical receiver is designed to convert the optical modulations into mechanical vibrations which induce the turbulence.

3. An actuator according to claim 2 in which the optical receiver includes a solid resonant element arranged to receive the modulated light and having a resonant frequency with matches the frequency of the modulations of the light

4. An actuator according to claim 2 in which the mechanical vibrations are pressure waves and in which the optical receiver is arranged so as to direct the pressure waves into the fluid flow path to induce the turbulence.

5. An optically controlled actuator substantially as described with reference to Figure 1 of the accompanying drawings and substantially as illustrated therein.

6. An optically controlled actuator substantially as described with reference to Figures 2 and 3 of the accompanying drawings and substantially as illustrated therein.