Pneumatic actuator

Abstract: A pneumatic actuator comprises a cylindrical cell (1) through which a normally laminar flow of air from a source (6) passes between an inlet jet (2) and an outlet jet (3) to provide an output pressure in outlet pipe (8). The cell (1) includes a piezo-electric element (9) arranged, in response to a suitably modulated oscillating electrical signal, to produce an acoustic signal that disturbs the laminar flow whereby air exhausts through exhaust ports (4) and the output pressure in the pipe (8) falls. The differing pressure outputs in the pipe (8) may be used to control the state of a pneumatic device such as a valve. The electrical signal is produced by a photo-diode (10), via a matching circuit (11), in response to a suitably modulated optical control signal generated by a laser source (12) and transmitted to the photo-diode (10) along a fibre optic cable (13). An actuator of the invention has extremely fast response times compared to prior pneumatic actuators, typically of the order of 5 milliseconds.

Description: This invention relates to pneumatic actuators. Pneumatically-actuated devices are, of course, very widely used in all sorts of fields, especially in the control field. Hitherto, it has been conventional practice to control such devices, such as fluid flow regulators, electrically using, for example, so-called I/P converters which produce varying pneumatic outputs in response to varying electrical input signals. By using such converters, remote and/or automatic control is possible. However, they do have disadvantages, especially as regards their response times, their susceptibility to electrical "noise" and, because of the need for relatively high power electrical control signals, their unsuitability for use in hazardous environments. In GB patent specification No 2165062A there is proposed a pneumatic actuator in which an optical control signal, as distinct from a relatively high power electrical control signal, impinges directly onto a photo-acoustic element that generates sound waves which induce turbulence in an otherwise normally laminar fluid flow path. The turbulence causes a change in output pressure which controls operation of the actuator. The photo-acoustic element thus relies on thermal effects and the actuator will therefore have, in particular, the disadvantage of a relatively slow response time. In accordance with the present invention, there is provided a pneumatic actuator in which the above disadvantages are overcome or at least mitigated particularly as regard response time. More particularly, the present invention provides, in its broadest aspect, a pneumatic actuator comprising: a) a cell having an inlet for pressurised gas, for example air, an outlet for said gas disposed opposite said inlet, and one or more exhaust outlets; b) means to control the flow of gas to said inlet so that, normally, a laminar stream of gas will flow through the cell from said inlet to said outlet thereby producing a pressure output at the outlet, and c) transducer means for producing, in response to an optical control signal, an acoustic signal for imparting turbulence to said laminer stream thereby causing at least some of the air in said stream to exhaust through said exhaust outlet(s) and thus a reduction in the value of said pressure output; characterised in that said transducer means comprises an electro-acoustic transducer responsive to an oscillating electrical signal and an opto-electrical transducer responsive to said optical control signal to provide, directly or indirectly, said oscillating electrical signal. The laminar stream of gas may be disturbed, and therefore rendered turbulent, by an acoustic signal having an appropriate frequency and minimum amplitude that is generated by feeding an appropriate oscillating electrical signal to the electro-acoustic transducer which may, for example, be a piezo-electric element, for example in the form of an annular disc surrounding said gas inlet. An actuator of the invention is, as will be noted, controlled by a primary optical control signal which is transduced by the opto-electrical transducer into an electrical signal, preferably via a matcher circuit. The opto-electrical transducer is preferably a photo-diode coupled to the electro-acoustic transducer by a matcher circuit, for example a transformer/inductance circuit. The optical source is therefore modulated at the afore-mentioned acoustic frequency and is preferably a coherent source, such as a laser. The light may be fed to the opto-electrical transducer by an optical fibre link, whereby the device may readily be controlled remotely. Optical fibre links have the advantage, relative to electrical cable links, of lower weight and volume and large signal band width. An actuator of the invention operates as follows. Normally, as indicated above, the air (or other gas) flows through the cell from the inlet to the outlet in a laminar stream and most of the air emerges from the outlet to provide an output pressure; in other words, a relatively small amount, if any, is lost through the exhaust outlet(s) of the cell. However, when the stream is disturbed in the manner described, most of the air will vent through the exhaust outlet(s) and so there is a considerable drop in the output pressure. The ensuing change in the output pressure may be utilised to control the operation of, for example, a pneumatic device such as a pressure regulator. Because an actuator of the invention may use very rapid response transducers (such as a photodiode and a piezo-electric device), its overall response time is very fast and it is of high sensitivity and stability. One embodiment of an actuator of the invention will now be described by way of example only with reference to the accompanying drawing. Referring to the drawing, the actuator comprises an elongate, cylindrical cell 1 having at one end a compressed air inlet jet 2 having a diameter of 0.5mm and, at the other end, an opposed air outlet jet 3 having a diameter of 0.5mm. The distance between the jet orifices is about 15mm. The cell 1 has a number of exhaust ports 4 formed in its wall adjacent to the outlet jet 3. The inlet jet 2 is defined by an end of a supply tube 5 which is connected to a source 6 of compressed air via a flow control valve 7. The outlet jet 3 is defined by an end of a tube 8. The inlet jet 2 is surrounded by an annular piezo-electric disc 9 which is electrically connected to a photodiode 10 via a matcher 11 that comprises principally a transformer and an inductor. A modulated light source 12, typically having a power of the order of a few milliwatts, is directed onto the photodiode 10 by an optical fibre link 13. During use, the flow control valve 7 is adjusted so that a stream of compressed air will normally flow, in laminar fashion, through the cell from the inlet jet 2 to the outlet jet 3, thereby producing an output pressure in the tube 8. The laminar stream may, however, be disturbed so as to render it turbulent by feeding an appropriately modulated light signal from the source 12 along the optical fibre link 13 and onto the photodiode 10 whereby an acoustic signal is generated by the piezo-electric disc 9. The frequency and amplitude of the acoustic signal are selected in order to create such a disturbance. In the embodiment described, at an input pressure in the tube 5 of 0,3 N/cm² (0.43 psi), the effective signal frequency is of the order of 17 KHz but this may vary with the geometry etc of the arrangement. The requisite frequency and amplitude will depend on the precise arrangement but for any given case they may be determined by simple experiment. Upon disturbance of the laminar stream, most if not all of the compressed air will exhaust through the ports 4, rather than through the outlet jet 3, and so the output pressure in the tube 8 will drop significantly. The tube 8 may be connected, usually via a pneumatic amplifier to, for example, the actuating section of a pressure regulator or on/off valve (not shown), the actuating section being responsive to the change in the output pressure in the tube 8. A pneumatic actuator constructed in accordance with the invention has the particularly desirable advantage of very small response times relative to those of, for example, known I/P converters and the type of device disclosed in GB2165062A, an advantage which those skilled in the art have long been attempting to secure, hitherto, however, without success. More particularly, an actuator of the invention will typically have a response time of 5 milliseconds or less.

Claims: A pneumatic actuator comprising: a) a cell (1) having an inlet (2) for pressurised gas, for example air, an outlet (3) for said gas disposed opposite said inlet (2), and one or more exhaust outlets (4); b) means (7) to control the flow of gas to said inlet (2) so that, normally, a laminar stream of gas will flow through the cell (1) from said inlet (2) to said outlet (3) thereby producing a pressure output at the outlet (3); and c) transducer means for producing, in response to an optical control signal, an acoustic signal for imparting turbulence to said laminar stream thereby causing at least some of the air in said stream to exhaust through said exhaust outlet(s) (4) and thus a reduction in the value of said pressure output, characterised in that said transducer means comprises an electro-acoustic transducer (9) responsive to an oscillating electrical signal and an opto-electrical transducer (10) responsive to said optical control signal to provide, directly or indirectly, said oscillating electrical signal. A pneumatic actuator according to Claim 1 wherein said electro-acoustic transducer (9) is a piezo-electric device. A pneumatic actuator according to Claim 2 wherein said piezo-electric device is in the form of an annular disc surrounding, and located adjacent to, an end of a tube (5) defining said inlet (2). A pneumatic actuator according to any one of claims 1 to 3 wherein the opto-electrical transducer (10) is interfaced with the electro-acoustic transducer (9) via a matching circuit (11). A pneumatic actuator according to any one of claims 1 to 4 wherein said opto-electrical transducer (10) is a photo-diode. A pneumatic actuator according to any one of claims 1 to 5 wherein said opto-electrical transducer is responsive to suitable laser or infra-red control signals. A pneumatic actuator according to any one of claims 1 to 6 wherein said opto-electrical transducer (10) is adapted to receive suitably modulated optical control signals transmitted to it along a fibre optic link (13).

Inventors: HU FANG-QUAN C O UMIST DEPARTM, PAGE MICHAEL, WATSON JOHN MERZA C O UMIST DE, HU, FANG-QUAN, C/O UMIST, DEPARTMENT OF, PAGE, MICHAEL, WATSON, JOHN MERZA, C/O UMIST, DEPARTMENT OF
Applicants: IMI NORGREN LTD, IMI NORGREN LIMITED
Application year: 1991
Main classification: F15C1
IPC codes: F15D1, F15C1