GB2153172A - A device for measuring pressure - Google Patents

Abstract

A device for measuring pressure has an encoder disc 6 rotatably mounted to respond to pressure changes sensed by e.g. a bourdon tube. The encoder disc is provided with a plurality of coded tracks divided into transparent and opaque portions the limits of these portions defining specific pressures to be measured. The disc lies in the light paths between a plurality of light sources and photosensitive detectors, corresponding in number to the number of tracks on the disc. The device has great versatility of application and is capable of operating a variety of switching functions by means of the considerable programmable facilities provided by the disc encoder. <IMAGE>

Description

SPECIFICATION A device for measuring pressure FIELD OF THE INVENTION The present invention relates to a device for measuring pressure having particular, but not exclusive, applicability to the control of pressure in a cryogenic surgical system used for destroying or manipulating tissue where continuous evaluation of operational cryogen pressure requirements is important.

BACKGROUND ART Pressure responsive devices for converting a gas pressure into an electrical signal indicative of that pressure, are known.

Amongst such devices are pressure transducers mainly of the strain gauge or piezo-electric type. The former operates by using a strain gauge mounted onto a substrate which stretches on application of pressure, causing a change in the electrical resistance of the strain gauge. This in turn varies the electrical voltage in the driving circuit.

The piezo-electric transducer on the other hand utilises the characteristics of a piezo-electric crystal onto which the pressure of the gas operates directly. A characteristic of such crystals is that under application of a direct force of pressure a voltage is produced which varies with that applied force.

The disadvantages of these known devices are their relative high cost, the complexity of the associated electronic circuitry necessary usefully to utilise the electrical output to perform various switching functions, and the inability to retain initial accuracy over life.

The pressure switch is a simple device in which an application of pressure at a certain intensity onto a sensing element makes or breaks an electrical contact to operate a simple associated control circuit. Each manufactured switch of this type is limited to operating at its own pressure. To operate at several different pressures therefore would require an equivalent number of switches making demands on physical space which frequently cannot be met. The cost is also a prohibitive factor.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a device for measuring pressure which removes or substantially removes the limitations and deficiencies of the prior art as above expressed.

According to the invention there is provided a device for measuring pressure comprising pressure sensitive means, encoder means movable in response to pressure changes sensed by said pressure sensitive means, and means responsive to the coding on said encoder means such that at predetermined positions thereof an output is provided representative of pressure being measured.

This arrangement has distinct advantages over the prior art mainly in that with its great versatility of design, one unit is able to operate many switching functions with relatively simple associated control circuitry and high accuracy.

Advantageously the encoder means is an encoder disc which is rotatable in response to a pressure change sensed by the pressure sensitive means. In this preferred embodiment the encoder disc has one or more radially positioned tracks divided into transparent and opaque portions, the number of tracks and divided portions on each track, being determined by the ranges of pressure that it is desired to monitor or sense and ultimately the number and type of switching functions the device may have to perform utilising the output therefrom. The encoder disc may be arranged to rotate such that each track is intercepted by a light beam between a light source and associated photosensitive detector, each detector thereby providing a means for correspondingly actuating associated electrical circuitry when the track changes from transparent to opaque and vice versa.

The encoder disc may be manufactured by a photographic process and therefore its copying in any quantities with great accuracy is virtually guaranteed.

Other features and advantages of the present invention will become apparent from the following description of a preferred embodiment taken with reference to the accompanying drawings wherein: Figure 1 is a sectional view through a device for measuring pressure according to an embodiemtn of the invention; Figure 2 is an end view of the device of Fig. 1; and Figure 3 is a view of a typical encoder disc design of the device of Fig. 1.

BEST MODES OF CARRYING OUT THE INVENTION The device, shown in Figs. 1 and 2 in particular, comprises a support body 1 to which is mounted a bourdon tube 2 fed with gas from a gas pressure inlet 3. The bourdon tube 2 operates under applied pressure in a manner as is well known in the art. This movement is amplified using a mechanical movement in the form of linkages 4, and a geared pinion 5.

A hairspring 5' prevents backlash and hysteresis in the system, and therefore the pinion 5 rotates proportionally to the applied pressure.

An optical encoder disc 6 is fixed to the pinion 5 of the mechanical pressure sensing movement. The disc 6 possesses a plurality of radially positioned tracks A to D having transparent and opaque portions, as shown more particularly in Fig. 3, and rotates with the plane of the disc 6 lying in the light path between a plurality of light emitting diode (LED)-photosensitive transistor pairs. A representative pair is shown in Fig. 3, wherein a light emitting diode (LED) 7 is mounted to a printed circuit board 8, and a receiving photosensitive transistor 9 mounted to a printed circuit board 10 attached to the support body 1, and between which the disc 6 rotates, this pair operating with respect to track A, on the disc 6 of Fig. 3. In general, to operate with the disc design of Fig. 3, four LED transistor pairs will be provided corresponding to the four tracks A to D.

As the encoder disc 6 rotates in response to movements of the bourdon tube resulting from gas pressure changes, the photosensitive transistor 9 effectively decodes the encoder disc 6 by detecting the transparent and opaque portions in track A, thus providing an output signal which can be used to effect one or more desired switching operations. In the case illustrated track A can be used to effect a switching function at 10 Ibs/sq inch, and at 750 Ibs/sq inch as will be more fully explained later.

An alternative arrangement to that shown in Fig. 2, for responding to rotational movements of the disc 6, is to mount an LED transistor pair on the same side of the encoder disc 6 which is partly transparent and partly coated on the side facing the LED transistor pair, with a light reflective material.

Light emitted by the LED in this arrangement is beamed towards the disc 6 which either passes through it and is lost thus breaking the associated circuit, or is reflected onto the transistor thus maintaining the circuit. The transparent part of the disc alternatively could be coated with a light absorbing medium which would have the same net effect.

A further variation is to make the disc of a polarised filter material. A second disc in the same plane and of similar material is fixed immediately above the rotating disc.

Using an LED-transistor pair arranged as shown in Fig. 2 light passes through the disc, and then rotating one disc relative to the other has the effect of varying the transparency of the disc system thus altering the intensity of the light collected by the photosensitive transistor.

In all the above arrangements each LED-transistor pair could be replaced by a magnet and Hall effect switch. In this arrangement the magnet is either rotating with the switch stationary or the magnet and switch are arranged similar to the LED and phototransistor pair of Fig. 2, but with the rotating disc cut away in parts to either allow or interrupt the magnetic flux path.

The pressure measuring device as above described is particularly useful in cryogenic surgery for controlling the various pressure requirements during the operating sequence of a cryogenic probe in a precise'manner.

Heretofore the operating pressure ranges have not been easy to monitor, and therefore optimum operational control has been iacking.

The main normal sequences of the pressure path of a cryoprobe to an operating condition is as follows.

1. Cryoprobe purge range (200 to 300 Ibs/in2) 2. End freeze probe range (650 to 700 Ibs/in2) 3. Standard freeze range (700 to 750 Ibs/in2) 4. Over pressure range (750 Ibs/in2 +) The encoder disc 6 shown in Fig. 3, in the device of the present invention, is designed to control such a sequence, using a single pressure regulator on the operating console of the cryogenic surgical system.

The maximum pressure range of the encoder disc is zero to 1000 Ibs/in2 over zero to 270 rotation. The disc therefore is provided with a 270 operating sector having four tracks A to D, each track having transparent and opaque portions as shown. The number of transparent and opaque portions in each track A to D, corresponds to the number of switching functions a particular track has to perform over the full pressure range.

The device of the present invention, with the encoder disc design of Fig. 3, will be mounted in the console of the cryogenic system, together with suitable electronic control circuitry, the design of which would be readily achievable to one skilled in the art and has not therefore been included herein, responsive to the output from the device, and used to illuminate a series of indentifiable lights which will tell the surgeon of the condition of the cryogenic probe as the pressure of the cryogen delivered to it is increased by actuation of the pressure regulator the various stages as set out above.

The pressure path of the system, and the manner in which the track design of the encoder disc of Fig. 3 controls the sequence of illumination of lights on the console at appropriate points in the pressure path, is illustrated in the following table.

Pressure Track on lbs/in2 LED Sequence Encoder Disc o All lights extinguished.

10 Green pressure light illuminates, A remains on until 750 lbs/in2 .

100-300 Orange purge light illuminates.

After 1 minutes duration light B flashes, continues until 300 Purge light extinguishes. B 650-750 Green and freeze light illuminates. C 700 End freeze light extinguishes. C 700-750 Green standard freeze light illuminates. D 750 Standard freeze light extinguishes. D Green pressure light extinguishes. A Red over pressure light illuminates. A 750+ Over pressure light remains on. Can only be extinguished by reducing pressure below 750 lbs/in2.

Clearly pressure regulation through the various stages to freeze pressure, in a non-erratic and steady fashion, will be possible using the device of the present invention, and instead of the need constantly to monitor a series of pressure gauges, and the constant close attention to detaii that this requires, a simple arrangement of sequentially illuminated coloured lights, suitably identified, is all that is required. The use of the encoder disc, with the capability of storing or entering a vast range of coded messages thereon, demonstrates the great versatility of the device, and in terms of the number of switching functions that may be realisable using relatively simple and uncomplicated control circuitry, illustrates a usefulness heretofore absent in the prior art.

Variations of the above described embodiments of the invention will be readily apparent to those skilled in the art yet within the scope of the invention as defined in the claims to follow.

For instance it would be quite feasible for the encoder means to be linearly responsive as well as rotationally so. Also, as touched upon earlier, the coding on the disc shown in Fig. 3, is not to be taken in any way as being limitative, and a large variety of information may be stored therein depending on requirements.

Claims (6)

1. A device for measuring pressure comprising pressure-sensitive means, encoder means movable in response to pressure changes sensed by said pressure-sensitive means, and means responsive to the coding on said encoder means such that at predetermined positions thereof an output is provided representative of pressure being measured.

2. A device as claimed in claim 1 wherein said encoder means is an encoder disc rotatable in response to a pressure change sensed by said pressure-sensitive means.

3. A device as claimed in claim 2 wherein said disc has one or more radially positioned tracks divided into transparent and opaque portions, the change from transparency to opacity, and vice versa, in each track corresponding to respective limits of a predetermined pressure range to be measured.

4. A device as claimed in claim 3 wherein said responsive means comprises a light source and a photosensitive detector therefor for each of said tracks, said encoder disc lying in the light path between said source and detector pairs such that the detector of each pair responds to rotatable positions thereof corresponding to a change from transparency to opacity, or vice versa, in a said track lying in a respective light path of a light source and detector pair.

5. A device as claimed in any preceding claim wherein said pressure-sensitive means is one of a bourdon tube, aneroid capsule or diaphragm.

6. A device for measuring pressure substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.