GB2107870A - Apparatus for measuring distances within elongate enclosures - Google Patents

Abstract

Apparatus for measuring a distance within a tube (12)-for example the height of the surface (14) of a fluid within the tube-comprises a transmitter (16) for transmitting a sound pulse into the tube, a receiver (20) for receiving a pulse reflected from the surface (14), and measuring means for determining the distance of the surface (14)-or other reflector- from the transmitter by measuring the time lapse between the transmitted pulse and a signal derived from the received pulse. The measuring means includes a variable gain amplifier that amplifies the signal derived from the received pulse before the signal is used in determining the distance, the amplifier gain increasing in at least approximate accordance with the reciprocal of the law of attenuation of the pulse of sound in the tube (12). <IMAGE>

Description

SPECIFICATION Apparatus for measuring distances within elongate enclosures This invention relates to apparatus for measuring distances within elongate enclosures such as tubes, pipes or the like.

It is known to measure subterranean pressures by means of a device known as a piezometer. A typical piezometer comprises a length of plastics tubing having an inside diameter of about 20 mm, the tubing being disposed in a borehole and having a porous pot disposed at its lower end to act as a filter. Above the filter, a waterproof seal is placed between the tube and the surrounding stratum. In use, water seeps into the tube via the filter and rises up the tubs. The level or height of the surface of the water is proportional to the pressure at the lower end of the tube. The water level can be measured by a variety of methods.

One method comprises lowering an electrical cable into the tube and ascertaining when it reaches the water surface by noting when current starts to flow through the cable. Another method, for remote operation, comprises lowering a smaller diameter tube into the first-mentioned tube and noting when pneumatic pressure is required to initiate flow of air from the smaller tube to below the water surface. It is also known to dispose a pressure transducer at the base of the hole. Such arrangements cannot provide accurate readings.

bne could envisage the use of sound to determine the water level. That is to say, sound (at an audible or ultrasonic frequency) could in theory be transmitted into the tubing and the water level determined by measuring the propagation delay involved in the receipt of sound reflected from the water surface. However, in practice such method involves the following disadvantage. Spurious echoes or reflections would be generated by other reflectors than the water surface, for instance from joints in the tubing. Further, the amplitude or level of the reflected signals falls of quickly with distance, whereby a spurious reflected signal from a position relatively near the sound source will be larger in amplitude than the signal reflected from a water level relatively farther from the source.It therefore becomes difficult or impossible in many cases to ensure that the measurement system employed does not wrongly identify a spurious reflected signal as a signal reflected from the water surface and therefore give a false reading.

According to the present invention there is provided apparatus for measuring a distance within an elongate enclosure, the apparatus comprising: transmitting means for transmitting a pulse of sound into the closure; receiving means for receiving a pulse of sound reflected from a reflector within the enclosure; and measuring means for determining the distance of the reflector from the transmitting means by measuring the temporal spacing between the transmitted pulse and a signal derived from the received pulse, said measuring means including variable gain amplifier means operative to amplify said signal derived from the received pulse before said signal is used in determining said distance, said means being such that its gain increases in at least approximate accordance with the reciprocal of the law of attenuation of the pulse of sound in the enclosure with time and/or distance.

Apparatus embodying the invention may be used for measuring the height of a fluid in the enclosure, in which case the reflector is constituted by the surface of the fluid. The invention has, however, more general applicability. For instance, by measuring the distance between the transmission means and a reflector provided in the enclosure, the length of an enclosure of variable length can be measured.

In apparatus embodying the invention, the drop in return signal strength with distance (erg. with the depth or height of the surface of a fluid in the enclosure) is at least approximately compensated for by the variable gain amplifier means, whereby the possibility of the apparatus responding falsely to a spurious pulse of sound reflected from something (e.g. a joint) located nearer to the transmitting means than the reflector (e.g. the surface of the fluid) can be reduced.

In an embodiment described below the sound pulse is attenuated in the enclosure in accordance with an exponential law whereby the gain of the variable gain amplifier means increases at least approximately exponentially with time to compensate therefor. However, such gain may vary in some approximate different manner if the law governing attenuation of the pulse of sound in the enclosure employed should depart from the exponential.

In a preferred arrangement, the measuring means comprises means to measure the amplitude of said signal after its passage through the variable gain amplifier means and operative to control the gain of the received signal path such that said signal is of a standard amplitude regardless of the amplitude of the received pulse.

This enhances accuracy since, assuming all such signals to be of substantially the same shape, a trigger or threshold circuit responsive to the signal achieving a predetermined level will be actuated at the same point on any said signal.

In the case where the apparatus is used for measuring the height of a fluid within the enclosure, the enclosure may wholly or partially confine the fluid. In some applications the enclosure may be open at least at its lower end, e.g. it may be a standpipe measuring the level of, for instance, a reservoir. In some applications, as is the case for the preferred embodiment described below, which is operative as a piezometer, the lower end of a tube functioning as the enclosure is closed only by a filter, i.e. it is effectively open in that it is not closed in a fluid-tight manner.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a vertical sectional view through an upper portion of a piezometer forming part of a fluid height measurement apparatus embodying the invention; Figure 2 is a highly schematic block circuit diagram of the fluid height measurement apparatus; and Figure 3 is a graph of return signal strength versus time to assist in explaining the manner of operation of the apparatus.

Figure 1 shows a head 10 coupled mechanically to the upper end of a conventional piezometer tube 12, typically an unplasticised polyvinyl chloride tube having an internal diameter of 20 mm. In a manner not shown in the drawings, the tube 12 extends down into a borehole in the ground and water can enter the lower end of the tube via a filter and rise up the tube, the level of the water surface 14 being proportional to the pressure at the lower end of the tube. The piezometer is operative to measure the distance d (Figure 1) between the surface 14 and a reference level in the head 10 and therefore to measure the pressure at the lower end of the tube 12.This is done, as explained in more detail below, by transmitting a pulse of sound at 5 kHz from a transmitter (electro-acoustic transducer) 1 6 acoustically coupled to the tube 12 via a slot 18 in the wall thereof, and measuring the time taken for the pulse to travel down the tube 12, be reflected from the surface 14 and be received by a pair of receivers (acoustic-electric transducers) 20 acoustically coupled to the tube 12 by slots 22.

Any suitable transducers can be employed for the transmitter 16 and receivers 20. By way of example, the transmitter 1 6 can comprise a miniature electromagnetic moving armature transmitter and the receivers 20 can comporise miniature capacitor/electret microphone capsules as manufactured for use in deaf aids.

The receivers 20 are disposed on opposite sides of the transmitter 1 6 and are spaced equidistantly from the transmitter. The receivers 20 are connected in opposition whereby the effect of direct sound from the transmitter 1 6 is reduced and extraneous noise affects both receivers equally and is cancelled out, thus reducing the susceptibility of the apparatus to interference.

The receivers 20 are spaced from each other by a distance at least approximately equal to half a wavelength of the 5 kHz acoustic signal. When a 5 kHz sound wave travelling up the tube reaches the upper one of the receivers 20, the opposite phase is simultaneously present at the lower one of the receivers, due to the half wavelength spacing, whereby the output signals of the receivers add together.

The transmitter 1 6 and the receivers 20 are mounted on a printed circuit board (PCB) 24 and, together with the PCB, are encapsulated or 'potted' in a resilient compound 26 for protection and acoustic isolation. Also encapsulated in the compound 26 are a temperature sensor 28 and heater coils 30. The heater coils 30 can be used to drive off condensation from the regions of the transmitter 1 6 and receivers 20.

The head 10 includes a casing 32 which, in use, is fitted in the top of the borehole. The space within the casing 32 is packed with acoustic wadding 38. The waddding 38, together with the heavy casing 32, helps to exclude external sounds.

A cable 40 pentrates the casing 32 to afford electrical connections to circuitry mounted on the PCB 24. Also, a tube 42 penetrates the casing 32 to equalise the air pressure in the head 10 with ambient pressure.

The electrical circuitry of the piezometer is shown in Figure 2. The circuitry includes a stable oscillator 50 which has a nominal operating frequency of 160 kHz and which produces, for example, a square wave output. The oscillator 50 is connected via a voltage controlled oscillator (VCO) 51 to the temperature sensor 28, the sensor being operative to provide a voltage that alters the oscillator frequency slightly with changes in temperature to compensate for changes in the speed of sound in air with temperature.

The output of the oscillator 50 is connected to a divider 52 which divides the oscillator output frequency by 32 (25) to produce a 5 kHz signal which is fed to the transmitter 1 6 via an electronic switch 54 and an output amplifier 56.

Via a trigger circuit 58, the output of the oscillator 50 is connected also to a counter 60, driving a display 62, and to dividers 64, 66.

The dividers 64, 66 divide the output frequency of the oscillator 50 by 214 to produce an output signal at a frequency of 10 Hz which is connected by a line 68 to a start/stop flip-flop 70.

The 10 Hz signal defines the frequency at which pulses of sound at 5 kHz are emitted by the transmitter 16: that is, ten such pulses are emitted every second. (The emission rate is decreased for longer distances.) Each pulse commences upon ciosure of the switch 54, which takes place on receipt (via a logic circuit 72) of a signal from the flip-flop 70 when the flip-flop is switched ON by a change in level on the line 68.

Lines 74 connect the divider 64 to the switch 54 via the logic circuit 72 to terminate each pulse after a predetermined number of cycles.

The counter 60 is controlled by the flip-flop 70 to start counting the 1 60 kHz pulses from the oscillator 50 when the flip-flop 70 is switched ON, i.e. at the start of transmission of each pulse of 5 kHz by the transmitter 1 6.

The receivers 20 are connected via respective individual amplifiers 80 to respective inputs of a differencing amplifier 82 connected to a controlled gain stage 84 whose gain increases exponentially with time over a period which, by virtue of connection of the stage 84 to the flipflop 70, commences with the start of transmission of each pulse. The output of the controlled gain stage 84 is connected via a Schmitt trigger 86 to the flip-flop 70 whereby the flip-flop is switched OFF again when the apparatus detects the receipt of a pulse reflected from the water surface 14 (Figure 1) as signified by the output level from the stage 84 exceeding the threshold level of the trigger 86. Upon switching OFF of the flip-flop 70 the counter 60 stops counting.The accumulated count is of course directly proportional to the time taken for the transmitted pulse to travel from the transmitter 1 6 down to the water surface 14 (Figure 1) and back to the receivers 20. That is to say, it is directly proportional to the distance d (Figure 1). The count is then transferred to the display 62 to display the distance d and/or is stored and compared with previous readings to ascertain the range of movement of the water.

In order to prevent each emitted pulse from stopping the counter 60 before an acho is received, the receiver output is turned off on emission of the pulse for an interval of time sufficient to allow the outgoing pulse to die away.

The output of the controlled gain stage 84 is also passed to an output measuring circuit 88 where the amplitude of the output is measured upon generation of an output signal by Schmitt trigger 86. The basic gain of the stage 84 is controlled in accordance with the value of the measurement made by the circuit 88 to ensure that the return signals are always of the same amplitude at the input of the Schmitt trigger 86 whereby the trigger operates at the same point on each such signal so that accuracy is enhanced.

The functioning of the (automatic) controlled gain stage 84 and the output measuring circuit 88 will now be described in more detail with reference to Figure 3, which is a graph of return signal strength with time.

Since the sound transmitted by the transmitter 1 6 is enclosed within the tube 12 (Figure 1), its amplitude diminishes exponentially with distance.

Therefore, in the absence of the stage 84, the return signal strength would vary with time (i.e.

with the value of the distance din Figure 1) in accordance with the exponential broken-line curve A in Figure 3. However, the exponentiallyincreasing gain of the stage 84 wholly or largely compensates for this feature in the abovedescribed arrangement whereby, over the designed operating range, the return signal strength is substantially independent of the value of the distance d, as represented by the solid line curve B in Figure 3. This feature enhances the ability of the apparatus to distinguish between a return signal (reflection) from the water surface 14 and a return signal from a joint of the tube 12 or other irregularity.To appreciate the reason for this, consider the three sets of pairs of curves shown in Figure 3, wherein the solid line pair of each set represent the behaviour of the illustrated apparatus and the broken-line pair of each set represent the situation without the stage 84, and wherein the higher curve of each pair represents a return signal from the water surface 14 and the lower curve represents a return signal from a tube joint or other irregularity. The exponential nature of the curve A (when the stage 84 is absent) causes difficulty. Thus, for each of the pairs of broken-line curves, the amplitude of the 'joint' curve is about the same as or even more than the 'water surface' curve of the pair on its right.If, therefore, the threshold level of the Schmitt trigger 86 is set at a level shown at C, though a signal from the water surface is greater than from a nearly joint, a signal from a joint considerably higher than the water surface may be above the threshold level of the Schmitt trigger 86 and the apparatus will fail to respond to subsequent signals from the water surface. With the present arrangement, however, this disadvantage largely disappears, and the threshold level of the Schmitt triggers can be set at a level shown at D so that the 'water surface' return signals are accepted and the 'joint' return signals are rejected over the whole of the illustrated range. The amplitudes of the 'water surface' and 'joint' return signals do not vary with the distance d.

The foregoing advantage will continue to apply in full only so far as the level of each return signal applied to the Schmitt trigger 86 is the same.

Thus, if the return signal level increased the apparatus could begin to respond to 'joint' return signal, whereas if it decreased the apparatus could stop responding at all. The above-described maintenance of the average return signal level or amplitude provided by the circuit 88 prevents this happening. It also gives rise to the advantage, mentioned above, that the Schmitt trigger responds to the same point on each successive return signal, thereby minimising timing inaccuracy.

The various frequencies mentioned above were given by way of example and may be varied if desired. The frequency of the oscillator 50 is chosen to be 1 60 kHz as a matter of convenience in view of the distance units employed because this frequency corresponds to approximately 2 mm of sound travel per count. The oscillator frequency can therefore be varied to a large extent as convenient.

Compensation for acoustic velocity variation with temperature is provided in the illustrated embodiment by a frequency offset loop whereby a frequency of 1 to 3 kHz derived from the VCO 51 governed by the temperature sensor 28 is added to the output from the stable oscillator 50 in such a manner as to maintain a constant relationship between the frequency of the input signal to the counter 60 and the velocity of sound.

The frequency (5 kHz in the above example) of the transmitted sound is believed subject to an upper limit. Thus, sound having a wavelength greater than the diameter of the tube 12 propagates along it approximately as a plane wave. At shorter wavelengths, however, the propagation is non-planar and the apparatus will not work or at least will not do so satisfactorily.

The maximum usable operating frequency (may) can be defined by the relationship K.V Fmax= D where V=the velocity of sound in air, D=the diameter of the tube 12, and K=a factor approaching unity.

A lower limit on the frequency of the transmitted sound is believed not to be so readily definable because it is determined by the desired resolution of the apparatus. Resolution is approximately directly proportional to frequency, so that operation at a frequency as close as possible to Fniax is desirable. There will be greater quantities of outside interference at lower frequencies. Also, resonance would occur at some point.

The invention can of course be performed in other ways that that described above by way of example. For instance, the transmitter 1 6 could be operated by a separate sine-wave oscillator, preferably locked to 1/32 of the frequency of the oscillator 50 for the sake of stability, as this should produce a cleaner signal.

The above-described apparatus may be designed to take readings when required i.e. 'on demand'. In a variant of the apparatus, designed to note the maximum and minimum values of the distance dover a predetermined period of time, a clock is provided to trigger the taking of readings at regular invervals and the readings are recorded, checked for 'quality' (i.e. for nonsensical or suspect readings) and the highest and lowest readings are stored for collection or transmission and from time to time.

Apparatus embodying the invention can be used in other applications than as a piezometer. It can for example be used for measuring tidal levels, river levels, or the level of water adjacent a dam, by connection to a tube extending down into the body of water in question, or be used with other liquids, e.g. a corrosive, inflammable or noxious nature. The apparatus is advantageous in such applications because it has no mechanical moving parts to jam, cannot be impeded in its operation by floating foreign bodies, and can avoid the erratic effects of such things as wave action.

Apparatus embodying the invention can of course be embodied in other ways than that described abov'e by way of example. For example, if the liquid level within the piezometer tube is below the limit of detection of the measurement apparatus but the liquid level movement is within a range measurable by the apparatus, the arrangement as shown in Figure 1 (i.e. at least the transmitting means and the receiving means) may be reconfigured to fit within the tube and to be capable of being lowered down the tube to a suitable distance above the liquid surface.

The apparatus is in fact more generally applicable than described above by way of example in that it can be used to measure the distance (within a tube or other elongate enclosure) between the transmitting means and some other reflector than the surface of a fluid within the enclosure. For example, an air-filled tube whose length may vary may be fitted with one or more sound reflecting devices whereby an apparatus as described above may be used to measure the length of the tube by measuring the distance between the transmitting means and such a reflecting device. (In this case, the tube may of course have any orientation.) In the case where there are a plurality of reflectors, the apparatus preferably incorporates a manually controllable time delay means so arranged as to turn off the receiver output until the emitted sound pulses have travelled a predetermined distance. This enables any of the reflectors to be selected and the associated distance to be measured.

Claims (8)

Claims

1. Apparatus for measuring a distance within an elongate enclosure, the apparatus comprising: transmitting means for transmitting a pulse of sound into the enclosure; receiving means for receiving a pulse of sound reflected from a reflector within the enclosure; and measuring means for determining the distance of the reflector from the transmitting means by measuring the temporal spacing between the transmitted pulse and a signal derived from the received pulse, said measuring means inciuding variable gain amplifier means operative to amplify said signal derived from the received pulse before said signal is used in determining said distance, said means being such that its gain increases in at least approximate accordance with the reciprocal of the law of attenuation of the pulse of sound in the enclosure with time and/or distance.

2. Apparatus according to claim 1 ,for effecting said distance measurement within an elongate enclosure in which the sound pulse is attenuated in accordance with an exponential law, wherein the gain of the variable gain amplifier means increases at least approximately exponentially with time.

3. Apparatus according to claim 1 or claim 2, wherein the measuring means comprises means to measure the amplitude of said signal after its passage through the variable gain amplifier means and operative to control the gain of the received signal path such that said signal is of a standard amplitude regardless of the amplitude of the received pulse.

4. Apparatus according to claim 1, claim 2, or claim 3, in combination with a generally vertical said enclosure and operative to measure the distance of the surface of a fluid in the enclosure from the transmitting means.

5. Apparatus according to any one of the preceding claims, wherein at least the transmitting means and the receiving means can be moved into a said enclosure.

6. Apparatus according to claim 1, claim 2 or claim 3, in combination with a said enclosure having at least one said reflector fitted therein.

7. Apparatus according to claim 6, wherein the enclosure comprises a plurality of said reflectors and the apparatus comprises manually controllable time delay means enabling the receipt of measurement of the received pulse to be delayed until the transmitted pulse has travelled a predetermined distance.

8. Apparatus for measuring a distance within an elongate enclosure, the apparatus being substantially as herein described with reference to the accompanying drawings.