GB2302946A

GB2302946A - Ultrasonic liquid level detector

Info

Publication number: GB2302946A
Application number: GB9613553A
Authority: GB
Inventors: Richard Hunter Brown
Original assignee: Whitaker LLC
Current assignee: Whitaker LLC
Priority date: 1995-06-29
Filing date: 1996-06-27
Publication date: 1997-02-05
Also published as: DE19621760A1; GB9513267D0; GB9613553D0; JPH09101191A

Description

2302946 ULTRASONIC LIQUID LEVEL DETECTOR is This invention relates to a

liquid level detector using ultrasonic energy for detecting the level of a liquid in a container.

In one form, the invention provides a switch-type yes/no operation indicating whether the liquid is above or below a fixed point. In another form, the invention provide's a measure of liquid level across a range.

A typical prior art arrangement for a yes/no operation uses a float to close a mechanical switch, or a magnet carried by a float to operate a reed switch. Such systems require intrusion into the container volume, and have limited reliability.

For measurement of liquid level across a range, a typical prior art arrangement uses a float to actuate a potentiometer connected in a meter circuit. This has similar drawbacks to those outlined above.

It is also known to measure liquid level ultrasonically by measuring the transit time required for an ultrasonic pulse to travel from the bottom of the container to the liquid surface and for a reflection to return to the bottom of the container. Such systems have good reliability, there being no moving parts, but are relatively complex and expensive. In addition, it is known to measure liquid levels by an array of transmitting and receiving devices spaced along the height of liquid within the container by passing an acoustic signal between the transmitting and receiving 30 devices. This is disclosed in EP 0 515 254 and W093/02340. According to the present invention, there is provided a liquid level detector comprising a piezoelectric element secured to a layer of material, the acoustic impedances of the piezoelectric element and the layer of material being such that an acoustic or ultrasonic signal generated by 40199A GB 1.. 1.

the piezoelectric element will propagate through the layer of material and will be transmitted or reflected at the opposite face thereof in dependence on whether or not, respectively, said opposite face is in contact with a liquid.

Preferably, the piezoelectric element comprises a layer of piezoelectric polymer film, most preferably of polarised PVW or P(VW-TrFE).

The film is preferably mounted on a printed circuit board having a copper surface etched to form one or more active signal electrodes.

The copper may be etched to f orm a plurality of independent signal electrodes which co-operate with a single layer of film to form an array of independent is ultrasonic transducers.

The copper may be etched to form an interdigital electrode structure comprising transmitter electrode(s) and receiver electrode(s).

The opposite face of the printed circuit board may be wholly or substantially covered by copper for connection as a ground plane.

Said layer of material may comprise part of the wall of a tank, or may form part of an enclosure encapsulating the detector to permit placement of the detector within a tank.

Typically, the layer of material has a thickness between 0.5mm and SOmm, and an acoustic impedance between 1 and 10 x 108 NSM-3.

Such a device may replace a reed switch actuated by a magnet contained within a float. Such reed switches are vulnerable to high impact or shock, and the metal contacts of the switch have a limited lifetime expressed in terms of the number of contact closures undergone prior to failure. The float assembly required may consume valuable 40199A GB space within the tank, and the assembly usually requires breaching of the tank wall to allow installation.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying 5 drawings, in which:- Fig. 1 is a cross-sectional view of a liquid level switch forming one embodiment of the invention; Fig. 2 shows waveforms illustrating the operation of the switch of Fig. 1; Fig. 3 is a cross-sectional view of a switch array forming another embodiment of the invention; Fig. 4 illustrates a modified version of the embodiment of Fig. 3.

Before referring to the drawings, the concept underlying the invention will be discussed.

An ultrasonic transducer is formed, preferably using piezoelectric polymer film such as polyvinylidene fluoride (PVDF) or copolymer p(VDF-TrFE). A f eature of such piezopolymer materials is their low acoustic impedance (typically 4. 1 x 108N S M73) and high internal damping.

These physical properties allow a f ast ultrasonic pulse to be transmitted into materials whose acoustic impedance is reasonably close to that of the piezoelectric transducer; this class of material includes common plastics. These may be used for forming tanks and containers by such processes as injection- or blow-moulding, and include materials such as polyethylene and polypropylene.

When the ultrasonic pulse has propagated through the wall material and arrives at the boundary, its behaviour is strongly af f ected by the medium present beyond the boundary. If the acoustic impedance of the medium is very low (for example, if the medium is air, gas, or vapour), then most of the acoustic energy will reflect from the boundary and very little energy will be transmitted into the medium. This will also occur if the acoustic 40199A GB and impedance of the medium is very high relative to that of the wall material. When, however, the acoustic impedance of medium is similar to that of the wall material, then a large proportion of the acoustic energy radiates into the medium, and a much smaller echo returns back f rom the boundary into the wall material.

The echo amplitudes can easily be estimated using the simple formula:

R = (Zw - ZJ / (Z,, + Z,) where Z, is the acoustic impedance of the wall material Z. is the acoustic impedance of the medium.

In the case where the medium is air or gas, then the return echo amplitude is effectively unity (i.e. virtually none of the energy is transmitted into the medium). The transducer could comprise a small area of piezoelectric polymer f ilm simply applied to the outside wall of a plastic tank. The f ilm element is driven by a voltage pulse, and the resulting echo creates an electrical response which is thresholded to determine whether the inner boundary or the plastic wall is wetted by liquid or not.

Referring now to Fig. 1, a tank containing liquid 10 is defined by a tank wall 12 of a plastics material such as polyethylene. A liquid level detector generally designated at 14 is secured to the tank wall 12. The detector 14 comprises a piezopolymer film 16 secured to a printed circuit board 18. The printed circuit board 18 is formed with etched copper layers on its front and rear surfaces, with the front surface carrying a copper electrode 20 surrounded by a copper border 22. The piezopolymer film 16 has its front surface metallised at 24 and is secured to the printed circuit board 18 by adhesive bonding. The major part of the rear surface of the printed circuit board 18 is covered by a copper ground 40199A GB plane 26 and this is electrically connected to the copper border 22 and the metallisation 24 by means of eyelets 28.

The electrode area 20 is connected by means of plated through hole 30 to an electrically isolated copper area 32 on the rear surface of the printed circuit board 18.

Thus, by electrically connecting the detector 14 as indicated in Fig. 1 the central copper area 20 acts as an active signal electrode and is electrically shielded from interference by the grounded regions 22, 24 and 26. A further advantage of the embodiment of Fig. 1 is that the copper signal electrode 20 acts to an extent as an ultrasonic mirror and thus aids the transmission of ultrasound into the plastic wall material.

When an electrical pulse is applied to the detector 14, an acoustic pulse A is propagated toward the interior of the tank. Fig. 2 shows a typical acoustic response.

The upper part of Fig. 2 shows the situation where the detector 14 is at a level where there is no liquid in the tank and thus air (or another gas) is present on the interior of the tank wall 12. In this situation there is a strong echo B from the wall/air interface. The lower part of Fig. 2 shows the situation where the detector 14 is at a level where the interior of the tank wall 12 is covered by a liquid 10. In this situation there is reasonably good acoustic matching between the tank wall 12 and the liquid 10 with the result that most of the acoustic energy C transmits into the liquid and only a weak echo occurs.

Fig. 1 shows a typical electronic arrangement for utilising this phenomenon, but other circuitry will be readily apparent to those skilled in the art. In Fig. 1, a pulse signal produced by a pulse generator 27 is applied to the active electrode 20 via plated-through hole 30, and the reflected signal is applied to a threshold detector 29.

40199A GB Fig. 3 illustrates an embodiment in which a detector assembly 31 is immersed in a liquid 32 within a tank (not shown) and is connected to the exterior via a cable 34.

The detector assembly 30 comprises a piezopolymer f ilm. 36 bonded to a printed circuit board 38 which is formed with a vertical array of electrode elements 40. This combination is enclosed within a plastic housing (for example, of high density polyethylene) 42, suitably with the piezopolymer film 36 bonded to one wall of the housing 42. The housing 42 may be sufficiently large to provide a space 44 in which associated electronics may be housed.

In this arrangement, an array of signal electrodes is readily formed using a single continuous printed circuit board laminated with a single piezopolymer film. It is a feature of piezopolymer films that small addressed areas within a single sheet may be treated as wholly independent of each other in terms of their mechanical behaviour. In the embodiment of Fig. 3, therefore, the electrodes 40 may be addressed sequentially to give an output signal in the form of a train of digital pulses which are simply counted to give a measure of liquid depth, the resolution of the system being set by choice of the number of electrode elements 40. A similar arrangement may be applied to the exterior of a plastic tank, rather than being immersed within the tank.

In an alternative embodiment, the signal electrode(s) may be split into two independent areas which are interdigital in form, such that one track may be designated as a transmitter electrode and the other as a receiver electrode. By this means, the electrical networks required to (a) generate a fast transmitter pulse and (b) receive an electrical response from the echo are electrically separated.

Fig. 4 shows schematically an arrangement of this type. A printed circuit board 50 carries transmitter 40199A GB electrodes 52 interleaved with receiver electrodes 56, the whole array being covered by a PVDF film 58.

In the case where the signal electrode is a simple single element, then the transmitter signal is potentially routed direct into the receiver network, and so there is a risk of saturation the receiver amplifier unless means are adopted to switch the receiver amplifier out of circuit during transmission.

In the case where the signal electrodes are interdigitated, then there is still a component of signal synchronous with transmission (generated by the direct and simultaneous piezoelectric response of the receiver electrode), but at a much lower amplitude, so that the risk or extent of amplifier saturation is greatly reduced.

This improvement is modified, however, by an accompanying overall loss in signal level, due to the reduced active areas.

An alternative solution to the problem of switching the receiver amplifier is to use a combination of inductive tuning and fast-recovery amplifiers. With a slightly tuned circuit (matching the capacitive reactance of the piezoelectric element with an inductive element selected to create a resonant network tuned to the mechanical resonance of the piezoelectric element), only a fraction of the transmitter drive signal is allowed to appear at the input terminals of the receiver amplifier. At the same time, the gain of the system is enhanced at the frequency of interest. Operating frequencies in the range of 1 to 10 MHz are found to be practical for readily-available piezoelectric polymer films, and tank wall thicknesses of a few millimetres.

Advantageously then, a sensor for determining if a substance, particularly a liquid, is present within a container, is presented where the sensor may be formed as a single unit and the detection is accomplished by 40199A GB 1 detecting whether or not a face opposite active transducers of the sensor is in contact with the substance, thereby eliminating the need for the acoustic signal to pass through the substance.

Claims

1. A liquid level detector comprising a piezoelectric element secured to a face of a layer of acoustically transmissive material, the acoustic impedances of the piezoelectric element and the layer of material being such that an acoustic or ultrasonic signal generated by the piezoelectric element will propagate through the layer of material and will be transmitted or reflected at the opposite face thereof in dependence on whether or not, respectively, said opposite face is in contact with the substance to be measured.

2. A detector according to claim 1, piezoelectric element comprises piezoelectric polymer film.

in which the a layer of

3. A detector according to claim 2, in which the film is of polarised PVW or P(VW-TrFE).

4. A detector according to claim 2 or claim 3, in which the film is mounted on a printed circuit board having a copper surface etched; so to form one or more active signal electrodes#

5. A detector according to claim 4, in which the copper is etched to form a plurality of independent signal electrodes which co-operate with a single layer of film to form an array of independent ultrasonic transducers.

6. A detector according to claim 4 or claim 5, in which the copper is etched to form an interdigital electrode 40199A GB 1 structure comprising transmitter electrode(s) receiver electrode(s).

and

7. A detector according to any of claims 4 to 6, in which the opposite face of the printed circuit board is wholly or substantially covered by copper for connection as a ground plane.

8. A detector according to any preceding claim, in which said layer of material comprises part of the wall of a tank.

is

9. A detector according to any of claims 1 to 7, in which said layer of material forms part of an enclosure encapsulating the detector to permit placement of the detector within a tank.

10. A detector according to any preceding claim, in which the layer of material has a thickness between 0.5mm and 50mm, and an acoustic impedance between 1 and 10 x 108 NSM3.

11. A liquid level detector substantially as hereinbefore described with reference to the accompanying drawings.

40199A GB