GB2312509A - Ultrasonic liquid sensor - Google Patents

Abstract

Liquid level sensor 2 comprises solid sections 36, 38 mounted on a signal generator 4 comprising a printed circuit board 12 and piezoelectric transducer 14. The transducer has a transmitter section 8 and receiver section 10, each comprising a plurality of discrete electrodes arranged in a column (18, 20, fig. 4). Ultrasound generated by the transmitter section is bounced off a first reflector surface 26 and second reflector surface 28 for reception by the receiver. The transmitter and receiver pairs (18, 20, fig. 4) below the surface of the liquid (21, fig. 4) are able to transmit and receive these signals, while those above the surface are not, thus the height of the liquid can be calculated. A long transmission path length T is provided along with a reduced sensor height H. Temperature can also be determined from the measured time between transmission and reception of the first reflected signal C, and transmission times can be used in combination with the temperature to determine the type of liquid being sensed.

Description

LIQUID SENSOR This invention relates to a sensor for determining the presence, level or type of liquid.

In European patent 51J254 a liquid level sensor is disclosed comprising a transmitter member having a plurality of individually addressable transmitter segments along the depth, and a receiver member spaced apart and facing the transmitter member for receiving the transmitter signals. Both the transmitter segments and the receiver are piezoelectric elements that generate and receive ultrasonic signals respectively. The transmitter segments that are immersed in liquid, transmit ultrasonic signals to the receiver through the liquid. The transmitter signals which are not immersed in liquid do not transmit ultrasonic signals to the receiver as air does not carry ultrasonic signals. The level of liquid is thus measured by determining which transmitter segment signals are received. There are a number of problems, however, with this design. One of the problems is that it is very costly, due to the provision of a transmitter member and a separate receiver member, which both need to be mounted on some sort of support, each having electrical circuitry and different electrical connections. Another disadvantage is that the design is not compact as in order to get a sufficiently long signal path, the receiver must be placed at a certain distance from the transmitter.

Some of these problems have been overcome in WO 9302340, where, in one of the embodiments the receiver and transmitter are disposed on a signal support and placed opposite a reflector. The provision of reflector not only doubles the signal path, but also eliminates the need for separate electrical connections, i.e. the transmitter and receiver can be interconnected to electronics which are on the same support. Such a support may for example be a printed circuit board where the electrodes of the ultrasonic transducer are patterned directly by circuit traces on the PCB and interconnected to electronic components mounted on the PCB. In the latter patent, the transmitters can also act as receivers, thus eliminating the need for separate receivers.

One of the applications for such sensors, is for measuring the levels of liquids in the various containers of an automobile, for example the fuel tank. Such sensors therefore need to be manufacturable in high volumes at low cost. They, however, must also be compact and robust, yet reliable. In this respect, one of the problems with the design disclosed on W09302340, is that the reliability of the sensor is sensitive to tolerances in the position and planarity of the reflector, and the compactness is dependent on the distance separating the reflector from the transmitter/receiver.

It is an object of this invention to provide a liquid sensor that is cost effective to produce in high volumes.

It is an object of this invention to provide a compact liquid sensor that is nevertheless reliable.

It is an object of this invention to provide a liquid sensor that can either be used to discriminate between different types of liquids, or to measure the level of liquid, or both.

Objects of this invention have been achieved by providing a sensor comprising a transmitter having a transducer for generating signals, a first reflector for re-directing the signals from the transmitter through a transmission path where liquid is receivable, and a second reflector for directing the signals received through the transmission path to a receiver.

Advantageously, a low profile sensor is provided with nevertheless a long transmission path.

The angles of the first and second reflectors can be adjusted uch that the signals transmitted therebetween ar directed subs-lantiall;r parallel to the plane within T~h7 ch the transmitter and receivers are substantially positioned. The transmitter and receivers could, for example, be piezoelectric transducer mounted on a printed circuit board, the reflectors being providing by a structure mounted to the printed circuit board. In an advantageous embodiment, the reflectors are -provided at substantially 450 angle with respect to the plane of the transmitter and receivers. By providing a solid structure with angled surfaces that act as the reflectors and a U-shaped recess positioned between the reflector surfaces for receiving liquid therein, the transmission path through the liquid would thus- be defined by the gap of the recess. As the gap is defined within the solid structure, a high degree of accuracy in the transmission path length can be achieved in a simple manner.

By providing the solid structure, a portion of the transmitted signal would be reflected from the interface with the liquid in the liquid transmission path, back to the transmitter via the reflector. In this case, if the transmitter also acts as a receiver, the transmission time of the signal through the solid can be determined.

As the transmission time of the signal through the solid is dependent on the temperature, the temperature can also be determined. The transmission time of the total signal through the solid and the liquid to the receiver, enables deduction of the transmission time through the liquid which is dependent not only on the temperature but also on the type of liquid, for example benzene or methanol.

The parameters of temperature and transmission time through the liquid can be used to determine the type of liquid that is being sensed. For certain applications this is particularly advantageous. For example, certain automobiles are able to function with different fusels, but require information on which fuel is being used for optimai functioning of the engine.

Rather than producing a solid structure mounted on the transmitter and receiver plane, a formed integral metal sheet or other cover member could also be provided with angled walls serving as reflectors. A long transmission path, and cost effective construction is achieved by the latter.

Exbodisents of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of an embodiment of this invention; Figure 2 is a cross-sectional view through a second embodiment of this invention; Figure 3 is a cross-sectional view through a third embodiment of this invention; Figure 4 is a plan view of the outlay of transmitters and receivers mounted on a support (e.g. a PCB); Figure 5 is a view similar to that of Figure 4, but of another embodiment; Figure 6a is an isometric view of a transmission path component of this invention; Figure 6b is a schematic plan view of the transmission path component of Figure 6a; Figure 7a is a view similar to that of Figure 6a but of another embodiment; Figure 7b is a schematic plan view sf the embodiment of Figure 7a; Figures 8,9 and 10 are plan views of further embos ments of transmission path components.

Referring to Figure , a liquid level sensor 2 comprises a signal generator 4, and a signal transission path component 6. The signal generator 4 comprises a signal transmitter section 8 and a signal receiver section 10. The signal generator 4 further comprises a support 12, which in this embodiment is a printed circuit board (PCB), and a signal transducer 14 mounted on the support 12, in this embodiment, comprising a piezoelectric film for generation and reception of ultrasonic signals.

The transducer 14 further comprises a conductive ground electrode 16 on one side, and transmitter and receiver electrodes 18,20, respectively on the other side.

Referring to Figure 4, the transmitter section 8 is shown comprising a plurality of discrete transmitter electrodes 18 (or segments) arranged in a column of juxtaposed electrodes extending along the depth D of a liquid to be measured. Receiver electrodes 20 are similarly arranged in a juxtaposed manner along the depth, where for each transmitter electrode 18 there is a corresponding receiver electrode 20 at the same depth.

The depth D is also indicated by the arrow D in Figure 1.

Referring to Figures 1 and 4, a portion of piezoelectric film 14 sandwiched between a transmitter electrode 18 and the ground electrode 16 is mechanically excited by a variation in electrical potential between the electrodes.

This mechanical excitation, generates ultrasound. The receiver functions in exactly the opposite manner whereby a mechanical excitation of the piezo film by an ultrasound wave generates potential differences between electrodes. The transmitters and receivers can thus both act as receivers and transmitters respectively.

Referring to Figure 5, a signal electrode 18,20 is provided and series as both transmitter and receiver.

Electronic control circuitry associated with the electrodes, first transmit electrical signals and then switch over intro reception mode for a certain window of time to capture the signals that were transmitted. The transmitter and receiver electrodes can be formed by patterned circuit traces directly on the printed circuit board 12, whereby the transducer 14 could comprise piezoelectric polymer film, for example a single sheet, positioned over the patterned printed circuit board.

Such an arrangement is particularly cost effective to manufacture, and provides great flexibility in the arrangement of the electrodes, whilst providing a convenient direct electrical connection between electrodes and electronic processing components positioned on the printed circuit board to process the electrical signals that are generated. As the electronic processing occurs directly on the sensor, only simple connection means with external components that receive these sensor signals is required, thereby providing a reliable and cost effective connection to the sensor.

The ground electrode 16 can be provided either by a thin conductive layer on the piezo film, (for example by vapor deposition) or could simply be the transmission path component 6 if it is provided in a conductive material.

The reception of signals is dependent on whether there is liquid in the transmission path T of the signal from the transmitter 18 to the respective receiver 20.

This process is well known from W09302340, whereby the transmitter segments 18 that are above the level of the liquid, transmit signals which are not received by the corresponding receivers 20 because the air gap in the signal transmission path cannot transmit ultrasound. On the other hand, the transmitter and receiver pairs 18,20 below the sur-ace of the liquid 21 (see Figure 4) are able to transmit and receive these signals. By determining which segments generate signals and which do not, the corresponding height of the liquid can be computed.

Referring to Figure 1, the transmission path component 6 comprises a gap 24 for receiving the liquid to be measured therein, a first reflector 26 and a second reflector 28. The reflectors are formed from a solid body 30, whereby outer surfaces 32,34 are angled with respect to the plane of the transmitter and receivers -(the transducer 14) by an angle of 45". Other angles could of course be considered, the important factor being that signals transmitted from the transmitter section 8 is received at the receiver section 10, although the symmetrical outlay as described in the embodiment of Figure 1 would seem to be the most sensible. The outer angled surfaces 32,34 act as reflectors to the transmission of signals, due to the difference in impedance of the mediums (solid-air or solid-liquid) with respect to the signal characteristics (in this case ultrasound). The surrounding material that interfaces with these surfaces 32,34, is in this case either liquid or air. The recess 24 separates a first section 36 of the body 30 from a second section 38, whereby the gap 24 is defined (in the first embodiment) by parallel sidewalls 40,42 respectively of the first and second sections 36,38.

A signal transmitted from the transmitter section 8 and received at the receiver section 10 is illustrated by the dotted line, the whole length of which will be called the transmission path T of the signal. The transmission path T comprises a first section S1 of transmission of the signal through the first solid section 36, a second transmission section S2 through the second solid section 38, and a third liquid section L across the gap 24 from side-wall 40 to side-wall 42. A signal emitted by the transmitter 18 therefore initially progresses perpendicularly away from the plane of the transducer 14, is bounced off the reflector 26 and progresses toward the side-wall 40, subsequently traversing the gap 24 when liquid is present, then entering the solid section 38, bouncing off the reflector 28 perpendicularly toward the corresponding receiver 20. There will also be a certain amount of reflection off the walls 40,42, the reflection C off the wall 40 being particularly strong in the absence of liquid. The reflection C is received by the transmitter 18 after bouncing off the reflector 26. The time of transmission of the first reflected signal that follows the path S1,C can be measured by switching the transmitter from transmission mode to reception mode to capture the reflected signal. The speed of transmission of ultrasound in a solid, depends on the temperature of that solid. By measuring the time of the reflected signal C, the temperature can thus be computed. In this respect, the use of plastic materials is more advantageous than metals for provision of the solid section 36, as the speed of sound in plastic varies to a greater extent with respect to temperature than the metal. Determination of the temperature by use of a plastic solid is therefore more accurate.

As already described, the transmitter and receiver pairs 18,20 positioned above the level of the liquid, will have liquid transmission paths L that do not traverse the liquid, such signals therefore not being received by the receiver. For the transmitter receiver pairs 18,20 below the level of the liquid, the time between transmission and reception of the signal can also be determined, whereby the time of transmission will depend on the speed of sound of the signal across the gap 24, i.e. the liquid transmission path L. The speed of transmission of sound in different liquids varies on the type of liquid and is a function of temperatre. It therefore is possible to determine the type c liquid present, using the temperature computation and the time of transmission. The sensor can therefore either be used to determine the type of liquid, or the level of liquid or both. Useful applications for the latter, is for example determining the level of fuel in an automotive fuel tank, whilst simultaneously providing information on the composition of the fuel. In certain countries different - fuels are used, for example methanol and benzene, whereby the automotive control electronics need to adjust parameters such as timing of ignition for optimal functioning of the engine. Another application may be, for example, where a single fuel is used (i.e.

diesel) but where water may be present in the fuel due to condensation (the water being at the bottom of the tank due to its higher density). Additionally, the level and therefore the amount of water could also be determined.

Many other applications for the sensor performing as a sensor for determining the type of liquid could of course be imagined without functioning-as a level sensor.

By provision of the first and second reflectors 26,28 the signal transmission path T can be provided with a component that is directed over the plane of the support 12 or transducer 14 thereby enabling provision of a long transmission path, but a low profile construction (i.e. the height H is reduced in comparison to an embodiment where only a single reflector is used as shown in W09302340). The longer transmission path increases the time of signal transmission, which provides a number of advantages, such as more accurate determination of the transmission time and slower electronics are needed to switch from transmission to reception mode for transmitters that also act as receivers. Furthermore, in the event where a printed circuit board 12 is provided, which needs a certain surface area and therefore width W for positioning of electrodes, circuitry and electronic components, the "surface area" as such is efficiently used by extending the transmission path T thereacross, whilst keeping the height H to a minimum. An additional advantage, is flat the reflectors 26,28 and the gap 24 can all be formed out of a single solid body, for example as shown in Figure 3, by the transmission path component 6'' which provides a simple way of ensuring accurate tolerances in the positioning of the relevant surfaces such as the reflector surfaces 26'', 28'', and the length of the liquid ransmission path L. The latter enables accurate computation of the signal transmission times, by having an accurate determined length for the transmission path.

Referring to Figure 3, the functioning and construction is essentially the same as that already described by the embodiment of Figure 1 except for the following differences. Similar features will be denoted with the same number, but with a double prime. In the embodiment of Figure 3, the first and second solid sections 36'', 38'' are joined together as an integral component by walls extending across the gap 24'' at an end 40 proximate the support 12, and an opposing end 42.

Other embodiments, where the sections 36'',38'' are joined only by the wall 40 or only by the wall 42 could of course also be considered. The provision of the walls 40,42 ensures accurate, stable dimensional positioning of the gap 24'' and reflector surfaces for an accurate provision of the signal transmission path length as discussed above. The outer wall 42 may further have the advantage of reducing slosh of liquid in a tank, whereby entry of the liquid into the sensor would occur through an orifice, (for example continuation of the gap 24'') at the lower end sf the sensor immersed in the liquid.

Referring to Figure 2, another embodiment of a sensor is short whereby the principle difference with the embodiment of figure 1 resides in providing the signal transmission component 6' as a shell without solid sections, such shell for example being formed from sheet metal. Walls 32',34' positioned at an angle with respect to the printed circuit board 12 provide the first and second reflector surfaces 26',28'. The signal generating component 4 can essentially be the same as that already described- in the previous embodiments. The liquid transmission path L' would comprise no solid transmission paths, and would therefore be equal to the complete transmission path from the transmitter section 8 to the receiver section 10. In other words, the gap 24 of the embodiment of Figure 1 is replaced by the gap -24' enclosed by the shell 30' of the signal transmission components 6'. In this embodiment, determination of the temperature is not effected as there is no solid reflected signal C as described with the embodiment of Figure 1. A very long transmission path in the liquid is however provided, thereby simplifying electronics as already discussed above, and furthermore the provision of the shell 30' enables a very cost effective and simple manufacturing of a liquid level sensor.

Referring to Figure 3, a shell 31 can also be provided around the signal transmission component 6'', the shell 31 serving to enhance the efficiency of reflection of the reflective surfaces 26'',28'' and furthermore providing protection to the sensor. A further protective shell 33 can be provided to cover the other side of the printed circuit board 12 to protect the electronic components thereon. Furthermore, the shells 31,33 may also provide electromagnetic shielding to reduce reception and transmission of electrcmagnetic noise.

Referring to Figure a and 6b, another embodiment of a signal transmission component 6 is shown whereby the gap 24 is stepped along the depth D, in other words the liquid transmission path L has different lengths -1,L2,L3 and ;4 at different depths. This arrangement enables the number of electrical connections and the number of electronic signals to be generated to be reduced. This can be explained as follows. For each of the different gaps L1 to L4, a transmitter/receiver electrode is electrically connected to a first common conductor 50, the transmitters 18 connected to the conductor 50 are thus all fired at the same time. The time of transmission through the transmission path T is however dependent on the time of transmission through the liquid, which is dependent on the length of the gap L1,L2,L3 and L4 respectively. As the gaps have different lengths, the signals from the transmitters 18 of L1 to L4 will be received at different times. By detecting the times at which signals are received, it can be determined which of the electrodes are below the level 52 of liquid. A second conductor 54 is likewise connected in common to a plurality of transmitters/receivers, one per respective gap L1 to L4. By having 4 steps as shown in the example of Figure 6b the number of conductors 50,54 required to interconnect all of the electrodes is thus reduced by a factor 4. The number of interconnections influences the price of the silicon chips to which they are connected.

By reducing the number of connections, this price is reduced, as well as the reduction in the number of circuit paths that need to be provided on the circuit board.

Instead of stepping the gap 24 as shown in Figure 6b, it is also possible to provide a continuously widening gap as shown in Figure 7b with a very similar effect. To enhance the effect of the gap, both of the solid sections 36,38 can be provided with steps as shown in Figure 8. In the event that only one conductor is desired, each of the electrodes could be provided with a stepped gap L1-L16 as shown in Figure 9. Figures 6a and 6b shown the respective transmission components of the figures 6b,7b in a three dimensional view.

Referring to Figure 10, the gap 24 is provided with oblique side surfaces 40,42 that reduce the amount of reflection bounced off the wall 42 back to the transmitt-er 18 in comparison to the embodiment of Figure 1. In order to have this effect it is also possible to provide only the side-wall 42 with oblique angles, whereby the side-wall 40 would be as in the embodiment of Figure 1. By reducing the echo of reflected signals, more simple electronic processing can be provided which does not require exclusion, filtering or determination of unwanted echo signals.

Claims (15)

1. A liquid sensor for detecting the presence of a liquid, comprising a transmitter section having at least one signal transmitter for generating a signal responsive to presence of liquid, a first reflector for re-directing the signal generated through a transmission path where the liquid is present, at least one signal receiver for capturing the reflected signal, and a second reflector positioned in the transmission path intermediate the first reflector and the receiver.

2. The sensor of claim 1 wherein the transmitter comprises a piezoelectric transducer.

3. The sensor of claim 2 wherein the transducer comprises a film of piezo-polymer.

4. The sensor of any one of the preceding claims wherein the transmitter and receiver are mounted on a printed circuit board.

5. The sensor of claim 4 wherein the signal transmitter and receiver comprise at least one electrode patterned on the PCB, and the PCB comprises conductive circuit traces that are interconnected to the one or more electrodes.

6. The sensor of any one of the preceding claims wherein the or each transmitter and receiver are common, the transmission and reception modes being switched by electronic control means.

7. The sensor of any one of the preceding claims wherein the transmitter section comprises a plurality of signal transmitter segments disposed along a direction of depth of the sensor for measuring the level of the liquid.

8. The sensor of any one of the preceding claims wherein the first reflector is arranged at substantially 450 with respect to a main plane comprising the transmitter section.

9. The sensor of any one of the preceding claims wherein a solid section is provided in the signal transmission path, the solid section comprising a third reflector for reflecting transmission signals back to the transmitter section.

10. the sensor of any one of the preceding claims wherein the sensor comprises a solid body having first and second solid sections comprising the first and second reflectors respectively, the reflectors being outer surfaces of the solid sections.

11. The sensor of claim 9 or 10 wherein the solid section or body comprises a gap traversing the signal transmission path for receiving liquid therein, the gap forming the liquid signal transmission path,

12. The sensor of claim 11 wherein the gap has a varying width over the depth of the sensor, such that the liquid signal transmission path varies in length over the depth of the sensor.

1 3. The sensor of claim 12 wherein the gap width varies in steps.

14. The sensor of claim 1 2 wherein the gap width varies substantially continuously over the depth.

15. The sensor of claim 10 wherein the first and second solid sections are joined together integrally by a wall extending across the gap.

1 6. The sensor of any one of claims 1 to 8 wherein the reflectors are provided by walls of a shell enclosing an area for receiving the liquid.

1 7. A liquid sensor constructed and adapted to operate substantially as hereinbefore described with reference to any one of the Figures of the accompanying drawings.