GB2312749A - Fluid level sensor has signal paths of different lengths - Google Patents

Abstract

Device 1 for determining the level of fluid 14 within a vessel comprises a plurality of sets of acoustic transmitting and receiving devices such as piezoelectric devices 6a-6f spaced along the vessel. The transmitters simultaneously generate an acoustic signal, to be reflected from a stepped reflector plate 10 for reception, only for those signal paths which pass through the fluid. The stepped reflector provides different signal path lengths between each transmitter and receiver, thereby providing a time delay between the returning signals so that different fluid levels can be measured. Circuitry is provided (figs. 18 & 19) to balance the amplitude of the response signals. The required number of connection pins, and hence the cost of manufacture, is reduced. Alternative arrangements for providing different acoustic path lengths are shown (figs. 5, 10, 12 & 13).

Description

FLUID LEVEL SENSING DEVICE This invention relates to apparatus for measuring the fluid within a vessel and in particular an apparatus that utilizes acoustic transmission characteristics in order to perform the measurement operation.

It is well known to use acoustic waves in order to measure how much liquid is contained within a vessel. In addition, it is known to use piezo electric material as both the transmitter and receiver of said acoustic waves.

A problem with all measuring devices or sensors is the degree of-resolution that is necessary to provide useable information. For example, consider a simple liquid vessel, such as a right cylinder. In a case like this and in other vessels, the volume of the fluid within the cylinder is easily determined if the height of the fluid therein can be determined accurately. Where an acoustic measuring device is to be used, a generator transmitter of acoustic waves and a receiver of those waves must be positioned relative to the fluid level such that signal transmission between transmitter and receiver occurs dependent on whether or not fluid at that particular location is present. As may be readily imagined, the more transmitter/ receiver pairs that are disposed along the height of the cylinder, the more accurate the determination of how much fluid is contained therein will be.

However, a problem with simply placing a large number of transmitter receiver pairs along the height of the tank is not only the cost associated with the particular transmitter and receiver devices, which can be drastically reduced through the use of Piezo electric material as is disclosed in (EP-A-O 515 254 and WO 93/02340), but also includes the expenses associated with the driving and detection circuitry necessary to operate the transmitter and receiver pair. For example, in a commercial fluid level sensor device such as that described, it would ultimately be desired to produce an application - specific integrated circuit in order to make the device work. If it were determined that 48 transmitter/receiver pairs spread out along the height of the vessel are necessary to provide the desired resolution, this application specific integrated circuit would require at least 48 input/output pins to connect to the 48 transducers plus pins for ground, power and data transmission. The number of pins required would ultimately easily exceed 50. While there are significant economies of scale involved in production of devices such as this and particularly with integrated circuit production, a limiting factor is the number of pins required. Therefore, it could be expected that an application-specific integrated circuit having twice the number of input/output pins than another application specific integrated circuit will be slightly less than double the cost. At present, the cost per pin would make a high-volume liquid level sensor with an application-specific integrated circuit having a high number of input/output pins marginally commercially feasible.

Therefore, while it is known to reduce the cost of the transmitter/receiver pair by the utilization of piezo electric material and the possibility of using the same piezo electric component as both the transmitter and receiver by switching between transmission and receive modes, it is necessary to reduce the number of pins that are necessary in an application-specific integrated circuit in order to have a commercially feasible device for such high volume industries as automotive and in particular fluid measurement in the numerous vessels therein.

A solution to the aforementioned problem is achieved by providing a fluid level measuring device for determining the level of fluid within a vessel where the measuring device comprises at least two acoustic transmitting and receiving devices where the transmitting and receiving devices are spaced at different levels along the vessel and where an acoustic signal is generated by each of the transmitters where said signal that travels along a corresponding path that is dependent upon the presence of the fluid to reach the receiver, the fluid level measuring device being characterized in that both of the transmitters are driven- to emit their respective acoustic signal simultaneously and the length of the path associated with the first pair is different than the length of the path associated with the second pair, thereby providing a time delay between the returning signals.

It is an advantage of this invention that by the use of a single excitation signal both transmitters may be driven to emit the necessary acoustic signal and as a result of the different path lengths associated therewith, the time delay enables the apparatus to measure different fluid levels; thereby enabling the number of pins associated with an application specific integrated circuit to be greatly reduced. In -addition, it is another advantage that the different path lengths associated with the acoustic signals can be established by providing a stepped member included in the path of the associated signals. In addition, it is advantageous that the aforegoing is simple to manufacture and of compact design and construction.

It may also be possible to utilize the present invention to determine if fluid is present in any number of tanks. In this instance the multiple transmitting and receiving devices are positioned to correspond to respective tanks and each tank having a characteristic distance between fluid and transmitter and receiver, whereby simultaneous firing in response to an input results in spaced output dependent on fluid presence in the respective tanks. Note, while the device configured in this fashion indicates only fluid presence, multiple structures could be provided along the fluid level in order to provide a measure.

In addition, as in the devices described above, the amplitude of signal is dependent upon the acoustic path length, a desirable circuit is provided to equalize the output indications.

The invention will now be described by way of example with reference to the following drawings, wherein; Figure 1 is a diagrammatic representation of a device incorporating the present invention; Figure 2 is a chart presenting the diagrammatical response of the device of Figure 1 versus time for water and methanol; Figure 3 is another diagrammatic representation of an enhanced version of the present invention of Figure 1; Figure 4 is a diagrammatic representation of another embodiment of the present invention; Figure 5 is yet another diagrammatic representation of an alternative embodiment of the present invention; Figure 6 is a perspective view of an apparatus incorporating the present invention as represented in Figure 3; Figure 7 is an exploded lower perspective view of the apparatus of Figure 6; Figure 8 is a side sectional view of the apparatus of Figure 6; Figure 9 is an end sectional view of the apparatus of Figure 6 taken along lines 9-9 of Figure 7; Figure 10 is a side sectional view of an apparatus incorporating the alternative embodiment of the invention presented in Figure 5; Figure Il is an end sectional view of the apparatus of Figure 10 corresponding to the end sectional view of Figure 8; Figure 12 is an alternative embodiment of the present invention used to determine fluid presence in multiple tanks; Figure 13 is an alternative embodiment of Figure 12; Figure 14 illustrates in graphical form the input signal into a device according to the present invention; Figure 15 illustrates in-graphical form the desired response signal of a device according to the present invention; Figure 16 illustrates in graphical form the expected response where each tank contains fluid; Figure 17 illustrates in graphical form an equalized response of Figure 16; Figure 18 provides an electrical schematic of a circuit to realize the equalized response of Figure 17; and Figure 19 shows a representational device incorporating the circuit of Figure 18.

With reference first to Figure 1, the invention is presented in diagrammatic form and will generally be described in basic terms. A liquid level measuring apparatus is shown in representational form generally at 1. A piezo electric film member 2 is provided that is metalized either by depositing metal thereupon through vacuum disposition methods or by the use of a foil or sheet metal plate that is stamped to have the desired characteristics of the front electrode. The metalization is constructed to provide a ground 4 and a plurality of transducer elements 6. The structure could be as follows: a front layer that is a continuous plane of foil, plate or metalization overlying a layer of piezoelectric polymer that is disposed over an electrode pattern formed upon a printed circuit board that carries the appropriate circuitry necessary on the opposite side thereof for operating the apparatus. The transducer elements 6 can function both as transmitters and receivers as is described in WO 93/02340, whereby electronic circuitry associated therewith switches their function between generation and sensing of acoustic pulses. The transducer elements are all coupled together in a gang circuit by traces 8 which would be disposed upon a printed circuit board where the piezo electric device has been mounted.

Note, the piezo electric device and the circuit traces shown thereupon have been rotated through 90 for ease of viewing and description. In addition, the printed circuit board and associated electronic circuitry and components have been omitted for clarity.

The piezo electric device is spaced apart from a reflector plate 10 formed of a material that is reflective to acoustic signals. The reflector 10 has a plurality of steps 12a-12f to correspond to the transducer elements 6a6f. A fluid 14 to be measured is disposed therebetween.

In this case, each of the transducer elements 6a-6f is located a different distance from the corresponding reflector surfaces 12a-12f. If each segment 6a-6f were to be fired individually, then switched into a received mode as described in the aforegoing references, then the return pulse from each transducer would show unique delay between transmission and reception according to the spacing for that segment. However, if all 6 segments of the gang are interconnected by a single circuit path 8, whereby all are fired as a result of a single initiating pulse, after which the transducer 6a-6f are switched to a receive mode, the return signal can be messaged to consist of up to 6 (6a-6f) discrete pulses (if fluid is present at all levels) that are separated in time according to the difference in distance of the reflector steps. The response for both water and methanol is shown in Figure 2 where the fluid level 14 would extend above the highest transducer. For the situation of Figure 1, only the first 3 main echoes would be present. In addition, as can be seen from the chart it is important that the array of transducers 6a-f may be switched out of the sensing mode before any secondary echoes return. This is accomplished by selecting the spacing between the reflector 10 and transducer array and the step height based on the transmission characteristics of the fluid. As the apparatus depends upon being able to capture and count these closely spaced echoes, the broad band with piezo polymers offers significant advantages.

Now with the basic understanding of the inventive concept, the device having a higher resolution is shown within Figure 3 generally at 100. In this device a reflector plate 110 is used that also has discrete steps 112a-112f thereupon. The reflector plate 110 is opposite a piezo electric element 102 including 48 transducer elements 116a-f-123a-f where all of the common transducers 116a-f, 117a-f, ..., 123a-f are joined together into separate gangs by a common signal path 108,128-134. This device has a ground contact 104 and 8 signal contacts corresponding to each of the gang tracks 108,128-134.

When a signal is fired into track 108, acoustic generation occurs by the transducers 116a-116f associated with that gang and a characteristic return signal is generated having a given number of pulses that the circuitry counts.

By proceeding sequentially or in some other pattern that does not allow interference between the various transducers a high resolution measurement may be made. It may also be desirable to fire all gangs at once and only switch one gang into the receive mode.

With respect now to Figure 4, an alternative embodiment of the basic device is shown. In this embodiment the structure is shown generally at 200. A standard planar surfaced reflector plate 210 is provided opposite the piezo electric structure 202 that is constructed as outlined above. Attached to the face of the piezo electric member 202 is a stepped transmission plate 250 that is transparent to acoustic signals generated by the piezo electric member 202; however, this transmission plate 250 has the effect of slowing the signals that pass across the thickness thereof. As the steps correspond to different path lengths for the acoustic signals that are still open or closed dependent on fluid presence, once again a return signal having spaced apart pulses will result each firing of multiple transducers.

a With reference now to Figure 5 device is generally shown at 300 that works on detecting the presence or absence of fluid along a stepped plate 302 such that a reflection is made at the interface with the fluid that is characteristic of whether or not fluid is present. A device such as this type is described in GB patent application no. 95/13267.6. An advantage of this device is that the transmission path while still dependent upon fluid presence, does not include the fluid in the path.

With reference now to Figures 1 and 2, an important aspect of this invention is whether or not there will be any interference between the main echoes which are generated due to reflections at the reflector plate and any subsequent secondary echoes that may be generated by way of additional reflections. It is important to note that the transducer can be switched out of the receive mode prior to any of these reflections returning to the system, thereby preventing the sensing of the additional/echoes. This is a mathematical problem that involves calculations at the speed of sound in the various fluids that may be present in the vessel and the step size of the reflector plates. For example, with respect to a system similar to that shown in Figure 1 a minimum spacing between transducer array and reflector of 4.5mm to a maximum spacing of 8.25mm would result in a minimum delay in water of first echo arrival of 5.81 micro seconds which corresponds to a 9mm path length and a maximum delay in methanol to the first echo of 8.16 micro seconds having a similar 9mm path length. This provides a minimum and maximum spacing between valid-echoes of .97 micro seconds to 1.36 micro seconds. As shown in Figure 2, the main echoes of either water or methanol would be separate from each other, thereby enabling the transducers to be switched off at a fixed point in time or stopping after counting to 6 pulses such that the secondary echoes would be ignored.

With reference now to Figure 6, a useable embodiment of the invention is shown generally at 500. The level sensor apparatus 500 would be fittable inside a vessel and includes a mounting flange 502 having a plurality of fasteners 504 thereabout for attaching the apparatus 500 to a mounting bracket therein and further assembly as described below. The apparatus 500 further includes a receptacle 506 wherein a plurality of terminals 508 are disposed for forming an electrical interconnection with the active components therein. The receptacle 506 extends outward as a port-like flange from a housing member 510 a sealing boot (not shown) could be attached thereto. The apparatus 500 includes input and output vents 512 at each end thereof to enable the fluid to be measured to pass through.

With reference now to Figure 7, an exploded view of the apparatus 500 is shown. The apparatus 500 includes a connector element 514 having a plurality of contacts 508 therein positioned in the receptacle 506. A sealing gasket 516 is provided such that a fluid tight seal exists at the receptacle 506. The housing 510 is constructed in a shell-like manner having a generally hollow interior including two-tiered pair of cavities 520,522. The upper cavity 522 is in conmunication with the input and output ports 512. A ledge 524 is formed where the upper and lower tiers 520,522 meet and the flange 502 extends outwards at the open upper surface. Note, if the housing werelout of metal it would have a shielding effect for the circuitry.

An active component 526 that includes the circuitry necessary to interconnect and operate the piezo electric sensor and transducers formed thereupon is formed in a board-like manner.

As described in the aforegoing patent applications, the structure of the active component 526 would involve a piezo electric material being laminated upon an upper surface 528 of a double sided printed circuit board that has the active components disposed on a lower side 530.

The components are encased within the lower tier 520 when the active component 526 is seated within the housing 510 and interconnected with the piezo electric structure by conventional means such as plated through holes. The active component 526 includes components thereupon that would be interconnected with the contacts 508 of the receptacle 506. As the active components are contained within the cavity 520, provided a sealing is placed on the upper surface 528 the components will be protected within a sealed cavity. A rectangular sealing gasket 532 is formed to seal around the outer edges of the active component 526 and against the ledge 524. A base plate component 534 includes a shroud portion 536 having a stepped upper edge 538 designed to engage the gasket member 532 and hold the gasket firmly against the housing 510 and active component 526. An inner surface of the component 534 includes a stepped reflector plate 540 having multiple steps 540a-540f that will be oriented opposite the transducers as discussed above. A flange 542 extends outward beyond the shroud and includes fasteners 544 for engaging the flange 502 of the housing 510.

With reference now to Figure 8, the assembled device 500 may be seen in longitudinal cross-section. In this view, the active device 526 is positioned at the lower tier 520 wherein the active components on the active device 526 are kept contaminant free. The fluid to be measured will be received in the upper tier 522 between the active device 526 and the stepped reflector plate 540 by passing through input and output ports 512. With further reference to Figure 9, the apparatus 500 is shown where the sealing member 532 is compressed by the shroud 536 so that it forms a contaminant tight fit about the housing 510, the active member 526 and the shroud 536.

The receptacle 506 contains a plurality of contacts 508 that are shown oriented to be connected with the active component 526.

With reference now to Figure 10 and Figure 11, crosssections that correspond to a device incorporating the principle of Figure 5. In this embodiment, as described above, the device 600 has a housing 602 with a first cavity 604 wherein the active devices on the active component 606 would be sealed therein. A stepped transmission plate 608 is provided that has a plurality of steps 608a-608f thereupon where the active component 606 is set such that the transducers thereupon switch between an on and off position in order to capture the signal if a fluid is present at the interface surface of the step 608a-608f.

With reference now to Figure 12 and Figure 13, an alternative embodiment of the present invention is presented. In this use, a multiple chambered tank is shown at 700 and 800 respectively. The tank 700,800 are shown to have three separate chambers 701,702,703;801,802,803. These chambers are separated by partition walls 704,705;804,805. In the embodiment of Figure 12, a piezoelectric sensor 706 is disposed along an outer wall 708 of the tank 700 such that it spans each of the three chambers 701-703. Each chamber 701,702,703 has a characteristic wall thickness 710a,710b,710c respectively. As will be apparent from the discussions above, as a result of these different wall thicknesses acoustic signals transmitted from the piezoelectric sensor device 706 will have different time characteristics depending upon the width of the walls 710a-c. With reference now to Figure 13, an alternative body is presented. In this embodiment, the tank 800 has a piezoelectric sensor 806 along an outer surface 808 thereof. Once again, the piezoelectric sensor 806 spans the three chambers 801-803. As an alternative construction to that shown in Figure 12, in this embodiment the wall 808 of the tank 800 has thickness portions 810a,810b,810c that are of equal thickness. In order to provide the time delay desired, spacers 812,814 having different thicknesses, or at least different acoustic path length thicknesses, are affixed to the outside wall 808 corresponding to the chambers 802-803.

In either of the embodiments discussed above, it is obvious that the piezoelectric member 706,708 would include transmission and receiving structures corresponding to the chambers according to the teaching described above.

With reference now to Figure 14, the piezoelectric elements described above would be excited by a pulse signal which would cause an acoustic signal to be transmitted therefrom. Ideally, the response from each of the receivers associated therewith would be approximately equal, as shown in Figure 15 and Figure 2. However, due to the difference in acoustic path length as a result of the varying wall thicknesses associated with the given transducers, what is more likely to occur is that the pulse received from the device with the shortest acoustic path length will have a greater amplitude than the pulse received from the device with the larger acoustic path length. This is best represented in Figure 16 when viewed in conjunction with the structure of Figure 12. Assuming all tanks 701-703 to be full-, the acoustic path length associated with the first chamber 701 would be equal to two times the wall thickness 710a, the acoustic path length of the signal associated with the second chamber 702 would be two times the wall thickness of 710b. The same applies for the third chamber. As can readily be observed, the acoustic path length is greater for the second and third chambers. As the acoustic signal is attenuated as it travels along its path length, the resulting amplitude of the received signal is decreasing as shown in Figure 16. While this may be acceptable, it would be more desirable to have equal signals as shown in Figure 15. In these cases it may be advantageous to connect passive electrical elements into the circuitry between each transducer element. Desirably a lossy transmission line is formed. Such a transmission line will desirably be driven from the end corresponding to the lowest natural amplitude response so that the progressive attenuation between the elements brings the final measured response from each sensing site into the same detection amplitude band.

With reference now to Figure 18, an electrical circuit schematic of the aforegoing is presented. The piezoelectric polymer film sensor represents a natural distributed capacitance C and the conductive paths linking the various sensor elements may incorporate increased Resistances. The resistance R may be realized by convoluted or serpentine track patterns on the piezoelectric element to enhance the effective length of the connection path, a resistive conductive path may be printed, or discreet resistive elements incorporated therein. It may also be desirable to use capacitive elements in place of this resistance.

With reference now to Figure 19, a device realizing the circuitry of Figure 18 is illustrated. It is important to note that this device may be used in a vertical position for determining liquid level measurements, as described with reference to Figures 1-11 above or across multiple tanks, as described with reference to Figure 12 and Figure 13 in order to determine the presence of fluid in corresponding chambers. The piezoelectric sensor of Figure 19 is reference generally by 900. The device 900 includes three transducer structures tl,t2,t3 that may either be a single element and switch between a transmit and receive mode or be configured as separate transmitters and receivers in order to generate and sense an acoustic pulse. In this embodiment, desirably, the longest acoustic path is associated with the first transducer t3 while the shortest acoustic path is associated with the transducer furthest away from the contact areas 902 from which the drive and sensing circuitry are connected to the device 900. A plated through hole 904 is provided along one of the contact areas 902 for connecting with a common ground 906.

In order to equalize the sensed response, resisters R are incorporated into the electrical circuitry such that the returned pulses are equal when a fluid is present.

Claims (8)

1. A fluid level measuring device for determining the level of fluid within a vessel, comprising at least two sets of acoustic transmitting and receiving devices for spacing along the vessel, the transmitting devices being adapted to transmit acoustic signals, which travel paths to the receiving devices dependent on the presence of the fluid, and to emit their respective acoustic signals simultaneously, the arrangement being such that the signal paths between the transmitting devices and their respective receiving devices are of different lengths, thereby providing a time delay between the returning signals, and circuitry for balancing the acoustic signals at the receiving side.

2. The fluid level measuring device of claim 1, wherein the transmitting and receiving devices are of piezo electric material.

3. The fluid level measuring device of claim 1 or 2, wherein the transmitting and receiving devices are the same element, whereby the transmitter emits the signal and is then electrically switched to a receive mode to detect the signal.

4. The fluid level measuring device of claim 1, 2 or 3, wherein the acoustic transmitting and receiving devices are oriented to face a reflector plate having a plurality of steps thereupon such that the sets of transmitting and receiving devices correspond respectively to the steps, the devices and reflector plate being spaced apart so that the fluid is disposed there between.

5. The fluid level measuring device of claim 1, 2 or 3, in combination with a vessel including multiple chambers, wherein the sets of transmitting and receiving devices correspond respectively to the different chambers.

6. The fluid measuring device of any one of the preceding claims, including resistance circuitry to balance the output signals of the devices.

7. The fluid level measuring device of any one of the preceding claims, wherein different acoustic path lengths are realized through the use of a spacer.

8. A fluid level measuring device constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.