Piezoelectric sensors

ES 2 149 358 T3

DESCRIPTION

Piezoelectric sensors.

This invention relates to a piezoelectric sensing probe for sensing a characteristic of a medium surrounding the probe. The invention has particular application to the perception of the level of diesel fuel in a diesel storage tank of the type used in domestic central heating systems. For a domestic user, it is essential to know when there is little diesel left in a tank of this kind, and for the driver of a tanker it is essential to know when the tank is full.

Apart from manual inspection and the use of a dipstick, the methods present to perceive the level of diesel in a storage tank make use of level gauges, for example of the Atkinson type, which are based on a visible check of the level of fuel. fluid in a transparent tube. However, such tubes frequently get dirty and / or blocked, and there is a substantial risk of fluid leakage.

GB-A-2097964 shows a pair of piezoelectric sensors spaced one above the other to detect the liquid level around a submersible pump. Each sensor is part of a respective oscillator circuit that only oscillates if the medium surrounding the element has the appropriate acoustic impedance, and each sensor is received in a recess in the outer shell of the pump.

Document EP-A-0515254 describes a fluid level indicator having vertically spaced piezoelectric emitters, mounted on an insulating support that also carries the conductors for transmitting electrical signals to the respective emitters. The emitters are driven in sequence, and the emitted signals are received by a common receiver mounted on a separate vertical bracket, spaced from the bracket carrying the emitters.

US-A-3220258 describes a liquid level detector in which spaced electro-acoustic transducers are supported on a rigid wall, plate or pole. Transducers can be piezoelectric devices and can form the resonant element of a crystal oscillator. Changes in the impedance or frequency of the transducer oscillation are detected to determine the level of the liquid.

Document EP-A-0384373 has an acoustic emitter at the bottom of a fuel tank to emit ultrasound pulses in the liquid, the pulses being reflected at the interface between light and heavy oils (diesel). A receiver at the bottom of the reservoir receives the reflected pulses and can combine with the emitter to form an electro-acoustic crystal. A probe rod extending into the fuel tank carries additional electro-acoustic receivers, one above the other, which receive the emitted sound waves and pass electrical signals through wires in a tube that is part of the probe rod.

In accordance with the present invention, there is provided a piezoelectric sensing probe comprising an elongated insulating support member supporting a plurality of piezoelectric sensing elements spaced along the member, the support member having respective conductors extending from one end of the member to each of the sensing elements for conducting electrical signals to and from each sensing element; Each sensing element is cantilevered on the support member, spaced relative to it by one or two pins in such a way that the sensing element is free to vibrate when an electrical signal is applied to the sensing element along the respective conductor. , and at least one pin provides an electrical connection between a conductor and each sensing element.

Preferably, an electrical connector is provided at the end of the support member to connect the respective conductors from each sensor element to a monitoring circuit for monitoring an electrical characteristic of the sensor elements dependent on the environment surrounding the sensor element. For example, in a fluid level detector, the monitoring circuit monitors a characteristic dependent on the presence or absence of a fluid at the height of the respective elements. The electrical characteristic may comprise, for example, the resonant frequency of the sensing element.

The support member preferably comprises a printed circuit board strip and the conductors then comprise conductive tracks on the board.

The piezoelectric sensing element preferably comprises a piezoelectric wax disk attached to a resonant metallic plate, the plate being secured to the printed circuit board in spaced relation thereto and being electrically connected to one or more of the conductive tracks of the circuit board. the printed circuit board.

A sensor incorporating the present invention can be used in a method of filling a storage tank with fluid supplied from a location remote from the tank. In this case, with the sensor probe inserted into the reservoir and with a radio transmitter coupled to the probe to transmit an output signal from the monitoring circuit, a first signal is transmitted from the sensor to a receiver at the remote location when the fluid from the reservoir reaches a first sensor element, and a second signal is transmitted from the sensor to the receiver when fluid from the reservoir reaches a second element of the sensor elements. In this way, the filling rate can be reduced in response to the first signal indicating that the tank is almost full, and filling can be terminated in response to the second signal indicating that the tank is full.

Consequently, when filling diesel storage tanks from tank vehicles, the tank driver can activate the transmitter so that the sensor signals can be transmitted from the storage tank to the vehicle without the tank driver. have to leave the vehicle.

To facilitate a more complete understanding

ES 2 149 358 T3 of the foregoing and other aspects of the present invention, an embodiment will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic plan view of a fluid level sensing probe and connector incorporating the invention;

Figure 2 is a plan view similar to that of Fig. 1, showing the probe without its connector;

Figure 3 is a schematic sectional view of the upper end of the probe mounted in a diesel fuel storage tank;

Figure 4 shows three different applications of the probe;

Figure 5 is an enlarged detail of Figure 1, showing the connection of one of the piezoelectric elements to the printed circuit board;

Figure 6 is a side view of the assembly shown in Fig. 5;

Figure 7 (A) shows a plan view of an alternative piezoelectric sensor ( flat pins );

Figure 7 (B) represents a side view of the piezoelectric sensor (bent pins);

Figures 8 (AD) show various stages of the formation of an elaostic contact for the sensor of fig. 7;

Figure 9 is a perspective view of the piezoelectric sensor of Figs. 7 and 8;

Figure 10 is a cutaway perspective view of a control box;

Figure 11 shows a front view of a ten point probe using the sensor of Figs. 7 - 9 and the control box of the

Fig. 10; and Figure 12 is a rear view of the probe of Figure 11.

Referring to Figs. 1-6 of the drawings, an elongated fluid level sensing probe 10 consists of piezoelectric ceramic probing elements 16 spaced along a strip 17 of printed circuit board. Each element consists of a thin resonant plate 30 of copper, brass or stainless steel, attached to a piezoelectric disk 31 of ceramic. Disc 31 has silver electrodes 32 connected to respective conductive tracks 18 of the printed circuit board. For clarity, tracks 18 are shown uniquely at the upper end of the probe.

When an electrical signal is applied to this element, the piezoelectric ceramic disk 31 expands or contracts slightly, causing the resonant metal plate 30 to bend. A positive voltage expands the piezoelectric disk and creates a convexly curved element while, instead, a negative voltage contracted the piezoelectric disk and created a concave element. A separate voltage generating island 33 is provided in the ceramic piezoelectric disk 31, which is used to feed back a signal at the resonant frequency of the assembly to the drive circuit.

In Figs. 1 and 2 the ceramic disc 31 is shown looking outwards but, in practice, as shown in Figs. 5 and 6, the disk 31 faces inwards towards the printed circuit board 17.

Each piezoelectric ceramic element 16 is mounted on the printed circuit board 17 by means of a short piece of rigid wire 64 (Fig. 6) that separates the ceraomic disk 31 from the tracks 18 of the board, while connecting a first selected one. of the tracks with an electrode 54 of the metal plate 30. The electrode 54 was positioned as close as possible to the nodal ring of the assembly, which is normally approximately 2/3 of the diameter of the plate 30 and, therefore, overlapped by the ceramic disk 31.

A second selected of the tracks 18 is connected by a flexible wire 51 with the silver electrode 32a of the ceramic disk 31, and a third selected track is connected by a flexible wire 50 with the silver electrode 32b of the generator island 33. voltage. The piezoelectric element 16 is of the self-activating type, in which the signal from the electrode 32b provides a feedback signal to a self-oscillating circuit, and the activation signal from the oscillating circuit is applied through the electrode 32a of the disk 31 and the electrode 54 of the metal backing plate 30.

In use, the probe 10 is introduced into a container containing the fluid whose level is being sensed. The upper end of the probe carried the monitoring circuit (not shown) and was provided with a connector 13 for the connection of a meter 60 (Fig. 4b) or an RF (radio frequency) transmitter unit 61a, 61b (Figs. 4a and 4c). The meter 60 will respond to the output of the monitoring circuit to provide an indication of the fluid level depending on the number of sensing elements immersed in the fluid. Similarly, transmitter unit 61a or 61b transmitted such indication as a modulated RF signal to a respective remote receiver 62a, 62b (Figs. 4a and 4b).

In a particular application, the probe is inserted into a diesel storage tank of the type used in central domestic heating systems. In this case, as shown in figs. 3 and 4, the connector 13 at the upper end of the probe is inserted into a gland 34 mounted on the upper wall 35 of the reservoir. The connector 13, with the upper end of the printed circuit board 17 exposed, then protrudes upward from the reservoir and can be provided with a protective cap 36 when not in use.

The upper part of the connector 13 comprises a conical head 46 with a vertical slot

ES 2 149 358 T3 through which the printed circuit board 17 extends. A bore 38 extends generally perpendicular to slot 37, and plate 17 is retained in slot 37 by a pin 39 in bore 38, which passes through a corresponding hole 55 in the plate. The canon head 46 provides a drain for any fluid that comes into contact with the connector. Aligned with the plate 17 and positioned on the outer edges of the connector, there are two pins 40. The pins 40 not only provide protection against accidental damage to the board but also act as guide pins when mounting the meter 60 or transmitter units. 61a, 61b on the sensor probe.

Figs. 1 and 2 illustrate a 2-point probe. Such a probe is particularly useful when a tanker driver is filling a domestic diesel storage tank. A first warning indication is then provided when the lower of the two piezoelectric elements 16 is submerged in the diesel, and a final "high" indication is provided when the upper element is submerged. The indications are transmitted by frequency modulation of an RF carrier signal from transmitter 61a to remote receiver 62a (FIG. 4) held by the tanker driver.

When it is important to monitor the level of a fluid as it rises or falls over a wide range, the number of piezoelectric elements spaced along the probe can be increased. For example, a 10-point probe could be used for a diesel fuel storage tank to provide information on the level of diesel fuel in the tank to a domestic or commercial user. The 60 meter (Fig. 4b) attached to connector 13 will then provide an appropriate reading as each piezoelectric element is exposed. It would also be possible to provide a remote meter 62b at the user's premises and transmit the data to a meter receiver using radio transmitter 61b (FIG. 4c) coupled to connector 13 and modulate or encode the RF carrier signal. In this case, the receiver may be connected to a telephone link and may automatically transmit a coded “new request” signal to a selected telephoanic number in response to a signal from the sensing probe indicating that the diesel level has dropped below of a predetermined minimum level.

For a 10-point probe, the PCB strip may be 15 mm wide and up to 10 meters long.

A circuit for operating each of the fluid level sensing probes described above is separately described in our co-pending application _________ based on and concurrently filed with UK application 9408176.7.

In another embodiment illustrated in Figs. 7-12, the piezoelectric element is not of the self-energizing type and lacks a separate voltage generator 33. In Figs. 7-9, piezoelectric element 116 comprises a resonant metal plate 130 attached to a smaller diameter ceramic piezoelectric disc 131, the plate 130 having two mounting pins 164A and 164B extending from the edge of the plate. Such elements are conventionally used as water generating elements in alarm circuits. The resonant plate 130 is formed from a thin metal sheet, by press molding, and notches 166A, B, C, D are cut into the plate at the junction with the pins. Application notches 167A, B are cut into the ends of the respective pins 164A and B, so that the pins can be engaged with corresponding holes in a printed circuit board (PCB) 17.

Tabs 164A, B, initially formed in the same plane as plate 130, are bent to a generally perpendicular position with respect to the plate. Notches 166A, B, C, D facilitate bending and improve the resonance properties of plate 130 in use. The pins are rigidly fixed to the PCB by soldering, so that the plate is supported with minimal damping from the vibrating plate; this is achieved in part by notches 166A, B, C, D, which reduce the size of the junction between the vibrating plate and the rigid lugs. Consequently, the pins provide high-impedance water blocking for vibrations and reduce shock absorption.

Tabs 164A, B are positioned at the ends of a string 168 through circular plate 130. The length of string 168 can be that of a full diameter, but a string length less than one diameter has been found to provide a desirable resonance mode on the board. The most preferred length of string is, as shown in Fig. 7A, approximately that which will allow four strings of similar length to be connected around the circular plate.

The flexible wires 50, 51 of the previous embodiment are replaced, in this embodiment, by a conductive elastic contact 151. The elastic contact 151 is connected at one end to a track of the PCB 17 and at the other end to the piezoelectric disk 131 of ceramic by means of soldered joints for electromechanical connections. Consequently, the spring contact provides the electrical path between the PCB track and the piezoelectric disk.

The elastic contact 151 is formed from a deformable metal strip by a press, and several strips can be formed together in a comb 152, as shown in FIG. 8A. The spring contact, illustrated removed from the comb in Figure 8B, comprises a body portion 155, temple portions 154, 156, a neck portion 157, and a head portion 158. In Figure 8C, body portion 155 has been bent by line 159 so that the plane of head 158 and neck 157 are substantially perpendicular to pins 154 and 156. Neck 157 has kinks, as indicated in Fig. 8C. This allows for better positioning of the spring contact so that the body portion 155 does not interfere with the resonant plate 116. In Figure 8D, neck 157 and head 158 have been bent to the loaded position, as indicated.

ES 2 149 358 T3

In manufacture, each spring contact 151 is positioned on a PCB with pins 154, 156 mounted in pre-drilled holes. The pre-formed piezoelectric element 116 is positioned on the PCB with its pins 164A and 164B mounted in corresponding pre-drilled holes. When the pins 164A and 164B are pushed into position, a previously welded contact point 171 (Figure 7) of the ceraomic disk 131 bears against the loaded head 158 of the contact spring 151. The piezoelectric element pins and the contact pins elastics are then soldered to respective tracks on the PCB. A hole 172 (Fig. 7B) of PCB 17, located directly under the previously soldered connection point 171, allows the heat from under the PCB to reach the solder, and therefore the sleeve, when the head 158 of the elastic contact is connected to the ceramic disk .

The position of the previously welded connection point 171, at a point on the edge of the ceramic disk, ensures that the connection has been made at a point with minimal vibration. This reduces the movement of the elastic contact, which, in turn, reduces wear and limits the damping effect of the spring.

In the embodiment of Figs. 7-12, the upper end of PCB 17 includes a connector 13 corresponding to that used in the embodiment of Figs. 1-6, and the connector 13 plugs into the base 123 of a control box 121 (Figs. 10 12). Control box 121 contains a microcontroller on PCB 122 as shown in Figure 10, and connects to the top of PCB 17 as shown in Figures 11 and 12.

The exposed upper end of the PCB 17 between the pins 40 of the connector 13, is received in a socket 124 within the base 123 of the box 121, the socket 124 having fixed electrical connections 125 to connect each of the PCB tracks 17 with the microcontroller (not visible in the figure) on the back of PCB 122.

The microcontroller circuit output terminals of PCB 122 are then provided on a protruding portion 126 of the PCB, arranged between a pair of pins 127a, 127b, so that a meter or radio transmitter can be mounted as shown in Fig. Four.

Figure 11 shows a front plan view of a multipoint sensing probe using the sensing elements of Figs. 7-9. Ten piezoelectric sensors 116 of the type shown in Figs. 7-9 are spaced along the front of PCB 17 and five printed tracks 117 extend below the board and connect to the lower five sensors.

Fig. 12 shows a plan view from behind of the probe shown in Fig. 11, the rear face of PCB 17 having five tracks 118 that connect with the five upper piezoelectric sensors 116 and a common track 119.

The sensor probes shown in Figs. 1-6 and in Figs. 7-12 can be used, in general, to detect the level of a fluid and, in particular, the level of diesel fuel in a diesel storage tank. However, the probes could also be used to detect other characteristics of the surrounding medium, such as the viscosity of a liquid and could also be useful, for example, in powdery materials technology.