GB2313473A - Piezoelectric sensor - Google Patents

Abstract

A piezoelectric film transducer 4 is mounted on a support 6 held to a PCB 8 to form a cantilever beam 9. Generating a signal impulse in the piezoelectric transducer causes the beam 9 to vibrate. Depending on whether the beam is free, damped by contact against an external object, or present within a fluid or powder, the vibration characteristics differ thereby enabling determination of such condition. Applications may, for example, be use as a switch or detector of presence of dry powder in a printer toner.

Description

PIEZOELECTRIC SENSOR This invention relates to a piezoelectric sensor, in particular a sensor in the form of a biasable beam.

Piezoelectric materials respond to variations in mechanical strain and are therefore typically used for event sensing (i.e. detection of vibration, shock, triggering of a switch etc.) A passive piezoelectric component does not however produce an output in static conditions i.e. a piezoelectric switch typically does not have a permanent on/off static condition, but rather detects an event. Certain piezoelectric sensors or switches are produced from piezoelectric polymer, typically in the form of a film, that is particularly cost effective and compact. Furthermore, film has other advantages such as the ability to be easily cut to different shapes, and also has a low mass.

It would be desirable to combine the latter characteristics in a sensor that determines not only events, but also operates in static conditions. It would also be desirable to improve the reliability of sensors, in particular, to be able to detect whether the sensor is functioning correctly or not. It would also be desirable to provide a versatile sensor that is able to sense presence of materials that are difficult to sense by conventional methods, such as the presence of dry powder, for example in a laser printer cartridge.

It is an object of this invention to provide a piezoelectric sensor operable in static conditions, and a reliable and versatile method of sensing therefor.

Objects of this invention have been achieved by providing the sensor according to claim 1, and method of sensing according to claim 7. Advantageously, a compact and versatile sensor able to detect a variety of static conditions is provided. Furthermore, self diagnosis is also enabled. A method of sensing that enables detection of powder or for detecting presence of liquids or external objects is also provided in a reliable and cost effective manner.

An embodiment of this invention will.now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is an isometric view of a sensor according to this invention; Figure 2 is a schematic cross-sectional view of a sensor according to this invention; Figure 3 is a graph of time versus voltage of analogue and digital output signals of the sensor of Figure 2 in the "offs condition; Figure 4 is a diagram similar to Figure 3, but with the sensor in an "on" condition (damped); Figure 5 is a graph of time against voltage of digital outputs of a sensor under different conditions.

Referring to Figures 1 and 2, a sensor 2 is shown comprising a piezoelectric transducer 4 mounted on a resilient beam 6 mounted on a printed circuit board 8 having electronic circuitry 10 thereon for processing the sensor signals. The beam 6 is mounted on an end 12 of the PCB at an attachment end 14, and extends therefrom to a free end 16 that extends beyond the PCB in a cantilever manner. The piezoelectric transducer 4 comprises a thin layer of piezoelectric polymer, or copolymer, for example provided in a film or deposited by other means on the support beam 6. On an upper side 18 of the transducer 4 is an electrode 20 that covers a large part, or substantially the whole, of the upper layer 18, in particular the portion extending beyond the attachment end 14. The underside surface 22 of the transducer 4 is in a similar manner provided with an electrode 24 (see Figure 2) sandwiched between the piezoelectric layer and the support 6. The electrode 20 may, for example, be the ground electrode, and the electrode 22 be the signal electrode. It may not be necessary to provide a signal electrode 22 on the piezoelectric layer 4 if the support 6 is a conductor (e.g. metal) that acts as the electrode.

The support 6 may, for example, be provided as a thin resilient printed circuit board. In the latter case, if a bottom side of the support is provided with a ground electrode, the sensor would be shielded from electromagnetic and electrostatic interference (the signal electrode being sandwiched or surrounded by a ground conductor). The electrodes are connected by rivets, or other means, to circuit traces on the printed circuit board for interconnection to the electronic circuitry 10.

Referring to Figure 2, a simplified cross-sectional view of a sensor similar to that of Figure 1 is shown (the same numbering is used). Application of a potential difference between the electrodes causes expansion of the piezoelectric layer, as illustrated by the arrow A. In the free beam embodiment elongation of the transducer 4 causes bending of the cantilever beam portion 9 of the sensor, in the direction B. A square signal pulse 24 is illustrated in a analogue signal graph denoted AS of Figure 3, which shows voltage as a function of time.

Figure 3 also shows the digital signal denoted DS derived from the analogue signal and transformed by the electronic circuitry 10. The initial voltage pulse 24 causes subsequent free vibration of the beam 9 that is dampened substantially only by internal damping characteristics of the beam. There is therefore a relatively slow decay of the vibration amplitude of the beam. The latter shall be denoted the "undamped" state (i.e. no external damping).

If the beam is damped, for example by application of the free end 16 against an external object or presence of the beam in a powder or liquid, the vibration of the beam will decay faster as illustrated by the output signals shown in Figure 4. Comparison of the digital signals DS of Figures 3 and 4 show significantly different results that enable determination of the state of the sensor, (i.e. whether it is damped or undamped). In the event of non-functioning of the sensor, the output signal may be very similar to the input signal, thereby making detection of the nonfunctional state also very easy. A mechanically defective or damaged sensor would also output a particular signal different from the undamped state as shown in Figure 3, which would also be detectable.

The vibration of the beam 9 behaves differently under different damping conditions, thereby enabling, with appropriate electronic circuitry, discrimination between a potentially wide variety of different states, for example whether the sensor was in a liquid, in a powder, in the air, or touching an external object. The sensor could thus act as a normal mechanical switch detecting presence of an external object, or as a sensor for detecting presence of liquid or powder. An example of an application of the latter is to indicate when a dry powder toner cartridge for a printer is empty. Additionally, a defective sensor can also be detected.

Detection of the presence of dry powder in a printer toner is particularly difficult with conventional means.

The use of a piezoelectric sensor as described above is particularly advantageous as the sensor is compact, cost effective and can be placed within the cartridge (housing) of a toner to detect when powder is at a low level. As the sensor gives a digital output, this can be easily integrated into the electronics of a printer, photocopier, etc. Apart from its simplicity, provision of the beam in a cantilever shape is advantageous due to the very distinct and therefore easily detectable difference in vibration behavior when damped or undamped.

Figure 5 illustrates some of the signals of the beam of Figure 2 in different conditions whereby digital responses to the input pulse IP are shown for the free beam FB, damped beam DB and for a malfunctioning (disconnected) transducer M.

It would of course be possible to imagine other sensor shapes other than a cantilever beam, for example a beam supported at both ends, or other shapes such as bending the flexible transducer into a cylinder or other three dimensional shape. The principal gist is to input a pulse signal to a piezoelectric transducer, and by analyzing the vibration characteristics, determine the condition of the sensor. The sensor may be manufactured very simply by laminating a piezoelectric polymer film to a polymer support in a large sheet, where the beam or complex shapes can be easily cut out of the sheet.

Electrodes may be produced on the film by chemical vapor deposition, sputtering, or a variety of other conventional methods. Production of piezoelectric film transducers is a well known procedure.

In order to detect a static condition, input pulses can be generated at periodic intervals, or at periodic intervals for a certain duration only after having been triggered by an event (for example when the sensor acts as a switch and contacts an external object thereby generating a signal due to the bending of the sensor).

Transforming the analogue signal to a simple digital signal as shown in Figures 5, can be dbne simply by transforming voltages that exceed a threshold into a digital signal of volume "1", whilst analogue signals below the threshold values produce digital output of zero.

Claims (13)

1. A piezoelectric sensor wherein a piezoelectric transducer is disposed in the form of a layer on a resilient support and comprises electrodes on its surfaces coupled to electronics, the length of the transducer being variable by application of a potential difference across the electrodes, and wherein the transducer and support comprise a resilient beam section mounted to a relatively rigid structure and adapted to vibrate freely, and the electronics comprise means for creating a voltage impulse across the electrodes to vibrate the beam and subsequently determine the characteristics of the beam vibration.

2. The sensor of claim 1 wherein the beam section is mounted in a cantilever manner.

3. The sensor of claim 1 or 2 wherein the transducer comprises polymer piezoelectric material, and is laminated to one side of the support which is in the shape of a thin layer.

4. The sensor of claim 3 wherein the support layer is made of a polymer.

5 . The sensor of any one of the preceding claims wherein the structure to which the transducer is mounted is a PCB.

6. The sensor of claim 5 wherein the PCB comprises electronics thereon for generating a pulse signal to excite the beam section in vibration.

7. A method of sensing the presence of powder or liquid or contact with an external object, comprising the steps of - providing a sensor having a transducer mounted in a manner to allow vibration of a section thereof - exciting the transducer by a pulse signal to cause it to vibrate - receiving and analyzing the signals generated by the transducer to determine the damping characteristics of the vibration section and thereby determine the presence of powder, or liquid, or contact with an external object.

8. The method of claim 7 wherein the sensor has the structure according to any one of claims 1-6.

9. The method of claim 7 or 8 wherein the pulse signal is generated intermittently.

10. The method of claim 9 wherein the intermittent pulse signal is only generated after sensing of an event such as contact of the sensor with an external object.

11. The method of any one of claims 7-10 wherein the signals received from vibration of the beam are analyzed in a manner so as to discriminate between a nonfunctioning sensor and a sensor that is functioning but damped.

12. A method of sensing the presence of powder, liquid or contact with an external object, substantially as hereinbefore described with reference to the accompanying drawings.

13. A piezoelectric sensor constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.