GB2388916A

GB2388916A - System for monitoring piezo-electric elements

Info

Publication number: GB2388916A
Application number: GB0211984A
Authority: GB
Inventors: Michael Geoffrey Broughton
Original assignee: Gent Ltd
Current assignee: Novar Systems Ltd
Priority date: 2002-05-23
Filing date: 2002-05-23
Publication date: 2003-11-26
Anticipated expiration: 2022-05-23
Also published as: GB2388916B; GB0211984D0

Abstract

A monitor (200) for a piezo-electric element (202) in which an electronic control arrangement produces a test signal for the element. The test signal has a fundamental frequency outside the audible frequency range. The control is arranged to monitor at least the response of the piezo-electric element to the test signal. The response may be arranged to be dependent on the capacitance of the piezo-electric transducer.

Description

C) 1 - 238891 6

Improvements in and relating to piezo-electric elements Phe present invention relates to piezo-electric elements and means for testing the performance of the piezo-electric element.

The use of a piezo-electric element for producing a sound in alarm systems is well known. A problem with such systems, is that to ensure they are ready for use it is important to test the system on a regular basis. Known methods of testing which involve the operation of the system to produce an audible sound are unacceptable in 10 many installations, such as for example a hotel which may be continuously occupied by guests. It is known to test the wiring interconnecting a number of alarm sounder devices by applying a test signal that is conducted by an end of line resistor across the wires at the end of the wiring run. This is known for safety critical audio systems, known as Voice Alarm Systems, which often use inaudible signals (typically using 15 high frequency such as 20KI-Iz) to monitor cabling that connects the control equipment to the loudspeakers. I his technique is only used to monitor the external cable and not the presence or operation of the loudspeakers. Such a test verifies that a transmission path along the cable run is intact. Such a test does not verify that each individual alarm sounder will operate to produce a sound in the event of an alarm 20 condition. Hence a serious problem exists with known alarm systems in that while it is recommended that such systems are tested regularly, and preferably once a week this is rarely done, and even when it is done known testing methods either do not test for 25 sounder faults, or if a sounder test is conducted, cause inconvenience to any persons in the building. The monitoring of a system for sounder faults is critical if failure of a sounder that may endanger life is not detected.

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According to an aspect of the invention there is provided a monitor for a piezo-electric element, comprising: an electronic control, the control having means for producing a test signal for the piezo-electric element, the test signal having a fundamental frequency outside an audible frequency range, the control being arranged to monitor at S least the response of the piezo-electric element to the test signal.

benefit of the control being arranged to monitor a piezo-electric element using a test signal having a fundamental frequency outside of an audible frequency is that such test may be conducted without operating the piezoelectric element so as to produce a 10 noticeable audible sound.

Another benefit of this invention is that it improves the reliability of an alarm system by enabling a continuously repetitive monitoring for sounder faults.

15 A further benefit of the present invention is that when sounders having a monitor according to the invention are used in an alarm system for a building, the alarm system may be arranged to monitor the operation of the whole system, that is including a Signal Storage Device, an amplifier, a transducer, a power supply, a feedback circuitry and connections between the various elements.

Preferably the response is arranged to be dependent on a capacitance of the piezo clectric element.

A benefit of the response being dependent on a capacitance of the piezoelectric 25 element is that a piezo-electric element has an intrinsic capacitance.

- 3 Prefcrably the control is arranged to indicate a fault when a value of capacitance of the piezo-electric element is below an acceptable value.

More preferably the control is arranged to indicate a change in the value of capacitance of the piczo-electric element.

5 A benefit of the control being able to measure the value of the capacitance of the piezo-electric element is that such a measurement may be used to indicate a range of faults. For example, a sudden change to a nil value would indicate that the piezo electric element had become disconnected.

10 Preferably the fundamental frequency is less than 20 hertz.

A benefit of the fundamental frequency being less than 20TIz is that this is outside of the audible frequency range for a normal person.

Preferably the fundamental frequency is greater than 1 9kliz.

15 A benefit of the fundamental frequency being greater than 1 9ki1z is that this is outside of the audible frequency range t'or a normal person.

Preferably the test signal comprises a substantially sinusoidal wavet'orm at the fundamental frequency.

O A benefit of the sinusoidal waveform is that an absence of harmonics reduces a risk of producing a sound in an audible frequency range.

l'referably the test signal comprises a short pulse of a sinusoidal waveform.

More preferably the test signal comprises a pulse of a single halt'waveform of a 25 substantially sinusoidal waveform at the fundamental frequency.

- 4 A benefit of the test signal comprising a short pulse of a sinusoidal waveform is that an amount of power consumed in performing the test may be reduced.

Preferably a maximum duration of the pulse is less than a period equal to a period of a 5 full wave cycle at the fundamental frequency.

AL benefit of the pulse having a duration of less than that of a full wave cycle at the fundamental frequency, is that an amount of power consumed in performing the test may be minimised.

10 Preferably the control further comprises a digital storage means, and the test signal is stored as a digital signal in a digital storage means.

A benefit of the test signal being stored as a digital signal is that a precisely repeatable signal may be reproduced for every test.

A further benefit of the test signal being stored as a digital signal is that close control 15 of the shape of the test pulse may be obtained.

Preferably a rate of change of voltage with respect to time of the test pulse is arranged to not exceed a value that would cause harmonics having an audible frequency to be generated. 2() A benefit of avoiding the generation of harmonics is that where a low frequency test pulse is used, a risk of generation of audible noise is reduced.

Preterably the electronic control further comprises a D.C. storage means to store electrical energy, and an amplifier arranged to supply the test signal to the piezo 25 electric element, the electrical energy being used to power the amplifier while the test signal is applied to the piezoelectric element.

c A benefit of the amplifier arranged to supply the test signal to the piezo-electric element is that the test signal may be amplified to produce a suitably high voltage across the piezo-electric element, and a benefit of the use of a l).C. storage means to store electrical energy to power the amplifier while the test signal is applied to the 5 piezo- electric element is that a switched motile power supply arranged to charge the D.C. storage means and / or otherwise to power the amplifier, may be switched off during the test, a source of noise that could produce audible harmonics.

According to another aspect of the invention, there is provided a method of 10 monitoring a piezo-electric element comprising: producing a test signal; the test signal having a fundamental frequency outside an audible frequency range; applying the test signal to the piezo-electric element; 15 detecting a feedback signal from the piezo-electric element; comparing the feedback signal with a set of values to determine the acceptability of the feedback signal.

20 Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure I is an illustration of a pulse width modulation technique suitable for use with a preferred embodiment of the present invention; Figure 2 is a block diagram showing a monitoring system for a piezo-electric element 25 according to a preferred embodiment of the present invention; Figure 3 is a schematic circuit diagram of the preferred embodiment of the present invention shown in F igure 2;

- 6 ligure 4 is a series of concurrent waveforms all being on the same time base, at a number of different points in the circuit shown in Figure 3 at the time a test is conducted, where: T7igure 4.1 is an analogue test waveform showing voltage with respect to time; 5 Figure 4.2 is a waveform showing the voltage applied across the piezo-elcctric element with respect to time; Figure 4.3 is a waveform of voltage at a point marked X on the circuit of Figure 3; Figure 4.4 is a waveform of a voltage feedback signal received when a fully 10 operational piczo-electric element is present in the circuit; and Figure 4.5 is a waveform of a voltage feedback signal received when the piezo-

electric element is not present in the circuit; Figure 5 is a flowchart showing a test method for testing the piezo-electric element using the preferred embodiment; and 15 Figure 6 is an alternative embodiment of the present invention.

From Figure 2 a monitoring system 200 is shown. having a piezoelectric transducer 202, which consists of a piezoelectric ceramic plate with a silver electrode on both 2() sides that is attached to a metal plate with adhesives. Sound is produced when an alternating current voltage in the audio range is applied across the two electrodes. This construction (i.e. two electrodes separated by an insulator) produces a substantial intrinsic capacitance, typically in the range of 30nr to 50nt for a typical piezoeiectric ceramic plate that is 25mm in diameter that is suitable for use in a sounder for an 25 alarm system.

In the preferred embodiment of the present invention, the capacitance of the piezoelectric transducer 202 allows it to be monitored by applying a short, pulse width

(A - 7

rnodulated (PWM), low frequency signal across it. A signal storage means 204 is provided to store the signal which is amplified by a signal amplification means 206.

Feedback means 208 receives a response from the piezo-electric element of the piez.o electric transducer 202 and relays the response to a control and monitoring means 210 5 which is arranged to monitor the response and provide an output indication of the condition of the piezo-electric element. The control and monitoring means 010 is preferably also arranged to initiate the issue of the test signal from the storage means 204 at a desired time. The control and monitoring means 210 preferably comprises a micro-controller, which is also arranged to output a signal 220 indicating the result of 10 the test when it is completed.

l o avoid false indications of a fault, the control and monitoring means 210 is preferably arranged to repeat the test at least once, and preferably twice to confirm the fault result before sending and indication of the test result to a remote control panel (not shown) that would be provided for example in a fire alarm system to control a 15 whole system or sub-system which would include many sounders.

Pulse width modulation is a common technique for transmitting analogue waveforms with low power digital circuitry. The principle is illustrated in Figure 1.

The PWM technique consists of a square wave with a variable on-to-off ratio, this ratio can be from O to 100 percent. Note that the modulation frequency must be 20 significantly higher than the analogue waveform' otherwise information will be lost and the analogue waveform will be distorted. preferred modulation frequency used for this implementation is above SOKI-lz.

If the piezoclectric transducer is present its internal capacitance will filter out the 25 modulation frequency and produce a substantial feedback analogue signal to a micro controller. If the piezoelectric component is not present the modulation frequency will not be filtered to the same degree and the feedback signal will be substantially reduced.

- - "Figure 3 shows a schematic circuit diagram 300 of the preferred embodiment of the present invention shown in leisure 2, where the analoguc monitoring signal (described with ret'erence to Figure 4) is stored in Digital Storage Device (2, 204), which is 5 controlled by the Micro-controller (1). The Micro-controller (1, 210) is programmed such that when a test is required it initially turns on the amplifier switch mode power supply (214) to charge up a storage capacitor (212). The storage capacitor (212) is a D.C. storage means used to store electrical energy. When the storage capacitor (212) is fully charged, the switch mode power supply (214) will be turned off. During the 10 test, the switch mode power supply (214) and any other circuit elements that may generate noise in the audio region (for example a PWM output that is used to calibrate the speed of the Digital Storage Device (2, 204) which is needed for voice message synchronization in the alarm mode) are turned off. Hence the production of audible noise that could arise during a test is avoided. The switch mode power supply for the 15 Micro-controller ( I 210) and Feedback Circuit (208) does not need to be turned off during the test because it has no affect on the monitoring signal.

The amplifier (206), which is required to produce a suitably high voltage across the piez.o-clectric element, is then powered from the storage capacitor (212) for the 20 remainder of' the test, which is lOOms.

From Figure 3 when a test is to be conducted, the Micro-controller (1) activates the Monitor Enable contro! lines to the Digita! Storage Device (2). The Digital Storage 25 Device (2) outputs phase and anti-phase, pulse width modulated signals, PWM I and PWM 2, to the amplifier biasing circuits (3,4,5,6).

I'he '1I' bridge circuit formed by switching transistors (7,8,9,10) then amplifies the applied signals. The inductors (1 1,12) filter out part of the high frequency components

present in the PWM signals, so that only the desired monitoring signal is seen across the piez.oelectric transducer ( 13). The effect of the capacitance ot' the piezo-electric element 13 is to produce a feedback signal (see Figure 4.4) which is fed into the feedback circuit (208).

5 In the f'ceclback circuit (20X), voltage divider resistors (14.15) reduce the voltage level applied to the voltage f'olkwer/limiting transistor (1G). Resistor (17) and capacitor (18) filter the feedback signal' whilst resistor (19) provides a slow discharge for capacitor (18).

I'he Micro-controller (1) samples the Feedback Signal periodically throughout the test 10 and sends a fault event to the control panel if the difference between the maximum and minimum reading is less than a predefined value.

The preferred embodiment present invention may be included in an intelligent low power voice sounder used for alarm purposes. Such an intelligent alarm sounder is 15 arranged to monitor the sound transducer or control circuitry by: a) Being provided with means to provide power to the alarm circuits of the alarm sounder while an alarm system to which the sounder is connected is in a quiescent state (that is, the sounder's control and monitoring circuitry is powered up but the sounder transducer is not operating) 20 b) 'I'he monitoring being arranged to be inaudible, so that people in the vicinity ol'the sounder are unaware that it is being tested.

A feature of intelligent alarm systems is that they are provided with a control panel that is capable of communicating, with alarm sounders in their quiescent state and is capable ot'instructing the sounder to operate.

25 On such an intelligent system it is possible to monitor the operation of the sound transducer where the sounder is in the quiescent state by selecting a monitoring signal that it is above or below the normal audible range of humans.

(,,, - 10

The preferred embodiment of the present invention makes use of an inaudible signal that has been specifically designed for monitoring a piezoelectric transducer.

henet'it of the present invention is that no components in the product incorporating the preferred embodiment have been added for the transducer monitoring. All of the 5 circuitry used in this invention is required to perform the main function of the product (voice alarm sounding). For example the Feedback Circuit is primarily used to determine the resonant f'recluency of the piez,oeleetric transducer to achieve optimal sound output.

I'he only costs associated with monitoring the transducer are a small memory area 10 (0.1 s) of the Digital Storage Device (2) for the analogue monitoring signal and a small amount of memory space in the Microc'ntroller ( I) for the monitoring software.

Figure 4. shows the analogue Monitoring Signal stored in the Digital Storage Device 15 2, 204 that is output as pulse width modulated signals PWMI and E'WM2. The graph shows the voltage of' the signal waveform on the vertical axis with respect to time on the horizontal axis. The Monitoring Signal is based on a single half sine wave that has been smoothed to remove high frequency components at each end? which would otherwise risk producing audible frequency harmonics. 'I'he frequency used in the 2() preferred embodiment is I OHz, but other suitable i'requencies with values lower than 2()Hz or higher than 2()KHz, may be used with corresponding modifications to component parts of the system such as the t'eedbaek circuit.

25 Figure 4.2 shows a wavcf'orm ot'ihe voltage that is applied across the piezoelectric transducer as a result of the production of the signal waveform of Figure 4.1. The main differences between this and the Monitoring Signal are the waveform has been inverted and in the graph of Figure 4.2 a slight voltage offset is introduced which is

Am - 11 seen as small voltage steps at the start and end of the waveform. This voltage offset is caused by a small error in the T'WM output of the Digital Storac Device of an actual example of this particular cmhodimcnt 5 Fixture 4.3 shows the wael'onn of a voltage seen at point X on the circuit diagram as a result of the production of the signal waveform of Figure 4.1. This wavei'orm is the output from one half of the EI bridge amplifier referenced to ground potential.

From figure 4.3, it may be seen that the signal is offset to the mid power supply point (causing the rise prom Ov to 22v at the beginning and the drop Ov at the end), and that 10 when the H bridge ampli tier is switched off at the end of' the waveform the output rises to the power supply potential (38v) for a short period. This is caused by an "anti click" feature in the PWM output circuit, that at the end of'the waveform momentarily puts both PWM outputs to the same state (high). 'I'he "anticlick" circuit causes the two transistors connected to the supply rail to turn on at the same time, which drives 15 troth ends of the transducer to the supply potential.

Figure 4.4 shows a voltage waveform for a normal Feedback Signal. 'the wavef'om shown in Figure 4.3 has been attenuated by the voltage divider resistors and then clamped by the voltage-limiting transistor. This clamping removes most ot' the :() positive signal in I inure 4.2 and causes the slight voltage steps at the beginning and near the end ofthe \vaveform. The filter components (17,18,19) cause the trailing edge of the wavcLorm to be extendecl.

leisure 4.5 shows a volta:c wavctorm for a Feedback Signal when there is an open 25 circuit fault. This waveform shows a reduced response to the monitoring signal caused by the absence of the piczoelectric transducer capacitance. 'l'hc stray capacitance within the circuit causes the small response to the half wave curve in the middle but removes the small voltage steps seen in Figure 4.4 completely.

- 12 Should a fault be present in the amplifier circuit 206 that prevents the production of the correct voltage amplitude for the test waveform shown in Figure 4.2, a voltage waveform of an intermediate shape between that shown in Figure 4.4 and that shown 5 in Figure 4.5 will be produced. Such an intermediate waveform may be used as an indication that the piezoeleetric transducer may not perform as intended, although there is an electrical connection to it.

10 In an alarm system having sounders having the preferred embodiment of the present invention, there are two factors that determine how frequently a test is performed.

I'hese factors are the overall power consumption of' the product and the need to report i'aults within a reasonable period of time. The power consumption is important because most alarm sounders operate on systems that have standby batteries to 15 provide energy for long periods if the normal AC mains supply fails. The test requires the full alarm power, which is approximately 80mW. A normal quiescent current for a typical sounder product is only 1/8 of this.

On this implementation the Lest programme has a total duration of 600ms (see Fig. 5) 20 and is preferably performed every hour.

If'a fault level is detected the micro-controller is preferably arranged to perform two more tests after 5 and I O.scconds before it transmits the fault message to the control panel. 25 The only condition that needs to be fulfilled before the test can be initiated is that the product is not already in the alarm mode. If an alarm occurs whilst the product is performing the test, the test will be completed before the product goes into the alarm mode.

my) - 13 Henee, from Figure 5 showing a flowchart for a test procedure for conducting a test with the preferred embodiment it maybe seen that a test sequence is initiated by the microcontroller 210. '1-his initiation may be as a result of the microcontroller being 5 programmed to initiate a test after an elapsed period of time, or may be initiated as a result of an external input to the microcontroller. The microcontroller checks to see if the sounder is in state where it is sounding an alarm. If it is in an alarm state, then the microcontroller is programmed to record that a test has been successfully completed 220, and finish the test cycle. If the alarm is not in an alarm state, the 10 microcontroller operates the sounder switch mode power supply 214 for a timed period so as to charge the storage capacitor 212. In a practical embodiment, SOOms has been found to be an adequate time period for charging the capacitor. After the timed period has elapsed the power supply 214 is turned off, and the monitoring signal is selected from the signal storage means 204, and the monitoring signal amplified by 15 the signal amplification means 206. A confirmation is received from the signal storage means 204 (that is part of a speech integrated circuit in a particular embodiment). If the confirmation is not received that the test signal has been successfully produced, it will be selected again. The microcontroller 210 is then programmed to record the feedback signal from the t'eedback circuit 208. When the 20 speech integrated circuit confirms that the test signal production has been completed, a maximum and a minimum values of the voltage of the feedback signal are compared. If the difference between the maximum and the minimum values is greater than an acceptable value (in a particular embodiment the acceptable value is I.S volts) then the microcontroller records a pass (no fault) result, otherwise if'the dii'ference 25 between the maximum and minimum values is less than the acceptable value, the microcontroller records a fault result. To minimise the risk ol'producing a spurious fail indication, the microcontroller is preferably programmed to only issue a hail output signal 22() to a system control panel if three consecutive tests each produce a fault result.

- 14 An alternative embodiment of the invention may be arranged so that a continuous output was produced from the L)igital Storage L)evice whenever the piezo-electric element was in its quiescent state. An advantage of this approach would be that a fault within the sounder would be detected immediately.

5 A disadvantage where a low power consumption was desirable would be that the quiescent power used by device would be substantially increased.

A preferred method for this alternative embodiment would be to store a longer inaudible waveform in the Digital storage device, such as lOHz sine wave with duration of 1 s. It would then be possible to continuously enable the Digital Storage 10 Device (with the Monitor Enable lines shown in Figure 3) so that it would output the monitoring signal as a continuous signal.

A further benefit of a continuous monitoring signal would he that there would be no harmonics generated by the test signal.

15 A second alternative embodiment of the invention may be arranged so that a circuit arrangement is provided that avoids the need for Digital Storage Device for monitoring, as shown in Figure 6.

In this system a Waveform (Jenerator and a Pulse Width Moclulator replace the Digital Storage Device.

90 The Waveform (generator produces the inaudible monitoring signal; an example of this would be a simple sinusoidal oscillator circuit.

The Pulse Width Modulator converts analogue monitoring signal into the PWM signals required by the Signal Amplification circuit.

25 'I'he second embodiment may be arranged to produce a continuous test signal. or a shorter intermittent test signal. Where an intermittent test signal is desired, a risk of' audible noise arising from harmonics produced at a start and an end of the intermittent signal may be reduced by using a waveform that increases gradually to the required

c\ - 15 test voltage amplitude over a number of cycles, and then reduces gradually over a similar number of cycles. The control and monitoring circuit being arranged to only monitor the feedback signal while the test voltage amplitude is at the required test voltage amplitude.

5 While the monitoring system of the present invention is described primarily with respect to testing alarm systems, and in particular to fire alarm systems, the same monitoring system may be used to test for the functionality of a piezo-electric element in a transducer where a drive circuit is used to drive the piezo-electric element, the drive circuit having a suitable response to the test signal of the present invention, such 10 that the response may be monitored to indicate the condition of the piezo-electric element.

Claims

c' - 16 CLAIMS

I. A monitor for a piezo-electric element, comprising an electronic control, the control having means t'or producing a test signal for the piezv-electric element, the test 5 signal having a fundamental frequency outside an audible frequency range, the control being arranged to monitor at least the response of the piezo-electric element to the test signal.
2. A monitor for a piezo-electric element as claimed in claim I, wherein the 1() response is arrangecl to be dependent on a capacitance of'thc piezo-elcctric element.
3. A monitor tor a piezo-electric element as claimed in claim 2, wherein the control is arranged to indicate a fault when a value of capacitance of the piezo-electric element is below an acceptable value.
4. A monitor for a piezo-elcctric element as claimed in any Blithe preceding claims, wherein the fundamental frequency is less than 20 hertz.
5. A monitor for a piezo-electric element as claimed in any of claims I to 3, 20 wherein the fundamental frequency is greater than 19kllz.
6. A monitor t'or a piezo-elcctric element as claimed in any of tile preceding, claims, wherein the test signal comprises a substantially sinusoidal waveform at the fundamental frequency.

hi\
7. monitor for a piezo-electric element as claimed in claim 6, wherein the test signal comprises a short pulse of a sinusoidal waveform.
8. monitor for a piezo-electric element as claimed in claim 7, wherein the test 5 signal comprises a pulse of a single half waveform of a substantially sinusoidal waveform at the fundamental f'recqucncy.
9. A monitor for a piezo-electric element as claimed in claim 8, wherein a maximum duration of the pulse is less than a period equal to a period of a full wave 10 cycle at the fundamental i'requcncy.
10. A monitor for a piezo-electric element as claimed in any of the preceding claims, wherein the control further comprises a digital storage means, and the test signal is stored as a digital signal in a digital storage means.
1 1. A monitor t'or a piezo-electric element as claimed in any of the preceding claims, wherein a rate of change of voltage with respect to time of the test pulse is arranged to not exceed a value that would cause harmonics having an audible frequency to be generated.
12. A monitor for a piezo-electric element as claimed in any of the preceding claims, wherein the electronic control further comprises a O.C'. storage means to store electrical energy. and an amplif'icr arranged to supply the test signal to the piezo electric element' the electrical energy being used to power the amplifier while the test 25 signal is applied to the picz.o-electric element.

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13. A method of monitoring a piezo-electric element comprising: producing a test signal; the test signal having a fundamental frequency outside an audible frequency range; 5 applying the test signal to the piezo-electric element; detecting a feedback signal from the piezoelectric element; comparing the feedback signal with a set of values to determine the acceptability of the feedback signal.

1 O
14. A monitor for a piez.o-electric element, substantially as hereinbefore described and with reference to the accompanying drawings.
] 5. A system including the monitor and piezo-electric clement of any one of claims 1 to 14.
16. An alarm system including the monitor and piezo-electric element of any one of claims 1 to 1 5.
17. A method ol monitoring a piezo-electric element substantially as lereinbefore 20 described and with reference to the accompanying drawings.