GB2388994A

GB2388994A - Piezoelectric sounder and drive circuit arranged to reproduce an alarm tone and a voice message

Info

Publication number: GB2388994A
Application number: GB0211986A
Authority: GB
Inventors: Michael Barson
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: GB2388994B; GB0211986D0

Abstract

A sounder for an alarm having a piezo-electric element (206)driven by a drive circuit (200) to produce an audible sound having a maximum audible frequency; the output of the drive circuit is at a high frequency above the maximum audible frequency; the piezo-electric element has a capacitance (214) which is part of a filter network (208) arranged to attenuate the high frequency.

Description

( - 1 Improvements in and relating to sound emitting devices The present

invention relates to sound emitting devices having an amplifier driving a piezo-electric element.

s Piezoelectric elements are used in sound emitting devices for their high sound output and low power consumption. For example, in fire alarm sounders? they may be used to provide an audible alarm signal. However, the use of piezo-electric elements to 10 produce sound for communicating verbal messages is limited by the difficulties in producing an adequate sound output efficiently over the required frequency range.

In particular, it is frequently desirable in an alarm system for a building to be able to produce an alarm signal, followed by a recorded voice message giving instructions for the actions to be taken by persons within the building. Known sound emitting devices 15 using piezo-electric elements to produce the audible alarm signal are unable to produce a voice message at a sufficient volume level, and at a sufficiently low power consumption level to enable an alarm system incorporating such devices to meet the requirements of accepted standards for alarm systems.

20 According; to the present invention, there is provided a sound emitting device comprising a piezo-electric element and an electronic drive circuit having an electrical output to drive the piezo-electric element to produce an audible sound having a maximum audible frequency, the electrical output having a high frequency, the high frequency being above the maximum audible frequency, wherein the piezo-electric 25 element has a capacitance, the capacitance comprising a part of a filter network arranged to attenuate the high frequency.

( - 2 A benefit of' the capacitance of the piezo-electric element forming a part of the filter network is that an audible sound, such as a recorded voice may be efficiently reproduced at a high sound pressure level.

Preferably the filter network further comprises at least an inductance connected in 5 series with the piezo-electric element.

A benefit of having an inductance connected in series with the piezoelectric element is that the filter may be arranged to efficiently attenuate the high frequency signals.

Preferably the inductance comprises two substantially identical inductors, one 10 connected to either side of the piezo-electric element.

A benefit of having two identical inductors is that the piezo-electric element may be positively driven by two amplifier outputs arranged to be 180 degrees out of phase with each other.

15 Preferably the electronic drive circuit further comprises a class D amplifier. A class D amplifier is hereinafter described as: An audio amplifier that comprises of a modulation stage (normally a pulse width modulation stage, however it could use another modulation technique), an ultrasonic 20 switching transistor stage and an output low pass LC filter stage.

In a class- D audio amplifier, a low level analogue audio input signal is converted into a variable PWM signal. The pulse width is proportional to the input signal and is used to drive a switching transistor stage. A pair of switching transistors in this stage turns 25 on and off in response to the PWM signal, supplying power to an output filter from the transistor supply rails.

( - 3 The switching transistors normally have a small delay so that one transistor turns off before the other one turns on, to prevent cross conduction.

A passive low pass LC filter network filters the PWM output waveform to reconstruct 5 the analogue signal.

Note that as the switching times of the transistors are small compared to the switching frequency and the voltage drop across the transistors during the 'on- time' is small compare to the supply rails, then the power losses are much reduced compared to a 10 linear amplifier.

Preferably the class D amplifier provides a pulse width modulated output at the high frequency, the modulation containing the audio signal.

15 A benefit of the modulated audio signal being carried by a pulse width modulated signal is that a highly efficient transfer of power to the piezo-electric element transducer may be achieved.

Preferably the high frequency is above 20kliz.

More preferably the high frequency is above 50kHz. Still more preferably the high frequency is above I ()OkHz. Yet more preferably the high frequency is above 200kHz. A benefit of the higher frequencies is that the filter may be arranged to be more 25 efficient.

( - 4 Preferably the class D amplifier is switched on only for a voice message playback.

A benefit of the class D amplifier being switched on only for a voice message is that the sound emitting device may have a low power consumption when it is not emitting a sound.

s Preferably the electronic drive circuit further comprises an alarm tone driver stage, which is arranged to produce an alarm tone sound.

A benefit of the electronic drive circuit comprising an alarm tone driver stage is that the sound emitting device may be arranged to efficiently provide an alarm tone with a 10 high sound pressure level.

Preferably the electronic drive circuit is arranged so that the alarm tone sound is produced before a voice message playback.

A benefit of the electronic drive circuit being arranged to produce an alarm tone sound 15 before a voice message is that a person in the vicinity of the sound emitting device will have their attention attracted by the alarm tone sound, prior to hearing the voice message. Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in whieh: Figure I is a Circuit diagram of an amplifier output stage driving a sound emitting transducer in a known bridged arrangement, otherwise known as a bridge tied load; 25 Figure 2 is a circuit diagram of an amplifier output stage driving a piezo-electrie element and filter network according to the present invention;

- 5 Figure 3 is a block diagram of an alarm system arranged to use the embodiment of the invention shown in Figure 2; and Figure 4 is a circuit diagram of a voice enhanced fire alarm sounder suitable for the alarm system of Figure 3, and having the embodiment of the invention shown in 5 Figure 2.

From Figure 1, a circuit 100 is shown having an amplifier output stage 102 and 104 in a bridged arrangement driving a sound emitting transducer 106 in a bridged arrangement. A filter network 108 is provided to attenuate a high frequency output 10 from the amplifier. The filter network 108 comprises inductors] 10 and 1 12 and capacitors 114 and 116. Hence, this circuit may be used to provide an audible sound output from the sound emitting device when supplied with a suitable pulse width modulated signal, having a pulse switching frequency higher than the maximum desired audible output frequency.

15 An arrangement, such as circuit 100, is often used to obtain a higher output drive level when driving a very high powered load. An example of such a high powered load would be a conventional moving coil loudspeaker having a high sound level power output. The arrangement of circuit 100 requires two amplifiers driven out of phase with each other and also requires the two sets of l,C output filters, as shown in Figure 20 1. From Figure 2? a circuit 200 according to the present invention is shown. (he circuit 200 comprises an amplifier output stage 202 and 204 in a bridged arrangement driving a piezo-electric element 206 in a bridged arrangement. A filter network 208 is 25 provided to attenuate a high frequency output from the amplifier. The filter network 208 comprises inductors 210 and 212 and a capacitance 214 of the piezo-electric element. Hence, this circuit may be used to provide an audible sound output from the piezo-electric element when supplied with a suitable pulse width modulated signal,

( - 6 having a pulse switching frequency higher than the maximum desired audible output frequency. It is possible to use the bridged arrangement of a class-D amplifier to drive a 5 piezoelectric element as shown in Figure 2, even though the piezo-electric element is a very high impedance load. In this arrangement the benefit comes from producing a higher voltage across the piezoelectric element and hence a higher sound pressure level (SPL). As the piezoelectric element is capacitive, then it is possible to remove both the filter capacitors that were required in the known circuit arrangement shown in 10 Figure I of the known bridged arrangement.

Hence in Figure 2, the piezoelectric element is directly connected only to the two output filter inductors, having values of inductance which are very high compared to what would be normally required in a known circuit as shown in Figure 1. The high values of inductance are required since the capacitance of the piezoelectric element is 15 low. Typically, the capacitance of a suitable piezo-electric element could be as low as 30nF to 40nF. The high inductance value of the inductors and hence its high resistance value does not result in a significant power loss, as the piezoelectric element is a high impedance load and the current is very low. More importantly the following losses associated with output filter capacitors are minimised. These 20 capacitor losses are given by the formula: (/2 Cl x Vsig x Vsig x Fsig) + (/2 C2 x Vsig x Vsig x Fsig) where Vsig and Fsig are the audio voltage and frequency of the signal. Hence, from the formula it can be seen that by utilising only the very low capacitance of the piezo-

electric element, significant improvements in efficiency may be obtained.

Figure 3 shows an embodiment of the invention in an alarm system 300 having an alarm annunciator 302. 1 he system 300 is provided with storage means 304 to store

- 7 alarm signals, such as a fire alarm tone. A voice storage means 306 is provided to store a recorded voice message. The alarm annunciator 300 is provided with an alarm output amplifier 312 and a voice output arnplifer 314 each driving a piezo-electric element 316. A filter network 318 is provided to attenuate a high frequency output 5 from the voice output amplifier. The filter network 318 comprises inductors 320 and 322 and a capacitance 324 of the piezo-electric element. The filter network 318 is arranged to filter a high frequency pulse width modulated signal from the voice output amplifier 314 to provide a sound output comprising the recorded voice message. To produce a suitably high sound pressure level output for an alarm tone, the piezo 10 electric element must be driven so that it resonates at its fundamental resonant frequency. Hence the configuration of the drive circuit is arranged such that the piezoelectric element may also be driven at its resonant frequency, during an alarm tone part of the message. An alarm control 310 is provided to receive and respond to an external stimulus 31 1, and to initiate and control the output of the voice message 15 and l or alarm tone.

Such an alarm tone as described with reference to Figure 3, would include for example a fire alarm tone. Such an alarm tone preferably is perceived as having at least two alternating tones, a higher tone and a lower tone. To arrange the embodiment of Figure 3 so that such a tone may be produced, the storage means 304 20 is arranged to store two different alarm signals, and the control means 310 is arranged to play these two different alarm signal alternately in succession.

Although shown as separate items, the alarm storage 304 and the voice storage 306, may be a single digital storage device, and likewise the alarm amplifier and the voice amplifier may be combined to achieve a reduction in shared components.

25 Hence the embodiment shown in Figure 3 may be arranged so that both tone and voice can then be played back using the class-D amplifier. The class-D amplifier preferably uses pulse width modulation (PWM) at a high switching frequency, which is preferably above the maximum audio frequency to be reproduced. This switching frequency is normally above the maximum audible frequency of 20KHz and is more 30 preferably higher than 200KIIz, for ease of filtering. A constraint that currently limits

( the maximum frequency is the distortion and efficiency of the switching transistors at the high frequency. he PWM switching frequency contains the modulated audio signal. The switching frequency is removed by the LC filter, whose corner frequency is set just above the maximum audio frequency, so that only an amplified audio signal 5 remains on the amplifiers output.

Note that delta-sigma modulation could alternatively be used in place of a pulse width modulated technique.

10 The arrangement shown in Figure 2 where the filter capacitance is wholly provided by the capacitance of the piezo-electric element, has a further advantage, as it becomes possible to monitor the capacitance of the piezoelectric element in order to check if it is faulty or missing. Such a monitoring arrangement may be arranged to measure the capacitance of the piezo-electric element by measuring its response to a signal.

15 Suitable signals could include a single pulse signal, or a continuous waveform. Such a signal could be stored as a digital waveform in the storage device 304 or 306 of the embodiment shown in Figure 3. Dynamic monitoring of a piezo-electric element may be arranged to show changes in the capacitance arising from a physical restraint of the element restricting or altering its dynamic response, or an absence of capacitance 20 arising from a failure of the piezo-electric element or of an inter-connection between the piezo-electric element and the monitor.

In the known circuit of Figure I, the effect of the capacitance of the external filter capacitors would prevent such monitoring of the piezoelectric element by monitoring its capacitance.

25 For the purposes of monitoring the capacitance of a piezo-electric element only, a single inductor could be used in the filter arrangement.

( Figure 4 shows an embodiment of the invention in an alarm annunciator 400, for use with an alarm system such as the alarm system 300 shown in Figure 3.

During the voice message playback period, transistors I and 2 are driven with one phase of a high frequency PWM signal containing the voice signal. Transistors 20 and 5 21 are driven with a PROM signal of the opposite phase.

Filter inductors 10 and 19 form a LC filter with the capacitance of the piezoelectric element 15. The filter removes the PWM switching frequency, so that the piezoelectric element 15, is only driven at the intended voice frequencies. The 10 amplified voice signal now appears across the piezoelectric element 15. It should be noted that no damping of the piezoelectric element is used, so that the maximum SPL can be obtained during voice playback with only a small loss of fidelity.

Diodes 8 and 9 are provided to block any ringing voltage on the audio signal from reverse biasing transistors 7 and 12. Transistors 16 and 18 are also arranged to block 15 any ringing voltage from being clamped to the ground potential.

During the tone period of the message, the class-D amplifier is turned off. Transistors I,2, 20 and 21 switch off as the PWM signal stops, whilst transistors 16 and 18 are turned on, as point E is pulled to a logic high. Transistors 16 and 18 connect the 20 ground potential to one end of the piezoelectric element 15. A complex digital drive waveform generated on point D, is applied to switching transistors 7 and 12. Capacitor 6 and resistor 7, form a differentiation network, so that transistor 7, only turns on during the falling edges of the applied waveform. Similarly capacitor 11 and resistor 13, form a second network, so that transistor 12 only turns on during the rising edges 25 of the applied waveform. The collectors of transistors 7 and 12, are connected to the piezoelectric element 15 at point P+. Current is now pulsed into and out of the piezoelectric element 15 during the rising and failing edges of the drive waveform.

- 10 Af'ter transistor 7, pulses current into the piezoelectric element 15, the voltage on Point P+, will rise to the supply level (Vcc) and as transistor 7 turns off, the piezoelectric element 15 is then free to resonate. Similarly after transistor 12, pulses current out of the piezoelectric element 15, the voltage across it will fall to zero and 5 the piezoelectric element 15 is then again free to resonate. 'I'he frequency of the digital waveform is adjusted so that the piezoelectric element 15, produces a harmonic frequency spectrum, which includes its resonant frequency.

Hence, the embodiment shown in Figure 4 enables the conf'iL;uration of the circuit to 10 be changed so that the piezoelectric element can be driven at its resonant frequency: whilst playing fire alarm tone signals without the capacitance of the output filters reducing its efficiency.

Claims

( CLAIMS

I. A sound emitting device comprising a piezo-electric element and an electronic drive circuit having an electrical output to drive the piezoelectric element to produce an audible sound having a maximum audible frequency, the electrical output having a 5 high frequency, the high frequency being above the maximum audible frequency, wherein the piezoelectric element has a capacitance, the capacitance comprising a part of a filter network arranged to attenuate the high frequency.
2. A sound emitting device as claimed in claim 1, wherein the filter network 10 further comprises an inductance connected in series with the piezo-electric element.
3. A sound emitting device as claimed in claim 2, wherein the inductance further comprises two substantially identical inductors connected to either side of the piezo-

electric element.
4. A sound emitting device as claimed in any of the preceding, claims, wherein the electronic drive circuit further comprises a class D amplifier' as hereinbefore described. 20
5. sound emitting device as claimed in any of the preceding claims, wherein the high frequency is above 20kHz.
6. A sound emitting device as claimed in any of the preceding claims, wherein the high frequency is above 50kHz.
7. A sound emitting device as claimed in claim 4, wherein the class D amplifier is switched on only for a voice message playback.

- 12
8. A sound emitting device as claimed in claim 7, wherein the electronic drive circuit further comprises an alarm tone driver stage, which is arranged to produce an alarm tone sound.

5
9. A sound emitting device as claimed in claim 8, wherein the electronic drive circuit is arranged so that the alarm tone sound is produced before a voice message playback.
1 O. A sound emitting; device, substantially as hereinbefore described and with 10 reference to the accompanying Figures 2, 3 and 4.