EP1507603A1

EP1507603A1 - Acoustic alarm having a piezo-electric element driven at multiple frequencies

Info

Publication number: EP1507603A1
Application number: EP03730331A
Authority: EP
Inventors: Michael Barson
Original assignee: Gent Ltd
Current assignee: Novar Systems Ltd
Priority date: 2002-05-23
Filing date: 2003-05-23
Publication date: 2005-02-23
Anticipated expiration: 2023-05-23
Also published as: AU2003241013A1; GB2388995A; ES2527050T3; GB2388995B; GB0211987D0; EP1507603B1; WO2003099468A1

Abstract

A sounder for an alarm having a piezo-electric element (1) and an electronic drive circuit (100) to drive the piezo-electric element to produce an audible sound perceived as a first tone at a first tonal frequency, the piezo-electric element having a dominant resonant frequency; the dominant resonant frequency is above the first tonal frequency and the electronic drive circuit is arranged to drive the piezo-electric element to produce the first tone sound; the piezo-electric element is also excited at the dominant resonant frequency while the first tone sound is produced.

Description

ACOUSTIC ALARM HAVING A PIEZO-ELECTRIC ELEMENT DRIVEN AT MULTIPLE FREQUENCIES

The present invention relates to alarms having a piezo-electric element for emitting a sound.

Magnetic buzzers or loudspeakers are used for alarm sounders, as these meet the requirement for producing a high sound pressure level (SPL) with frequency components in the range 500Hz to 1000Hz, to comply with the British Standard BS 5839 : Part 1. A problem with these known sounder designs is the high power consumption, typically > 0.5 W is required to produce > lOOdBA. Further, these sounders may also have a high minimum working voltage.

The high current required to power sounders is a major limitation in fire alarm systems. In general, voltage drops on cables limits the total sounder load and maximum cable runs possible. This limitation becomes very acute in addressable systems where a large number of both sounders and smoke detectors are required to work on the same cable pair.

Piezoelectric elements, may be used in sounder designs to reduce the power requirements.

Most known sounders generate frequencies well above lKHz and will not be part of the following discussion. A known technique of driving a piezoelectric element with a square wave at a third of the resonant frequency is shown in Figure 9 and 10. With this known technique, not only is the resonant frequency not excited to the same extent (over 6dB down compared to the present invention with the same piezoelectric element), performance is further degraded as a required warble tone is produced by driving the piezoelectric element both sides of the resonant frequency.

Note that a higher sound pressure level (SPL) is possible using a nodal mounted piezoelectric element, this also would give the advantage of a feedback output from the element (piezoelectric transformer effect). A problem with this known arrangement is that the nodal mount element will only oscillate at a single high frequency above lKHz and is therefore will not meet the requirements for alarm systems in buildings.

To produce a distinct tonal difference, means the more away from resonance both tones must be perceived to be, hence a lower efficiency and output SPL will be obtained. To indicate this, if such a sounder had a warble tone with a typical 200Hz shift in its drive frequency, then the higher frequency tones would change by 600Hz, or be off resonance by +300Hz and -300Hz, which is clearly a problem as only a small change in frequency will degrade the performance of the piezo-electric element and very significantly reduce the sound pressure level produced.

To illustrate this, Figure 9 shows the ringing waveform on the piezoelectric element, whilst Figure 10 shows the resulting frequency spectrum produced using a simple square wave drive at a third of the resonant frequency. It should be noted that the ringing waveform in Figure 9 is shown in an ideal resonant condition when the piezoelectric element is driven by a 923Hz signal, which cannot occur in practice if different tones are to be produced. To produce two distinctly separate tones, say

200Hz apart, the low tone would be produced by driving the piezo-electric element at 823Hz, and the high tone by driving at 1023Hz. For each of these tones, the detrimental effect on the amplitude of vibration of the piezo-electric element, as compared with the amplitude that would be obtained at the resonant frequency would be very significant, and an inadequate sound pressure level would be produced.

According to the present invention, there is provided an alarm sounder 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 perceived as a first tone, at a first tonal frequency, the piezo-electric element having a dominant resonant frequency, the dominant resonant frequency being above the first tonal frequency, the electronic drive circuit being arranged to drive the piezo-element to produce the first tone sound, the piezo-electric element being also excited at the dominant resonant frequency while the first tone sound is produced.

A benefit of the piezo-electric element being also excited at the dominant resonant frequency is that a high sound pressure level may be obtained.

Preferably the electrical output is further arranged to produce a second tone, at a second tonal frequency, the first tone frequency being higher than the second tonal frequency, the piezo-electric element being also excited at the dominant resonant frequency while the second tone is produced.

A benefit of the piezo-electric element being arranged to also produce a sound at a second tonal frequency while the piezo-electric element is also being excited at its dominant resonant frequency is that the alarm sounder may be arranged to produce two alternating sounds of different perceived tonal frequencies, having similarly high sound pressure levels.

Preferably the electronic drive circuit further comprises a digital signal generator and the electrical output is a digital signal. A benefit of the digital signal generator is that precise control may be obtained over the electrical output to the piezo-electric element.

Preferably the digital signal is controlled by a variable square wave control frequency, which is a multiple of the dominant resonant frequency.

A benefit of a variable square wave control frequency is that the control electronics may be simplified.

Preferably the electrical output comprises a digital waveform arranged to pulse drive the piezo-electric element in to a constantly reinforced multi-resonant condition.

A benefit of the digital waveform arranged to pulse drive the piezo-electric element in to a constantly reinforced multi-resonant condition or state is that the sounder is able to produce and maintain the sound at the first or second tonal frequency.

Preferably the electrical output comprises a waveform having a plurality of superimposed frequencies, at least one of the frequencies having a frequency arranged to stimulate resonance of the piezo-electric element at the dominant resonant frequency.

A benefit of the waveform having a plurality of superimposed frequencies, is that different sounds may be produced, while the at least one frequency ensures that the dominant resonant frequency is stimulated, thus producing a high sound pressure level.

Preferably the electronic drive circuit is arranged so that the electrical output comprises a waveform having a plurality of superimposed frequencies, at least one of the frequencies having a frequency arranged to stimulate resonance of the piezo- electric element at the dominant resonant frequency, and at least another of the frequencies varying with time so as to produce a sound with a rising or falling tone.

A benefit of the at least another frequency varying with time while the at least one frequency stimulates resonance of the piezo-electric element at the dominant resonant frequency, is that a rising or a falling tone may be produced while substantially maintaining a high sound pressure level.

A further benefit is that, by using complex drive waveforms, a very low profile fire alarm sounder design may be produced. Such a sounder can produce an output SPL of >100dBA, with a rich frequency spectrum, whilst using < 0.1W of input power.

Preferably the electronic drive circuit is arranged to monitor a dominant resonant response of the piezo-electric element to the electrical output and is further arranged to adjust the square wave control frequency to obtain a maximum dominant resonant response of the piezo-electric resonant element.

A benefit of the electronic drive circuit being arranged to monitor a dominant resonant response of the piezo-electric element to the electrical output and being further arranged to adjust the square wave control frequency to obtain a maximum dominant resonant response of the piezo-electric resonant element is that any drift of the actual frequency of the dominant resonant frequency may be detected. Hence, the electronic drive circuit is thus arranged to compensate for any change in the actual frequency at which the dominant resonance occurs.

Preferably a sound pressure level produced at the first tone and at the second tone are within 15dB of each other.

More preferably a sound pressure level produced at the first tone and at the second tone are within 3dB of each other. Preferably a sound pressure level produced at either tone, is within 21dB of a sound pressure level produced by the piezo-electric element when it is driven by the electrical output at a third harmonic of the dominant resonant frequency.

More preferably a sound pressure level produced at either tone, is within 15dB of a sound pressure level produced by the piezo-electric element when it is driven by the electrical output at a third harmonic of the dominant resonant frequency.

A benefit of the tones having similar high sound pressure levels is that the tones will be audible above an ambient noise level.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a schematic circuit diagram for an alarm sounder according to the present invention.

Figure 2 shows a drive and ringing voltage waveform with respect to time resulting from a 11000101 (00010111) data sequence generated during the operation of the circuit of Figure 1;

Figure 3 shows a drive and ringing voltage waveform with respect to time resulting from a 000101 data sequence generated during the operation of the circuit of Figure 1;

Figure 4 shows a drive waveform and a resulting piezoelectric ringing voltage waveforms with respect to time generated during the operation of the circuit of Figure

1; Figure 5 shows a DC feedback level, which is monitored by the microcontroller during initial calibration;

Figure 6 shows an audible harmonic frequency spectrum for data sequence 11000101

Figure 7 shows an audible harmonic frequency spectrum for data sequence 000101

Figure 8 shows an audible frequency spectrum generated from the calibration waveform of Figure 4.

Figure 9 shows a known piezoelectric ringing waveform obtained if driven by a square wave at a third of the resonant frequency; and

Figure 10 shows a known frequency spectrum resulting from the square wave shown in Figure 9.

From Figure 1 an electronic drive circuit 100 for an alarm sounder is shown, the circuit being arranged to drive the piezo-electric element 10 to produce an audible sound. The piezo-electric element has a dominant resonant frequency that is stimulated when the audible sound is produced. When resonating at the dominant resonant frequency, the power consumed by the piezo-electric element for a given sound pressure level is at a minimum. Hence, when the electronic drive circuit is driving the piezo-electric element 10 to produce a particular sound, the overall sound pressure level may be significantly enhanced for a given power consumption, if the piezo-electric element is also excited at the dominant resonant frequency. A suitable waveform for producing a first audible sound at a high perceived first tone, while also exciting the piezo-electric element at its dominant resonant frequency is described with reference to Figure 3 below, and a suitable waveform for producing a second audible sound at a low perceived second tone, while also exciting the piezo- electric element at its dominant resonant frequency is described with reference to

Figure 2 below.

The perceived tone from Figure 3 would include a frequency of 923Hz, while that of Figure 2 would include a frequency of 693 Hz. Hence, a difference between the two perceived tones would be 230Hz, and the tones would sound distinctly different.

In a second embodiment of the invention, a measured sound pressure level output from the piezo-electric element when driven at resonance by the calibration waveform of Figure 4 was in excess of lOOdB. A measured output from the piezo-electric element when driven by the waveform of Figure 3 was less than 3dB lower. When the element was driven by the waveform of Figure 2, it was less than ldB lower than the output level produced when driven by the Figure 3 waveform.

In a third embodiment of the invention, the first and second tones produced a sound pressure level in excess of lOOdB, for an electrical power input of less than lOOmW.

For the third embodiment, a desired electrical power input is less than 75mW.

The microcontroller 1 and the shift register 2 and the multiplexor 3 comprise a digital signal generator 120. The output signal from the digital signal generator 120 is amplified by the output amplifier 130, which comprises switching transistors 8 and 9. A feedback circuit 140 is provided so that the electronic circuit may be arranged to monitor the dominant frequency response of the piezo-electric element as an output frequency is varied over a calibration frequency range. A peak hold detection circuit 150 is provided to enable a peak resonant response of the piezo-electric element to be detected.

Although the electronic drive circuit in the embodiment shown and described with reference to Figure 1 uses a digital signal generator to produce a digital waveform arranged to stimulate the piezo-electric element to produce different sounds while also resonating at the dominant resonant frequency, an alternative embodiment not shown in the figures is arranged to provide a suitable waveform by superimposing a plurality of waveforms from an analogue signal generator, so that an output signal is produced that produces an audible sound perceived as a first tone, at a first tonal frequency, the piezo-electric element having a dominant resonant frequency, the dominant resonant frequency being above the first tonal frequency, the electronic drive circuit being arranged to drive the piezo-element to produce the first tone sound, the piezo-electric element being also excited at the dominant resonant frequency while the first tone sound is produced.

In the alternative embodiment a suitable feedback circuit may also be provided so that the electrical output frequency from the analogue drive circuit may be adjusted to ensure it will stimulate the piezo-electric element to resonate at the dominant resonant frequency.

An advantage of the embodiment using a digital signal generator, is that the power supply to the piezo-electric element may be easily produced as a pulsed electrical output, while with an analogue signal generator the waveform would more easily be produced as a continuous electrical output signal. With a pulsed output, a further improvement in efficiency may be obtained, since the piezo-electric element may be allowed to ring after a pulse, rather than being driven again immediately, and hence electrical power consumed by the piezo-electric element and losses in the drive circuit is reduced.

An advantage of using the shift register and multiplexor to produce the digital signal, is that a microprocessor having a low clock frequency may also be used for other tasks, such as communication with a remote control panel. Hence, savings may be made in the power consumption of the sounder, and in overall component costs.

The present invention uses complex drive waveforms to pulse drive a piezoelectric element in to a constantly reinforced multi-resonant condition. The dominant resonant frequency is stimulated even when the sounder produces a warble tone with two clearly distinct tones below IKHz. A feedback control loop maintains the optimum drive conditions of the complex waveforms, enabling a small, efficient and more aesthetic sounder design to be produced.

A large piezoelectric element (50mm diameter) that is edge mounted in a Helmholtz chamber and coupled to a folded horn is a practical way of producing such a sounder, which has a frequency response below IKHz.

Such a piezoelectric element will have a number of resonant peaks in this arrangement, however a dominant resonant peak will always exist. To obtain the lowest possible power dissipation and highest possible sound pressure level (SPL), the piezoelectric element needs to be driven at this dominant resonant frequency. This produces a number of fundamental problems, the first is a requirement that a fire alarm sounder must produce more than a single distinct tone. A second problem is that a suitable piezoelectric element with a high SPL will have its dominant resonant frequency above IKHz. A further problem is that the resonant frequency is subject to initial manufacturing tolerances as well as a drift during its useful lifetime due to environmental conditions and ageing.

These problems are overcome by aspects of the present invention.

The generation of the piezoelectric drive waveforms are described below in detail, and shown in Figures 2, 3 and 4.

When the sounder is operated to produce the second tone, shown in Figure 2, 8 bits of data are parallel loaded into an 8-stage shift register 2, under the control of microcontroller 1. One of the outputs Q6 or Q8 of the shift register is selected to feedback into its own serial input (IN), by selecting control line A0 of multiplexor 3. The microcontroller 1 now selects the serial mode of the shift register 2, using control line P/S. The microcontroller 1, then generates a free running square wave frequency on port line CLK, which is used to serially clock the data around the shift register 2, so that it circulates in a continuous loop.

It should be noted that the microcontroller 1 is able to adjust the square wave frequency it generates, hence the pulse width of the clocked data bits.

The multiplexor is arranged to circulate a 8 bit data output for the waveform of Figure 2, and a 6 bit data output for the waveform of Figure 3.

The loop of circulating data forms a complex waveform which we will see contains a fundamental frequency and a related harmonic frequency, which is ultimately used to drive the piezoelectric element 10. Only three waveforms will be examined in any detail, although more are clearly possible. The first waveform shown in Figure 2, is constructed from a data sequence as follows:

P8 P7 P6 P5 P4 P3 P2 PI

1 1 0 0 0 1 0 1

In this case the Q8 output of the shift register 2 is fed back into its own serial input. If we assume the bit pulse width is 180.5uS, it can be seen that a fundamental frequency exist in the waveform, which depends on its cycle period, as follows:

1 / ( 8 bits * 180.5uS )= 693 Hz

There also exists a higher 4th harmonic frequency generated at:

1 / ( 2 bits * 180.5uS )= 2770 Hz

The second waveform to consider shown in Figure 3, is constructed from a data sequence of:

P6 P5 P4 P3 P2 PI

0 0 0 1 0 1

In this case the Q6 output of the shift register 2 is fed back into its own serial input. Note that in both of the above cases the microcontroller 1 has set the data bit P5 to a logic low. If we again assume that the bit time is 180.5us, then a fundamental frequency will now exists at:

l / ( 6 bits * 180.5uS )= 923 Hz

There will also exist a higher 3rd harmonic generated at:

l / ( 2 bits * 180.5uS )= 2770 Hz

The third waveform shown in Figure 4, is constructed from a data sequence as follows:

P6 P5 P4 P3 P2 PI

0 1 0 1 0 1

This is a simple square wave, which is used for initial calibration. In this case the Q6 output is fed back into the shift registers serial input, however the data bit P5 is in this case set to a logic high. Finally if we assume the bit time is again 180.5us, then a simple square wave will produce a fundamental frequency of:

1 / ( 2 bits * 180.5uS )= 2770 Hz

This is identical to the 3rd and 4th harmonics of the previous complex waveforms, which is in fact the dominant resonant frequency (Fr) of the piezoelectric element.

The drive waveforms generated on output Q8, are applied to switching transistors 8 and 9. Capacitor 5 and resistor 7, form a differentiation network, so that transistor 9, only turns on during the rising edges of the applied waveforms. Similarly capacitor 4 and resistor 6, form a second network, so that transistor 8 only turns on during the falling edges of the applied waveforms. The transistors collectors are connected to the piezoelectric element 10 at point P+, the other side of the element is connected to 0 V ground. Current is now pulsed into and out of the piezoelectric element 10 during the rising and falling edges of the drive waveforms.

After transistor 8, pulses current into the piezoelectric element 10, the voltage will rise to the supply voltage level (Vcc) and as transistor 8 turns off, the piezoelectric element 10 is then free to resonate. Similarly after transistor 9, pulses current out of the piezoelectric element 10, the voltage across it will fall to zero and the piezoelectric element 10 is then again free to resonate. In this resonant period, a ringing voltage will be produced before transistor 8 turns on again.

If the applied current pulses occur at the piezoelectric elements dominant resonant Frequency (Fr), then the ringing voltage on the piezoelectric element 10 after transistor 9 turns on, will ring up to a maximum value, just before transistor 8 turns on. This ringing voltage, indicated in Figure 4, is sensed by a detection circuit.

A peak hold detection circuit is used to produce a DC voltage level which is monitored by the microcontroller 1, on an analogue to digital port (ANl). The sounder is initially calibrated during its manufacturing by the microcontroller 1 applying waveform 3, the simple square wave drive waveform, to the piezoelectric element 10 and then frequency sweeping, by adjusting the clock rate of the shift register 2 (CLK) in distinct frequency steps. The square wave clock duration is increased by 2uS at each step, to lower its frequency and is maintained for 40mS, so that the DC level at

ANl is stable enough for analogue to digital readings to be taken. A wide frequency capture range is used for this initial calibration, which is sufficient for the expected variance in any piezoelectric elements resonant frequency. The frequency that corresponds to the highest DC level will be the dominant resonant frequency (Fr) of the piezoelectric element. The DC level obtained during initial calibration is shown in Figure 5.

Divider resistors 11 and 12 drop down the voltage level applied to the peak detection transistor 14. Resistor 15 and capacitor 16 filter and hold the peak voltage level applied to transistor 14, whilst resistor 17 provides a slow discharge for capacitor 16.

As only a part of the voltage on the piezoelectric element 10 indicates that it is in a resonant condition, then the rest of the voltage waveform must be blanked off from peak detection transistor 14.

The blanking network consists of a clamp diode 13 and transistor 21. The diode 13 conducts when the drive waveforms are at a logic low. This blanks out the voltage caused by transistor 8 turning on. Transistor 21 is also pulsed on to blank the falling edge period of the piezoelectric voltage from being applied to transistor 14.

Once the dominant resonant frequency (Fr) of the piezoelectric element 10 is known, then the corresponding clock rate of the shift register 2 is then stored by the microcontroller 1. The microcontroller 1 from now on will use this same clock rate, however the complex waveforms of Figures 2 and 3 are now used to drive the piezoelectric element 10 into a multi-resonant condition. The value of the DC level is now also stored by the microcontroller.

If the sounder is switched on, and the resonant frequency has shifted from its initial value, the DC voltage feedback level to the microcontroller 1 will have dropped compared to its stored valve. The microcontroller 1 now executes a mini-resonant search using a complex waveform to find the optimum operating frequency. If the DC voltage level has dropped below a fixed threshold and the sounder is an addressable type, then this fault condition will be communicated to its control panel.

From the waveforms of Figures 2 and 3, we can see that the piezoelectric element 10 is strongly driven at its dominant resonant frequency (Fr), which is the 4th harmonic and 3rd harmonic respectively of the complex drive waveforms basic frequencies.

The microcontroller 1 is able to switch between the two complex drive waveforms to produce a warble tone. Figures 6 and 7 show the frequency spectrum produced in each case. What is clear, is that a very rich harmonic frequency spectrum is produced in both cases, whilst the same dominant resonant frequency (Fr) has been generated. This gives maximum efficiency with two widely separated low frequency tones in the range 500Hz to IKHz.

Figure 8 shows the frequency spectrum produced due to the calibration drive waveform of Figure 4. Note that the peak output is always at the same dominant resonant frequency (Fr) in all cases.

It will be appreciated that, although described with reference to sounders, and particularly to sounders for alarm systems in buildings, the invention is also suitable for use with alarm sounders for use with vehicle alarms, or sounders such as are used for warning devices for vehicles, for instance as reversing warning sounders, or emergency service vehicle sounders.

Claims

1. A sounder 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 perceived as a first tone, at a first tonal frequency, the piezo-electric element having a dominant resonant frequency, the dominant resonant frequency being above the first tonal frequency, the electronic drive circuit being arranged to drive the piezo- element to produce the first tone sound, the piezo-electric element being also excited at the dominant resonant frequency while the first tone sound is produced.

2. A sounder as claimed in claim 1, wherein the electrical output is further arranged to produce a second tone, at a second tonal frequency, the first tone frequency being higher than the second tonal frequency, the piezo-electric element being also excited at the dominant resonant frequency while the second tone is produced.

3. A sounder as claimed in claim 1 or 2, wherein the electronic drive circuit further comprises a digital signal generator and the electrical output is a digital signal.

4. A sounder as claimed in claim 3, wherein the digital signal is controlled by a variable square wave control frequency, which is a multiple of the dominant resonant frequency.

5. A sounder as claimed in claim 4, wherein the electronic drive circuit is arranged to monitor a dominant resonant response of the piezo-electric element to the electrical output and is further arranged to adjust the square wave control frequency to obtain a maximum dominant resonant response of the piezo-electric resonant element.

6. A sounder as claimed in any of the preceding claims, wherein the dominant resonant frequency is above 1kHz.

7. A sounder as claimed in any of the preceding claims, wherein the tonal frequency is within a frequency range between 500Hz and 1kHz.

8. A sounder as claimed in claimed in any of claims 5 to 7 when dependent on claim 5, wherein the electronic drive circuit is arranged to adjust the square wave control frequency within a narrow range while the first or second tone is being sounded to discover the frequency of the dominant resonant response.

9. A sounder as claimed in claim 8, wherein the electronic drive circuit is further arranged to monitor a feedback voltage level from the piezo-electric element while the piezo-electric element is being sounded, and to compare the feedback voltage level with an acceptable minimum voltage level.

10 A sounder, substantially as hereinbefore described and with reference to the accompanying drawings numbered from Figure 1 to Figure 8.