Abstract:
A sounder control system, including a converter circuit for converting a drive signal to a signal for activating a sounder and a drive circuit for generating the drive signal. The drive circuit includes a microprocessor for receiving at least one input signal representative of one of a plurality of control parameters for the sounder control system and for adjusting the drive signal on the basis of the at least one input signal, such that the sounder exhibits a predetermined sound pressure level characteristic.

Description:
BACKGROUND INFORMATION 
     The present invention relates to a sounder control system. 
     Sirens employed in vehicle alarm systems include piezoelectric sounders or speakers which are powered by a voltage supply from the vehicle battery. In the event that supply from the vehicle battery fails or is disconnected, a backup battery is provided to ensure the siren can still be activated. Most vehicle alarm systems are configured, for example, to activate the siren if the vehicle battery is disconnected when the alarm system is armed. The backup battery however is normally only able to provide about a 6-9V supply to the siren, instead of the normal 12V supply provided by the vehicle&#39;s battery, and this gives rise to a significant drop in the performance of and sound pressure level generated by the siren. The other difficulty encountered with existing vehicle sirens is that the sound pressure level generated varies considerably across the frequency range for which the siren is to be activated, primarily because of a sounder&#39;s resonance behaviour. It is desirable to be able to provide a vehicle siren which generates a predetermined sound pressure level regardless of supply voltage, signal frequency, temperature or component tolerances. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a sounder control system, including a converter circuit for converting a drive signal to a signal for activating a sounder and a drive circuit for generating said drive signal. The drive circuit includes a microprocessor for receiving at least one input signal representative of one of a plurality of control parameters for the sounder control system and for adjusting the drive signal on the basis of the at least one input signal, such that said sounder exhibits a predetermined sound pressure level characteristic. 
     The present invention also provides a sounder control system, including a converter circuit for converting a drive signal to a signal for activating a sounder and a drive circuit for generating the drive signal. The drive circuit includes a microprocessor for receiving a voltage signal representative of the level of a supply voltage for the sounder control system and for adjusting the drive signal on the basis of the voltage signal, such that the sounder exhibits a predetermined sound pressure level characteristic. 
     The present invention further provides a sounder control system, including a converter circuit for converting a drive signal to a signal for activating a sounder and a drive circuit for generating said drive signal. The drive circuit includes a microprocessor for receiving a temperature signal representative of the temperature inside a housing of the system and the sounder and for adjusting the drive signal on the basis of the temperature signal, such that the sounder exhibits a predetermined sound pressure level characteristic. 
     The present invention also provides a siren control system that includes a transformer with a secondary coil connected across a sounder and a primary coil, a switch connected to said primary coil to cause current to flow in the primary coil when activated, and a control device for controlling activation of the switch such that said sounder exhibits a predetermined sound pressure level characteristic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of sound pressure level versus frequency for a piezoelectric siren. 
     FIG. 2 is a circuit diagram of a first embodiment of a siren control system according to the present invention. 
     FIG. 3 is a timing diagram of a PWM signal. 
     FIG. 4 is a circuit diagram of a second embodiment of a siren control system according to the present invention. 
     FIG. 5 is a circuit diagram of a third embodiment of a siren control system according to the present invention. 
     FIG. 6 is a graph of sound pressure level versus frequency for a siren control system. 
     FIG. 7 is a graph of sound pressure level versus supply voltage for a siren control system. 
    
    
     DETAILED DESCRIPTION 
     The sound pressure level generated or output by a piezoelectric siren peaks at one frequency and drops off dramatically for other frequencies, as shown in FIG.  1 . The graphs  2  and  4  of FIG. 1 show the sound pressure level generated when the drive or activation signal is swept across 1800 to 3600 hertz to produce a wailing sound from the siren. The first graph  2  is the levels generated when a 12 volt DC supply from the vehicle battery is available and the second graph  4  is the levels produced when a backup battery of the vehicle alarm system is used to provide the voltage supply for the siren. 
     A first siren control system  6 , as shown in FIG. 2, is for the siren of a vehicle alarm system. The siren includes a piezoelectric sounder or speaker  8  and the control system  6 . The control system  6  includes a transformer  10 , a microprocessor  12 , a field effect transistor (FET)  14  and two bias resistors  16  and  18 . The piezoelectric sounder  8  acts as a capacitor and is connected in parallel to the secondary coil  20  of the transformer  10 . The primary coil  22  of the transformer  10  is connected between a voltage supply line  24  and the drain of the transistor  14 . The voltage V supply  of the line  24  is normally the voltage of the vehicle battery, about 12 volts DC, or the voltage provided by the backup battery of the alarm system, normally 6 volts, if the vehicle battery is disconnected or is unable to supply the vehicle battery voltage. The gate of the transistor  14  is connected to an output port  26  of the microprocessor  12  via the first bias resistor  16 . The source of the transistor  14  is connected to ground by the second bias resistor  18  which also acts as a current limiter. 
     Activating the transistor  14  to connect the source to the drain causes current to be drawn through the primary coil  22  so as to generate a secondary current in the secondary coil  20  which charges the sounder  8  to cause it to emit sound. Once the sounder  8  is sufficiently charged, the transistor  14 , acting as a switch can be deactivated so as to allow current to be drawn through the sounder  8  as it discharges. The higher the supply voltage supplied, the higher the charge of the sounder  8 . The current in the secondary coil  20 , and activation of the sounder  8 , is controlled by a pulse width modulation (PWM) signal  30 , as shown in FIG. 3, generated at the output  26  of the microprocessor  12 . The sound pressure level or energy generated by the sounder  8  is dependent on the electrical energy or current supplied to the sounder  8 , which is governed by the time and frequency for which the transistor  14  is activated or switched on. The transistor  14  is switched on for the entire width W of a pulse of the PWM signal  30 . A decrease or increase in the pulse width W correspondingly decreases or increases the sound pressure level generated by the sounder  8 . The frequency of the sound generated by the sounder  8  is determined by the frequency f of PWM signal  30  which relates directly to the period T between pulses. The siren is therefore swept over a range of frequencies between 1800-3600 hertz by gradually decreasing the period T of PWM signal  30 . The pulse width W is increased or decreased depending on the level of the voltage V supply  on the line  24  as detected at an input  28  of the microprocessor  12 . The pulse width W and the period T of the PWM signal  30  at any given time is determined by values stored in memory of the microprocessor  12 . The microprocessor  12  calculates the periods T for the required frequency sweeps and the pulse widths W from data stored in look-up tables in the memory of the microprocessor  12 , and which is accessed using the frequency f as a pointer. The level of the supply voltage V supply  is also used as a pointer to data used to determine to output pulse width. As discussed below, other control parameters, such as temperature, may also be used as pointers. The values used to determine the output PWM signal are chosen and stored to ensure a predetermined sound pressure level is obtained over the entire frequency band of the sweep, regardless of the level of supply of the supply voltage V supply . For example, the sound pressure level may be set at 116 dba for the desired frequency sweep. Of course the sound pressure levels and the frequencies which can be used will differ depending on a country&#39;s regulations and the type of the piezoelectric sounder  8  which is used. Therefore the data used to generate the pulse widths W and the periods T, i.e. the basic duty cycle, is selected and stored accordingly with these parameters in mind. The input port  28  connects V supply  to an analog to digital converter of the microprocessor  12  which uses the converted signal as a pointer to the processor&#39;s calibratable (EEPROM) memory. The level of the supply V supply  is used to select a pulse width correction factor C v  from a look-up table which is used to adjust the width W at the output port  26 . The output pulse width (OPW) at the output port  26  is then C v  ×W. 
     If a temperature sensor  37  is placed on a printed circuit board of the system  6 , an electrical signal generated by the sensor  37  is fed back to the microprocessor  12  on a line  36 . The temperature value t represented by this signal is then used with the frequency f and the supply voltage V supply  as pointers for the look-up tables to obtain the output PWM signal. The temperature t relates to the ambient temperature inside a sealed housing of the siren which includes the system  6  and the sounder  8 . The temperature t is used to access a temperature correction look-up table to obtain a temperature correction factor C t . The pulse width at the output port  26  is then calculated by the microprocessor  12  as follows 
     
       
           OPW=C   v   ×C   t   ×W   (1) 
       
     
     The microprocessor  12  calculates, as described above, the pulse widths W and periods T for a basic pulse to produce a predetermined basic sound pressure level characteristic. The pulse widths W and periods T are calculated using the frequency f. The frequency f can also be used to access a frequency correction look-up table to obtain a frequency correction factor C f . The pulse width at the output port  26  can then be adjusted as follows 
       OPW=C   v   ×C   t   ×C   f   ×W   (2) 
     The control system  6  employs efficient and close control of the piezoelectric sounder  8  by obtaining feedback concerning the sound energy generated by the sounder  8 . Sound energy feedback enables the sounder  8  to be driven at maximum efficiency whilst taking into account tolerances of the transformer  10  and the sounder  8  as well as temperature drifts of the components. Feedback concerning the sound energy can be obtained by monitoring either the current of the primary coil  22 , the current of the secondary coil  20  or the voltage across the secondary coil  20 . 
     A signal representative of the primary current of the coil  22  is taken from the source of the transistor  14  and inputted into an analog input  32  of the processor  12  via a diode  34 , as shown in FIG.  2 . The diode  34  has a cathode connected to the input  32  and its anode connected to the source of the transistor  14 . A grounded capacitor  38  is connected across the input  32 . Upper and lower current limits are stored in the microprocessor  12  so as to define an acceptable primary current operating range for the piezoelectric sounder  8 , and the microprocessor  12  modifies the pulse width W at the output  26  to ensure the current sensed at the input  32  is within the predetermined range. The pulse width W is incremented or decremented until the sensed current falls within the predetermined range. The level of the feedback signal can be used to generate a value X to adjust the pulse width at the output port  26  at predetermined intervals. For example, the output pulse width can be derated or increased by X % every Y ms. At this time, the output pulse width could be determined by              OPW   =       C   v     ×     C   f     ×     C   t     ×     (     1   +     X   100       )     ×   W             (   3   )                                
     In a second control system  40  as shown in FIG. 4, the secondary current is alternatively supplied to the input  32  by connecting the anode of the diode  34  to a connection point between the secondary coil  20  and a resistor  42  placed between the coil  20  and the sounder  8 . A third control system  44 , as shown in FIG. 5, illustrates a further alternative where a voltage representative of the voltage across the secondary coil  20  and the sounder  8  can be obtained from between two resistors  46  and  48  connected across the sounder  8  and provided to the anode of the diode  34 . The resistors  46  and  48  provide a voltage divider across the secondary coil  20 . For the third control system  44 , the operating range defined in the microprocessor  12  is with reference to the voltage across the second resistor  48 . Therefore current and voltage sensing can be provided and fed back to the input  32  with only the addition of a minor number of passive components. 
     The siren control systems  6 ,  40  and  44  are able to control the sound pressure level (SPL) characteristic of the sounder  8  to the extent that a desired or predetermined SPL characteristic can be produced regardless of the level of the supply voltage. The graphs of FIGS. 6 and 7 illustrate three SPL characteristics, with reference to frequency in FIG.  6  and with reference to supply voltage in FIG.  7 . The natural characteristic of the sounder  8  is represented by the fine line, and FIG. 6 shows how the natural characteristic includes a resonant point  50  and tapers off on either side of the point  50 . FIG. 7 shows how the SPL increases linearly with supply voltage until a characteristic point of destruction  52  of the sounder  8  is reached. To prevent destruction of the sounder  8  at high supply voltages, and to provide a constant SPL regardless of supply voltage, the SPL frequency response is set to be constant over a predetermined frequency range by adjusting the natural characteristic to produce a desired predetermined characteristic as shown in bold in FIGS. 6 and 7. An alternative flatter frequency characteristic, as shown by the dotted line in FIG. 6, may be desired to prevent the SPL dropping off at high supply voltages, as shown by the dotted line in FIG.  7 . 
     The control systems  6 ,  40  and  44  produce a constant sound pressure level output across a frequency range thereby fully utilising the piezoelectric speaker. The constant sound pressure level output is also achieved independent of supply voltage, and local requirements and restrictions can be taken into account. 
     The control systems  6 ,  40  and  44  are closed loop control systems which allow optimal use of the piezoelectric sounder  8  and the available supply voltage whilst compensating against component tolerances, and temperature tolerances, which is particularly advantageous when the sounder  8  is driven by a backup power source, such as the backup battery. A constant high sound pressure level can also be generated from the sounder  8  through operation close to the destruction point of the piezoelectric sounder  8 , thereby extending the operating range of the siren system. 
     The control systems  6 ,  40  and  44  whilst particularly advantageous for piezoelectric sounders  8  can also be used beneficially with other sounders, such as loudspeakers.