Abstract:
An alarm sounder, which incorporates a piezo-electric output transducer, can be silently monitored using a variable frequency square wave. An initial frequency, close to the upper limit of human hearing, is coupled to the sounder. The transducer draws very little current at this initial frequency. The frequency of the square wave is systematically reduced, and the current draw is continually monitored. A high current indicates a low impedance type of fault. A low current throughout the frequency range indicates a potential high frequency type of fault.

Description:
FIELD 
     The application pertains to audible alarm indicating output devices, or sounders. More particularly, the application pertains to substantially silent monitoring of alarm sounders. 
     BACKGROUND 
     Modern analogue addressable fire alarm systems use many loop powered alarm sounders controlled by microcontrollers, to alert people in protected areas to the presence of a fire alarm condition. Many alarm sounders use piezo-electric transducers (a piezo) to reduce the current consumption of the sounders in the alarm condition. Typically these analogue addressable systems can continuously monitor all outstation types on each addressable loop for faults, to ensure the system can be relied on to detect fires and alert people. In the case of alarm sounders, the actual sounder output can normally only be switched on and verified during regular tests with the system in the alarm state. 
     While it would be an enormous benefit to continuously verify that the alarm sounder can actually provide its correct output, background monitoring has always proved difficult to successfully implement especially with sounders using a piezo element. In known systems, complex monitoring waveforms need to be generated, so that background monitoring is normally only available on speech variants. However, as a relatively large acoustic output during the background monitoring has always proved to be unavoidable, its use in bedrooms for example, is clearly unacceptable. 
     One way to guarantee reliable fault detection of the sounder would be to require that the monitoring frequency be fixed at a relatively low in-band frequency. This configuration would produce a monitoring current high enough to provide reliable discrimination. This however would prevent the monitoring from being silent, and it would limit its general use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an embodiment hereof; 
         FIG. 2  is a flow diagram of aspects of operation of the embodiment of  FIG. 1 ; and 
         FIG. 3  is a flow diagram of additional aspects of the method of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     While disclosed embodiments can take many different forms, specific embodiments hereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof, as well as the best mode of practicing same, and is not intended to limit the claims hereof to the specific embodiment illustrated. 
     Silent monitoring, in accordance herewith can be provided for a fire alarm piezo-electric sounder that uses a class-D drive amplifier which normally produces attention tones between 500 Hz and 1 KHz. Significant higher harmonics are also normally produced through-out the audio band, and optionally, the sounder may be used to produce speech messages. The class-D amplifier uses the audio output providing piezo-electric transducer as a filtering capacitor. The volume is normally controlled by adjusting the supply voltage feed to a storage capacitor connected to the amplifier. 
     Silent monitoring can be implemented by periodically producing a high frequency square wave, starting at a frequency close to the upper limit of hearing of approximately 20 KHz. The amplifier is set to the maximum volume, and then the supply rail to the amplifier is turned off, allowing the amplifier to be energized by a local storage capacitor coupled to the supply rails. 
     Normally the amplifier will draw a small current due to the piezo being driven at a frequency far outside the speech band and higher than the upper corner frequency of the amplifier. This also causes the amplifier to attenuate the monitoring frequency so it becomes inaudible. The drop in voltage on the storage capacitor over a fixed period is directly proportional to the current used and is monitored by local control circuits, which could include a microcontroller, in the alarm sounder. 
     If the piezo element or, the amplifier draws a very high current, then this is clearly a low impedance type of fault and can easily be detected by the local control circuits as such. If the current is lower than expected, then this could be a high impedance type of fault; however it could also be due to the very high efficiency of the amplifier, the variation in its impedance at this high frequency or due to the component variation of the particular piezo element and storage capacitor used. While all the variations could be initially calibrated out during manufacturing, it is know that component values will change with the effects of temperature and age. 
     In one aspect hereof, the alarm sounder generates a square wave monitoring frequency at descending frequency steps below 20 KHz, with the transition between steps carefully controlled to minimise the audio content. If an adequate, but not excessive monitoring current can be detected at any step, then this process will stop and the test will have been passed, if however the current is always too small then the test will only stop and fail at a drive frequency well into the audio band. 
     The overall effect of this optimising technique is to produce a preferred monitoring frequency that will reliably monitor the sounder and give the lowest possible audio output and therefore annoyance. If an open circuit type of fault is discovered, the sounder will actually be tested at its maximum volume and at a drive frequency which is consistent with the sounder&#39;s normal operation during an alarm. In this case it is a certainty that a real fault must exists, however even this will be a silent test. 
       FIG. 1  illustrates a block diagram of an alarm/monitoring apparatus  10  which incorporates silent testing of one or more alarm sounders in accordance herewith.  FIG. 2  and  FIG. 3  illustrate aspects of methods  100 ,  200  respectively of testing such sounders. 
     Apparatus  10  includes an alarm/monitoring control unit, or panel  12  of a type generally known to those of skill in the art. The unit  12  is in bidirectional communication with a plurality  14  of substantially identical alarm sounders  14   a ,  14   b  . . .  14   i  . . .  14   n  via a medium  16 . The medium  16  could be implemented, for example as an electric cable. 
     The unit  12  can communicate information and commands to and receive information from members of the plurality  14  along with smoke, fire, or intrusion detectors, without limitation as would be used in monitoring a region R and providing alarm related information to individuals in that region. 
     Sounder  14   i  is representative of members of the plurality  14 . A description thereof will suffice for the other members of the plurality  14  as well. 
     Sounder  14   i  is carried by a housing  20  which could be mounted on a surface in the region R to provide audible alarm indicating outputs. Sounder  14   i  receives commands, and other information along with electrical energy from unit  12  via medium  16 . Sounder  14   i  can also communicate status or test results to the unit  12  via medium  16  and interface circuits  12   a . If desired, sounder  14   i  could be in wireless communication with unit  12  and receive its electrical energy from a local source, without limitation. 
     Housing  20  of sounder  14   i  carries a programmable control unit, or microcontroller,  22   a  along with pre-stored control software  22   b . Housing  20  also carries volume control circuits  24 , storage capacitor  28 , monitoring circuitry  30 , a class-D amplifier  32  a piezoelectric audible output transducer  34  and an A/C load  36 . 
     In operation as discussed below, microcontroller  22   a  periodically background tests the alarm sounder  14   i  using a test signal when the alarm sounder is not active to determine if it is capable of giving an audio alarm when required. This test signal starts at an inaudible high initial frequency close to 20 KHz. First, the quiescent current taken by the class-D amplifier  32 , the monitor circuitry  30  and the hold-up time of the storage capacitor  28  is measured in a calibration test  200  discussed further relative to  FIG. 3 . 
     The microcontroller  22   a  sets the volume to 0%, as at  202 , using a PWM control line  40 , which drives the volume control circuit  24 . The volume control circuit  24  supplies a voltage supply level on line  26  to a class-D amplifier  32  and hence controls its volume. Microcontroller  22   a  then sets a PWM drive frequency, line  40   a , to a very low duty cycle of just less than 0.5%, as at  206 , at the initial start frequency of 20 KHz, as at  204 . The audio envelope generated in this step change is masked by the fact that the volume is set to 0%. 
     The volume is then ramped up to its maximum 100% level, as at  208 , using the PWM control line  40  over a number of seconds, so that the frequency content of the envelope appearing on the output of class-D amplifier  32  is too low to be audible. Storage capacitor  28  is now charged up to its maximum voltage, which is equal to the regulated input voltage  42  obtainable from the medium  16 . Microcontroller  22   a  now turns off the volume control circuit  24 , as at  210 , by setting the PWM control line  40  to 0%. 
     Storage capacitor  28  now slowly discharges at a rate dependent on the actual value of the capacitor  28 , the static circuit loading and the dynamic loading caused by the finite switching losses of the class-D amplifier  32 . The piezoelectric transducer  34  causes almost no loading because of the very small duty cycle of the class-D amplifier  326 . 
     After about a one second discharge period, as at  212 , microcontroller  22   a  measures the monitor circuitry  30  using an analogue to digital converter connected to ADC port line  44 , as at  214 . This calibration reading is termed ADC1 and could be the result of a number of samples averaged together to filter noise. It should also be understood that this ADC1 value could also be checked to see if it is in an expected range, so that many other hardware faults could be determined. 
     With respect to  FIG. 2 , following on from the calibration test, of  FIG. 3 , the duty cycle of the initial test frequency is slowly increased to 50%, as at  102 , to produce a square wave drive waveform and the volume is also slowly increased to a maximum 100%, as at  104 , by the microcontroller  22   a . In both cases the rate of change is limited so that the frequency content from the class-D amplifier  32  remains inaudible. The output of the class-D amplifier  32  is now at the maximum drive power for the piezoelectric transducer  34 , for this particular frequency. Microcontroller  22   a  now turns off the volume control circuit  24 , as at  106 , so that storage capacitor  28  will discharge at a higher rate determined mainly by the loading from the piezoelectric transducer  34 . 
     After about one second, as at  108 , the microcontroller  22   a  measures the monitor circuitry  30  using an analogue to digital converter connected to the ADC port line  44 . This reading is termed ADC2, as at  110 , and may be the result of a number of samples averaged together. 
     A small NC load  36  may be placed asymmetrically on the piezoelectric transducer  34  to increase its high frequency loading and the class-D amplifier  32  may be driven only on the opposite side of the piezoelectric transducer  34 , with a single ended output  38 . The A/C load  36  could be just a capacitor, with a value that is small compared to the capacitance of the piezoelectric transducer. 
     Microcontroller  22   a  now removes the calibration value ADC1 from the first test measurement ADC2, giving a delta measurement to determine the load impedance of the alarm sounder. If the delta measurement is too high, as at  112 , it indicates an excessively high current caused by a low impedance fault, either due to the piezoelectric transducer  34  or class-D amplifier  32  being faulty. Microcontroller  22   a  then reports back to the control panel  12  that a short-circuit fault exists on the alarm sounder  14   i , as at  114 . 
     If however the delta measurement is too low, it does not necessarily mean that an open circuit fault has occurred, it could be that the loading caused by the class-D amplifier  32  driving the piezoelectric transducer  34  at such a high frequency is just too small to measure reliably, as the test frequency is far outside the normal operational range of an alarm sounder, such as  14   i.    
     If the delta measurement is too low, as at  116 , then the microcontroller  22   a  goes through a process of reducing the test frequency by a small amount (say by 1 KHz) and repeating the above test measurement (as at  104 - 110 ) to obtain a new value of ADC2. As the test frequency moves closer to the normal operational frequency of the alarm sounder, then the load current taken by the class-D amplifier  32  due to the piezoelectric transducer  34  must increase if the alarm sounder is really fault free. The microcontroller  22   a  will then finish the silent monitoring when it detects that an open circuit does not exist. This will be at a frequency that exactly minimizes the audio output noise and maximizes the robustness of the measurement. 
     If however the delta measurement is too low on each new test frequency and the test frequency has reached the normal operational frequency range of the alarm sounder, say for example as low as 3 KHz, then at this minimum frequency it is certain that a real open circuit fault must exist and the microcontroller  22   a  will stop the test and report an open circuit fault to the control panel. Note that in this condition the monitoring has also remained silent even while the test frequency is well into the audio band and at its maximum volume. 
     Assuming a successful test of the alarm sounder resulted in no faults being found, then the square wave test frequency remains on for about ten seconds, as at  120 , so that the storage capacitor  28  is fully discharged i.e. the class-D amplifier  32  falls to 0% volume, as at  122 , before microcontroller  22   a  ramps the test frequency off to end the test. This process again ensures that the frequency content of the audio envelope is masked. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.