Abstract:
A dual sensor detector incorporates a fire sensor, such as a smoke or heat sensor, and a gas sensor. Control circuitry is coupled to the sensors. In response to a sensed fire condition, such as due to heat or smoke, a constant sample rate sampling parameter, such as a sample time interval or a drive amplitude, is increased for the second sensor so as to increase its signal-to-noise ratio and resolution. The second sensor will be operated with the increased sample interval or drive amplitude so long as the first sensor continues to exhibit the detection of a condition. When the first sensor drops outs of the detection of a condition, the alterable parameter of the second sensor is reset to its quiescent state which draws a lower current value.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention pertains to ambient condition detectors. More particularly, the invention pertains to such detectors which incorporate multiple sensors wherein an output signal from one of the sensors is used to alter a performance characteristic of a second sensor.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known to incorporate more than one sensor into an ambient condition detector. Tice U.S. Pat. No. 5,831,524, entitled System and Method for Dynamic Adjustment of Filtering in an Alarm System assigned to the assignee hereof discloses such a system. The noted Tice et al patent addressed apparatus and methods for altering a form of processing of an output from a sensor.  
           [0003]    Another known multiple sensor detector incorporates a smoke sensor which is used to alter a sample rate of a gas sensor. In the absence of a signal from the smoke sensor, the gas sensor samples at a first relatively low rate. In the presence of an alarm indicating signal from the smoke sensor, the sampling rate of the gas sensor is substantially increased to thereby shorten its response time to emitting an alarm indicating signal.  
           [0004]    There continues to be a need for devices and methods of operating multiple sensor detectors so as to further enhance signal to noise ratio, and shorten response time while at the same time reducing average current.  
         SUMMARY OF THE INVENTION  
         [0005]    A variable parameter detector which incorporates at least two sensors can be switched between first and second modes of operation depending on an output signal from one of the sensors. In the absence of an alarm indicating signal from the first sensor, an output signal from the second sensor, which is sampled at a constant rate, is processed with one of its alterable parameters having a first value. In response to the first sensor changing state and emitting a detected condition indicating signal, the alterable parameter of the output signal of the second sensor is driven from a first value to a second value during the period of time where the first sensor is exhibiting the detected condition. When the second sensor has a parameter which is exhibiting the second value, its performance, using a selected indicium, is altered so as to improve over-all detector response.  
           [0006]    The alterable sensor parameters can be selected from a group which includes an alterable sample interval, an alterable sample drive amplitude, an alterable sample drive time parameter, an alterable sample drive frequency parameter, and an alterable sample drive modulation parameter. In one embodiment, a sample interval of the second detector can be switched from a relatively short interval, used in the absence of an alarm indicating signal from the first sensor, to a longer sample interval used in the presence of an alarm indicating signal from the first sensor.  
           [0007]    So long as the second sensor is being operated with a relatively short sample interval, as an exemplary alterable parameter, it will draw a relatively low average current. In this operational mode, the second sensor may well have a lower-than-desired signal-to-noise ratio given a relatively short sample interval. However, it will exhibit a relatively low average current draw. Further, in the presence of large concentrations of the sensed condition, it will produce an output indicative of an alarm condition. For example, in the presence of a fast flaming fire, when the second sensor is a gas sensor, it can be expected to have a gross gas response, in the absence of an alarm indicating signal from a first sensor implemented as a fire sensor, that can be detected even with a short sample interval.  
           [0008]    Where the first sensor starts to exhibit an alarm condition, based on its sensing technology, and causes the second sensor to enter an altered parameter state, for example by increasing the sample interval or drive amplitude of the second sensor, the signal-to-noise ratio will increase, and the resolution increases. The average current increases during the time of the longer sample interval or increased drive amplitude. However, this increased current is only exhibited in the presence of an alarm indicating output from the first sensor. Hence, over a long interval of time the average current will continue to be relatively low. In yet another aspect, if the first sensor should in some way fail, the second sensor more likely than not will continue to function at the lower resolution, lower current mode and will still respond to relatively large increases in spaced sensed ambient condition.  
           [0009]    In yet another aspect, the average current can be reduced by pulsing one of an emitting element and a sensing element in a gas sensor with a pulse width less than the response time of the respective element. By selecting a pulse width that is less than the respective response time, coupled with a relatively long sample period, a further reduction in average current can be achieved. Additionally, the short activating pulse widths can be supplied at increased amplitudes to increase power. This in turn compensates for shorter pulse widths and keeps applied energy at acceptable levels.  
           [0010]    Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is block diagram of the system in accordance with the present invention, and  
         [0012]    FIGS.  2 A- 2 C are timing diagrams illustrating aspects of operation of the system of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    While this invention is susceptible of embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.  
         [0014]    [0014]FIG. 1 is a block diagram of a system  10  which incorporates two ambient condition sensors  12 ,  14 . The sensors  12 ,  14  have outputs which are coupled to a control element  18 . The control element  18 , as those of skill in the art will understand, could be implemented as hardwired logic or could incorporate a processor programmed with pre-stored instructions all without departing from the spirit and scope of the present invention.  
         [0015]    The sensors  12 ,  14  respond to different types of ambient conditions. For example, the sensor  12  could be implemented as a smoke sensor or a heat sensor using any one of a variety of available technologies. In one implementation, a photo-electric smoke sensor could be used.  
         [0016]    The sensor  14  could be implemented as, for example, a gas sensor. A typical example includes carbon dioxide sensors. It is known that carbon dioxide can be sensed using a variety of technologies including non-dispersive infrared technologies such as photo-acoustic as well as various types of thermal pile technologies. The exact nature and characteristics of the gas sensor are not a limitation of the present invention.  
         [0017]    As illustrated in FIG. 1, in the system  10  the control element  18  is coupled to a sensor  12  via a control  12   a  and receives signals from the sensor  12  via an output line  12   b . Similarly, the control element  18  is coupled to sensor  14  by a control  14   a  and receives output signals on a line  14   b.    
         [0018]    In accordance with the present invention, the sensor  12  produces outputs indicative of smoke or heat in accordance with the respective technology as illustrated by graph  16   a  of FIG. 2A. As illustrated in FIG. 2A as smoke or heat increases over a period of time, an output from sensor S 1 , via line  12   b , is received by control element  18  and processed. It will also be understood that some portion of the processing could be conducted by sensor  12  without departing from the spirit and scope of the present invention. In one aspect, the system  10  can establish that the sensed ambient condition, such as smoke or heat, has crossed a pre-established threshold, AL TH , which is regarded as being indicative of the presence of a sufficient level of the respective ambient condition as to represent a detected condition state.  
         [0019]    Simultaneously with receiving an output from sensor  12 , the control element  18  has been receiving sampled outputs from sensor  14 . As illustrated in FIG. 2B, control element  18  via line  14   a  transmits variable width, constant period sample control signals to sensor  14 . The sensor  14  is thus operated in two different modes.  
         [0020]    In mode M 1 , the sensor  14  is sampled with a sample time on the order of 5 milliseconds. This results in a relatively low resolution, gross gas measurement with a relatively low signal-to-noise ratio. Representative sample periods could be, for example, in a range of 3 to 8 seconds.  
         [0021]    In mode M 1 , sensor  14  is functioning at a very low average current level. In this mode, the sensor  14  is functional to detect the level of carbon dioxide in the ambient atmosphere and is usable for detecting large fires or large changes in carbon dioxide concentration. While the signal-to-noise ratio is relatively low in this mode, sensor  14  can be expected to appropriately respond to carbon dioxide levels in the ranges of 1000 parts per million or larger. Thus, large quantities of carbon dioxide are detectable. Such quantities can be present either alone or as a by-product of a large fire even in the presence of noise on the order of 300 parts per million.  
         [0022]    Control element  14  can maintain a running average of sample values from detector  14  while in mode M 1  which can be used to suppress some of the noise. Changes in carbon dioxide on the order of 600 to 1000 parts per million can be quickly detected despite the fact that the smoke or thermal sensor  12  may not as yet have generated a sufficient signal for the control element  18  to have detected the presence of an alarm condition.  
         [0023]    Where the output from sensor  12  has in fact crossed an alarm threshold, as illustrated in FIG. 2A, the sensor  14  is switched via control element  18  to a second mode, M 2 . In mode M 2 , sensor  14  is sampled at the same rate but with a substantially longer sample interval. For example, instead of a 5 millisecond sample interval, the sensor  14  can be sampled for 20 milliseconds. This in turn substantially improves the signal to noise ratio making it possible to detect changes in carbon dioxide which exceed 200 parts per million.  
         [0024]    In mode M 2 , noise is reduced to on the order of 50 parts per million as a result of a substantially longer sample interval. Thus, a higher resolution lower noise signal is present in mode M 2 . In contradistinction to the mode M 1 , in mode M 2 , sensor  14  when active, draws a substantially higher current perhaps 600 microamps versus 200-250 microamps as in mode M 1  operation.  
         [0025]    Signals from sensor  14  can be processed with a different running average when in mode M 2 . For example, this average can be implemented by operating in mode M 1  for nine samples and then switching to mode M 2  for one sample. With an exemplary sampling period of 5 seconds, the mode M 1  average will be updated every five seconds for nine samples. The mode M 2  signal will be updated every tenth sample, every 50 seconds. Average current flow required for sensor  14  with this type of averaging is the average of the current required for the updates, namely:  
         [9*250+1*600]÷10=285 microamps.  
         [0026]    The above described averaging process takes advantage of improved resolution and improved signal to noise ratio in the M 2  mode of operation and requires 285 microamps of current as opposed to the 250 microamps of current in the M 1  mode of operation. This is still significantly less than operating the sensor  14  in the M 2  mode of operation continuously which produces an average current on the order of 600 microamps.  
         [0027]    Those of skill in the art will understand that the number of samples in the running averages can be changed as the function of how often the average is updated. For example, in mode M 1 , a running average with a time constant on the order of 128 samples can be implemented. In mode M 2 , a time constant of 16 samples can be used to achieve a similar time reference for measuring the change in carbon dioxide concentration. Other averages or filtering processes can be used without departing from the spirit and scope of the present invention.  
         [0028]    Further with respect to FIG. 2, when the signal  16   a  from the sensor  12  drops below the pre-alarm or alarm indicating threshold, the control element  18  reverts to the M 1  mode of operation of sensor  14 .  
         [0029]    It will be understood that a variety of processing criteria could be used with the output of sensor  12  to switch modes of operation of sensor  14 . These all come within the spirit and scope of the present invention. Alternate criteria include rates of increase of the signals on line  12   b  or various types of patterns indicative of fire.  
         [0030]    Other drive characteristics of sensor  14  can be altered provided the sampling period is maintained at a constant value, such as 5 seconds, 10 seconds or the like. Alternates include changing the amplitude of the sample drive signal, changing a frequency parameter within the sample drive signal, or, altering a modulation parameter of the sample drive signal. FIG. 2C illustrates the process described above, FIG. 2B, where the drive amplitude to the sensor  14  is modulated.  
         [0031]    In yet another aspect of the invention where sensor  14  includes a source of radiant energy, such as is the case with a photo-acoustic carbon monoxide sensor, either the source of radiant energy or the sensor, a microphone, can be activated, pulsed or sampled, for duration that is shorter than the response time of one of the transmitter or the receiver. This produces a very short pulse and results in a very low average current. For example, where a receiver has a response time, defined to be the time interval between 10 percent to 90 percent of full output signal to a designated input which could be on the order of 100 milliseconds, the respective transmitter could be pulsed for less than 100 milliseconds. Where the transmitter is pulsed at a fixed rate, illustrated in FIG. 2, for example with a period of three to eight seconds, the average current will be reduced.  
         [0032]    Where the system  10  is to be coupled to a medium M, such as a wired medium which is part of an alarm system, devices, such as system  10 , can be powered off of electrical energy received from the medium M. In such environments, it is desirable to be able to reduce the current per unit since numerous detectors, such as the system  10 , might be coupled to the medium M. By reducing the average current as described above, additional detectors, such as the system  10 , can be coupled to the same wired medium M than is the case for higher average current detectors.  
         [0033]    In another alternate, the electrical energy received from the medium M by the system  10  can be increased where the control element  18  energizes or pulses the sensor  14  with an increased voltage. Pulsing the transmitter or source in sensor  14  for a time interval less than its response time, but with a higher voltage, makes it possible to increase the energy delivered to the source or transmitter. Thus, where the sensor  14  is a photo-acoustic carbon monoxide detector, for example, the source of radiant energy such as a light bulb, or, light emitting diode can be energized with extra large voltage but for a time interval less than its response time. Alternately, where the sensor  14  is a thermal pile gas sensor, a source of radiant energy such as a photo emitter or heater element can be activated with a higher voltage pulse width a pulse with less than the response time of the respective device.  
         [0034]    Those of skill in the art will understand, the above-noted variations and combinations produce detectors having lower average currents. This makes it possible to successfully energize an increased number of detectors, such as the system  10 , from medium M.  
         [0035]    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.