Abstract:
An apparent motion detector is provided with multiple response levels at differing degrees of sensitivity. The motion detector is based on use of a pyroelectric infrared sensor and conventional circuitry which generates an output signal the strength of which reflects transient temperature changes occurring within a field of view and distinguishable from background heat levels. The detector&#39;s response varies with the strength of the signal using a plurality of LED&#39;s which emit different colors or are driven at different intensities. The system generally will have a field of view defined by a lens system which is translucent to visible light and transparent to infrared. The lens system doubles as a back screen projection system for display of the indicator light.

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure relates to passive infrared sensor-based motion detectors. 
     2. General Description 
     Motion detectors based on infrared sensing elements represent a specialized type of passive infrared (PIR) sensor called Passive Infrared-based Motion Detectors (PID). PIDs are routinely applied to security systems and for automatically actuated lighting systems which are intended to be triggered by movement of an object or body through the sensor&#39;s coverage area. PIDs do not literally detect “motion”, but rather detect transient changes in temperature occurring within usually small and changing portions of the coverage area. These transient changes, or “apparent motion”, can stem from movement of a warm bodied person or vehicle into or through the coverage area, producing a local area of higher temperature against the cooler background of the coverage area. Such transient changes can also be associated with movement of a body or an object colder than the background through the coverage area. An example of this would be ice floating on a warmer river. 
     A PID is usually built using a plurality of pyroelectric thin film sensors which are combined with appropriate circuitry to make the PID insensitive to changes in ambient temperature generally detected across the coverage area while remaining sensitive to the localized temperature changes likely to be associated with movement of objects of a different temperature in or through the coverage area. Ambient temperature changes generally occur more slowly and across more of the coverage area than changes observed upon the movement of objects through the area allowing generation of signals generally indicative of object movement based on transient localized changes in temperature in the coverage area. In a literal sense then, PID motion sensors could be considered “apparent” motion detectors since the changes they detect are not exclusively caused by motion of objects, however, the term “motion detector” is commonly used in the art. 
     Unfortunately, transient changes can also stem from events which have little or nothing to do with the movement of “objects of interest”. Handling of localized, transient changes of temperature which are not the result of motion of an object of interest, but are instead associated with other events, for example wind induced motion of tree branches or changes in cloud cover, is an issue for PID based systems. Unusually large objects, or objects exhibiting a substantial deviation from ambient temperature may pass through peripheral portions of the coverage area (or just outside the intended coverage area) giving rise to undesirable triggering of lighting or security alerts (“false” triggers). 
     The sensor elements in PIDs produce what is basically an analogue signal subject to variation over time depending upon changes in the heat output, or reflection, within its field of view. Signal events must be accurately correlated with the motion of warm bodies of significant size moving into or out of the focused sensing zones. A warm person walking into a sensing zone can generate a relatively large change in the infrared energy in that sensing zone, causing the circuitry to interpret that change in infrared energy as motion. One difficulty a sensor has is distinguishing between signals which indicate object motion in the coverage area, in which case it is desirable to activate the lights, and other signals which are not caused by object motion but mimic the signal resulting from object motion and thus which result in activation of security lights. These false triggers or nuisance activations need to be minimized. Minimization of false trigger events has demanded careful aiming of PIDs if maximum sensitivity is to be employed. As a consequence, less than maximum PID sensitivity is sometimes used with less than optimal aiming of the device as a tradeoff to minimize installation time. 
     Prior art PIDs have been built which have included a secondary light which is activated whenever a signal is generated which is of sufficient magnitude to trigger activation of the primary lighting or to activate an alarm. In some applications the secondary light is activated for a relatively brief period compared to the primary lighting. The secondary light provides several advantages. One advantage is that initial installation is aided since an installer can determine the sensor coverage area by watching the secondary light turn on and off. The “on period of the secondary light is generally much shorter than the “on” period for the primary lights and often corresponds closely to when an object is actually in the coverage zone as opposed to the main lights which may have an “on” period of several minutes. Another advantage is that the secondary light can be left active during daylight hours when the primary lighting is inoperable to save power. Still another advantage is that the secondary light can serve as an indicator that the primary lighting has failed. 
     Prior art PIDs have often been used as part of an area security system for detecting intruders within a secured area. Upon detection of an intruder the primary lighting or an alarm is activated. A secondary light, if present, also turns when the intruder enters the secured area. While the primary lighting typically remains on for an extended period of time, for example five minutes, the secondary light typically turns off after just a few seconds and then, if additional object movement is detected within the coverage area, turns on again. The secondary light can flash on and off if the intruder moves around in or further into the coverage/secured area. However, intruders often vacate a secured area in response to activation of the primary lighting, particularly if the intrusion is innocent. 
     While PIDs have been built to accommodate ambient conditions, changes in ambient conditions can still affect the precise borders of a PID&#39;s coverage area and/or the minimum temperature differentials between an object and its background required to activate the primary lighting or alarm. False triggers, or failures to respond, may be more or less likely when the weather does not match the conditions prevailing on installation. Small changes in ambient conditions may cause signals that were just below the threshold necessary to activate the primary lights or alarm. A typical example would be a PID installed at the front of a house and inadvertently aimed so that traffic passing in front of the house was within its field of view but far enough away that the signals generated by passing cars were not large enough to activate the system&#39;s primary response. Should a vehicle later exhibit a substantial temperature contrast with the environment, often stemming from colder weather conditions, the signal may become large enough to trigger the system&#39;s primary and secondary response. 
     SUMMARY 
     A passive infrared motion detector has multiple response levels at differing degrees of sensitivity. The motion detector is based on use of a pyroelectric infrared sensor and on circuitry which generates an output signal the strength of which reflects transient temperature changes occurring within a field of view and which is distinguishable from background heat levels. The detector&#39;s response varies with the strength of the signal. One vehicle for expressing the response is generation of an indicator which varies with the strength of the output signal. This can be effected a number of ways. For example, a plurality of LED&#39;s which emit light in different colors may be used. Alternatively, a single LED can be illuminated at an intensity which varies with signal strength. Auditory systems can be used. Typically these preliminary responses are staged to occur upon comparison to a plurality of reference levels of decreasing sensitivity. The system&#39;s ultimate response, illumination of an area or triggering of an alarm for example, occurs when the output signal magnitude reaches the least sensitive reference level. The system generally will have a field of view defined by a lens system which is translucent to visible light and transparent to infrared light. The lens system can double for back screen projection of the indicator light output. For motion detection systems based on ranging and/or Doppler shifts occurring within a coverage area and basing a response on strength of the return signal, analogous indication mechanisms and provision of multiple reference levels may be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a passive infrared motion detector. 
         FIG. 2  is a block diagram of motion detection circuitry for a passive infrared sensor. 
         FIG. 3  is a block diagram of motion detection circuitry for a passive infrared sensor exhibiting multiple concurrent sensitivity levels. 
         FIG. 4  is a graphical illustration of a possible output signal from a passive infrared sensor. 
         FIG. 5  is a graphical illustration of a possible output signal from a passive infrared sensor against a plurality of sensitivity trigger levels. 
         FIG. 6  illustrates the horizontal dispersion of detection zones within the coverage area for a passive infrared sensor based motion detector. 
         FIG. 7  illustrates distance limits for a horizontal pattern of detection zones within the coverage area of a passive infrared sensor. 
         FIG. 8  graphs a possible output signal from a passive infrared sensor. 
         FIG. 9  is perspective view of an apparent motion detector incorporating multiple trigger level indicators. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. Furthermore, example sizes/models/values/ranges may be given. These are not intended to be limiting in most cases. In circuit diagrams, well-known power and ground connections, and similar well-known elements, may be omitted for the sake of simplicity of illustration. 
     Referring to  FIG. 1 , a PID based sensor  10  is illustrated comprising a housing  12  and a lens system  14  installed on the housing. Lens system  14  is preferably translucent to the visible light spectrum, but transparent to the infrared spectrum, which can help mitigate false triggers from visible light sources. 
     Referring to  FIG. 2 , a block diagram of representative PID circuitry  20  uses a transducer  21  based on thin films of pyroelectric material. The transducer(s)  21  is connected to an amplifier  22  (typically a differential amplifier). The transducer  21  is exposed to infrared radiation B radiated by an object A moving through or within the coverage area of a lens system  14 . The transducer  21  is located behind the lens system  14  which together define a coverage area for a PID based sensor. A preferred transducer assembly is the Nippon Ceramic model RE200B (described at http://www.nicera.co.jp/pro/ip/pdf/pdfip001.pdf). The signal B, received by the transducer  21  could come from the reflection of a signal from a microwave or ultrasonic source (not shown), or direct radiation from the moving object as in the case of passive infrared motion sensors. Transducer  21  converts the infrared signals from the moving object into an electrical signal, which is amplified and filtered by the amplifier  22  of the PID circuitry  20 . The output signal from the amplifier  22  is directed to the input of level detector  23  which compares the amplitude of the output signal to a primary reference level. If the amplitude of the output signal is of greater magnitude than the primary reference level, the level detector  23  sends a trigger signal to the logic block  24  indicating that a signal level consistent with detection of an object in motion is present. Logic block  24  may use one or more additional criteria, such as requiring multiple signals from the level detector or requiring low ambient light conditions, before activating the attached lighting  25 . The level detector  23 , primary reference level and logic  24  could also be implemented in software. The choice of the primary reference level is made to obtain the desired sensitivity, and range, while avoiding an excessive number of false triggers. If the motion sensing system is attached to a security system that activates an alarm, the tendency would be to limit the sensitivity to avoid false alarms. 
     The field of view or coverage area of a PID is usually defined by the focusing lens  14  disposed between an active surface of the sensor element  21  and the environment. In a typical application the focusing lens  14  is a polyethylene lens (preferably a Fresnel Lens) placed and shaped to collect energy in the form of a pattern of sensing zones (see  FIG. 6  or  7 ) covering a portion of a broader field. The energy within the sensing zones is magnified. This arrangement is used to extend the range over which infrared energy can be detected. 
     A problem with this system lies in its application to motion sensing. The sensor element produces what is basically an analogue signal subject to variation over time depending upon changes in the heat radiated by objects and background within its coverage area. Signal events must be accurately correlated with the motion of warm bodies of significant size moving into or out of the focused sensing zones. A warm person walking into a sensing zone can generate a relatively large change in the infrared energy in that sensing zone, causing the circuitry to interpret that change in infrared energy as motion. A difficulty with such systems has been setting the coverage area such that, to the extent possible, only objects in motion through the coverage area drive the output signal level to a greater magnitude than the reference level and thereby trigger the primary response of the system. Signal levels which have a relatively high probability of resulting from peripheral or non-motion events should not exceed the reference level. To achieve this result the coverage area should be set to avoid locations likely to host peripheral or non-motion based events. 
       FIG. 3  is a block diagram of motion detection circuitry  30  modified to implement multiple detection or reference levels. Level detectors  31 ,  32 ,  33  monitor the output signal from amplifier  22  and compare the amplitude of the output signal to each of three secondary reference levels; a, b, and c. In a conventional installation at least two of the three reference levels, and more typically all three secondary reference levels, are set to a lesser absolute magnitude than the primary reference level. As a result, level detectors  31 ,  32  and  33  successively generate discrete responses to the output signal as it reaches progressively larger absolute magnitudes (see  FIG. 5 ). In other words, levels c, b and a, in that order, represent progressively increased sensitivity at the cost of the increasingly greater likelihood of false triggers. Secondary logic  34  is connected to receive the outputs from level detectors  31 ,  32 ,  33 , and potentially other signals relating to certain necessary conditions, such as ambient light level or the position of control switches (not shown), to activate: a 1st indicator light  35  if the output signal is of greater absolute magnitude than level a; a 2nd indicator light  36  if the output signal is greater than level b; and a 3rd indicator light  37  if the output signal is greater than level c. It is not necessary that the exogenous conditions required for a response at any particular reference level be the same as those for another reference level. For example, the responses at secondary reference levels might be insensitive to ambient light levels while the response to the output signal exceeding the magnitude of the primary reference level might be inhibited at high ambient light levels. A secondary reference level might be set to the same magnitude as the primary reference level, but the response made conditional on different exogenous conditions. Secondary reference levels of course might be set higher than the primary reference level to be used to indicate bounds tighter than the current coverage area. 
     The indicator lights  35 ,  36 ,  37  are preferably low intensity lights such as LED&#39;s that are clearly visible to a person looking directly at the motion sensing system but which do not provide enough illumination to light up the area surrounding the motion sensing system. As a result, a higher degree of false triggers can be tolerated for activation of indicator lights  35 ,  36 ,  37  since they do not draw as much attention nor are as distracting as illumination of the higher wattage security lights  25 . In one embodiment, the first indicator light  35  would be a low level, amber colored LED package activated in response to the output signal exceeding reference level a. Reference level a may be chosen to represent a four factor increase in sensitivity compared to the primary reference level. Since the infrared radiation from a moving object received at the PID  10  tracks the inverse square law, the result will be a doubling in effective range of PID  10 . This would be expected to generate a substantial increase in false triggers. Similarly, the second indicator light  36  is arranged to have a sensitivity that is a factor of 2 greater than the sensitivity used to activate the security lights  25 . The second indicator light  36  would also be an amber LED, but driven at a higher level than the first indicator light  35 , so that it is illuminated at a brighter level than the first indicator light  35 . The first indicator light  35  and the second indicator light  36  could be the same LED driven at two different levels. Finally, the third indicator light  37  could be a red LED set to a sensitivity level quite close to the sensitivity used to activate the high wattage security lights  25 . 
     On approaching a PID which incorporates the circuitry of this disclosure and the indicator light system of the first embodiment, an intruder could see a low level amber glow as he approaches a PID motion sensing system, indicating that he has been detected. As the intruder moves closer, the amber LED  35  will glow more intensely, or change color, in response to the intruder&#39;s motions providing a clear indication that he has been detected and an implied warning that the system is active and about to respond to the intruder. If the intruder moves still closer, the red LED  37  will activate as a final warning that a response is imminent. Any further motion toward the motion sensing system will activate the high wattage security lights  25  and/or set off an alarm. The result is three additional layers of deterrence that occur before the intruder has activated the high wattage lights  25  and/or the alarm system. 
       FIGS. 4 and 5  graphically illustrate a typical output signal from amplifier  22  as a moving object A approaches a motion detection system.  FIG. 4  illustrates the response of a system having only a primary reference level setting while  FIG. 5  illustrates a system incorporating a plurality of reference levels including a primary reference (defined as the reference level which results in generation of the systems intended response) and a plurality of secondary, higher sensitivity levels. As the object A approaches, or moves (as indicated by arrow C), into coverage zone rays  60  of the coverage area CA (see  FIG. 6 ), the amplitude of the output signal grows in magnitude approaching the positive and negative primary reference levels. The amplitude of the output signal can eventually become greater than the primary reference level. At this point in time, the logic element  24  receives a signal from the level detector  23  and turns on the security lights  25 . Once the primary reference level has been exceeded, the security lights  25  would typically remain on for a predetermined minimum period even with loss of the trigger signal. 
       FIG. 6  shows a simplified diagram of the coverage zones  60  within the coverage area CA for a typical PID motion sensor. For simplicity in illustration, only six zones  60  are shown. To further simplify the drawing, only the horizontal plane is depicted. In actual applications the zone coverage is replicated, or something similar is provided, in the vertical plane. The coverage zones  50  are typically a plurality of pyramid shaped volumes extending outwardly from the face of the lens system  14 .  FIG. 6  includes a representation of a warm object A moving into one of the sensing zones  60  (as indicated by arrow C). When outside the sensing zone, only a very small fraction of infrared energy radiated by object A reaches the PID system. However, once inside a sensing zone  60 , the lens system  14  focuses a much higher percentage of the IR energy onto the transducer  21 . The change in incident IR energy on the transducer  21  from low intensity to high intensity results in a relatively large signal at the transducer  21  output. A typical response is that after a 0.4 seconds delay the high wattage lights  25  are activated. In this amount of time, a person may have traveled two or three feet further into the motion sensing zone  60 , making it difficult to judge where the edge of the motion sensing zone actually began. In most cases this will give the installer the impression that the motion sensing zone is considerably narrower than it actually is. 
       FIG. 5  shows positive and negative reference levels for level a, level b, and level c. This allows the effective comparison of the absolute value of the output signal with the secondary reference levels, and the responsive illumination of the LEDs indicating which sensitivity level is being triggered. There are multiple indications from the indicator lights  35 ,  36 ,  37  of the magnitude of the apparent motion detected. LED indicator lights  35 ,  36  and  37  are preferably illuminated for relatively short periods compared with the activation period for an alarm or the primary lighting following a triggering event at the respective levels. In effect a variety of staged levels of response are produced. 
       FIG. 7  illustrates generation of apparent motion signals that are created by objects substantially beyond the normal range of the PID. Two motion sensing zones  50 , with extensions  52  are shown. Possible borders  54  between the zones  50  and extensions  52  are illustrated. The position of borders  54  would be quite fluid, being subject to the amount of heat radiated by and potentially by the speed of the object A to be detected. Borders  54  indicate only a maximum range of the PID on an average day for an average human being. A person or object moving into the motion sensing zone  50  would activate the high wattage lights  25 . A person A moving into an extended sensing zone  52  (indicated by arrow D) would result in generation of an output signal as graphically illustrated in  FIG. 8 . Here the output signal does not exceed the primary reference level and the high wattage lighting  25  is not activated. However, since the output signal does exceed level a, the first indicator light  35  is activated. 
       FIG. 9  illustrates location of the invention that places the supplemental indicator lights  35 ,  36 ,  37  behind the lens system  14  of a PID  40  used for focusing infrared light on the sensor element of PID  91 . The indicator lights  35 ,  36 ,  37  are arranged to illuminate the entire lens system  14  which is typically translucent to visible wavelengths of light. Illumination of the relatively large surface area of the lens  14 , results in a display that is more visible to an intruder or installer than would be the relatively small surface areas of the individual LEDs. A sensor element is placed in recess  91  to avoid direct exposure of the pyroelectric sensor films to light emitted from LED indicator lights  35 ,  36 ,  37 . Alternative arrangements of indicator lights based on an LED are possible. LEDs have become available which may be varied in color output. It is possible that the response of the system could be continuous variation of the color output, intensity output or flash rate depending upon signal strength. The LEDs will illuminate the lens system  14  from behind relative to the exterior of housing  12 . Since lens system  14  is translucent to visible portions of the spectrum the lens system will appear to glow to an outside observer due to back screen projection on the lenses from the indicator LEDs  35 ,  36  and  37 . 
     These concepts may also be extended to any motion detection system that generates proportional signals, e.g. microwave and ultrasonic systems. The system is applicable for false trigger identification, verifying coverage range and makes aiming easier.