Abstract:
A passive infrared sensor (PIR) for detecting infrared radiation which includes a lens positioned in a sensor housing, and a pyroelectric element electrically connected to a circuit board within a filter housing positioned in the sensor housing. A microprocessor is electrically connected to a main circuit board and controls a liquid crystal display (LCD) attached to the sensor housing. The lens overlaps the LCD. The LCD has LCD regions corresponding to lens regions of the lens. Using the microprocessor, the LCD regions selectively prevent radiation energy from passing to the pyroelectric element, and the LCD regions selectively allow radiation energy to pass to the pyroelectric element. A signaling device communicates an alarm signal indicating when radiation energy within a specified wavelength band reaches the pyroelectric element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a passive infrared sensor (PIR) or device for detecting infrared radiation, and more specifically, a PIR sensor which includes a lens overlapping an LCD positioned in a housing for selectively preventing and allowing radiation energy to reach a pyroelectric sensor. 
     BACKGROUND OF THE INVENTION 
     Currently, pyroelectric sensors are used in intrusion detection devices to identify intruders. Pyroelectric elements are sensitive to infrared light at wavelengths emitted by the human body, i.e., a wavelength band of about 7 to 25 μm. However, pyroelectric elements are also sensitive to broadband radiation which includes ultraviolet, infrared, and visible light. Much of this radiation is outside the wavelength band emitted by humans. To minimize false alarms, a typical pyroelectric sensing device, used in intrusion detection contains a window (or filter) which filters, i.e., minimizes the transmission of wavelengths, for example, below 5 μm. More specifically, the window is typically formed using a substrate which may be comprised of silicon. Silicon absorbs radiation energy below 1.1 μm and passes radiation energy above 1.1 μm. Filtering of the wavelengths from 1.1 to 5.0 μm is achieved by placing layers of other materials on the silicon substrate. The material in these layers must pass the wavelengths of interest (7.0 to 25.0 μm), while filtering the wavelengths from 1.1 to 5.0 μm. Each material by itself can either absorb or reflect some of the wavelengths not passed. 
     More specifically, known pyroelectric sensing devices may include a printed circuit board including one or more pyroelectric elements. The pyroelectric elements are electrically connected to a microprocessor. If an electrical signal from the pyroelectric elements satisfies preset conditions, the microprocessor will transmit an alarm signal to an alarm system or monitoring device. The window/filter is formed using a substrate including a plurality of coating layers. The coating layers transmit, reflect, absorb, or cause destructive interference of radiation being focused at the window from a radiation source. A secondary filter may be placed in front of the window such that window is a primary filter working in conjunction with the secondary filter to selectively reflect and pass radiation energy. 
     Knows pyroelectric sensing devices are inherently susceptible to detecting stimuli not associated with intrusion which results in false alarms and/or false detections. Specifically, pyroelectric sensing devices are susceptible to the radiation energy produced by automobile head lights and other light sources emanating from outside the region being protected, but penetrating into the field-of-view of the pyroelectric device, and ultimately onto the pyroelectric device package. The energy produced by automobile head lamps can be sufficient to cause an alarm in a pyroelectric sensing device. False alarms in intrusion systems are a significant distraction and loss of man hours for the police force, and also can be costly in fines to the owners of the security systems. 
     Additionally, known intrusion detection systems include passive infrared sensors which detect intruders moving within a field of view by measuring the temperature gradient caused by an intruder. Also, known systems include devices for monitoring a volume of space encompassing a field of view, as disclosed in U.S. Pat. No. 7,145,455, issued to Eskildsen et al. The devices may include a micro electro-mechanical system having mirrors arranged in an array for reflecting IR energy to an IR energy detector which is then converted to an output signal and monitored for determining when an intrusion has occurred. Further, the mirrors are angularly adjusted to detect or cover a desired field of view. 
     A drawback of these known devices includes the necessity of moving the device or detection system to achieve IR detection in a desired field of view. Further, micro electro-mechanical systems and mirror arrays are expensive to manufacture, as well as, difficult and expensive to maintain and repair. 
     Additionally, current PIR devices may be built with one or more fixed fields of view designed into the lens array. In the case of multiple fields of view, there is no distinguishing between these different fields of view since each view may cause a non-unique alarm when a sensor detects radiation. Thus, there is no indication of the direction from which the sensed radiation came from or ability to ignore signals from a particular direction. 
     Additionally, current approaches to solving for false alarms also include augmenting the blocking ability of the pyroelectric detectors window/filter to block unwanted radiation energy. Typically, this includes adding materials, sometimes pigmenting agents (e.g. Zinc Sulfide) to the lens to make the lens more opaque to white light or visible light (energy radiation at wavelengths which the human eye can see) while passing IR (infrared) energy/radiation, or may include addition of a secondary filter. Typically, the amount of a white light absorbing substance added to a passive infrared (PIR) intrusion detector lens to ensure ignoring car headlights is significant, and has an adverse effect on lens transmission in the infrared realm, which may impair the ability of the pyroelectric sensor to detect an intruder. Lens transmission may be reduced by at least 30% in the IR wavelength band between 5 and 25 μm when adequate amounts of pigmentation are added. 
     Another approach to solving the problem of false alarms is adding a secondary filter to an intrusion detector to ensure that the pyroelectric sensing device ignores car headlights. Secondary filters add significantly to the cost of the intrusion detector and may reduce the IR transmission by approximately 20%. Thus, when intrusion detectors incorporate secondary filters to ensure the pyroelectric sensing device ignores car headlights, the detector may not detect an intruder because the secondary filter reduces the amount of energy that will reach the pyroelectric elements. Further, secondary filters also alter the optical path between each lens element and the pyroelectric elements, which may distort the intended protection. 
     Additionally, energy between 0.4 and 1.8 μm reaching the pyroelectric detector, for example from an automobile headlamp, is significant and may result in a pyroelectric detector signal sufficient to cause a motion sensor to send an alarm. Specifically, the typical pyroelectric filter does not transmit energy in this wavelength band because the energy is absorbed by silicon and coating layers. However, as the filter absorbs this energy, the energy is converted into heat. This heat is re-radiated at a longer wavelength, passes through the filter and is detected by the pyroelectric element(s). Typical pyroelectric filters used today may contain layers which cause destructive interference in the 1.8 to 5.0 μm wavelength band. 
     Another drawback to current pyroelectric sensing devices is the susceptibility of the window/filter to absorb energy in close proximity to the sensing elements (ie, the housing and most significantly the optical filter). Although the pyroelectric window/filter blocks energy below 5 μm, a large portion of this blocking comes in the form of energy absorption and a smaller portion from destructive interference and reflection. The absorbed energy is converted into heat, which is re-radiated at wavelengths that pass through the filter to the sensitive pyroelectric elements, thereby generating an electrical response leading to a false alarm from detection of the energy source. 
     It would therefore be desirable to provide a PIR device and method for intrusion detection which achieves IR detection in a desired field of view without the necessity of moving the device or detection system, or the necessity to reflect or redirect IR radiation to a pyroelectric sensor. It would also desirable to provide a PIR device and method that can prevent unwanted energy from reaching the pyroelectric sensors without producing heat and the undesirable re-radiation of energy. Thus, the desired PIR device would substantially eliminate false alarms/detections without the shortcomings of current devices and methods. It would further be desirable to provide a pyroelectric sensor which prevents visible and near infrared radiation (NIR) energy from reaching the pyroelectric filter. Also, it would be desirable to simplify manufacturing, reduce costs, and improve reliability of current PIR devices. 
     SUMMARY OF THE INVENTION 
     In an aspect of the invention a passive infrared sensor comprises a sensor housing including a first circuit board and a filter housing including a second circuit board. The filter housing is positioned within the sensor housing. A filter is positioned in the filter housing and the filter is transparent to a first specified wavelength band of radiation. The filter blocks a second specified wavelength band of radiation outside the first specified wavelength band of radiation. A liquid crystal display (LCD) is attached to the sensor housing. At least one pyroelectric element is electrically connected to the second circuit board. A microprocessor is electrically connected to the first circuit board. At least one lens is attached to the housing and overlapping the LCD. The lens has at least one lens region corresponding to at least one LCD region. The at least one LCD region is controlled by the microprocessor to selectively prevent radiation energy from passing to the pyroelectric element and to selectively allow the radiation energy to pass to the pyroelectric element. The microprocessor receives an electrical signal generated from the at least one pyroelectric sensor and initiates an alarm signal when radiation within the specified wavelength band reaches the at least one pyroelectric sensor. 
     In a related aspect, the lens is a Fresnel lens. 
     In a related aspect, the lens is a Fresnel lens and the LCD regions angularly correspond to a lenslet of the Fresnel lens. 
     In a related aspect, the Fresnel lens and LCD are combined, and a front protective layer of the LCD is directly scribed with at least one Fresnel lens pattern. 
     In a related aspect, the Fresnel lens and LCD are combined such that a front polarizer of the LCD is directly scribed with at least one Fresnel lens pattern. 
     In a related aspect, the Fresnel lens and the LCD are concave. 
     In a related aspect, the lens overlapping the LCD is in spaced relation with the LCD to define a gap therebetween. 
     In a related aspect, the gap has a substantially constant width dimension between the LCD and the lens. 
     In a related aspect, the at least one sensor includes a plurality of sensors. 
     In a related aspect, the LCD is coupled to the lens using an adhesive. 
     In a related aspect, the lens and the LCD are convex. 
     In a related aspect, the lens and the LCD are concave. 
     In a related aspect, the microprocessor initiates the alarm signal when the radiation within the specified wavelength band reaches the at least one pyroelectric element from multiple lens regions. 
     In a related aspect, the microprocessor initiates the alarm signal when the radiation within the specified wavelength band reaches the at least one pyroelectric element from multiple lens regions in a specified sequence. 
     In another aspect of the invention a method for detecting infrared radiation comprises providing a sensor housing including a first circuit board and a filter housing including a second circuit board; positioning the filter housing within the sensor housing; positioning a filter in the filter housing, the filter being transparent to a first specified wavelength band of radiation and the filter blocking a second specified wavelength band of radiation outside the first specified wavelength band of radiation; attaching a liquid crystal display (LCD) to the sensor housing; electrically connecting at least one pyroelectric element to the second circuit board; electrically connecting a microprocessor to the first circuit board; attaching at least one lens to the sensor housing and overlapping the LCD; corresponding at least one lens region of the lens to at least one LCD region; controlling the at least one LCD region using the microprocessor for selectively allowing or preventing radiation energy from reaching the at least one sensor; and signaling when the radiation energy reaches the at least one pyroelectric element. 
     In a related aspect, controlling the LCD regions includes allowing radiation energy to pass toward the at least one pyroelectric element from a specified LCD region; and preventing radiation energy from passing to the at least one pyroelectric element from another specified LCD region. 
     In a related aspect, the method further includes spacing the LCD and the lens to define a gap therebetween. 
     In a related aspect, the lens is a Fresnel lens and further including angularly corresponding the LCD regions to corresponding lenslets of the Fresnel lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings, in which: 
         FIG. 1  is a side elevational cross-sectional view of a pyroelectric sensor according to an embodiment of the invention depicting a lens, an LCD, pyroelectric elements mounted on a circuit board and a microprocessor mounted on a main circuit board; 
         FIG. 2  is rear elevational view of the LCD shown in  FIG. 1  depicting LCD regions as clear and opaque; 
         FIG. 3  is a front elevational view of a Fresnel lens depicting a plurality of lenslets; and 
         FIG. 4  is a cross sectional view along line A-A in  FIG. 3  depicting the lens and the lenslets shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary embodiment of a passive infrared (PIR) sensor  10  according to the present invention is shown in  FIGS. 1 and 2 . A lens embodied as a Fresnel lens  16  having lenslets  18  and is arcuately shaped and attached to a sensor housing  15 . An LCD  14  includes a rear surface  14   b  and the LCD is flexible in the embodiment shown in  FIG. 1 , but in other embodiments may be flat as well as rigid and mating with a similarly shaped lens. The LCD  14  includes a front protective layer embodied as a front polarizer  14   a  (shown in  FIGS. 1 and 4 ) for polarizing plane incident light in an arbitrary direction. The polarizer passes only those lightwaves whose associated electromagnetic fields are oriented in a predetermined “polarizing direction.” The polarizing direction lies in a plane parallel to the surface of the polarizer. 
     The Fresnel lens  16  overlaps the LCD  14 , and a source of radiation energy, represented by element  11 , is positioned in front of the lens  16 . The Fresnel lens  16  is typically arcuately shaped as shown in  FIGS. 1 and 4 , and the LCD  14  is also arcuately shaped to fit in mating relation with the Fresnel lens  16 . The Fresnel lens  16  and the LCD  14  are in space relation to each other and thereby define a space or gap  13  therebetween. The space  13  remains constant between the lens  16  and the LCD  14 . One advantage of the flexible LCD  14  is that it conforms to the curved surface of the lens  16 . The LCD includes LCD regions  22  positioned behind the lenslets  18 , such that each LCD region  22  of the LCD  14  is behind a corresponding lenslet  18  of the Fresnel lens  16 . 
     Further referring to  FIG. 1 , a filter housing  62  includes a filter or window  66  for selectively allowing or preventing specified wavelengths of radiation energy access to the pyroelectric elements  24 . It is understood that a particular or a range of wavelengths of radiation energy may be prevented from reaching the elements  24  by way of filtering methods using the window  66  known in the art. 
     In an alternative embodiment, the Fresnel lens  16  and LCD  14  are combined into a single subassembly for reducing manufacturing costs. In such an embodiment, the front polarizer  14   a  or a protective layer over a polarizer attached to the front of the LCD  14  is directly scribed with Fresnel lens pattern(s). 
     In another alternative embodiment, the filter  66  may be omitted and individual filters (which may have differing characteristics) may be affixed to, or printed upon, the rear surface  14   b  of the LCD  14 . In this embodiment, the device can effectively measure different spectral bands of the radiation source  11 , and perform additional processing to identify specific alarm conditions or reject specific false alarm conditions based on the spectral characteristics of the source. 
     The Fresnel lens  16  is shown in an exaggerated form in  FIGS. 1 and 4  to more easily depict the lenslets  18 . The Fresnel lens  16  may be molded using a piece of flat plastic, which is then bent or curved so that all the radiation through the lens is directed toward the pyroelectric element or sensor  24  beneath the lens  16 . The lenslets  18  are lens regions of the Fresnel lens  16 . Each lenslet  18  includes a different lens thickness and refraction index. The radiation energy source  11  may include, for example, infrared energy emitted by a person, and other sources of radiation energy such as car headlights. Instead of one section of the lens directly in front of the pyroelectric element, the curved Fresnel lens  16  can direct a multiplicity of angles of light through the lenslets  18  to the pyroelectric elements  24 . Pyroelectric elements  24 , only two of which are shown for illustrative purposes, are mounted to a printed circuit board (PCB)  50  mounted in the filter housing  62 . The lens  16  selects the angles of light by selecting which LCD region  22  to make clear  22   a  and thus allow light to pass therethrough. Thus, the lens  16  is able to receive radiation from a select angle without physically moving the lens as would be necessitated by a flat lens with a single section passing radiation to the pyroelectric elements  24 . 
     In another embodiment, for example, the LCD may be flat, or may be a plurality of other geometric shapes, such as rectangular, different curvatures such as convex or concave. Correspondingly, for example, the lens would also be flat and of the same geometric shape as the LCD or vice versa. Alternatively, the lens may be, for example, convex, concave or another type of lens other than a Fresnel lens. 
     Referring to  FIGS. 1 ,  3  and  4 , in operation, the Fresnel lens  16  includes lens regions or lenslets  18  and a center lens region  17 . The lenslets  18  are shown in cross-section in  FIGS. 1 and 4  and have a saw-tooth profile. As shown in  FIG. 3 , the Fresnel lens  16  includes a center lens region  17  with the lenslets  18  concentrically positioned around the center lens region  17 . Referring to  FIGS. 1 and 2 , the LCD regions  22 , only four of which are shown for illustrative purposes, are either opaque  22   b  or clear  22   a . The lens  16  overlaps the LCD  14  such that the LCD regions  22  correspond to lenslets  18 . The microprocessor  54  selectively controls the LCD regions  22  to be “on” or “off”, i.e., opaque  22   b  or clear  22   a , respectively. When the LCD regions are clear  22   a , the pyroelectric element  24  can receive radiation energy through the lenslet  18  and the clear LCD region  22   a . Thus, radiation energy from a specific direction defined by the positioning of the lenslet  18  may emanate from, e.g., a person, to a sensor  24 . When radiation energy in a specified wavelength range reaches the pyroelectric elements  24  the microprocessor  54  determines whether an alarm state of “on” should be initiated to indicate that an intruder has been detected. 
     The microprocessor  54  on the main circuit board  82  controls which LCD region  22  is being used, i.e., allowing radiation through to the pyroelectric elements  24  by making a cleat LCD region  22   a , or blacking out a select LCD region  22   b  to prevent any radiation from reaching the pyroelectric elements  24 . For example, the device  10  can scan through the LCD regions  22  in a selection process for selecting which LCD region meets specified parameters. Thus, the device  10  can distinguish between different fields of view and assign different alarm messages to each field of view. A central station operator can effectively reprogram the protected field of view remotely by instructing the alarm panel to process certain alarm messages and ignore others. 
     Regarding the sensor  10  operation and referring to  FIG. 1 , radiation energy is allowed to pass through clear LCD regions  22   a . When the pyroelectric window/filter  66  passes wavelengths of interest, i.e., specified wavelength bands of radiation energy, the absorption of radiation by the pyroelectric element  24  causes the elements  24  to heat up. The pyroelectric elements  24  generate an electrical signal proportional to the rate of temperature change as a result of the pyroelectric effect. The electrical signal from the pyroelectric elements travels via electrical connections in the PCB  50  in the filter housing  62 , and is received by the main PCB  82 . Thereafter, the electrical signal is amplified by amplifier  88  and processed by the microprocessor  54 , both mounted on the main printed circuit board  82 . The microprocessor  54  determines an alarm state (i.e., alarm “on” or “off”) of the PIR device  10  by first determining if an alarm threshold is achieved. The alarm threshold is attained when the electrical signal strength is greater than a predetermined value. At that point, the sensor  10  using the microprocessor  54  sends an alarm signal to an alarm system  92  which may be an alarm system control panel  92 . The alarm signal may be sent to the alarm system  92  using wired or wireless technology which may utilize a radio transmission. This is achieved by the microprocessor  54  removing power from a relay  96  on the main PCB  82  which opens the relay or alarm circuit. The open circuit is interpreted by the alarm system  92  as an alarm “on” state. An alarm can be generated from the alarm system  92 , as well as, transmitted to a remote receiving device, a monitoring station, and to alert emergency personnel using, for example, wired or wireless technology. 
     Thus, the sensor  10  can discern the spatial location of the incoming radiation using the LCD regions, and can allow or prevent radiation from a specified angle from entering the device without physically moving the device. Furthermore, the device can thereby prevent false alarms by, for example, preventing radiation energy from a known location from entering the sensor  10  by making opaque LCD regions  22   b  which face appliances, heater vents, or other sources of IR energy. 
     In another embodiment of the invention, the sensor  10  can require a specific heat motion pattern to occur before the microprocessor sends an alarm “on” signal. For example, each lenslet  18  and LCD region  22  may represent a zone and a specific heat pattern may be required from each zone before the microprocessor sends an alarm “on” signal. Further, a multi-zone method may be used where each zone has a specified angular requirement, e.g., the radiation must be received by the pyroelectric sensors from a first specified direction in a first zone, and from a second specified direction in a second zone. The specified direction may be, for example, generally up or down, or from a particular angular direction. 
     In another embodiment of the invention, the microprocessor initiates a scan of the LCD regions and denotes which LCD regions corresponding to spatial zones in a room or area are initiating the electrical signal from the pyroelectric sensors. Thus, the microprocessor builds a picture of where the IR sources are located in its surroundings. This information can be used either to assign specific alarm loop numbers to particular spatial zones, or to add extra false alarm immunity, e.g., by requiring an IR source to appear in one zone then move completely to another zone before the alarm signal is initiated. Further, the IR source may be required to appear in multiple zones, particular zones in a specified sequence, and/or at a particular time or within specified times before an alarm signal is initiated. 
     While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in forms and details may be made without departing from the spirit and scope of the present application. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated herein, but falls within the scope of the appended claims.