Sensors using detector arrays

Details Sensors using detector arrays Sensors using detector arrays

1998-01-13

2001-05-29

Ham, Seungsook (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S339140, C250S339150

Reexamination Certificate

active

06239433

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application corresponds to European Patent Application No. 97300196.9 filed Jan. 14, 1997.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to sensors for identifying the occurrence of events.
2. Description of Related Art
Radiation Detectors
Single and multi-element electromagnetic radiation detectors are currently manufactured for a variety of applications. In particular, pyroelectric infrared radiation detectors with from one to four elements are manufactured in large quantities for use in, for example, passive infrared intruder alarms, flame detecting fire alarms, gas analysis, and pollution monitoring systems. These detectors are typically made by the assembly, using more or less conventional semiconductor assembly processes, of pyroelectric chips with transistor and resistor chips into a suitable enclosure with an infrared transmitting window.
Two-dimensional arrays of pyroelectric detectors are also manufactured for thermal imaging applications. These arrays have a large number of elements, typically 10,000 or more, in order to obtain the best possible spatial resolution in the thermal image. The requirement for a large number of elements, and the consequent requirement for a small element pitch (typically 100 &mgr;m or less), tends to make the manufacturing cost of these arrays relatively high, and therefore unsuitable for low cost applications.
Flame Detectors
One of the methods used to detect fires in a fire alarm system is to use a radiation detector to detect the radiation emitted by a flame. There is a range of electromagnetic radiation wavelengths at which this may be done, but current flame detectors generally operate in either the ultraviolet (UV) or the infrared (IR) regions of the spectrum.
In the case of UV flame detectors, wavelengths shorter than 0.3 &mgr;m are used because the atmospheric ozone layer is opaque in this region and therefore false alarms due to solar radiation are largely eliminated. There are, however, various other sources of false alarms, such as lightning and arc welding.
In the case of IR flame detectors, normal practice is to look for radiation given off by hot carbon dioxide (CO
2
) gas, in the wavelength region around 4.3 &mgr;m, and to look for the characteristic random flicker frequency of a flame, typically in the region 2 Hz to 20 Hz. There are many other sources of infrared radiation which could give false alarms, but in general they do not give a signal which varies in the same way that a flame flickers, and additional discrimination can be obtained by comparing the signal within the band 4.1 &mgr;m to 4.6 &mgr;m, where hot CO
2
radiates, with the signal from a region outside this band, where hot CO
2
does not radiate. Hot bodies will radiate both within and without this spectral band.
In spite of these precautions, both UV and IR flame detectors are subject to false alarms. Also, because they are generally based on a single detector, or two detectors viewing the whole scene in different wavelength bands, they do not give any information regarding the spatial distribution of incident radiation, and thence the position of the flame.
Movement Sensors
Pyroelectric detectors are commonly used, in conjunction with sectored infrared lenses, in intruder alarms or for automatically switching on lights. The detector senses the infrared radiation emitted by a person and detects the movement of a person against the ambient background. Because these systems generally use one, two, or, in some cases, four detector elements which, by means of the sectored lens, view the whole of the scene being covered, an alarm condition is registered for any detected movement within the total scene, with no information provided on the location or direction of the movement. They also give no discrimination between persons who must be detected and false targets such as modulated sunlight (e.g., by moving tree leaves and branches), modulated heat sources (e.g., fans or curtains moving in front of radiators), or animals. Similar problems apply to sensors which are used to monitor the movement of vehicles, the problems being more severe for any external application where the field of view is not limited by walls and there are many more potential sources of false alarms.
One example of a known movement detector is disclosed in WO-A-9210812. This document describes an optical movement detection system as part of a direction sensitive counting and switching device for use, for example, to count people boarding a bus. The optical system includes an array of sensor elements arranged in one or more double rows with the sensor elements being connected in pairs to a common rear electrode. Thus, the sensors are not individually monitored.
Thermal Imaging
The multi-element, two-dimensional pyroelectric arrays used in thermal imaging systems are usually provided with a mechanical chopper system for modulating the infrared radiation from a scene. The signals from the various elements of the array are read out in phase with the chopper in a serial raster scan fashion, and subsequently processed to produce an image which is compatible with normal video standards.
BRIEF SUMMARY OF THE INVENTION
The current invention is aimed at providing both temporal and spatial information about the infrared radiation emitted from a scene in order to improve the performance of sensors and systems used, for example, in flame detectors for fire alarms, person or vehicle detectors in intruder alarms or monitoring systems, and detectors for monitoring various properties in process monitoring systems.
One aspect of the present invention provides a sensor comprising an array of detectors and an optical collection means arranged so that spatial information from a scene is focused onto the array, and a readout means for monitoring signals from the elements of the array and identifying the occurrence of an event within a particular part of the field of view, characterized in that, in use, the readout means monitors signals from each of the various elements of the array independently and identifies the occurrence of an event by comparison of these signals. Preferably the array is a two-dimensional array of infrared detectors and the optical collection means is preferably an infrared transmitting lens. The array will preferably include between 10 and 10,000 elements, and typically have at least 64 elements but not more than 1,024 elements. Thus, the current invention uses an infrared detector array of relatively few elements compared with arrays for thermal imaging, thus providing a much lower cost device, but of adequate size to provide sufficient spatial information to improve significantly the performance of flame detectors, motion detectors, and similar sensors.
The array is preferably mounted directly onto and in electrical contact with an integrated circuit which provides the means of reading out the signals from the individual elements and may also enable selective addressing of the elements. The signals from each element may be examined for the frequency content of the signal and compared with the signals from some or all of the other elements. The system has the capability of selecting and concentrating on a particular set of elements in which a signal of interest is observed. In a preferred configuration, the readout circuit is connected to a further circuit such as a microprocessor which monitors the output from the various elements of the array in terms of signal amplitude and frequency. Thus, the current invention allows addressing of the separate elements of the array in a random or non-sequential manner, unlike a typical thermal imaging system where the elements are addressed sequentially, and provides means of comparing the signal amplitude and frequency from one element with that from any other element. Uncooled thermal imagers using pyroelectric detector arrays employ mechanical choppers to modulate the incoming radiation;

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