Low cost detection technique

Details Low cost detection technique Low cost detection technique

2001-03-12

2004-07-27

Le, N. (Department: 2858)

Electricity: measuring and testing

Measuring, testing, or sensing electricity, per se

Frequency of cyclic current or voltage

Reexamination Certificate

active

06768294

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to circuitry for frequency domain signal measurement in general and more particularly but not exclusively to such circuitry incorporated into intruder detection systems, energy efficiency systems and the like.
BACKGROUND OF THE INVENTION
There are many kinds of detectors for the purposes of break-in and security, energy control and other purposes, including PIR detectors, microwave detectors, ultrasonic detectors, infrasonic detectors, shock detectors and the like. In all of these detectors, it is the practice to use very sensitive sensors that generally produce a very low signal, which must be amplified in order to allow for the processing of signals.
As is known, one of the main problems in the field of alarm systems is the undesirable, relatively high rate of false alarms. In order to reduce the number of false alarms, there now exist very sophisticated signal processing circuits, sometimes using micro-processors, and these attempt, with the help of sophisticated algorithms and A/D circuits, to better distinguish a real alarm from a false alarm. The accepted solutions are relatively expensive and costly, and the market is very price-sensitive.
A further drawback of sophisticated signal processing is the multiplicity of components which such an approach requires. Every additional component leads to a corresponding reduction in reliability and increases the sensitivity of the system to external RF noise, thus leading to additional false alarms. Reliability of the apparatus and the prevention, or at least reduction, of false alarms are very important issues in the field of security systems.
Passive Infrared Detectors (PIR)
PIR detectors are very popular today in the field of burglar alarm systems and energy control. These detectors use a pyroelectric sensor (explained in U.S. Pat. No. 5,077,549 Col. 1/13-48 and U.S. Pat. No. 5,414,263 Col. 1/12-54, the contents of which are hereby incorporated by reference).
The pyroelectric sensor is connected to a band-pass filter/amplifier having a very high gain of several thousands (generally 5,000). Reference is accordingly made to U.S. Pat. Nos. 4,570,157, 4,468,658, 5,309,147, 4,364,030, 4,318,089, 4,612,442, 4,604,524, the contents of which are hereby incorporated by reference. In these patents the signal is amplified and fed into a window comparator or other voltage comparator, and when the signal exceeds a threshold voltage, the alarm is activated.
In recent years, with the appearance of microprocessors, very sophisticated signal processing methods have been adopted. U.S. Pat. No. 5,077,549, the contents of which are hereby incorporated by reference, describes an alarm based on the principle of signal integration (equivalent to measuring the energy). In this patent it is important to measure the exact form of the signal in order to transform the signal into useful information. In this patent too, use is made of a similar high gain band-pass filter/amplifier.
An additional patent worth noting is U.S. Pat. No. 5,693,943 to Visonic, in which an exact analysis of the form of the signal is used to make a decision regarding a real or false alarm. In this patent too, use is made of high gain amplifiers. Again, it is very important to keep track of the exact form of the signal in order to make the right decision. Similar problems can be seen in U.S. Pat. No. 5,870,022. The contents of both of these patents are hereby incorporated by reference.
In all of the examples cited above and in many others, it can be seen that the electronic circuits contain a large number of components of various kinds, which raise the price of the product and reduce reliability. An additional problem stems from the fact that, due to the weakness of the signal which is generated by the pyroelectric sensor, it is customary to use high-gain amplifiers (between 1,000 and 10,000) and relatively narrow (0.2-8 Hz) pass bands to overcome environmental interference. In these amplifiers, which are low frequency, use is often made of high value capacitors with low leakage. This makes the products more expensive and in particular causes a substantial reduction in reliability, which may be responsible for certain types of false alarm.
The combination of very high amplification and very narrow band pass at low frequency and the use of AC coupling between amplification stages may cause the amplifier to distort the form of the signal. Ringing problems are known as are DC Offset, overshoot and other problems, and these may cause the signal produced by the amplifier to differ significantly from the original signal of the pyroelectric sensor. This, in turn, causes various signal processing problems in detectors which use comparators, and in particular with more sophisticated detectors which analyze the form of the signal (see U.S. Pat. No. 5,084,696, the disclosure of which is hereby incorporated by reference, and U.S. Pat. No. 5,870,022 which is referred to above).
In order to reduce amplifier gain and to improve the processing of the signal, attempts have been made to use high resolution A/D circuits, see for example, U.S. Pat. Nos. 4,546,334 and 5,693,943 referred to above.
Using such a technique, it is possible to reduce some of the levels of amplification which are used in circuits with a window comparator. However, the cost of the circuit rises due to the use of the A/D converter and reliability is not necessarily improved. In recent years, with the appearance of micro-processors comprising internal A/D converters, the use of A/D converters has been expanded—See U.S. Pat. Nos. 5,629,676 and 5,237,330, the contents of which are hereby incorporated by reference. Such use can reduce the required amplification and allows just one amplification stage. However, such processors are more expensive than regular non-A/D processors.
An additional problem, when using PIR detectors, is the question of their immunity to radio frequency interference (RFI) and electromagnetic interference (EMI), which is a main factor in the design of alarm systems with PIR detectors and others. This is a consequence of the low signal levels, and the use of high gain amplifiers with high impedances.
It is also worthwhile noting, regarding PIR detectors, that it is customary to compensate for the effect of the temperature difference between the body of an intruder and room temperature. This can be done directly by means of altering the gain of the amplifier's analog circuit—see U.S. Pat. Nos. 4,195,234 and 4,943,712, the contents of which are hereby incorporated by reference,—or it can be done more exactly using software and a microprocessor—see U.S. Pat. No. 4,546,344, the contents of which are hereby incorporated by reference, and U.S. Pat. No. 5,629,676, referred to above.
There are PIR detectors of various kinds on the market which use two or more pyroelectric sensors and sophisticated signal processing. These detectors are sometimes called QUAD. See for example patents: EPO198,551, GB 2170952, 4,614938, 4,618,854, 4,704,533, 4,697,081, 4,746,910, 4,912,748, 4,943,800, the contents of which are hereby incorporated by reference.
In the above-mentioned patents each sensor has a separate amplification circuit, such that in practice, the problems discussed above are magnified.
Another kind of detector is a combination of a PIR detector and a detector based on a different technology, such as microwave (MW) or ultrasonic. These are generally called DUAL detectors.
The following patents, EP O147,925, U.S. Pat. Nos. 4,660,024, 4,772,875, 4,833,450, 4,882,567, 5,077,548, 5,216,410, 5,276,427, 5,331,308, the contents of which are again incorporated by reference, show PIR detectors combined mainly with microwave detectors, such that the alarm is activated only when both individual detectors have been activated. All of the above-mentioned patents show PIR detectors which suffer from the above-mentioned problems.
Other Kinds of Detectors
As was explained above with regard to DUAL detectors comprising both PIR and MW, there are detectors which use sensors of vari

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