Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-03-16
2001-08-28
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S666000, C200S08600A, C361S179000, C049S264000
Reexamination Certificate
active
06281803
ABSTRACT:
FIELD OF THE INVENTION
The disclosed invention relates to control mats used for presence detection in applications involving automatic pedestrian doors, and other situations. More particularly, it relates to the monitoring of control mat electrical integrity, providing warning of and compensating for deterioration of control mat performance.
BACKGROUND OF THE INVENTION
Control, or switch mats, which are typically used in such applications as actuating automatic pedestrian doors and providing safety areas around potentially dangerous machinery, function as simple electrical switches that operate under the weight of an object or person. The most common variety consists of a pair of flat parallel steel plates separated by an array of small, compressible, electrically insulating spacers, sealed within a tough vinyl jacket. Electrical leads are attached to the two plates. When an object or person of sufficient weight stands on the mat, the steel plates are forced into physical, and electrical, contact, thereby causing the electrical resistance between the mat leads to drop to a low value indicative of the presence of the person or object on the mat. When the object or person is removed from the mat, the contact between the steel plates is broken and the electrical resistance between the mat leads assumes an essentially infinite value indicative of the absence of sufficient weight, and, hence, the person or object, on the mat.
Control mats afford a number of distinct advantages over other methods of presence detection, such as microwave and infrared (IR) beam sensors. Unlike beam sensors, the detection area of a control mat is, generally, rectangular so that, in a pedestrian door application, control mats can provide detection coverage right up to the threshold and side-to-side to the door jambs. The detection area cannot be displaced by the sensor, in this case the mat, accidentally or intentionally being struck or tampered with, as can a beam sensor. Control mats are insensitive to variations of line voltage and, if proper interference suppression measures are employed, cannot be interfered with by nearby RF (radio frequency) transmitters or IR radiation from sunlight. Beam sensors, on the other hand, cannot be made completely immune to interference from RF and IR radiation because they use these media as their primary input. Also, control mats respond strictly to the weight of the person or object standing on them and they do not depend for detection upon any other special properties or characteristics, such as the state of motion or the microwave or IR reflectivity of the detected person or object, as do beam sensors.
Shortcomings of Control Mats
Control mats, while considered to be the most reliable known method for detecting the presence of a person or object (See U.S. Pat. Nos. 4,467,251, 4,560,912, 4,590,410 and 4,888,532), suffer from the serious disadvantage that they are electromechanical devices that are, unlike solid-state beam and other sensors, subject to physical wear and deterioration. Consequently, maintenance of the superior reliability of control mats for presence detection depends critically on careful mat maintenance and the replacement of mats whose electrical performance characteristics have deteriorated below acceptable limits (U.S. Patents, ibid). The ANSI (American National Standards Institute) A156.10 national standard for automatic pedestrian doors specifies an elaborate manual procedure for determining the functional integrity of control mats. However, this procedure is time-consuming, labor intensive and, furthermore, requires that the door be literally, or effectively, taken out of service while the test is being performed. As a practical result, the ANSI control mat test is not applied on a regular basis to maintain the safety of control mats used on automatic pedestrian doors. Often, the first time the ANSI test is applied to a mat is after an accident has already occurred due to severe mat performance deterioration.
Manufacturers of control mats and automatic pedestrian doors that use control mats specify that a Daily Safety Check be performed at least once a day to verify proper actuation of the door by the control mat. Unfortunately, the Daily Safety Check is far too crude a test to be able reliably to uncover subtle, yet important, changes in the electrical performance characteristics of a control mat. For example, mats often begin to fail by developing “bad spots” that do not respond as well as the unaffected areas of the mat to the presence of people and objects. Such bad spots often go undetected by the Daily Safety Check because the person performing the check does not verify proper actuation of the door from all areas of the mat. In fact, the instructions for performing the Daily Safety Check provided by manufacturers do not specify that all areas of the mat need to be checked. Moreover, aside from the failure to check all areas of the mat, the weight of the person making the test is very likely to exceed by a considerable margin the minimum weight that a properly functioning mat should be able to detect. Consequently, the mat may appear to be operating properly when the Daily Safety Check is performed, but, in fact, could already be in a severely deteriorated condition that could result in a serious failure and accident when a person of only slightly less weight steps onto the mat.
A further shortcoming associated with the use of control mats follows from the fact that the control electronics used with the mat to determine if a person or object is, or is not, present on the mat neither senses nor compensates for the possible deterioration of the electrical performance characteristics of the mat. Rather, in most cases, the control electronics “concludes” that there is a person or object on the mat when the mat resistance drops below some preestablished threshold value and, conversely, that there is nothing present on the mat when the mat resistance rises above the threshold value. This very simple algorithm tolerates some variation of the mat resistance. For example, the lower resistance value can rise above the ideal value of zero, and the upper resistance can fall below the ideal condition of an essential open circuit (infinite resistance). However, this algorithm is only tolerant of the variation; it neither senses the existence of the variation nor compensates for it. Consequently, the control electronics currently used with control mats incorporates no mechanism for detection and warning of the deteriorating condition of the mat to which it is attached. Likewise, the control electronics in current use with control mats exhibits no capacity to compensate for the deterioration of the mat so that the correct presence detection signal is generated even though the electrical performance characteristics of the mat have deteriorated significantly. The ability to compensate in this manner would afford an important margin of public safety by assuring that the door continues to be actuated properly and appropriately, even though the control mat is failing and would otherwise be incapable of proper operation of the door.
Accordingly, it is a purpose of the disclosed invention to provide a means for early detection of the deterioration of the electrical performance characteristics of control mats so that such mats can be identified and removed from service in a timely manner, thereby minimizing the possibility of accidents and injury. It is also a purpose of the disclosed invention to provide compensation for the deteriorating condition of control mats so that the correct presence detection signal is generated when the mat would otherwise be incapable of proper operation of the door.
PRINCIPLE OF OPERATION
When a control mat is functioning properly, the electrical resistance exhibited across the mat terminal leads is essentially infinite (open circuit condition) when there is no person or object on the mat. Conversely, when sufficient weight is placed on the mat—as when a person or object is on the mat—the electrical resistance exhibited across t
Hofsass Jeffery
La Anh
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