Emergency lighting test system and method

Electric lamp and discharge devices: systems – Automatic substitution of the power supply

Reexamination Certificate

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C315S129000

Reexamination Certificate

active

06285132

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related in general to the field of emergency lighting and, in particular, to systems and methods for self testing emergency lighting systems.
2. Description of the Related Art
Emergency lighting is required by most safety codes in the United States. Emergency lights provide temporary lighting in the event of a power failure. During normal operation, power is provided from power mains to operate the lamp and to charge a backup power source (e.g. a battery). When power from the mains is interrupted, the backup power source provides power to the lamp for a limited time (typically 90 minutes).
It is desirable to test emergency lights periodically to ensure proper operation. A typical prior art self test is initiated by a person pressing a button or switch on the lighting unit. A simple voltage and/or current test is performed and a light or buzzer is activated if the test fails.
There are several problems with the prior art. One problem is that safety codes typically require a brief (e.g. 30 second) test every month and a longer (e.g. 90 minute) test each year. The prior art requires a person to initiate, monitor and manually record each of these tests. Consequently, testing is easily neglected, records of the tests are easily lost and costs for personnel to do the testing and track the testing are incurred.
Other problems are associated with self testing of lighting systems which use fluorescent lamps. Fluorescent lamps differ from non-fluorescent lamps in that a high frequency signal is needed to power them. During normal operation AC line power is converted to a higher frequency to efficiently power the fluorescent lamp. When the main power source is interrupted, DC power from the backup power source is converted into a high frequency signal to power the fluorescent lamp.
A problem with the prior art is that only the backup power source and the fluorescent lamp are tested. Other parts and circuits may not be directly tested. This is due to the simple methods used in the prior art to perform the self test. For example, the prior art typically tests the backup power source by simply disconnecting the main power from the system and measuring the voltage across the backup power source. If the voltage drops below a predetermined limit a failure light or LED is activated. This self test method does not detect other failures in the system.
Similarly, the prior art tests fluorescent lamps by simply sensing the current to the lamp. This is done by measuring the voltage across a resistive element and computing current using Ohm s law. There is a correlation between current into a fluorescent lamp and proper lamp operation. If the current is outside of predetermined limits the lamp is determined to be failed and an LED is activated.
Some of the problems with this technique are that a resistive element introduces power loss, interferes with normal operation of the circuit, may require a high tolerance resistive element, may require a high resolution analog to digital (A/D) converter, is unlikely to detect problems in other parts of the system and can fail to detect a failed lamp under some conditions. For example, in the prior art, it is possible that the inverter circuit may consume enough current to “fool” a controller into determining that the lamp and inverter are operating normally when, in fact, either or both are not working properly.
Examples of prior art testing techniques and deficiencies are found in U.S. Pat. No. 5,666,029, issued Sep. 9, 1997, to McDonnell, which is incorporated herein by reference.
Clearly there exists the need for an improved emergency lighting test system and method which automatically perform lighting system tests, automatically keep records of testing, do not require high tolerance components, do not require high resolution A/D converters, are a simple design and provide thorough self testing.
BRIEF SUMMARY OF THE INVENTION
The invention discloses an emergency lighting test system and method for automatic scheduling and testing of emergency lighting systems. A typical emergency lighting system comprises a lamp and a backup power source (e.g., a battery). The invention provides a microcontroller (i.e., the controller) which controls testing of the lighting system and automatically schedules tests of the lighting system. Automatic self testing reduces costs, keeps the lighting system in compliance with safety codes and automatically keeps records of the tests. The invention also uses two novel diagnostic test methods to evaluate the condition of fluorescent light type emergency lighting systems.
Fluorescent light type emergency lighting systems are unique in that an inverter circuit is required to convert DC power from the backup power source into a high frequency power signal. The two novel diagnostic tests use this high frequency signal to thoroughly test the system.
The first novel test method evaluates the condition of the backup power source and other parts of the system circuitry. This test functions as follows. The main power source is disconnected so that the backup power source provides power for the lamp. The high frequency signal generated by the inverter circuit is over sampled. Over sampling requires sampling a signal at a sufficiently high rate to successfully determine or extract desired data. The preferred embodiment uses an 8 bit A/D converter to sample the high frequency signal at four times the nominal frequency of the high frequency signal. The controller analyzes the samples taken by the A/D converter and determines a peak voltage for each of multiple predetermined time periods. The resulting multiple peak voltages are averaged to determine an average peak voltage.
The average peak voltage from the inverter is representative of the voltage of the backup power source. If the average peak voltage falls below a predetermined lower limit, it indicates a backup power source failure or a circuit problem. The lamp is turned off, a failure is flagged and a battery fail LED is activated.
The second test evaluates the condition of the lamp and the inverter. This test functions as follows. The main power is disconnected and the backup power source is providing power for the lamp. The high frequency signal is conditioned for interfacing with the microcontroller. In the preferred embodiment, a filter shapes the signal into a square wave and reduces the voltage such that the filtered signal can be interfaced with a digital circuit. The filtered signal is input to a counter which reduces the raw frequency by 1024. The filtered and reduced signal is input to the interrupt of the microcontroller which counts the interrupts and determines the frequency.
A frequency outside of a predetermined range indicates a failed lamp or failing inverter circuit. A frequency of zero indicates a probable failed inverter circuit. By using this testing technique, it is possible to better determine if the inverter or lamp has failed. The lamp is turned off, a failure is flagged and a frequency fail LED is activated. A lamp failure is predicted by the controller as the frequency trends toward the limits of the predetermined range. A predictive warning status is flagged or stored into memory.
Therefore, an object of the invention is to provide an improved system and method for testing emergency lighting systems.
A feature of the invention is a controller having current time of day and date data and a testing schedule.
Another feature of the invention is a storage means for recording test report data.
Another feature of the invention is an A/D converter sampling the high frequency output of an inverter circuit.
Another feature of the invention is a controller capable of computing a maximum voltage using samples from the A/D converter.
Another feature of the invention is a controller determining the condition of a fluorescent lamp as a function of the frequency of the inverter output.
Advantages of the invention include reduced operating cost, reliable scheduling of tests and reliable record keeping of t

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