Abstract:
An apparatus and method adapted to detect a failure in at least one lamphead of an emergency lighting system includes a signal generator and a failure sensor. The lamphead receives power from a power connection. The signal generator generates a monitoring signal having a voltage, which is operatively coupled to the power connection. The failure sensor detects a failure in the lamphead and modifies the voltage of the monitoring signal. The emergency lighting system may also include a plurality of lampheads. In this case, the failure sensor may detect a failure in at least one of the lampheads, and identify which of the plurality of lampheads has failed. The failure sensor may also be adapted to reduce the voltage of the monitoring signal in response to detection of the failure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/860,357 filed Nov. 21, 2006, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a diagnostic circuit for use with emergency lighting. In particular, the present invention identifies failures in remotely located emergency lighting lampheads. 
   2. Description of the Prior Art 
   Emergency lighting is used in many types of facilities to provide DC battery-powered lighting in the event the main AC power supply is temporarily inoperative. Examples of these facilities include schools, hospitals, government offices, hotels, motels, industrial buildings, multiple unit dwellings, shopping malls, and airports. In many cases, these structures are very large and require that emergency lampheads be placed at several different locations to provide adequate coverage. Fire safety codes typically require that emergency lighting be tested periodically to ensure proper operation during an emergency. With a system employing many separate lampheads at scattered locations, these tests become labor intensive and time consuming. 
   Emergency lighting systems ordinarily include a battery to supply temporary power to one or more lampheads during an AC power loss, a charger for charging the battery from the AC power supply during standby operation, and a relay or other type of switching device for connecting the lampheads to the battery during loss of AC power. If diagnostic circuitry is provided in a central unit rather than in the remote lamphead, testing for proper battery and charger operation is facilitated. However, centrally located diagnostics make it difficult to check for proper operation of individual lampheads. Specifically, problems that can cause failure in particular lampheads include a defective, burned out, or improperly connected lamp or wire, which is difficult to detect from a central location. 
   In addition, remote lampheads that are connected to each other and the central battery and charging unit in a parallel “daisy chain” arrangement make it difficult for a diagnostic circuit located at the central unit to perform separate tests on each lamphead to identify the specific lamphead that requires service. Thus, if the central monitoring or diagnostic circuit merely shows that one of the lampheads is inoperable, but does not specify the identity or location of that lamphead, the system must be placed in emergency mode to visually identify the inoperable lamphead. 
   Determining operability at remote lampheads is also difficult since there is a need to minimize the number of lines between the remote lampheads and the central monitoring unit. Therefore, connecting additional wires between the remote lampheads and the central monitoring unit to support diagnostic functions adds to the cost of the system and increases the potential for additional failures. Further, the expense and complexity of diagnostic circuitry is ordinarily such that it is not practical to replicate the required circuitry at each remote lamphead. 
   Therefore, it is desirable to provide a diagnostic apparatus and method that can readily determine whether a failure exists in an emergency lighting system and can identify the particular lamphead that has failed. 
   SUMMARY OF THE INVENTION 
   An apparatus adapted to detect a failure in at least one lamphead of an emergency lighting system formed in accordance with one form of the present invention, which incorporates some of the preferred features, includes a signal generator and a failure sensor. The lamphead receives power from a power connection. The signal generator generates a monitoring signal having a voltage, which is operatively coupled to the power connection. The failure sensor is adapted to detect a failure in the at least one lamphead and modify the voltage of the monitoring signal. 
   The emergency lighting system may also include a plurality of lampheads. In this case, the failure sensor may be adapted to detect a failure in at least one of plurality of lampheads and identify which of the plurality of lampheads has failed. The failure sensor may also be adapted to reduce the voltage of the monitoring signal in response to detection of a failure in the lamphead. The apparatus is coupled to the power connection without additional wiring to indicate failure in the lamphead, and the power connection preferably includes only two power cables. 
   The monitoring signal may include a pulsed and/or time-varying signal, and the failure sensor may include a microcontroller. Thee failure sensor may be adapted to encode information associated with failure in the lamphead on a power signal. The monitoring signal may be filtered to reduce false indications of failure in the lamphead, and the apparatus may be adapted to operate with 6-24 volts. 
   A method adapted to detect a failure in at least one lamphead of an emergency lighting system formed in accordance with one form of the present invention, which incorporates some of the preferred features, wherein the lamphead receives power from a power connection, includes generating a monitoring signal including a voltage, coupling the monitoring signal operatively to the power connection, detecting a failure, and modifying the voltage of the monitoring signal in response to detection of the failure. 
   The method may include detecting a failure in at least one of plurality of lampheads, and identifying which of the plurality of lampheads has failed. The method may further include reducing the voltage of the monitoring signal in response to detection of the failure  14 . The method may include coupling the apparatus to the power connection without additional wiring to enable indication of failure in the lamphead. The power connection may include two power cables, and the monitoring signal may include a pulsed signal and/or time-varying signal. The failure sensor may include a microcontroller, and the method may include encoding information on a power signal associated with failure in the lamphead. The method may also include filtering the monitoring signal to reduce false indications of failure in the lamphead, and operating the apparatus with 6 volts to 24 volts. 
   These and other objects, features, and advantages of this invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are block diagrams of a failure detection system in accordance with the present invention adapted to be used in an emergency lighting system. 
       FIG. 2A  is a schematic diagram of a failure detection circuit adapted for use in an emergency lighting system. 
       FIGS. 2B and 2C  show waveforms of a monitoring signal in normal mode and lamp failure mode, respectively. 
       FIG. 3  is a simulation diagram of a circuit used for simulating the fault detection circuit shown in  FIG. 2A  with a plurality of lampheads. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to a method and apparatus adapted to detect lamphead failures in an emergency lighting system. Single lamp failures can be detected and associated with a particular lamphead in accordance with the present invention. The apparatus preferably requires only two power cables or connections that typically connect the battery unit and lamphead to transmit a failure signal without needing any additional wiring in case of a mains power failure. 
   Alternative methods of detecting failures in emergency lighting systems utilize impedance measurements of a multi-lamp circuit. A slightly higher value than the measured impedance value, such as 10% more than the measured value, is preferably stored in memory as a threshold. This threshold is then preferably compared by a microcontroller with future measurements of the impedance, which if higher trigger an alarm condition that indicates a failure in the particular multi-lamp circuit being tested. 
   The advantages of this approach include simplicity of operation; the ability to use standard, unaltered lampheads; and automatic calibration of impedance measurements. Unfortunately, this solution is only efficient for use with emergency lighting systems having a few lampheads. Further, this approach suffers from only a generalized lamp failure that cannot specify the particular lamphead or lampheads that have failed. If such an approach is used in large emergency lighting systems, the resulting lack of resolution in impedance changes may mean that several lampheads must fail before any one failure becomes detectable. 
   The apparatus formed in accordance with the present invention preferably incorporates a limited impedance signal generator, which is located on a battery charger board, and a lamp failure detection circuit located in each lamphead. When a lamphead failure occurs, the failed lamphead preferably pulls down or reduces the voltage of a pulsed or time-varying monitoring signal provided by the signal generator. The peak value of the monitoring signal is then preferably detected by a microcontroller, compared to the normal value of the monitoring signal, and used to indicate failure in at least one of the lampheads. 
   Thus, the apparatus only requires the two power wires ordinarily used to provide power to each of the lampheads and can preferably operate with 6-24 volts and any number of lampheads. The apparatus is also independent of lamp power or size. The microcontroller can also filter the monitoring signal to avoid false indications of failure, which may be caused by extraneous noise on the monitoring signal. The microcontroller preferably compares the monitoring signal to the normal value substantially continuously when the battery is being charged, periodically, and/or upon user request at other times. The failure detection circuit located in the lamphead preferably has a reduced footprint of less that about one (1) square inch by, for example, using surface mount technology to minimize impact on the retrofit of existing lampheads. 
   The type and/or quantity of information transmitted between the lampheads and battery charger may be augmented or enhanced by incorporating, for example, a microcontroller in the failure detection circuit located in the lamphead that encodes such information on the power lines. In this way, transmitted information may be tagged with the identity of the source of the information, which would substantially simplify the service of lampheads by enabling failed lampheads to be found without a time-consuming physical search of the facility. Likewise, information transmitted by, for example, the battery charger could be tagged with the identity of its source, intended recipient, and/or destination. 
     FIGS. 1A and 1B  are block diagrams of a failure detection system formed in accordance with the present invention and adapted for use in an emergency lighting system. A power signal  12  is preferably provided to a centrally located signal generation circuit  14  that is remote to the lampheads. A lamp fail sensing signal or monitoring signal  18  is provided to a microcontroller-based battery charger  20  that is also preferably located remotely from the individual lampheads. The microcontroller preferably controls a silicon controlled rectifier SCR 1  that provides gating for a battery V 1  to be charged by the power signal  12 . In transfer mode, when there is an AC power failure, switch SW 1  is preferably closed to provide battery backup power to the lamp  16  from the battery V 1 . 
   In accordance with the present invention, a failure detection circuit  22  shown in  FIG. 1B  is preferably substituted for the lamp  16  shown in  FIG. 1A  across nodes A and B in each of the lampheads. The failure detection circuit  22  preferably includes a fail sense circuit  24 , a gate drive circuit  26 , a MOSFET  28 , a resistor R 5 , and a transistor Q 3 . The fail sense circuit  24  preferably detects whether there is a lamp failure, for example, whether the lamp presents an open circuit. The gate drive circuit  26  preferably provides a gate driving signal to the MOSFET  28  with appropriate electrical characteristics. 
     FIG. 2  shows a more detailed schematic diagram of the failure detection circuit  10 , which is preferably located in each of the lampheads of an emergency lighting system. The power signal  12  is preferably provided by an AC signal generator  30  and is full-wave rectified by four (4) diodes D 1 -D 4  configured as a full-wave bridge circuit  32 . 
   The signal generation circuit  14  preferably includes resistors R 3 , R 4 , R 5 , R 71 , R 1 , diodes D 7 , D 12 , D 13 , and transistor Q 3 . Resistors R 71 , R 3 , and diodes D 12 , D 13 , are preferably electrically connected in series. Resistor R 1  is preferably connected from the power signal  12  to ground. Zener diode D 7  is preferably connected to a point between resistors R 71 , R 3  and ground. Resistor R 4  is connected to a point between resistor R 3  and the anode of diode D 12  and ground. Resistor R 5  is preferably connected to the collector of transistor Q 3 . The emitter of transistor Q 3  is connected to ground. 
   The fail sense circuit includes resistors R 6 , R 7 , R 8 , R 10 , transistor Q 4 , and diode D 11 . Resistors R 7 , R 8  are preferably connected in series between the base of transistor Q 3  and the collector of transistor Q 4 . The emitter of transistor Q 4  is preferably connected to resistor R 5 , and the base of transistor Q 4  is connected to resistor R 8 . The lamp  16  is preferably connected in series between diode D 13  and resistor R 8 . Diode D 11  is connected to a point between resistors R 6 , R 7  and a resistor R 10 . Resistor R 10  is preferably connected in series from the cathode of diode D 11  to ground. 
   Transistors Q 3  and Q 4  are preferably used to detect an open lamp circuit or failed lamp  16 . If the lamp  16  has failed, the monitoring signal will preferably have a peak voltage of about 1.2 V. If the lamp  16  is operable, the monitoring signal will preferably have a peak voltage of about 3.2 V. Transistor Q 4  is preferably implemented as a high gain transistor, such as 2N5087, which helps maintain the loading of the complete circuit high. A high gain transistor is able to work at a lower current and consequently higher resistance values can be used in the detection circuit. Transistor Q 3  is preferably configured as a generator pump. Diode D 11  is preferably used to stop transistor Q 3  from turning on in transfer mode when battery backup is being supplied to the lamp  16 . 
   The gate drive circuit  26  preferably includes transistors Q 1 , Q 2 , diodes D 6 , D 9 , D 10 , resistors R 11 , R 12 , R 13 , and capacitor C 2 . The anode of diode D 9  is connected to the cathode of diode D 13 , the anode of Zener diode D 10  is connected to resistors R 11 , R 12 ; and the cathodes of diode D 9  and Zener diode D 10  are preferably connected to each other. The emitter of transistor Q 1  is also connected to the cathode of diode D 13  and the anode of diode D 9 ; and the base of transistor Q 1  is connected to the collector of transistor Q 2  and the cathodes of diodes D 9 , D 10 . The collector of transistor Q 1  is connected to the base of transistor Q 2  and resistor R 13 . Resistors R 13 , R 12  are connected in series between the base of transistor Q 2  and ground. Resistor R 11  is connected in series between the emitter of transistor Q 2  and the gate of MOSFET Q 5 . Capacitor C 2  is connected from the gate of MOSFET Q 5  and ground. The cathode of Zener diode D 6  is connected to the gate of MOSFET Q 5 , and the anode of Zener diode is connected to ground. 
   It is desirable for the circuit shown in  FIG. 2  to exhibit good stability over a temperature range of about 0-100° C. The upper portion of this range is highly possible due to proximity of the circuit to the lamp  16 . Performance and stability of the circuit is improved if h fe  is stable by using transistors, such as 2N4403. Use of general purpose transistors, such as 2N3904, as transistors Q 2  and Q 4  exhibit poor performance at extreme temperatures due to variations in the gain (h fe ) of the transistor over temperature. 
   The MOSFET  28  is preferably chosen with V DS  about 60 V and current high enough to handle an inrush current from the lamp (I Dpulse ). Also, the zero voltage drain current (I DSS ) of the MOSFET is preferably chosen with the lowest possible value since this will sink current in standby mode, which will enable the highest number of lampheads to be used with the detection circuit. 
   In normal mode, when the lamp  16  is operational, a rectified 3.2-volt peak voltage (shown in  FIG. 2B ) is preferably present at node C as the monitoring signal. The microcontroller in the charging unit  20  (shown in  FIG. 1A ) preferably interprets this voltage as indicating that the lamp  16  is operational preferably using an analog-to-digital input. Transistor Q 4  is maintained in an off state by current flowing through resistor R 8  and the lamp  16 . No current is provided at the base of transistor Q 4  since voltage at the admitter of Q 4  is equal to voltage at the base of Q 4  in the off state. Therefore, transistor Q 3  is also in the off state. In addition, power MOSFET Q 5  and the associated drive circuitry  26  are off. 
   In the lamp failure mode, when the lamp  16  is not operational, transistor Q 4  is preferably turned on with base current from resistors R 8 , R 10  which provide connections to ground. This current is provided to the base of transistor Q 3 , which turns transistor Q 3  on as well. With current leaking through both transistors Q 3  and Q 4 , voltage provided at node C as the monitoring signal is reduced to no more than approximately 1.5 volts (shown as in  FIG. 2A ) which is substantially less than the peak voltage of the monitoring signal shown in  FIG. 2B  during the normal mode. The microcontroller in the charging unit  20  (shown in  FIG. 1A ) preferably interprets this voltage as indicating that the lamp  16  is not operational. 
   When the lamp  16  is operational and in transfer mode, that is, when battery backup is being provided by battery V 1 , power to the gate drive circuit  26  is preferably on through action of the current polarizing diode D 10 , which maintains the base current of transistor Q 1  by ensuring about 4.7V across the collector and emitter of transistor Q 2  forcing transistor Q 2  into saturation. Transistor Q 1  is preferably used as positive feedback from transistor Q 2 , which drives transistor Q 1  deeper into saturation. Thus, the gate drive circuit  26  will turn off when the base current to transistor Q 2  and its gain (h FE ) are too low. This preferably occurs at about 1.7V. Capacitor C 2  with resistor R 11  preferably smooth any effect of relay bounce to drives the MOSFET Q 5  with a ramp signal. Transistor Q 3  in the fail sense circuit  24  is preferably kept off with diode D 11 , which preferably drains current coming from transistor Q 4  as it is turned on. Failure of the lamp is preferably not detected while in transfer mode. 
     FIG. 3  is a schematic diagram of a circuit used for simulating the fault detection circuit  10  with a plurality of lampheads  34 . 
   Thus, the apparatus and method for failure detection formed in accordance with the present invention provides is able to detect and identify failure of individual lampheads in an emergency lighting system using only the wires ordinarily used to provide power to the lampheads. The apparatus and method is also able to operate with voltages from about 6-24V and is independent of lamp power or size. 
   Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.