Method Of Monitoring Airport Runway End Identification Lamps

Abstract

A control device for a pair of flashing airport runway end identification lamps which automatically shuts down both lamps when either lamp ceases to flash, and thereby avoids the pilot confusion which could result from a single flashing lamp. This device monitors the surges of line current which flow due to flashing of the lamps and compares a signal, which varies according to the magnitude of the surges, with a threshold, and, if the comparison indicates that both lamps are not flashing, automatically disconnects the power to both lamps.

Description

BACKGROUND OF THE INVENTION

In runway end identification service, one capacitor-discharge lamp is generally installed on each side of the runway at the touchdown end and the two are timed to flash simultaneously at approximately half-second intervals by a motor-driven switch installed in one of the lamp housings. In such service it is desirable that both lamps be shut down if either fails to flash.

Many airports have, in addition to the runway end identification lamps, sequenced lights which help lead the pilot in landing. An airport lighting system using sequenced approach lights is described in U.S. Pat. No. 2,734,180, by W. L. Pennow, issued Feb. 7, 1956 entitled "Approach Light System." Typically, such systems have a monitor for detecting multiple lamp failures of the sequenced lights. This monitor cannot reasonably be adapted for detecting failures of the simultaneously flashing runway end identification lamps, however, as it monitors current within an individual lamp power supply rather than line current to a pair of lamps. To modify existing installations using this monitor, an additional wire would be needed across the runway. This monitor also lacks failsafe provisions.

It is, of course, quite desirable that a runway end identification lamp a control device, the failure of which could cause serious pilot confusion, should be failsafe. Failsafe, as used herein, means that if any one solid state device in the control device should fail by either opening or shorting, so as to prevent the control device from fulfilling its purpose, both lamps would be shut down.

SUMMARY OF THE INVENTION

A control device is disclosed for use with simultaneously flashing lamps which constitute part of an airport runway lighting system. This device would be operable to disconnect the flashing lamps when one of the flashing lamps fail.

The control device includes a means for detecting the surge current, a means for producing a prescribed threshold or other control value, a threshold switching device, or other comparator means, and a disconnecting means. The surge current detection means monitors surges of line current flowing from a power source directly or indirectly (via the lamp power supplies) to the flashing lamps and produces a signal which varies according to the magnitude of the surges of current. The surge current, signal and control value (threshold) are calibrated such that the control value is indicative of the flashing lamps operating properly, for example by the surge current signal being larger than the threshold when the lamps are operating properly and smaller than the threshold when one lamp fails. The threshold switching device (comparator means) receives the surge current signal and compares the surge current signal to the control value (threshold) and generates the disconnect condition when the surge current signal differs from the control value in a predetermined manner such as falling below the threshold. The disconnecting means is electrically connected between the power source and the flashing lamps (typically, between the power source and the power supplies of the lamps) and disconnects the flashing lamps (typically indirectly by deenergizing the power supplies) from the power source when the disconnecting means is actuated by the disconnect condition.

Preferably there is also provided means for remote indication of lamp status and as well as failsafe operation. Preferably there is also provided a means for starting operation such that the lamps are not prematurely shut down before the surge current signal has stabilized and also such that the control device can be reset by temporarily deenergizing the power source.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to the accompanying drawings.

FIG. 1 is a block diagram showing a preferred configuration of the control device and the relation of the control device to the lamps and the power source.

FIG. 2 is a schematic diagram showing a typical configuration of the control device and component values for that configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1 there is shown the relation of the control device with respect to the power source and the two flashing lamps. The control device is electrically connected between the power source and the power supplies for the capacitor discharge lamps. The device monitors the line current flowing from the source of power to the power supplies after the lamps are flashed. A flash timer controlling both flashing lamps is within the housing of one flashing lamp. The configuration of such flash timers is well known in the art.

FIG. 1 also shows the relationship of the preferred elements within the control device. The surge current detection means has a low impedance and is effectively in series with the lamps. The control device is designed to be energized from the same power source. With the use of a low impedance detector and by the use of the same power source, the control device need only be connected to the power lines and is therefore conveniently added to existing installations. The surge current detection means feeds a signal to the threshold device which, when the surge current signal is greater than the threshold, will keep the amplifier on. Thus, the output of the waveform generator is amplified and used to maintain the disconnecting means in a connected status. When the signal falls below the threshold, the threshold device turns off the amplifier. The lack of amplifier output is the disconnect condition which will, unless prevented by the starting means, cause the disconnecting means to disconnect the flashing lamp power supplies from the power source.

The set of electrical contacts operates simultaneously with the disconnecting means thereby facilitating remote indication of the status of the flashing lamps.

The starting means, when power first becomes available, causes the disconnecting means to connect the lamp power supplies to the power source, and to continue to be connected for a predetermined length of time.

Failsafe operation is provided by; generating the disconnect condition from the absence of an AC signal coupled through a capacitor to the gate of an SCR, the SCR controlling the flow of power from an AC supply to the actuating coil of the disconnecting means (the lack of AC signal causing the SCR to cease supplying half-wave power to the disconnecting means), and providing a fuse in series with the actuating coil of the disconnecting means and a diode in parallel with the actuating coil of the disconnecting means with the diode connected anode to anode with the SCR (the fuse being sized to blow when subjected to the half-wave signal without any series load when the SCR shorts). Failure of semiconductor devices in the circuitry prior to the capacitor (by going into saturation or by opening) will prevent coupling of the AC signal through the capacitor and thereby cause shutdown of the lamps. If the rectifier opens, there will be no DC for the disconnecting means. If the rectifier shorts, the fuse will blow.

Preferably the flashing lamps are capacitor discharge lamps, and the surge current is the line current which flows to the power supplies which recharge the capacitors after the lamps have been flashed, but the control device is applicable to other lamps, for example flashing incandescent lamps.

With reference to the values of FIG. 2, the schematic diagram, terminal L1 is connected to the hot side of the 120 or 240 volt, 60 cycle power line and L2 to the neutral. The power for the two flashing lamps comes into L1, through the primary of transformer T1 and through contacts 1K2 of relay K2 and out terminal 6 to the flashing lamps. The neutral also goes to the flashing lamps. Transformer T2 is connected across the line and provides power to operate the control device. The primary of T2 is tapped for 120 or 240 volt operation and serves also as an autotransformer for the 120 volt time-delay relay K1 (K1 comprises the starting means).

In principle, the surges of voltage of the secondary transformer T1 is analyzed and, if it is of an appropriate magnitude, the circuitry holds K2 (the disconnecting means) closed keeping the power on the flashing lamps. If one of the lamps fails to flash, the surges of voltage drop and the circuitry lets K2 drop out, shutting down the lamps. However, K2 has to be closed in some other way to start the system and this is achieved by time delay relay K1 which remains closed for 15 seconds after power is first applied. While K1 is closed, K2 is held closed (even if a disconnect condition is generated) and power is supplied to the lamps. This enables the surge current signal to build up and the circuitry to remove the disconnect condition if both of the lamps flash. Should one lamp fail to flash, the surge current signal from one lamp alone is not enough and any time after the first 15 seconds K2 can be opened and cut power to the lamps. Typically if a lamp fails during operation the power to the lamps will be shut off approximately 2 to 3 seconds after a lamp starts to miss flashes. K1 stays open and the system stays off. The control device can be reset by removing power from the line for approximately 15-20 seconds, thereby letting K1 cool off enough to close, and then reapplying power.

More particularly, as a result of the energy passing from current transformer T1 through diode D1 into the capacitor-resistor network C1-R1, the surge current signal is produced. To get a signal that will carry over from maximum to maximum, the pulsating AC from T1 is rectified by a diode D1 and charges capacitor C1 (T1, D1, C1, and R1 comprise the surge current detecting means). A steady AC is taken from the secondary of T2, the power transformer, via R2 and is reduced to a square wave with a peak-to-peak amplitude of approximately 1.2 volts by the back-to-back diodes D3 and D4. The DC across C1 is 10-12 volts and is added to the square wave so that the signal at the wiper arm of calibrating potentiometer R1, relative to ground, is positive DC with about 10 percent square wave ripple.

With no surge current signal at Q1, Q2, and Q3 have no input and are cut off. When the surge current signal rises above the 5.6 volt threshold of the zener diode D2, the excess over 5.6 volts appears across R3 and is applied to the base of Q1. When the excess reaches 0.6 volts, Q1 starts to conduct and this establishes a threshold in the system (the threshold of zener diode D2 and the 0.6 volt forward drop of Q1). R1 is calibrated so that this threshold occurs midway between the signal with one lamp flashing and the signal with two lamp flashing. The operating point is about 1.3 volts above the threshold and, to prevent Q1 from saturating, the emitter develops 1.3 volts of bias across R4 raising the emitter of Q1 so that the square wave on the signal will turn the transistor on and off. Capacitor C5 holds the bias on the emitter between the pulses of the emitter current.

When Q1 is conducting, current is draWn from the base of Q2, and Q2 turns on and off in synchronism with Q1. The resulting square wave at the collector of Q2 is coupled by capacitor C7 to the gate of SCR Q3. Thus Q3 is turned on during the half-cycles when the upper end of the secondary of T2 is positive with respect to ground. The coil of relay K2 is connected from this end of the secondary via fuse F2 to the anode of SCR. Q3 acts as a gated rectifier, rectifying the secondary voltage of T2 and feeding DC to the coil of K2. C4 serves to smooth pulsations in the DC across to coil of K2.

As long as surge current signal is above the control signal, the circuitry holds disconnecting means K2 closed. If the surge current signal falls below this control signal, as it does if one lamp stops flashing, Q1, Q2 and Q3 go back to a cut off state and K2 drops out.

This circuitry protects against semi-conductor device failure. If Q1 or Q2 shorts, the resulting DC of the collector of Q2 is blocked from the gate of Q3 by C7 which charges to the supply voltage and lets Q3 turn off. If Q3 should short, it will conduct on both positive and negative half-cycles, and, because the coil of K2 is bypassed by D7, a heavy current will flow on the negative half-cycles and blow fuse F2, and K2 will drop out. In general, any malfunction of a semiconductor component that would prevent the control device from serving its purpose will result in K2 dropping out and shutting down of the lamps.

While it is convenient to have the control device powered from the same power source, it is possible to supply the power for the control device from a separate power source, for example a 120 volt, 60 cycle lighting circuit. If the control device is supplied from a separate power source, however, it will be necessary to make special arrangements for resetting and initializing the circuit.

Alternative, though possibly less desirable, means are also possible for detecting the surge current and producing a signal which varies according to the magnitude of the surge current. A one-tenth ohm resistor, for example, could be placed in the line from the power source to the flashing lamps, and the signal across the resistor fed to an amplifier and then to an averaging circuit or a peak detection circuit to produce a surge current signal.

Alternative means could be used for failsafe operation. A transformer, rather than a capacitor could be used, for example, to couple the AC signal which exists when there is no shutdown condition.

Claims

What is claimed is:

1. A control device for simultaneously flashing lamps which constitute part of an airport runway lighting system, said device operable to disconnect said flashing lamps when one of said flashing lamps fail, said control device comprising:

a. surge current detection means which monitors surges of current flowing from a power source to said flashing lamps and produces a signal which varies according to the magnitude of said surges of current;

b. a means for producing a control value indicative of said flashing lamps operating properly;

c. comparator means which receives said surge current signal and compares said surge current signal to said control value and generates a disconnect condition when said surge current signal differs from said control value in a predetermined manner; and

d. disconnecting means which is electrically connected between said power source and said flashing lamps and which, when actuated by said disconnect condition, disconnects said flashing lamps from said power source.

2. The control device as specified in claim 1, wherein said control device is designed to be energized from said power source and said surge current detection means has a low impedance and is effectively in series with said lamps, whereby said control device can be conveniently added to existing installations, and wherein said control device is designed to be failsafe.

3. The control device as specified in claim 2, including a starting means which, when power first becomes available, causes said disconnecting means to connect said lamps to said power source, and for a predetermined time thereafter prevents said disconnect condition from actuating said disconnecting means.

4. The control device as specified in claim 3, wherein:

a. a set of electrical contacts which operate simultaneously with said disconnecting means is provided to facilitate remote indication of the status of said flashing lamps;

b. said flashing lamps are two capacitor discharge lamps; and

c. said surge current detector comprises a current transformer and said surge current signal is produced as a result of energy from said current transformer passing through a diode into a resistor-capacitor network.

5. The control device as specified in claim 3, wherein failsafe operation is provided by:

a. said disconnect condition being generated from the absence of an AC signal coupled through a capacitor to the gate of an SCR;

b. a flow of power from an AC supply to the actuating coil of said disconnecting means being controlled by said SCR; and

c. a fuse being provided in series with said actuating coil of said disconnecting means and a diode being provided in parallel with said actuating coil of said disconnecting means, which diode is connected anode to anode with said SCR, said fuse being sized to blow if said SCR shorts and subjects said fuse to half-wave current without any series load.