Abstract:
A Style A PAPI system comprises a plurality of identical lamp housing assemblies, each with one or more lamps and each housing assembly including a tilt circuit. A master control includes a microcontroller that supplies an inverted 1 KHz, 50% duty cycle pulse signal to each tilt circuit through a corresponding blocking diode. Each of the tilt circuits includes an optical pendulum tilt switch that responds to tilting of the housing assembly. DC power is developed in each tilt circuit from one phase of the pulse signal. A pair of steering diodes and a voltage divider network enable return pulses to be supplied back to the microcontroller via the same wires that deliver the pulse train. The returned pulses exhibit different voltages corresponding to tilt and no tilt conditions of the tilt switch. Visual indications of the status of each tilt circuit are presented at the tilt circuits and at the master control. The microcontroller disables all of the lamps in the system unless a no tilt voltage is received from every lamp housing assembly.

Description:
BACKGROUND OF THE INVENTION AND PRIOR ART 
   The present invention relates generally to Precision Approach Path Indicator (PAPI) visual guidance systems for aiding pilots in landing an aircraft. Specifically, the PAPI system defines a desired vertical approach angle to a runway and indicates to the pilot, via colored lights, whether the angle of approach of his aircraft to the runway is within the desired approach angle. The colored lights are produced in a number of Lamp Housing Assemblies (LHAs), as will be described below. The Federal Aviation Administration (FAA) establishes the standards for PAPI systems in the United States, whereas the standards for foreign PAPI systems may differ. Therefore it should be understood that, while the present invention is described with respect to the FAA endorsed systems, its application should not be considered as limited to FAA endorsed systems. 
   The components in an FAA Style B PAPI system are powered by the well-known and widely used constant alternating current (AC) loop employed in most of the world&#39;s airport lighting systems, whereas the components in an FAA Style A PAPI system are powered in parallel, directly from utility line power. In any PAPI system there are a number of important considerations, among them being: power consumption; number and type of lamps; size of the Lamp Housing Assemblies (LHAs); system reliability; ease of installation and service; safety with respect to exposed wiring and high voltages; ease of detection and identification of lamp/housing problems; environmental impact of components used; and minimization of the number of wires and interconnections. 
   The PAPI system generally comprises an array of two or more LHAs, each of which may contain two or three individual lamps. The LHAs are located adjacent the side of a runway and precisely aimed to define a correct vertical approach angle for guiding an incoming aircraft. Generally, each LHA is fitted with an optical filter to present a white light when the aircraft is too high, i.e., above the correct approach angle, and a red light when the aircraft is too low or below the correct approach angle. When the aircraft is too high, all of the LHAs are seen as white lights, when the aircraft is too low, all of the LHAs are seen as red lights and when the aircraft is within the correct approach angle, one-half of the LHAs in the array present a white light and one-half present a red light. The PAPI system usually comprises either two LHAs or four LHAs, with each LHA having either two lamps or three lamps. A two LHA system will therefore show: two red lights when the aircraft is too low; one white and one red light for a correct approach; and two white lights when the aircraft is too high. A four LHA system will also indicate intermediate positions within the correct approach angle. Thus the light indications will be: four red for too low; one white and three red for slightly low; two white and two red for correct approach angle; three white and one red for slightly high; and four white for too high. 
   The PAPI system also includes a tilt detection arrangement and tilt switch control circuitry for disabling the entire LHA array should the physical attitude or position of any of the LHAs be displaced by a predetermined amount for a predetermined time. This is necessary since the color of the light seen by the pilot could be erroneous and create a potentially hazardous situation should an LHA position be disturbed sufficiently to change its aiming. The choice of PAPI system selected is determined by a number of factors, such as airport size and location, traffic density, economics and the like. For example, some airport installations use multiple PAPI systems located at differing distances (touch down points) along the runway to accommodate aircraft having different landing requirements. The present invention is useful in all PAPI Style A systems. 
   Current state-of-the-art FAA Style A PAPIs supplied by Multi Electric Mfg. Inc., the assignee of the present invention, include two or more LHAs, each of which is mounted above ground adjacent to the runway and connected to a master control via underground conduit or buried cabling. It will be appreciated that a minimum number of wires and connection points in the PAPI system is a desirable objective with respect to cost, installation and maintenance. Also, it is desirable to minimize the amount of above-ground equipment to avoid damage to or from vehicles and aircraft. The LHAs are connected through break-away type connectors that are designed to readily separate in the event of contact with a moving object or vehicle. The connectors are also poled to minimize exposure of high voltages in the event of a connector separation. 
   The system of the invention uses optical pendulum tilt switches that require a power connection for a light-emitting diode (LED) and a signal connection for a photo transistor detector which, but for the invention, would undesirably add to the number of wires and connections. The arrangement provides a visual indication of a tilt condition, identifying the particular LHA or LHAs involved, at both the master control and the LHA, reduces system cost and complexity and enhances system manufacture, installation and service. 
   SUMMARY OF THE INVENTION 
   The present invention PAPI Style A system is characterized by a microprocessor controlled optical pendulum tilt switch arrangement having a multiplexed power and signal line resulting in a minimal number of wires. 
   OBJECTS OF THE INVENTION 
   A principle object of the invention is to provide a novel PAPI Style A system. 
   Another object of the invention is to provide a PAPI Style A system of greater simplicity and lower cost than the prior art. 
   A feature of the invention is the provision of a tilt arrangement incorporating a multiplexed power and signal line in a PAPI Style A system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects features and advantages of the invention will be apparent upon reading the following description in conjunction with the drawings in which: 
       FIG. 1  is a block diagram of a PAPI Style A system incorporating the invention; 
       FIG. 2  is a circuit diagram of the tilt arrangement portion of  FIG. 1 ; 
       FIG. 3  illustrates the pulse voltage developed at terminal A; and 
       FIG. 4  illustrates the voltages produced at terminal B under “tilt” and “no tilt” conditions. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a PAPI Style A system  10 , constructed in accordance with the invention, includes four identical LHAs  11 ,  12 ,  13 , and  14 . Each LHA includes a plurality of lamps and suitable lenses (not shown) for displaying either a white light or a red light, depending upon the vertical viewing angle, to an approaching aircraft, as discussed above. A power transformer  15 , having a center-tapped secondary winding and a multiple tap primary winding, is connected to a conventional AC source of power through a block  16 , labeled Relays. The lamps in the LHAs are powered from the secondary winding of transformer  15  and may have their intensity adjusted, and be turned on and off by operation of one or more relays (not shown) in block  16 . Details of these operations are not disclosed as they have no bearing on the present invention. 
   A master control, indicated by the components within the dotted-line box  20 , includes a DC power supply  21  that is energized from block  16 . Power supply  21 , in turn, supplies a block  22  for producing an unregulated 12 volts output V 1 , a block  23  for producing a regulated 12 volts output V 2  and a block  24  for producing a regulated 5 volts logic voltage output V 3 . A microcontroller  25  is energized by voltage V 3  (at pin  14 ) and controlled by an oscillator  26  (at pins  15 , 16 ). Microcontroller  25  has an output ( 10 ) coupled to a time division multiplex arrangement (TDM)  30 , four outputs ( 6 - 9 ) for controlling red LEDs  27 , an output ( 12 ) for controlling a green LED  28  and an output ( 13 ), coupled to block  16 , for turning the LHAs on and off. TDM  30  provides four inputs to microcontroller  25  (at pins  1 , 2  and  17 , 18 ) and four outputs, one each to four tilt circuits  31 ,  32 ,  33  and  34 . A tilt circuit is included in an associated one of the LHAs as indicated by the dashed lines. 
   A tilt condition results (and a tilt indication is generated) when any of the LHAs in the system experiences a predetermined magnitude of change in its physical orientation. The criteria for generating a tilt indication may be established by the airport operator or other controlling authority so that temporary disruptions of orientation due, for example to strong wind gusts or the like, do not result in disablement of the PAPI system. Since a physically displaced LHA may yield erroneous information as to the desired descent angle for an incoming airplane, all of the lamps are disabled in response to a tilt condition occurring in any of the LHAs. The system is automatic and should a tilt condition be removed, operation of the LHAs returns to normal. 
   Referring to  FIG. 2 , the LEDs are shown as red LEDs  27   a - 27   d  and a green LED  28  that are resistively connected to pins  6 - 9  and  12 , respectively, of microcontroller  25 . Regulated 12 volts DC voltage V 2  is supplied to one end of resistors  41  and  44  and to the collector of a switching transistor  45 , the emitter of which feeds a terminal A. The other end of resistor  44  is connected to: the base of transistor  45 ; the collector of a step up and inverter transistor  48 ; and the anode of a diode  43 . The cathode of diode  43  is connected to the other end of resistor  41  and to circuit ground, through a capacitor  42 . It will be observed that the circuit grounds of the system are isolated from the earth ground as indicated by the different symbols used. Unless otherwise noted, the term ground refers to a circuit ground. 
   The base of transistor  48  is connected to the junction of a pair of resistors  46  and  47 , with the other end of resistor  46  being connected to pin  10  of microcontroller  25  and the other end of resistor  47  being connected to the emitter of transistor  48  and to ground. Microcontroller  25  generates a 1 KHz, 5 volts DC, 50% duty cycle pulse train at pin  10 , which is stepped up to 12 volts DC and inverted by transistor  48 . Transistor  48  causes transistor  45  to conduct, thus producing a corresponding pulse train at terminal A (as best seen in  FIG. 3 ). 
   Terminal A is coupled to the anodes of blocking diodes  49 ,  50 ,  51  and  52 , the cathodes of which are connected to tilt circuits  31 ,  32 ,  33  and  34 , respectively. Since the individual circuits in time division multiplex arrangement  20  that supplies the tilt circuits  31 - 34  are identical, only the first will be described in detail, it being understood that the discussion applies to the others as well. The cathode of blocking diode  49  is connected to a terminal B to which is also connected one terminal of a Transorb  53  and a resistor  54 . The other terminal of Transorb  53  is connected to earth ground and the other terminal of resistor  54  is connected to ground through a resistor  55 . The junction of resistors  54  and  55  is connected to pin  2  on microcontroller  25  via a resistor  56 . Transorb  53  serves to suppress transient voltages, and the combination of resistors  54 - 56  form a voltage divider arrangement for converting the potential at terminal B to logic values for application to microcontroller  25 . It will be appreciated that the Transorbs and voltage divider networks for the other tilt circuits are connected to corresponding pins of microcontroller  25 . 
   Terminal B is connected a pair of oppositely poled steering diodes in tilt circuit  31 , i.e., to the junction of the anode of a diode  70  and the cathode of a diode  71 . The cathode of diode  70  is connected to: a bypass resistor  72 ; the anode of a green light-emitting LED  74 ; the collector of a tilt sensor photo detector transistor  76 ; a current limiting resistor  78 ; and, through a relatively large ( 101   f ) filter capacitor  80 , to ground. The other end of bypass resistor  72  is connected to the junction of the anode of diode  71 , the emitter of a driver transistor  75  and, through a resistor  73 , to ground. Bypass resistor  72  and resistor  73  will be seen to form a voltage divider network between the cathode of steering diode  70  and ground. The other terminal of LED  74  is connected to the collector of driver transistor  75 . An LED  79  is coupled between the other end of current limiting resistor  78  and ground. A slotted pendulum  77 , which together with LED  79  and photo detector transistor  76  forms a tilt switch, is pivotally secured to its associated LHA  11 . The slot in pendulum  77  normally permits light from LED  79  to pass and impinge on photo detector transistor  76 , causing it to conduct. It will be appreciated that LED  79  and photo detector transistor  76  are securely affixed to LHA  11  and therefore move along with the LHA, whereas pendulum  77  swings on the LHA to maintain its vertical orientation. Should the LHA move sufficiently, the light beam between LED  79  and photo detector transistor  76  will no longer pass through the slot in pendulum  77 , causing photo detector transistor  76  to cease conduction. 
   Thus the optical pendulum tilt switch develops an indication whenever the physical attitude of the LHA has been altered by a predetermined amount, i.e., to the extent that it no longer can be relied upon to accurately define the correct vertical approach angle. This change, signifying the possible occurrence of a tilt condition, is sensed by the microcontroller, which after a predetermined time interval that is selected to minimize spurious operations, verifies that a tilt condition exists and generates a signal to disable all of the lamps in all of the LHAs 
   Under normal operating conditions, when LHA  11  is not in a tilt condition, transistor  75  is conducting and green LED  74  is illuminated. The same holds true for the other LHAs  12 - 14  and their corresponding tilt circuits  32 - 34 . Also, green LED  28  in master control  20  is illuminated (from pin  12  of microcontroller  25 ) when none of the LHAs are in a tilt condition. Therefore, the master control  20  and the individual LHAs illuminate a green LED when the PAPI system is normal, i.e., not in tilt. As will be seen, should an LHA experience a tilt condition, the green LED in the master control and in the LHA in tilt will be extinguished and the red LED in the master control corresponding to the LHA in tilt will be illuminated, thus simplifying servicing of the system. Simultaneous tilt of more than one LHA results in extinguishing the green LED in the master control and the affected LHAs and illumination of the corresponding red LEDs in the master control. Master control  20  disables the lamps in all of the LHAs when microcontroller  25  detects a tilt condition in one or more of the LHAs and also whenever the appropriate voltages from the tilt circuits are not returned, regardless of the cause. Thus, a broken wire or a defective tilt circuit will result in disabling all of the LHAs. Thus the PAPI system is constantly supervised, which is an important safety feature. 
   Tilt circuit operation will be described for tilt circuit  31 , it being understood that the other tilt circuits  32 - 34  operate in a similar manner. The stepped up and inverted 1 KHz, 50% duty cycle pulse from microcontroller  25  at terminal A passes through blocking diode  49  and appears at terminal B, to which tilt circuit  31  is connected. Current from this pulse passes through diode  70  and “pump” charges capacitor  80  to approximately 12 volts DC. Since LED  79  and current limiting resistor  78  are connected in parallel with capacitor  80 , LED  79  is energized and emits light as long as the system is powered on and capacitor  80  is charged. In a no tilt condition, this light passes through the slot in pendulum  77  and impinges on photo detector transistor  76 , causing it to conduct. Driver transistor  75  is activated causing the illumination of green LED  74 . Conduction of driver transistor  75  diverts current from bypass resistor  72  causing the voltage at the anode of diode  71  (terminal B) to rise to the voltage across capacitor  80 . (In a tilt condition, transistor  75  is cut off and the voltage at the junction of bypass resistor  72  and resistor  73  is much lower.) 
   Each time the inverted pulse voltage at terminal A swings low, the voltage at terminal B (the anode of diode  71 ) is sampled by microcontroller  25 . (When the inverted pulse voltage at terminal A swings high, the anode voltage of diode  71  is higher than its cathode voltage and it is reverse biased. Blocking diode  49  prevents the incoming pulses from TDM  30  in the master control from affecting the pulses sampled by microcontroller  25 . It will also be appreciated that the signals returned to microcontroller  25  are divided down to a 5 volts DC logic scale by the voltage divider arrangement comprising resistors  54  and  55 . Since the pulses at terminal A are 180° out of phase with the pulses sampled by microcontroller  25 , when microcontroller  25  outputs the TDM signal (at its pin  10 ) it checks the incoming signals (at pins  1 , 2  and  17 , 18 ). Should a logic low return signal be received at one or more of the pins (for the predetermined amount of time) microcontroller  25  illuminates red LED  27   a  (via pin  6 ), disables green LED  28  (via pin  12 ) and sends a signal to block  16  (via pin  13 ) to operate the relays for disabling the lamps in all of the LHAs. Each of the other tilt circuits operates in the same manner and each is simultaneously monitored for a tilt condition, with a tilt signal in any of the LHAs resulting in all of the lamps in all of the LHAs being extinguished. 
     FIG. 3  shows the waveform at terminal A and  FIG. 4  illustrates the waveforms at terminal B, for both a tilt and a no tilt condition. Bear in mind that these waveforms are displaced by 18° from the TDM signal (not shown). T 1  and T 2  indicate successive periods of the inverted 1 KHz, 50% duty cycle pulse train from microcomputer  25 . Because of the inversion, period T 2  corresponds to the positive cycle of the TDM signal from pin  10  of microcomputer  25 . During interval T 2 , microcontroller  25  samples the voltage at terminal B. The waveforms indicate a logic low (indicative of a tilt condition) of approximately 0 to 1 volts, and a logic high (indicative of a no tilt condition) of around 5 to 6 volts. 
   To recapitulate, for a normal no tilt condition, transistor  76  conducts, effectively shorting out bypass resistor  72 , thus causing diode  71  to conduct, raising the voltage on terminal B to approximately the voltage across capacitor  80  and reverse biasing diode  49 . This high voltage is converted to logic levels by voltage divider action and detected as a logic high by microprocessor  25 . For a tilt condition, transistor  76  is switched off allowing the voltage divider network; consisting of bypass resistor  72  and resistor  73 , to place a lower voltage on the anode of diode  71 . Diode  49  is still reverse biased and this lower voltage, when converted to logic levels, is detected as a logic low by microprocessor  25 . Note that if the tilt circuit in the LHA should be disconnected the voltage from diode  71  will not be present and a logic low will be detected by microcontroller  25 . 
   What has been described is a novel PAPI Style A system of improved cost and efficiency and that is more readily and accurately installed and maintained. It is recognized that numerous changes in the described embodiment of the invention will occur to those skilled in the art, without the need for inventive skill. Therefore, the scope of the invention is to be limited only as defined in the claims.