In the early 1990s, Mike Shuler of EdgeCom Inc. was tasked with developing a new standardized airfield lighting control system for the US Navy. Airfield lighting functions include runway and taxiway edge lights, approach lights, strobe lights, PAPIs, helipads and many more. The Navy has approximately 50 air stations worldwide and none are alike regarding their airfield layouts, lighting equipment and control configurations.
Constant current regulators, high voltage switches, and other remotely controllable lighting equipment associated with each airfield lighting control function are typically located inside various lighting vaults distributed around the airfield. Therefore, the new control system had to be flexible enough to handle many different control configurations – not only at each airfield, but at each lighting vault.
As part of the new system, a Vault Control Unit (VCU) would be designed, built and installed inside each lighting vault to implement these control functions remotely from the ATC Tower. Each VCU would contain:
- Data communications equipment to interface with the airfield lighting computer in the ATC Tower
- Discrete I/O modules to implement control commands from the lighting computer in the ATC Tower
- Remotely controllable switches and relays to interface directly with the lighting equipment for on/off and brightness controls.
The switches and relays in each VCU would be inherently latching, so that loss of control power or data communications between the ATC Tower and the lighting vault(s) would not result in the airfield lights suddenly dropping out. Finally, the new VCUs would be required to provide manual backup controls for all the airfield lighting functions in each vault.
Mr. Shuler developed a rotary stepping switch assembly for use in the VCU that utilizes a bi-directional solenoid from NSF Controls Ltd as a key part of its design.
The rotary switch portion of the assembly features six isolated ganged 6-position control circuits. Four of these six control circuits are the non-shorting type (break-before-make), and the other two are the shorting type (make-before-break), offering a multitude of control options to implement the various existing control requirements at all 50 air stations.
This bi-directional rotary stepping switch assembly can probably best be described as a “multi-pole, multi-position, manually overridable latching relay.”
The switch assembly consists mainly of a rotary switch, a bi-directional rotary stepping solenoid, and a connectorized 7-foot-long control cable. A flexible shaft coupling connects the ¼” rotary switch shaft to the bi-directional rotary stepping solenoid shaft. The 50-conductor cable extends all of the rotary switch contacts and the solenoid control leads to a 50-pin connector, which mates with a DIN-rail mounted 50-pin breakout module inside the VCU. The breakout module provides screw terminals for control cables/conductors for local lighting equipment external to the VCU (e.g., constant current regulators, strobe controllers, contactors, etc.). The CW and CCW solenoids are pulsed from output modules inside the VCU to sequentially increase or decrease the intensity of specific airfield lighting functions on the airfield. The pluggable nature of the stepping switch assembly allows for quick and easy troubleshooting and replacement.
Each new VCU typically has one or more rotary stepping switch assemblies mounted to the interior side of the VCU door panels. The free ends of the solenoid shafts extend through the door panel where manual control knobs are attached. By manually operating these control knobs, the maintenance tech can independently control each of the lighting functions connected to that VCU.
Regardless of whether the switch assembly is remotely operated from the ATC Tower or manually operated via the control knob on the door panel, the same control circuits in the rotary switch are being switched to control the airfield lighting functions. This feature alone obviated the need for banks of transfer relays that were required in the old system to transfer control from the ATC Tower to the lighting vault. Truly, some of the old lighting vaults had walls full of transfer relay cabinets – you could almost feel the building shake when control was transferred between the ATC Tower and the lighting vault!
It should be noted that due to the obsolescence of many of the components used in this “new” 30-year-old airfield lighting system, the Navy is currently in the process of installing their newest system design for airfield lighting control. One of the few components that has been carried over from the previous system to the newest system is the rotary stepping switch assembly, built by EdgeCom with rotary solenoids from NSF Controls, LTD.