A Boeing 747-400 is 232 ft. long, has a wing span of 212 ft. and a tail height of 64 ft.. It weighs close to 875,000 lbs. at takeoff. The Superbay Maintenance Hangar at the San Francisco International Airport can accommodate up to four of these massive airplanes at one time. The hangar has eight doors—four on each opposite sides—allowing access to this huge space measuring 500 x 540 ft, about the size of two football fields.
Each hangar door (Figure 1) consists of two inner and two outer doors, each measuring 130 ft wide and 90 ft high to allow the planes to be towed in and pushed out using tow vehicles. Each door is made of two halves, each 65-ft wide, and weighs 74,000 lb. The door sections are mounted on dedicated rails (much like a train track), and the rails are offset so that adjacent doors can open and close without interfering with their neighboring door.
MicroSmart Pentra PLCs from IDEC (www.idec.com) are almost lost in this huge structure, yet they provide vital control and monitoring of the doors to allow the coming and going of airplanes as large as a 747-400s.
An approaching train activates the advance detector loop sensor, which sends a signal to the PLC. The PLC delays for 15-25 seconds, depending on the time before the train is due to arrive at the intersection (Figure 2), which is a function of the train’s speed and the sensor’s distance from the intersection. The PLC instructs the traffic light controller to stop vehicular traffic through the intersection. As soon as the train hits the release loop sensor on the other side of the intersection, the PLC instructs the traffic signal controller to resume normal operations.
Pilot Construction Management (www.pilot-construction.com ) in San Francisco was the general contractor in charge of retrofitting and modernizing the hangar doors, which is used by American Airlines and United Airlines for A-Check and B-Check airplane maintenance, as well as engine and flap replacement.
A major part of the retrofit was upgrading the door controls in each hangar, including the eight doors, controls, motors and gear drives. The previous control system consisted of multiple relays, contactors, timers, antiquated power track systems, and miles of wiring, requiring continuous maintenance, repair and upkeep.
The doors were constructed in 1969 using relays, contactors and chain drives to control the speed of door travel. The components were so old that parts for repair were scarce or non-existent. The power panel and power track were so old that their components were discontinued over 20 years ago, and airport personnel had to actually manufacture these components just to keep the doors running (Table).
When this project started, the airport’s engineers saw an opportunity to introduce new technology and bring the drives and the controls into the 21st century. Also, because of the new upgrades, the function of the hangar was re-evaluated and additional aircraft maintenance work, such as weigh and measure procedures that could not previously be performed, were added to the function of this hangar.
“Weigh and Measure” is one of these additional special tasks on the periodic maintenance schedule of the aircraft. It requires all doors to be closed and almost zero air movement within the hangar. Although the doors are often left open in pleasant San Francisco weather, all doors must be closed when weighing an airplane because any wind will create lift on the wings and affect the weight of the aircraft. Closing all eight doors manually can be quite a task for a single operator, but automation greatly simplifies this work.
Engineers from San Francisco International Airport’s Design and Construction Department attended one of IDEC’s three-day PLC Programming classes in Sunnyvale, CA, and selected IDEC PLCs for the project because of their simplicity, capability and expandability.
The airport engineers designed the control system and logic themselves, while Pilot Construction was in charge of the electrical contractors, control contractors, mechanical contractors, precision removal of all old equipment, installation of new wheels, structural upgrade on doors, and all other renovation tasks.
Pilot called upon Jensen Instrument Company (www.jenseninstrument.com), a very reputable distributor and systems integrator in San Mateo, California, to provide the programming and integration solution for the PLCs and HMIs to meet SFO’s requirements for the project.
Each door is comprised of two halves, and is driven by two sets of drives. Each drive is controlled by one Schneider variable frequency motor drive (VFD, Figure 3). Each drive is capable of driving the entire door; however, the airport engineers required redundancy of drives for each door. To further increase reliability, the airport engineers borrowed the quick replacement concept from NASCAR, and designed a “Drive Cart.” Using a Meltric DSN20 Decontactor (a quick disconnect electrical coupling for the 480 volt power system), an engineer can replace the motor and gear drive in less than 10 minutes should there be any need to replace a drive.
Figure 3: Schneider VFDs working in tandem move the 70,000-lb hangar doors.
Each door has an IDEC MicroSmart Pentra PLC (Figure 4) that connects to the two VFDs via Modbus, communicating in ASCII via this RS485 connection. The PLC also has inputs for on/off switches, photoelectric sensors, and a laser sensor that checks for people or objects in the path of the door.
If a train is coming from the opposite direction, the PLC will wait until both trains clear the intersection before allowing the traffic light controller to resume normal operation. The PLC can stop both trains if traffic congestion or other problems make it impossible to stop vehicular traffic.
After the PLC serves the trains, it waits until the traffic controller has served the opposing vehicle phases or directions. Figure 4: An IDEC Pentra PLC controls the two VFD drives that open and close the door. Each hangar has eight doors, and each door has a PLC controller.
The sequence of events for opening a hangar door starts when the operator is notified that an airplane is approaching the hangar for entry through a certain door, or is ready to depart the hangar. From the IDEC HG4G touchscreen Human Machine Interface (HMI, Figure 5), the operator starts the door open sequence. Because each hangar door has inner and outer doors, the operator has to command each door to open from that door’s touchscreen. Similarly, when an airplane completes its passage through the door, the operator uses the touchscreens to command the PLCs to close the inner and outer doors.
The anti-collision laser sensor devices were designed by the airport engineers, and Pilot Construction implemented them into the newly retrofitted doors.
The eight PLCs controlling all eight doors are centrally located, making it easy for a single operator to close all eight doors when needed for an airplane weighing. He or she simply walks from one PLC touchscreen to the next, closing each door in turn. Plans for the future are to have a Master PLC that can control all doors, and set up each door PLC as a slave. Also, a wireless connection from a remote location will be set up to serve as a fire control.
Figure 5: From this touchscreen HMI, the operator can instruct the PLC to open or close the hangar door.
Before opening or closing the door, the PLC first checks the status of the VFDs via Modbus to ensure that they are engaged. The doors have no brakes and the drives and the inertia of the door act as positioning brake. If a VFD is not functioning, the operator can manually disengage it for maintenance and safety purposes. Once the VFD drive is replaced or repaired, the PLC can re-engage the drive and put it back into operation.
Next, the PLC checks the anti-collision laser sensor to make sure nothing is in the path of the door. If the laser sensor detects an obstruction during the door operation, the PLC stops the motor and the drive. As a redundant and backup system to the laser sensors, radial limit switches are also installed at each end of the doors to mechanically detect any obstruction. The IDEC’s PLC monitors the laser sensors and limit switches continuously during the door operation.
As part of the retrofit, during all door operations the airport engineers designed the PLC to illuminate an LED directional light and announce the message, “Door Closing, Please stay Clear of the Door” over a loudspeaker continuously, while the LED directional light signals the direction of door travel to the hangar occupants.
The PLC monitors all safety sensors to ensure that the doors are moving properly; if the doors stop for any reason, the PLC analyzes the problem, informs the operator, and displays a troubleshooting screen on the HMI.
The PLC has web server capability which allows access to the PLC from mobile devices and the Internet via any browser. The PLC has both Ethernet and Modbus communications, and Jensen Instrument and the airport engineers decided to use Modbus to connect to the Schneider VFDs.
Jensen Instrument programmed the PLCs and HMIs, following the Airport engineers’ design parameters. IDEC’s Automation Organizer software provides two packages for programming: WindLDR for programming the PLC in relay ladder logic and function blocks, and IDEC’s WindO/I-NV2 for programming the touchscreen HMI.
WindLDR allows online editing and simulation, so Jensen was able to fully simulate the PLC logic before installation, critical in this and many other applications. WindO/I-NV2 provides tools for programming graphical screens with a library containing 5,000 symbols, which simplified the programming effort and saved design time.
Using Automation Organizer, Jensen programmed the system to allow touchscreen control of all functions, display device status, trigger alarms, present troubleshooting displays, calculate scaling for the laser sensors, and keep track of the time the drives are in use for maintenance purposes.
After installation of the eight PLC systems, Jensen downloaded PLC and HMI programs and performed start up. Thanks to the simulation capability of the IDEC system, startup of the first door and fine adjustment took less than a day. As the other doors came on line, Jensen started up the other PLCs using a cookie cutter approach, and put the doors in operation within a few hours. Also, Jensen was able to easily add features and options as the users discovered the capability of the new system.
One such added option was interlocking the “Man Doors” that are installed as part of the hangar doors. These Man Doors are standard-sized, fire-rated, all-steel doors that are embedded and constructed as an integral part of the hangar doors, which allow aircraft mechanics to enter and leave the hangar when the main doors are closed. After installation of the "Man Doors," and at the request of the hangar employees, Jensen added the programming logic onsite for the PLC and HMI to interlock the Man Doors with the operation of the hangar doors. If any of these Man Doors are open, the hangar door will not operate but will display on the HMI why the door is not running, and which Man Door is left open.
The system has been very reliable and trouble-free since the first set of doors went online in March 2014. The overall cost of the hangar door renovation was about $3,300,000. However, each IDEC Micro Smart Pentra PLC cost less than $200, making the PLCs one of the most cost effective solutions in the industry.
# # #
Table: Reasons to Automate Door Operation