PLCs Control Massive Hangar Doors at San Francisco International Airport

Fig. 1: IDEC PLCs control the opening and closing of 70,000-lb. doors to allow airplanes as large as a 747 to enter and leave the hangar.

By Jenna Castro, San Francisco International Airport, Design & Construction Department

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 opposite sides—allowing access to this huge space measuring 500 x 540 ft., about the size of two football fields.

As shown in Fig. 1, each hangar door 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 lbs. 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.

Revamping the Hangar

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 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.

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” requires all doors to be closed and almost zero air movement within the hangar. 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 distributor and systems integrator in San Mateo, CA, to provide the programming and integration solution for the PLCs.

Micro PLCs and Big Doors

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. 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” that allows an engineer to replace the motor and gear drive in less than 10 minutes.

Each door has an IDEC MicroSmart Pentra PLC (Fig. 2) 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.

Fig. 2: 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 HMI (Fig. 3), 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 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.

Fig. 3: 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. 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 PLC monitors the laser sensors and limit switches continuously 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.

Programming the PLC

The IDEC MicroSmart Pentra PLC supports 32-bit processing, has floating-point math functions, Modbus master and slave capabilities, seven communication ports, up to 512 digital I/O and 56 analog I/O, and can be expanded easily if needed.

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. 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 startup. 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.

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.

Jenna Castro is Project Manager in the San Francisco International Airport Design & Construction Department.