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Clever drives for clever docking

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A BOEING 767 has a typical wing span of 52m, a tail height of 17m, an overall length of 61m, and is composed of more than three million parts. During heavy maintenance periods the aircraft spends several weeks stationed in a hangar, while up to 75 technicians scurry around like ants on an anthill, performing a series of safety checks and inspections.

In order to grant personnel access to all areas of the aircraft during these inspections, the vehicle is parked into what is known as a ‘heavy maintenance docking system’. The docking system consists of a number of multi-level platforms (or docks), abutting the aircraft’s external panels. Because any one docking system is used to service multiple aircraft models, its design traditionally consists of a number of manually positioned versatile mobile stands.

Sydney automation engineering group, CIES (Complete Industrial Engineering Solutions), in collaboration with SEW-Eurodrive , has developed an innovative new docking system design incorporating electronic control founded on intelligent drives. “Once we introduced the idea of using the on-board smarts to control the docking, this opened up a revolutionary architecture concept,” says Nick Bent, CIES director.

Park n go

The configuration of standard docking systems comprises four types of docks that provide access to the all areas of the plane, including the fuselage (port and starboard), wings, horizontal stabiliser, and vertical fin (port and starboard). Some of these docks are suspended from the ceiling on runners; some are grounded mobile support structures.

Motors, situated at the extremities of the platforms, drive each of the docks into position around the aircraft, where they remain for the entire maintenance procedure. According to Bent, the key feature of this design is the ability to skew the docks parallel to the fuselage. In traditional docking systems, the crew can spend up to six hours parking the aircraft and positioning the docks to be perfectly in-line with the docking installation.

“The skewing function allows for greater tolerances in aircraft positioning because the dock will automatically adjust to the lay of the plane,” said Bent. This reduces the parking time to less than two hours and when compared to previous systems comes as close to ‘park n’ go’ as you can get. Of course from a practical sense there is also a significant saving when considering emergency and breakdown repairs.

“Normally in plants we talk of downtime. In this sector it’s uptime that’s important,” said Tim Hawkins, SEW-Eurodrive sales manager for NSW. “How long a plane is actually up in the air making money, as opposed to spending time in the hangar for maintenance.”

Movement of each of the docks is powered by a total of 19 SEW-Eurodrive helical-bevel geared motors electronically controlled by Movidrive application inverters. “The motors are in tight spaces, interfacing with quite complex screw-jacking and line shaft systems,” said Hawkins.

“That’s why we suggested the helical bevel geared motors, which are the strongest 90-degree gears in our range--and supremely compact.”

The skewing function is made possible by the smarts built into the drives. “The engineer can program the dimensions for different aircraft models straight into the drives,” said Hawkins. “Platform position sensors feedback to the motor control, providing a reference for where, and in what direction, the plane is actually sitting.”

Sharing the load

The skewing functionality or ‘free run mode’ is one of many functions made possible by SEW-Eurodrive’s application modules, embedded in the Movidrive inverter.

The inverter’s controller area network (CAN)-based communications protocol, S-bus, communicates between drives. With a multitude of motors driving each dock, the S-bus connection allows the devices to share the load--literally.

This co-ordination is achieved using the Movidrive’s internal synchronous operation (ISYNC) function. ISYNC enables a group of motors to be synchronised in either a time-controlled or position-dependant manner. One drive is nominated the ‘master’ drive and runs on a positional program reliant on encoder feedback. The other drives, or ‘slaves’, match the behaviour of the master defined by the specific programming.

“Having intelligence integrated within the drive ensures you can achieve tight integration between process control logic and synchronisation feature, which allowed for the successful implementation of the auto stabilisation function,” said Bent. Additionally the availability of the integrated process control allowed for the docks to be controlled and operated within a economical and safe pre-defined envelope.

“The docking system, as now structured, allowed for the replacement of the legacy system which occupied significant floor space,” Bent said.

The extra space provided by the new design has now been set up as workshops and storage space, to help streamline the full maintenance process.

Integrated safety

Retrofitting the docking system into the existing hangar presented a significant challenge to CIES.

“In order to reduce that envelope without threatening the safety of the aircraft, we put in place a number of safeguarding features,” said Bent. Anti-collision detection systems were integrated into the docks: proximity and intrinsically safe rated photo electric sensors were installed to prevent contact between the aircraft panels and the docks, the edges of which were lined with physical bumpstrips, as a final protective barrier.

“Safety is of the upmost priority in this sort of installation,” said Bent. From an official stand-point, all designs, panels and installation wiring must comply with local and international wiring and hazardous area standards. Practically, says Bent, a hole in the fuselage of an aircraft full of aviation fuel presents an obvious and considerable risk--which is unacceptable.

The true risk is associated with movement of the docks near and around the aircraft. For example, docks that have been skewed must be re-aligned before being retracted. This is primarily to ensure that minimal stress is placed on the screw-jack structure of each platform.

The intelligent control allows the system to automatically realign the dock, moving the side closest to the aircraft up or out first. This removes the responsibility from the operator doing it manually, where the wrong end may be moved first inadvertently, thus bringing the other end into contact with the plane.

“The old way of arranging each dock involved one guy operating the radio control pendant with two spotters: one on the dock and one on the ground” said Bent.

Traditional docking systems rely on motors being started and stopped via simple direct on line (DOL) or soft starters to drive each of the docks back and forth. This purely electromechanical design can result in considerable platform-swing during the positioning manoeuvre.

“When trying to dock up close the traditional system can be a bit of a ‘hit and miss’ operation,” said Bent.

Importantly, ‘close docking’ fulfils a safety requirement for both the engineers working several metres above ground level, and for those below, who are at risk of being hit by dropped tools or equipment. In legacy docking systems, unless the aeroplane is parked dead-straight, a relative gap will always exist at one end of the platform.

“This is the basis of why the skewing functionality is so important. It gives the system flexibility, 250mm in the traverse and 150mm in the vertical, to be precise,” said Bent.

The use of the drives also introduces intelligent ramping and speed control for dock relocation, allowing operators to position the docks within 25mm of the aircraft.

SEW -Eurodrive 03 9933 1000.

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