Ground crews talk sense on generator maintenance

Sensor Technology: Aircraft servicing times are being reduced, enabling more flying hours, thanks to a new fully automatic generator testing station developed by MEL Aviation in Sudbury, Suffolk.

A key element in the design of the station is the use of a TorqSense non-contact torque sensor, which can be deployed instantly for high performance acceleration and deceleration tests of aircrafts’ generators. Conventional sensors would, by contrast require the fitting and aligning of fiddly slip rings, an expensive process when considering the down time costs of an airliner or military jet.

MEL Aviation is a foremost full service and repair provider for the overhaul of aircraft systems and components. It also designs, manufactures and supports military aircraft systems and military ground-support equipment (GSE). Civil airline customers include British Airways, Virgin Atlantic, Monarch, BMI and Saudi Arabian airlines. On the military side it works with the UK MOD and BAE Systems, on the Eurofighter and Nimrod programmes, and has manufactured oxygen masks for use in military aircraft.

Gary Parker, one of the company’s leading engineers explains that their 24 hour service is supported by sophisticated customer care and quality assurance cultures, one stop shop capabilities and a passion for excellence.

“For the most part we work with specifically designed technology and purpose-built test equipment. This breeds a culture of total responsibility for total reliability.”

When a civil aircraft isn’t flying, it isn’t making money. When a military aircraft is on the ground, defences are weakened. But an unsafe aircraft in the air is a liability to everyone, so they are constantly rotating through ground servicing schedules to ensure their ongoing air and battle worthiness.

Every modern aircraft has at least one on-board generator to produce electrical power (most have two or more in a redundancy configuration for utmost reliability). These need to be ground tested regularly, typically every 500 to 1000 flying hours, hence the need for MEL’s test station.

Our GTR station is in essence very simple. It uses a great big electric motor to turn the generator, rapidly accelerating and decelerating it up to 10,000rpm or more to simulate extreme flight conditions, such as in ground hugging for radar evasion or an emergency descent of an airliner.”

Various sets of instruments measure the electrical output, bearing performance etc.

The innovation of the GTR is that it is fully automatic, the test steps being controlled and monitored by a PLC system requiring the very minimum of human intervention i.e. selecting what type of test is to be performed from a menu and pressing the start button. The software language for the GTR is a propriety SCADA package using Proficy iFix with dynamic communication links to a Telemecanique PL7 ladder logic embedded in a TSX premium programmable logic controller. All necessary functions are embedded in the Proficy iFix core construction architecture. The dynamic link is written in visual basic.

“TorqSense has the pivotal role of measuring the rotational speed – as a torque value - of the input shaft, which defines the simulated speed profile. It is the primary parameter of the test; if it is not monitored with 100 per cent accuracy throughout the whole test all data is invalid and precious time is wasted.”

TorqSense is a surface acoustic wave (SAW) based device. In a TorqSense transducer, surface acoustic waves are produced by passing an alternating voltage across the terminals of two interleaved comb-shaped arrays, laid onto one end of a piezoelectric substrate. A receiving array at the other end of the transducer converts the wave into an electric signal.

The frequency is dependent upon the spacing of the teeth in the array and as the direction of wave propagation is at right angles to the teeth, any change in its length alters the spacing of the teeth and hence the operating frequency. Tension in the transducer reduces the operating frequency while compression increases it. To measure the torque in a rotating shaft, two SAW sensors are bonded to a shaft at 45deg to the axis of rotation. When the shaft is subjected to torque, a signal is produced which is transmitted to a stationary pick up via a capacitive couple comprising two discs, one of which rotates with the shaft, the other being static.

“It imparts no extra load onto the shaft, so calculations are simple. In short it is simple, robust, reliable and – dare I say it – idiot-proof. As such it is ideal for the sort of demanding work we do.”

MEL expects each test station to have a working life of at least 30 years and for the TorqSense transducers to perform for all this time. In both military and commercial flight operations, this is a typical expectation, with all investments being planned with long term amortisation and reliability never compromised.