Turning up the temperature

29 July 2009

Dependability and reliability are essential quality criteria for wind power equipment. At the heart of these products and installations are a group of critical yet often hidden components such as magnetostrictive transducers and sensors used for pitch control, yaw position, speed indicators, lock pin positions and level/leak detection.

A competitive advantage goes to those system builders who only use such transducers and sensors in their wind turbines, which have demonstrated flawless function over the entire life cycle. To reach this flawless function conventional testing for product qualification within specification is often no longer sufficient. Sensor specialist Balluff, routinely subjects its sensors and linear displacement systems to HALT testing in its own testing facility.Here the components are subjected to an accelerated ageing process within three to five days, thereby simulating the total of all load conditions within the life of a sensor in time lapse fashion. Possible weak points can thereby be reliably detected and corrected as early as the development phase.

In a HALT test the sample is subjected to ever increasing mechanical, thermal and combined loads at precisely defined intervals until it fails. This time lapse indicates the function and destruction limits of the product, thereby enabling the design team to greatly extend the final product’s life expectancy before failure. Information about long-term behaviour is gathered, with greater reliability than the final result compared with conventional testing for product qualification within specifications.

The Balluff test fixture, 230 cm x 164 cm x 227 cm (H x W x D) provides suitable conditions for such testing. With a temperature range of -100° C to +200°C and a temperature change speed of 70 K/min, significant thermal stress is produced. The rapid cooling of the testing chamber air is accomplished using liquid nitrogen. Vibration is produced on a testing table 106 x 106 cm which is vibrated using 9 pneumatic cylinders. Vibrations with three linear and rotary degrees of freedom of up to 50g in a frequency range of 1 to 10,000 Hz are generated.

Balluff also offers testing in the HALT facility as a contracted service.

HALT testing is standard practice in the aircraft industry and is used in the automotive industry by many of the major manufacturers. One defining observation from companies using HALT is that the problems that occur after a few hours of HALT testing in the laboratory are the same as those that occurred in the field after several years of service life.

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