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Rugged rotary encoders withstand hard start-stop conditions

Integrated circuit board improves device reliability

By Roy OConnor, Contributing Editor -- Design News, November 4, 2002

Tje ,pde; 5820 rotary encoder from Kubler is mounted directly on the engine crankshaft without the need for extra mounting hardware or couplings.

Owing to the various design constraints affecting car engines, starter motors tend to be unique to the engine model. Consequently, engineers at manufacturers like Bosch have to thoroughly test each new model for reliability in an endurance test to be sure that it will meet the rigorous requirements. The equipment on these test rigs, including the rotary encoders used to measure the engine speed, must also withstand the stresses involved.

When the intense jerking, vibrations, and high ambient temperatures in the tests proved too much for the original encoder, Bosch engineers tested the 5820 encoder from Fritz Kübler GmbH, a German-based company that manufactures a range of counters, meters and encoders. The encoder, marketed in North America by Turck Inc. (Minneapolis, MN), has a IP66 level of protection and a compact hollow-shaft that enables it to be mounted directly on the crankshaft, eliminating the need for any coupling. Over a period of 18 months, Bosch mounted about 100 of the encoders on starter-motor testrigs. The rigs put the starters through up to 60,000 cycles, which means that the engines start up to six times each minute.

A significant factor in producing a high reliability rotary encoder is the 6-layer circuit board which holds all the components. This "integrative technology" reduces connections and improves rigidity.

Markus Wiedmann, Kübler's Marketing Director, highlights the success of the replacement program: "Conditions on the starter-motor test rigs are so severe that almost none of the previous encoders survived the first year of operation. But during the 18 month test period with the new encoders, only one failed and that was because of a broken engine crankshaft." Bosch is now replacing the encoders on all the testrigs by Kübler encoders. Wiedmann points out that apart from the inherent high shock and vibration resistance of the encoder, patented "integrative technology" also contributes to the encoder's reliability.

A multi-layer technique enables all components, including a special optoelectronic ASIC (Application Specific Integrated Circuit), to be mounted on a single printed circuit board. This method of construction has advantages in ruggedness and reliability over circuit boards assembled together with screws according to the sandwich principle. Other features important for reliability are the temperature compensation, which provides a constant signal level under varying temperature conditions, and the specific compensation employed by Kübler to combat the ageing process in the encoder's light emitting diode.

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