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01/04/2007
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Rear wiper systems have to cope with significant performance demands, writes Roger Bishop.
They have to function across a wide range of temperatures, deal with voltage variations, and cope with severe degrees of glass contamination by dirt, ice and liquids.
The latest, electronically controlled rear wiper, designed by Johnson Electric, is claimed to overcome some of the limitations of current systems and allows the same motor to be used across several vehicle platforms. It also provides program timing flexibility and can adjust to system design changes and market trends – in other words, ‘on-the-fly’ changes to vehicle platform models.
Described as a second-generation system, it is said to be 40% lighter, 20% smaller and to have 60% fewer moving parts than the first-generation design.
A traditional mechanical system comprises an electric motor with a shaft mounted worm gear, a mating helical gear, and a mechanical crank mechanism. The crank mechanism consists of a connecting rod, coupled on one end to a pin mounted on the helical gear. The opposite end of the rod is formed into a gear that mates with a second gear on the wiper output shaft. A balance link joins the output shaft to the connecting rod. One end of the connecting rod rotates with the helical gear and creates reciprocating motion at the balance link. The balance link translates this into a bidirectional, limited angle rotation of the output shaft.
Johnson Electric describes its mid-1990s first-generation electronic system, which replaces the mechanical elements, as “rather conservative”. The control method resulted in sudden speed and direction changes at each end of the wiper travel.
Analysis showed that big improvements in noise and end of travel position accuracy could be achieved by using a sinusoidal motion profile that provides smoother transition of motion at the end of travel, quieter operation and more accurate positioning with less overshoot. However, a sinusoidal motion profile requires more sophisticated sensing – delivered in the shape of a 16-pole magnet attached to the motor shaft and two Hall sensors that send rotor position signals to a microprocessor.
The final development step in the development of the second-generation system was the integration of the electronics into the gear motor assembly. To achieve this, the electronics package size had to be drastically reduced. The first iteration used an ASIC chip to halve the 68 parts. The final design further reduced the count to just 14 parts – six active and eight passive – with 52 components integrated into the mixed signal (analogue/digital) ASIC.
The latest system is completely programmable for different vehicles and customer requirements. Disruptions on the wipe surface, or at the end of the wipe angle, are compensated for. The motor starts softly with smoothly increasing speed and reaches peak speed at the glass mid-position. The motor then decelerates smoothly until it reaches the end of travel, where motor reversal occurs. This ensures an exceptionally quiet operation. Wiping precision is 0.2deg.
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Author
Roger Bishop
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