Speed sensors insensitive to EMI

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6 February 2008 Analogue, Mixed Signal, LSI

New speed sensor technology for industrial electric vehicles has been developed by SKF to cope with the harsh environmental demands created by the switch to more powerful, compact AC motor designs.

The growing trend towards replacing DC motors with AC designs is resulting in new challenges for designers of mechatronics solutions in control and sensing applications.

While there are space-savings and material cost reductions in adopting more compact and powerful AC drive motors, the greater compactness of motor design means that all the related components such as the electronics, bearings etc, are closer together and have to exist and perform in an even more demanding environment. For certain components such as the sensor electronics, the increasingly harsh electromagnetic and physical environments can be a real issue.

The main issue is that the existing sensor bearing technology employing Hall Effect devices, used mainly to detect the speed, direction and position of the drive shaft, cannot operate optimally or as expected because of interference by the strong magnetic fields generated by the higher motor currents (between 600 and 800 A) of these AC motors.

SKF has developed a new generation of speed sensors for electric AC motors that are said to be insensitive to the effects of strong magnetic environments. As importantly, the sensor can be integrated into the bearing design, continuing the general trend towards increased component integration and 'plug-and-play' solutions for electric vehicle systems.

The new motor encoder unit, designated the BMH series, uses an inductive microcoil technology, where two arrays incorporating inductive microcoils sense electromagnetic (EM) pulses that are bounced off a moving metallic target wheel that rotates with the bearing inner ring. The microcoils transform the EM signal into current, which is then converted into a voltage by a Schmitt Trigger that produces the pulses which are directly related to the speed of rotation. The signal created is clearly detectable and separable from any background noise and provides a versatile signal output.

One of the attributes of this sensing system is that it is valid for extremely low speeds and can provide accurate measurements over the range from zero up to about 20 000 rpm. Many other speed-sensing techniques have difficulty in accurate measurements at very low speeds. Another feature of this high performance motor encoder unit is its ability to work at ambient temperatures up to 150°C.

For more information visit www.skf.com

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Analogue, Mixed Signal, LSI

Speed sensors insensitive to EMI

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