An introduction to high resolution patterned magnetoresistive sensors and their application in digital tachometers

High resolution magnetoresistive digital tachometers have recently been introduced to the pulp and paper industry. This industry, by its own admission, has been technologically conservative due to the high cost of down time for equipment failure. In spite of this conservatism, high resolution patterned magnetoresistive (MR) digital tachometers have very quickly gained acceptance with both the motor and drive manufacturers as well as the end user. The combination of this sensor and a magnetic pulse wheel allows five times the sensitivity of previous magnetic technologies utilizing a Hall effect sensor and gear tooth wheels. This technology, coupled in an innovative mechanical and electrical digital tachometer design, allows the use of interchangeable sensor modules, enhancing ease of maintenance and serviceability for mill personnel. The unique properties of the MR sensor/magnetic pulse wheel combination result in a high resolution digital tachometer with a high degree of immunity to environmental contamination. This improved reliability is extremely important in many mill applications. The purpose of this paper is to introduce this technology and to explain its attributes for usage in the pulp and paper industry