Bearing sensor unaffected by EM fields

Written by: Tom Shelley | Published:

Today, AC motors normally consume around 400 to 600 Amps, but future motor designs are heading towards the 800 to 1,000 Amps range, with the focus on extreme power density and high current.

Problem: Today, AC motors normally consume around 400 to 600 Amps, but future motor designs are heading towards the 800 to 1,000 Amps range, with the focus on extreme power density and high current. This higher current means that traditional Hall Effect sensors will be limited when it comes to measuring and monitoring bearings. Therefore, sensors that are unaffected by electromagnetic interference is the goal.

Solution: Bearings manufacturer SKF has developed a sensor bearing unit that is unaffected by external electromagnetic fields. The unit has the normal bearing function of carrying and transmitting load smoothly, but can also give shaft speed and position data, for further processing, to precisely control the motor and optimise the motor's performance and efficiency.

Currently undergoing qualification tests but due to launch in March 2005, the new unit incorporates novel encoding technology and signal processing electronics for acquiring the speed and position data and transferring it to the motor controller. This involves integrating contactless, high frequency, inductive micro-coil sensing technology into the unit.

A unique electronic layout induces an electric pattern in the micro-coil sensor. This pattern is modified by the presence or not of the teeth of the metallic impulse ring. The phase shift of this pattern is then processed via a dedicated ASIC, generating digital signal outputs. In this way, the signal detection and processing is thoroughly separable from the background electromagnetic fields.

Applications: The device is ideal for applications in high power AC drive motors, including heavy duty conveyors, escalators, lifts, airport trolleys, golf carts and fork lift trucks. The units can operate in extreme conditions of heavy vibration and thermal shocks, making them robust enough to survive in severe mechanical and electrical environments. Motor designers will benefit from being able to integrate the encoding solution inside an electromagnetic motor brake or very close to the coils of the stator, leading to a more compact design. DP


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