Magnetic Resolver: A Design-To-Cost Alternative to Conventional Variable Reluctance Resolvers for the Electric Drives Control

Many synchronous electric motors require a very accurate position sensor compatible with a sinusoidal control. The purpose of such a control is to enable an efficient and smooth operation enhancing the comfort by limiting vibrations. In some cases related to mechanical constraints, we have to deal with through-shaft design. One can quote for examples power drives for Electric or Hybrid Electric Vehicles as well as for Electric Power Steering motor. More generally, these sensors need to keep a simple and robust design and a restricted number of parts as they are submitted to high vibration levels, a wide temperature range and speeds of several krpm. In order to meet such requirements, MMT has developed a magnetic sensor principle offering a competitive alternative to the conventional inductive resolver type sensors. The basics of this solution is a through shaft angular position sensor using one or two Hall-effect probes. These Hall IC measure the angle of the magnetic field generated by a ring or a disc magnet. The magnetic field angle is shaped in order to meet very demanding accuracy requirements (+/−1° of electric error). The use of two probes allows enhancing even more the accuracy and provides as well the design with an intrinsic robustness against external magnetic perturbations coming from the motor coils. The paper deals with this sensor principle showing that the challenge to meet less than +/−1° of electric error on rotary position sensors with motor mechanical shaft diameters up to 100 mm was taken up.