The best way how to use resolvers

Resolvers are one of the most common sensors for detecting the rotor position in servo drives. Due to their electrical and mechanical materials and high robustness similar to those of servo motors they are recommended like second to none for those applications. However, prevalently used frameless resolvers are open sensor kits without any bearing. Their feature of not being stand-alone sensor-units with well defined mechanical interfaces requires the design engineer having a certain comprehension of their electro magnetic behaviour in order to obtain the best possible accuracy. Electromagnetic interference by the stray fields of motor and brake is one issue, but there are a couple of other mechanical, magnetic and electrical reasons which could cause perceptible distortions of the resolver signals with results in noise and harmonic ripple on the angular information as well as on derived velocity information. As the increasing requirements on dynamic and stiffness of control loops needs higher and higher control loop gains, this may lead to extreme ripple of motor current, causing vibration, noise, increased temperature and a reduction of efficiency. This paper will give an overview of the sources of angular errors and how they will affect the resolver signals. Finally it will give hints how to reduce or, if possible, how to avoid these errors.