Mosolver: An Optimized Servo System

Interesting Projects Blog

April 23 2024 | Donald P. Labriola PE

The Mosolver is a combination Motor and Resolver all built on the same magnetic structure. The Resolver sense coil is added into the existing motor stator.  The digital drivers for the motor are used not only to generate torque from the motor, but to also excite the resolver sense coil. The ripple current associated with the motor PWM drive sets up a time varying magnetic field. As the rotor rotates relative to the stator poles, the magnetic paths from the stator to the rotor vary, deflecting the time varying magnetic field through different portions of the sense coils. The resulting sensor signals produce sine and cosine waveforms when properly demodulated using the chopper drive timing.

Operation down to zero speed is available as the sense coils are not based on back-EMF, but rather senses a portion of the time varying magnetic field induced by the ripple current inherent with a PWM drive. The resulting motor with integrated sensor are the same size and weight as the motor alone.

The sensor itself is a passive coil built onto a polyamide flex circuit set within the stator pole structure. The polyamide flex sensor is operational over a wide temperature range, with materials available from cryogenic to over 400C, with low outgassing for vacuum applications, and in radiation resistant formulations. This makes the resolver sensor as rugged as the motor into which it is integrated. Note that the sensor flex circuit is only flexed once to insert it into the motor, so flex life is not an issue.

The electronics used to control the Mosolver adds only a couple of op-amps to the basic DSP driven digital controller/driver. These are used to buffer the sensor signal for two spare ADC channels in the DSP used in the digital driver/indexer/controller. The resulting sensor system provides both commutation information, and fine position feedback for servo control, with a resolution of 32,000 counts per revolution.

The cutaway view of the Mosolver shows the “4-leaf clover” sense coil associated with each phase within the stator tooth structure, with the stator pole pieces being driven by the standard phase windings.

The Mosolver is a compact, robust, and inexpensive servo system.

The Mosolver is a structurally simple and robust high resolution Hybrid Servo for harsh environments. It is compact and rugged, and eliminates most of the additional controller hardware normally associated with a standard resolver feedback system while also being much more robust than a common encoder. As the magnetics are shared between the motor and its feedback, no alignment or phasing is necessary – they are inherently aligned.

The Mosolver hybrid servo provides a high torque constant and wide operational speeds.

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