How Resolvers Work
Resolvers are absolute angle sensors that use entirely passive transformer technology. Resolver offer reliability benefits over other technologies and are perfectly suited in extreme temperature, oily, dusty or even radio active environment. Resovers are now supported in all new and updated version of Roboteq's brushless motor controllers.
Resolvers is an analog position sensor that is absolute over a single turn. A resolver is a rotary transformer where the magnitude of the energy through the resolver windings varies sinusoidally as the shaft rotates. A resolver control transmitter has one primary winding, the Reference Winding, and two secondary windings, the SIN and COS Windings. The Reference Winding is located in the rotor of the resolver, the SIN and COS Windings in the stator. The SIN and COS Windings are mechanically displaced 90 degrees from each other. In a brushless resolver, energy is supplied to the Reference Winding (rotor) through a rotary transformer. This eliminates brushes and slip rings in the resolver and the reliability problems associated with them.
Roboteq newest brushless motor controllers can interface to Resolvers. They provide an AC excitation voltage of 10Vpp of 8kHz to the Reference Winding. The induced voltages in the SIN and COS Windings are equal to the value of the Reference Voltage multiplied by the SIN or COS of the angle of the input shaft from a fixed zero point. Thus, the resolver outputs provides two AC voltages whose ratio represents the absolute position of the input shaft. (SIN θ / COS θ = TAN θ, where θ = shaft angle.) Because the ratio of the SIN and COS voltages is considered, any changes in the resolvers’ characteristics, such as those caused by aging or a change in temperature, are ignored. An additional advantage of this SIN / COS ratio is that the shaft angle is absolute. Even if the shaft is rotated with power removed, the resolver will report its new position value when power is restored. Commercial Resolvers have a 2:1 transformer ratio, meaning that the SIN and COS output voltage will reach 5Vpp at the max positions.
The principal challenge of interfacing to Resolvers is the AC nature of the signals. The Roboteq algorithm uses very precise synchronization between the Sinusoid signal generation and the ADC capture so that the SIN/COS voltage are measured precisely at the peak of the sine wave. Capture is typically within 0.5 degree resolution which is the precision that is typically specified by the Resolver device itself. After a one time calibration procedure to capture the 0o mechanical reference and the min/max voltage swing over a full rotation, the resolver output is used by the controller to generate the 3 phase sinusoidal voltage that goes to the motor to create the rotation.