Further guidance
A motor resolver signals to a motor inverter changes in motor rotor position with time. The motor inverter uses the signal for precise closed-loop control of motor speed and position. As they are analogue sensors, motor resolvers can (in theory) provide infinite resolution of rotor position.
The resolver unit comprises a rotor winding circuit (in a dual winding arrangement), fixed to the motor rotor shaft, and three separate stator windings, consisting of an exciter winding and two output windings.
The exciter stator winding and part of the rotor winding form a rotary transformer; when the motor controller creates a sinusoidal alternating current in the former a similar alternating current is induced within the latter. The associated windings are arranged so that the rotor position does not affect the induced current.
As the alternating current induced in the stator winding circuit flows around the other part of the rotor winding circuit, it induces alternating currents in the stator's two output windings. The output windings are arranged so that the amplitudes of the induced currents are dependent on the relative angle of the rotor winding.
The two output windings are 90 degrees apart. Therefore, the induced current in one will vary according to the sine of the rotor angle, whereas in the other it will vary according to the cosine. Hence, when measured across an output winding, the output voltage will oscillate at the excitation frequency but with an amplitude modulated by either the sine or cosine of the rotor angle.
Resolver testing procedures
When carrying out motor resolver testing it is necessary to be aware of different possible internal combustion engine (if hybrid) and/or motor-generator configurations. The resolver may need the vehicle to be either driven, idling whilst stationary, or jacked up with the wheels turned manually to produce an output. The required method will depend on the vehicle and the diagnostic requirements. In all cases, you must strictly adhere to manufacturer operating procedures and safe working practices.
Resolver voltage measurement
As motor resolver output voltage measurements are made across each output circuit, an oscilloscope with a floating input or differential probe must be used. Oscilloscopes having common ground inputs are unsuitable for making these differential measurements.
Resolver voltage evaluation
The motor resolver output voltages can be evaluated using PicoScope's Math channels feature, as shown in the above figure.
The relative angle of the rotor is given by the Math channel:
360/(2*pi)*atan(LowPass(B/C,2500))
The associated Math channel signal in the graph view clearly shows how the rotor position varies.
The effect of the relative rotor position on the output voltage amplitudes of the induced output winding currents (i.e. the envelope) is given by the Math channels:
lowpass(B/A,500)
lowpass(C/A,500)
The associated Math channel signals in the graph view clearly show the modulation effect of the rotor position on the output voltage amplitudes.
Resolver failure modes and effects
Motor resolvers have no onboard electronic components and avoid direct electrical connections. Therefore, they are well suited to harsh environments, have no wires to limit rotation and no wearing brushes. However, they may be susceptible to the following faults:
- Standard circuit faults (short, open and high resistance) on one or more single winding circuits
- Shorts between sensor output circuits
- Slippage of the resolver stator relative to the motor rotor.
