Resolvers

A resolver is a transducer that uses a stator winding and rotor winding to produce waveforms for shaft angle measurement for positioning. The generic term for all of these types of transducers is synchro. ill. 1a shows an example of a resolver with the rotating coil removed from the stationary (stator) coil. This diagram also shows two different types of winding relationships between the stator and rotor coils. In ill. 1b notice that the stator uses three coils that are Y (wye) connected, and the rotor uses a single coil. In ill. 1c you notice the stator has two separate coils that are mounted at 90° to each other. If the transducer uses three coils connected in a Y (wye), it's generally called a synchro, and if the stator has two windings it's generally called a resolver.

Above: ill. 1 (a) A resolver with its rotor removed from its stator. (b) The stator windings connected in a Y (wye) configuration. When the windings are connected in this configuration, the transducer is generally called a synchro. (c) Two stator windings are mounted at 90° to each other. When the windings are connected this way, the transducer is generally called a resolver.

In the operation of the resolver, the rotor is excited with ac voltage. The stator and rotor will act like a generator and a voltage will be produced in the S1 to S 3 winding and a voltage that is out of phase by 90° will be produced in the S2 to S 4 winding. ill. 2 shows the waveforms produced by each stator. From the diagram notice that the phase difference between waveforms of the two windings is 90° at all times. Each waveform represents one full revolution of the rotor shaft (360°).

Above: ill. 2: Waveforms from the two stator windings of a resolver. Notice that the two waveforms are always 90° out of phase.

One of the earliest applications of resolvers was to indicate the position of large guns on U.S. Navy destroyers. In these early resolvers, the sum of the two voltages was sent to an amplifier whose output was connected directly to a motor. In this configuration, a setpoint voltage was also sent to the amplifier, and the motor would begin to turn the guns. This would also turn the shaft of the resolver until the output voltage from the resolver would provide an amount of voltage equal to the setpoint. Since the setpoint voltage and the resolver voltage are out of phase, the two voltages would cancel each other when they were equal, which would mean the sum of the voltage sent to the motor would be zero and the motor would stop moving. It was also important in this early application that the gun mount could only make one revolution. These early resolvers were not useful without modification in applications where the motor shaft turned multiple rotations.

The resolver can determine the location of the rotor within 1° anywhere in one revolution. Since the resolver is attached to the motor shaft to determine the location of the motor shaft, it can be directly connected or it can be connected through a set of gears. If gears are used, a coarse resolver and a fine resolver can be used to determine the location of the shaft anywhere along the entire movement. The coarse resolver is geared so that it makes only one revolution over the entire range of travel, and the fine resolver is geared so that it makes one revolution every 12 inches. Resolvers are typically used in robot applications and machine-tool application where the location of a robot axis or machine position must be determined continually.

Another advancement in resolver technology occurred when op amps became refined. The op amp has the ability to compare the voltage between the two stator waveforms and determine the exact location of shaft rotation within 0.001 degrees. The op amp can also be used to detect which waveform is leading or lagging the other. This indicates if the rotation of the shaft is clockwise or counterclockwise.

Troubleshooting a resolver is simple because it acts like a generator and its windings act like a transformer. The simplest test for the resolver is to test for ac exciter voltage at the R2 and R4 terminals of the rotor. If the exciter voltage is present, a voltage should be present at the S1 and S3 stator terminals and the S2 and S4 stator terminals because the relationship between the rotor and stator windings is essentially the same as the relationship between the primary and secondary windings of a transformer. This relationship will be present whether or not the rotor shaft is turning. When the resolver is rotating, the waveform of the stator voltages will be a sine wave like an ac alternator, which can be displayed on an oscilloscope. if an exciter voltage is present, but one or both of the stator voltages are not present, either the rotor or the stator windings have an open circuit. One can disconnect the resolver and test both the rotor and the stator windings for continuity. If any of the windings have an open, the resolver must be replaced.

The second type of problem that occurs with the resolver is the wires that connect the stator and rotor windings to the resolver control circuit may develop an open. Since the resolver must be mounted near the motor shaft, and the detection circuit is mounted near the controls, the amount of wire between these two may be significant, and it may have one or more terminal connections between them. The wires can become loose at any of these terminal connections or they can develop an open anywhere in between. One can determine if the wiring has a problem by testing for exciter voltage at the source (resolver control circuit) and then at the resolver. If voltage is present at the control end of the circuit but not at the resolver end, one of the two wires has an open. The stator circuit can he tested in a similar manner, except the voltage is developed at the stator and it uses the wires to get to the controller. This means that voltage should be tested at the stator and then at the controller. If voltage is present at the stator but not at the controller, an open has developed in the wiring.