Resolver Interface Circuit Design

A resolver is a rotating transformer whose two secondary windings report the shaft angle as a pair of amplitude modulated signals. It survives heat, vibration and radiation that would destroy an optical encoder, and in exchange the electronics has to excite it and then recover an angle from two analog signals.

How the Windings Work

The rotor carries one winding, excited with a sine wave usually between one and twenty kilohertz. The stator carries two windings at right angles, so the coupling into each depends on the angle between the rotor and that winding.

The result is a sine cosine pair: one secondary produces the excitation multiplied by the sine of the angle and the other the same signal multiplied by the cosine. Dividing one by the other removes the amplitude of the excitation and leaves the tangent of the angle, which the converter tracks.

The transformation ratio is the fraction of the excitation that appears at a secondary when it is aligned. A typical ratio is about half, so a four volt excitation produces a two volt signal at the peak, which is a comfortable level for the following circuits.

The excitation signal

The excitation signal drives a winding with an inductance of tens to hundreds of millihenries, so the buffer has to supply a reactive current and remain stable into that load. A power operational amplifier or a small audio amplifier in a bridge configuration is the usual choice.

An excitation at a low frequency reduces the reactive current but slows the conversion, because the angle information is recovered from the envelope and the envelope changes at the excitation rate. Ten kilohertz is a common compromise between bandwidth and reactive current.

The excitation source should be sinusoidal rather than a square wave. A square wave contains harmonics that the resolver transforms unevenly, and the resulting ripple appears in the angle output. A filtered sine is worth the extra components.

Buffering and Loading

Driving the primary directly from an amplifier gives the cleanest waveform and the least phase shift, at the cost of a dedicated amplifier. A transformer coupled drive isolates the amplifier from the winding and allows a step up, but its own phase shift changes with frequency and temperature.

Where several resolvers are multiplexed onto one excitation source, the switch resistance and the cable capacitance add to the load. The amplifier has to remain stable into the worst case load, which is usually the combination of the longest cable and the lowest winding inductance.

The buffer output should be protected against a short. A resolver winding that fails to a short circuit presents a low impedance, and an unprotected amplifier will not survive the event. A series resistor sized for the worst case is cheap insurance.

Reading the sine cosine Channels

The two secondary channels are differential in most resolvers, and a differential receiver removes the common mode that the windings pick up from the motor. The input impedance should be high enough not to load the winding, and matched between the two channels so that their phase shift is the same.

The cables to the two channels should be identical in length and type. A difference of even a few centimetres changes the phase of one channel relative to the other, and a phase error appears directly as an angle error, worst at the angles where one channel is small.

Filtering at the input removes the switching noise of the drive and any pickup, and it also adds phase shift. The filter on both channels must be identical for that reason, and the corner should be well above the excitation frequency and well below the switching frequency of the drive.

phase shift and Its Correction

phase shift between the excitation and the received signals comes from the winding inductance, the cable capacitance and the input filters. The converter measures the ratio of the two channel amplitudes, and it assumes the channels are in phase with the excitation; a shift makes the recovered angle wrong.

The effect is not a constant offset. A phase shift produces an error that varies with the angle at twice the rotation rate, so it appears as a ripple in the reported position rather than a fixed bias. That signature is a useful clue when diagnosing a resolver installation.

Some converters include a programmable phase compensation that removes the error at the excitation frequency. Where the converter has none, the compensation is done by adjusting the input filter or by correcting in the controller software from a table measured on the bench.

angle tracking and Conversion

An angle tracking converter multiplies the two channels by the sine and cosine of its internal estimate and drives the difference, which is the sine of the error angle, to zero. The loop then tracks the rotation continuously rather than sampling it.

A tracking converter gives a smooth output with a defined bandwidth, and its acceleration error is set by the loop gain. A high loop gain follows a fast acceleration and passes more noise, so the bandwidth is chosen from the dynamics of the axis rather than from the converter.

A sampling converter with an analog to digital converter on each channel and an arctangent in software is cheaper and slower. It suits a slowly rotating shaft, and it needs the two channels sampled simultaneously, because a delay between the samples appears as an angle error proportional to speed.

Layout, Cabling and Noise

Run the excitation pair as a twisted pair and the two received channels as separate twisted pairs, each with its own return. Sharing a return between channels introduces crosstalk that appears as an angle error at the rotation frequency.

The screen of the resolver cable is earthed at the controller end, and the resolver housing is usually earthed to the machine frame. Where the two earths differ, an isolated input stage removes the problem entirely, and the same reasoning applies as for any differential measurement described in our guide to mixed signal board design.

Keep the resolver signals away from the motor phase cables. Those cables carry the switching current of the drive and radiate a field that couples into any parallel conductor; where they must run together, the resolver cable should be in its own screened conduit.

Verification and Common Faults

Verify the installation by turning the shaft slowly through a full revolution and plotting the reported angle. A curve that deviates twice per revolution indicates a phase shift, while a deviation once per revolution indicates an amplitude mismatch between the two channels.

A reading that jumps to a fixed value points to a channel that has failed or to a broken winding. Measuring the winding resistance and the transformation ratio at the connector distinguishes a cable fault from a resolver fault.

A reading that is noisy only when the drive runs points to coupling from the motor cables. Check the screening, the earth at the controller end and the routing. The release checks that keep such an interface consistent from unit to unit are collected in our PCB design release checklist, and the assembly points we inspect are listed in judging PCB quality.

FAQ

Why does my resolver reading ripple twice per revolution? A phase shift between the excitation and the received channels produces exactly that pattern. Check the input filters and the cable lengths.

What excitation frequency should I use? Between one and twenty kilohertz, with ten kilohertz a common choice. Higher frequencies reduce the reactive current but add cable effects.

Can I read a resolver with two ADCs and software? Yes, for a slowly rotating shaft. The two channels must be sampled at the same instant or the angle error grows with speed.