Analysis of Sin/Cos Signal Distortion in Reluctance-Type Resolvers Under Strong Electromagnetic Interference -EV Resolver Sensor

In applications such as new energy vehicle electric drives and industrial servos, reluctance-type resolvers are widely used as motor rotor position sensors. They contain no semiconductor chips, withstand high temperatures, and resist vibration -they appear very “rugged.” However, many engineers encounter a puzzling issue on site: once the motor power increases and the PWM switching frequency rises, the resolver’s sine/cosine output signals develop glitches, flat-topping, envelope distortion, and other anomalies, leading to angle jitter, torque ripple, and even controller faults.

I. What Does a Normal Resolver Sine/Cosine Signal Look Like?

The structure of a reluctance-type resolver is relatively simple: the stator carries an excitation winding and two spatially orthogonal output windings, while the rotor is made of specially shaped silicon-steel salient poles with no windings. A high-frequency sinusoidal excitation is applied to the excitation winding:

When the rotor rotates by an angle θ, the two output windings induce respectively:

In practice, each signal is differential, e.g., S1-S3 and S2-S4. As can be seen, the resolver output is a high-frequency carrier whose amplitude is modulated by the rotor angle θ . On an oscilloscope, the waveforms appear as two envelopes that vary with angle as sine and cosine, respectively.

The decoding chip -the Resolver-to-Digital Converter (RDC) -demodulates these signals, extracts the envelopes, and calculates the angle via  

II. Where Does Strong Electromagnetic Interference Come From

The resolver is typically mounted at the motor end, surrounded by a typical strong electromagnetic interference environment:

· High-speed switching of IGBTs/MOSFETs: The PWM carrier frequency of the motor controller is usually in the range of a few kHz to tens of kHz, but the switching edges are only tens to hundreds of nanoseconds, with spectra extending up to tens of MHz.

· Power busbars and phase cables: Fast changes in high current and high voltage generate strong electric and magnetic fields.

· Contactor and relay operations: Produce arcing and transient pulses.

· Power converters: Ripple and harmonics from switching power supplies.

Once these interferences couple into the resolver signal lines, they superimpose on the otherwise clean sine/cosine signals.

III. How Interference Enters the Signal Chain

There are four main paths through which interference enters the resolver signal chain:

1. Radiative coupling
Power lines and switching nodes act like transmitting antennas, while resolver wiring harnesses act like receiving antennas. When resolver harnesses are routed parallel to power cables, high-frequency electromagnetic fields induce voltages directly on the harnesses.

2. Conductive coupling
Interference enters the resolver system through the excitation power supply lines or the decoder board power supply, and then couples into the signal side.

3. Ground potential differences
Large currents cause voltage drops on ground traces or metal housings, creating unequal ground potentials between the resolver end and the decoder end, forming common-mode voltages.

4. Crosstalk between lines
When resolver harnesses run parallel to phase cables, busbars, etc., parasitic capacitance and mutual inductance between lines couple interference over.

IV. Typical Manifestations of Distortion

On an oscilloscope, the sine/cosine signals under strong EMI may appear in the following forms.

1. High-frequency glitch superposition
Spikes and ringing synchronized with PWM edges appear on the carrier. This is because high-frequency currents generated during switching moments couple into signal lines through parasitic capacitance. Glitch amplitudes may reach tens of mV or even hundreds of mV, while normal differential signal amplitudes are typically only a few volts, resulting in a significant drop in SNR.

2. Common-mode to differential-mode noise
Differential signals are theoretically sensitive only to the voltage difference between the two wires and have rejection capability against common-mode interference. However, actual cables, connectors, and filter networks are never perfectly symmetrical. When high-frequency common-mode interference enters, part of it is converted into differential-mode voltage and superimposed on the sine/cosine signals. This noise often appears as periodic "fuzz" on the envelope.

3. Clipping and saturation
When interference amplitude is large, and the superimposed signal peak exceeds the RDC input range or the op-amp supply rails, the waveform tops are "clipped." Clipping directly destroys the sin/cos envelope, causing severe nonlinear errors in the decoded angle.

4. DC offset drift
Low-frequency components in the interference, ground loop currents, or rectification effects may cause a DC offset in the signal, making the waveform asymmetrical. RDC decoding typically assumes pure AC signals; offset leads to angle errors and speed ripple.

5. Harmonic distortion
Nonlinearity of magnetic materials and intermodulation between interference and the excitation carrier generate harmonic components. Once harmonics enter the RDC, they may be demodulated into high-order harmonic errors in the angle, manifesting as torque ripple or noise.

6. Sine/cosine amplitude imbalance and phase shift
If the two signal paths for sin and cos have different filter parameters, cable lengths, or connector contact impedances, the two channels are affected differently under strong interference, resulting in amplitude mismatch or phase shift. The RDC calculates the angle based on sinθ/cosθ; amplitude or phase imbalance directly produces angle errors.

V. Impact of Distortion on the System

Signal distortion is not merely a "visual issue" on the oscilloscope; it directly reflects on system performance:

· Angle output jitter: Noise demodulated by the RDC causes high-frequency jitter in the angle value, amplifying speed feedback noise.

· Periodic angle errors: Envelope distortion and harmonics cause angle errors that vary periodically with rotor position, leading to dq-axis current fluctuations and torque ripple.

· Faults and loss of lock: Severe interference may cause the RDC tracking loop to lose lock, resulting in angle jumps and triggering overcurrent or overvoltage protection.

· System performance degradation: In electric vehicles, this may lead to reduced motor efficiency, increased vibration and noise, and even compromise vehicle safety.

VI. How to Identify and Troubleshoot

In practical debugging, the following methods can quickly determine whether resolver signals are affected by strong EMI:

· Use differential probes to measure S1-S3 and S2-S4, avoiding single-ended probes that may introduce ground loops.

· Observe both the excitation signal and the output signals simultaneously to determine whether interference is introduced through the excitation side.

· Compare waveforms with the motor stationary vs. running, and with PWM switching on vs. off, to determine whether interference originates from the power circuit.

· Use the oscilloscope's FFT to analyze the interference spectrum and check whether it coincides with the PWM carrier frequency or edge harmonics.

· Measure the common-mode voltage of the two lines to ground to assess the common-mode interference level.

VII. Suppression Measures

Suppressing distortion requires simultaneous efforts on three fronts: the interference source, the coupling path, and the receiver.

1. Optimize wiring harness and shielding

· Use shielded twisted pairs for resolver signal lines, with the shield grounded at a single point at the decoder end to avoid ground loops caused by double-ended grounding.

· Route signal lines separately from power cables, avoiding long parallel runs; if crossing is unavoidable, cross them perpendicularly.

· If necessary, use metal conduits or double-layer shielding to improve high-frequency shielding effectiveness.

2. Filtering and protection

· Add common-mode chokes and RC low-pass filters at the RDC input to suppress common-mode noise and high-frequency glitches.

· The filter cutoff frequency should be higher than the resolver excitation frequency while avoiding excessive phase delay, which would otherwise introduce new angle errors.

· Add protection devices such as TVS tubes to prevent transient overvoltages from damaging the decoder circuitry.

3. Layout and grounding

· Place the decoder board as close as possible to the resolver connector to shorten analog signal traces.

· Separate analog ground from power ground, connecting them at a single point to prevent large currents from flowing through the analog ground.

· Pay attention to shield continuity in connector selection, ensuring the shield is not broken at the connector.

4. Excitation side treatment

· Use shielded twisted pairs for excitation lines as well, and add filtering to the excitation power supply.

· Avoid sharing connectors or harnesses between excitation signals and power cables.

5. System and algorithmic measures

· Choose RDC chips with strong anti-interference capability, such as those with built-in synchronous demodulation and digital filtering.

· In software, apply filtering, rate limiting, and plausibility checks to the angle signal to prevent individual interference pulses from causing control anomalies.

· Optimize PWM edge slew rates and add snubber circuits to reduce interference at the source.

VIII. Conclusion

The magnetoresistive resolver is inherently a rugged sensor, but its sine/cosine outputs are low-level analog signals that are susceptible to distortion under strong EMI. Understanding the sources of interference and coupling paths, and implementing proper shielding, filtering, grounding, and layout, are essential to ensuring the accuracy and reliability of angle feedback.

For engineers, the waveform on the oscilloscope is the best "health report." The earlier signal distortion is detected, the lower the probability of system issues.