A resolver is a rugged rotary position sensor built like a small transformer, with no electronics inside the sensing element itself. It reports shaft angle through a pair of analog signals, a sine and a cosine, whose amplitudes vary with the shaft's rotation. Because it is essentially copper windings on iron, with no fragile optics or on-board circuitry, a resolver survives heat, vibration, and contamination that would degrade an optical encoder. This page explains how the sine and cosine windings encode angle, how a resolver-to-digital converter turns those analog signals into a usable position, and why resolvers are chosen for harsh-duty industrial motors.
Resolver in one line: A resolver is a transformer-based analog position sensor that reports shaft angle through sine and cosine output windings, decoded into a digital position by a resolver-to-digital converter. Having no internal electronics or optics, it tolerates high temperature, vibration, and contamination far better than optical encoders, which is why it is favored for feedback on rugged, harsh-environment servo motors.
A resolver works on transformer principles. A rotor winding is energized with an AC reference signal, and two stator windings are positioned ninety degrees apart mechanically. As the rotor turns, the coupling between the rotor and each stator winding changes with the shaft angle, so the AC signal induced in the two stator windings is modulated by the sine and the cosine of that angle respectively. The result is two output signals whose relative amplitudes encode exactly where the shaft is pointing.
Reading a single winding is ambiguous, because the same amplitude occurs at more than one angle, but the pair together is unambiguous within a revolution. The ratio of the sine and cosine amplitudes uniquely identifies the shaft angle throughout a full turn. This is why the two windings are essential: one channel says how much, the pair together says exactly where. The measurement is inherently analog and continuous, with no discrete steps in the sensing element itself.
Because all the resolver contributes is windings and an iron core, the actual position information leaves the device as raw analog sine and cosine signals riding on the excitation frequency. There is nothing in the sensor to translate that into a number; the resolver simply presents these two modulated waveforms and relies on downstream electronics to extract the angle from them. That simplicity is precisely what makes the sensor so robust.
Turning the resolver's raw sine and cosine signals into a usable position is the job of a resolver-to-digital converter, or RDC. This electronic circuit, located in the drive or a dedicated interface rather than in the resolver, demodulates the two signals to recover their amplitudes and computes the shaft angle from them. It continuously tracks the angle as the shaft turns and outputs a digital position that the servo control loops can use, much as they would use an encoder count.
The RDC does more than a single measurement. Because it tracks the angle continuously, it can also derive shaft velocity from the rate of change of the angle, which is useful to the velocity loop in a servo drive. Keeping this conversion circuitry in the drive, away from the motor, is deliberate: it places the temperature-sensitive electronics in a benign environment while leaving only the rugged, passive winding assembly out on the hot, vibrating motor.
This division of labor is central to the resolver's appeal. The sensing element endures the harsh conditions because it has nothing delicate in it, and the intelligence that interprets its signals lives where conditions are controlled. The system as a whole delivers reliable position feedback, but the vulnerable part has been moved off the machine. It is a fundamentally different architecture from an optical encoder, which necessarily puts its light source, disc, and photodetectors right at the shaft.
The reason to choose a resolver over an optical encoder is almost always the environment. An optical encoder relies on a light source, a precisely patterned disc, and photodetectors, all of which can be degraded by high temperature, mechanical shock, vibration, dust, oil mist, and moisture. A resolver has none of those vulnerabilities. Its windings and iron core tolerate high operating temperatures, shrug off vibration and shock, and are unaffected by the dust and contamination that would obscure an optical disc.
This makes resolvers a natural fit for demanding industrial and mobile motors: heavy machinery, traction and off-highway equipment, motors mounted close to hot processes, and anywhere a sealed, robust feedback device is worth the tradeoffs. Resolvers are typically absolute within a single turn, immediately reporting angle at power-up within that revolution, and their analog nature means there are no counts to lose in the sensing element. The tradeoff is the need for the RDC and generally coarser resolution than a high-line optical encoder, so the choice comes down to whether ruggedness or ultimate resolution matters more.
For operations that monitor equipment remotely, the resolver's durability aligns well with hard-to-reach installations where a fragile sensor would be a liability. On motors at unmanned or distant sites, feedback that keeps working through temperature swings and vibration reduces the number of trips prompted by a failed sensor. A cloud SCADA platform such as Merobix can trend the position, speed, and any drive-reported feedback health from these axes, so operations sees how motion equipment is behaving across scattered sites and can rely on a feedback source chosen precisely because it holds up in conditions people cannot easily visit.
A resolver is a small transformer: an AC-excited rotor couples into two stator windings offset by ninety degrees, producing output signals modulated by the sine and cosine of the shaft angle. The ratio of those two amplitudes uniquely identifies the angle within a revolution. The raw analog signals are then decoded by a resolver-to-digital converter into a usable digital position.
A resolver-to-digital converter, or RDC, is the electronic circuit, located in the drive rather than the sensor, that demodulates the resolver's sine and cosine signals and computes the shaft angle from them. It tracks the angle continuously, outputs a digital position for the servo loops, and can also derive shaft velocity from the rate of change of angle.
Resolvers are chosen for harsh environments. Having only windings and an iron core with no optics or on-board electronics, they tolerate high temperature, vibration, shock, dust, and moisture that can degrade an optical encoder. The tradeoff is the need for a resolver-to-digital converter and generally coarser resolution, so ruggedness is weighed against ultimate precision.
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