Encoder vs Resolver: Differences, Pros and Cons, and How to Choose
Encoder vs resolver compared — how each works, signal types, accuracy, durability, cost, and how each connects to a PLC or drive for motion feedback.
Encoder vs Resolver: The Fast Answer
An encoder converts mechanical position or motion into a digital pulse signal — typically quadrature A/B/Z — that a PLC high-speed counter input or servo drive reads directly. A resolver is a rotating transformer that produces two analog sine/cosine voltages representing shaft angle; a dedicated resolver-to-digital converter (RDC) chip or interface card must convert those signals before the controller can use them.
Encoders are less expensive, simpler to interface, and dominate light-to-moderate industrial environments. Resolvers are ruggedized, require no electronics at the sensor itself, and remain the preferred choice in extreme temperature, vibration, and contamination environments such as aerospace, military, and heavy traction applications.
The sections below break down exactly how each device works, how to wire them to a PLC or servo drive, and how to decide which one belongs in your application.
What Is an Encoder?
An encoder is an electromechanical transducer that converts shaft rotation (rotary encoder) or linear travel (linear encoder) into an electronic signal a controller can count or decode. Encoders are found in virtually every motion-controlled machine: CNC machining centers, robotic joints, conveyor positioning systems, and servo-driven press axes.
Types of Encoders
Incremental encoders produce a pulse train as the shaft rotates. Each pulse represents a fixed angular increment. The controller accumulates pulses in a hardware counter to track position. A Z-channel (index pulse) fires once per revolution and is used to establish a home reference. Incremental encoders are simple, low-cost, and capable of very high pulse-per-revolution (PPR) counts.
Absolute encoders output a unique digital word for every shaft position within one revolution (single-turn) or across many revolutions (multi-turn). Power loss does not cause position loss. Most modern absolute encoders communicate over a serial interface — SSI, EnDat, HIPERFACE, or BiSS-C — reducing wiring complexity to a handful of signal pairs.
How an Optical Encoder Works
Inside a typical optical incremental encoder, a code disc with precisely etched transparent and opaque segments rotates between an LED emitter and a photodetector array. As the disc turns, light pulses are detected and converted to square-wave electrical signals on channels A and B, offset 90° in phase (quadrature). The quadrature relationship lets a counter determine both direction and position. Magnetic encoders replace the optical disc with a magnetized ring and Hall-effect sensors, offering better immunity to contamination.
What Is a Resolver?
A resolver is a rotating transformer — a purely electromagnetic, all-analog device. The rotor carries a single excitation winding; the stator carries two windings oriented 90° apart. When the rotor is driven by an AC reference signal, the magnetic coupling between rotor and stator varies sinusoidally with shaft angle, producing two outputs:
- Sin output: amplitude ∝ sin(θ)
- Cos output: amplitude ∝ cos(θ)
Where θ is the mechanical shaft angle. These two analog voltages allow the downstream converter to calculate exact shaft position using the arctangent relationship, with full 360° absolute-within-one-revolution knowledge at all times.
The critical point: there are no active electronic components inside a resolver. The sensor body is a wound coil assembly with high tolerance for heat, shock, vibration, and electromagnetic interference (EMI). The electronics live external to the sensor, typically inside the drive or an RDC module in the control cabinet.
How Each Connects to a PLC or Servo Drive
This is the most practical and least-covered aspect of the encoder vs resolver decision — and it directly affects your cabinet design, wiring cost, and programming approach.
Connecting an Encoder to a PLC
A standard quadrature incremental encoder wires to a high-speed counter (HSC) input module on the PLC. Most major platforms provide dedicated HSC cards:
- Allen-Bradley / Rockwell: 1756-HSC module (ControlLogix) or 1769-HSC (CompactLogix); also on-board HSC inputs on MicroLogix 1100/1400 and Micro800 series.
- Siemens: FM 350-1/FM 350-2 counter modules (S7-300/400), or the on-board fast counter inputs on the S7-1200/S7-1500 (up to 1 MHz).
- Mitsubishi: QD62 high-speed counter module (Q-series); FX5-C1AD on FX5U.
- Beckhoff EtherCAT: EL5101 incremental encoder interface terminal (1 MHz, RS422).
The HSC module counts rising and falling edges on both A and B channels (4× decoding or ×4 mode), multiplying effective resolution by four. For a 1,000 PPR encoder using 4× decoding, the controller sees 4,000 counts per revolution. The Z-channel is wired to a dedicated index input on the module and used in the PLC program to latch a home position on startup.
In the PLC program, you read the accumulated count as an integer or DINT tag. Velocity is derived by measuring counts over a timed sample window — many HSC modules do this in hardware. For a deeper look at how high-speed counter inputs fit into the broader PLC I/O architecture, see our guide to PLC programming basics and fundamentals.
Absolute encoder wiring depends on the protocol. SSI encoders need a dedicated SSI interface card or a fieldbus module that bridges to the drive network. EtherCAT-native absolute encoders (e.g., HIPERFACE DSL on a Beckhoff system) communicate over a single cable pair using industrial communication protocols.
Cable requirements: encoder signals are typically differential (RS-422) for noise immunity. Maximum recommended cable length varies by PPR and clock rate — 100 m is achievable at low PPR; above 100 kHz, keep runs short and use twisted-pair shielded cable with shield grounded at one end.
Connecting a Resolver to a Drive or PLC
A resolver cannot connect directly to a standard PLC digital input or HSC card. It requires a resolver-to-digital converter (RDC), also called an R/D converter. An RDC takes the excitation signal and the sin/cos feedback voltages, tracks the shaft angle using a Type II tracking loop, and outputs a parallel or serial digital position word (typically 10- to 16-bit resolution).
In servo drive applications, the RDC is usually integrated into the drive itself. The motor's resolver connects to the drive's dedicated resolver feedback port (typically a 9-pin or 15-pin D-sub connector carrying the reference, sin, cos, and shield lines). The drive handles the RDC math internally and exposes the position data to the motion controller over the network (EtherCAT, SERCOS, Profidrive, etc.).
When connecting resolver feedback to a PLC for standalone position monitoring — rather than through a servo drive — you need an external RDC module. Manufacturers such as Renishaw, Heidenhain, and Analog Devices (the AD2S series ICs) supply both chip-level and DIN-rail-mounted RDC solutions. The RDC module outputs a parallel digital word or an SSI/EnDat stream that a standard absolute encoder interface card on the PLC can read.
Excitation signal: the resolver requires an AC reference voltage, typically 2–7 V RMS at 2–20 kHz, generated by the RDC or drive. This is an additional wiring requirement compared with an encoder — the excitation pair runs from the drive/RDC out to the resolver and returns on the sin/cos pairs.
For VFD and servo drive integration context, see our guide to VFD programming with PLC control.
Signal Type and Output Comparison
| Parameter | Incremental Encoder | Absolute Encoder | Resolver |
|---|---|---|---|
| Output signal type | Digital pulse (A/B/Z) | Serial digital (SSI/EnDat/BiSS) | Analog sin/cos AC |
| Power loss behavior | Position lost | Position retained | Position retained (after RDC) |
| Interface at controller | HSC counter module | Encoder interface card | RDC + interface card / drive port |
| Cable count | 4–6 wires | 4–6 wires | 6–8 wires (incl. excitation) |
| Signal bandwidth | Very high (>1 MHz) | Protocol-limited | Bandwidth set by RDC tracking rate |
Accuracy and Resolution
Encoders achieve very high intrinsic resolution. A 13-bit single-turn absolute encoder delivers 8,192 unique positions per revolution. High-end optical encoders with 26-bit resolution (67 million counts/rev) are available for precision metrology and semiconductor equipment. Within a given encoder, accuracy depends on disc manufacturing quality and bearing eccentricity; typical accuracy is ±1 to ±5 arc-minutes for industrial grades.
Resolvers typically offer 10- to 16-bit resolution after RDC conversion (1,024 to 65,536 counts/rev). The analog-to-digital conversion step introduces quantization error; the RDC tracking bandwidth also limits dynamic accuracy at high shaft speeds. The fundamental resolver accuracy (before RDC) depends on winding symmetry — industrial resolvers are rated ±2 to ±10 arc-minutes. High-precision military-grade resolvers approach ±0.5 arc-minutes but at significant cost.
For most servo motor applications, both devices deliver more than adequate accuracy. The resolution advantage of high-resolution encoders becomes relevant in precision positioning, electronic gearing, and high-speed coordinated motion — common in motion control applications that demand sub-arc-minute repeatability.
Environmental Durability
This is where resolvers have a decisive edge.
| Parameter | Encoder (optical/magnetic) | Resolver |
|---|---|---|
| Operating temperature | −20 °C to +100 °C typical | −55 °C to +155 °C typical |
| Vibration (IEC 60068-2-6) | 10–20 g | 50–100 g |
| Shock | 50–100 g | 100–500 g |
| IP rating | IP54–IP67 (housing dependent) | IP65–IP67 (no internal PCB to seal) |
| EMI immunity | Moderate (sensitive to high dV/dt) | Excellent (fully passive) |
| Contamination | Optical: sensitive to oil/dust; Magnetic: robust | Very robust (sealed wound assembly) |
The absence of active electronics inside a resolver is the core durability advantage. There are no LEDs to degrade, no photodetectors to contaminate, no internal ICs to fail from heat cycling. This makes resolvers the standard feedback device on traction motors in electric vehicles, aerospace actuators, military turret drives, and heavy steel-mill equipment where temperatures routinely exceed what encoder ICs can tolerate.
Cost Comparison
Encoders are generally less expensive for the sensor itself. A basic 1,000 PPR industrial incremental encoder costs $30–$150 USD. Mid-range absolute encoders with SSI output run $100–$400. High-resolution multi-turn absolute encoders (EnDat, 23-bit+) can reach $500–$1,200.
Resolvers have a moderate sensor cost ($60–$300 for standard industrial units) but carry a system-level cost premium: the RDC module or the drive's built-in resolver port. Standalone DIN-rail RDC modules add $150–$600 to the BOM. If your servo drive already includes a resolver input, this overhead disappears.
Total installed cost is often similar once wiring labor, cable, and interface hardware are counted. The cost argument favors encoders in benign environments; resolvers become cost-competitive when the alternative is protecting an encoder against extreme conditions (hermetic housing, cooling, EMI shielding).
Application Suitability
Choose an encoder when:
- The operating environment is clean, temperature-controlled, or moderate
- High resolution (>16 bit) or very high bandwidth is required
- You are using a standard PLC with HSC inputs and want simple, direct wiring
- Budget is constrained and environmental conditions do not justify resolver cost
- The motor or axis already ships with an encoder (most standard servo motors do)
Choose a resolver when:
- Operating temperature exceeds +100 °C or drops below −20 °C
- High shock and vibration are present (heavy machinery, off-road vehicles, aerospace)
- Extreme EMI environments exist (large welding systems, traction drives)
- Long-term reliability in sealed, unserviceable locations is paramount
- The servo drive platform you are using has a native resolver input port
Also consider types of industrial sensors for related position and proximity sensing technologies that complement encoder or resolver feedback in a full machine control architecture.
Side-by-Side Comparison Table
| Criterion | Encoder | Resolver |
|---|---|---|
| Operating principle | Optical/magnetic pulse generation | Electromagnetic transformer (sin/cos) |
| Output | Digital pulses or serial data | Analog AC voltages |
| PLC/drive interface | HSC module, encoder interface card | RDC converter, drive resolver port |
| Resolution | Up to 26+ bit | Typically 10–16 bit (RDC-limited) |
| Accuracy | ±1–5 arc-min (industrial) | ±2–10 arc-min (industrial) |
| Temp range | −20 to +100 °C | −55 to +155 °C |
| Vibration tolerance | Moderate | Very high |
| EMI immunity | Moderate | Excellent |
| Sensor cost | $30–$1,200 | $60–$300 |
| System cost | Lower (direct to PLC card) | Higher (RDC required unless in drive) |
| Power loss position | Lost (incremental) / retained (absolute) | Retained (after RDC) |
| Typical applications | Servo axes, CNC, conveyors, robotics | Aerospace, EV traction, heavy industry |
Frequently Asked Questions
What is the difference between an encoder and a resolver?
An encoder is a digital sensor that produces pulse or serial-digital position data; the signal is read directly by a PLC counter input or drive encoder port. A resolver is an analog electromagnetic transformer that produces sine/cosine voltages proportional to shaft angle; those voltages must be processed by a resolver-to-digital converter before the controller can use them. Encoders are simpler to interface and lower cost; resolvers are more durable in extreme environments because they have no active electronics at the sensor.
Is a resolver more accurate than an encoder?
Not inherently. High-resolution optical or magnetic absolute encoders typically outperform resolvers in raw accuracy and resolution because the analog-to-digital conversion step in a resolver system (the RDC) limits resolution to 10–16 bit and introduces tracking-loop error at high shaft speeds. Industrial-grade resolvers are typically accurate to ±2–10 arc-minutes; high-end encoders can reach ±1 arc-minute or better. However, in vibration and high-temperature environments where encoder optics or electronics degrade, a resolver may deliver more consistent accuracy over service life.
Why use a resolver instead of an encoder?
The primary reasons are extreme environmental demands. Resolvers operate reliably at temperatures from −55 °C to +155 °C, tolerate very high shock and vibration loads (100 g and above), and are immune to contamination because there are no internal electronic components. They are also inherently absolute — the shaft angle is always known from the sin/cos ratio without requiring a homing cycle. For these reasons they are standard in aerospace actuators, electric vehicle traction motors, military systems, and heavy industrial drives.
Can a resolver replace an encoder?
Yes, in most servo motion applications — provided the drive or control system has a resolver input (or an RDC module is added). Resolution and bandwidth requirements must be checked: a resolver after RDC conversion typically provides 12–16 bit resolution, which is sufficient for most servo axes but may fall short of high-precision positioning applications that need 23-bit or higher absolute encoders. Cable runs and excitation wiring also differ. In practice, many servo drive platforms support both feedback types via interchangeable feedback modules, making substitution straightforward at the hardware level.
