Encoders vs Resolvers: Choosing the Right Sensor for Precision Motion Control

What Are Resolvers?

Resolvers are electromechanical devices that measure angular position and velocity in motion control systems. They consist of two main components - a rotor and a stator.

The rotor is the rotating part attached to the shaft whose position needs to be sensed. The stator is the static part wrapped around the rotor, forming the transformer windings.

The rotor is excited by a reference AC signal, which induces voltages in the stator windings based on the rotational angle. The angular position can be precisely determined by measuring these output sine and cosine voltage signals.

There are two common types of resolvers:

1. Rotary resolvers: Used in rotary applications like motors, drives, and rotary tables. The rotor rotates continuously with the shaft over multiple revolutions.
2. Linear resolvers: Used in linear motion systems in a limited stroke of a few inches. The rotor moves linearly back and forth to sense position.

Resolvers find widespread use in motion feedback systems across industrial machines, robot arms, antenna platforms, assembly lines, and more. They can provide absolute position data, which is advantageous for recovery after power loss. Their durability makes them well-suited for harsh environments involving high temps, vibration, and contamination - a key benefit over optical encoders.

What are the Advantages of Using Resolvers?

Resolvers stand out for their ultra-robust construction and reliability to perform consistently through demanding operating conditions that cripple other position sensors. Here are some of the notable benefits:

Durability in Harsh Environments

• With no optics or electronics, resolvers are extremely resistant to dust, dirt, and moisture and can operate at very high temps exceeding 200°C. Their encapsulated design is also highly tolerant to shock, vibration, radiation, EMI/RFI noise.

High Temperature Tolerance

• The absence of heat-sensitive components allows resolvers to work reliably at temperatures exceeding 200°C, which is usually beyond the scope of encoders. This permits their deployment around engine compartments, turbine systems, etc.

Absolute Position Sensing

• Resolvers directly provide absolute angular position data rather than relative, eliminating the need to initialize or recalibrate after power loss. This aids in quick recovery and positioning accuracy.

Overall, resolvers set new benchmarks for robustness and standalone functioning without environmental control. Both rotary and linear variants deliver a high accuracy of ±30 arc-seconds at fast response times under 10 milliseconds.

Not prone to incremental count errors, they outlast other sensing solutions in challenging dynamics involving extremes of speed, pressure, and temperatures, to name a few.

Are There Limitations With Resolvers?

While resolvers clearly outmatch optical encoders for durability, they do have some inherent drawbacks stemming from their working principle:

Complex Signal Processing

The sine/cosine output signals require additional hardware and software for precise conversion into usable position data. This adds to system complexity and overall costs.

Size and Weight

Resolvers employ bulky multi-pole transformers in their construction. They can be considerably larger and heavier than compact optical encoders. This hinders applications where tight-weight budgets exist.

Lower Resolution

The highest resolution resolvers achieve is around 18 bits (approx. 4 arc-seconds). While sufficient for most motion control needs, optical encoders can potentially reach finer resolutions exceeding 24 bits for nano-positioning systems.

Limited Maximum Speed

Large rotary resolvers have a maximum rotational velocity limit of around 20,000 rpm. This constrains deployment in ultrahigh-speed systems, unlike smaller optical encoders.

Overall, the resolver’s lower part count, lack of electronics/optics, and simple, rugged design boost field reliability despite complex outputs and sizing. Precision below 10 arc-seconds is enabled through advanced signal processing techniques - on par with many enclosed optical encoders.

What are Encoders?

Encoders are electronic devices used to measure angular or linear position by converting motion into digital signals that can be decoded. They rely on optical, magnetic, or capacitive effects to generate pulses that indicate incremental movements.

Some common types of encoders include:

1. Optical Encoders: Use light beams and gratings/code-wheels to produce signals based on motion. Can be incremental or absolute.
2. Magnetic Encoders: Detect changes in magnetic fields from magnetized rotors via Hall Effect sensors. Provide incremental pulses.
3. Capacitive Encoders: Sense positions via capacitance changes between rotating plates/scales to output incremental signals.

In incremental encoders, only relative motion from a reference can be tracked by counting pulses. Absolute encoders additionally encode positions into unique signal combinations to facilitate instant position readout.

Encoders find wide-ranging use in motion systems across industrial machines, high-precision instruments, robotics, and electronics. Popular applications exploit their high resolution, high-speed capabilities, and precision digital interfaces. 

See here to learn more about Encoders

What Are the Advantages of Using Encoders?

Encoders leverage various sensing principles to provide position feedback with compelling benefits like:

High Resolution and Precision

Optical encoders can achieve extremely fine resolution of just a few arc-seconds using diffraction grating scales. This enables precise nano-positioning systems.

Compact and Versatile

Small encoders are available in tape scales, discs, and rods that can measure from fractional mm to meters flexibly. Easy to embed in tight spaces.

Digital Interface

Unlike analog resolvers, encoders provide position data digitally for direct interfacing with modern controllers, drives, and data systems.

Overall, encoders excel in delivering very accurate and repeatable position measurements down to microns or less. Absolute optical variants avoid risks of position loss during power off/on cycles through inbuilt battery backup or multi-turn counting capabilities.

High-resolution incremental types provide precision speed feedback, too. Digital communications facilitate seamless integration with motion control networks and automation software.

What Are the Limitations of Encoders?

While encoders outshine on precision and integration, they have some weaknesses arising from their sophisticated optical/electronic construction:

Sensitivity to Contamination

Dust, fibers, fluids, and oils can obstruct/interfere with the optic sensor inside encoders, leading to signal degradation or complete failures.

Limited Temperature Range

Most encoders employ electronics and LED/laser sources and have a standard operating temperature of -20C to 85C with options for -40C to 120C, which is still applicable for many motor or automotive conditions.

Noise Susceptibility

Long cable runs make encoder signals prone to electrical interference, which gets superimposed as position errors demand shielded wires.

Fragility and Shorter Lifespan

High-resolution glass/tape scales wear out over time. Also, mechanical knocks can easily displace/damage the internal optics. The mean time between failures is lower than resolvers.

Engineers must take precautions to protect encoders from adverse particulates, temperatures, vibrations, shocks, and moisture levels for reliable functioning. Backlash, motor noise, and encoder nonlinearities/hysteresis also affect metrological performance. Proper installation and handling are key to utilizing their best-in-class resolution capabilities.

Read more about encoders in inhospitable environments.

Choosing Between Resolvers and Encoders

When designing precision motion control systems, the decision to use a resolver versus encoder comes down to factors like:

Accuracy Requirements

If ultrahigh accuracy below 10 arc-seconds is needed for nanometer-level positioning, encoders are a clear choice. For robust feedback in the tens of arc-minutes range, resolvers may suffice.

Environmental Conditions

Harsh environments with high temperatures, shock/vibration, radiation, or corrosive elements favor the sturdy resolver. Encoders demand protection in such scenarios.

Speed and Dynamics Needs

With high max rotational speeds of 100,000+ rpm, encoders better serve very high-speed systems over resolvers limited to ~20,000 rpm.

Interface and Integration Complexity

Encoders provide plug-and-play digital integration. Resolvers require additional analog-digital converters and sine/cosine decoding hardware.

Budget Considerations

Incremental encoders provide the most economical solution. High-end absolute optical encoders and resolvers with advanced signal processing can be pricier.

Finding the Right Balance for Precision Motion Systems

When comparing resolvers versus encoders, you can see how both technologies will continue playing indispensable and complementary roles across industrial motion control applications.

With resolvers becoming smarter and encoders getting tougher, opportunities exist to combine their strengths for synergistic outcomes. Thanks to their balanced evolution, motion systems engineers now enjoy great flexibility in component selection as per budgets and functional requirements.

Do you need more help choosing the best option for your industrial needs? For over 50 years, EPC has provided tailored motion-sensing devices to industry leaders globally. Reach out to us for recommendations or customization support. To explore resolver or encoder options optimized for your needs, check out our specialized portfolio of extreme environment and high-accuracy feedback transducers.

Contact us today to learn more about how EPC can enhance the performance of your motion systems.

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