Optical Encoder
Optical encoders, built around diffraction gratings and photoelectric detection, convert rotary or linear motion into electrical signals. They are the mainstream position-feedback devices in CNC machine tools, semiconductor platforms, precision metrology, robotics, and high-end automation. Thanks to high resolution, low sub-division error (SDE), and excellent repeatability, optical encoders are widely used for nano- to micrometer-level positioning.
What Is an Optical Encoder
An optical encoder is a sensor that acquires position information via a light source → optical grating → photoelectric detection chain. Its core comprises a periodically structured scale/disk and a readhead: within a specified optical gap, the readhead detects the fringes or interference formed by transmission/reflection of the grating. After processing by an analog front end (AFE) and interpolation/decoding circuitry, it outputs incremental A/B (optionally with Z reference), sine/cosine 1 Vpp, or absolute position serial data (SSI/BiSS/EnDat, etc.).
By motion type, encoders are classified as rotary optical encoders (disks) and linear optical encoders (scales). Compared with indirect estimation through ball screws or racks, linear encoders enable direct measurement, significantly suppressing positioning errors from backlash, lead error, and thermal expansion; rotary encoders provide high-resolution angle and speed feedback to motors and rotary stages.
Working Principle
1) Imaging / Moiré
- Structure: Light source (LED/VCSEL) → collimation/imaging optics → mask/phase grating → photosensitive array.
- Mechanism: Relative motion between the scale and the reference grating inside the readhead produces Moiré fringes, yielding near-sine/cosine signals; interpolation plus amplitude/phase correction then produce high-resolution displacement.
2) Interferential / Phase Grating
- Mechanism: The phase difference between diffraction orders varies linearly with displacement; superposition forms high-purity sinusoidal signals, enabling lower SDE and higher resolution potential.
3) Transmission vs Reflection
- Transmission: Glass/ceramic substrate; high SNR and good linearity—ideal for clean environments and high-precision positioning.
- Reflection: Metallized or coated reflective gratings; compact and installation-friendly, but more contamination-sensitive—requires good sealing and air purge/scrapers.
4) Incremental vs Absolute Output Logic
- Incremental: A/B quadrature pulses (90° phase shift); direction determined by phase lead/lag; Z reference once per travel/revolution.
- Absolute: Unique code at any position (Binary/Gray), with optional diagnostics, temperature, and status registers.
Approximate linear resolution: Δx ≈ p / (N × M)
where p = grating pitch, N = analog interpolation factor (e.g., 100×), M = digital multiplication factor (typically 4×).
Equivalent angular resolution (rotary): θ_res = 360° / (lines × 4)
Taxonomy
- By motion: Linear optical encoder / Rotary optical encoder (disk)
- By output: Incremental (TTL/HTL/RS422, 1 Vpp/11 µApp) / Absolute (SSI, BiSS-C, EnDat 2.2, parallel)
- By optics: Transmission / Reflection; Imaging / Interferential; Amplitude grating / Phase grating
- By enclosure: Open (high dynamics, low friction) / Sealed (IP65–IP67, resistant to oil mist and coolant)
- By range/substrate: Glass/ceramic (low CTE), steel tape (long travel), coated reflective scales, etc.
Key Components & Signal Chain
- Light source & optics: LED/laser; collimation/focus determine illumination uniformity and temp drift; aging and power closed-loop recommended.
- Grating/disk: Pitch
p, duty, and phase accuracy set signal purity and SDE. - Photosensor array / AFE: Multi-channel sampling; auto gain/offset/phase balance (ABC).
- Interpolation / encoding ASIC: Amplitude/phase correction, ellipse compensation, digital filtering and jitter suppression, protocol encoding, line drivers.
- Physical layer: RS422 differential, 1 Vpp / 11 µApp; termination impedance and cable layout (shielding/grounding) strategies.
Outputs & Interfaces
| Output mode | Typical signal | Notes |
|---|---|---|
| Incremental square wave | A/B (+Z), TTL/HTL/RS422 | PLC high-speed counting; speed/position loops; prefer differential for long runs |
| Sine/cosine | 1 Vpp, 11 µApp | Ultra-high-resolution interpolation; SDE and jitter depend on chain quality |
| Absolute serial | SSI, BiSS-C, EnDat 2.2 | Single/multi-turn absolute, diagnostics/temperature/alarm registers |
| Fieldbus | EtherCAT, PROFINET, CANopen | Multi-axis sync, distributed clocks, online configuration |
Bandwidth estimate (linear): f_max ≈ (v / p) × edges
where v = linear speed; edges = valid edges per period (e.g., 4× multiplication).
Key Specifications
| Specification | Typical range / notes |
|---|---|
| Resolution | Linear: 1 µm → 1 nm; Rotary: ≤ 24-bit equivalent |
| Linear accuracy | High-end optics: ±1–±3 µm/m; Standard: ±3–±10 µm/m |
| Repeatability | Linear: ≤ ±0.1–±0.3 µm; Rotary: sub-arcsecond achievable |
| SDE (Sub-Division Error) | High-quality 1 Vpp chains: ±20–±80 nm |
| Jitter | Tens of nanometers; dominated by AFE and clock phase noise |
| Phase/duty error | A/B 90° ±(1–5)°; duty 50% ±(2–10)% |
| Speed capability | Linear > 1 m/s; Rotary up to > 12,000 RPM (interface-dependent) |
| Environmental rating | IP40 (open) to IP67 (sealed); IEC 60068-2 vibration/shock |
Installation & Error Sources
- Gap and attitude (pitch/roll/yaw) deviations → amplitude imbalance and higher SDE
- Abbe error: offset × angular error; route the measurement line through the motion center or compensate in software
- Cosine error (small-angle approx.):
e ≈ (L × θ²) / 2 - Disk eccentricity / shaft runout (rotary): introduces fundamental/2nd-order angular harmonics
- Thermal expansion (CTE): glass/ceramic = low CTE; steel tape needs thermal compensation and floating mounts
- EMC/grounding: single-point grounding, proper shielding and return paths to avoid common-mode/loop noise
Calibration & Compensation
- Linear mapping: Use a laser interferometer/ballbar to capture the error curve, then load into the controller LUT
- Thermal drift compensation: Combine scale/frame temperatures with a CTE model for real-time correction
- Amplitude/phase & ellipse correction: Adaptive balancing prior to interpolation
- Reference strategy: Distance-coded reference marks shorten homing distance and improve repeatability
Comparisons
| Technology | Resolution/accuracy | Contamination/environment | Typical range | Main drawbacks |
|---|---|---|---|---|
| Optical encoders | ★★★★★ | ★★★ | Medium/long | Sensitive to contamination/condensation; tight installation tolerances |
| Magnetic encoders | ★★☆ | ★★★★ | Long | Weaker linearity and SDE performance |
| Inductive/capacitive | ★★★ | ★★★★ | Medium | Susceptible to nearby metal/eddy currents or moisture |
| Resolver/potentiometer | ★★ | ★★★★★/★ | Medium/short | Lower resolution or less intuitive interface |
Applications
CNC (linear and rotary axes), CMM and metrology platforms, semiconductor alignment/exposure/inspection, precision motion stages, robot joints and gearbox backlash monitoring, medical imaging/radiotherapy, print & packaging synchronization, SMT/inspection, and high-speed handling.
Maintenance & Troubleshooting
- Routine: Periodic cleaning (lint-free cloth + suitable solvent), check cable bend radius/shielding, monitor humidity and condensation
- Common symptoms & remedies:
- Missed pulses/edge loss: Excessive gap, contamination blockage → adjust alignment/clean/add sealing and air purge
- Increased SDE/jitter: AFE/interpolation chain noise, poor grounding → optimize power, routing, and termination
- Absolute comms failure: SSI/BiSS/EnDat parameter or polarity mismatch → verify frame length, CRC, timing, impedance
- Angular harmonic error (rotary): Eccentricity/runout → improve concentricity and bearing stiffness; apply harmonic compensation
Selection Guide
- Target accuracy/repeatability (µm/m or arcsec) and dynamic speed
- Operating principle (transmission/reflection; imaging/interferential) and pitch
p - Output interface (A/B/Z, 1 Vpp, SSI/BiSS/EnDat, fieldbus) vs controller bandwidth
- Enclosure & environment (open/sealed; IP rating; coolant/dust)
- Mechanical & thermal design (gap/attitude tolerances; CTE; floating mounts)
- Compensation & diagnostics (error mapping; temperature/status registers; online alarms)
- Lifecycle (serviceability of cables/readheads; spare availability; calibration capability)
Standards & References
- IEC 60529:2020 (IP protection ratings)
- IEC 60068-2 (vibration/shock/temperature-humidity)
- IEC 61000-6-2 / -6-4 (industrial EMC immunity/emissions)
- ISO 230-2 / ISO 230-3 (machine-tool positioning & thermal tests)
- ISO 10360 (CMM verification)
- ISO 14644 (cleanroom requirements)
Summary: Mastering the principles, specifications, interfaces, and installation/compensation of optical encoders enables high-accuracy, robust, and diagnosable long-term operation under demanding conditions.