Linear Encoder
Linear encoders are used to convert linear displacement into electrical signals (digital or analog). They can directly measure the linear motion of stages, slides, or workpieces, serving as core positioning sensors in CNC machine tools, coordinate measuring machines (CMMs), semiconductor equipment, and high-end automation platforms. Compared with indirect displacement calculation via lead screw/rack, linear encoders provide "direct measurement", significantly reducing the effects of lead screw thermal expansion, backlash, and transmission errors on positioning accuracy.
What is a Linear Encoder
A linear encoder consists of a scale (Scale/Rule) and a readhead (Readhead): the scale contains periodic structures (gratings, magnetic tracks, inductive patterns), while the readhead moves above it with a specified gap, reads position information, and outputs electrical signals. According to output type, encoders can be incremental or absolute; according to sensing principle, they can be optical, magnetic, inductive, capacitive; according to construction, they can be open-type or sealed/enclosed.
Working Principle
Optical
- Structure: Glass/ceramic grating scale (or stainless steel tape) + LED/laser light source + photodiode array.
- Mechanism: Transmission/reflection interference or moiré fringe imaging, demodulated and interpolated by the readhead.
- Features: High resolution (nm level), linear error as low as ±1 μm/m; sensitive to contamination and condensation.
Magnetic
- Structure: Magnetic tape with fixed pole pitch (steel or flexible) + Hall/AMR/GMR/TMR sensor array.
- Mechanism: Readhead senses the varying magnetic field waveform as displacement changes, calculates position.
- Features: Resistant to oil, vibration tolerant, large installation tolerance, long measuring range (tens of meters); lower resolution and accuracy than high-end optical encoders.
Inductive
- Structure: Excitation and induction coils in the readhead; scale with metallic patterns or conductive targets.
- Mechanism: Coupling and eddy current effects vary with gap and position; readhead demodulates phase/amplitude to obtain displacement.
- Features: Pollution resistant, high-temperature capable, immune to EMI; accuracy between optical and magnetic.
Capacitive
- Structure: Electrode array + scale with periodic conductive patterns.
- Mechanism: Capacitance matrix changes with displacement; readhead uses phase lock/decoding to obtain displacement.
- Features: Compact, low power; sensitive to moisture and nearby metal effects, requiring good shielding and grounding.
Types & Constructions
1) By Output Signal
- Incremental: Outputs quadrature A/B pulses (with optional Z/reference pulse). Requires counter in controller; can provide 1 Vpp sine/cosine analog signals for interpolation.
- Absolute: Provides unique code at any position; can be single-track or distance-coded reference marks, commonly using SSI, BiSS-C, EnDat.
2) By Packaging & Protection
- Open-type: Exposed readhead, high dynamic response, low friction; suitable for clean/semi-clean environments.
- Sealed/Enclosed: Scale and readhead inside sealed housing, with wipers/air purge for protection; suitable for machine tools and harsh environments.
3) By Scale Material & Form
- Glass/ceramic gratings: Very low thermal expansion (CTE), μm/m accuracy; requires stable support and temperature control.
- Steel/stainless steel tapes: Long measuring range, can be coiled; larger CTE, requiring thermal compensation.
- Magnetic tapes (adhesive/rail-mounted): Flexible installation, contamination resistant; less linear accuracy than high-end optical.
- Inductive/capacitive conductor patterns: Rugged and environment resistant.
Outputs & Interfaces
| Type | Output Style | Typical Use |
|---|---|---|
| Incremental | A/B (+Z), TTL/HTL, RS422 | PLC high-speed counting, velocity/position loops |
| Sine/Cosine | 1 Vpp, 11 μApp | High-resolution interpolation (×4 ~ ×10,000) |
| Absolute | SSI, BiSS-C, EnDat 2.2 | Absolute position, diagnostics, temperature/status |
| Fieldbus/Ethernet | CANopen, EtherCAT, PROFINET | Multi-axis sync, long distance, online diagnostics |
Max output frequency vs. velocity (incremental square wave):
f_max ≈ (V / Pitch) × Edges_per_cycle
- V: Linear velocity (mm/s)
- Pitch: Scale pitch (mm/cycle)
- Edges_per_cycle: Number of edges per cycle (e.g. ×4 multiplication)
Key Specifications
| Spec | Meaning / Typical Range | Notes |
|---|---|---|
| Resolution | 5 μm → 0.1 μm (magnetic/steel tape); 1 μm → 1 nm (optical/interferometric) | Related to interpolation factor |
| Accuracy | ±3 ~ ±10 μm/m (magnetic); ±1 ~ ±3 μm/m (high-end optical) | Expressed in μm/m or ppm |
| Repeatability | < ±0.1 ~ ±0.5 μm (high-end optical) | Affected by environment and installation |
| Subdivision Error (SDE) | ±20 ~ ±80 nm (premium 1 Vpp systems) | Periodic interpolation error |
| Jitter/Noise | Tens of nm | Linked to analog chain and power noise |
| Reference marks | Single, distance-coded, bidirectional | Used for homing or absolute reference |
| Measuring Length | 0.1 m → 30+ m | Long ranges require stitching/compensation |
| Ride Height | 0.1 ~ 1.0 mm (by principle) | Also limited by pitch/roll/yaw tolerance |
| CTE | Glass/ceramic: ~0.5–2 ppm/K; Steel: ~10–17 ppm/K | Determines thermal compensation |
| Protection rating | IP40 (open) → IP67 (sealed) | Consider oil mist, coolant, dust |
Installation & Geometric Errors
- Gap/attitude tolerances: Control ride height and angular deviations (pitch/roll/yaw); exceeding limits causes signal loss and increased SDE.
- Abbe Error: Offset between measurement axis and motion axis multiplied by angular error creates extra displacement error; encoder line should align with motion axis or be modeled.
- Cosine Error: Encoder axis not parallel to true motion axis introduces projection error.
- Thermal coupling & mounting: Steel/magnetic tapes should allow floating/stress-relieved mounting to avoid warping; glass/ceramic scales require isothermal support.
- Grounding & shielding: Single-point grounding of readhead and controller; shield layer properly grounded to prevent loops and coupling noise.
Calibration & Compensation
- Linear compensation: Use laser interferometer, ballbar, or gauge blocks to calibrate full range and generate error mapping tables (pitch/roll/yaw + straightness/flatness).
- Thermal compensation: Correct based on CTE and machine temperature distribution; advanced systems combine readhead temperature with thermal models.
- Reducing SDE: Use high-quality 1 Vpp signals, optimize analog chain and interpolation; maintain stable gap and alignment.
- Reference strategies: Distance-coded marks reduce homing distance; bidirectional marks improve repeatability.
Comparisons with Other Technologies
| Technology | Advantages | Limitations | Applications |
|---|---|---|---|
| Linear Encoder (Optical) | nm-level resolution, μm/m accuracy, excellent dynamics | Sensitive to contamination/condensation, strict installation | CNC, CMM, semiconductor platforms |
| Linear Encoder (Magnetic) | Contamination resistant, tolerant, long range | Lower accuracy and resolution than optical | Automation, heavy machinery |
| Laser Interferometer | Highest accuracy, traceable | Expensive, sensitive to environment/path | Calibration, metrology |
| LVDT/Eddy Current | Rugged, high short-range repeatability | Limited range, non-linearity | Stroke limits, precision control |
| Rotary encoder + leadscrew | Low cost, mature technology | Errors from thermal expansion/backlash/lead | Medium/low accuracy positioning |
Applications
- CNC machine axes (X/Y/Z): Maintain high positioning accuracy and surface quality under thermal load.
- CMM/metrology microscopes: Nanometer interpolation + error mapping ensures dimensional traceability.
- Semiconductor equipment: Lithography/alignment/inspection; requires nm-level jitter and low SDE.
- High-end printing/packaging/SMT: Long travel, high speed, precise synchronization.
- Medical equipment: CT/MRI tables, radiotherapy positioning.
- Logistics & heavy equipment: Long-travel magnetic tape systems, robust in harsh environments.
Maintenance & Troubleshooting
Routine Maintenance
- Regular cleaning (optical: lint-free cloth + IPA; magnetic: remove ferromagnetic particles). Maintain stable gap and alignment.
- Check cable bend radius and clamp condition to prevent core breakage/shield damage.
- Monitor environmental temperature, humidity, coolant spray to avoid condensation/liquid ingress.
Common Issues
| Symptom | Possible Cause | Solution |
|---|---|---|
| Signal dropout/missing pulses | Excessive gap, misalignment, contamination | Adjust ride height/attitude, clean scale, check rigidity |
| Excessive jitter/SDE | Interpolation noise, poor grounding | Improve shielding/grounding, use better PSU/cables, shorten length |
| Large linear error | Improper mounting, missing thermal comp. | Reinstall (floating mount), calibrate + load error map |
| Absolute comm. failure | SSI/BiSS/EnDat mismatch | Verify timing/polarity/frame/CRC, check cable impedance |
| Unstable reference | Misused/damaged reference marks | Clean, check reference configuration |
Standards & References
- IEC 60529:2020 (IP protection ratings)
- IEC 60068-2 series (vibration/shock/temp/humidity/salt mist)
- IEC 61000-6-2/-6-4 (industrial EMC immunity/emission)
- ISO 230-2 / ISO 230-3 (machine tool accuracy & thermal characteristics)
- ISO 10360 series (CMM verification & testing)
- ISO 14644 series (cleanroom standards for semiconductor/metrology)
Note: Applicability depends on equipment and industry; always confirm with manufacturer documentation and actual conditions.
Selection Guide
- Accuracy target: Define positioning/repeatability goals and allowable linear error (μm/m).
- Principle choice: Clean/high precision → optical; dirty/vibration/long range → magnetic or inductive.
- Outputs & Interfaces: Incremental for velocity loops; SSI/BiSS/EnDat or EtherCAT for precision & multi-axis sync.
- Mechanical & Installation: Confirm travel length, scale material, CTE, tolerances, mounting method (floating/adhesive/clamp).
- Environment & Protection: For coolant/dust/thermal drift → sealed type with proper IP, air purge/wipers.
- Compensation & Diagnostics: Check support for error mapping, temp/status monitoring, online diagnostics/alarms.
- Lifecycle factors: Cable/readhead replacement, spare part availability, calibration & service support.
By understanding the principles, construction, and installation requirements of linear encoders—and applying error modeling, thermal compensation, and standardized verification—engineering teams can achieve high accuracy, long life, and diagnostic capability for linear positioning and speed control in complex industrial environments.