Differences Between Magnetic Scales and Precision Code Discs: How to Select Magnetic Encoding Products

Author: AIC Engineering (骏材磁应用团队) | Material: | Industry:

Differences Between Magnetic Scales and Precision Code Discs:

How to Select Magnetic Encoding Products

Application Pain Points: The Positioning Accuracy Dilemma

Design engineers working on precision motion control—whether in CNC machine tools, semiconductor wafer handling, robotic joints, or medical imaging gantries—face a critical decision early in the system architecture phase: should the feedback element be a linear magnetic scale (magnetic grid ruler) or a rotary precision code disc (magnetic encoder disc)? The confusion is understandable because both products encode position information magnetically, both interface with similar Hall-effect or magnetoresistive (MR) sensor ICs, and both promise non-contact, wear-free operation.

Yet the engineering trade-offs diverge sharply. A magnetic scale offers continuous linear position feedback over travel lengths from tens of millimeters to several meters, but demands tight mounting tolerances, substrate flatness, and environmental sealing against ferrous contamination. A rotary magnetic code disc encodes angular position in a compact form factor, but resolution is constrained by pole count, disc diameter, and interpolation electronics. Selecting the wrong topology—or specifying the wrong pole pitch, magnetization pattern, or sensor gap—can result in position errors that exceed the servo loop's ability to compensate, leading to scrapped product, costly redesigns, or missed project timelines.

This article provides a trend-aware, first-principles framework for making this selection decision confidently.

Material and Topology Comparison Table

The table below compares the two primary magnetic encoding topologies across parameters that directly affect your system-level design decisions.

What this means for your design: If your motion axis is linear and travel exceeds ~50 mm, a magnetic scale is almost always the natural choice. For rotary axes or compact joints (robotics, gimbal systems), a code disc wins on packaging. The grey zone—short-stroke linear actuators or arc segments—is where careful pole-pitch and signal-chain analysis becomes essential.

First-Principles Derivation: Signal Strength and Resolution Limits

Principle 1 — Magnetic Field Periodicity and Sensor Signal

A magnetic encoder (scale or disc) generates a spatially periodic magnetic field. For a sensor at air-gap distance $g$ above a magnetized surface with pole pitch $\lambda$ (one N-S pair = one full period), the fundamental component of the normal flux density decays exponentially with gap:

where $B_0$ is the surface flux density determined by the magnet material's remanence $B_r$ and the magnetization depth.

What this means for your design: Halving the pole pitch $\lambda$ (to double raw resolution) causes the signal amplitude to drop exponentially for the same air gap. For example, moving from $\lambda =2\mathrm{m}\mathrm{m}$ to $\lambda =1\mathrm{m}\mathrm{m}$ at g=0.5 mm reduces the signal by a factor of $e^{-\pi (0.5)/1}/e^{-\pi (0.5)/2}\approx 0.46$—a 54 % signal loss. This directly impacts signal-to-noise ratio (SNR), which limits interpolation depth and therefore effective resolution. Engineers pursuing finer pitch must either tighten the air gap (raising mechanical cost and contamination risk) or select a higher-$B_r$ magnet material (raising material cost). This is the fundamental trade-off governing the industry trend toward sub-millimeter pole pitches.

Principle 2 — Angular Resolution of a Rotary Code Disc

For a disc of effective magnetic radius $R$ with $P$ pole pairs, the angular resolution before interpolation is:

After $N$-fold electronic interpolation (typically 256× to 4096× with modern sensor ICs), the achievable resolution becomes $\Delta \theta ={360}^{\circ }/(P\times N)$. However, interpolation only works reliably when the input sinusoidal signal has adequate amplitude and harmonic purity—both of which degrade as $P$ increases on a fixed diameter (because the equivalent linear pole pitch λ=2πR/P shrinks, triggering the exponential decay from Principle 1).

What this means for your design: Simply requesting "more poles" from your magnet supplier does not guarantee higher system resolution. Beyond a critical pole count (dependent on diameter, gap, and material), the signal degrades faster than interpolation can compensate. A well-designed magnetic code disc balances pole count, magnet material grade, and specified air gap to sit in the optimal SNR window. This is why magnetization pattern design and quality verification are as important as raw material selection.

Design Parameter Recommendations

Based on current industry best practices and the physics above, the following parameter ranges serve as starting-point guidelines:

• Linear magnetic scale pole pitch: 1.0–2.0 mm for general industrial servo (achievable resolution 1–5 µm after interpolation); 0.5 mm for high-precision stages (requires gap ≤ 0.3 mm and MR sensors).
• Rotary code disc pole count: 32–64 pole pairs for Ø20–Ø40 mm discs; 100–200 pole pairs for Ø50–Ø80 mm discs. Exceeding these ranges demands careful signal-chain validation.
• Air gap specification: Design nominal gap at 60–70 % of the maximum allowable gap to provide safety margin against thermal expansion, bearing play, and assembly tolerance stack-up.
• Material selection: Bonded NdFeB (typical $B_r$ 0.45–0.70 T) for room-temperature precision applications; bonded SmCo or specially stabilized NdFeB for environments exceeding 120 °C.
• Magnetization uniformity: Pole-to-pole amplitude variation should be held within ±3 % to maintain interpolation accuracy; this requires precision magnetization fixtures and 100 % inspection.

A Magnetic Design Review Checklist covering air-gap budget, thermal derating, harmonic distortion limits, and mounting tolerance stack-up is strongly recommended before freezing your encoder subsystem design.

Precision Magnetic Encoding Components

The AIC Engineering team provides end-to-end support for magnetic encoding product development, directly addressing the design challenges outlined above:

- Magnetic Encoder and Magnetic Scale Customization: The team designs and manufactures both linear magnetic scales (rubber-magnet tape and rigid strip formats) and rotary multi-pole code discs (injection-molded rings, compression-bonded rings, and arc segments) with pole pitches down to 0.5 mm and pole-count uniformity verified to ±2 %.

- Hall IC Matching and Complete Signal-Chain Solutions: Selecting the right sensor IC (Hall, AMR, TMR) to pair with a given magnet topology is non-trivial. The AIC Engineering team provides sensor-magnet co-design recommendations, including air-gap sensitivity analysis and interpolation-error budgeting, so that system resolution specifications can be met without over-engineering the mechanical assembly.

- Permanent Magnet Product Quality Inspection: Every magnetic scale and code disc undergoes surface-field scanning and harmonic analysis to guarantee that pole pitch accuracy, amplitude uniformity, and waveform purity meet interpolation requirements—before shipment.

- Rapid Prototyping (3–7 Days): For design validation and servo-loop tuning, the AIC Engineering team offers fast-turn prototype magnetic scales and code discs, enabling engineers to verify signal quality on actual hardware within one design-iteration cycle.

Action Checklist

1. Define your measurement axis geometry and travel: Determine whether your application is fundamentally linear, rotary, or arc-segment, and specify the required resolution and accuracy at the system level (not just at the encoder output).
2. Run an air-gap budget analysis: Using the exponential decay relationship, verify that your chosen pole pitch delivers adequate signal amplitude at the worst-case mechanical gap in your assembly tolerance stack-up.
3. Request pole-pitch and uniformity data from your magnet supplier: Insist on quantified magnetization uniformity (% variation) and harmonic distortion metrics—these directly determine your achievable interpolation depth and position accuracy.
4. Contact the AIC Engineering team for customized magnetic encoding product design and rapid prototyping support. Whether a linear magnetic scale with sub-millimeter pole pitch or a high-pole-count rotary code disc matched to a specific Hall/MR sensor IC is required, the team delivers application-specific solutions from concept through volume production.

Visit https://www.aicmagnetics.com today to schedule a free engineering consultation and accelerate your precision magnetic encoder development.

References

1. E. Ramsden, Hall-Effect Sensors: Theory and Application, 2nd ed., Newnes/Elsevier,
2. P. Campbell, Permanent Magnet Materials and Their Application, Cambridge University Press,
3. R. Pallás-Areny and J. G. Webster, Sensors and Signal Conditioning, 2nd ed., Wiley,
4. IEC 62828-3:2020, "Reference conditions and procedures for testing linear and rotary magnetic encoders."
5. J. Fraden, Handbook of Modern Sensors, 5th ed., Springer, 2016, Chapter 6: Magnetic Sensors.
6. Sensitec GmbH, "Magnetoresistive Sensor Technology for Linear and Rotary Encoders," Application Note, 2019 (publicly available at sensitec.com).