Temperature vs. Magnetic Encoder Ring Accuracy: How to Choose the Right Material

Is Your Encoder Accuracy Still There When Temperature Rises 10°C?

Magnetic encoders are prized for dust immunity, oil resistance, and maintenance-free operation. But there’s a silent killer that’s often overlooked — temperature.

The magnetic field strength of an encoder ring drifts with temperature. When a robot joint goes from cold start to full load, with internal temperature surging from 20°C to over 100°C, the ring’s remanence (Br) may have already dropped by 10–20%. What does that mean for servo control precision? Not “close enough” — it means potentially blowing straight through your error budget.

The Root Cause: Why Do Magnetic Materials Drift With Temperature?

To understand thermal drift, you have to start with the basic physics of permanent magnets.

The Origin of Magnetism: Electron Spin and Magnetic Domains

The magnetism of permanent magnets fundamentally comes from electron spin magnetic moments. In ferromagnetic materials, the electron spins of neighboring atoms align in the same direction, forming microscopic magnetic domains — each domain acts like a tiny bar magnet. When all domain moments are aligned in one direction (magnetization), the material exhibits macroscopic permanent magnetism.

Temperature Is the Disruptor

Temperature is essentially thermal vibration of atoms and electrons. The higher the temperature, the more intense the lattice vibrations, and the stronger the thermal agitation working against the ordered alignment of electron spins. Here’s the process:

1. Domain wall migration becomes unstable: Thermal vibrations destabilize the boundaries (domain walls) between magnetic domains, causing some domains to begin flipping direction
2. Net magnetic moment declines: As more domains deviate from the aligned direction → the overall net magnetic moment (remanence Br) drops
3. Reaching the Curie Temperature: When temperature reaches the material’s Curie point, thermal agitation completely destroys all domain ordering — the material loses ferromagnetism entirely and becomes paramagnetic

Each material has a different Curie temperature — NdFeB: ~310–370°C, SmCo: ~700–800°C, Ferrite: ~450°C. Below the Curie point, the decline in remanence is reversible and approximately linear — this is the physical meaning of the temperature coefficient.

Why Do Different Materials Drift So Differently?

The temperature coefficient depends on how strongly the material’s crystal structure “locks in” the intrinsic magnetism:

• SmCo (−0.035%/K): The 4f electron orbital angular momentum of samarium and cobalt contributes exceptionally strong magnetocrystalline anisotropy — the crystal lattice exerts a very powerful constraint on magnetic moment orientation. Thermal vibrations struggle to flip moments, giving SmCo the lowest drift.
• NdFeB (−0.12%/K): While Nd also has 4f electrons, the crystal field anisotropy of Nd₂Fe₁₄B is weaker than SmCo’s, making magnetic moments more susceptible to thermal agitation. The upside: high iron content → high saturation magnetization → strongest field at room temperature.
• Ferrite (−0.20%/K): In spinel or magnetoplumbite structures, the indirect exchange interaction (superexchange) between magnetic ions is inherently weaker, making thermal disruption of magnetic order more pronounced. Unique trait: coercivity increases with temperature.
• Rubber Magnet (−0.18~−0.20%/K): Ferrite powder dispersed in a rubber matrix — the magnetic powder is diluted, further weakening the effective magnetic anisotropy. Drift is similar to pure ferrite.

In one sentence: The stronger the crystal lattice locks down the magnetic moment direction, the lower the thermal drift — SmCo is the most stable, rubber magnet drifts the most.

Temperature Coefficient: How We Measure Drift

As established, the remanence (Br) of all permanent magnets declines with rising temperature. The metric that quantifies this is the Temperature Coefficient of Remanence (α), expressed in %/K — the percentage drop in field strength per 1°C increase.

Example: An NdFeB encoder ring with 75 mT field strength at room temperature (20°C). When the joint reaches 120°C (Δ100K), with α = −0.12%/K:

100K × (−0.12%/K) = −12% → 75 mT drops to 66 mT

And encoder chips have strict minimum field thresholds. Take the iC-MU family: the minimum external field strength (H_ext) required at the chip surface is 15 kA/m (approx. 18.7 mT in air). Once remanence falls below this threshold, SNR collapses and angular error spikes — not a graceful degradation, but a cliff-edge failure.

Five Magnetic Ring Materials — Vastly Different Thermal Behavior

Choosing a magnetic ring isn’t just about dimensions and pole counts. The material determines the thermal envelope. Here’s how they compare:

Quick Summary:

• Ferrite: Higher field strength than rubber magnets, operating temperature up to 250°C, α = −0.20%/K. Suitable for industrial applications requiring moderate precision with budget or size constraints.
• NdFeB: Strongest field, the go-to choice for industrial robotics. But above 120°C, you need high-coercivity grades (N42SH, N38UH, etc.) — otherwise irreversible demagnetization risk escalates sharply.
• SmCo: Drifts only 1/3 to 1/4 as much as NdFeB. Over the same 100°C rise, NdFeB loses 12% while SmCo loses just 3.5%. The trade-off: higher cost and more challenging machining.
• Rubber Magnet: Ferrite powder in a rubber/polymer binder — drift similar to ferrite. Flexible and low-cost, ideal for less demanding applications, but limited to 80~140°C.

Beyond Field Strength — Three Dimensions of Temperature Impact

1. Reversible vs. Irreversible Demagnetization

Field loss that recovers when temperature drops is reversible loss (governed by the temperature coefficient). Loss that doesn’t come back is irreversible demagnetization. NdFeB is most vulnerable to irreversible loss under combined high temperature + strong demagnetizing fields (high current, open-circuit conditions). SmCo, with its extremely high intrinsic coercivity (Hcj), is virtually immune to this failure mode.

2. Thermal Expansion of Ring Geometry

The magnetic ring and its metal backplate (AL6061 / stainless steel) have different coefficients of thermal expansion. Across wide temperature swings, thermal stress in the bonding layer can cause micro-cracks and pole-pitch shifts, introducing additional angular error. This isn’t a magnetic problem — it’s a thermo-mechanical coupling problem.

3. The Encoder Chip’s Compensation Ceiling

Modern encoder ASICs (such as the iC-MU family) automatically compensate for field strength variation — but only while the field remains within the chip’s rated operating window. The iC-MU family requires field strength at the chip surface to stay between 15–100 kA/m (approx. 18.7–125 mT in air). Once temperature pushes the field outside this window, the AGC (Automatic Gain Control) saturates, compensation fails, and errors skyrocket.

iC-MU Series: Magnetic Field Operating Window Explained

Temperature affects the ring’s field output — and the encoder chip has a hard “pass/fail” line for the field it receives. Using iC-Haus iC-MU, iC-MU150, and iC-MU200 as examples:

Field Strength Operating Range

Only when the input field falls within the 15–100 kA/m range can the chip’s internal AGC (Automatic Gain Control) and interpolation logic function correctly.

What Happens When Field Drops Below Threshold?

Signal quality degradation: Insufficient sine/cosine signal amplitude → increased measurement noise → reduced effective resolution.

NON_CTR Error: This is the most dangerous scenario. With insufficient field, the system’s phase margin shrinks dramatically — especially with high pole counts (e.g., MPC=64) or low-quality magnetic targets. A NON_CTR error means the system cannot recover the correct absolute position after a restart, forcing the joint through a complete recalibration sequence.

Installation Guidance: Air Gap Is Critical

The typical distance between sensor surface and magnetic target surface (air gap) is recommended at 0.4 mm. Too large a gap → insufficient field at the chip → risk of falling into the low-field danger zone. Too small → mechanical contact risk, and field strength may exceed the upper limit.

iC-MU Compatible Magnetic Ring Material Classification

iC-Haus explicitly states in the iC-MU family datasheets: chip performance is highly dependent on the quality of the magnetic target. Material selection, pole width accuracy, field uniformity, and installation air gap — each directly impacts the final measurement resolution and system reliability. With this in mind, recommended and compatible materials for iC-Haus iC-MU series chips are classified into three tiers:

Selection principle: For applications demanding high resolution and high performance (servo joints, surgical robots), iC-Haus recommends NdFeB or SmCo. For industrial, large-diameter rings (large rotary tables, wind turbine pitch control), NBR/HNBR-based ferrite composites are the standard — meeting large-format molding requirements at manageable cost.

Tier 1: High Energy-Product Materials (Precision Feedback)

Tier 2: Composite & Rubber-Based Materials (Off-Axis Rings)

Off-axis encoder rings commonly use magnetic powder embedded in polymer matrices:

• Ferrite Magnetic Material — Sintered ferrite, stronger field than rubber magnets, operating up to 250°C, widely used in industrial applications requiring moderate precision
• NBR (Nitrile Butadiene Rubber) + Ferrite — Most common magnetic layer combination, excellent cost-performance ratio
• HNBR (Hydrogenated Nitrile Rubber) + Ferrite — Superior temperature and chemical resistance, suited for high-performance magnetic tracks
• Bonded NdFeB — NdFeB powder in polymer matrix, combining high magnetic performance with complex geometry molding capability

Tier 3: Hub / Substrate Materials

The non-magnetic carrier of the encoder ring is critical for thermal expansion matching and mounting stability:

OTV Sensing: Full-Spectrum Material Solutions

As a leading precision magnetic encoder target supplier, OTV Sensing offers a complete material portfolio covering all iC-MU family compatibility requirements:

Magnetic Layer Materials

• NBR Rubber Magnet: Standard solution — NBR (Nitrile Butadiene Rubber) + ferrite composite, best cost-performance, operating range −40~140°C, ideal for most industrial encoders and collaborative robot joints
• NBR + Ferrite: Nitrile rubber-based ferrite composite, the most cost-effective magnetic layer option
• HNBR + Ferrite: Hydrogenated nitrile rubber base, superior temperature and chemical stability, suitable for high-performance magnetic tracks
• Ferrite Magnetic Material: Sintered ferrite, stronger field than rubber magnets, upper temperature limit 250°C, for industrial encoders requiring moderate precision
• Sintered NdFeB / SmCo: High-coercivity or samarium cobalt solutions tailored to specific high-temperature operating conditions

Hub / Substrate Materials

• AL6061 Aluminum Alloy: Lightweight design choice
• Stainless Steel (SUS304 / AISI416 / AISI430F): Corrosion-resistant, EMC-compatible, recommended for harsh environments

All encoder rings use precision molding + magnetization processes, with pole width accuracy matched to iC-MU, iC-MU150, and iC-MU200 series chips. Full custom dimensions — from OD, ID, and thickness to pole count, magnetization direction, and material combination, all configurable to your requirements. Typical installation air gap 0.4 mm, ensuring chip-side field strength falls within the optimal 15–100 kA/m range.

Three Questions to Ask Before Specifying a Magnetic Ring

1. What is the steady-state joint temperature? Peak temperature? — This determines the material class (NdFeB / SmCo)
2. What is the encoder chip’s field input window? — At maximum temperature, does the remanence still fall within the chip’s rated range?
3. Are there rapid temperature transients from cold start to full load? — Repeated thermal cycling accelerates bonding layer aging; long-term reliability must be evaluated

Answer these three questions, and magnetic ring material selection becomes straightforward.