Giant Magneto Impedance (GMI): Delivering Hysteresis-Free Sub-Arcsecond Motion Feedback

In high-precision motion control, selecting a feedback sensor requires balancing positional resolution against real-world mechanical tolerances. While optical glass discs deliver high accuracy, they require extremely tight axial alignments and sealed housings to prevent failure from dust or liquid contamination. Standard magnetic encoders provide environmental ruggedness, but are limited by magnetic hysteresis and non-linearity.

Patented Giant Magneto Impedance (GMI®) technology, developed by FLUX GmbH in Braunau am Inn, Austria, bridges this gap. By utilizing high-frequency electromagnetic induction within specialized ferromagnetic foils, GMI achieves sub-arcsecond resolution and absolute accuracy without magnetic hysteresis, maintaining total immunity to environmental dust and axial runout.

“FLUX encoder technology bridges the gap between optical-grade precision and magnetic-grade robustness, delivering uncompromised absolute feedback in the most challenging industrial environments.”

1. The Physics of Giant Magneto Impedance (GMI)

The Skin Effect & Impedance Modulation

The physical foundation of GMI lies in the skin effect—the tendency of a high-frequency alternating current (AC) to distribute itself near the surface (the “skin”) of a conductor. The skin depth (δ_s) is defined by:

where ρ is the electrical resistivity of the conductor, f is the frequency of the AC excitation current, and μ is the magnetic permeability of the material.

In a FLUX GMI encoder, the stator houses an absolute GMI sensor coupled with a thin ferromagnetic GMI layer (foil). The rotor carries a high-precision absolute magnetic ring. When the rotor moves, its magnetic field alters the local magnetic permeability (μ) of the GMI layer on the stator.

This shift in permeability causes a corresponding change in skin depth (δ_s), which modulates the high-frequency AC electrical impedance of the GMI layer. The absolute GMI sensor reads these local impedance variations, and the integrated evaluation electronics translate them in real-time into a precise digital position.

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Why GMI is Hysteresis-Free and Jitter-Free

Conventional magnetic sensors (such as Hall-effect or Magnetoresistive AMR/GMR/TMR sensors) measure magnetic flux lines directly and are susceptible to bulk magnetic hysteresis, the physical lag of magnetic domains during direction changes. This lag introduces non-linearities and positioning errors during oscillation or reverse movements.

Because FLUX GMI technology measures high-frequency AC impedance changes in a specialized thin foil rather than tracking magnetic domain rotation, there is zero magnetic hysteresis (none). The system scans the scale and delivers real-time, jitter-free positioning signals with an exceptionally high signal-to-noise ratio (SNR), making it highly suitable for high-dynamic closed-loop servo systems.

2. Holistic 360° Scanning vs. One-Point Scanning

A common issue with traditional optical, magnetic, and Hall-effect encoders is their reliance on segment or “one-point” scanning. Under this approach, a single sensor head reads a localized portion of the rotor scale. Any eccentricity (the displacement [e] between the rotor’s geometrical center and the actual axis of rotation) introduces significant errors.

In a “one-point” scanning system, eccentricity generates a sinusoidal positioning error (δ) over a complete rotation, which can be calculated using the formula:

where:

• δ is the encoder eccentricity error in arcseconds.
• e is the radial eccentricity (half of the physical runout) in micrometers (μm).
• D is the outer diameter of the encoder in millimeters (mm).

Eccentricity occurs statically due to manufacturing tolerances and mounting misalignments, and dynamically under operational loads, vibration, or thermal expansion. While static errors can be partially calibrated out at commissioning, dynamic eccentricity cannot be compensated for in one-point scanning systems.

FLUX 360° Holistic Scanning averages out the eccentricity error across the entire circumference of the rotor. By scanning the scale continuously around 360°, GMI encoders inherently cancel both static and dynamic eccentricity errors, providing stable sub-arcsecond positioning even with up to 0.20 mm of mechanical runout.

Eccentricity Error Comparison: FLUX GMI vs. One-Point Scanning

3. FLUX GMI Product Portfolio & Technical Specifications

FLUX designs and manufactures bearingless, absolute frameless GMI encoders in two main configurations to suit different space envelopes and requirements: the GMI-ANG (Angle) series for ultra-precision applications, and the GMI-ROT (Rotary) series for compact, versatile integrations.

FLUX GMI-ANG (Angle Encoder) Specifications

Optimized for high-accuracy and high-stiffness installations (such as rotary tables, gimbal axes, and robotic joints), the GMI-ANG series features robust IP67 stainless steel housings and sliding fits with dowel-pin holes for quick centering.

FLUX GMI-ROT (Rotary Encoder) Specifications

Designed for space-constrained integration in servo motors, cobots, and actuators, the GMI-ROT series offers a lightweight profile with an axial stack-up of just 8 mm (including the air gap). It features a choice between IP67 aluminum or steel housings.

Note: GMI-ROT-150 is available exclusively in an lightweight aluminum housing configuration.

Alternative Technology: FLUX Inductive Encoders (IND)

In addition to GMI, FLUX has developed a patent-pending inductive absolute technology designed for space-critical setups. The IND-ROT series features an open, bearingless PCB design that is completely immune to electric and magnetic fields, weighing a mere 14 g and measuring less than 6 mm in thickness, making it the smallest inductive absolute encoder on the market. For extreme land, sea, and air environments, the IND-MAX series delivers encapsulated, high-precision inductive feedback.

4. Operational Specifications & Integration Guidelines

Mounting Tolerances & Calibration-Free Commissioning

A core mechanical advantage of FLUX GMI encoders is their large mounting tolerance, eliminating the need for delicate air-gap shimming or in-field signal adjustments.

• Nominal Air Gap: 0.30 mm
• Axial Tolerance: ±0.25 mm (GMI-ANG) / -0.20 mm to +0.50 mm (GMI-ROT)
• Radial Tolerance (Runout): up to 0.20 mm

FLUX encoders require no signal or accuracy calibration during installation. Stators are equipped with a multi-color status LED (Red/Yellow/Green) to provide instant visual feedback on alignment quality during mechanical positioning, making setup fast and reliable.

Environmental Limits & Mechanical Durability

Because FLUX encoders have a bearingless design, there is no mechanical friction or wear, rendering them maintenance-free over their entire operating lifetime. They are tested under extreme conditions and feature the following ratings:

• Operating Temperature: -40°C to +85°C (standard); -40°C to +125°C (extended temperature option “E”)
• Storage Temperature: Down to -55°C
• Vibration Resistance: EN60068-2-6, 20g (55 Hz to 2000 Hz)
• Mechanical Shock: EN60068-2-27, 200g (6 ms half-sine pulse)
• Sealing: IP67 protection rating against dust, condensation, and solvents

Digital Communication Interfaces

To match modern motion controllers, FLUX GMI encoders support multiple high-speed serial protocols. Users can select from:

• BiSS-C: BIS10 (recommended for resolutions up to 24 bits) and BIS20 (for resolutions starting at 25 bits and higher). Serial interfaces BIS00 and BIS21 are also available.
• SSI: Multiple serial options (SSI00, SSI01, SSI03, SSI04) depending on frame rates and clock options.
• SPI: High-speed SPI communication (SSI02 interface option) for direct microcontroller integration.
• Incremental (A/B/Z): High-frequency digital line driver outputs (INC00, INC01, INC02) for legacy controller compatibility.