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MEMS MFXSTR

Sub-mg precision inertial sensors that capture the vibration and acoustic signature of bogies and door mechanisms at industrial sampling rates.

What it is

MEMS (Micro-Electro-Mechanical Systems) integrate mechanical and electronic components at micrometre scale on a single silicon chip. In IN-SIGHT, the MFXSTR sensors are high-frequency IMU (Inertial Measurement Unit) units from STMicroelectronics that combine a triaxial accelerometer, a triaxial gyroscope and, in the Pod B variant, a wideband MEMS microphone.

The key differentiator of these sensors versus traditional industrial solutions is their sub-mg (milli-g) resolution, which makes it possible to detect subtle changes in the vibration profile of bearings or door mechanisms weeks before the degradation becomes perceptible to the maintenance team.

The MEMS manufacturing process etches the mechanical structures (suspended masses, polysilicon springs, capacitive sensing electrodes) by lithography on 200 mm silicon wafers, with tolerances of ±0.1 µm.

Role in IN-SIGHT

MEMS sensors are the entry point of all vehicle health data. IN-SIGHT uses two configurations:

Pod A — Bogies and suspension

  • Sensor: High-frequency triaxial accelerometer mounted on the bogie via a neodymium magnetic adapter.
  • Measures: Bearing vibration (1–3 kHz), wheel impacts (50–500 Hz) and suspension resonances (0.5–20 Hz).
  • Sampling rate: 6,667 Hz for full capture of the bearing spectrum.

Pod B — Door mechanism

  • Sensor: Combination of a low-latency accelerometer + MEMS microphone to capture the acoustic profile.
  • Measures: Open-close cycle (duration, peak acceleration, end-of-travel impacts), noise level and wear signals in guides and actuators.
  • Sampling rate: 3,200 Hz standard, with a 6,667 Hz burst during open/close events.
Non-intrusive installation: The pods are fixed with high-retention magnetic adapters (≥ 50 N). They require no drilling, welding or access to the vehicle wiring.

Physical sensing principle

The MEMS accelerometer works by differential capacitive sensing: a silicon proof mass suspended by springs moves with acceleration. The displacement changes the capacitance between fixed and moving electrodes, and an integrated ASIC converts that capacitive difference into a digital voltage.

Suspended mass (Si)
      │  ← acceleration → │
  ┌───┴───┐           ┌───┴───┐
  │ C_fix  │     Δd    │ C_fix  │
  └───────┘           └───────┘
       └──── ΔC = ε·A/d² · Δd ────┘
                    │
              ASIC sigma-delta
                    │
              16-bit SPI output

This architecture makes it possible to detect accelerations from 0.06 mg RMS (noise floor) up to ±16 g full scale, with an effective resolution of 0.488 mg/LSB in the ±16 g range.