Methods 2
Method 2: Interferometry and Spectrometry¶
Methods that uses the properties of light, either phase of a coherent emitter or spectrum.
1 - Frequency-Modulated Coherent Laser Radar (FM-CLR)¶
Laser Radar technology represents the optimal convergence of range, precision, and surface versatility for this application. Unlike traditional LiDAR, which uses pulsed time-of-flight (incoherent detection) and suffers from lower precision (\(>1 \, \text{mm}\)), Coherent Laser Radar uses Frequency Modulated Continuous Wave (FMCW) modulation and heterodyne detection.
- FMCW Instead of firing short pulses of light, an FMCW system emits a continuous beam. However, the frequency of this beam is constantly changing—usually in a "sawtooth" or "triangular" pattern. This is known as a chirp. Because the frequency is changing at a known rate, the difference in frequency between the transmitted light and the received light is directly proportional to the time it took to travel.
- Heterodine The "magic" happens when the returning light meets a portion of the original light that never left the device (called the Local Oscillator or LO). This is heterodyne detection. When these two light waves interfere on a photodetector, they produce a beat frequency. Distance Calculation: The farther away the target, the larger the frequency gap (\(f_{beat}\)).Velocity (Doppler): If the target is moving, it adds a Doppler shift to the return signal. By using a triangular chirp (ramping up and then down), the system can solve for both distance and velocity simultaneously.
2 - Interferometric Displacement Sensors¶
If the "deformation" consists primarily of expansion along the beam axis (Z-axis), or if the 0.2 mm requirement is a loose upper bound and the user desires the highest possible fidelity (e.g., detecting the onset of thermal expansion), Interferometric Sensors are the superior choice.
Michelson Interferometer:¶
The light beam coming from the laser source is split into a reference beam and a measurement beam by a beam splitter. After reflection, the two beams interfere with each other. The change in the interference intensity allows the determination of the target's positional change. A disadvantage is that the reference beam must be perfectly stable to measure only the target displacement and not a combination of target and reference mirror motions. As the reference beam is affected by multiple error sources, like air refraction index changes or thermal expansion, perfect stability de facto cannot be achieved.
Fabry-Perot Interferometer:¶
If the beam splitter of the Michelson interferometer is replaced with a semi-transparent surface, the measurement beam interferes with the portion of light reflected by the semi-transparent surface. This eliminates the need for a reference arm, thus eliminating a major source of perturbation and allowing for outstanding measurement stability and compactness.
Digital Shearography¶
Digital Shearography is an interferometric technique that measures the gradient of deformation (slope) rather than the deformation itself.
Principles and Capabilities
A laser illuminates the object, and a shearing device (like a wedge prism or Michelson interferometer) in front of the camera lens creates two overlapping images of the object. These images interfere, creating a fringe pattern sensitive to the derivative of displacement (\(\partial w / \partial x\)).
- Sensitivity: It is exceptionally sensitive to local strain concentrations, making it the industry standard for Non-Destructive Testing (NDT) to find subsurface defects, delaminations, and cracks.
- Robustness: Shearography is a "common-path" interferometer, meaning both interfering beams travel from the object. This makes it much more robust against environmental vibration than holography.
- Data Interpretation: The output is a "zebra stripe" fringe pattern representing slope. To get a sub-millimetric displacement map (the shape of the bend), this pattern must be numerically integrated (unwrapped). This process is mathematically complex and prone to errors at the boundaries, especially with complex shapes.
- Optical Difficulty at 3 Meters: Shearography requires the object to be illuminated with coherent laser light. Illuminating a large metal plate uniformly from 3 meters away requires a high-power expanded laser beam. Passing this expanded beam through a small, potentially stressed vacuum window can ruin the wavefront coherence due to the window's birefringence and thickness variations.
Commercial Availability: Systems like the Dantec FlawExplorer or LTI-5100 are designed for this, but they are typically optimized for finding flaws (micrometer-sized defects) rather than measuring the global shape of a bending plate.
NOTE: DIC sensor vs Shearography
The choice of method depends heavily on the definition of "bending in time."
| Feature | 3D Stereo-DIC | Shearography |
|---|---|---|
| Primary Output | Displacement (\(u,v,w\)) & Strain | Displacement Gradient (Slope) |
| Measurement Type | Full-field (Snapshot) | Full-field (Snapshot) |
| Static Deformation | Excellent(Stable over time) | Good (but requires integration) |
| Dynamic Deformation | Good (limited by frame rate) | Good (limited by frame rate) |
| Resolution @ 3m | \(10 \mu m\)-\(50 \mu m\) | \(1 \mu m\)-\(10 \mu m\) |
| 3m Feasibility | High(with telephoto & multi-window) | Medium (Laser illumination difficult) |
| Vacuum Window | Requires Refraction Correction | Sensitive to birefringence |
3 - CONFOCAL CHROMATIC SENSORS¶
Chromatic confocal method for optical measurement technology for measuring distance and thickness has been established as one of the mature methods available to industry and research. Incident white light is imaged through a chromatic lens to yield a continuum of monochromatic light along the z-axis, thereby “color coding” the optical axis. When an object is present in this color field, a single wavelength is fixed to its surface and then reflected back to the optical system. The backscattered beam passes through a filtering pinhole and is then acquired by a spectrometer. The beam’s specific wavelength is calculated to precisely determine the position of the surface in the measurement field. Chromatic confocal technology allows reliable, accurate and reproducible dimensional measurements with high resolution.
PRODUCT COMPARISON¶
NIKON apdis-mv4x0¶
Coherent Laser Radar uses Frequency Modulated Continuous Wave (FMCW) modulation and heterodyne detection.
https://industry.nikon.com/en-gb/products/laser-radar/apdis-mv4x0/
The Nikon APDIS (formerly Metris Laser Radar) is the primary industrial embodiment of this technology. It is specifically engineered for large-volume, non-contact inspection in aerospace and automotive sectors, often replacing large CMMs or Laser Trackers where contact is impossible.
The APDIS MV430 specifications indicate a working range of 0.5 m to 30 m , placing the 3-meter standoff distance in the "sweet spot" of the instrument's performance envelope.
- Distance Accuracy: The system specifies a Maximum Permissible Error (MPE) for distance measurements of \(20 \, \mu\text{m} + 5 \, \mu\text{m/m}\).
- Application to User Scenario: At a range of 3 meters, the maximum error is:
$$ E_{dist} = 20 + (5 \times 3) = 35 \, \mu\text{m} \quad (0.035 \, \text{mm}) $$
surfaces can vary from highly polished (low emissivity) to oxidized or matte. * Heterodyne Detection: The APDIS uses a coherent detection scheme where the reflected signal is mixed with a local oscillator. This provides a signal gain of up to 100 dB, allowing the system to measure surfaces with less than 1% diffuse reflectivity. * Ambient Light Immunity: The coherent nature of the detection acts as an extremely narrow spectral filter. This is critical if the vacuum chamber contains heating lamps or viewports that admit ambient light; the Laser Radar is immune to incoherent interference, ensuring robust measurements even in illuminated chambers.
PRO
- Distant measure
- Gimbal to mutipoint probe
CONTRA
- Not clear if can measure through window glass
- COST
- To verifiy angle available geometrically (the pivot point is distant from the window)
NOTE: high-end systems (APDIS/ATS600) represent a significant investment (\(\approx \$135k\text{--}150k\)) , they are though one solution that guarantees the 0.2 mm precision in this constrained environment. Low-cost sensors will probably fail on physics (beam divergence, triangulation geometry) or precision. For a critical vacuum test, the cost of the metrology could be justified.
ATTOCUBE IDS3010 FPS3010, FPS1010¶
The attocube IDS and FPS interferometer is a high-precision, industrial laser-based displacement sensor priced around $29,000 for a single unit. It features 10 MHz data acquisition, Picometer resolution, and up to 3-axis measurement capabilities.
The IDS3010 operates by directing light from a semiconductor laser through an optical fiber to a sensor head which is a purely optical component based on the described Fabry-Pérot principle. At the end of the optical fiber 4% of the light is reflected to form the reference beam, while the rest is collimated or focused by the sensor head optics into a beam aimed at the target. The measurement beam reflects off the target and when re-entering the fiber, it interferes with the reference beam. This interference signal is sent back through the fiber to a detector, which displays a sinusoidal interference intensity based on the target's position. To detect positional changes within a fraction of the light's wavelength and determine direction, the system generates a second, 90° phase-shifted cosine signal through high-frequency wavelength modulation.
The IDS3010 features a compact base module housing, the semiconductor laser diode, and electronic controls. The semiconductor laser's wavelength is kept extremely stable by a gas cell filled with acetylene gas, which features naturally constant absorption peaks at certain wavelengths. A control loop adjusts the laser wavelength with an expanded uncertainty (k=2) of just ±0.3 pm, certified by NIST. This precise wavelength reference ensures the accuracy and traceability necessary for reliable high-precision measurements.
https://www.directindustry.com/prod/attocube-systems-ag/product-50096-1322105.html
For example the Attocube IDS3010 utilizes fiber-based Fabry-Perot interferometry.
- Precision: The resolution is in the picometer range (\(10^{-12} \, \text{m}\)), with accuracies certified to sub-ppm levels. This is orders of magnitude beyond the user's request, effectively eliminating instrument error entirely.
- Working Distance: While interferometers are often associated with short ranges, Attocube offers collimated sensor heads (e.g., M12/C7.6) capable of measuring up to 30 meters , and focused heads (D4/F17) for rough surfaces.
- Surface Roughness: Traditional interferometers require mirrors. The IDS3010's focused heads are designed to measure on diffusive surfaces (rough metal, ceramic, glass) by capturing backscattered light.
- Vacuum Compatibility: The sensor head itself is passive (glass fiber and lens) and can theoretically be placed inside the chamber if a fiber feedthrough is available. However, for the user's "looking through a window" requirement, the sensor head is compact (\(\varnothing 1.2\text{--}14 \, \text{mm}\)), allowing it to peer through even the smallest apertures without vignetting.
PRO
- 3 measures very fast and accurate (nanometers) drectly displaced in VACUUM if fiber can be passed.
- Do not need reflective material or special targets
CONTRA
- Does not measure an absolute position but ONLY RELATIVE DIPLACENMENT.
- Do be identified if the relative displacenment can be distant in time.
- PRICE could be high
High temperature sensors: confocalDT IFS2407-xHT¶
confocalDT IFS2407 sensors are designed for high precision measurement of distance, roughness and thickness. These confocal chromatic sensors offer a high numerical aperture, a small light spot diameter and tolerate large measurement angles, which means that the sensors can also be used for roughness measurements. In addition, the sensors impress with nanometer accuracy.
- Reliable measurement at ambient temperatures up to 200 °C
- Measuring ranges (mm): 0.8 | 2 | 4
- Temperature range: max. 200 °C
- Linearity: max. < ±0,18 µm
- Resolution: max. < 6 nm
- Stainless steel housing 1.4404
- Vacuum-suitable up to UHV
PRO
- can be vacuum tolerant, they can be mounted on back holding structure
- Absolute measurement from the mount point
CONTRA
- Single point of measure per probe
- Need to be close to the measurement point
- Small range of measure (0.8 | 2 | 4 mm)