2024-07-09 22:00:00
THE laser interferometry techniques are widely used to make metrological dimensional controlsThey combine a laser source with a perfectly controlled optical frequency and an interferometric assembly to produce relative or absolute distance measurements.
Compared to other approaches, Laser interferometry technique is the only method for high-precision dimensional measurement over a dynamic range typically from 0 to 10 m and a resolution at the nanometer scale (from 0.1 to 1 nm). The distance measurement can be performed continuously on a fixed cooperative target (mirror, cube corner, reflecting sphere) or in translational motion with a maximum displacement speed of the order of 0 to 1 m/s.
It is therefore widely used for the periodic calibration of translational movements on machine tools or the characterization of mechanical actuators (displacement range, absolute precision and resolution), such as motorized or piezoelectric micrometric translation tables.
The technique of homodyne measurement uses a light wave with a single optical frequency. It corresponds to detecting by interferometry the optical phase difference between a reference light signal and the signal reflected on the mechanical part whose position is controlled.
The technique of heterodyne measurement is based on the beat detection to determine the gap between two optical frequencies. One optical frequency constitutes the reference and the other optical frequency is reflected on the mechanical part to be controlled. These homodyne and heterodyne laser interferometry techniques only allow the measurement of a variation in distance and are therefore techniques of relative dimensional measurement. They also require that the measure is carried out without discontinuity during the entire movement of the mechanical part, which constitutes a strong constraint in real conditions of use.
To obtain an absolute distance measurement, the laser source undergoes a continuous optical frequency sweep. This frequency sweep approach has several advantages:
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the distance East measured absolutely ;
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the measurement may be interrupted during a move without the absolute distance information being lost.
On the other hand, it requires a seamless optical frequency scanning, which results in a significant constraint on the laser technology implemented.
For all of these techniques, absolute precision and resolution of distance measurement are directly related to the quality of control exercised over the optical frequency (wavelength) of the laser source and to corrections made to account for variations in the refractive index of air crossed by the laser beam during measurement (parasitic effects linked in particular to variations in temperature, pressure and humidity).
Of the Differential assemblies with multiple optical axes of simultaneous measurements allow additional measurements of tilt angles of mechanical parts. These configurations are also suitable for determining the pitch, roll and yaw angular defects during continuous translation on mechanical actuators.
After some basic reminders on the different laser sources (He-Ne laser, semiconductor laser) commonly used for metrologyTHE Theoretical principles of laser interferometry in homodyne or heterodyne setups or during a continuous scan of the optical frequency will be presented.
Of the examples of industrial laser interferometers will then be detailed. The performances (dynamic, precision, resolution and bandwidth) of these commercial systems will be analyzed, as well as the associated accessories:
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internal or external optical systems to instrument the part to be checked;
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external module for sensor measurement and real-time compensation of influence quantities.
THE Main applications of laser interferometry will also be mentioned.
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