Laser Doppler distance sensor using phase evaluation

Abstract

This paper presents a novel optical sensor which allows simultaneous measurements of axial position and tangential velocity of moving solid state objects. An extended laser Doppler velocimeter setup is used with two slightly tilted interference fringe systems. The distance to a solid state surface can be determined via a phase evaluation. The phase laser Doppler distance sensor offers a distance resolution of 150 nm and a total position uncertainty below 1 microm. Compared to conventional measurement techniques, such as triangulation, the distance resolution is independent of the lateral surface velocity. This advantage enables precise distance and shape measurements of fast rotating surfaces.

Distance measurement technique using tilted interference fringe systems and receiving optic matching

“…It has to be pointed out that the measurement uncertainty of 1.2 μm holds for distance measurements at different surface segments and includes the speckle pattern specific phase error. In contrast, the presented results in [7] and [8] are only valid for displacement measurements of one and the same recurring surface segment, i.e., equal speckle pattern. Thus, distance measurements of unknown rough surfaces with a total uncertainty of about 1 μm are provided by this novel optical setup.…”

“…However, its total measurement uncertainty of about 20 μm is too high for applications, e.g., in working lathes. In order to achieve a lower distance uncertainty, a laser Doppler distance sensor with phase-difference evaluation (P-LDDS) has been investigated [7].…”

“…Due to the tilted fringe systems, the time difference, i.e., the phase difference ϕ, of the amplitude-modulated scattered light signals from the two interference fringe systems depends on the axial position z of the rough surface. By evaluating the phase difference ϕ employing the cross-correlation function, the position is determined via the a priori known calibration curve zϕ [7]. Assuming plane wave fronts, the phase difference function ϕz can be described as a linear function ϕz s · z ϕ 0 ;…”

“…The mismatched setup results in a standard deviation of 2.2 μm and a maximum deviation of the mean value of about 8 μm, whereas the matched setup exhibits a standard deviation of 0.8 μm and a maximum deviation of the mean value of only 1.5 μm. Assuming a uniform probability density of the unknown systematic measurement deviations [7], the total measurement uncertainty can be calculated by…”

Laser Doppler distance sensors using phase and frequency evaluation

“…Measured statistical distance uncertainties σ z in dependence of the object velocity v for the LDD phase sensor and the two triangulation sensors (TS)8 .…”