Optics: measuring and testing – By polarized light examination – With birefringent element
Patent
1989-04-20
1991-06-25
Willis, Davis L.
Optics: measuring and testing
By polarized light examination
With birefringent element
G01B 902
Patent
active
050261630
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to straightness interferometer systems for use in measuring the transverse deviations from a nominally straight path of a moving object such as a machine component.
A straightness interferometer system is known, for example, from UK Patent No. 1,409,339 in which a laser beam travelling along a principal beam axis is split into two secondary beams by a Wollaston prism. The two secondary beams deviate from the principal beam axis by equal small angles, and are directed at two separate plane reflecting surfaces which are mounted in mutually fixed relationship at an angle to each other which is bisected by the principal beam axis. The angles of deviation of the secondary beams from the principal beam axis, and the angle between the plane reflecting surfaces are such that the secondary beams are reflected back along their original paths into the prism and are re-combined in the prism. The combined beam returns along the principal beams axis.
Such a system has certain disadvantages. Firstly the re-combined beam is returned from the prism to the laser where it is possible that it will adversely effect the stability of the outgoing laser beam. Secondly, because the angular deviations of the secondary beams from the principal beam axis are very small, there is a limit as to how close the plane reflectors can be put to the Wollaston prism before the interferometer breaks down and the signal is lost. The distance between the Wollaston prism and the plane reflectors at this point is hereinafter referred to as the dead path. In addition, the parts of the above-described system, particularly the Wollaston prism, have to be very accurately made which makes them expensive. For example, in the manufacture of the Wollaston prism, not only does the total deviation, i.e. the angle between the two beams, have to be accurately controlled, but also the angle of deviation of each individual beam from the principal axis has to be substantially equal to avoid asymmetry of the system. Also the plane reflectors have to be accurately located relatively to each other and to the Wollaston prism to ensure that the reflected beams re-combine in the prism to provide the interference fringes used for measurement. Thus the alignment of the components of the above-described interferometer is not easy.
The invention as claimed in the appended claims overcomes one or more of the above disadvantages by providing a beam splitter which produces two outgoing secondary beams a first one of which is directed parallel to the principal axis of the light beam and the other one of which is directed at a small angle to the principal axis.
In one embodiment of the invention the first secondary beam is directed co-linearly with said principal axis.
The asymmetric arrangement of secondary beams thus produced has the advantage that the beam splitter used is easier, and thus less costly, to manufacture since only one angle of deviation has to be controlled. Also the alignment problem is eased by having one undeviated beam.
A Rochon prism will produce the desired asymmetric secondary beams but this, although the most inexpensive solution, has the same dead path limitation as the prior art arrangement described above.
The dead path limitation can be cured in a more expensive embodiment by the use of a pair of square deflectors, e.g. hollow pentacubes or other optical devices to provide a periscope effect. Using a pair of hollow pentacubes also has advantages in that they are insensitive to pitching movements and less sensitive to yawing movements of a machine component to which they are attached.
The invention as claimed in the appended claims provides in another embodiment thereof a retro-reflector in the form of a roof top prism for returning the outgoing secondary beams to the beam splitter. By this means the returning light beams are displaced from the paths of the outgoing secondary beams, thus ensuring that the returning combined beam is displaced from the principal axis and does not interfere with the original light beam.
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REFERENCES:
patent: 3458259 (1969-07-01), Bagley et al.
patent: 3523735 (1970-08-01), Taylor
patent: 3790284 (1974-02-01), Baldwin
patent: 4180328 (1979-12-01), Drain
patent: 4436424 (1984-03-01), Bunkenburg
"An Interferometer for `Straightness` Measurement", Nature, Mar. 26, 1955.
"Proposals for Laser Based Straightess and Flatness Gauges and Two Dimensional Coordinate Measuring Instruments", G. J. Parkinson 2/5/71.
Chaney Raymond J.
McMurtry David R.
Koren Matthew W.
Renishaw plc
Willis Davis L.
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