Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1988-10-25
1990-08-21
Willis, Davis L.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
33503, 250237G, G01B 902
Patent
active
049500791
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to measuring apparatus for use in measuring relative movements of two objects, which may, for example, be two parts of a machine such as a co-ordinate measuring machine or a machine tool.
Various forms of scales and scale reading apparatus are known, for example, opto-electronic and magnetic. Opto-electronic scales usually include diffraction gratings which require a large number of marks to be placed on them with great accuracy in their manufacture and are thus expensive. Magnetic scales are inherently less accurate than opto-electronic scales but are cheaper to manufacture.
Both of the above types of scale and scale reading apparatus need interpolation devices to improve their resolution so that measurements can be made to a resolution of .+-.0.5 microns. Interpolation introduces a non-linear error into the resultant readings which is difficult to compensate.
By contrast interferometers can measure directly with a much greater resolution, for example of the order of 0.3 microns, but suffer from the problem that changes in the local refractive index of air will cause variations in the interferometer reading.
In broad terms these problems are overcome in the measuring apparatus according to the present invention, by the combination in the above-mentioned measuring apparatus of a laser interferometer with a scale and a scale reader, the arrangement being such that the scale reader provides an accurate determination of the relative positions of the two objects with reference to the marks on the scale, and the interferometer measures the relative movements of the scale and scale reader in order to provide an accurate determination of the relative positions of the two objects at positions between the scale marks. The scale reader, on detecting a mark outputs a signal which is used to update the instantaneous reading of the measurement of the relative positions of the scale and scale reader given by the interferometer.
The readings from the interferometer detector system may be provided either in the form of an interpolation measurement i.e. the distance measurement from the last scale mark, or as a measurement of the total distance moved by the scale reader from a datum position. It is to be understood that the datum position may be any known position including one of the scale marks.
Upon receipt of the signal from the scale reader the instantaneous reading of the interferometer may be recorded. If an interpolation measurement only is required, the counter of the detector system is updated by being re-set to zero and starts a new interpolation measurement, or, if a total distance measurement is required, the interferometer is up-dated to the accurate scale reader measurement and continues counting from the updated reading.
In one aspect of the invention, in order to provide the accurate determination of the relative positions of the two objects with reference to marks on the scale, the relative positions of at least some of the scale marks are accurately determined either indirectly by calibration, if the marks are not themselves put on the scale with great accuracy, or directly by the scale reader measurements if the marks are themselves accurately relatively positioned. If the marks are sufficiently closely spaced, no allowance need be made for the effect of ambient atmospheric conditions on the interferometer reading.
One advantage of this invention is that the scale can be used to correct the interferometer for changes in the refractive index of air.
In another aspect of the invention the scale markings can be relatively widely spaced and need not be positioned on the scale with the accuracy needed for the present optical scales, provided that the relative positions of the scale markings are accurately established by calibration under controlled conditions after manufacture. Upon receipt of the signal from the scale reader the interferometer reading is checked against the reading of the scale reader and a correction factor is calculated and applied to subsequent interferomete
REFERENCES:
Doi et al., "An Automatic Calibrating Apparatus for Standard Scales by Counting Interference Fringes", Applied Optic, vol. 3, No. 7, pp. 812-823, 7/1964.
Chaney Raymond J.
McMurtry David R.
Koren Matthew W.
Renishaw plc
Willis Davis L.
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