GB1104081A - Improvements in or relating to apparatus for measuring lengths - Google Patents

Abstract

1,104,081. Selective signalling. CARL ZEISS JENA VEB. 1 July, 1965, No. 27968/65. Heading G4H. [Also in Divisions G1, G2 and H1] The invention relates to apparatus for measuring lengths or displacements involving the use of a photo-electric interferometer the output of which controls a cathode-ray tube. The displacement measured is that of a triple-reflector member 10 along a scale 13. As the prism is displaced, a photo-electric microscope 12 travels with it to detect scale divisions 14 and produces pulses which are amplified 15 and transmitted to an analyser 16 for counting. The interferometer (e.g. a Michelson interferometer) comprising light source 1, condenser lens 3, diaphragm 5, collimater 6 and filter 7 together with fixed, reference triple reflector 11 and displaceable, test triple reflector 10, mirror 23, partially reflecting plate 9, and objectives 18 and 24, produces two phase-displaced sets of curved interference fringes respectively moving across slits 19, 20. The intensity changes are detected by photodetectors 25, 26 the outputs of which are used to control the movement of the electron beam in cathode-ray tube 31 by the connection of the detectors to the pairs of deflecting plates 27, 28 and 29, 30 respectively. The beam is caused to follow a rotary motion and sweep out a cone shape. A fixed sector disc 33 which may have its sectors electrically insulated from one another, is mounted inside the tube 31 adjacent the screen 32. Each time the beam sweeps across a sector, a pulse is produced which is amplified (at 34) and fed to the analyser 16, where pulses are combined with those from the detector 12 to obtain measurements to fractions of a scale division. In another embodiment (Fig. 2, not shown) the sector disc (33) is outside the cathode-ray tube light from which having passed through the disc interspaces is detected by a detector (35) outside the C.R.T., to produce pulses indicating the fractions of a scale division as before. Alternatives to a Michelson-type interferometer are possible. Reflectors 10, 11 may be replaced by mirrors or right-angled prisms and the phase shifting member 8 by a suitable compensator.