Patent GB-774403-A (GB)
Title: A method and apparatus for testing whether the axes of two relatively movable members are aligned with reference to a predetermined axis
IPC: G01B11/00
Filed: 1954-05-19
Abstract: 774,403. Checking alignment photo-electrically. SOC. GENEVOISE D'INSTRUMENTS DE PHYSIQUE. May 19,1954 [May 23, 1953], No. 14750/54. Class 40 (3). A method for testing whether the axes of two relatively movable members are aligned with reference to a predetermined axis comprises means for projecting a parallel beam of light defining said axis along the axis of one of the members towards a photo-cell and at least one element mounted on the other member which is oscillated transversely to the beam to periodically intercept it so that the ratio between successive time intervals elapsing between the interceptions of the beam depends on the degree of misalignment of the axes relative to the predetermined axis. A beam of light 3 from source 1 is passed through aperture 15 in the end of envelope 14 and along its axis to a photo-cell 7. The envelope is fixed to a base 18 movable in three mutually perpendicular directions on slides 9, 10, 5 by hand-wheels. Two elements V, H, which are perpendicular to each other and to the axis of the beam and are mounted on blades 17, 16, are caused to oscillate across the beam in directions perpendicular to their lengths by electromagnets fed from dephasing circuit E with equal frequency voltages in phase quadrature. When the intersection of the equilibrium positions of elements V, H lies on the axis the output of the photo-cell which is amplified at 22 consists of two series of evenlyspaced negative pulses v, h corresponding respectively to the passages of the two elements through the beam, whilst if the intersection is off axis the series of pulses are unevenly spaced, the unevenness in each series corresponding to the extent of displacement in the direction of oscillation of the element causing that series. The amplified output on line e is differentiated and amplified at AD to two series h<SP>1</SP>, v<SP>1</SP> of sharp positive pulses which are fed to a pair of parallel channels each comprising a modulator M<SP>1</SP>, M<SP>2</SP> and biased rectifier R<SP>1</SP>, R<SP>2</SP>. In the modulators the pulse waveforms are superimposed on square waveforms i<SP>1</SP>, i<SP>2</SP> differing in phase by 180 degrees which are generated in G. In one modulator output the v<SP>1</SP> pulses are superimposed on the high parts of the square waveform, whereas in the other the h<SP>1</SP> pulses are superimposed on the high parts. The rectifiers are biased sufficiently to just block the high parts of the square waveform so that their outputs VII, VIII respectively comprise only v<SP>1</SP> and h<SP>1</SP> pulses. Successive pulses in these two unevenly-spaced sets actuate the respective electronic switches C<SP>1</SP>, C<SP>2</SP> causing reversal of direct currents generated by battery B and passing through measuring instruments L<SP>1</SP>, L<SP>2</SP> respectively. The latter have such mechanical or electrical inertia that they read the average values of the respective discontinuous currents. When the line of intersection of the elements is on the beam axis the total time for which current flows in the two directions is the same whereas when it is not the intervals between the switch-operating pulses in each channel are alternately longer and shorter than when in the on axis state so that net currents corresponding to the displacements from the axis in mutually perpendicular directions are obtained in meters Ll, L<SP>2</SP>. The meters may be replaced by devices controlling motors producing mutually perpendicular motions to restore base 18 to its on axis condition. Specifications 555,103, 581,930, [both in Group XL], 684,435 and 686,274 are referred to.