Patent GB-1407817-A (GB)
Title: Method and apparatus for determining the position of a road  or railway track
IPC: E01B35/00
Filed: 1972-07-20
Abstract: 1407817 Aligning railway track photo-electrically MATISA MATERIEL INDUSTRIEL SA 20 July 1972 [9 Aug 1971] 34055/72 Heading G1A [Also in Divisions B7 and H4] A railway track or a road is aligned or levelled by projecting a laser beam from one reference point so that it sweeps in a plane to scan a target of photo-detectors at a second reference point, a screen being located in relation to an intermediate point of the track or road so that it casts a shadow on the target, the electrical signals from the target being processed to compare the target sweep times on either side of the shadow. As shown, Fig. 1, a straight track is aligned by mounting a radiation emitter comprising a laser 1, a rotating prism 3 and a diaphragm 4 at a reference point A on the track so that the beam sweeps at constant velocity through an angle sufficient to scan the whole width of a target 6 mounted at a second reference point B. A track alignment machine (not shown) carries a screen 5 located at an intermediate point P of the track and this screen casts a shadow 5a on the target 6 which comprises an array of photo-cells 6a which produce an output signal 7 which is analyzed to compare the sweep times of the beam on either side of the shadow to produce an error signal which is used to control the track aligning machinery to move the point P to the point H. Alternatively if the displacement of the track is too great for the shadow of the screen to fall on the target the screen 5 may be moved relative to the point P by means of a servosystem controlled by the error signal generated from the target to position it at point H, the distance of the screen from point P being measured to provide an error signal which is applied to the track aligning machine. The signal 7 from the target is processed by removing signals due to background radiation and leakage currents by means of a threshold circuit and using the resulting signal to gate an oscillator having a constant frequency and amplitude, the sweep times then being measured either by counting the number of oscillations on either side of the shadow or by determining the lengths of the wave trains by measuring their energies.  The system may also be applied to aligning a curved section of track, Fig. 3 (not shown) the screen being arranged at a lateral distance from the point to be aligned equal to the offset required to produce the desired track curvature. The target 6 may be connected to the track aligning machine so that the distance between the screen 5 and the target 6 is constant. This means that as the machine is moved along the track, the reference point A remaining stationary, the length of the reference base line A-B varies and this distance is determined by measuring the scanning time across the whole width of the target to enable the correct offset at the point being aligned to be calculated. The system may also be used for levelling the track and two systems may be arranged in orthogonal planes for simultaneous alignment and levelling, discrimination between the two beams being achieved by polarization.