Patent GB-1476190-A (GB)
Title: Distance measuring devices
IPC: G01S17/08
Filed: 1974-09-13
Abstract: 1476190 Photo-electric distance measuring AERONUTRONIC FORD CORP 13 Sept 1974 [13 Sept 1973] 40010/74 Heading G1A [Also in Division H1] The position of a specular or diffuse reflecting surface with respect to a lens is found by using the surface and the lens as a reflector of a laser resonator, or of a secondary resonator coupled to a laser, and analyzing the laser output photoelectrically. In Fig. 1 the surface 15 and lens 16 combine to form a resonator reflector for a He-Ne laser tube 10 operating at 1À15Á, the other resonator reflector being at 13. When the surface 15 is in the so called focal saddle of the lens 16, i.e. in a region close to its focal point, the active medium in tube 10 laser, and part of the laser output passes through reflector 13 and is detected at 18. The lens 16 can be moved to and fro until lasing begins, and the position of the surface 15 can be read on a scale associated with the lens. Another lens 17 collimates the beam so that the lens 16 can be moved without incurring resonator losses. The dimensions of the focal saddle, which in effect control the accuracy of the measurement, are defined. In Fig. 4 surface 15 and lens 16 co-operate with a reflector 20 to form a resonator coupled to a continuously oscillating laser but not containing the active medium. This resonator has a path length modulator 21, and when surface 15 is in the focal saddle the modulation in the resonator can be detected in the laser output. In Fig. 5 an arrangement similar to Fig. 1 is used as a profile measurer. Surface 29 co-operates with lens 16 to make active medium 10 lase when the surface is in the focal saddle; the lens is vibrated to and fro by Piezo mount 27, the detector 18 giving a signal every time the distance between the surface and the lens is such that lasing occurs. The profile is scanned in the X direction by moving the head 26, and an oscilloscope plots the profile by displaying the X displacement against the voltage on the Piezo mount at the moment of lasing. The laser medium and the detector can be mounted on head 26 if desired. In Fig. 6 (not shown) the onset of lasing when the surface is in a given position is used to control a lathe. A workpiece is turned until its surface is reduced to a size where lasing commences, the photo-detector signal controlling a lathe programmer. In Fig. 7 the laser can operate in one or other of two orthogonal polarization modes depending on resonator length. Tube 50 has end windows which are only slightly non-parallel, and the main resonator comprises reflector 51, linear birefringent element 53, circular bi-refringent element 54, and output mirror 52. The secondary resonator comprises mirror 52, quarter wave plate 56, lenses 57 and 16, and surface 15. A polarizer 55 blocks one of the orthogonal laser outputs, these are elliptical modes. Initially the laser operates in one of its two possible modes, but as the surface 15 is moved into the focal saddle the laser output flips to the other mode; the output then flips back as the surface continues to move, a change occurring every quarter wavelength of the laser output. Thus the actual position of the surface 15 within the saddle can be found by counting the flips. In Fig. 8 the above system is modified by adding a path length modulator 62 to the secondary resonator, and also by making the length of the main resonator variable by moving housing 60. The laser output flips when housing 60 moves, and also when a surface is in the focal saddle, the latter flipping being due to modulator 62 and being at a higher frequency F M  than that due to the housing motion, F T . The laser is initially set up so that a reference surface 64 lies in the saddle, this being detected by the onset of high frequency flipping; the surface is then removed, and the housing is driven forward by driver 61 until the high frequency flipping starts again due to the surface of a workpiece 69 being in the saddle. By counting the flips produced by the housing movement the distance from the original reference position to the workpiece is found. It is also possible to make the housing advance a pre-set distance from the original reference positions. These two possible types of operation are selected by a switch 65, the pre-set advance being set by entering a number of flips at 76.