Patent GB-868219-A (GB)
Title: Device for measuring the width or diameter of fixed or movable tapes, wires, tubes, bars and the like
IPC: G01B11/08
Filed: 1958-11-25
Abstract: 868,219. Photo-electric measurement of dimensions. INGBER, O. H. Nov. 25, 1958 [Nov. 29, 1957; May 5, 1958], No. 37898/58. Class 40(3) [Also in Groups XIX, XX, XXX, XXXV and XXXVI] The transverse dimension of a movable wire, cable, tube, tape, &c. is measured photoelectrically by apparatus comprising a source of luminous flux, at least one diaphragm between the source and photo-cell, a surface having successive zones of different transparency and an optical system forming an image of the object to be measured on the diaphragm. Light from a source 11 (Fig. 6) passes around the object 13 and an image is formed by an optical system 14 on the diaphragm 15. The output of the photocell 17 will depend on how much light is obscured by the object and is displayed on a meter 19. A toothed "modulator" disc 2 is interposed between the diaphragm and the photo-cell and normally the width of the image is equal to the pitch of the teeth so that the photo-cell output does not vary as the disc rotates. If the dimension of the object changes, the image will no longer occupy this space on the modulator and a variation in photo-cell output will be obtained. The position of the lens 14 and/or the disc 2 is adjusted until the photo-cell output is constant once more, and the dimension is indicated by the distance between the pointers on the scale 24. The modulator can have a single aperture only, in which case the fluctuations are produced by reflecting the light beam from an oscillating mirror or by illuminating the object from a flying-spot scanner. In another modification (Fig. 10) the teeth are replaced by long radial slots which co-operate with a stationary disc 88 having a spiral aperture and another stationary disc 87 with a single radial slot. The angular position of the disc 88 is then adjusted until the width of the slot visible through the disc 87 is equal to half the width of the image. In another form (Fig. 12) the modulator teeth are of gradually increasing width so that the photo-cell output will be zero only once per revolution of the disc. By means of a stroboscopic lamp energized by the photo-cell output (see below) the particular tooth whose width equals half the image width is illuminated and can be indicated by a pointer 27 and movable scale 26; the scale can be stationary and the pointer movable. When the edges of the diaphragm are crossed by the edges of the slots in the disc an "interference" signal is generated, and this may be reduced by suitable choice of shape and dimensions of the diaphragm aperture. This aperture may be equal to a multiple of the pitch of the disc, or may consist of two staggered portions of equal or unequal widths (Figs. 14-17, not shown), whereby two separate "interference" outputs are obtained which cancel one another. If the modulator teeth are of variable width (Fig. 19) it is necessary to have the diaphragm aperture in three portions of which one (A) is twice the depth of the others (B, C), the three being displaced relative to one another by an amount equal to an odd multiple of the mean width of the slots. The algebraic sum of the three interference signals generated is then much less than any one alone (Fig. 20, not shown). Alternatively the edges of the diaphragm may be inclined to those of the modulator, or their transparency may change gradually instead of abruptly; the low-frequency components of the output can then be eliminated electrically. Further, there may be injected an equal and opposite signal derived from a rotating magnetic or photographic track carried by the disc, or the beam may be compensated by another derived from a similar diaphragm displaced angularly. Such compensation is unnecessary if the whole of the luminous flux passes through the modulator disc. The modulator can be a cylinder instead of a disc. Machine construction (Fig. 22). Light from a source 11 passes round the object 13 and is focused by a lens-system 14 and prism-system 34 on to the diaphragm 15, behind which is the modulator disc 2 (of the type shown in Fig. 19) and photo-cell 17. A stroboscopic lamp 25 illuminates the scale(s) on the modulator disc viewed in conjunction with a reference mark 43. Knobs 36, 37 control the positions of the lenssystem 14 and prism-system 34 to vary the size of the image. The prism-system (Fig. 23) is mounted on a block 46 which slides on a rail 44 and has rollers engaging another rail 45. A spring clip 50 engages a lock nut 52 to maintain the system in a given position. A similar arrangement is provided for the lens-system (Fig. 25, not shown). The lens and prism systems can thus be altered by predetermined amounts, and the scale viewed in conjunction with the modulator disc can be changed simultaneously. An auxiliary scale carries marks to indicate percentage deviation from a standard or from the initial value of the dimension of the object, an adjustable pointer being set accordingly before measurement starts. Indication can be effected by a dark line on a clear background or vice versa, or by the separation of two contrasting areas. The scale may be a flat disc or a drum, in which case the mark can be straight, curved or helical. The indication can be traced graphically and reproduced photographically by using a photosensitive layer. The wire or tape is guided by two devices 32 each comprising a plain roller 35 and a grooved roller 54 (Fig. 27) to which are attached segments 57, 57a pivoted at 58, 58a and secured by pins 59, 59a. These pins (Fig. 30) engage a spring-loaded member 61 and urge the rollers into contact with one another. The rollers are disengaged by a lever 64 and cam 64b when the wire &c. is to be inserted or removed. Flexible pads or rollers (not shown) minimise vibration of the wire &c. If the latter travels through water at any stage, the effect of droplets on its surface can be avoided by passing it through a drying zone or by carrying out the measurement under water. Alternatively, a circuit can be included to block the photo-cell output when this changes abruptly. For objects such as tapes whose cross-section is not circular, the apparatus can be duplicated to measure the dimension in a different direction, or the object can be turned periodically. Alternatively a plurality of light beams from the same or different sources can be switched (mechanically or with a polarizer) to illuminate the object successively from different directions. Circuitry (Fig. 32). The photo-cell 17 receives light from a modulator of the type of Fig. 19, and its output is amplified at 66, 68 and 70, and passed through two symmetrical choppers 67, 69. A demodulator 71 removes most but not all of the A.C. component, and the residual signal is inverted at 72 and amplified at 73. A cam 28a is driven from the modulator shaft and by contacts 74 biases off valve 73 once per revolution for the period when the width of the modulator teeth visible changes abruptly. The remaining signal is demodulated at 75, amplified at 76 and again biased at 77 to complete the suppression of the interference signal. The resulting short pulse is again demodulated at 78, amplified at 79 and used to trigger a monostable multivibrator 80. The output of the latter is chopped at 81, differentiated at 82 and fed to the stroboscopic lamp 25, which thus flashes whenever the output of the photo-cell is a minimum, i.e. when the width of the image is equal to the instantaneous pitch of the modulator teeth. The number corresponding to the measured dimension will then appear behind the pointer 43 through the reading gate (Fig. 22). Instead of a stroboscopic indication, the lamp can be energized from a bi-stable multivibrator over a period extending from the instant of minimum photo-cell current either backwards to the beginning of the cycle or forwards to the end of the cycle. The mean current supplied to the lamp, or to an auxiliary photo-cell illuminated thereby, is then a measure of the dimension and can be read on a meter or transmitted to a remote point. Remote indication can also be effected by feeding the lamp with alternating or pulsating current which is started or stopped when the photo-cell output is a minimum, and counting the pulsations. Alternatively, a remote indicating scale and lamp can be energized in synchronism with the local ones. Remote operation of the optical system and rangechanging can also be effected. The mean value of rapidly-varying readings can be obtained by allowing a phosphorescent screen to be illuminated with ultra-violet rays from the flashing lamp. Another method of measurement is to generate a linearly-varying voltage synchronized with the modulator and to determine its value when the photo-cell output is a minimum. This method is suitable when the modulator has only one aperture (see above) and the voltage can be the sweep voltage of the cathode-ray tube used as flying-spot scanner, or can be generated from the oscillating mirror by blocking a light beam falling on an auxiliary photo-cell, moving a plate of a capacitor forming part of a potential divider, or moving a coil in the field of another coil. The voltage representing the dimension is obtained on an integrating capacitor. Alternatively, the number of pulses in the photo-cell output between the start of the cycle and the moment when the multivibrator is synchronized can be counted. It is stated that this method is less sensitive to vibration of the object being measured.. Determining deviation from standard, and maximum and minimum values. In Fig. 33 the beam from the object passes through the modulator disc 104 to the photo-cell 106, and contacts 109 suppress the signals when the pitch of the disc changes abruptly, as before. Coaxial with the modulator disc is a shaft 112 inside a further hollow shaft 113, carrying arms 114, 115 respectively (see also Fig. 34). The modulator disc carries three arcuate slots 132, 133, 134 at different distances from its centre,