GB890380A - Method and apparatus for web thickness control - Google Patents

Abstract

890,380. Automatic thickness control systems. GOODYEAR TIRE & RUBBER CO. Feb. 23, 1959 [April 14, 1958], No. 6109/59. Class 38 (4). [Also in Group XL (b)] In a radiation thickness gauge including a plurality of sensing positions spaced across the width of a moving web, film, &c., the indicating device associated with each position is arranged to give a " null " indication for webs &c. of various thickness determined by the setting of a calibrated control common to all the sensing positions. In the embodiment shown in Fig. 1, which is concerned with controlling (in production) the thickness of a web, film, &c. 3, each sensing station (three only being shown) comprises a radiation source 1, an ionization chamber 2, an electrometer tube 8 and a load circuit 10. During initial setting up (switches 13 and 20 in positions shown) each of the latter devices is adjusted to give a " null " response on an associated visual indicator when the radiation sources are completely screened, after which switch 13 is moved to contact 16 and maximum radiation (no web &c. present) is allowed to fall on the ionization chambers 2 and the potentiometers 12 adjusted to again induce " null " indications. A plurality of calibrating webs, films, &c. of known absorption are then successively introduced between the sources 1 and ionization chambers 2 and, with switch 13 on contact 14, potentiometer 18 is adjusted to produce (for each thickness of web &c.) a null indication in the associated visual indicators and the control knob of potentiometer 18 is calibrated in terms of the thickness of each web. In order, periodically, to check the uniformity of response of each sensing station, the radiation sources are screened and with switch 13 on contact 15 and switch 20 closed a " backing-off " voltage proportional to an arbitrary web &c. thickness and determined by resistor 21 is introduced into the circuit and the response uniformity determined by the uniformity of the departure of the indicating devices from the " null " positions. Fig. 2 shows the schematic lay-out of the thickness gauge in an automatic arrangement for controlling the thickness of a continuously produced web &c. 3, the radiation sources being contained in a housing 22 (provided with a compressed air-operated shutter system actuated by a cylinder 23 controlled by a push-button system 24 . . . 27) and the ionization chambers contained in a housing 28. Referring to Figs. 2 and 3, the output signals from the latter are supplied to a circuit 30 (considering, e.g. the signals derived from terminals 58, 59, 60, Fig. 1) from which they are supplied to the visual indicating devices in housing 35, and to integrating R.C. networks 55, 56, 57 in unit 37. The integrated signals are then supplied in turn, via a stepping switch 51 to the control grid of an electrometer tube 46 the cathode-derived output of which (terminals 62, 63) is supplied (by connections not shown) to the pen control motor 89 of a chart recorder 40. A second stepping switch 71, ganged to switch 51 and operated by a stepping relay 75, controls the supply of rectified A.C. from the secondary of transformer 69 to a relay 70, 72 which operates the chart driving motor 73 so that, as the various sensing positions are " scanned " by the stepping switches a visual record of variations in web &c. thickness is produced on the record chart. In addition to this, the pen control motor 89 operates a potentiometer 82, connected in a bridge circuit including a zeroing potentiometer 80 across the secondary of transformer 69 and the voltage derived at the slider of potentiometer 82, which is a measure of the thickness of the web &c., is supplied to various outputs 88 via a third stepping switch 84 ganged to the stepping switches 51 and 71. The variations in amplitude and polarity of the voltages at outputs 88 then represent, respectively, the extent and direction (i.e. increase or decrease) of variations in the web &c. thickness at the various sensing positions " scanned " by the stepping switches and such voltages are then employed in association with motors, selsyns &c. in unit 44 (Fig. 2) to control the movement via a mechanical device 45 (Fig. 2), of a calender roll or doctor blade &c. which operates to maintain the thickness of the web &c. at the value determined by the setting of the calibrated potentiometer 32 (Fig. 2) equivalent to potentiometer 18 (Fig. 1). Suitable substances for the radiation sources are the radio-isotopes Carbon 14 and Strontium 90 and the radiation gauges may comprise devices using photo-multiplier tubes instead of ionization chambers.