GB1372438A - Moisture measuring and control apparatus - Google Patents

Abstract

1372438 Automatic control of dryness STRANDBERG ENG LABORATORIES Inc 23 Aug 1972 [27 March 1972] 39194/72 Heading G3R [Also in Division G1] Apparatus for controlling a variable, e.g. the dryness of a moving web of material, comprises means for measuring the variable and producing respective signals when the measured value is above or below a reference value by more than predetermined amounts, means responsive to said signals for correcting the variable and means for preventing said response except during periods of predetermined duration at recurring intervals (at a predetermined frequency). As shown, a web 13 is drawn through drying or damping means 11 and its moisture content is determined by the resistance between rollers 12 which is measured by a thermionic valve circuit and operational amplifiers at 16. Also Fig. 2A (not shown). The reference value is set at 17 and the width of the "dead band" at 19; respective "dry" and "wet" signals outside this band appear on lines 20, 21 to control switching transistors Q1, Q2 in relay circuits 22, 23. The relays control a reversible motor driven potentiometer controlling the speed of a web drive motor 14. However, the transistors Q1, Q2 are normally biased off by a signal on line 38, except for periods determined by a timer 30 which may be selectively different for wet or dry errors. At the end of each period a transistor switch Q19 is closed to start another timer 31 which resets 30 after a prescribed interval so that the cycle is repeated. The cycle is eventually interrupted by opening of a further transistor switch Q8 when the error falls to within the prescribed limits (so that there is no signal on line 35). Q8 is prevented from closing for a time set on another timer 28 which is started by closure of "material running" contacts 33. Closure of Q8 starts a further timer 29 which controls another switch Q12; this prevents corrective action for short term errors. Thus, at start up, switch 33 closes, after a delay due to 28 an error will close Q8, if the error persists for a time set on 29 the appropriate correction relay is closed for a time set at 30, at the end of which an interval set on 31 must occur before the relay is again closed, which happens if the error is still present. The timers may be electro-mechanical but preferably comprise RC transistor circuits Figs. 2B, 2C (not shown). If the error reaches limiting values, e.g. set on a dryness meter scale, the machine is stopped. The upper measuring roller may be smooth, knurled or spiked according to the nature of the web material.