GB526771A - Improvements relating to the automatic control of variable conditions - Google Patents

Abstract

526,771. Automatic control systems. KENT, Ltd., G., and IVANOFF, A. March 25, 1939, No. 9423. [Class 38 (iv)] Variables in general.-An " on-off " controller. e.g., a fluid valve, is operated so that it remains constantly in one of its extreme positions when the sense of the deviation of a variable from a desired value is the same as the sense in which the variable is changing, but alternates between these positions to obtain a median effective setting when these two senses differ. A switch 4 is operated at a frequency high compared with the natural frequency of the variable, so that a switch member 9 responsive to the deviation and a switch member 17 responsive to the sense in which the variable is changing are effective for equal (or different) alternating periods, upon the electrical operating mechanism B of the valve V; the relation between the periods may be dependent on the magnitude of the deviation and the rate of change of the variable. An arm 13 moves a spindle 12 anti-clockwise in accordance with an increase in the value of the controlled quantity and carries with it a cam 11 and one element of a slipping clutch 18 actuating the member 17 against a contact 15, so long as the quantity is increasing, and against a contact 14, whilst it is decreasing. A cam 10 loose on the spindle 12 is adjusted to set the desired value of the quantity and engages one of two limbs 6, 7 of a bifurcated arm on a pivot 8 carrying contacts 1, 2 with which a contact 3 on the arm 9 co-operates. Thus the contacts 1, 3 engage when the variable is below the desired value and the contacts 2, 3 when above. To avoid errors due to the time taken by the arm 17 in transferring its contacts 16 from engagement with contact 14 to contact 15 or vice versa a contact 21 is arranged to energize a heating coil on the arm 17 which, being bimetallic, is carried suddenly by flexure on heating and on cooling, from one contact to the other in the time normally taken by the variable in changing its trend, i.e., one half cycle of change.