Patent GB-773395-A (GB)
Abstract: 773,395. Electric control systems. HOBART MANUFACTURING CO. July 23, 1954 [Aug. 24, 1953], No. 21548/54. Class 40 (1). [Also in Group XXXVII] A weighing-machine in which a lever is deflected proportionally to the weight actuates an indicator and recorder through a motordriven follow-up system. Energization of the motor is delayed slightly following closure of the follow-up switch to preclude operation of the motor by vibration effects. The motor is driven at low, medium or high speed according to the magnitude of the deviation of the follow-up member, and is dynamically braked to prevent hunting. To preclude backlash errors the follower member is always brought to rest in the balance position while moving in the same direction. When an article is removed from the scale the motor drive is disabled for a short delay time sufficient for another article to be placed on the scale so that the follower mechanism does not have to be driven to zero between weighings. This delay time is ended immediately the new article is placed on the scale. As shown, the weighing platform 10 is carried on a lever system 11-12-14 so that the right-hand end of the lever 12 is deflected downwardly according to the weight in opposition to a spring 15 or a counterweight. The lever 12 has an extension 25 carrying contacts 30-32-34- 36-38 co-operating with corresponding contacts 31-33-35-37-39 of a follower 27 driven by a motor 50 which also drives a counter type weight indicator 54 and a printer 60. At " balance " the contacts on the lever arm 25 are disengaged, and the contact pairs 32-33, 34- 35, 38-39 close successively as the deflection of the arm 25 increases downwardly. The contact pairs 30-31, 36-37 similarly close successively for upward deflection of the arm 25. Further contacts 20-21 close when the platform is loaded to release the no-load relay N, which together with relays U, D, A, C, P are operated at no-load. Precluding operation by vibrations. Vibrations of the scale mechanism cause momentary closure of the " fine " contacts 30-31 or 32-33 which shunt, respectively, the up and down relays U, D by resistors 88, 89 but the shunting does not persist long enough for the relays in question to fall out. Weighing operation. When the platform 10 is loaded contacts 32-33 close to shunt the down relay D as above described. After its delay period relay D relapses and in turn releases the control relay C to supply both the motor armature 51 and field 52 in parallel from the low voltage D.C. source 82. The motor drives the follower member at slow speed until balance is restored when relay D closes immediately to short-circuit the armature but the control relay C is delayed in pulling up by a condenser 100 to maintain the motor field for its delay period to effect dynamic braking of the motor. If the initial deflection of the lever arm 25 is sufficient to close contacts 34, 35 a medium speed relay M is also energized and connects the motor as a series motor directly across the A.C. source 70 through a resistor 105. A resistor 106 is shunted across the armature so that the motor now runs at medium speed. If the lever arm 25 deflects sufficiently to close contacts 38-39 a fast relay F is also energized to cut out resistor 106 and short-circuit 105 so that the motor runs at full speed. As the balance point is approached and contacts 38-39 open, dynamic braking of the motor to medium speed is effected by the reinsertion of resistor 106. When the relay M is cut out by opening of contacts 34-35 dynamic braking to slow speed is effected by the shunt field. Printing. When the control relay C reoperates at a new balance position the printer motor 61 is energized to perform a printing cycle during which the follower switch is disabled and a ticket printed showing the weight registered at 54. At the end of the cycle the print relay P is released at cam-operated contacts 96 and can be energized only when the load has been removed from the scale to reenergize the no-load relay N. This relay also prevents printing at zero load. Anti-backlash. If the follower 27 should overshoot the balance position while travelling downwardly contacts 30-31 close to release relays U, A and energize relay M so that the motor runs at medium speed in the up direction. If contacts 36-37 close the motor will run at fast speed. The upward motion continues beyond the balance point until contacts 34-35 close to re-operate relay A when the motor reverses and as described above the follower 27 is driven downwardly to the balance point. Holding. When a load is removed from the scale the contacts 30-31, and possibly 36-37, close to initiate upward drive of the follower 27 as above described. The no-load relay N is, however, operated as the load is removed and disconnects the A.C. supply 70 from the motor 50 for the release time of a hold relay H, this relay being energized when N releases and deenergized as N re-operates; the release time is sufficient for another article to be set upon the scale whereupon relay N releases and the follower is driven according to the new load. If no load is applied to the scale the follower is driven to the zero position, after the delay period of relay H as described above under antibacklash. Specification 773,396 is referred to.