Patent GB-701585-A (GB)
Title: Improvements in control systems
IPC: G05D16/20
Filed: 1950-10-26
Abstract: 701,585. Control of A.C. motors. BAILEY METERS & CONTROLS, Ltd. Oct. 26, 1950 [Oct. 26, 1949], No. 26106/50. Class 38(3) [Also in Groups XXXV and XXXVIII] A system for maintaining a variable at a set value automatically comprises means sensitive to the variable for exerting a " standardizing " control on a fluid pressure signal which is converted into a corresponding electrical signal which controls an electric motor operating to restore the variable to said set value; standardizing control comprises a combination of positioning control and floating control, i.e. the magnitude of the fluid pressure signal is conjointly proportional to the deviation of the variable from the set value and to the time integral of such variation. In the embodiment of the invention. described, the controlled variable is furnace pressure but it may be temperature, humidity, rate of flow or position. As shown a pressure sensitive device 1 positions the stem 2 of a pilot valve 3 of the type disclosed in Specification 425,507, [Group XII], to establish in pipe 4 a fluid loading pressure related to the furnace pressure which is applied to a standardizing relay 5 of the type disclosed in Speoification 461,389, [Group XXIX], to establish in pipe 7 a pressure which is conjointly proportional to the deviation of the furnace pressure from a set value and to the time integral of such deviation. Pipe 7 is coupled via a manual-automatic selector valve 9 to a bellows 8 which positions the movable core 10 of a transformer T comprising a primary 11 and two series-opposed secondaries 12 and 13 connected by leads 14, 15 to the secondaries 12<SP>1</SP>, 13<SP>1</SP> of a similar remotely located transformer R, primaries 11, 11<SP>1</SP> being energized by the same A.C. source and a resistance 16 being connected in series in lead 15. When the two cores 10, 10<SP>1</SP> are in corresponding positions there is no voltage across resistance 16 but any departure from this balanced condition produces an A.C. error signal of amplitude and phase dependent on the magnitude and direction of the unbalance and a fraction E of this voltage is applied in parallel to the grids of two thyratrons 24, 25 whose anodes are energized in push-pull from transformer 26 so that the interlocked relays 27, 28 are selectively energized in dependence on the phase of the error signal. The anodes and grids of power thyratrons 29, 30 are energized in push-pull from transformers 37 and 38, the voltages from the secondaries of the two transformers being in phase opposition so that the thyratrons are normally cut off but are selectively fired on energization of relay 27 or 28 which short circuits grid resistance 31 or 32 thereby selectively energizing interlocked relays 33, 34 which control the energization and direction of rotation of a three-phase motor 20 coupled via reduction gearing 35 to a damper 21 controlling the furnace pressure and via a cam 36 to position the core 10<SP>1</SP> so that the damper and core 10<SP>1</SP> move in correspondence with movements of core 10 resulting in a positioning type of control with floating characteristics superimposed.