Patent GB-676390-A (GB)
Title: Improved speed control mechanism for prime movers
IPC: G05D1/06
Filed: 1950-05-19
Abstract: 676,390. Fluid-pressure servomotor-control systems. GENERAL MOTORS CORPORATION. May 19, 1950 [May 24, 1949], No. 12543/50. Class 135 [Also in Group XXXV] A speed control system wherein a hydraulic servomotor adjusts the pitch of an aircraft propeller 5, Fig. 8, comprises, for automatic fine speed control, a three-phase alternator 15, Fig. 4, directly connected to the propeller, and two voltage rectifier-filter combinations 120, 138 fed from one of the phases of the alternator which provides reference and speed proportional voltages respectively. These voltages are applied to a bridge circuit 137, the output of which, varying in dependence on speed variation of the propeller from a fixed datum is applied to a multi-vibrator and amplifier circuit 169, the pulse output of which controls relay combinations 534, 541 which in turn controls a solenoid actuated valve 14, Figs. 5 and 8, to control the application of fluid pressure to the servomotor controlling the propeller pitch. The system also includes a fluid pressure operated automatic coarse speed control, Fig. 8, and over-riding manual control mechanism, Fig. 3. The electronic fine speed control mechanism, Figs. 4 and 5, is also sensitive to rate and integral components of the speed deviation. Over-riding manual control (Fig. 3). By movement of control levers 9 the pilot causes unbalance of an electrical bridge comprising potentiometers 339 and 342 thereby energising a polarised relay 345 to energise one or other of relays 365, 366 depending on whether a higher or lower propeller speed is required. Energisation of the relay 365 causes the field winding 374 of motor 381 to become energised thereby rotating the motor and shaft 155 to rebalance the bridge circuit through contact 357 of potentiometer 342, to reset the datum level of operation of the fine speed control through potentiometers 273 and 139, and to adjust the datum level of the coarse speed control and decrease the propeller pitch by means of linkages 8, 7, 500, 502, 86, 87, 90 and plunger 33 of hydraulic selector valve 30, Fig. 8. Energisation of the relay 366 causes rotation of the motor 381 by means of field windings 375 in the opposite direction with corresponding resettings and increase of propeller pitch. Automatic electronic fine speed control (Figs. 4 and 5). One phase of the alternator 15 driven by the propeller, feeds, through a transformer 122, the rectifier-filter combination 120 which, owing to the action of a voltage regulator 132 provides a reference d.c, voltage for input to the bridge circuit 137 at terminals 135, 136. The same phase of the alternator also feeds the other rectifier-filter combination 138 through a transformer 510 and manually set potentiometer 139 which provides across bridge terminals 150, 152 a voltage proportional to propeller speed. The bridge output, across points 157, 158, being of magnitude and polarity corresponding to the extent and sense of propeller speed deviation from the preset value, is applied to the grids of valves 191, 192 of the multivibrator 169. The valves of the multivibrator 169, when the bridge is in balance alternately conduct for equal intervals of time, but, as the cathodes of valves 191, 192 are connected to the point 184, when the bridge is out of balance one of the grids has a positive bias applied thereto and the other a negative bias so that one valve remains conductive for longer than the other, and the multivibrator produces alternate pulses of unequal duration. The pulse output of the multivibrator is applied to power valves 230, 231 so that one set of alternate pulses is applied through valve 230 to control relay 534 and the other set through valve 231 to control relay 541. Under normal conditions, therefore, the relays are energised for equal intervals of time and, over contacts 530, 540 and 532, 542, cause equal alternate energisation of the solenoids 62, 63 of the hydraulic valve 14. Alternate equal pulses of fluid pressure are therefore applied through pipes 53, 54 to the hydraulic propeller pitch controlling servomotor. When the bridge is out of balance, however, the pulses are of unequal duration and the pitch controlling hydraulic servomotor will receive unequal pulses of pressure so as to adjust the propeller pitch in a sense to correct the propeller speed deviation from the preset valve. The voltage applied to the grids of multivibrator valves 191, 192 is modified in accordance with rate of change of propeller speed by the action of resistor capacity networks, 167, 902, and 168, 901 which act as differentiating networks to the voltage applied to the bridge across resistors 504, 506, thus producing a voltage proportional to rate of change of propeller speed across the resistors 901, 902 for superimposing on the grids of valves 191, 192 respectively. The integral voltage component is derived by automatic adjustment of potentiometer 149 in the speed proportional voltage input circuit to the bridge, the adjustment being made at a rate proportional to the frequency difference between the output of the alternator 15 and that of a preselected alternating frequency signal source 240, the correct frequency of oscillation of which for a particular speed is determined by the manually set potientiometer 273. The oscillator 240 comprises two oscillator valves and a gating valve 960, the oscillator output being applied to a discriminator 241 comprising valves 314, 315, 316, rendered conductive and non-conductive at the oscillator frequency by the action of valve 960 and to which are applied the three phases of the alternator 15. The outputs of valves 314, 315, 316, are applied the windings 308, 309, 310 of a three-phase induction motor 154 which operates shaft 446 to control the setting potentiometer 149. Condensers 317, 318, 319 connected across motor windings 308, 309, 310 respectively serve to integrate the plate current flowing when the valves 314, 315, 316 are rendered conductive. Thus, when the frequency of the alternator varies from that of the oscillator 240 the motor 854 is rotated in accordance with the beat frequency produced to adjust the potentiometer 149 and provide the integral voltage component. Automatic hydraulic coarse control (Fig. 8). The hydraulic regulator is contained within the propeller hub and includes a pump system 20 driven by rotation of the propeller, pressure being applied through line 21 to a control valve 22 and high pressure relief valve 23. A high pressure line 24 is connected to a pressure reducing valve 26, and to the solenoid operated valve 14 of the fine speed control system. A low pressure line 28 extends to a speed sensitive valve unit 29 and to a selector valve 30, the plunger 33 of which is set manually by the linkage 86, 87, 90. The valve 30 is connected over lines 38, 41 to a distributor valve 27 connected in turn to the two chambers 51, 52 of the propeller pitch controlling servomotor by passages 49, 50 which also communicate through lines 64, 65 to opposite sides of a shuttle valve 66 in the control valve 22. Thus pressure applied to the propeller servomotor is also applied to one end of the shuttle valve 66 so that pressure may be applied to a piston 68 the opposite side of which is exposed to pressure from line 21. Piston 68 is also urged downwardly by centrifugal force and by a spring 70 so as to partially close a port 71, thus controlling the pressure in line 24 and, by operation of the piston in chamber 73, allowing an auxiliary propeller driven pump 77 to discharge to drain through port 75. Should the pressure of pump 20 fail ports 71 and 75 are closed and the auxiliary pump supplies the line 24 against the action of a ball valve 78. The pressure reducing valve 26 reduces the pressure in line 24 to a lower and steadier value, the new pressure being communicated over line 28 to the speed responsive valve 29, the plunger 35 of which is urged upwardly by centrifugal force and downwardly by pivoted lever 80 and spring 80<SP>1</SP>, the pivot 81 being set by the manual control linkage 86, 87. Upon an increase of speed, plunger 35 moves upwardly to uncover port 82 and allow pressure to be applied through selector valve 30 to chamber 39 of the distributor valve 27. Since the diameter of chamber 39 is greater than that of chamber 40 to which pressure is also applied through valve 30, the piston 44 moves downwardly to uncover port 48 exposing high pressure to chamber 52 of the propeller servomotor and adjusting the pitch to reduce the speed. A follow-up is provided by horizontally movable link 83 and vertically movable porting sleeve 35a of the valve 29. Should the speed decrease, the plunger 35 moves downwardly to expose port 82 to drain so that fluid may be exhausted from the chamber 39 allowing the piston 44 to move upwardly under the action of pressure applied to chamber 40, thus exposing high pressure to chamber 51 of the servomotor to effect an increase in propeller speed. A limited amount of movement of piston 44 is allowed owing to the small diameter of ports 47, 48 and it is within this range that the fine control is effective through valve 14. A motor operated pump 92 is provided to supply pressure through ball valve 93 should the pumps 20 and 77 become ineffective through slow speed or stoppage of the propeller. Negative pitch and feathering control (Fig. 8). Any movement by the manual control of the linkage member 90 so that a pin 91 of the plunger 33 remains in the horizontal groove of the member 90 allows coarse and fine governed pitch action. If, however, the linkage is operated by the pilot so that the pin 91 rides into the grooves 90f or 90n the plunger 33 is set for feathered or negative pitch action respectively. In the first case the plunger is moved upwardly allowing low pressure to flow from line 28, groove 31, passage f, line 38 and chamber 39 to open port 48 for extreme increased or feathered pitch, oil in chamber 40 b