US 2849072 A
Description (OCR text may contain errors)
Aug. jz6, 195s l c. B. BRAHM 2,849,072 VARIABLE SENSITIVITY SPEED CIRCUIT FUR VARIABLE PITCH PROPELLERS Filed June 23, 1955 ,een #masks/50 /N VE N T 0R CHARLES B. BRA/-IM amos afm-E wm/65 By 7'/ ,sans a@ DGEEE/J-c. ATTORNEY nited States Patent VARIABLE SENSiTI'i/ITY SPEED ClRtCUllT FOR VARIABILE-PITCH PROPELLERS Charles B. Brahm, Windsor Locks, Conn., assigner to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application June 23, 1955, Serial No. 517,544 11 Claims. (Cl. 170--16.2)
This invention relates to a propeller control mechanism and particularly to a speed control having variable sensitivity to minimize hunting.
An object of this invention is a device which will reduce the sensitivity of propeller control mechanism for relatively small speed errors.
A further object is mechanism limiting the value of a rate-of-change-responsive signal opposing the signal producing the change.
A further object is an electric speed responsive propeller pitch control having a rate-of-pitch-change responsive mechanism opposing said speed responsive control and having means limiting the effect of said mechanism.
A still further object is a control mechanism having a low sensitivity around the control point and a higher sensitivity when approaching the control point.
Fig. l is a schematic drawing showing a controllable pitch propeller with speed responsive control mechanism incorporating a rate responsive modifying circuit.
Fig. 2 is a chart showing the effect of the sensitivity varying mechanism.
In the speed control of propellers it has been found that reducing the sensitivity of the speed error signal producing mechanism as the propeller approaches the on-speed position tends to reduce hunting and stabilize the system. However, in order to maintain proper control for large speed errors, it is necessary to utilize the full sensitivity of the control, so as to provide a large correcting force for a large speed error.
In the embodiment chosen to illustrate this invention a speed error signal indicating the extent of variation from a non-speed condition is produced by electrical means, and after being amplified is utilized to proportionally actuate a hydraulic pitch changing mechanism to regulate the propeller pitch and thus control the speed of the propeller-driving engine by controlling the load on the engine. The rate of pitch change for each unit of speed error is made great enough to prevent material speed variations from the selected or datum speed, but such a rate requires a control so sensitive that the mechanism will be subject to hunting or overshooting. In order to reduce or prevent such hunting, the sensitivity of the control, for small speed changes, is reduced by supplying a signal opposing the speed error signal. The opposing signal is a signal responsive to the rate of propeller pitch change, which will reduce the sensitivity of the control for small speed changes.
In order to provide suilicient sensitivity for large speed errors and rapid pitch changes a limiting mechanism is placed in the rate responsive signal circuit which will limit the amount of the opposing rate signal to a predetermined amount. Hence, for small speed errors which will give a comparatively slow rate of pitch change the opposing rate signal may be a relatively large proportion of the error signal to lthus reduce the effective pitch changing signal, or in other words, reduce the sensitivity of the circuit. -For large error signals `which will produce a large rate of pitch change the opposing signal is limited in amount so that the opposing effective rate signal will be a small proportion of the error signal which will give a circuit or increased sensivity.
In the embodiment illustrated an engine l drives a well known type of propelle;y such as a controllable pitch propeller l2 having a hydraulically actuated piston 14 actuating cam mechanism 16 geared to the several propeller blades 18, only one of which is shown, to change the blade pitch. A shaft 20 geared to the blades 18 transmits the blade motion and position through gearing, including a differential gearing 22 to a gear 24 located outside of the propeller. The `differential gearing 22 may be of any desired type suitable for transmitting motion from a rotating part to a non-rotating part so that the nal gear of the train` will move in accordance with the movement of the initial gear of the train with respect to its rotating support. Such a gear mechanism is shown and described in more detail in the patent to Longfellow et al. 2,664,960 issued January 5, 1954, and patent to Richmond 2,666,960 issued January 19, 1954, to which reference may be made for further details of the gear train. Gear 24 which is the last gear of the train is mounted to rotate around a stationary shaft 26 and is actuated by movement of the planet gear assembly 28. Gear 24 meshes with a gear 36 connected with an internally threaded sleeve 32 held against axial or longitudinal movement by shoulders 34 and stationary ring 36. An externally threaded member 38 meshes with the internal threads of member 32 and is connected with a movable portion 4@ of a control valve 42. Valve 42 receives uid under pressure from a pump 44 taking fluid from reservoir 46. Valve 42 selectively directs the fluid under pressure to one side or the other of piston 14 to control the propeller pitch. Threaded member 3S is internally splined to' receive a splined shaft 43 held against longitudinal movement by collars 50 and stationary post 52. Shaft 48 is connected to gear S4 which is actuated by rack 56 connected to a piston 5S which is hydraulically controlled by a speed responsive mechanism.
The speed responsive mechanism includes a propor-` tional solenoid 60 actuated by speed responsive mechanism, presently to be described, controlling a valve 62 receiving pressure fluid from a pump 64 and a reservoir 66 and selectively directing the pressure fluid to one side or the other of the piston 58.
The mechanism thus far described operates in a well known manner to control the propeller pitch and thus Control the engine speed. A speed error signal transmitted to the proportional solenoid titl will actuate valve 62, piston 58, gear 54 and shaft 43 to turn the screw threaded member and move valve member 4d longi- 1 tudinally to actuate the piston 14 and change the propeller pitch. Changing of the propeller pitch will actuate shaft 20 and through the diferential gearing 22 actuate gear 24 and gear 30 to turn the internally threaded sleeve 32 which being held against longitudinal movement will return the valve member 40 to the shutoft' position shown in the drawings. Screw member 38 is held against turning with the internally threaded member 32 by the splined shaft 48 which is held against rotation except when motion is called for by the proportional solenoid 60.
A generator 68 driven by the engine l0 supplies alternating current to a bridge 70 including a condensor 72 to provide an indication of speed error, three legs of the bridge being resistors and the fourth being a condensor. A condenser having a different reactance with different frequencies will unbalance the bridge upon changes in speed and provide the speed. error indication. A rectifier C) 74 along with filtering mechanism connected across one side of the bridge will give a negative signal and a rectifier 76 connected across Vthe other side of the bridge including the condenser 72 will give a positive signal. The two signals are compared through resistors 78 and B to give a final error at 82 which will be negative for overspeeds and positive for underspeeds.
The convention used with respect to the rectifier showing is that the arrow indicates current flow from plus to minus as opposed to electron fiow.
The speed error signal is fed between resistors 83 and 84 where it is transformed to a correction signal and fed to the well known chopper amplifier 86 where the correction signal is amplified. The amplified correction signal is then fed to the proportional solenoid 60 to start the mechanism in motion to change the propeller pitch.
In the mechanism thus far described, a signal and propeller pitch change motion is produced which is proportional to the speed error. It has been found that, in a system sufiiciently sensitive to give a rate of pitch change sufficient to keep the speed control within the desired limits of speed variation, the system is so sensitive when approaching the on-speed position that it will overshoot and hunt to an undesired degree. In order to prevent this unstable and hunt condition while retaining the sensitivity for large speed errors, mechanism has been provided to induce an opposing signal to the i amplifier 85 to thus reduce the sensitivity of the system at the control point.
This opposing or negative feedback signal is provided by a rate-of-pitch-change responsive mechanism including a gear S8 driving a screw threaded member 90 having a nut 92 threaded thereon and movable therealong by rotation of the screw 90. Nut 92 carries a potentiometer arm 94 moving along a potentiometer 96 connecting a fixed voltage source 98 with ground. Arm 94 is connected to a resistor 100 and a condenser 102 to a point between resistors 104 and 106 connected with the chopper amplifier 86. A pair of opposed rectifiers 103 and 110 are connected in parallel with one side connected to ground and the other side connected between condenser 102 and resistors 104 and 106. Movement of arm 94 being geared directly to the blades 18 will move at a rate proportional to the rate of change `of the propeller pitch and will therefore change the potential on -condensor 102 at a rate proportional to the rate of change of the propeller pitch. The signal sent through condensor 102 to resistors 104 and 106 opposing the speed error signal will, therefore, be proportional to the rate of change of the propeller pitch. This is desirable for small pitch changes near a control point but it would reduce the sensitivity of the system for large pitch changes. In order to prevent such a sensitivity reduction for large pitch changes, rectifiers S and 110 will conduct ofi any voltages generated above a predetermined voltage. The voltage limit is determined by the contact potential of the rectifiers 108 and 110 and may be increased by putting several rectifiers in series so that any desired voltage limit may be obtained. With the rectifiers I0?, and 110 in the circuit any voltage which may be created by rapid pitch changes will be limited to the predetermined selected voltage of the rectifier deadband. Thus limiting the opposing voltage obtainable for rapid pitch changes will retain the sensitivity of the circuit for rapid pitch changes since the limiting opposing voltage will be a very small fixed portion of the speed error voltage for large errors.
On the other hand, the opposing or feedback voltage for small speed errors will be a variable voltage and will be a much larger proportion of the speed error v-oltage and will therefore reduce the sensitivity of the system for small errors or, in other words, for small errors the speed error signal opposed by the rate of change signal will result in an output signal to the proportional solenoid 60 which is a much smaller proportion of the speed error signal than would be the case for large speed errors.
The effect of this signal limit mechanism can be shown graphically. Fig. 2 is such a graph. It will be understood that the blade-angle change rate, or the number of degrees per second of blade angle pitch change, bears a direct relation to the strength of the signal output of the amplifier 86. The graph shows R. P. M. error as the X axis and blade angle change rate, or degrees per second as the Y axis. The dotted line indicates the rate at which the blade angle would change for any particular speed error without any feedback and it will be noted that the ratio remains substantially constant throughout the entire range.
rIhe full lines indicate the rate with the limited feedback connected in the circuit. From this graph it will be noted that for small speed errors, the rute of pitch change is small, or in other Words, the sensitivity of the system is low. However, after a selected speed error is reached, at which point the feedback signal is limited, then the rate of pitch change will increase with an increase speed error in the same manner that it increased before the feedback system was applied. lt is therefore apparent from this graph that the system will have a low sensitivity for small speed errors around the control point but will have much higher sensitivity when approaching the control point from a material speed error or variation.
Another feature of the mechanism shown is that for extremely small variations in speed, the speed error signal is first transmitted to the amplifier and then tov the pitch change mechanism and it is not until a pitch change is effected that an opposing signal is produce and fed to the amplifier to oppose the speed error signal so that for very small speed errors the full amplification or sensitivity is obtained. In other words, the back-lash or lost motion in the mechanism from the speed error detector to the blade actuated potentiometer will allow full sensitivity until motion is imparted to the potentiometer arm. This will tend to quickly correct any speed errors and prevent any material speed variation excursion.
It is to be understood that the invention is not limited to the specific embodiment herein illustrated and dcscribed, but may be used in other ways without departure from its spirit as defined by the following claims.
l. In a control System for a controlled device receiving an error signal varying in accordance with the extent of the error to be corrected, means transforming said error signal into a proportional correction signal said controlled device including mechanism making a correction at a rate proportional to the value of the correction signal, means producing an opposing signal proportional to the correction rate, means combining said opposing signal with said correction signal in opposition thereto, and means limiting the value of the opposing signal to a preselected fixed value.
2. In a speed control system, means creating a speed error signal proportional to the deviation of the speed of the controlled device from a selected speed datum. means transforming the speed error signal into a speed changing movement of the conrtollcd device for returning the speed of the device to the datum speed and eliminating the speed error, and means creating a feedback signal, opposing said error signal, proportional to the rate of changing movement of the controlled device, and means limiting the value of said feedback signal to a predetermined amount when said rate exceeds a predetermined value.
3. In a enginefpropeller speed control system, means creating a speed error signal proportional to the deviation of the engine speed from a selected speed datum, means transforming said error signal into pitch changing movement of said propeller to vary the load on the engine and return the engine speed' to the datum speed and eliminate said speed error, and means responsive to the rate of pitch change creating a feed-back signal, means connecting said feed-back signal in opposition to said speed error signal to reduce the signal fed to said transforming means, and means preventing said opposing feed-back signal from exceeding a preselected value when said created feedback signal exceeds said value.
4. A control for a controllable pitch propeller cornprising speed responsive means, pitch changing means and means connecting said speed responsive means with said pitch changing means including means changing said pitch at a rate proportional to the speed error, and means responsive to the rate of pitch change reducing the proportion of the pitch change rate to the speed error and meansv effective above a selected rate of pitch change less than the maximum rate of pitch change limiting the reducing effect of said pitch change responsive means to a preselected value.
5. Pitch control means for a controllable pitch propeller comprising speed responsive means including means creating a speed error signal having a value proportional to the speed error, hydraulically actuated means responsive to said speed error signal for changing the propeller pitch to eliminate the speed error creating said error signal, means responsive to the rate of pitch changing movement creating a rate signal opposing said speed error signal to reduce the sensitivity of the control means and means connected with said rate signal creating means preventing said opposing signal from exceeding a preselected value when said rate signal exceeds said value.
6. Propeller pitch control means comprising hydraulic pitch changing means, means creating an electrical speed error signal proportional to the variation of said propeller from a datum speed, an amplifier producing an amplified signal from said error signal, electro-magnetic means actuating said hydraulic pitch changing means proportional to said amplified signal, means creating an electrical rate signal proportional to the rate of pitch change, means feeding said rate signal to said amplifier in opposition to said error signal, and means limiting the value of said opposition signal when said rate signal exceeds a preselected Value.
7. A control as claimed in claim 6 in which the limiting means is a pair of opposed rectiers connected in parallel grounding said rate signal for all values above the contact potential of said rectiliers.
8. In combination with a propeller having pitch control mechanism, means creating a pitch changing signal, follow-up mechanism actuated by blade pitch changing movement eliminating said signal, electrical mechanism creating a speed error electrical signal, electrical mechanism actuated by said pitch changing mechanism creating a pitch change rate electrical signal, rectiers limiting the value of said rate signal, means combining said two electrical signals in opposition, V:deans amplifying said combined signals7 and means responsive to the polarity and value or' said amplified signals actuating said means creating said pitch changing signal.
9. In a propeller control system means transforming an electrical error signal into a pitch changing signal said transforming means having a selected ratio of input to output, means reducing the effecting ratio of said transforming means for relatively small error signals comprising means creating an electrical signal responsive to the rate of pitch change opposing said error signal, and means restoring said effecting ratio for relatively large error signals comprising means limiting the value of said opposing electrical signal to a predetermined value corresponding to a selected rate of pitch change.
10. In a control system for a controlled device having a substantially fixed datum and subject to deviation from said datum, means creating an error signal dependent on the extent of deviation of said controlled device from said datum, means transforming said error signal into a correction signal, means responsive to said correction signal at a rate proportional to the value of said correction signal for eliminating the deviation causing said error signal, said transforming means having a selected substantially constant ratio of correction signal to error signal and means reducing said correction signal by amounts proportional to said rate when the rate is below a selected value, and means preventing the value of said reducing means from exceeding a predetermined amount at all rates above said selected value.
ll. A control for a controllable pitch propeller comprising speed responsive means, pitch changing means and means connecting said speed responsive means with said pitch changing means, including means changing said pitch at a rate proportional to the speed error, means responsive to the rate of the pitch change reducing the proportion of the pitch change rate to the speed error and means effective above a selected rate of pitch change between zero and the maximum rate preventing the reducing elr'ect of said rate responsive means from exceeding the value attained at said selected rate.
References Cited in the le of this patent UNITED STATES PATENTS 2,382,847 Baumann Aug. 14, 1945 2,455,364 Hays Dec. 7, 1948 2,656,498 Goodwin a Oct. 20, 1953 2,669,312 Dinsmore et al. Feb. 16, 1954