Actuator rate saturation compensator

A device for compensating for actuator rate saturation is disclosed. The ice is an improvement to a servomechanism of the type wherein a command signal and a position feedback signal from a member being positioned are summed to produce an error signal for driving an actuator to position the member in accordance therewith. The improvement is a means for inverting the error signal when the ratio of the slope of the plot of the position feedback signal versus time to the slope of the plot of the command signal versus time is less than one and the polarity of the error signal is opposite to the polarity of the slope of the command signal, and the absolute value of the error signal is not increasing. Application of the invention to the flight controls of an aircraft is disclosed.

BACKGROUND OF THE INVENTION 
This invention relates generally to apparatus for compensating for actuator 
rate saturation in servomechanisms, and more particularly to such 
apparatus for use with flight control actuators. 
With the advent of more and more high performance jet aircraft, actuator 
rate saturation, wherein a phase lag develops between changes in command 
to the actuator and response from the aircraft, is becoming an 
increasingly critical issue in the flight controls arena. Actuator rate 
saturation can occur due to limitations in the hydraulic system or the 
physical limitations of the actuator itself. 
In hydraulic system design there is a trade-off between high flow rate and 
high pressure. At high dynamic pressure (q), when airspeed is greatest, a 
very high hydraulic pressure is required to overcome the large hinge 
moments and effectively control the actuators. Under this circumstance, 
the actuator piston should be designed to have a large crosssectional 
area, so as to reduce the hydraulic pressure level required. At low 
dynamic pressure, a large flow rate is required for actuator control due 
to reduced control surface effectiveness at low speeds. The large flow is 
especially critical in the power approach situation. If an actuator is 
designed to have a large piston area for high q situations, low q 
conditions can only be satisfied with the actuator operation at a very 
high rate. 
High performance jet aircraft, particularly short takeoff and landing 
(STOL) designs, must operate in both the high q/supersonic and low q/power 
approach situations. As a result of these conflicting demands on the 
actuator, rate saturation can occur, causing a phase lag problem, which in 
turn causes poor flight control performance and, often, pilot induced 
oscillations (PIOs). Another situation often occurs in which the pilot 
utilizes many of his control surfaces simultaneously. This results in 
reduced hydraulic pressure, which can cause actuator rate saturation due 
to the hydraulic limits of the actuator itself. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to compensate for the rate 
saturation of an actuator used to control the movement of a member. 
It is a particular object to eliminate poor flight control performance by 
compensating for the rate saturation which can occur in a flight control 
actuator. 
It is yet another object to reduce the occurrence of pilot induced 
oscillations in aircraft. 
Briefly, these and other objects of the invention are accomplished by an 
improvement to a servomechanism for driving an actuator to control the 
position of a member, the servomechanism being of the type wherein a 
command signal and a position feedback signal from the member are compared 
to derive an error signal for controlling the actuator. The improvement 
comprises a means for inverting and sustaining inverted the polarity of 
the error signal when a ratio of the slope of the plot of position versus 
time (hereinafter "slope") for the position feedback signal to the slope 
of the command signal is less than one and the polarity of the error 
signal is opposite to the polarity of the slope of the command signal and 
the absolute value of the error signal is not increasing. 
These and other objects, advantages, and novel features of the invention 
will become apparent from the following detailed description of the 
invention when considered in conjunction with the accompanying drawings 
wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like characters designate like or 
corresponding parts throughout the several views, there is shown in FIG. 1 
a compensator 10 of the present invention. It is shown in context with its 
use in a servomechanism 12, which includes an actuator 14, used to control 
the movement of a member 16. In such a conventional servomechanism 12, a 
command signal A inputs to a summer 18. A position feedback signal B from 
member 16 also inputs to summer 18, where it is summed with command signal 
A to produce error signal C, which in turn drives actuator 14. A typical 
application of such a mechanism would be in the flight control system of 
an aircraft, wherein the command signal A is the signal from the pilot's 
control input, actuator 14 is the hydraulic drive, and member 16 is one of 
the control surfaces on the aircraft. In such a flight control system, 
error signal C drives actuator 14 through a servovalve (not shown) which 
controls hydraulic flow to actuator 14. 
In a conventional servomechanism 12, the error signal C is indicative of 
the extent to which member 16 is not where the command signal A is 
directing it to be. Therefore, actuator 14, which is driven by error 
signal C, should be responding at the level necessary to move member 16 to 
the desired position. Under certain circumstances, however, servomechanism 
12 becomes rate saturated and member 16 cannot track command signal A. 
This is shown in FIG. 2a, where the slope of the plot of the position 
feedback signal B versus time is less than the slope of the plot of the 
command signal A versus time. In this situation, member 16 will not change 
direction with the command as long as error signal C remains the same 
polarity. Actuator 14 will continue to move member 16 in the same 
direction under these circumstances in spite of a change in direction of 
command signal A, in an effort to drive the error signal C to zero. When 
error signal C does change polarity, shown on the graph of FIG. 2a as the 
point when command signal A and position feedback signal B intersect, 
position feedback signal B will change direction or slope. 
Compensator 10 is designed to compensate for this failure of actuator 14 to 
track a change in direction of command signal A, or rate saturation. It 
does so by sensing when member 16 is traveling in the opposite direction 
to which it is being commanded to travel under circumstances of rate 
saturation, and inverts the polarity of error signal C to drive actuator 
14 in the opposite direction. FIG. 2b, wherein the command and position 
feedback signals are plotted as functions of time, shows how 
servomechanism 12 operates when using compensator 10 of the present 
invention. The slopes referred to below are the slopes of these plots. 
Compensator acts to switch the polarity of error signal C and sustain it 
switched when a ratio of the slope of position feedback signal B to the 
slope of command signal A is less than one, and the polarity of error 
signal C is opposite to the polarity of the slope of command signal A. 
Additionally, the compensator 10 will only continue to operate when the 
absolute value of error signal C is not increasing. This last condition is 
present to prevent signals A and B from diverging, as shown in portion 1 
of FIG. 2b. 
FIG. 1 shows a schematic of a preferred embodiment of compensator 10. A 
slope ratio determining means 20 first determines when the ratio of the 
slope of position feedback signal B to the slope of command signal A is 
less than one and outputs a signal D indicative thereof. Signal D is 
enabling if the ratio is less than one, in which case compensator 10 is 
said to be armed. Inputs to slope ratio determining means 20 are signal 
A', which is indicative of the slope of signal A, and signal B', which is 
indicative of the slope of signal B. Signal A' is derived by passing 
signal A through first transfer function 22, which may be a discreet 
delay, and comparing the output therefrom to signal A via first summing 
junction 24. Signal B' is derived in the same manner using a second 
transfer function 26 and a second summing junction 28. Slope ratio 
determining means 20 may be an appropriately arranged operational 
amplifier 30 or any other well-known device capable of comparing B' and 
A', in combination with a limiter 32 for determining when the value from 
the operational amplifier is less than one. 
Next, a polarity determining means 34 determines when the polarity of error 
signal C is opposite to the polarity of slope A' of command signal A, and 
outputs a signal E, indicative thereof. Signal E is enabling if C and A' 
are opposite in polarity, and non-enabling if they are the same. Polarity 
determining means 34 may be two polarity determiners 36 and 38, which 
receive signals A' and C, respectively, and output only their polarities, 
and an exclusive OR gate 40 which receives the signals therefrom. 
An absolute value determining means 42 then determines whether the absolute 
value of the error signal C is increasing or not with time, and outputs a 
signal F, indicative thereof. Absolute value determining means 42 may be a 
first signal stripper 44 in parallel with a series combination of discreet 
delay 46 and a second sign stripper 48. Error signal C passes through both 
to be received by a comparator 50, which outputs signal F. Signal F is 
enabling if signal C is increasing with time, and non-enabling if it is 
not. 
An interrupting means 52 is arranged to output an enabling signal G only as 
long as compensator 10 is armed and the output F from absolute value 
determining means 42 is nonenabling. Interrupting means 52 may be a first 
AND gate 54 connected to receive as input both signal F and output from a 
J-K flip-flop 56, also part of interrupting means 52. The output of first 
AND gate 54 is connected to the K input of flip-flop 56, which has as its 
J input signal D from slope ratio determining means 20. Signal G outputs 
from J-K flip-flop 56. In this way, signal G will become non-enabling as 
soon as the absolute value of error signal C begins to increase and 
compensator 10 was previously armed. 
Polarity altering means 58 is connected to invert and sustain inverted 
error signal C when signal E and signal G are enabling, which is when 
signals A' and C have opposite polarities, B'/A' is less than one, and 
signal C is not increasing. Polarity altering means 58 may be a second AND 
gate 60 for receiving the signals and an inverter 62 connected to the 
output of the second AND gate for altering the polarity in accordance with 
the output received thereby. Inverter 62 is connected between summer 18 
and actuator 14. 
Some of the many advantages and novel features of the invention should now 
be easily apparent. For instance, a compensator has been provided for 
compensating for the effects of rate saturation of a servomechanism which 
is used to control the position of a member. The invention is particularly 
useful in the flight control of an aircraft, where it can reduce the phase 
lag between change in direction of commands to the flight controls and 
response by the aircraft. The aircraft will now change directions with the 
command even under conditions of heavy demand during which the flight 
actuator becomes saturated. The likelihood of pilot induced oscillations 
occurring is thereby reduced. 
Other embodiments and modifications of the present invention may readily 
come to those of ordinary skill in the art having the benefit of the 
teachings presented in the foregoing description and drawings. For 
instance, the invention can be readily implemented with either analog or 
digital devices that are well-known in the art. Therefore, it is to be 
understood that the present invention is not to be limited to such 
teachings presented, and that such further embodiments and modifications 
are intended to be included within the scope of the appended claims.