Electricity: motive power systems – Positional servo systems – 'reset' systems
Reexamination Certificate
2000-02-11
2001-07-10
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
'reset' systems
C318S609000, C318S432000, C318S433000, C318S434000
Reexamination Certificate
active
06259223
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for executing phase compensation in a control system of a motor-driven vehicle, in which a time derivative for an output signal from a circuit contained in the control system is limited relative to a time derivative for an input signal to the circuit, if the time derivative for the input signal exceeds a predetermined value. The invention also relates to apparatus for executing phase compensation in the control system of a motor-driven vehicle, which includes a circuit comprising limiting means for limiting the time derivative of an output signal in relation to the time derivative for an input signal to the circuit, if the time derivative for the input signal exceeds a predetermined value, set by the limiting means.
BACKGROUND OF THE INVENTION
All (vehicle) control systems have physical actuators, e.g., electrical motors, hydraulic servo valves, etc. These actuators all have rate limits due to limits in power supply, hydraulic pressure, etc. Thus, all control systems inherently include restrictions with regard to the rate at which a new command from the driver of the vehicle, i.e., a change in the input signal into the control system, can give rise to corresponding changes in the physical output signal from the control system. If the time derivative for the input signal exceeds a certain value, the time derivative for the output signal is limited in relation to the time derivative for the input signal. This limitation is known as the rate limitation of the control system. When very rapid changed in the input signal are executed, for example when the input signal to the control system consists of an excessively rapid (or large) sinusoidal signal, a phase shift occurs between the input signal and the output signal. That is, the output signal is subject to a time delay in relation to the input signal. This phase shift leads to impairment of the performance of the vehicle and, in the worst case, can give rise to instability.
In aircraft applications, a so-called PIO (Pilot In-the-loop Oscillation) can occur when an excessively rapid change in the input signal causes the rate limitation of the control system to be exceeded. This can occur if an unforeseen circumstance causes the pilot to execute rapid and large movements with the control stick of the aircraft. The phase shift which occurs because of the rate limitation of the control system amplifies the oscillations. In the worst case these oscillations become divergent, which can result in loss of control over the movements of the aircraft.
The aforementioned rate limitation is more noticeable in aircraft which constitute a so-called unstable system. In this type of aircraft, the control surfaces of the aircraft are affected not only by the signals from the pilot, but also by stabilization signals generated in the control system, which are dependent on values obtained from sensors at different points in the aircraft.
One way of reducing the aforementioned problems in control systems with rate limitation involves the introduction of phase compensation when the rate limitation is active. Such phase compensation must meet the following requirements:
A. reduce the phase retardation in the case of sinusoidal input signals;
B. minimize the dynamic retardation for rapid ramp and steps;
C. provide the same input and output signal when the input signals are sufficiently slow.
D.
Different methods of executing phase compensation in control systems with rate limitation are previously disclosed in Buchholz, J. J. (1993): “Time delay induced by Control Surface Rate Saturation ”, Zeitschrift f. Flugwissenschaften und Weltraumforschung, Springer Verlag, Vol. 17, pp. 287-293; A'harrah, R. C. (1992); “Communique with DLR and others”, NASA HQ, Washington DC, Jul. 14, 1992; and Chalk, C. R. (1992); “Study of a Software Rate Limit Concept”, Calspan Flight Research Memorandum 635, Buffalo, N.Y. These methods use logical conditions (if-then-else) to establish whether a phase compensation requires to be executed in the control system. However, these conditions call for a jump to be made between at least two different dynamic behaviors for the respective methods. Thus, input signals can always occur which give rise to an undesired output signal. For this reason, none of these methods is suitable to be implemented in a control system with rate limitation.
A method for executing phase compensation in a control system of the described type solving the above mentioned problems is disclosed in U.S. Pat. No. 5,528,119.
It is an object of the present invention to provide a method and apparatus for executing phase compensation in a control system with rate limitation being an alternative to the above mentioned prior art methods.
BRIEF DESCRIPTION OF THE INVENTION
One object of the invention is to provide a method for executing phase compensation in a filter for such systems as defined in the claims by use of a phase compensation filter which has a nonlinear feedback of the output signal to the input. More specifically, when there is an error between an input signal and an output signal to the phase compensation filter, an error signal is formed. If the absolute value of said error signal exceeds a predetermined value, a level k of the nonlinear amplification is chosen. The input signal is low pass filtered and differentiated, whereupon it is multiplied with the nonlinear amplification level k for forming a product signal &dgr; which is added to the input signal and led to a conventional rate limiting device thus forming the final output signal of the phase compensation filter.
The output signal produced in the filter after subtracting the product signal &dgr; is in phase with the input signal sent to the filter device.
The level of phase compensation may be determined as a function of the derivative of the input signal and the error between input signal and output signal by the choice of a wanted filtering and the level of nonlinear amplification k.
When the level k of the nonlinear amplification is zero or when the differentiated input signal is zero the phase compensation filter acts as a conventional rate limiting device
One advantage with a filter as disclosed according to the above presented aspect of the invention is that the phase compensating filter is stable against influence from noise and resonance disturbances on the dominating input signal.
Another advantage with the filter according to the above described aspect of the invention is that the filter upon input step formed signals acts as a conventional rate limiting device during a time interval when the time delay between the input signal and the output signal should be as short as possible. This is a difference between this invention and the method disclosed in U.S. Pat. No. 5,528,119, where the output signal has an exponential decay towards the input signal. Depending on the specific situation either of the two methods may be advantageous.
Yet another advantage with a filter according to the invention is that the filter is active when it is needed only, which may happen during rare occasions or perhaps never.
FIG. 1
shows an example of the effect of a phase compensating filter according to the invention as a curve y, the output signal, (continuous line) as a response from the filter to an input signal illustrated by curve u (dashed line) in the figure. A corresponding signal as a response from the same input signal when using a conventional rate limiting device is shown as a dotted curve c.
It may be noted that the response to the step function as indicated by the curve for the input signal u more rapidly is followed by the output signal y from the filter according to the invention compared to the output signal from a conventional rate limiting device.
The invention is primarily adapted for use in aircraft control but may as well be used in any type of vehicle control system.
According to a further aspect of the invention a device for carrying out the method is disclosed. Said device is described in the claims and further in the descriptio
Nappi Robert E.
SAAB AB
Smith Tyrone
Swidler Berlin Shereff & Friedman, LLP
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