Abstract:
A method and an apparatus for controlling aircraft rudder movement are disclosed. The system including a yaw damping control portion integrated with a directional compensation rudder control portion, such that the system may simultaneously provide yaw damping control and directional compensation.

Description:
TECHNICAL FIELD 
     The present invention generally relates to control systems. More particularly, the present invention relates to control systems suitable for controlling movement of an aircraft rudder. 
     BACKGROUND OF THE INVENTION 
     An aircraft often includes one or more control systems designed to control the aircraft rudder position during flight. Such systems are generally configured to manipulate the rudder position to (1) stabilize the aircraft during flight or (2) provide directional compensation when one or more aircraft engines loose power. 
     Stabilization or yaw damping control systems are generally designed to manipulate the aircraft rudder position to compensate for wind gusts, turbulence, phenomena such as Dutch roll, and the like. Typical yaw damping control systems include a motor or apparatus to move the rudder and a feedback control loop designed to control the motor and thus the rudder position. 
     Directional compensation systems are designed to facilitate directional control of the aircraft when the aircraft looses all or most of the power from one or more engines. For example, directional compensation systems are often employed to reduce an amount of force a pilot is required to apply to a rudder control system when one engine fails on a dual engine aircraft. An amount of force reduction that the compensation system provides may vary, depending on various factors such as the differential force provided by one or more engines on the aircraft, the type of aircraft, and aircraft manufacturer preferences. For example, an aircraft with relatively small engine thrust may not require any directional compensation, while aircraft with relatively large engine thrust would generally include a compensation system configured to facilitate rudder position, such that no more than about 150 pounds force is required by a pilot to maneuver the rudder to compensate for the engine power loss. 
     Aircraft including both stabilization and directional compensation systems generally include separate motors and control devices dedicated to each system. Although dedicating motors and control devices for each system may allow for relatively easy design of each of the respective systems, using two separate systems may be problematic in several regards. For example, aircraft including two separate rudder control systems generally operate such that only one rudder control system can function at any given time. Thus, the stabilization system generally does not operate when the directional compensation system is employed. Accordingly, improved aircraft rudder control systems that simultaneously provide both stabilization and directional compensation rudder control are desired. 
     Another problem associated with aircraft including dedicated rudder control systems is that such aircraft generally include superfluous control devices and/or rudder movement motors. Accordingly, improved rudder control systems which use a single control device and a single motor to provide both yaw damping stabilization and directional compensation control are desired. 
     SUMMARY OF THE INVENTION 
     The present invention provides improved apparatus for controlling aircraft rudder movement and position. The way in which the present invention addresses the deficiencies of now-known rudder control systems is discussed in greater detail below. However, in general, the present invention provides a single system suitable for simultaneously providing yaw-damping stability and directional-compensation rudder control. 
     In accordance with one exemplary embodiment of the present invention, a rudder control system includes a yaw damping stability portion integrated with a directional compensation portion. The integrated system includes a yaw damping command signal generator, a bias command signal generator, at least one summing junction configured to combine signals from the yaw damping and bias command signal generators, and a motor configured to receive the summed yaw damping and bias command signals and move an aircraft rudder in response to the summed signal. 
     In accordance with an exemplary embodiment of the present invention, the control system includes a bias command feed forward path configured to transmit a signal representative of engine thrust differential (difference in thrust between two or more engines on an aircraft) to the motor. In accordance with one aspect of this embodiment, the feed forward path includes a wash out filter and a lag filter. In accordance with another aspect of this embodiment, the feed bias command forward loop path includes one or more gain devices to facilitate turning of the control loop. 
     In accordance with a further embodiment of the present invention, the aircraft rudder control system includes a motor rate feedback loop configured to provide negative feedback to the system to reduce damping motion speed. 
     In accordance with another embodiment of the present invention, the aircraft rudder control system includes a motor current feedback path configured to a feedback signal based on motor load. 
     In accordance with yet another embodiment of the present invention, the bias command signal is transmitted through a feedback path including a signal filter configured to diminish the input bias command signal over time. 
     In accordance with a further exemplary embodiment of the present invention, the control system includes a second motor current feedback path configured to facilitate fine tuning or the rudder control motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be derived by referring to the detailed description and the claims, considered in connection with the figure, wherein: 
     FIG. 1 is a schematic illustration of a feedback control system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present invention generally relates to control systems. More particularly, the invention relates to an aircraft rudder control system. Although this invention may be used to provide rudder movement control to a variety of aircraft, the invention is conveniently described below in connection with providing rudder control to a multi-engine airplane. 
     FIG. 1 illustrates a schematic representation of a rudder control system  100  in accordance with an exemplary embodiment of the present invention. System  100  is generally configured to provide both yaw damping control and directional compensation to a rudder of an airplane. 
     As illustrated, system  100  includes a yaw damping command source  110 , a bias command source  120 , a yaw damping feed forward path  130 , a bias command feed forward path  140 , a first motor current feedback path  150 , a motor rate feedback path  160 , a second motor current feedback path  170 , and a motor  180  configured to manipulate and control rudder position. 
     Yaw damping command source  110  is generally configured to provide an electronic signal indicative of an airplane&#39;s yaw rate or desired rudder position. In accordance with one exemplary embodiment of the present invention, command source  110  provides a signal indicative of an airplane&#39;s yaw rate, and the yaw rate signal is derived from a gyroscope and appropriate gain circuitry. 
     Bias command source  120  is generally configured to provide an electronic signal indicative of a thrust differential between two or more engines on the airplane. For example, bias command  120  may produce a signal indicative of engine power loss of an airplane engine. In accordance with one embodiment of the invention, bleed air pressure sensors are placed on the airplane engines and the sensors produce a signal indicative of airplane engine thrust. 
     Yaw damping feed forward path  130  is generally configured to transmit a signal from yaw damping command source  110 , sum the yaw damping command signal with any bias command signal and any feedback signals, and transmit the summed signal to motor  180 , such that motor  180  is manipulated in accordance with the summed or manipulated yaw damping command signal. 
     In accordance with one exemplary embodiment of the invention, path  130  includes a first summing junction  190 , a second summing junction  200 , a gain device  210 , a third summing junction  220 , a fourth summing junction  230 , a servo motor command source  240 , and motor  180 . Summing junction  190  is suitably configured to combine the yaw damping command signal from source  110  with a bias command signal from path  140 . Similarly, summing junctions  200 ,  220 , and  230  are respectively configured to sum signals from feedback paths  150 ,  160 , and  170  with the yaw damping command from source  110 . Gain device  210  is suitably configured to provide signal amplification in path  130 . 
     Path  140  is generally configured to provide a manipulated bias command signal from source  120  to summing junction  190  relatively quickly, and to eventually fade out the bias command to zero over time (over path  140 ), while mitigating any overshoot of the desired rudder position. In accordance with the exemplary embodiment of the invention illustrated in FIG. 1, path  140  includes a wash out filter  250 , a lag filter  260 , and an amplifier  270 . Wash out filter  250  reduces the amplitude of the bias command signal over path  140  over time such that the original bias command over feed forward path  140  diminishes over time. Lag filter  260  increases an amount of time it takes the bias command signal to travel from source  120  to summing junction  190  over path  140 . Although it may be desirable to provide the bias command to junction  190  and eventually the manipulated bias command to motor  180  relatively quickly, some delay within system  100  may be desirable to reduce or eliminate control command overshoot, while maintaining a sufficient signal to manipulate motor  180  as desired. Wash out filter  250  and lag filter  260  configuration may vary from application to application. However, in accordance with an exemplary embodiment of the present invention, for a small, twin jet business aircraft, wash out filter  250  includes a filter represented by the LaPlace transform equation,            t   i        S           t   i        S     +   1                            
     where t i  is 0.5; and filter  260  includes a filter represented by the LaPlace transform equation          1         t   2        S     +   1       ,                          
     where t 2  is 3.5. 
     Path  150  is designed to provide negative feedback for system  100  based on servo motor current, which is indicative of a load applied to motor  180 . In accordance with one embodiment of the invention, path  150  includes a first summing junction  290 , a first amplifier  300 , a second summing junction  310 , a second amplifier  320 , a third summing junction  330 , and a filter  340 . Path  150  also includes a secondary feedback path  155 . Summing junction  290  combines a motor current amp signal (from motor  180 ) and a bias command signal from source  120 . Summing junction  310  combines the summed current amp and bias command signal from junction  190  with an integrated signal from feedback path  155 . Summing the signal from junction  290  with a signal from feedback loop  155  provides an estimate of the servo command rate error. Similarly, signals from junction  310  and from loop  160  are combined at junction  330 . Summing the rate signal from path  160  and the summed signal from junction  310  provides negative feedback to path  150 , which provides an estimated servo motor rate feedback signal based on the measured motor rate and the servo load. 
     Filter  340  is suitably configured to convent the estimated servo motor rate feedback signal from junction  330  to an estimated servo motor position feedback signal. In accordance with one embodiment of the present invention, filter  340  includes an integrator. 
     Path  160  of system  100  is generally configured to provide negative rate feedback to system  100  to slow the speed at which motor  180  and consequently the rudder move as the rudder approaches its desired position. System  100  also includes feedback path  170  configured to provide high frequency damping to the server motor to facilitate rudder position control. 
     Amplifiers  210 ,  270 ,  300 ,  320 , and  350 , configuration may vary in accordance various embodiments of the present invention. However, when system  100  is used in connection with a Sino-Swearingen SJ30-2 aircraft, amplifier  210  has a gain constant of four percent per degree-motor, amplifiers  270  and  300  have a gain constant of 7500 degrees-motor per amp multiplied by the inverse of the normalized dynamic pressure, amplifier  300  has a gain constant of 2, and amplifier  350  has a gain constant of 75 percent per amp. 
     Although the present invention is set forth herein in the context of the appended drawing figure, it should be appreciated that the invention is not limited to the specific form shown. For example, while the invention is conveniently described above in connection with a dual-engine aircraft, the system may be used with other multi-engined aircraft. Various other modifications, variations, and enhancements in the design and arrangement of the method and apparatus set forth herein, may be made without departing from the spirit and scope of the present invention as set forth in the appended claims.