Abstract:
An aircraft drive system having a fully electric speed-proportional control and drive system for the nose wheel of an aircraft in which variable steering authority is accomplished by monitoring of the signal from a tiller and rudder pedals by a nose wheel controller in conjunction with the speed of the aircraft, in order to command and power an electromechanical steering, actuator to achieve speed-proportional variable steering authority. This provides a system with a quicker response time than current hydraulic-based systems which can help reduce nose wheel shimmy, eliminate certain nose wheel steering failure modes associated with hydraulic systems, and reduce weight when used in small-to-medium sized aircraft.

Description:
[0001]    This application claims priority to and the benefit of European Patent Application 15198291.5, filed on Dec. 7, 2016, the contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a nose wheel steering system for an aircraft. More specifically, it relates to a fully electric speed-proportional system in which steering inputs from an electronic tiller and/or rudder pedals, together with ground speed data from speed sensor(s) and a nose wheel position sensor are sent to an electric controller, which automatically determines the correct steering target angle output for the aircraft speed with a software based algorithm, before determining the correct polarity and magnitude of power to deliver to an electromechanical drive actuator in order to steer the nose gear to the correct target angle computed by the controller. 
       BACKGROUND OF THE INVENTION 
       [0003]    During the ground regime of a flight (taxiing, take-off and landing), modern aircraft including business jets, helicopters and airliners achieve directional control-using landing gear located beneath the front portion of the fuselage, comprised of one or two individual wheels, called the nose wheel. At low speeds, the nose wheel is required to have a high steering angle range to enable a high degree of maneuverability. When the aircraft is moving at high speeds, for example, during landing or take-off, the nose wheel is required to turn only at small angles in order to make minor adjustments to the aircraft direction. Making large adjustments to the nose wheel angle at high speeds is dangerous and can cause a lack of control of the aircraft. 
         [0004]    In order to account for the above, present electronically controlled nose wheel steering systems may have a continuously variable gain or dual mode gain steering control system, which may be automatically or manually selected depending on the speed of the aircraft. In this manner, while the plane is moving at low speeds, i.e. during taxiing, the sensitivity between the nose wheel control and the nose wheel is high, allowing the pilot to impart large nose wheel steering angle deflections with small control inputs via the tiller or rudder pedals. As the aircraft gains speed, the sensitivity between the control and the nose wheel may be automatically decreased by an electronic controller or manually selected to a reduced sensitivity mode by the pilot by means of a switch, or a secondary control input i.e. steering with the rudder pedals instead of the tiller, such that the same input to a control will cause the nose wheel to turn a smaller angle, thus maintaining a finer degree of control over the aircraft, preventing over-controlling of the aircraft at high ground speeds. 
         [0005]    The specific systems used presently tend to be hydraulic or electro-hydraulic systems in which an electrical output from the control system commands a servo-directional valve of a hydraulic system or a direct mechanical input controls a directional valve of a hydraulic drive system, which controls the direction of movement of a hydraulic actuator, such that the nose wheel is turned hydraulically. 
         [0006]    An example of such a system can be found in U.S. Pat. No. 3,753,540, which discusses using a servo system in combination with an electro-hydraulic control system to mechanically increase the steering range of a hydraulic steering system as an aircraft slows down. 
         [0007]    More recent systems have attempted a slightly different approach, such as those discussed in U.S. Pat. No. 8,473,159 B2 which relate to a variable gain electronic control of hydro-mechanical system for control of the nose wheel by means of a programmable electronic controller. 
         [0008]    Hydro-mechanical systems and electro-hydraulic systems have various issues associated with them. Hydraulic systems are dependent on servo valves and shut-off valves, which-can be susceptible to contamination and wear, and can be difficult to maintain without fluid leakage. 
         [0009]    Further, systems in which electrically commanded and mechanically commanded servo valves are used to control steering direction are particularly sensitive to contamination induced uncommanded steer, which can result in dangerous runway excursions. Hydraulic based steering systems are also susceptible to nose wheel shimmy vibration (speed wobble) in certain conditions. Hydraulic systems are also regarded as somewhat heavy for a given power output, particularly in small-to-medium sized aircraft. 
         [0010]    The present invention aims to overcome these problems by providing a system which is fully electric in its command system and its power system (fly by wire &amp; power by wire), such that contamination induced failure modes associated with hydraulic systems are not possible and maintenance is much simpler. 
       SUMMARY OF THE INVENTION 
       [0011]    In order to solve the problems associated with the prior art, the present invention provides a nose wheel steering system for an aircraft, comprising: at least one nose wheel control input, each at least one nose wheel input being arranged to produce an electrical output signal proportional to the position of the at least one nose wheel control input; a controller, with variable gain at least in part based on speed, arranged to receive the electrical output signal(s) from the at least one nose wheel control input, amplify them individually to compute desired target steering angles and to output an electrical power output signal; and an electromechanical nose wheel drive means arranged to receive the electrical power output signal from the controller. 
         [0012]    As will be appreciated, the present invention provides several advantages over the prior art. For example, an electromechanical system provides an improved response rate due to the shorter control loop required to connect the system, due to the deletion of servo-electric torque motors and valve spools needed in present electrohydraulic servo valves, which have a response lag time associated with them, and the elimination of dependence on a hydraulic power supply. Further, an electromechanical system allows for more precise control over steering, due to the use of infinitely variable command and feedback position sensing, combined with direct step-less electronic power control of the actuator. A fully electric speed-proportional nose wheel steering system also provides a control system that is much lighter than hydraulics-based systems when used in small-to-medium sized aircraft, due to the lack of hydraulic tubes, hoses, valves, filters and accumulators which are needed to power a hydraulic nose wheel steering system. 
         [0013]    Furthermore, an electromechanically driven nose wheel steering system innately has improved resilience to nose wheel shimmy (speed wobble) issues due to the elimination of hydro-mechanical backlash, improved response rate of the system, and more precise control. The command-feedback control loop of an electromechanical system shall also have a refresh rate that can be tuned to dampen the natural harmonic frequency of the nose wheel steering system, thereby eliminating ‘shimmy’. 
         [0014]    Crucially, due to the elimination of hydraulic servo valves, an all-electric nose wheel steering system is immune to hard-over uncommanded steering events associated with hydraulic blockages and contamination, which may lead to stuck valves that can lead to dangerous runway excursions on take-off or landing. 
         [0015]    Traditional mechanical and hydraulic nose wheel steering systems also typically embody a centering cam system in the shock absorber, in order to center the nose wheel in preparation for landing gear retraction, extension and landing, however centering cams are subject to fatigue, and can cause nose wheel steering hard-over events if they fail, but with a continuous duty electromechanical steering system centering, cams are not needed, as the controller can automatically center the nose wheel by continuously monitoring its position. 
         [0016]    The at least one nose wheel control input of the nose wheel steering system may comprise a hand tiller and/or rudder pedals. These provide a simple steering input means for the pilot(s). 
         [0017]    The nose wheel steering system may also comprise an angle position sensor arranged to relay a current nose wheel angle to the controller. In this manner, the controller knows the angle of the wheel, and can adjust its output signals accordingly. 
         [0018]    The nose wheel steering system may further comprise a ground speed sensor arranged to relay a current ground speed of the aircraft to the controller. This provides the advantage of up-to-date ground speed, allowing the controller to adjust the variable gain of the nose wheel control input without relying on external speed data. 
         [0019]    The nose wheel steering system may comprise an air speed system arranged to relay a current air speed of the aircraft to the controller. This provides the advantage of up-to-date air speed, allowing the controller to adjust the variable gain of the nose wheel control input without relying on external speed data. 
         [0020]    The nose wheel steering system may also comprise a weight-on-wheels microswitch arranged to send an electric signal to the controller when a weight is detected on the wheels of the aircraft. This allows the controller to switch between using the ground speed and the air speed to adjust the variable gain of the nose wheel control inputs. 
         [0021]    The electric output from the control may be of differing polarities depending upon which way the nose wheel is intended to turn. This provides a simplified means of controlling the actuation, removing the need for complicated electrical signals to determine nose wheel turning direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Other advantages and benefits of the present invention will become apparent from a consideration of the following description and accompanying drawings, in which: 
           [0023]      FIG. 1  shows a structural view of the present invention; and 
           [0024]      FIG. 2  shows a graph of the integrated electronic steer authority of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0025]    A fully electric speed-proportional nose wheel steering system  100  in accordance with an exemplary embodiment of the present invention is shown in  FIGS. 1 and 2 . 
         [0026]    The present invention provides an integrated nose wheel control and drive system for an aircraft which provides means to collect the system electronic inputs via a data concentrator function, then a means of detecting, interpreting and computing those inputs with software, and a means of commanding and powering a final electromechanical drive to steer the nose gear. 
         [0027]      FIG. 1  shows a structural diagram of an embodiment of the invention. A nose wheel steering command system  100 , comprising inputs from the rudder pedals  102  and a hand tiller(s)  103 . The rudder pedals  102  and the hand tiller  103  are located in the cockpit  109  of an aircraft. The rudder pedals  102  and the hand tiller  103 , in operation, send electrical command inputs to a nose wheel controller  101  by use of digital rotary variable differential transformers (RVDTs), via an ARINC 429/MIL-STD-1553 databus or equivalent. The electrical command inputs vary depending on the position of the tiller  103  and the pedals  102 , which are commanded by the pilot(s). Alternatively, the rudder pedals  102  and hand tiller  103  may send electrical command inputs through an analogue RVDT, a variable potentiometer or a hall effect sensor. 
         [0028]    In accordance with any revision of RTCA DO-254 or equivalent and any revision of RTCA DO-178 or equivalent, the controller is a dual redundant controller comprising two separate boxes i.e. two individual controllers performing the same functions in parallel. However, these two individual controllers will be referred to as a single controller, for the sake of simplicity. 
         [0029]    The system further comprises ground speed and air speed sensing systems  108 ,  104 . The ground speed system  108  is comprised of one or more wheel speed transducers using passive inductive magnetic sensors. The air speed data shall be provided to the nose wheel controller  101  via an ARINC 429/MIL-STD-1553 interface with an on-board air data computer (separate system), which is normally installed on an aircraft to provide airspeed information to the pilots by interpreting pitot pressure. The ground speed and air speed sensing systems  108 ,  104  send electrical signals to the nose wheel controller  101 , which vary depending on the measured speed of the aircraft. 
         [0030]    A weight-on-wheels microswitch  105  detects whether the aircraft is on the ground or in the air, and sends a signal to the nose wheel controller  101  via an ARINC 429/MIL-STD-1553 interface to switch between using ground speed or air speed accordingly. 
         [0031]    A steering angle sensor  107  detects the current steering angle of the nose wheel and relays it to the nose wheel controller  101 . The steering angle sensor  107  can be an RVDT, a potentiometer, a hall effect sensor or any other suitable device. 
         [0032]    The nose wheel controller  101  is integrated via an ARINC 429/MIL-STD-1553 databus, or any other suitable device, and receives digital inputs from the rudder pedals  102 , hand tiller  103 , ground speed and air speed systems  108 ,  104 , weight-on-wheels microswitch  105  and the steering angle sensor  107 , in order to determine a target steering angle using an algorithm, and a percentage error or delta comparison value (between the current steering angle and commanded steering angle) which is used to instantaneously determine the correct polarity and magnitude of the power supply to the final electric drive, to achieve the desired angle. The ground speed system  108 , air speed system  104 , weight-on-wheels microswitch  105  and the steering angle sensor  107  are all associated with the landing gear  110 . 
         [0033]    The nose wheel steering actuator  106 , also associated with the landing gear  110 , is part of a nose wheel steering electromechanical final drive, which may be of multiple configurations. The electromechanical drive comprises an electric motor which drives a recirculating ball jackscrew connected to a rack-and-pinion steering output gear and a steering tube. 
         [0034]    Alternatively, the electromechanical drive means can include an epicyclic gear set driving a nose wheel steering tube via a reduction gearbox. The skilled reader will understand that other arrangements are possible within the scope of the invention. The electric motor may be an AC motor, a DC motor, a brushless DC motor, or any other suitable device. Similarly, other gear systems are readily applicable where necessary in specific systems. 
         [0035]    The electromechanical drive incorporates the steering angle sensor  107 , and sends instantaneous actual steering position data to the nose wheel controller  101 . A feedback loop is used to compare the instantaneous angle to the commanded steering angle (percentage error, or delta value) in order to make adjustments. 
         [0036]    During a taxiing regime, a pilot will use the tiller  103  and the rudder pedals  102  to adjust the direction of an aircraft. Each of these sends a digital signal to the nose wheel controller  101 . The nose wheel controller  101  adjusts the sensitivity of the signal according to the signal received from the ground speed system  108 . 
         [0037]    At low speeds, a large turning angle is required for the nose wheel in order to increase maneuverability of the aircraft around an airport or airfield. As such, the sensitivity between the tiller  103  and rudder pedals  102  and the nose wheel is high, allowing the pilot to turn the aircraft through large angles with minimal effort. As the aircraft increases speed, for example, during take-off, only small adjustments are made to the angle of the aircraft. Making large adjustments to the angle at high speeds is extremely dangerous, and so the nose wheel controller  101  restricts the turning angle, reducing the risk of an accident. 
         [0038]    The nose wheel controller  101  uses the signals received from the ground speed system  108  to adjust the steering authority using an algorithm based on an exponential or logarithmic relationship, illustrated by a curve such as the variable gain curve according to the graph shown in  FIG. 2 . 
         [0039]      FIG. 2  is representative of one or more possible exponents representing gain curves of one or more input controls, e.g. the rudder pedals  102  and tiller  103 . Such curves may be expressed in the manner of y=log b  x or y=b x  in terms of exponential gain inversely proportional to speed, or as shown in  FIG. 2 , an expression such as y=x.2 −(t/h)  akin to exponential decay, or a half-life where gain is inverse-exponentially proportional to speed. Other formulaic expressions may be used to represent a continuously variable gain curve of steer authority vs speed. 
         [0040]    As will be appreciated by the skilled reader, other variable gain schemes are possible within the scope of the invention. The adjusted signal is then output to the drive actuator part of the electromechanical drive means to adjust the direction of the aircraft. 
         [0041]    The steering angle sensor  107  continuously relays the actual steering angle to the nose wheel controller  101 . The nose wheel controller  101  can then make adjustments to the output signal sent to the drive actuator  106 . 
         [0042]    As the aircraft leaves the ground, the weight-on-wheels microswitch  105  detects that the aircraft in flight, and may center the nose wheel for retraction. When in flight with the landing gear extended, the nose wheel controller  101  then uses signals sent from the air speed system  104  in order to determine the speed of the aircraft, this provides an alternative means to determine nose wheel steering gain when the wheel may not be turning, such as immediately before landing. 
         [0043]    It is to be appreciated that many of the features described above in the exemplary embodiment are readily replaceable depending on the specific needs of a particular system.