Abstract:
Methods and apparatus for controlling the attitude of a mobile platform with a resolution suitable for vernier attitude control. In a preferred embodiment, a method includes flowing fluid through an orifice of an aerodynamic surface. The method also includes modifying a boundary layer of the aerodynamic surface with the flowing fluid. Another preferred embodiment provides an aerodynamic member of a mobile platform. The aerodynamic member includes an aerodynamic surface, an orifice, an actuator, and a fluid moving member. The orifice is in the aerodynamic surface and the actuator is subject to friction and backlash. The fluid moving member communicates with the orifice and causes the fluid to flow through the orifice to modify the boundary layer of aerodynamic surface.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to control surfaces for mobile platforms and, more particularly, to actuators for vernier control of aircraft attitude. 
   BACKGROUND OF THE INVENTION 
   Emerging precision flight applications are generating control surface actuator performance requirements that exceed the capability of electrical and hydraulic actuator technology. For example, a vehicle performing a carrier landing must keep within ±12 in of the projected landing approach flight trajectory near touch-down in order to avoid a wave-off and to successfully hook the arrestor cable. The control requirement for naval style mid-air refueling is even more demanding than the requirement for the carrier landing. In the refueling scenario, a low-on-fuel vehicle has to intercept a fuel hose basket to within ±6 inches while the tanker maintains a constant flight trajectory. While not quite as demanding on the aircraft as the naval scenario, Air Force style refueling still requires the low-on-fuel vehicle to maintain its position relative to the tanker within ±4 ft. Meanwhile, the boom operator aboard the tanker has to “fly” the 40-ft long boom to within ±6 inches of the bobbing fuel probe on the low-on-fuel aircraft. Thus, the ruddevators (i.e. small wings on the refueling boom) must be controlled rather accurately. The need for an aircraft to hook a cable, or intercept a refuel drogue, or the like translates into control surface deflections so small that they lie beneath the range of the minimum achievable backlash in available actuator gearheads and linkages. Moreover, static friction in the control systems causes resistance to the small movements associated with the vernier attitude control required for such demanding operations. These nonlinearities limit the size of the smallest control surface movement that can be reliably repeated. Even perfect actuator position feedback along with unlimited control system throughput cannot avoid the detrimental effects of the nonlinearities. 
   Automatic control, during operations requiring vernier control, is therefore not currently achievable. Manned vehicles have succeeded in carrying out these difficult piloting tasks only because pilots adapt to nonlinearity (e.g. the friction and backlash in the control system) in subtle ways that heretofore have defied duplication in software. The nonlinearities impeding the automation of these tasks, though, cannot be eliminated because they are inherent in the available actuator technology. 
   Attempting to work around the resulting dead band using smaller rate command steps is possible, at least theoretically. Doing so would require boosting actuator loop gain and controlling pitch rate, roll rate, or yaw rate primarily through pulse width modulation of the actuator commands. However, this solution invites limit-cycle oscillations (e.g. actuator “hunting”) that reduce actuator life through excessive fatigue. Other previous attempts at controlling pitch, roll, and yaw rates include morphing airfoils into new shapes by using electrical or hydraulic actuators that drive hinged flaps or variable camber surface-bending mechanisms. Although these previous control systems have performed “coarse” attitude control satisfactorily, in the future, much faster, higher resolution control will be needed than can be achieved with this technology. 
   Another work-around might involve using an electro-hydraulic “coarse” position actuator supplemented with a smaller “fine” position control actuator that has higher bandwidth capability. The smaller actuator&#39;s loop gain could also be increased so that the vehicle will follow the fine actuator the majority of the time. The coarse actuator would therefore quietly serve in a secondary role for causing relatively large attitude changes. Unfortunately, all of the problems previously discussed with respect to the larger actuator would also apply to the fine actuator. Plus, because of the high gain required for the small actuator, instabilities could develop in the “fine” control system, 
   Thus, a need exists for a mobile platform attitude control system that provides vernier attitude control sufficient to maintain a mobile platform within about 6 inches, or less, of a target trajectory. 
   SUMMARY OF THE INVENTION 
   It is in view of the above problems that the present invention was developed. The invention provides methods and apparatus for providing vernier attitude control for a mobile platform. More particularly, the present invention provides a high-resolution high-bandwidth control system that is completely free of backlash and static friction. 
   In a first preferred embodiment, a method includes flowing fluid through an orifice of an aerodynamic surface. The flowing fluid modifies a boundary layer associated with the aerodynamic surface to control the attitude of the mobile platform. The quantity of fluid flowing through the orifice defines the resolution of possible control actions. Mobile platforms employing the method of the current embodiment may automatically land on an aircraft carrier or automatically refuel. Of course, the mobile platform may be a marine vehicle or even an unmanned aircraft. Further, the modification of the boundary layer can oppose the effect caused by the actuator and can result in maintaining the mobile platform within about 6 inches of the target. Moreover, the modification of the boundary layer may change the roll, yaw, or pitch of the mobile platform as if a control surface moved by as much as about 1 degree or more. Preferably, the fluid flow may be perpendicular to the aerodynamic surface and may separate the boundary layer from the surface. Additional fluid flows may be in the opposite direction of the first fluid flow and may be on opposite sides of the surface from the first fluid flow. Additionally, the effect of the modification of the boundary layer may be automatically sensed and used to further modify the flow of the fluid. 
   In a second preferred embodiment an aerodynamic member of a mobile platform is provided. The aerodynamic member includes an aerodynamic surface, an orifice, an actuator, and a fluid moving member. The orifice is in the aerodynamic surface and the actuator is subject to friction and backlash. The fluid moving member communicates with the orifice and causes the fluid to flow between the fluid moving member and the orifice in order to modify the boundary layer of the aerodynamic surface. Of course, the aerodynamic member may be adapted for either subsonic or supersonic movement. 
   A third preferred embodiment provides a mobile platform with a vernier attitude control system. The mobile platform includes an aerodynamic surface, an orifice and an actuator, acting as a fluid moving member. The orifice is in the aerodynamic surface and the actuator is subject to friction and backlash. The fluid moving member communicates with the orifice and causes the fluid to flow between the orifice and itself to modify the boundary layer of the aerodynamic surface. 
   In yet another preferred embodiment, the present invention provides an automatic, vernier, attitude, control system. The system includes an attitude sensor, a comparator, and an active flow control actuator. The comparator compares the attitude sensed by the sensor to the desired attitude and commands the actuator based on the comparison. The resolution of control provided by the system is defined by the flow rate of the active flow control actuator. 
   Advantageously, the control systems provided by the present invention do not require a constant system gain. Inertial measurement unit sensor feedback (for example) can therefore be used to compensate for even wide variations in actuator gain. Further, because the control systems provided by the present invention are free of static friction and backlash, the systems enjoy essentially infinite resolution and high bandwidth while experiencing only negligible delay between the issuance of a command for corrective action and the completion of the corrective action. 
   Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  illustrates a mobile platform constructed in accordance with the principals of the present invention; 
       FIG. 2  illustrates an aerodynamic member of the mobile platform illustrated in  FIG. 1 ; 
       FIG. 3  illustrates a cross section taken along line  3 — 3  of the aerodynamic surface of  FIG. 2 ; and 
       FIG. 4  illustrates a method in accordance with the principals of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the accompanying drawings in which like reference numbers indicate like elements,  FIG. 1  illustrates a mobile platform constructed in accordance with the principals of the present invention. 
   The mobile platform  10  uses active flow control (AFC) jet actuators (electromagnetic or piezo) to provide vernier control of the lift, drag, and pitching moments that act on it. An exemplary AFC actuator is disclosed in U.S. Pat. No. 6,713,901 issued to Hassan et al and entitled, “Linear Electromagnetic Zero Net Mass Jet Actuator” which is incorporated herein as if set forth in full. The interaction of the oscillatory jet provided by the AFC actuators with the mean flow over the aerodynamic surface, and the resulting modulation in the aerodynamics forces acting on the surface, requires only a small fraction of a second. In contrast, conventional control surfaces used to alter the lift, drag, and pitching moments of an aerodynamic surface must overcome their inertia that, in turn, causes their response time to be inherently long. Therefore, AFC-equipped vehicles can respond to unsteady gusts quicker, hold altitude more accurately, and can track desired flight trajectories more accurately than vehicles equipped with conventional electrical or hydraulic actuators. In particular, because the small oscillating flow rate of the AFC actuators defines the attitude resolution achievable via the present invention, an order of magnitude improvement in vernier attitude control is provided by the present invention. 
   With continuing reference to  FIG. 1 , a mobile platform  10  is shown including a fuselage  12 , a pair of wings  14 , and a vertical stabilizer  16 , and a pair of horizontal stabilizers  18 . More particularly, the wings  12  each include ailerons  22  and other types of control surfaces. Likewise, the horizontal stabilizers  18  each include an elevator  24  while the vertical stabilizer includes a rudder  26 . During the majority of time, the user of the aircraft  10  desires for it to move in a curvilinear trajectory through the atmosphere while being approximately at equilibrium with its environment. However, natural and man-made disturbances (e.g. wind gusts and the wakes of other aircraft) cause unbalanced forces and moments to act on the aircraft  10 . If left uncorrected, these unbalanced forces push the aircraft  10  from its desired attitude and desired trajectory  28 . The pilot or autopilot reacts by adjusting the position of the control surfaces (e.g. the ailerons  22 , the elevators  24 , or rudder  26 ) to steer the aircraft  10  back toward the desired trajectory  28  in a manner well known in the art. Backlash, friction, and other non-linear aspects of the aircraft control system limit how quickly the adjustments can be made as well as limiting the resolution, precision, and repeatability with which the adjustments actually occur. The present invention provides apparatus and methods that essentially eliminate the undesired delay, inaccuracy, imprecision, and non-repeatability associated with using previously available approaches for vernier control for maintaining a desired trajectory. 
   Turning now to  FIG. 2 , a top plan view of the horizontal stabilizer  18  of  FIG. 1  is illustrated.  FIG. 2  also shows the lifting surface  30  of the stabilizer  18 , the elevator  24 , a pair of conventional actuators  34 , and a plurality of AFC actuators  36 . The traditional actuators  34  adjust the position of the elevator  24  relative to the lifting surface  30  to assist in providing coarse control of the aircraft  10  attitude. The AFC actuators  36  are shown as being distributed in an array on the elevator  24  and lifting surface  30 , although such an orderly pattern need not be used to practice the invention. For instance, just one AFC actuator  36  could be employed in some applications. While the AFC actuators  36  on the horizontal stabilizer  18  are shown on the top, and toward the trailing edge, of the horizontal stabilizer  18 , the AFC actuators  36  could also be on the bottom and in more forward locations than the positions shown. 
   Now referring to  FIG. 3 , a cross section of the horizontal stabilizer  18  is illustrated. For clarity only the lifting surface  30  has been shown, although, the present invention applies to the elevator  24  as well as other aerodynamic surfaces of the aircraft  10 . As those in the art understand, an aerodynamic “surface” is actually a body such as a wing, airfoil, stabilizer, or control surface with two sides (e.g. upper and lower or starboard and aft sides) and is generally intended to interact with the fluid flowing adjacent to it. The cross sectional view shows the stabilizer  18  in relation to the airflow as designated by the arrow  40 . The airflow approaches the leading edge of the stabilizer  18  at an angle of attack a (i.e. the stabilizer  18  moves through the air at the angle of attack) and forms a boundary layer  42  and  44  on both the upper and lower sides of the stabilizer  18  respectively. It will be understood that the boundary layers  42  and  44  shown are simplified representations of complex phenomenon. The pressure of the boundary layers and  44  acting on the area of the upper and lower sides of the stabilizer  18  create a net force on the stabilizer  18 . Thus, a change in either boundary layer  42  or  44 , or the area of the stabilizer  18  that they act against, will cause a change in the lift provided by the stabilizer  18 . 
   At any given distance from the fuselage  12  (i.e. a station) the stabilizer  18  has a chord length d 1  between the leading edge and the trailing edge. Positions on the stabilizer  18  at any station may be described with reference to a fraction of how far along the distance d 1  the position is, as measured from the leading edge. Thus, a position that is 60% of the distance d 1  from the leading edge is referred to as being at the 0.60 position. 
   As shown by  FIG. 3 , the stabilizer  18  of the present embodiment includes three actuators  36  and three orifices  37 A,  38 A, and  39 A. The orifices  37 A,  38 A, and  39 A allow fluid to communicate between the actuators  36  and the boundary layer region  42 . Also, the three orifices  37 A,  38 A, and  39 A are preferably positioned one each at the 0.70, 0.80, and 0.90 positions on the stabilizer  18 . As disclosed in U.S. Pat. No. 6,713,901 each of the Zero Net Mass Jet Actuators  36  acts by causing oscillating jet/suction pulses at the associated orifice  37 A,  38 A, or  39 A. The oscillating flows at the orifices  37 A,  38 A,  39 A cause vortices in the boundary layer  42 . In turn, the net force caused by the boundary layer  42  on the stabilizer  18  changes and alters the pressure acting on the stabilizer  18  downstream of the orifice  37 . The aircraft  10  (of  FIG. 1 ) therefore experiences a change in the forces acting on it and responds accordingly. A second set of orifices  37 B,  38 B, and  39 B is provided on the under side of the stabilizer  18 . The second set of orifices  37 B,  38 B,  39 B is ported to the side of the actuators  36  opposite the side of the actuators  36  to which the first set of orifices  37 A,  38 A, and  39 A is ported. Orifices  37 B,  38 B, and  39 B thus take advantage of the oscillating flow from the other side of the actuators  36 . The effect the actuators  36  have on the stabilizer  18  of the aircraft  10  is multiplied accordingly. 
   Two dimensional simulations on a T-33 have shown that the system illustrated by  FIG. 3  can produce a pitch moment equivalent to that caused by a deflection of the elevator of up to 1.21 degrees. This is more than sufficient to eliminate limit cycle oscillations in the coarse pitch control loop and the resulting ±10 foot (305 CM) altitude wandering that traditional attitude control systems produce. The simulations also showed that, preferably, the orifices  37 ,  38 , and  39  should be positioned relative to the surface of the stabilizer  18  in a manner so that the airflow produced thereby is initially perpendicular to the applicable boundary layer  42  or  44 . Thus, when the AFC actuators  36  cause the oscillating air flow, the resulting interaction of the airflow with the boundary layers  42  and  44 , and the effect of the actuators  36  on the aircraft  10  attitude, is maximized (without causing an appreciable increase in drag). 
   The simulations typically assumed a zero angle of attack, a free stream Mach number of 0.55, a Reynolds number of 5.25 million per foot (fully turbulent flow), an altitude of 20,000 feet (6096 meters), and an air temperature of −12.2° F. Effectiveness of the AFC system for a non-zero elevator flap deflections was also verified. Simulations were likewise successfully run on a T-33 rudder, and on a T-33 aileron, to verify the capability of AFC actuators to provide control authority with respect about the yaw and roll axes of the aircraft. Thus, the vernier control authority provided by the present invention is sufficient for a wide range of applications including, but not limited to those previously discussed (i.e. re-fueling and carrier landings). Since the two-dimensional simulations assumed an infinite span for the elevator, those skilled in the art will understand that the simulation results are conservative. Three-dimensional simulations are expected to show improved vernier attitude control authority even if fewer actuators are included in the simulation. 
   For applications involving higher free-stream subsonic Mach numbers, supersonic conditions, or where a shock wave is present, simulations also showed that the AFC actuators can produce significant pitch moments (on an order commensurate with as much as a 47% increase in lift associated with the surface area under consideration) if the actuators are placed near the foot of the shock. 
   A method in accordance with the present invention is illustrated in  FIG. 4 . The method  100  generally includes determining whether a mobile platform is deviating from a desired attitude (or trajectory), choosing to correct the deviation with either the vernier or coarse control system, and correcting the deviation accordingly. More particularly, the method  100  includes incorporating an AFC actuation system into a new, or existing, aircraft at operation  102 . Generally, in parallel with operation  102 , a controller can be programmed in operation  104  to control the vernier (and coarse) actuators. The actual attitude of the aircraft and the desired attitude are compared, as shown at operation  106 . Depending on the nature of any deviation from the desired attitude, one or more of the axes of the aircraft may be selected about which corrective action will be taken, as indicated at operation  108 . 
   For each of the selected axes, a determination is made in operation  110  as to whether the vernier control system, the coarse control system, or both will be used to correct the deviation. Generally, the determination is made based on whether the correction falls within the deadband of the coarse actuator  34 . In other words, if the coarse actuator  34  cannot provide the resolution required for the corrective action, the oscillating jet actuator  36  is selected. Otherwise, the coarse actuator  34  can be used. If the coarse system is to be used, then commands are sent to the coarse actuator(s) in operation  112 . If not, then the coarse actuator  34  is left in its current position as shown at operation  114  and the command is sent to the vernier actuator as shown in operation  116 . In turn, the vernier actuator  36  modifies the boundary layer thereby controlling the attitude in operation  118 . The method  100  then determines if the deviation has been corrected (as indicated at operation  120 ). If not, the method  100  loops back to operation  106  and continues until the deviation is eliminated. If the deviation has been corrected, the method  100  continues in operation  106  until the next deviation is sensed. Appropriate commands are then generated as previously described. 
   In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. In general, the agile attitude control capabilities provided by the system and method of the present invention can improve the handling characteristics of any aircraft, missile, refueling boom, or other mobile platform. Further, the system and method also eliminates the inaccuracy, imprecision, and non-repeatability associated with maintaining a desired attitude using previously available approaches for attitude control. Moreover, the system and method enables the automation of many demanding mobile platform applications because the nonlinearities of traditional control systems are eliminated. By substituting, or supplementing, traditional actuators with the AFC actuators, the present invention also provides an order of magnitude improvement in the resolution of attitude control systems for mobile platforms. 
   The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. 
   As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. For example, while the foregoing description expressly discussed control of mobile platforms in air, the present invention can also be applied to mobile platforms moving through water or other liquids. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.