Patent Application
20070251313
This invention relates generally to flight control systems that are incorporated into air vehicles, and more specifically, to methods and systems for determining angle of attack and angle of sideslip using flow sensors.
Angle of attack (AOA) and angle of sideslip (AOS) are parameters used by aircraft, missiles, and unmanned aerial vehicles (UAVs) for flight control. Angle of attack is the angle between an airfoil chord line and the relative wind. Angle of sideslip is the angle between the airfoil chord line and the flight path. In current systems there are two ways in which AOA and AOS are sensed. In a first method, separate mechanical AOA and AOS probes are used that protrude from the air vehicle. In the second method, a combination of either six precision absolute pressure sensor transducers or two precision absolute pressure sensors and two precision differential pressure sensors are used.
Both of the above described solutions for determining AOA and AOS are high cost, and both are complex in that they require high precision sensors, precision modeling algorithms, and complex controller software and electronics. In addition to the high cost, these systems are of excessive size and weight and therefore are not typically suitable for small, emerging air vehicles such as missiles and UAVs.
In one aspect, a system for determining angle of attack (AOA) and angle of sideslip (AOS) of an air vehicle is provided. The system comprises a plurality of mass flow sensors, at least a portion of which are mounted to result in a differential in air flow across the respective mass air flow sensors, and an electronic controller configured to receive signals from the flow sensors. The controller is configured to produce digital electrical signals representative of the angle of attack and the angle of sideslip for the air vehicle.
In another aspect, a method for determining one or more of AOA and AOS for an air vehicle is described. The method comprises receiving air flow data from a plurality of mass air flow sensors, at least a portion of which are positioned to result in a differential in air flow across the portion of the sensors, and determining, using a controller, at least one of the angle of attack and the angle of sideslip for the air vehicle from the received air flow data.
In still another aspect, a mass air flow sensor system is provided that comprises a probe, a plurality of mass air flow sensors, and a controller. The probe comprises a plurality of flow ports formed therein, where a first plurality of these flow ports is positioned to create a differential airflow therethrough based on an angle of attack of an air vehicle, and a second plurality of these flow ports is positioned to create a differential airflow therethrough based on an angle of sideslip of the air vehicle. The plurality of mass air flow sensors includes a first of the mass air flow sensors positioned within a path created between the first plurality of the flow ports and a second positioned within a path created between the second plurality of the flow ports. The controller is configured to receive signals from the flow sensors and determines at least one of the angle of attack and the angle of sideslip for the air vehicle based on the differential mass air flows through the flow ports.
System 10 further includes a processor 20 which receives flow data from flow sensors 12, 14, and 16. While referred to herein as processor 20, those of ordinary skill will recognize that processor 20, in alternative embodiments, will include memory, and/or input/output controller functionality so that processor 20 can provide AOA and AOS data to external systems, and alternatively user interfaces (e.g., displays) within an air vehicle.
Still referring to
In one embodiment, flow sensors 12, 14, and 16 incorporate microbridge technology, such that flow sensors 12, 14, and 16 are configured to operate in harsh environments. In addition, flow sensors 12, 14, and 16 are capable of sensing extended air mass flow rates of more than 30 grams per square centimeter per second. In one embodiment, flow sensors have a range of 1000:1 (i.e. the sensors can sense mass flow rates from 30 gm/cm2/sec to 0.030 gm/cm2/sec). Flow sensors 12, 14, and 16 are also bi-directional, that is, they can measure flow in both directions.
Microbridge mass air flow sensors measure actual mass flow of a gas media. Actual flow is driven by a pressure gradient flowing from a higher pressure region to a lower pressure region. Sensors 12, 14, and 16 measure air mass flow which is a function of the gas density, the gas density being a function of pressure and temperature. Sensors 12, 14, and 16, in one embodiment, are compensated for temperature and therefore do not vary as a function of temperature due to altitude changes.
Mass flow sensor 12, 14, and 16 translate to air flow sensors using the following equation:
Q=
where the mass flow rate of air,
For air, the mass can be calculated based on its composite makeup, in mass/mole, e.g., about 78.8% nitrogen (N2) which is about 28.01 grams per mole, and about 21.2% oxygen (O2) which is about 32.00 grams per mole. For this calculation the approximate 0.9% of argon, and 0.03% of carbon dioxide in the air can be ignored, and the result is that air is about 28.854 grams per mole.
The relationship between pressure and altitude is:
Ps=Po(1−6.875586×10−6 Hp)5.25596; for Hp<36089.238 ft, and Ps=6.68322×exp[−4.806(Hp−36089.238)]; for 36089.238 ft<Hp<65616.7977 ft. Static temperature, in Kelvin, can be approximated from the following equation:
T=(−0.00189×altitude+15)+273.15.
In one embodiment, pressure is determined, for example, utilizing GPS altitude and then calculating the pressure using the equations above. However, errors relative to GPS altitude are considered in alternative embodiments.
By placing the ports (e.g., the α1, α2, β1, and β2 ports) in precise locations on a surface, for example, on surface 60 of hemispherical probe 50, then differential air flow can be measured by the respective flow sensors 14 and 16 and the differential results are proportional to AOA and AOS for the air vehicle (i.e., AOA is proportional to Vα1-Vα2, and AOS is proportional to Vβ1-Vβ2, where V is a velocity).
The variability of Vαand Vβas a function of pressure altitude and temperature can be eliminated by adding another flow sensor to measure the total flow (i.e. Vtotal shown in
AOA=(Vα1-Vα2)/kVtotal and AOS=(Vβ1-Vβ2)/kVtotal, where k is a function of the angle of the flow ports, more specifically, the angle between the total flow port, Vtotal, and the individual flow ports (the α1, α2, β1, and β2 ports).
The above described embodiments for determining AOA and AOS provide a low cost, and simple solution for the determination of AOA and AOS which can be utilized in small air vehicles such as missiles drones, and other UAVs. In addition to the low relative cost, as compared to pressure sensor based systems, the above described embodiments are relatively small in size and weight and therefore are especially suitable for small air vehicles of the type above described.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
