Control System Development and Flight Testing of the Tiger Moth UAV

There is a need to provide high-resolution video imagery to the sensor operators of current and future gunships while supporting missions above an area of interest. Sensor operators need to see beyond the visual range of the main platform, peer through weather, and monitor multiple geo-separated or dispersing targets. The Lite Machines Tiger Moth UAV is designed to fill this need. The objective of the work presented herein was to improve the inner loop control laws of the Tiger Moth UAV through control system modeling, optimization and flight tests. Laboratory tests were conducted to identify aircraft sensor and servo dynamics. A bare-airframe hover/low-speed dynamics model was developed from piloted frequency sweeps. The identified components and dynamics model were combined with a Simulink representation of the control laws to form a validated analysis model which was used in CONDUIT to optimize the attitude loop feedback gains. Flight tests with the optimized gains showed improved performance. Finally, the improvements were demonstrated to the U. S. Air Force in untethered flight tests conducted at Camp Atterbury, Indiana, in December of 2011. Notation ax, ay, az Longitudinal, lateral, and vertical accelerations [m/s] h Height of point of rotation above the aircraft’s center of gravity [m] Iy Pitch moment of inertia [kg m] m Mass of the aircraft [kg] p, q, r Roll, pitch, and yaw angular rates [deg/s] u, v, w Body velocities [m/s] δlat, δlon, Roll, pitch, and heave control inputs [deg] δcol δΔQ Differential torque – yaw control input [%] φ, θ, ψ Roll attitude, pitch attitude, and heading [deg] ( )ʹ Test stand value with rotation about a pivot point other than the aircraft’s center of gravity Presented at the American Helicopter Society 68th Annual Forum, Fort Worth, TX, May 1-3, 2012. This is a work of the U.S. Government and is not subject to copyright protection in the U.S. DISCLAIMER: Reference herein to any specific commercial, private, or public products, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favoring by the United States Government. The viewing of the presentation by the Government shall not be used as a basis of advertising. Introduction The Tiger Moth V6.1 UAV (Figure 1) developed by Lite Machines Corporation is an outgrowth of the Voyeur UAV which was developed for the U.S. Navy as a sonochutelaunched UAV for use on the Lockheed P-3 Orion and the Boeing P-8 Poseidon. Lite Machines is currently working under a U.S. Air Force Phase III SBIR contract to refine the Tiger Moth V6.1 control system for more extensive airlaunched flight tests. The Tiger Moth is a 3.5 pound, electrically-powered, unmanned helicopter having an 18 inch vertically elongated body with 30 inch diameter, counter-rotating, coaxial rotors that fold for storage inside a tube. Each rotor system is driven by a separate electric motor and includes both cyclic and collective pitch controls which are actuated by a common set of servo actuators located midway between the Figure 1. Tiger Moth rotors. Directional control is achieved using differential rotor torque. The Tiger Moth airframe consists of a hollow aluminum spine tube extending the full length of the body which acts as both a rigid structural element and a conduit for electrical wiring. Power to drive the rotors is transmitted by electrical wiring running through the spine tube instead of mechanical shafting thereby reducing mechanical complexity and weight. The spine-tube style airframe can withstand high launch loads and has been tested by the Navy to withstand 100 Gs during a sonochute launch. When configured for hovering flight, the Tiger Moth flies in a near-vertical orientation. A GPS guidance and telemetry system is mounted above the rotors for unobstructed access to the sky. A lower payload module houses sensors including infrared and/or visible-light video cameras, a telemetry system for transmitting digital data back to a command center, and possibly a warhead to disable small vehicles. The current effort is part of an on-going program sponsored by the Air Force Research Laboratory (AFRL) to develop a disposable, air-launched, off-board sensor for a variety of manned aircraft. The requirements of this program are challenging from a control system perspective. After being ejected from a manned aircraft flying at over 200 knots, the sensor vehicle must fall or fly to a lower altitude, automatically configure itself for flight, then transmit stable sensor imagery to a remote operator for up to one hour. It must travel at high speeds between widely separated waypoints, hover for extended periods of time, and operate in high and gusting winds. While considered disposable by aircraft operators, the vehicle may be hand recovered and reused by ground forces. This paper describes work jointly performed by Lite Machines and the U.S. Army Aeroflightdynamics Directorate to improve the hover/low-speed performance of the Tiger Moth. The effort involved conducting laboratory tests on individual components as well as the complete aircraft, identifying a state-space model from flight test data, developing and validating an analysis model, and optimizing the control system based on the analysis model. Results of the optimization are discussed and compared with flight test observations. Finally, a brief overview of the Air Force demonstration flight tests is presented. Laboratory Tests In order to develop a useful analysis model, it is important to first accurately model all the components of the aircraft. To this end a series of laboratory tests were performed by Lite Machines. Automated frequency sweeps were used to identify the dynamics of the flight sensors and servos actuators. Servo position data was recorded as the servos were commanded to sweep through a range of frequencies. From this data, transfer function models of the servos were developed as shown in Figure 2. A time delay alone (H = e) was sufficient to model the servos with a cost of J = 2.2 which indicates the model is nearly perfect. Validation of the inertial measurement unit (IMU) sensor was achieved by performing frequency sweeps with the Tiger Moth rotors turning at full speed and mounted on a programmable robotic arm capable of 0.001 degree resolution and rotating the entire airframe and IMU. This produced a more controlled flight environment where the sensor signal noise could be analyzed in the context of airframe and rotor vibration. It was also possible to check the angular kinematic consistency of all IMU data to ensure proper sensor orientation and scale factors. Translational kinematic consistency was verified by mounting the test stand in a car, and driving the car down a road while collecting velocity and acceleration data with the onboard GPS and IMU. The majority of the laboratory testing during this program was performed on either the single-axis yaw test stand shown in Figure 3a or the fully articulated pitch/roll test stand shown in Figure 3b. All articulated axes were supported by low-friction instrument bearings to ensure high fidelity measurements. Due to the location of the aircraft’s center of gravity, it was not possible to clamp the aircraft in the pitch/roll test stand so that rotation occurred about the center of gravity. This was accounted for in the subsequent calculations. The pitch and roll moments of inertia of the aircraft are identical and were determined by performing a swing test with the aircraft mounted in the pitch/roll test stand [1]. From the damping envelope and damped natural frequency -20 0 20