Control System Development and Flight Testing of the Tiger Moth UAV

Abstract

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 t