# Autopilots for Ultra Light Weight Robotic Birds Automatic altitude control and system integration of a sub-10 g weight flapping wing micro air vehicle

This article discusses the altitude control of a flapping wing micro aerial vehicle (MAV). Although the power requirement can be high for suboptimal designs, flapping flight can be an efficient way to transport a mass over a distance (citation not found: Norbegr). For example, an optimized flapping flight can require 27% less aerodynamic power than its optimal steady-flight counterpart at the scale smaller than a bird (citation not found: JWang). This potential for increasing efficiency as well as increased maneuverability has motivated a renewed interest in flapping flight among researchers. However, it is difficult to understand the autonomous flight of bird-like micro robots due to limited sensing payload capacity. This article describes the first step in tackling this problem, which is the automatic altitude control of such a vehicle.

Of the flapping MAVs available, the Golden Snitch (citation not found: Hsu), named after the device referenced in the popular Harry Potter series, is the platform used for this investigation (see Figure \ref{fig:gs}). This device has a 20 cm wingspan and a gross takeoff weight (GTOW) of 8 g, which includes its fuselage, flapping wings, tail wings, a battery, a motor, and a set of gear system. The flapping wings are driven by a motor with a four-bar linkage system. By adjusting the lengths of the four bars, various stroke angles can be employed. The stroke angle of the Golden Snitch is designed around $$53^{\circ}$$ (citation not found: Hsu).

Theoretical investigations of the aerodynamics of flapping flight span over a century (e.g., (citation not found: Lighthill)(citation not found: Weis) and citations therein). Variations of the positional angle of fore and hind wings during flight of locusts (Schistocerca gregaria) have been determined (citation not found: Weis). Experimental studies (citation not found: Parslew), (citation not found: Couceiro2), (citation not found: ellinton), (citation not found: willmott) have enabled a better understanding of the natural wing articulation by insects in hover and forward flight. External or onboard cameras have been used for navigation and guidance for the flight altitude of the Delfly II MAV (citation not found: delfly2). Unlike the experiments in this article that employ stereo vision, only one external camera was used in (citation not found: delfly2) to determine the scaled $$y$$-coordinate in the Delfly II experiments. A proportional control (P-control) was designed to stabilize the flight with a scaled $$y$$-coordinate as the feedback quantity. As a result, the throttle required trimming in advance. Modeling, dynamics, and control laws for flapping flight has been investigated (citation not found: wang)(citation not found: Couceiro1). Optical methods for navigation and guidance have been developed (citation not found: Bermudez)(citation not found: clchen). The robotic birds in (citation not found: delfly2) and (citation not found: Bermudez) are twice the weight of the Golden Snitch, affording them a greater sensing payload capacity than would be possible for the Golden Snitch. Consequently, the aim of this article is not to develop a new or more sophisticated control law. Rather, this article considers the practical problems of the control law implementation and system integration using existing control technology.

The article begins by providing some background on the design and development of the mechanisms that enable the Golden Snitch to fly. Then the equations of vertical motion for this particular robotic bird are derived. Aerodynamical and dynamical coefficients are obtained from experimental data. To keep the overall weight of the vehicle as low as possible, a commercial infrared (IR) transmission module that weighs less than 1 g was selected, which unfortunately constrains the communication capability. B non-intrusive method to replace a traditional onboard computer and sensors is also discussed in this article. By using stereovision, the three-dimensional position of the Golden Snitch can be obtained (citation not found: clchen). Further details about the stereovision system are provided in the side bar on “How Does a Stereovision System Work?”. All navigational information and control commands are computed on the ground station, and transmitted to the air vehicle. The control commands generated using a modified proportional feedback is shown be sufficient for achieving height control. The validity of the approach is demonstrated in numerical simulations and flight tests.