Unmanned Aerial Vehicles (UAVs) — autonomous or remotely controlled pilotless aircraft — have been recently thrust into the spotlight for military applications, for homeland security, and as test beds for research. In addition to these functions, there are many space applications in which lightweight, inexpensive, small UAVS can be used — e.g., to determine the chemical composition and other qualities of the atmospheres of remote planets. Moreover, on Earth, such UAVs can be used to obtain information about weather in various regions; in particular, they can be used to analyze wide-band acoustic signals to aid in determining the complex dynamics of movement of hurricanes.

This Photograph of the Avionics-and-Sensors System shows the critical sensors used for flight control.

The Advanced Sensors and Electronics group at Langley Research Center has developed an inexpensive, small, integrated avionics-and-sensors system to be installed in a UAV that serves two purposes. The first purpose is to provide flight data to an AI (Artificial Intelligence) controller as part of an autonomous flight-control system. The second purpose is to store data from a subsystem of distributed MEMS (micro-electromechanical systems) sensors.

Examples of these MEMS sensors include humidity, temperature, and acoustic sensors, plus chemical sensors for detecting various vapors and other gases in the environment. The critical sensors used for flight control are a differential-pressure sensor that is part of an apparatus for determining airspeed, an absolute-pressure sensor for determining altitude, three orthogonal accelerometers for determining tilt and acceleration, and three orthogonal angular-rate detectors (gyroscopes). By using these eight sensors, it is possible to determine the orientation, height, speed, and rates of roll, pitch, and yaw of the UAV. This avionics-and-sensors system is shown in the figure.

During the last few years, there has been rapid growth and advancement in the technological disciplines of MEMS, of onboard artificial-intelligence systems, and of smaller, faster, and smarter wireless telemetry systems. The major attraction of MEMS lies in orders-of-magnitude reductions of power requirements relative to traditional electronic components that perform equivalent functions. In addition, the compactness of MEMS, relative to functionally equivalent traditional electronics systems, makes MEMS attractive for UAV applications. Recent advances in MEMS have made it possible to produce pressure, acceleration, humidity, and temperature sensors having masses in subgram range and possessing sensitivities and accuracies comparable to those of larger devices.

Some flight-control sensors, including pressure sensors, incorporate supporting circuitry that enables adjustment of their ranges (to values different from those set at the factory) in order to satisfy mission needs. Hence, the pressure sensors can be set to measure pressures in certain ranges (in effect, an absolute-pressure sensor can be set to be sensitive to a specific altitude and/or a differential-pressure sensor can be set to be sensitive to specific airspeed). If the altitude and airspeed requirements of the UAV are changed, the sensor ranges can be adjusted accordingly. The accelerometers incorporate circuitry that adjusts their offset output voltages so that an onboard analog-to-digital converter (16-bit ADC) can center on their stable voltages. The data from the various sensors are multiplexed via the ADC, and the data are then gathered by an onboard microcontroller. The microcontroller determines the sample rate for each sensor and processes the digitized sensor data into a serial stream at a user-programmable rate.

An interface between this avionics-and-sensors system and an external system can be established at any of several points in the circuitry — which point depending on the type and level of control needed by the external system. For example, the serial data stream is sent to an onboard UART (Universal Asynchronous Receiver/ Transmitter), the 0-to-5-volt output of which can be utilized directly by an external controller or processor. In addition, the data stream is also sent to an onboard RS-232 level converter chip, enabling a direct serial-port connection to an external computer when this avionics-and-sensors system is operated in a laboratory. The onboard microcontroller can be utilized in two ways: enabling an external microcontroller or computer to simply receive and respond to the sensor data, or controlling the flow of the sensor data.

The UAV control system provides (1) external connections to flight sensors, (2) the computational resources necessary to make flight decisions, and (3) a simple interface through which control can be exerted over the servomotors that actuate the flight-control mechanisms of the UAV.

This work was done by Qamar Shams of Langley Research Center. For further information, contact the Langley Innovative Partnerships Office at (757) 864-8881. LAR-16736-1

NASA Tech Briefs Magazine

This article first appeared in the April, 2006 issue of NASA Tech Briefs Magazine.

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