Technological advances have driven the evolution of aircraft flight-control systems. In the dawn of 20th-century aviation, the Wright Brothers used cables and warping of wing surfaces to change the shapes of flight-control surfaces. As engineers designed aircraft capable of flying faster, higher, and farther, the forces needed for controlling the aircraft surpassed the physical abilities of pilots. Hydraulics were introduced to provide the pilot with the capability to manipulate the flight-control surfaces on the wings and tail of the aircraft. NASA Dryden Flight Research Center (DFRC) has been instrumental in effecting the continuing evolution of aviation technology by exploring ways of replacing hydraulics with "power by wire" actuators; an instance of this effort was reported in "Design-ing Electrically Powered Actuators for Aircraft" (DRC-9609), NASA Tech Briefs, Vol. 21, No. 10 (October 1997), page 84.

Figure 1. Spread-Spectrum RF Data Links would supplant or supplement wire communication links between the flight-control computer and the actuators for the flight-control surfaces.

On the verge of the next millennium, NASA DFRC is once again involved in the next major aviation milestone: a wireless flight-control system (WFCS), in which radio-frequency (RF) links would supplement or supplant wire flight-control connections. The ultimate goal is to develop a closed-loop WFCS that will be used to either (1) back up a wired system and provide redundancy for enhanced safety and reliability or (2) replace the wired system and decrease the size, weight, and cost of the affected aircraft.

The WFCS concept (see Figure 1) involves the insertion of spread-spectrum RF data links into the communication paths between the flight-control computers (FCCs) and the actuators for the flight-control surfaces at the aile-rons, rudder, and elevators. For a demonstration of the feasibility of this concept, a prototype system (see Figure 2) was designed and demonstrated on the F-18 Iron Bird test facility at Dryden Flight Research Center to establish the ability of the RF links to provide adequate control capability. For demonstration purposes, the testing was limited to the signal paths between the FCCs and an aileron on the Iron Bird.

Figure 2. A Prototype WFCS System was tested to establish the capabilities of its RF link. The testing was limited to the signal paths between the FCCs and the actuator of one aileron.

The RF hardware in the test system includes an input module that receives and sends analog signals to and from the FCC and the control-surface actuators. These analog signals are converted to and from digital values by analog-to-digital and digital-to-analog circuits. The converted analog signals are passed to buffers that are set up in the memory spaces of WFCS computers. From these spaces, digital control movement data are encoded into messages that contain the movement information as well as synchronization and channel-identification data. These messages are then passed to the baseband processor of a spread-spectrum radio transceiver. Here, the baseband data are multiplied with randomized redundant data and sent to a modulation processor.

The modulation processor commands the RF section of the transceiver by phase-encoding the data "chips" to be transmitted. The RF section then transmits the data via a phase-modulated carrier signal. The receiver performs the inverse process that results in an analog signal that replicates the analog command input from the FCC.

For the proof-of-concept test, the data-link hardware was constructed in "breadboard" form. There was no attempt to miniaturize the hardware at this stage of development. The idea was only to prove that the bandwidth of the spread spectrum was sufficient to satisfy a requirement for timing in an inner control loop. Power for the data link was provided by bench-top power supplies. Considerable effort was spent to make the power-supply lines between the bench supplies and the data-link hardware as short as possible so as to shield the power supply lines from outside electromagnetic interference. The data-link transceivers were placed at locations for which there was no line of sight between the transmitting and receiving antennas; thus, communications during the test took place entirely by multipath propagation. The antennas were simple monopole structures with no ground planes.

The WFCS hardware was completely software-driven, with the exception of the analog input and output needed to satisfy the analog front ends on the FCCs and the actuators. The software was designed carefully to maintain a minimum of phase and latency error while sampling the control signals at sufficiently high rates to insure accurate representations of the waveforms without filtering that would inherently introduce phase errors. Analog signals were captured from several locations in the system during the test to verify that the wireless system was reproducing the analog control signals with adequate fidelity. Manual commands were also used to test the full dynamic range of the wireless system and its ability to move the aileron to its maximum extent.

The system performed reliably after installation and debugging. Noise was reduced by shielding the power-supply wires against electromagnetic interference along their full lengths. Junction boxes that contained pass-through wiring were covered by foil tape; this provided shielding against electromagnetic interference (including RF interference) for the signal paths that remained hard-wired. Considerable experimentation was done to explore the size and distribution of the near-field diffraction and interference patterns associated with the hangar and the placement of the airframe in the line-of-sight path between the wireless transceivers.

The wireless system provided control of the aileron that was equal to the control provided by the production system supplied with the aircraft. The hardware was switchable into and out of operation at any time. One box controlled the signal paths through the aileron actuator. Another box controlled the signals from the FCC. When the control switches were placed in the "pass through mode" position, the Iron Bird flight-control system operated normally. When the control switches were placed in the "RF mode" position, the WFCS digital RF data link passed the control data for the actuator servo valve to control movement of the aileron.

The WFCS was tested for frequency, stability, and amplitude response, using the same test criteria ordinarily used to determine the fitness of the aircraft for flight. The RF system satisfied all the closed-loop-control timing requirements. The RF spread-spectrum system operated in under the non-line-of-sight condition throughout the test, with no direct path between the transceivers. In fact, the devices were positioned so that the entire bulk of the F-18 was in the center of the direct path between the two transceivers.

The demonstrated feasibility of the prototype system provides a solid base on which to proceed to the next stage of development. Future efforts will include more extensive testing in progressive phases and implementation of carrier suppression and code diversity multiple access (CDMA) techniques. The development effort is expected to culminate in a flight test that will include flight-dynamics demonstrations with and without the RF system in operation. The test is also expected to include operations in jamming environments designed to simulate worst-case accidental interference conditions.

This work was done by Sangman Lee, Linda Kelly, and Arthur Lavoie of Dryden Flight Research Center and Karl Kiefer and Kevin Champaigne of Invocon Inc.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Karl Kiefer
Invocon Inc.
19221 IH-45 South Ste. 530
Conroe, TX 77385

Refer to DRC-99-08

Motion Control Tech Briefs Magazine

This article first appeared in the February, 2000 issue of Motion Control Tech Briefs Magazine.

Read more articles from the archives here.