SABER Homepage
Cutting Edge Technology
University of Alaska FairbanksDuckering 221
Fairbanks, AK 99775
SABER, an acronym for Semi-Autonomous BEam Rider, is a microwave supported platform (helicopter). The platform will consist of a Kyosho Concept 30 DX model helicopter retrofitted with an Aveox AVX-1412/7 electric motor. Power for the platform is obtained from a rectenna (rectifying, ie. AC to DC, antenna) array attached below the airframe of the 30 DX. The rectenna will convert an incident 2.45 GHz beam to DC which will be conditioned by appropriate circuitry to make the voltage levels suitable for the various SABER subsystems. A rectenna is the combination of a dipole antenna, a diode, and an impedance matching network. The rectenna converts radio frequency energy from free space waves into direct current energy available at the rectennas output terminals. SABER will sense the incident microwave beam for attitude (rotational) and position (with respect to transmitter) determination. The only external control to the platform will be a preprogrammed flight regime (a communication system may make the flight duration controlable). The flight programming will consist of two binary dip switches to input maximum height attained during flight, and maximum flight duration. SABER is otherwise fully autonomous. SABER is specified to be stable, robust against reasonable disturbances, and to remain on station to within 10 cm on any axis with zero steady state error for step disturbances. SABER will be flown outdoors as a public demonstration of microwave power transmission.
The concept of microwave power transmission has been examined for a very long time both in theory and in the laboratory. To date there have been no commercial implementations of microwave power transmission beyond feasibility study. The most promising application for microwave power transmission is a solar power satellite. A very large solar array at geosynchronous orbit converts solar energy to electrical energy which is transmitted as microwaves to an Earth station. At the Earth station rectennas convert the microwave energy to direct current which can be either converted to alternating current for direct distribution through a power grid, or can be used to split hydrogen from water. The hydrogen is then liquefied and transported to wherever needed. Another application that is being studied a great deal worldwide concerns aerosats. An aerosat provides the same services as a satellite, remote sensing, communications, etc., however the aerosat remains within the Earth's atmosphere. The aerosat can be either a fixed wing craft in a stationary flight pattern, or a rotary wing craft at hover. Microwave power transmission has been proposed as a means to provide a continuous power source to the aerosat. Canada has successfully flown a prototype fixed wing microwave powered aerosat as part of their SHARP study.
SABER is intended to promote public interest and understanding concerning microwave power transmission. SABER will also fulfill a more tangible role as it is the culmination of a three part study of which the first two portions were completed over twenty years ago under the direction of Mr. William C. Brown, formerly of Raytheon. The first study, completed in 1965, concerned a rotary wing supported platform guided by tethers [1]. The platform obtained all of its power from an incident 2.45 GHz beam, converted to DC via a rectenna array. The second study, completed in 1968, also concerned a rotary wing supported platform. In the second study the platform was guided by an on board controller which was fully autonomous. Power was supplied to the platform through an umbilical cord. The unique point to the study was that the controller obtained information about five out of six degrees of freedom from sensors placed in an incident 10 GHz beam. The sixth variable was the height of the platform, and that was externally controlled by a human operator. A third study was envisioned in which elements from the first two studies would be combined to build a rotary wing supported platform which derived all of its power, its attitude, and its position from an incident 2.45 GHz beam utilizing an autonomous controller. The height of the platform would be directed by an external human operator. The third study was never conducted. Mr. Brown has spent the last twenty years determined to complete the third study, and SABER will realize that goal.
Figure 1 is a block diagram for SABER indicating the flow of energy through the system from AC input to the output thrust from the rotor blades. AC energy is converted to DC energy and supplied to a magnetron by a controlled current source. The magnetron converts the DC energy into microwave energy at 2.45 GHz. The microwave energy is then radiated from the slotted waveguide array (SWGA) in a tight beam. A portion of the microwave beam is intercepted by a rectenna array. The rectenna array converts the microwave energy back into DC energy that is used by the rotary winged platform (helicopter) to provide control and support, enabling the platform to stay aloft indefinitely.
In Figure 1 the blocks labeled Magnetron and Slotted Waveguide Array form part of a modular transmission system, an electrically steerable phased array antenna (ESPAM), developed by Mr. Brown. The magnetron is replaced by a magnetron directional amplifier (MDA) in the ESPAM. The MDA is capable of significant tuning in both its frequency of operation and in its output power level, while still maintaining the high efficiency that is characteristic of magnetron devices. The slotted waveguide array is a light weight, highly directional array capable of mass fabrication. When a large number of the MDA/SWGA elements are combined together with suitable processing power, they form an efficient phased array perfectly suited for microwave power transmission at 2.45 GHz. SABER will be a demonstration vehicle for one ESPAM element, minus the MDA. The transmission system will radiate more than one kilowatt of power.
The rectenna array will consist of 99 rectenna elements from the original JPL Goldstone experiment in which over 30 kW of power was received over a one mile distance [3]. Each element is a complete rectenna consisting of dipole, diode, and matching section. The elements will be reconditioned and joined to form a rigid, light weight, two-dimensional array capable of supplying power to the SABER subsystems. Figure 2 is a six element detail of a section of the rectenna array illustrating how the elements will be arranged. The final array will consist of nine strings of eleven elements each in offset geometry. The output of the rectenna array is routed through power conditioning circuitry in order to provide voltage levels that are suitable for the other subsystems. The need to dump unneeded energy absorbed by the array is handled by the power conditioning circuitry. A key point of this design is that the rectenna elements themselves form part of the array structure.
Figure 2
The sensor design utilizes four slot antennas and appropriate circuitry to provide roll, pitch, forward translation, and side translation information to the helicopter controller. Figure 3 is a photograph of the pitch and forward translation sensor circuit. The outputs from each pair of slot antennas are routed into power splitters. One output from each power splitter is fed into a rectifying circuit. The outputs from the rectifying circuits, two per sensor, are then connected to a differential amplifier that produces an output proportional to translational motion. The other outputs from the power splitters are fed into ring couplers. Outputs are taken from the ring couplers and manipulated by circuitry to provide outputs proportional to rotational motion. These signals are possible because the microwave beam varies in both magnitude and phase symmetrically in a plane parallel to the transmitting antenna face. A pair of crossed dipoles at the center of the helicopter body and mounted below will provide yaw information with suitable circuitry.
Figure 3
The control system for SABER will be implemented in a single microcontroller. Forward_translation-inclination-yaw, side_translation-vertical_translation-roll, and motor_speed-power_dumping are the control divisions. Figures 4 through 6 are block diagrams for each subcontroller. The subcontrollers use state space methods to provide the necessary control laws. The control algorithms will be implemented on a Motorola 68HC711E9 microcontroller. Due to the intense microwave environment in the vicinity of the rectenna the control system circuitry will be shielded and all exposed wiring will be protected as necessary.
Figure 4
Figure 5
Figure 6
The communication subsystem is primarily for downlinking state information from the helicopter to the ground crew. An uplink is desired to provide start/stop and abort signals to the helicopter control system, but may not be feasable. The system will be implimented in infra-red technology. Radio frequency systems are not suitable due to the intense energy enviroment surrounding the helicopter during flight.
SABER will be a semi-autonomous rotary wing supported platform. All power for SABER will be extracted from an incident 2.45 GHz microwave beam and converted to DC by a rectenna array. SABER will sense all six degrees of freedom from sensors in the incident beam. The control systems will provide stable control and will be robust enough for outdoor demonstration. SABER will promote the concept of microwave power transmission with the general public as a safe and realiable technology. This project is the work of a few dedicated students at the University of Alaska Fairbanks, and the tireless efforts of Prof. Joe Hawkins and Mr. William Brown.
[1] W. C. Brown, Experimental Airborne Microwave Supported Platform, Techniques Branch Rome Air Development Center Research and Technology Division Air Force Systems Command, Griffis Air Force Base, NY, Tech. Rep. RADC-TR-65-188, Dec. 1965.
[2] W. C. Brown, Experimental System For Automatically Positioning A Microwave-supported Platform, Raytheon, Burlington, MA, Tech. Rep. PT 1751, June 1968.
[3] W. C. Brown, Electronic and Mechanical Improvement of the Receiving Terminal of a Free-Space Microwave Power Transmission System, NASA, Tech. Rep. CR-135194, August 1977.
