Abstract:
The power distribution control element significantly improves the efficiency by which solar energy is distributed and controlled to large phased array antenna assemblies by providing current directly from photovoltaic cells to lithium-ion battery cells through a neural-network based charge controller. The small current required to operate each transmit/receive module is provided from an adjacent battery cell rather than a large centralized battery assembly located in the spacecraft bus. In the preferred embodiment, the charge control is regulated by a back-propagation neural network.

Description:
RELATED APPLICATION 
     The present application is based on the Applicants&#39; U.S. Provisional Patent Application Ser. No. 60/149,305, entitled “Neural Network Controlled Power Distribution Element,” filed on Aug. 17, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of electrical power systems. More specifically, the present invention discloses a neural network power distribution control element for a phased array antenna and similar distributed systems. 
     2. Statement of the Problem 
     Conventional space-borne phased array antennas, communication satellites and signal intelligence satellites currently use large solar arrays, heavy battery assemblies, and complex power distribution systems to operate. Large-area phased array antennas require enormous power to function. They tend to be heavy because of the area required by hundreds of thousands of radiator elements needed to accomplish the mission. Typically multiples of radiator elements are combined onto a single transmit/receive (TR) module and multiple TR modules are combined into a radio frequency (RF) tile, which provides a convenient building block with which to work. Ideally, each TR module is driven from a low voltage current (i.e., 3 to 4 volts). 
     Present power systems for such phased array antennas collect energy though the solar panels that is sent to a central voltage regulator and power conditioner from which large-capacity batteries (usually multiple 60 to 200 amp-hour) are charged. The batteries supply power to the bus and payload through a power distribution unit, typically at 28 volts. The voltage at the RF tiles is dropped to significantly lower levels through a voltage de-boost circuit that distributes the power to the TR modules. There are significant losses inherent to the cumulative inefficiencies associated with all of the steps from the solar collection to the TR modules. 
     For example, standard power systems for such phased array antennas use nickel-hydrogen or nickel-cadmium batteries, both of which have significant drawbacks associated with them, including life limitations due to depth of discharge, heavy packaging constraints, and reconditioning requirements. Lithium-ion batteries are a promising technology because they offer much lighter, more efficient assemblies. Several limitations stand in the way of their development. The individual cells don&#39;t interact well together and require separate charge controls. For 60 to 200 amp-hour battery assemblies, significant challenges in charge control, thermal dissipation, cell scalability and other technical problems face battery engineers. Also, in order to achieve long life from each battery (i.e., 50,000 cycles) the depth of discharge has to be limited to less than 10 percent, meaning that the overall size of battery becomes too large to take advantage of the high energy density ratio that Li-ion technology offers. Substantial research and development efforts have been dedicated to overcoming these deficiencies. 
     Solar energy can be converted into electricity by means of solar cells composed of various chemistries. One of the most efficient solar cell technologies is galium-arsenide dual junction, which can be as good as 25% efficient and development efforts promise 30% efficiencies in the near future. However, they are susceptible to degradation from radiation and require cover slides for protection, thus adding weight to the solar array. Furthermore, the cells within each string are connected in series which boosts the voltage to a higher level. The strings are then connected in parallel to send current from the solar array to the regulator, conditioner and battery through a heavy wire bundle. The fact that strings are wired in series results in the loss of an entire string should a single cell be lost due to cell failure, broken connection, shadow, etc. 
     3. Solution to the Problem 
     The present invention addresses many of the shortcomings associated with conventional power distribution systems for phased array antennas. The present system provides current directly from solar cells to lithium-ion battery cells through a charge control regulated by a neural network at each battery. The small current required to operate each TR module is provided by an adjacent battery, rather than a large centralized battery assembly located on the spacecraft bus. 
     This approach eliminates the need for the majority of power distribution components that are traditionally used to operate phased array antennas. This technology eliminates or replaces heavy components such as voltage regulators, power distribution units and wire harnesses with significantly lighter, less complex elements. It allows for small currents and small voltages to provide power to the TR modules avoiding loss due to long cable runs. It enables the use of lithium-ion cells that have much higher energy density ratios by eliminating the technological problems that are associated with large capacity lithium-ion battery assemblies. Use of lithium-ion technology represents about a 70% cost reduction in battery assemblies. The incorporation of a neural network charge controller increases battery life and eliminates the need for thousands of lines of software code and computations. This idea will yield significant improvements in costs associated with manufacturing, assembly and testing. Less efficient but much lighter and much less expensive solar cells such as copper indium diselenide (CIS) or amorphous silicon fabricated on an Upilex® mylar substrate to be used in conjunction with this concept offering further weight and cost savings and both contribute greatly to long life and graceful degradation of the payload. 
     This revolutionary approach can be used to reduce the weight and cost of any phased array space-borne antenna system. Also any electronic system requiring small voltages distributed over large areas would be potential candidates for utilizing this technology. 
     SUMMARY OF THE INVENTION 
     The present invention significantly improves the efficiency by which solar energy is distributed and controlled to large phased array antenna assemblies. By providing current directly from solar cells to lithium-ion battery cells through a neural network, charge control is accomplished at each battery using a microprocessor. The small current required to operate each TR module is provided from an adjacent battery cell rather than a large centralized battery assembly located in the spacecraft bus. In the preferred embodiment, the charge control is regulated by a back-propagation neural network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more readily understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a simplified block diagram of present invention. 
     FIG. 2 is an exploded perspective view of one RF tile assembly in the present invention. 
     FIG. 3 is a perspective view corresponding to FIG. 2 showing the normal stacked configuration of an RF tile assembly. 
     FIG. 4 is a block diagram of the neural network battery charger  30 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In large-area phased array antennas, hundreds of thousands of radiator elements may be required. Typically, multiples of radiator elements are combined onto a single transmit/receive (TR) module and multiple TR modules are combined into an RF tile which represents a convenient building block with which to work. Each RF tile is driven by a low voltage current (i.e., 3 to 4 volts). Because phased array antennas typically occupy large areas that tend to radiate heat rapidly, keeping the TR modules warm enough is usually of more concern than keeping them cool. The back of the antenna is an ideal location for solar arrays because of the excellent surface area on which to locate solar cells. The close proximity of the solar cells with the RF tiles provides thermal control benefits as well under most conditions. 
     The present system provides electrical DC power directly to the individual RF tiles  52  from solar cells  10  in relative close proximity. FIG. 2 is an exploded perspective view of one RF tile assembly in the present invention. FIG. 3 is a perspective view corresponding to FIG. 2 showing the normal stacked configuration of one RF tile assembly. Each RF tile  52  consists of four TR modules  54  that receives a 3.6 volt current from a single lithium-ion battery cell  40 , and each TR module  54  consists of four radiators  56 . The configuration shown uses a 3-inch square tile with 16 radiator elements  56  spaced equally at 0.75 inch on center. Preliminary analysis indicates that a Li-ion battery cell  40  (2-inch square by 0.1 inch thick) will accommodate conservative average power requirements of the RF tile  52 . An innovative neural network microprocessor chip described in detail below controls the battery cell  40 . Ultra-small, lightweight copper polyimide connections  12  and  42  are used to connect the solar cell  10  to the battery neural network microprocessor and battery  40 , and the battery  40  to the RF tile  52 . 
     By individually controlling each cell with a microprocessor, we can bypass the classical problems of charge control of Li-ion batteries. The battery cell  40  can be sized to meet the power requirements of the RF tile  52 . These exceptionally small battery sizes allow the benefits of Li-ion technology to be realized without incurring the problems of large batteries. Li-ion technology has advantages over other batteries not only in terms of weight, but can also operate over a wider range of temperatures. Another benefit of using small batteries is that much of the battery mass (i.e., base plate, end plate, etc.) is eliminated. Conceivably, the battery cell  40  could be packaged within the RF tile  52  structure thereby acquiring further weight reduction. 
     Because of the low voltages and currents required by the RF tile  52 , the small batteries  40  are ideally suited to incorporate the copper polyimide flex patch connections  12 ,  42 , which contribute greatly to reliability and ease of manufacturing. The flex patch  12 ,  42  can be designed specifically for specific applications and launch environments. The microprocessor charge control chip can be integral to the fabrication of the cell  40 . The need for voltage boost regulators, power distribution units and de-boost electronics to and from the spacecraft bus are eliminated as well as the wire harnesses. Depending upon mission requirements, potentially 450 pounds of equipment can be eliminated with this concept. 
     Solar cell sizing requirements are easily met with various chemistries. Copper indium diselenide (CIS) can be applied to this application with two immediate benefits. One is that when applied to a UPILEX® mylar substrate, it provides an exceptionally light and flexible solar collector. The other is that it lends itself to this modular, self-contained concept in a way that a single manufacturer could integrate the solar cell, battery and microprocessor in a single facility. The RF tile  52 , battery  40 , and solar collector components  10  can be parallel processed, assembled, and tested in a geometrically flat configuration prior to folding into the normal, stacked geometry as shown in FIG.  3 . 
     Manufacturability, testability and reliability requirements can be accommodated simultaneously to produce a substantially less expensive, robust system. Another associated benefit is that solar array string failures are eliminated because the cells are not necessarily wired in series. With this approach, each cell is independent of the others, and thus the effect of a failure in a series configuration is eliminated. 
     Back-Propagation Neural Network to Control the Charge Logic of a Lithium-Ion Battery. 
     The current “state of art” battery chargers use microprocessor control to charge the battery based upon calculations derived from telemetry consisting of battery temperature, battery voltage, and battery charge current. The microprocessor relies upon software programmed onto the chip. Different code must be used for different usage environments and the battery control logic must deal with a nonlinear, battery chemical system. 
     The present system uses a back-propagation neural network  30  to control battery charging as illustrated in FIGS. 1 and 4. As illustrated in FIG. 1, one or more solar cells  10  generate a low voltage current that powers the charge electronics  20  used to charge a battery  40 . In turn, the battery  40  powers the load  50  (e.g., an RF tile assembly). A back-propagation neural network  30  receives inputs from a series of sensors monitoring the battery voltage, charge current, and temperature, and outputs a charge control signal to the charge electronics  20 . It should be understood that other parameters indicating the state of the battery could be monitored by sensors and used as inputs to the neural network in addition to, or in place of the parameters listed above. 
     FIG. 4 provides a more detailed block diagram of the neural network battery charger  30 . The general concept of using a back-propagation neural network to control battery charging has been disclosed by Harvey, “The Use of Neural Networks In A Smart Battery Charger” (M.S. degree thesis, University of Missouri—Rolla, 1995). However, this thesis did not apply this concept specifically to lithium-ion batteries or to the general field of phased array antennas. 
     Returning to FIG. 4, the telemetry received from the battery  40  includes the battery voltage, charge current, and temperature. These inputs are subjected to a linear transformation and are normalized in block  31  to produce an input vector having values in the approximate range of 0.2 to 0.8 for the neural network  32 . The neural network  32  preferably has three layers, with three nodes in the input layer, seven nodes in the intermediate layer, and one node in the output layer. A linear transform  33  can also be applied to the output of the neural network  32  to produce a charge control signal having a desired range for the charge electronics  20 . 
     The neural network  30  has previously been trained using sets of input data to produce an acceptable charge control signal. It should be understood that other types of neural networks could be substituted, or that other configurations of back-propagation neural networks could be used. 
     The uniqueness of this approach deals with the ability of the network to be “trained” from actual orbital charge/discharge data, eliminating the need for thousands of lines of code, computation, etc. and supplying a robust charge control capable of dealing with the nonlinear battery system. The neural network can also take into account the variation of required charge due to changing orbital configuration such as changing sun/eclipse time. This is classically a problem with charge logic design, since battery capacity must be maintained at the same high level even when the load on the batteries becomes lessened due to shortening eclipse time. Such a compromise shortens battery life due to stress upon the battery system. With neural network control, the battery state of charge and peak voltage will be adjusted as needed for the changing battery load due to shortening or lengthening of the eclipse period. The complex relationships in a battery between state of charge, voltage, current, and temperature can be learned by the neural network. The only required inputs to the network are from a training set of data consisting of cell voltage, temperature, and charge current over time. 
     Other Fields of Use 
     The present invention can also be applied to a broad spectrum of other types of electronic devices, not only in aerospace, but the automotive industry and numerous commercial markets. In fact, any electronic device whose constituent components operate with small power supplies (such as cellular telephones, hearing aids, calculators, automobile circuits, etc.) would be a potential candidate for the present invention. The light source does not have to be the sun and the photovoltaic cells are not limited to any particular chemistry. For instance, solar-powered calculators operate efficiently using ambient room light. 
     Because modular power elements in the present invention incorporate “smart” charge control, the need for separate charge equipment can be eliminated. Equipped with one of the present modules, a calculator could be solar powered and still have a long memory for programs and data storage. Cell telephones could be populated with sufficient power modules to operate without the need for recharging within obvious limits. Small modules could potentially be developed to fit into a hearing aid eliminating the need for replacement batteries. Home fire and smoke detectors could be equipped with these modular elements to mitigate the risk of dead batteries. 
     An automotive application using the present invention could be employed to provide standby power for many of the circuits in a car. In the event that the main battery goes dead, is damaged or removed, the functions of door locks, radio settings, seat positions, GPS functions (included with On Star System) would not be affected. 
     The potential applications in aerospace are equally numerous. All system components included in attitude control system, control and data handling, navigation systems and communications can be designed using the present invention to incorporate modular power supplies for the various processes within these subsystems. Space-borne electrical power systems may evolve to an entirely new level of technology as this idea becomes available. 
     The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims.