Домашнее чтение, перевод №2, страницы 1-3. (Некоторые готовые переводы и другие ДЗ), страница 2

One resistive grid, ofwhich there are a total of 16, is divided into two sections (A and B sides) that can be monitored for changes inresistance. As a method of safeguarding against the effects of temperature fluctuations imparted due to orbitalheating and cooling, the bifurcation of thegrid allows for more precise monitoring ofthe subsystem by comparison resistancechange between sides.

Only the centralportion of the grid was used duringhypervelocity tests due to permitcontainment in the available test chamber.The test article consists of Duroid 6002board material with a grid of TicerTechnologies NiCrAlSi resistive materialphoto lithographed into 1500 resistive linescomprised of 75 microns width of resistivematerial and 75 microns of spacing.The organization of the resistive gridfor the flight model is as follows, seeFigure 3.

One grid is divided into twosections that each can be monitored forresistive change. The total resistance of thegrid as well as the differential resistancebetween the two sections is measured. Thiswill help differentiate resistance changesFigure 3. One of the four panels of the DRAGONS instrument.due to temperature effects from thoseresulting from particle impacts. The differential measurement also increases the precision of the measurements.During engineering model development, the resistive grid had been divided into four sections to minimize thethermal effects on the resistance measurements.

This was determined to be unnecessary and four sectionsnecessitated twice the amount of electronics than a division of each grid into two. Four grids, each 0.25 metersquare, makes up a panel, or 1/4th of the entire flight model, so 4 panels, a total of 16 grids arranged four by four,constitutes the flight test model. Each panel is 0.5m x 0.5m for a total area of 0.25 m2 for a total area of 1 m2. Thepresently used orbital debris model anticipates approximately 1000 hits per year for an array of this size orapproximately 3 impacts per day.Preliminary analysis has shown that the smallest change in resistance is 0.5 ohms per section of the grid whenparticle impacts break one line of resistance.

Therefore, resistance measurements must be stable to within 0.25 ohmsfor the resistance measurement to differentiate betweentemperature changes and particle impacts. Testsconfirm that for constant temperature the resistancestability is within this limit. For varying temperature,the stability of the resistance at a given temperature iswithin this criterion but the resistance change is greaterthan this criterion but repeatable over the maximumtemperature excursions anticipated. Consequently, itwill be necessary to have a temperature measurement ateach panel.B.

Acoustic SubsystemDesigned and constructed at the United StatesNaval Academy was a low velocity test facility. Itsgoal was to support subsystem and system level testingof the acoustic subsystem. First, a technique had to beFigure 4. Sample acoustic data.developed to consistently attach the acoustic sensors tothe panel that ensured a consistent waveform. This was done by adding an insulating layer of low viscosity4American Institute of Aeronautics and Astronauticscyanoacrylate adhesive (superglue) to the back of thepanel, 3.54 inches (90 mm) from each of the corners,and allowed to dry. The exposed leads of the acousticsensors were insulated with Kapton tape.

The sensorswere then attached using additional superglue. Carehad to be taken to assure that there were no airbubbles under the sensors.To conduct the particle impact test, an Air ForceCondor air rifle was used. The rifle can fire particlesat 1450 ft/s (442 m/s). While not near the expectedimpact velocities, the velocity is sufficient todetermine the effect of impact particles on variousboard materials.

Eight different sized particles andtypes of particles were used: brass pellets of size0.78mm, 1.58mm, 0.5mm 1.0mm, 2.0mm, 3.0mm, anylon pellet of 2.39mm, and a Teflon pellet of3.18mm. The acoustic wave form was used toFigure 5. USNA supersonic test facility.determine the particle impact location on the board aswell as the speed and material properties of the particle. In order to capture an acoustic wave form, a highspeed/data rate A/D board made by National Instruments was used to digitize and store the waveforms. Their dataanalysis program called LABVIEW SignalsExpress was used to display and analyze the waveforms.The lab setup required a trigger system to initiate data capture, created when a sabot containing the impactingparticle hit a stripper box containing another acoustic sensor.

Upon detection of an impact, the electronics captures2 msec of signal from the acoustic sensors. It stores this data for download: 1 msec of pre-impact data and 1 msecof post-impact data. The sample rate is 250k samples per second. This data is also useful for determining the impactparticle characteristics at low impact velocities. The time of the reception of the initial front of the wave at each ofthe sensors is used to determine the particle impact location to correlate with the change in resistance of the resistivegrid.

Orbital impacts would be at a much higher velocity, but this system is sufficient to determine the instrumentdesign characteristics and support testing. Impacts studies at near orbital velocity are also being undertaken at thehypervelocity facility at the University of Kent.C. Electronics SubsystemThe DRAGONS electronics engineering model, see Figure6, is a functionally equivalent copy of the final design but usescommercially available off the shelf components instead ofradiation hardened components. However, each component wasselected on the basis of having both a radiation hardenedversion and a commercial non-radiation hardened analog.

Theengineering model is composed of four identical printed circuitboards. Each detects and records data from the complete arrayof sensors on a panel: 4 acoustic sensors and the A and B sidesof the 4 resistive grid sensors totaling 12 channels of data.Because of part shortages from suppliers, substitute parts had tobe installed on the boards, and several post-fabricationengineering changes were required, Functional verification ofboth the acoustic data recording electronics, as well as theresistive data recording hardware, was completed in December2010.Figure 6.

The electronics subsystem.The electronics system receives the output of the acoustic and resistive grid subsystems from a panel and stores thedata for transfer to the host spacecraft communication system when the latter interrogates the DRAGONSinstrument at its convenience. The acoustic data consists of the waveforms for each of the acoustic transducersrecorded at a rate of 250k samples per second.

The analog data for the acoustic subsystem are converted from analogto digital by a 12 bit A/D and recorded continually in a FIFO that stores 2 msec of data. When the signal on one ofthe acoustic channels exceeds the threshold, data are recorded for an additional 1 ms and the data input into the5American Institute of Aeronautics and AstronauticsFIFO is terminated and stored in memory. Thus,the transmitted data for the acoustic system willconsist of the time of acquisition, data identifiers,and 1 msec of data prior to the impact and 1 msecof data subsequent to the impact for each of the 4acoustic channels in a panel. Data for the resistivegrid will be taken periodically at approximately 5minute intervals.

The sum and differenceresistances of the A and B sides of the fourresistive grids are digitized by the 12 bit A/Dconverter for a data rate of ~1024 bits per samplefrom all 4 panels including overhead. A sampleinterval of 5 minutes results in a data rate of~296kb/day.

Thus the total data rate is 448 kb perday including allowance for false alarms and errordetecting coding. The schematic of the electronicsis given in Figure 7.Figure 7. Electronics Subsystem Schematic.The hardware controller is implemented using VHSIC Hardware Description Language (VHDL). The controllerdirects signals from the 12 inputs through various ports and stores them in the First-In-First-Out (FIFO) dual portmemory modules.

The controller then reads all data from the FIFO and routes them to storage with time and dataidentifiers. On the request from the communication system of the host spacecraft, the controller will output the datafor transmission. Three FPGAs are used for the controller, one for the acoustic data, one for the resistive grid data,and one to interface with the host’s communication system.

Should any one FPGA fail, the other two are able tocontinue to run the instrument. For testing, Altera Cyclone IIchips are being used in place of the radiation hardened ACTELFPGAs because they are reprogrammable and thus allow fortroubleshooting of the software. After testing is complete and thesoftware code is finalized, the radiation hardened ACTEL FPGAswill be programmed and mounted on the electronics board.Functional verification of both the acoustic data recordinghardware, as well as the resistive data recording hardware, wascompleted in December 2010.

Figure 8 shows the electronicsundergoing continuing systems level tests expected to becompleted in summer 2011. External equipment used in thecontinued testing of the electronics system includes two voltagesources, a function generator to simulate the clock and a digitaloscilloscope to read the output data.Figure 8. Electronics Subsystem Testing.VI.Current StatusIt was a challenge in the early stages of the DRAGONS project to find a resistive grid that can maintain a nearlyconstant resistance in the space environment and provide consistent line breaks of the resistive grid during impacttests at the hypersonic velocity test facility at the University of Kent (UoK). First, FR4 circuit boards and OhmegaPly resistive material were used, but early testing resulted in larger resistivity changes with temperature andintermittent jumps in resistance at constant temperature. A Ticer resistive material applied to a Duroid 6002 boardwas then evaluated and significantly improved the test results.