Command and Data Handling: Design

Transcription

1 Command and Data Handling: Design Mark Campbell AA420 Space Design Outline Driving Issues and Requirements mission sensors and actuators communications radiation Sotware approaches simple versus OS Protocols Types o Architectures Design Approach Reerences: Larson and Wertz Stone, Harold E.; Microcomputer Interacing; Addison-Wesley Publishing Company; Reading, MA; Blau, Mike; C&DH SW Homepage, NASA TRACE project, Questions to mentors 1

2 Driving Issues and Requirements Mission amount, importance o science data on-board calculations required Number, type, and rate o sensors and actuators Radiation Communication windows and the amount o data sent/received Vibration/thermal characteristics o boards Power required Complexity in sotware design Cost Mission Objectives and Science Data The mission objectives and importance o the data collected drive the on-board memory type and size. Is the science data very important, such that it cannot be stored in RAM but requires radiation hardened memory or ROM? How many communication passes are there to downlink the data? How much data is required or downlink (and thus storage)? Is there on-board decision making or other high computationally intensive sotware? Rendezvous closed loop system video downlink rendezvous sensors 2

3 Sensors and Actuators Sensors are required or health (temperature, voltage, current, pressure, uel, ) decision making (attitude determination, navigation and rendezvous) Each sensor must have a type (digital or analog) data rate word size number o sensors A table o sensors and each o the above characteristics should be started asap Example: Dawgstar Sensors Telem etry S am p ling W ord Size Required Data Item Type Quantity Rate (Hz) (bits) R ate (bps) Power Solarce lvoltage in sensor voltage Powerbus voltage outsensor voltage Battery charge sensor voltage PowerManagement Therm al PRT sensor tem perature ThermalControl heaters C om m unications GPS powerlevelsensor voltage GPS tem perature sensor tem perature Antenna tem p tem perature Transm itpower voltage Kalman Filter G PS C&DH CPU powerlevelsensor voltage CPU tem perature sensor tem perature FaultDetection Monitor errordetection FaultCorrection errorcorrection Telem etry Processing system health Command Processing system management Propulsion Capacitorvoltage sensor voltage Prope lantsensor voltage D.I.Capacitorsensor voltage ADCS Magnetom eter xyz position Gyro xyz rotation Horizon Sensor pitch and rol Tem perature sensor tem perature ThrusterControl thrustercontrol OrbitPropagation orbitdetermination ErrorDetection O rbiterror Science Absolute electron density Impedance data PCM Data Rate (bps)

4 Radiation Radiation can work in primarily two ways: 1. Single Event Upsets - usually a bit lip (sotware) or a hardware anomaly (latchup) caused by a short burst o radiation (such as a solar lair). Latchups are much more dangerous. 2. Constant bombardment o radiation over time Single event upsets are usually helped by using a WatchDog timer on the CPU using a second CPU to watch the primary Constant bombardment is a unction o location in space time o mission neither o which should aect our mission Radiation Hardened components can help with both types o radiation, but they are VERY expensive (>$30K) Other Issues Communication windows (i.e. how long can we talk) and power drive the amount o data that can be sent to the ground. This is a big system driver or small satellites in LEO Downlink options or Universities: own ground station other universities purchase time rom commercial ground stations Vibration/thermal characteristics o boards are they classiied as MIL spec? High thermal loads on boards is an important driver How much continuous power is required or the board? What is the cost o the board? What is the mass o the board including wiring? 4

5 Dawgstar Requirements Hardware Capabilities Digital I/O Lines RS-232 in addition to above lines 4 Analog Input Signals 64-8 bus width / addressable memory space 25-bit or higher Boot Image Memory 256k (Fuse-Link ROM on light board, EEPROM on development boards) System Memory 1 MB FIFO Data Memory 8 MB Processor Speed MHz Operating Environment Industry Standard Compliance Operating Temperature Cooling Mechanism Structural Stability Total Radiation Dose Single Event Upset Threshold SEU Error Rate Hardware Watchdog Timer 2-1 Development, Support, etc. VxWorks OS Support 1 Flight Board, 3 Development boards Lead Times: 2 weeks or development boards, 6 months or light board Educational Discount Mil-Spec 883, Level B NOTE: compliance with this standard is equivalent to the requirements listed below. -55 to +85 C Conduction cooled 40g shock loads, 12g RMS random vibe krad MeV/mg/cm2 10E-10 10E-8 errors/bit/day Physical Characteristics Mass Power consumption 0.1 kg W Simple Sotware Design: Loop While Spacecrat OK sample attitude sensors sample health sensors sample navigation and other science sensors save health and science sensors to o-board memory queue calculate attitude command calculate position command send out commands to attitude actuators send out commands to propulsion End Interrupt or: communications anomalies 5

6 Operating System: Scheduling o Tasks Holding E Executing Process P3 P2 E E Executing w/ R1 Executing w/ R2 P1 E E Preempted Time A operating system: is more complex than a simple scheme allows easy multi-tasking Communication Protocols and Inormation Flow A Terminal Node Controller (TNC) packetizes binary serial data into a communications protocol ormat. An alternative is an internal modem in the transmitter/receiver Ater packetizing the data, the TNC modulates it using requency shit keying (FSK) Frames and packets are two methods o ormatting data or storage and transmission rames are ixed packets are variable Packets add lexibility to the telemetry system Each major subsystem can package its data dierently Data can be transmitted as generated Standard communication protocols can be used many EDAC sotware exists Examples X.25, TCP/IP, ETHERNET 6

7 AX.25 Packet Structure ################################################################# # Packet ID #RT Bit # DATA # CRC # ################################################################# Packet ID: Identiies which satellite is sending the data RT Bit: Real Time bit identiies whether the data is real time, or processed data Data: Data ield (larger ield means aster transer, but more risk) CRC: allows the satellite to determine i the packet has been corrupted Centralized Architecture ADDITIONAL MEMORY GPS TERMINAL NODE CONTROLLER FLIGHT COMPUTER TT-8 BOARD WITH AN MC68332 I/O BOARD PROPULSION SUBSYSTEM THERMAL ACTUATORS ANALOG MULTIPLEXING SCHEME POWER SUBSYSTEM INTERFACING CIRCUITRY SCIENCE EXPERIMENTS CAMERA ARRAY GYRO MAGNETOMETER 7

8 Distributed Architecture data bus SCIENCE EXPERIMENTS ACTUATORS FLIGHT COMPUTER TT-8 BOARD WITH AN MC68332 GPS PROPULSION SUBSYSTEM CAMERA ARRAY MAGNETOMETER THERMAL GYRO Data bus supports all data or the system components are nodes that use standard communication protocols to exchange data, interrupt usually a parallel network o wires POWER SUBSYSTEM Sotware Issues How does inormation low: serially pull interrupt Mission goals, cost requirements will drive the decision o using an operating system The best approach to sotware design is to deine modes o operation initial checkout diagnostic nominal attitude control rendezvous docking servicing... 8

9 How the CPU gets Inormation Thermal/Power ADCS Pull Pull Interrupt Interrupt C&DH Interrupt Interrupt Communications Propulsion Interrupt Pull Pull Pull GPS Plasma Impedance Probe The two most common approaches are pulling data (i.e. sampling when the CPU requires data) and interrupt. C&DH: Functional Diagram OFF EVERYTHING OK!! SEPARATION FROM SHUTTLE DIAGNOSTICS TEMP PROBLEMS COMMAND & DATAHANDLING BATTERY NEEDS CHARGING LOW POWER BAD THERMAL FORMATION KEEPING UPLINK INSTR SCIENCE MODE DOWNLINK/ UPLINK TIME FOR EARTH COMM. ATTITUDE CONTROL ATTITUDE CONTROL MAXIMISE SOLAR CHARGING FORMATION MANEUVERING PROPULSION ROTATE AWAY FROM SUN 9

10 C&DH: States o operation Diagnostics Check battery charge Check temperature sensors Everything OK - go to science mode Science Mode Attitude control gyroscope, magnetometer, camera Science measurements Health monitoring GPS and Crosslink monitoring Low Power DAWGSTAR is in light orient to max solar intake shut down inessentials while charging DAWGSTAR is cold sleep till light side Bad Thermal Low temperature use heaters Excessive temperature shut down device rotate away rom sun C&DH: States o operation (cont.) Propulsion Attitude and position determined beore entering propulsion state Fire thrusters 1, 2,..., n or x seconds Formation maneuvering Based on GPS, Crosslink, and Uplink instructions Only a ew times during mission duration Attitude Control Downlink / Uplink Mode Uplink instructions to memory address Downlink data rom memory address Health monitoring Attitude Control Formation Keeping Based on GPS and Crosslink data Attitude Control 10

11 Design Approach List design drivers, associated calculations, narrow requirements Fill out more detailed speciication based on template, list the requirements or your system Make a table o the sensors, data rates, numbers, type List modes o operation Estimate processing required or each mode o operation Using speciications, do a trade study on the dierent commercial o the shel board options you may need to make some small changes to the board, such as added RS-232 ports remember MUXing! Purchase a board a start the initial prototyping (remember lead time!) 11