UAS Telemetry. Objective

Transcription

1 UAS Telemetry UAS270 Avionic Systems F18 Objective primary objective: to downlink UAV status information data to remote pilot, mission supervisor, etc. secondary objective: provide uplink path for C&C from GCS UAS270 Avionic Systems 2 UAS Telemetry 1

2 Example UAS with Telemetry project for a smart pod uav pod UAS270 Avionic Systems 3 Telemetry Data flight mode rx signal strength (RSSI) battery/fuel usage/status GPS quality (# satellites, DOP) UAS position, heading, speed (air, ground, vertical) flight path, waypoint status, home MinimOSD other: command acknowledgement payload status EagleTree Vector OSD UAS270 Avionic Systems 4 UAS Telemetry 2

3 Telemetry Data Transfer data transfer is accomplished using "RS 232 style" serial data transfer method each ASCII character is represented by a unique 7 bit binary code (next slide) then framed with start/stop/parity bits before being transferred 1 bit at a time over a single, unidirectional connection at a particular baud rate (bits/sec) which both ends agree upon baud rate determines the bit time via UAS270 Avionic Systems 5 Hex Hi Lo Bits NUL DLE P ` p SOH DC1! 1 A Q a q STX DC2 2 B R b r ETX DC3 # 3 C S c s EOT DC4 $ 4 D T d t ENQ NAK % 5 E U e u ACK SYN & 6 F V f v BEL ETB 7 G W g w BS CAN ( 8 H X h x HT EM ) 9 I Y i y A 1010 LF SUB * : J Z j z B 1011 VT ESC + ; K [ k { C 1100 FF FS, < L \ l D 1101 CR GS - = M ] m } E 1110 SO RS. > N ^ n ~ F 1111 SI US /? O _ o DEL KEY NUL = Null SOH = Start Of Header STX = Start of Transmission ETX = End of Transmission EOT = End of Text ENQ = Enquiry ACK = Acknowledge BEL = Audible Bell BS = BackSpace HT = Horizontal Tab LF = Line Feed VT = Vertical Tab FF = Form Feed CR = Carriage Return SO = Shift Out SI = Shift In DLE = Data Link Escape DCx = Device Control x NAK = Negative Acknowledge SYN = Synchronization char. ETB = End Transmission Block CAN = Cancel EM = End of Medium SUB = Substitute ESC = Escape FS = Field Separator GS = Group Separator RS = Record Separator US = Unit Separator DEL = Delete UAS270 Avionic Systems 6 UAS Telemetry 3

4 RS 232 Data Transfer framing: each character transferred gets its own start bit and stop bit, optional parity bit start bits are always 0, stop bits always 1 parity used for error checking may be even, odd, fixed at 0, or unused data bits are sent/received lsb to msb characters may be sent back to back lines idles at '1' when no data to transfer ex: 2 chars using 8 data, 1 stop, no parity: UAS270 Avionic Systems 7 RS 232 Example what does the message Hello! look like over an RS 232 style comm link???? standard baud rates 110, 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, , , so, how long will it take to transfer (a) each bit and (b) entire message at xxx baud???? UAS270 Avionic Systems 8 UAS Telemetry 4

5 Two way Data Transfer normally, 1 wire or channel provides unidirectional data transfer from A to B simplest way to implement bidirectional data transfer is to have 2 wires/channels! and a common ground A B additional wires sometimes used for 'handshaking' signals UAS270 Avionic Systems 9 Protocols when multiple data bytes are transferred over a comm link, they are often packaged into data packets according to one or more protocols ex: TCP/IP, SMTP, HTTP/HTTPS more robust error detection/correction is then handled by the protocol than simple parity which is only 50% accurate UAS270 Avionic Systems 10 UAS Telemetry 5

6 Data Packets aka "message", is the unit of transfer in a protocol typical data packet fields packet ID/type number packet sequence number source address destination address payload (the actual data being transferred!) error check (usually at end) UAS270 Avionic Systems 11 one such protocol of interest is MAVlink an open source protocol commonly used with modern UASs (also UGSs and USSs) between unmanned vehicle and GCS used in PixHawk, Mission Planner, QGroundControl packet layout (v1.0): UAS270 Avionic Systems 12 UAS Telemetry 6

7 MAVlink Packet Details Field name Index (Bytes) Purpose Start of frame 0 Denotes the start of frame transmission (v1.0: 0xFE) Payload length 1 length of payload (n) Packet sequence 2 System ID 3 Component ID 4 Each component counts up their send sequence. Allows for detection of packet loss. Identification of the SENDING system. Allows to differentiate different systems on the same network. Identification of the SENDING component. Allows to differentiate different components of the same system, e.g. the IMU and the autopilot. Message ID 5 Identification of the message the id defines what the payload means and how it should be correctly decoded. Payload 6 to (n+6) The data into the message, depends on the message id. CRC (n+7) to (n+8) Checksum of the entire packet, excluding the packet start sign (LSB to MSB) UAS270 Avionic Systems 13 MAVlink Packet Application G.C. software Mission Planner QGroundControl UgCS Tower "terminal emulator" RealTerm experiment: custom software to capture, decode, and display MAVlink packet contents UAS270 Avionic Systems 14 UAS Telemetry 7

8 MAVlink Packet Capture: RealTerm straight ASCII display mode shows a lot of control (binary) characters not fit for human consumption! UAS270 Avionic Systems 15 MAVlink Packet Capture: RealTerm hex display mode shows each byte received as 2 hex digits packet structure can be discerned MsgID=0, "HEARTBEAT" UAS270 Avionic Systems 16 UAS Telemetry 8

9 MAVlink Decoder Program written in 'C' approx. 200 lines of code payload contents extracted & interpreted UAS270 Avionic Systems 17 Telemetry Systems: External FMU based external solution via a direct, hardwired (RS 232 or USB) link or a set of wireless telemetry transceivers 915 MHz: USA 433 MHz: Europe SiK Telemetry Radio one popular open source implementation updated version at mrobotics.io sik telemetry radio.html UAS270 Avionic Systems 18 UAS Telemetry 9

10 Telemetry Systems: RC based RC provided an extension to 2.4GHz 1 way C&C links to also downlink telemetry data over same link some RC system manufactures now provide telemetry features in their systems ex: Futaba, Graupner, FrSky, Spektrum sensors, datum points UAS270 Avionic Systems 19 Futaba Telemetry via "T FHSS" receivers using SBUS2 SBUS2 = "2 way" sensors: GPS, voltage, current, temperature, altitude RPM, airspeed sensors ensors UAS270 Avionic Systems 20 UAS Telemetry 10

11 FrSky Telemetry via receivers with "Smart Port" sensors: GPS, Vbatt, Ibatt, temperature, Variometer, airspeed, RPM, fuel flow rc.com/product category/sensors UAS270 Avionic Systems 21 Setting Up Telemetry: FrSky telemetry data display is supported by certain FrSky transmitters such as our Taranis X9D demo: Nate UAS270 Avionic Systems 22 UAS Telemetry 11

12 Telemetry Flight Parameters altitude attitude, AoA heading turn indicator engine RPM temperature: engine, OAT vertical speed (VS) airspeed (AS) groundspeed (GS) fuel supply Sensors voltage current temperature tachometer (RPM) barometric pressure (altitude) gyros, accelerometers (attitude) pitot tube (airspeed) GPS (position) magnetometer (heading) UAS270 Avionic Systems 23 Voltage Sensing determines voltage of an unknown input by performing analog to digital (A/D) conversion on a "sample" of the input being monitored (V M ) then scaling the binary result by an appropriate conversion factor (C F ) block diagram: V M xc F ADC B M ex: monitoring V of a 6S LiPo using an 8 bit ADC with V REF =5V UAS270 Avionic Systems 24 UAS Telemetry 12

13 Current Sensing determines amount of current (I) extracted from a battery by accumulating periodic instantaneous current measurements over many time intervals instantaneous current found by monitoring voltage drop across a very low value resistor and exploiting Ohm's Law but high power! (wattage) ex: monitoring up to 100A UAS270 Avionic Systems 25 Temperature Sensing modern solution is to use a solid state precision integratedcircuit (IC) device then perform A/D conversions on V OUT UAS270 Avionic Systems 26 UAS Telemetry 13

14 Tachometer Sensor an instrument that measures the rotational speed of an engine or motor, typically in revolutions per minute aka tach, rev counter, RPM gauge so UAS270 Avionic Systems 27 Atmospheric Pressure atmospheric pressure / BP is the pressure in the Earth's atmosphere i.e. "weight of the air" measured in force per unit of area in Pascals, = 1 newton per sq. meter (1 Pa = 1 N/m 2 ) standard atmosphere (atm) = 101,325 Pa which also = mbar = inhg = 14.7 psi atmospheric pressure is directly affected by altitude and by temperature and humidity to lesser degrees pressure decreases 12 Pa per meter of altitude increase UAS270 Avionic Systems 28 UAS Telemetry 14

15 Altitude Sensing atmospheric pressure changes by 12 Pa for each 1 m altitude change modern UAV solution is to use a silicon pressure sensor IC device Invensense ICP 101xx BP & temp sensor IC ±1 Pa accuracy or 8.5 cm! temperature sensor provides temperature stability perfect for UAV/drone applications axis/icp 101xx UAS270 Avionic Systems 29 Airspeed Sensing measure of speed of movement through the air typically in knots (kts, 1 kt = 1 nautical mph = 1.15 mph) method: use a pitot tube and compare ram pressure against static pressure pitot tube: a device that measures dynamic pressure ( ) UAS270 Avionic Systems 30 UAS Telemetry 15

16 GPS system of satellites (24 active + 7 spare) used to geolocate a receiver's position orbiting at 20,000 km altitude started in 1973 as Navstar by US DoD for military applications, currently operated by USAF (Space Command) requires direct visibility to minimum of 4 satellites more accurate results with more satellites original accuracy of 5m (2000), improved to 30 cm (2018) UAS270 Avionic Systems 31 GPS Operation based on precise distance to satellites each having highly accurate atomic clocks where speed is speed of light (3x10 8 m/s) and a mathematical process known as trilateration to find intersection point of at least 3 satellites (for 2D fix, 4 for 3D fix) results in lat/lon/alt of the GPS receiver "How does GPS work?": UAS270 Avionic Systems 32 UAS Telemetry 16

17 GPS Accuracy dilution of precision (DOP) GPS errors: 1. atmospheric refraction which modifies (lengthens) signal path 2. ephemeris (satellite time & location) caused by slight position and time synchronization errors of satellites; although they maintain accuracy to 1 ns! 3. multipathing from "echos" off of ground structures/features 4. GDOP (geometric dilution of precision) result of separation between satellites in view your own gps applications part 2/2/ UAS270 Avionic Systems 33 GPS Accuracy results of GPS errors PDOP: 3D position DOP HDOP: horizontal DOP VDOP: vertical DOP TDOP: time DOP lower DOP (better accuracy) achieved by: more visible satellite sources increased separation DOP value interpretation Value Quality Rating >20 Poor Fair 5 10 Moderate 2 5 Good 1 2 Excellent <1 Ideal UAS270 Avionic Systems 34 UAS Telemetry 17

18 GPS Receivers GPS receivers are available in both standalone and component form factors component receivers attach to FMU via dedicated port UAS270 Avionic Systems 35 Magnetometer a device that measures/outputs its orientation with respect to the Earth's magnetic field produces "heading angle" like a compass (0 359 ) used in UAVs to stabilize yaw heading sometimes in combination with GPS heading UAS270 Avionic Systems 36 UAS Telemetry 18

19 References MAVlink, Oct MAVLink Developer Guide, Oct Pilot's Handbook of Aeronautical Knowledge, FAA "How GPS Works", toponz/articles/how gps works/how gps works.html, Nov "Trilateration vs Triangulation How GPS Receivers Work", triangulation gps, Nov "GPS Accuracy: HDOP, PDOP, GDOP, Multipath & the Atmosphere", accuracy hdop pdop gdop multipath, Nov UAS270 Avionic Systems 37 UAS Telemetry 19