MAVS Current Measurement

1. MAVS Current Measurement Nobska Development, Inc. Albert J. Williams 3rd Archie Todd Morrison III

2. Current Measurements • Mechanical Sensors • Electro-Magnetic Sensors • Doppler Sensors • LDV • Acoustic Doppler • Acoustic Doppler Profiling • Acoustic Travel-Time Sensors

3. Mechanical and EM Sensors • Mechanical • Rotor, vane – operate at very low power but have poor response in low flows and in oscillatory motion • Fan type – operate at low power, have good response in oscillatory flow but exhibit dead band in low flows • Electro-Magnetic • 2-D, no dead band but response is poor to accelerating flow, far field of electric field makes calibration awkward in a tow tank

4. Doppler Sensors • LDV – Laser Doppler velocimeters • Narrow range of particle densities over which it has good signal, requires many realizations to obtain low velocity noise • Acoustic Doppler single point sensors • df=(f1-f0)=2f0(u/c)cos(a), where a is the angle between the acoustic beam and the current • ADV sensor is bistatic and measures velocity components at a point. There are three acoustic paths • Acoustic Doppler current sensors suffer from weak returns if there are few scatterers and may lock onto spurious returns from supporting structures with zero velocity. Scattering volume is remote from sensor head and fairly large

5. Doppler Sensors • Acoustic Doppler profilers (ADCP) • An entire profile of velocity is obtained • Monostatic, three or four acoustic axes diverge from transducer head • Pulse burst must decay before next burst is transmitted • Range depends on frequency, 300kHz gives about 150 meters range with good scatterers • Individual burst has large ambiguity in velocity, many pings must be averaged to obtain speed resolution desired

6. Doppler Sensors • Acoustic Doppler profilers (continued) • Velocity resolution*repetition rate = constant for any particular frequency • Range*velocity resolution*repetition rate = constant allowing frequency to be selected for desired range. This tradeoff limits use of ADP for turbulence measurements as well as large ensonified volume • Broadband encoded bursts in BBADCP permit effectively more pulses at once and this constant is increased proportionally

7. Acoustic Travel-Time Sensors • Defined acoustic path along which fluid velocities are averaged • Depends only on the ability of the fluid to transmit sound, no penalty for moderate attenuation or for perfect clarity • Linear velocity response through zero velocity

8. Acoustic Travel-Time Sensors u c + u c - u L

9. Applications of Current Measurement, Requirements • Transport • Average over waves and mooring motions, typically 20 seconds • Long deployment duration, typically 3 to 12 months • Excellent directional resolution and low velocity threshold, linearity over large range in velocities • Waves • Bursts of duration capable of resolving spectrum at modest sampling rate, typically 20 minutes at 2 Hz • Burst interval related to wave development, typically 2 hours • Boundary layer turbulence • High frequency sampling, typically 20 Hz • Fine spatial resolution, typically 1/3 of distance from the boundary

10. Current Meter Moorings and Rigid Supports • Mooring cage • Carry mooring tension around current sensor • Bottom Tripod • Sensor inside tripod or sticking up in cage, MAVS illustrated • In-line support • Near bottom with float, MAVS illustrated at Juan de Fuca hydrothermal vent • Piling • Rigid mount above bottom in shallow water

11. Nobska Nobska - MAVS3 Modular Acoustic Velocity Sensor 3rd Generation • "Standard" MAVS3 • with CTD option

12. Memory • TT8 • Flash (ROM) SCP • RAM Run-time • EEPROM Configuration Deployment Definition • MAVS • Flash Card Data Configuration Deployment Definition

13. Net Relative Velocity, Tilt, Heading HEADING COD END DIRECTION OF TOW

14. MAVS Specifications • Sensitivity and linearity • 0.05 cm/s sensitivity, 0.5% linearity (corrected to that in 2003)

15. MAVS Specifications • Velocity zero offset stability • 0.5 cm/s per deployment, no net drift with time • Demonstrated sensitivity to pressure and temperature on deployment in gel to 2000 m

16. MAVS Specifications • Cosine response • Horizontal cosine response within 5% through 360° • Vertical cosine response within 10% to 30° and within 20% to 45° • On-axis response is 20% low but turbulence is minimum • Tow tank shows test run with sensor at +45°

17. MAVS Specifications • Compass • Three magneto inductive axes in orthogonal array • Tilt measured with two axis solid state accelerometer for resolution of earth magnetic field onto horizontal plane • Second socket for remounting of tilt sensor for horizontal deployment of MAVS

18. MAVS Specifications • Tilt • Linearity within 1° out to 50° • Temperature compensated to 0.3° of tilt between 5°C and 45°C • Works equally well inverted or on 90° bracket for horizontal mounting

19. MAVS Specifications • Toughness • Carried by submarine landslide 550 m down Monterey Canyon. Still recording pressure although sensor rings sheared off and tube bent 90°. • Fouling sensitivity • Stops measuring only when flow is totally blocked as in illustration • Insensitive to particles, bubbles, or perfectly clear water. Only hard obstruction or swim bladder stops acoustic signal

20. MAVS Carried 550 m in Monterey Canyon Landslide and Buried Alive • MAVS on MBARI Frame with Homer acoustic beacon • ROV manipulated sand lifter exhuming frame • Upper portion of MAVS exposed • MAVS on deck, sensor stripped of rings and bent 90°

21. MAVS Operation • Setup with PC and deck box: Menu mode • Run a terminal emulator: Crosscut, Hyperterm, TTOOLS, Kermit, etc. • Connect through the endcap to the deck box, which provides power and a DB9 connector for the PC’s RS232 serial comport • Configure the comport for 38,400 baud, 8 bits, no parity • Turn on power and type Control C

22. MAVS Operation • Setup with PC and deck box: MAVSoft mode • Bring up MAVSoft on the PC • Connect through the endcap to the deck box, which provides power and a DB9 connector for the PC’s RS232 serial comport • Turn on power

23. MAVS Operation • Set time: Menu mode • When screen says “Change time & date?” type y <enter> • Type month_day_year_hour_minute_second <enter> • Set time: MAVSoft Mode • Push button “SET TIME” • Push button “Send PC Time to MAVS”

24. MAVS Operation • Calibration: Menu mode, select Calibration • Select Velocity Offsets: V <enter> • Place sensor in still water • Type M, <enter>, <enter> • Accept if STDV values are less than 0.15 cm/s • Select Compass Offsets: C <enter> • Follow instructions • Select Tilt Offsets: T <enter> • Follow instructions • Save constants and exit • Type x, <enter>

25. MAVS Operation • Calibration: MAVSoft mode, select MavsTerm • Select Calibration: 3 <enter> • Select Velocity: V <enter> • Place sensor in still water • Type M, <enter>, <enter> • Accept if STDV values are less than 0.15 cm/s • Select Compass Offsets: C <enter> • Follow instructions • Select Tilt Offsets: T <enter> • Follow instructions • Save constants and exit • Type x, <enter>

26. MAVS Operation • Deploy: Menu mode, select Deploy • Frame, select MAVS, earth Cartesian, or earth speed and direction • Logging enabled as append or overwrite • Beware of overwriting data before reading it but beware of starting with a partially filled compact flash • Measurement frequency, number of measurements per sample • Samples per burst, burst interval • Delayed start (Start Time), duration of deployment • G <enter> to get analysis of battery and data storage duration • There are opportunities to revise deployment parameters

27. MAVS Operation • Deploy: MAVSoft mode, from Control Panel select Parameters, press UPLOAD • Select frame (MAVS, earth Cartesian, or earth speed and direction) • Enter measurement rate, number of measurements per sample, samples per burst • Set burst interval and bursts per file • Establish the deployment schedule • Select Enable Logging to Flash Card and check append or uncheck for overwrite • Press DOWNLOAD to put these parameters into MAVS • In Control Panel push DEPLOY • As data appear, select START LOGGING and name file for capturing data on the screen.

28. MAVS Operation • Offload: Menu mode • Control C to get control (may need to do this three times if MAVS is in a Low Power Sleep) • 7 <enter> • Capture conversation with terminal (alt z in Crosscut) • Offload: MAVSoft mode • Select Offload • Name file to which the data will be captured

29. MAVS Operations • Flash Card Ops: Menu mode • Control C to get control • 2 <enter> • Note data files with selection 2 • Data files contain data • Config.bin, Deploy.bin, and f_count.bin are required for unpacking data • Select 4 <enter>, open MAVS case, and remove compact flash card • Copy card contents to PC with Microsoft copy command. Then return compact flash to MAVS and type <enter> again.

30. MAVS Data Analysis • MAVSPack • Run MAVSPack selecting the folder into which the contents of the compact flash were copied. This converts from binary files to ASCII data files and ASCII configuration and deploy files. • Plot • Load into Excel • Select columns for time, u, v, w and other ancillary parameters. Time may be generated from hours, minutes, and seconds as fractional hours • Select plot as scatter plot • Or load into Matlab • Generate m files for data manipulation and plotting

31. MAVS Deployment Choices • Low energy flow • Long deployments, infrequent samples • Wave “interference” • Average over wave or mooring motion, measurement frequency high enough to resolve motion • Wave spectra • Measurement frequency high enough to resolve highest frequency wave • Burst long enough to resolve wave bands • Burst interval short enough to resolve change in wave climate • Turbulence • Highest frequency to resolve turbulent eddies, burst to extend duration of deployment

32. Batteries for MAVS • Basic alkaline • 4.8 ah at 12 volts • Lithium • 9.6 ah at 12 volts • Double lithium • 19.2 ah at 12 volts • Power consumption is reduced by allowing time between bursts so MAVS can go into low power sleep

33. Data Storage for MAVS • Compact flash memory • 16 MByte CF • 64 MByte CF • 256 MByte CF • Other capacities available to 1 GByte or more • Industrial rating required for temperature range of application

34. Real Time MAVS • RS232 output • 100 foot limit • RS485 output • Half duplex, not supported by MAVSoft • More than 1000 feet possible • Power source • 12 volt low impedance (must provide 2 amps for 15 ms each measurement) • 24 volt with internal regulator, typically 60 ma

35. Ancillary Sensors for MAVS • Pressure • Omega or Druck, 20 meters to 6000 meters full scale range • External temperature • Precision of 0.03°C and accuracy of 0.1°C • Internal temperature standard with precision of 0.03°C and accuracy of 0.5°C • Conductivity • Aanderaa inductive head, range 0 to 75, resolution 0.02, and accuracy 0.2 mS/cm • Optical turbidity • OBS, LBSS, or SeaPoint • Analog voltage from external source

36. Analog Output MAVS • Integration with existing data systems • Output voltage range +/- 5 v for +/- 180 cm/s • External strobe possible • Calibration signals provided

37. Other Specials for MAVS • Bent sensor • 90° right or left bend of sensor from body • 90° up or down bend from body • Inverted sensor, vertical up or down • With solid state compass and tilt meter, this is standard, selectable by user from the setup menu • Horizontal sensor • Internal tilt sensor must be moved from vertical socket to horizontal socket and the change entered in the setup menu

38. Setup Menu for MAVS • Configuration parameters may be changed in the setup menu, accessible with a password to prevent accidental entries • S <enter> WHIPR <enter> gives entry • Orientation can be selected • Ancillary sensors can be disabled or enabled • X <enter> stores new configuration

39. Downloading New Versions of MAVS Firmware • Enter TOM8 monitor • Type q <enter> nobska <enter> • Change communications setup for a baud rate of 9600 • Download and open new distribution software, MAVSxy_z.zip • MPREPxyz.rhx and MAVSxy_z.ahx is the new software release to load to MAVS • In Crosscut select Tattletale, Load S-record, and MPREPxyz.rhx • Type G, select previous version from the menu • Next load MAVSxyz.ahx • Accept the burn to flash memory • Type G • Change baud rate back to 38,400