MAVS Current Measurement

Nobska Development, Inc.Albert J. Williams 3rd

Archie Todd Morrison III

Current Measurements

Mechanical Sensors Electro-Magnetic Sensors Doppler Sensors

LDV Acoustic Doppler Acoustic Doppler Profiling

Acoustic Travel-Time Sensors

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

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

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

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

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

Acoustic Travel-Time Sensors

uc

LtLR +

=uc

LtRL −

=

u

c + u c - u

L

tcu

c

Lu

uc

Lut

2

22 2

222

∆⋅=⇒≈−

=∆

L

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

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

Nobska - MAVS3Modular Acoustic Velocity Sensor

3rd Generation

"Standard" MAVS3 with CTD option

Nobska

Memory

TT8 Flash (ROM) SCP RAM Run-time EEPROM Configuration

Deployment Definition MAVS

Flash Card DataConfigurationDeployment Definition

Net Relative Velocity, Tilt, Heading

HEADING

DIRECTION OF TOW

COD END

MAVS Specifications Sensitivity and linearity

0.05 cm/s sensitivity, 0.5% linearity (corrected to that in 2003)

-20 0 20 40 60 80 100 120 140 160-50

0

50

100

150

200MAVS3 10104 Red - original transistors, Green - 2N2369A, Blue - 2N2369A without capacitors

Delay [ns]

Velo

city

readin

g [

cm/s

]

-20 0 20 40 60 80 100 120 140 160 180-2

-1

0

1

2

3

Velcity reading [cm/s]

Velo

city

err

or

[cm

/s]

-20 0 20 40 60 80 100 120 140 160 180-30

-20

-10

0

10

20

Velcity reading [cm/s]

Err

or

[%]

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

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1Filtered Velocity, Averaging Length: 100

Record Number

cm/s

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°

Vertical Cosine Response

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-90 -70 -50 -30 -10 10 30 50 70 90

Elevation Angle

Co

rre

cte

d s

pe

ed

re

ad

ing

/to

w s

pe

ed

-30%

-25%

-20%

-15%

-10%

-5%

0%

5%

10%

15%

Pe

rce

nt

err

or

Scale Error

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

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

Tilt vs Pitch Reading

0

10

20

30

40

50

60

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Tilt degrees

Pit

ch

de

gre

es

Pitch

Tilt1005 Inverted, Corrected

-1

-0.5

0

0.5

1

1.5

2

5 15 25 35 45

Celsius

De

gre

es

of

tilt

PitchRoll

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

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°

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

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

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”

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>

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>

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

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.

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

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.

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

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

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

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

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

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

Analog Output MAVS

Integration with existing data systems Output voltage range +/- 5 v for +/- 180

cm/s External strobe possible Calibration signals provided

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

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

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