file: asimet_general.html
28 Feb 2008

ASIMET General Information

The ASIMET suite of instruments are an evolution based on the IMET modules developed for the WOCE (World Ocean Circulation Experiment) program. The suite consists of seven modules (including 2 wind module options). Current production (as of 2008) uses Ver 4.x firmware and optional Compact Flash storage.

A low power data acquisition computer is used on buoys and various MS-DOS PC computers are used on ships. An ARGOS / GPS unit is recommended and can be packaged with the data acquisition computer. Sensors, configurations, and radiation shields were extensively tested under the IMET program and are maintained or improved for the ASIMET suite. For ship systems, a low-speed acoustic modem is used for relaying sea surface temperature data from deep inside the ship to a central data logger.

WHOI provides calibration services to the community and a standard price list is available. At the present time, calibration is recommended every 6 to 8 months. When there is a good history on drift from several calibration cycles for each module, this recommendation may be modified.


General Module Description

Housing material on all modules is Grade2 titanium with an underwater design (o-ring seals) at the end caps. There is a closure bolt at the bottom end cap for access to the electronics. Interconnection is made by underwater pluggable connectors and polyurethane cables. Mounting brackets are available with 102 mm (4") fiberglass channel and spring clips for easy attachment. The fiberglass channel can be attached to other structures in various ways.

The standard meteorological sensor(s) interface to a front end board that in turn communicates with the C530 logger board. The C530 board can receive power either from internal batteries or an external source. It communicates with the outside world using an IMET communications protocol in either RS232 or RS485 (9600, N, 8, 1). The C530 board has provision for a 8 megabyte PCMCIA flash memory card to store data, with Ver 3.x firmware. Current production units handle a Compact Flash card in a Sandisk SDAD-38 adapter; any size Compact Flash card larger than 8 MB may be used (as of early 2008, the smallest easily available cards are probably 256MB). In a standalone configuration, the unit operates on internal batteries and stores data on a flash card. In a system configuration, the unit receives external power, communicates with a central data logger and does not have a flash memory card installed.

All of the sensor specific interface takes place in a front end board with A/Ds, serial ports, amplifiers, etc., being controlled by a microprocessor. There is also an EEPROM for storing calibration constants and any other information of use. Except for WND, all calibration and conversion to engineering units take place in the sensor front end board. The front end board gets power from and has two way RS485 communications (9600, N, 8, 1) with the C530 board. The basic measurement would typically be a 5 second scalar wind speed and direction for the WND unit or one sample of relative humidity and air temperature for the HRH. A typical current consumption for a front end board is a few milliamps at 12 vdc. The current requirements from the sensors are then added to this. Wherever possible the sensors are powered up in a duty cycle for minimum average power.

In the case of the WND unit, the C530 board then uses this basic measurement in combination with the calibration data and the proper algorithm for the unit and provides data in ASCII engineering units averaged over a one minute sample period. An external data logger must interrogate the C530 once per minute to get a full data set. Each one minute data set is stored in RAM and then a one hour record is stored in the flash card memory if a card is installed. Communications with the C530 board is the standard IMET communications protocol as described in the ASIMET Module Operations document.

Typical Operation

An external data logger would typically transmit in serial ASCII an address and command i.e. #WND01C. The module would reply to this with the calibrated data stored in the module buffer. This buffer is updated once per minute with new data. The data is also serial ASCII i.e. from the wind command: 2.25, 3.55, 4.2, 7.2, 0.5, 010.4, 022.0, 5.1, 7.2 where 2.25 (meters/sec) is vector averaged wind toward the east, 3.55 (m/s) is vector averaged wind toward the north, 4.2 (m/s) is the scalar average wind speed, 7.2 (m/s)is the max 5 sec wind speed, 0.5 (m/s) is the min 5 sec wind speed, 010.4 (deg) is the last vane, 022.0 (deg) is the last compass, 5.1 (deg) is the average tilt around the X axis, and 7.2 (deg) is the average tilt around the Y axis. Other commands include a help menu, command #WND01H, that lists all of the commands available in the module, a list command, #WND01L, that returns the module address, serial number, software version, calibration date and calibration constants. This is most helpful for collecting a configuration file on the modules installed and storing that file with the resulting data files.

Flexible Configuration

The C530 board can communicate in either RS232 or RS485 by setting a jumper and moving the power/comm cables before power up (see mode configuration in the Module Operations document). RS232 is used for single port single module configurations and also for bench operation with a laptop while RS485 may be used for multiple modules on one bus.

This configuration provides a very flexible approach to data collection. The advantage of the IMET design is this addressable digital data bus and the feature of having calibration constants stored in a module for ease of keeping the calibration constants with the data. Because of the addressable digital data bus concept, the number and types of modules can be highly variable without any hardware changes. Note that RS485 permits up to 32 ports on one transmission line without a repeater and if repeaters are used, the module suite can be very large.


For further information...

WHOI DGE (WHOI Design Group for Electronics) maintains a web home page with the latest versions of documentation for ASIMET (and other instrumentation). This web page is: http://frodo.whoi.edu/asimet/