Single And Multi-Channel Operation; Theory Of Operation - Geokon 8020-59 Instruction Manual

Vibrating wire to analog converter

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4. SINGLE AND MULTI-CHANNEL OPERATION

In addition to working as a stand-alone device to interface a single transducer to a Data

Acquisition System (DAS), multiple channels may be configured in similar fashion to the Single

Channel. The 8020-59 works in conjunction with the 8032 Multiplexer and the user's Data

Acquisition System to select either 1 of 16 or 1 of 32 transducers.

The 8020-59 was developed with multiplexing in mind and uses the same control signals as the

Geokon model 8032 Vibrating Wire Multiplexer (for control signal timing requirements, refer to

the model 8032 Multiplexer Instruction Manual). With a simple control program executing on

the host DAS, it is capable of working with 16 vibrating wire transducers (with thermistors), or

32 transducers (without thermistors). All setup parameters for each channel are stored in internal

EEPROM memory, requiring no backup battery and can be retained for years.

The 8020-59 is powered from either a 12V or 24V supply (nominal), and draws approximately

70mA when taking readings. When it is "shut-off", or between scans when multiplexing, the

total current consumption of the 8020-59 is less than 10μA (12V) or 16.5mA (24V).

The 8020-59 setup and configuration utility runs on a PC operating under the Windows operating

system and communicates to the 8020-59 via the 8020-59's RS-232 port. A "Command Line"

interface is provided in the 8020-59 internal firmware, with a command set that allows all

functions to be easily set up and calibrated.

The 8020-59 provides excitation for a VW transducer using a swept frequency pluck that is

optimized for the selected gage type. The period of the resulting VW signal is measured using a

25 MHz clock. This measurement is performed 512 times, resulting in an average period being

stored in internal memory. The microprocessor then converts the average period into digits and

scales the output accordingly, for the specific transducer being read. This information is then sent

to a 20-bit D/A converter, which outputs a voltage (0-5V) that is directly proportional to the

average digits. Post-processing circuitry converts this voltage to a non-isolated 4-20mA current

signal. In addition, the 20-bit D/A is put through a self-calibration routine at the start of each

measurement cycle to minimize errors due to temperature and power supply variations. The

resulting 0-5V and 4-20mA signals are brought to the output connector for connection to the

input(s) of the host DAS.

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