Principle Of Operation - ABB 268D/V Operating Instruction

PRINCIPLE OF OPERATION

Resin potting
Inductance Coils
& Magnetic
Cores
Capillary tubing
Isolating
diaphragm
Process
chamber
Fig. 2a - Primary Unit
The instrument consists of two functional units:
- Primary Unit
- Secondary Unit
The Primary Unit includes the process interface, the sensor
and the Primary electronics; the Secondary Unit includes the
electronics, the terminal block and the housing. The two units
are mechanically coupled by a threaded joint. Electronics are
based on custom integrated components (Application Specific
Integrated Circuit - ASIC).
Output meter
(option)
Primary Electronics
Printed Circuit
Sensor Diaphragm
with Ferrite Disks
Capillary tubing
Isolating
diaphragm
Process
chamber
Surge protector
(option)
Terminal
block
Housing
Fig. 2b - Secondary Unit
The principle of operation of the Primary Unit is as follows. The
process fluid ( liquid, gas or vapour ) exerts pressure on to the
sensor diaphragm via flexible, corrosion-resistant isolating
diaphragms and capillary tubing containing the fill fluid (see
Fig. 2a).
As the sensor diaphragm deflects in response to differential
pressure changes, it simultaneously produces variations in the
gap between two fixed magnetic circuits (comprising coil and
ferrite core) positioned on both sides of the measuring
diaphragm. As a result, the inductance of each coil changes.
The two inductance values L1 and L2, and the sensor
temperature ST are combined in the primary electronics to
provide a proprietary standardized signal.
In the manufacturing process the sensor output characteristics
are compared with reference pressures and temperatures: the
"mapped" parameters are then stored in the memory of Primary
electronics.
The measured values and the sensor parameters are transferred
to the Secondary Unit, where a microprocessor computes
precise primary output linearisation, compensating for the
combined effects of sensor non linearity, of static pressure and
temperature changes. In the permanent memory of the
secondary electronics are stored the transmitter
information:
- non modifiable data such as the serial number, the UID
(Unique Identifier), the manufacturer's name and device type,
the hardware and software version of the electronics.
- the modifiable data such as the final trimming and calibration,
in other words, all data that can be changed by the user
through the configuration devices.
External Zero/Span
adjustments
RFI filter
Electronics
specific
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