Page 219 - Instrumentation Reference Book 3E

Attributes and features of microprocessor-based and intelligent transmitters 203

zero and span, accessing data regarding the appli- advantages include the fact that the measurement
cation, installed position, and service history can signal is not affected by changes in the loop resist-
be implemented. The term “intelligent” has come ance, transmitters from one manufacturer can be
into use to identify transmitters with such facilities. interchanged with those from another and, within
The concept of transmitters which not only quite wide limits, individual instruments such as
provide a measurement signal but are also cap- alarms and indicators can be added without
able of two-way communication over the same affecting the accuracy of measurement. Further-
circuit raised two issues, namely: (i) the need for more, the power needed to energize the sensor
a standard which will enable transmitters from and the signal conditioning circuits is provided
different manufacturers to be interchanged (as is within the live zero. However, the only informa-
possible with the old 4-20 mA transmission stand- tion that can be communicated is the measure-
ard); and (ii) the need for a standard which meiit signal, which is transmitted from the
enables the enhanced capabilities to be exploited. transmitter to the center or control room.
It also led to the possibility of replacing the indi- A dominant influence in the evolution of pro-
vidual circuits between transmitters and control cess measurement systems is the need to reduce the
rooms with a data highway. The term “fieldbus” cost of ownership, Le., the cost of the transmitter
has come into use to describe any form of data itself plus the cost of installing and commissioning
highway which supports communication between it in the plant and subsequently maintaining it.
field devices and a control room. Developments in the basic sensing elements have
A fieldbus has many advantages over the con- exploited CAD methods to enhance silicon strain
ventional 4-20 mA system, the most immediate of gauge technologies and the microfabrication of
which are the reduced cost of wiring. commission- both capacitance and resonant sensing elements.
ing, and maintenance, as well as increased versatil- These have progressed in parallel with develop-
ity and enhanced functionality. Work started in ments in solid state electronics, particularly the
1985 to develop a single international standard advances in low power microprocessors and ana-
but, in the intervening period, proprietary and log-to-digital converters. A further way of reducing
national standards were developed which are the cost of manufacture is to reduce the weight and
now competing to become the accepted industry size of the device. If the transmitters which were
standard. In spite of this confused situation, the generally available a decade ago are compared with
introduction of fieldbuses is now having a dra- those currently available, the improvements which
matic effect on the control industry as advances have been achieved are significant.
in adjacent technologies, such as signal processing,
neural networks, fuzzy logic, and sensor arrays.
improve the power and versatility of measurement 13.4 Attributes and features of
and control systems. One of the first field comniu- microprocessor-based an
nication protocols to be successfully exploited intelligent transmitters
commercially was the HART Field Communication
Protocol, supported by the HART Communication Attributes and features which are provided in
Foundation, an outline descriptioii of which microprocessor-based and intelligent transmitters
is given in Section 13.10.4. are described hereafter, but the detailed imple-
One ofthe many problems which has existed at
large process plants is the variety of transmitters mentations may differ from one manufacturer to
another as well as from one type of transmitter
needed to meet the operational requirements and to another, and some of the features may not
the costby inventory of complete instruments as
well as spare components which have to be car- be provided. Most of the intelligent features or
attributes can be addressed from an appropriate
ried. This is due in part to the fact that, hitherto,
most of the transmitters were analog devices and hand-held terminal (sometimes described as a
hand-held communicator or a hand-held inter-
their basic design was such that the range of span face) or from a suitably equipped control room or
adjustment was limited, so that many different computer system although, in some instances, the
primary sensors were required. The introduction communication can be effected via a locally
of a secondary sensor and a microprocessor sys- mounted operator interface.
tem has greatly increased rangeability and accur-
acy of transmitters so that, for pressure or
differential pressure measurements, the oper- 13.4.1 Microprocessor-based features
ational requirements in most process plants can 13.4.1.1 Linearization, characterization, arid
be covered by two transmitters of each type. correction of the priniary Sensor characteristic
The 4-20 mA transmission standard provided
an excellent basis for the development of process The presence of memory and computing power at
instrumentation during the past three decades. Its the transmitter permits the signal to be conditioned