Page 249 - Instrumentation Reference Book 3E

Fieldbus function and benefits 233
concerned the electronics rather than the primary loaded with Device Description files (DD) that
sensor. describe the devices in the field (or GSD in case of
However, based on years of experience and Profibus), a single tool such as the Smar SYSC-
study of sensor failure modes, some manufac- CON software can configure a plethora of devices
turers now offer transmitters that diagnose not from different manufacturers without inter-
only themselves but also the sensor. An added operability problems. An additional advantage
advantage of the FOUNDATION Fieldbus is can be achieved using FOUNDATIONTM Field-
that it not only specifies how to present the status bus. Because the control strategy programming
to the user, but the function block programming language is part of the same specification as the
language has also been designed to provide auto- communications for device configuration and
matic shutdown action in case of sensor failure. maintenance, strategy building and device man-
Particularly interesting is the development seen in agement, as well as network management, can be
valve positioners over the past few years. For performed from the same single software tool. In
example, the Smar FY302 pneumatic valve pos- the case of the Smar SYSCON software this
itioner performs diagnostics of itself, the actuator means that users can drag and drop from the
and the valve. thereby providing the operator strategy directly into the devices.
with status for the entire valve package.
13.12.7 Measuring directives
13.12.5 Plant diagnostics
It is likely that many process measurements
A considerable amount of potentially useful which at present are not subject to weights and
information regarding the process, beyond the measures surveillance will become so in future.
value of the process parameter being measured. Directives requiring further integration of diag-
is available at the interface between the process nostic routines into these instruments, data back-
and the primary sensor. This information can up, etc., are to be expected.
be recovered by spectral and statistical analysis
of the measurement signal if the frequency
response of the transmitter is adequate. In 13.12.8 Further developments of intelligent
transniitters
effect, it provides a window through which the
status of the associated process can be assessed The accuracy and versatility achieved by the pre-
and changes detected to provide warning of sent generation of microprocessor-based and
incipient failures. However, as most of the intelligent transmitters is such that there is little
transmitters at present in use have been real advantage to be gained from further
designed to operate in a process control loop, improvement in these respects. For temperature
and as it is generally accepted that there is no and differential pressure transmitters where
useful information in those components of the accuracies of 0.1 percent of the measured value
measurement signal having frequencies greater are claimed, the point has been reached where it is
than about 5Hz, the amount of useful informa- necessary to question whether the transmitted
tion which can be recovered using currently signal has the significance implied by the number
available transmitters is very restricted. How- of digits in the displayed value.
ever, devices that locally sample and cache For most process measurements, the noise
information at a higher speed during the test component can range from 1 percent to 30 percent
phase and then communicate it at a more lei- of the mean value and the damping applied may
surely pace are already on the drawing board. be anything from 1 to 60 s. It is then necessary to
consider how realistically the displayed value
represents the process variable.
13.12.6 Hand-held interfaces (hand-held
terminals or hand-held communicators) The focus has now shifted towards the valve
positioners that until now have been contributing
Using fieldbus technology there is an unbroken significantly to significant process variability.
digital chain from the fieid instruments to the Until recently positioners. due to their imprecise
operator workstation. Therefore it is possible to mechanics and simple analog electronics, have
diagnose, configure, and otherwise interrogate a been unable to respond to demands in position
device from the computer without having to ven- change by Iess than a few percent. Modern pos-
ture into the field with a hand-held. There is no itioners like the Smar FY302 use a non-contact
need to find the right connection point at the position sensing method eliminating mechanical
junction box. The workstation runs the engineer- linkages, and perform all characterization and
ing and maintenance application that allows the positioning algorithms in software, thereby being
user to configure and diagnose the devices, and able to modulate with a precision of a fraction of
manage maintenance. Because these tools can be a percent.