Page 216 - Instrumentation Reference Book 3E

I3 Microprocessor-based and

intelligent transmitters

E. H. HIGHAM and JONAS BERGE

13.1 Introduction corrected, and (ii) by enabling a secondary sensor
to be included, so that secondary effects on the
The evolution in the design of transmitters has primary sensor can be compensated. To differ-
been influenced, on the one hand, by the require- entiate the conventional transmitters (Figure
ments of users for improved performance coupled 13.1) from those in which corrections are applied
with reduced cost of ownership and, on the other, to the primary sensor signal, using a microproces-
by developments which have taken place in adja- sor to process information which is embedded in
cent technologies such as computer aided design memory (Figure 13.2), or those in which a micro-
(CAD), microelectronics, materials science, and processor is used in conjunction with a secondary
communication technologies. The most signifi- sensor to derive corrections for the primary sensor
cant advances have resulted from the emergence signal (Figure 13.3), the term “microprocessor-
of low power microprocessors and analog-to-digi- based” has come into use.
tal converters which, in conjunction with the basic The fact that a microprocessor can be incorpor-
sensor circuits, can function on the limited power ated in a transmitter has also provided an oppor-
(typically less than 40 mW) available at the trans- tunity to move from a regime in which only the
mitter in a conventional 4-20 mA measurement measurement signal is transferred from the trans-
circuit. This has provided two distinct routes for mitter to a receiver, such as an indicator or con-
improving the performance of transmitters: (i) by troller, to one in which the microprocessor
enabling non-linear sensor characteristics to be not only implements the microprocessor-based

I
0 Controller
0
0
Transmitter
0
0
8 Power
I supply
Figure 13.1 A conventional 4-20 mAtransmitter circuit

Sensor b A/D Micro- DIA 4-20 mA
converter processor __e
converter measureLent
circuit