Page 239 - Wastewater Solids Incineration Systems

Instrumentation and Control 201

improvements allowed devices to randomly talk (broadcast) on a piece of wire, with an
identified talker and identified target (listener) but was heard by many.
4.3.2 Distributed Control System
A DCS was similar to a DCC in that a central computer or mainframe still performed
process calculations. But instead of directly controlling an element, it transmitted set-
points, or parameters, for the desired action to the hardware that would operate or
move a valve or other device. Therefore, it exerted “supervisory control” rather than
“direct control.” The “local” hardware was intelligent enough to control the actions but
needed oversight for the process set points. As with the DDC systems, communications
were encoded onto wire pairs through MUX/deMUX. The remote field units were
known as remote terminal units (RTUs) and responded to supervisory signals from
master terminal units (MTUs). The RTUs at their inception were capable of only primi-
tive ladder control: to turn on an output (for example, a pump) and recognize a state of
output and communicate that back to the MTU. As the price of electronics has plum-
meted, field devices have grown increasingly “smart” to the point that they rival the
level of sophistication found only in a mainframe computer a few decades ago.

4.4 Supervisory Control and Data Acquisition

In the water and wastewater treatment industries, the term SCADA has been used
most often to refer to a system that communicates by radio, performs low-level con-
trol such as controlling pumping stations, and communicates back liquid levels of
elevated water towers and other similar control processes. A central control room
interface is typically provided to display an overview of the status of the controlled
system, known as the human-machine interface, or man-machine interface. Today’s
radio and computer systems have replaced early hard-wired systems that typically
controlled a primitive “light board” to display status conditions.
Simply restated, SCADA “talks” to various remote locations from a central loca-
tion and relays a command or request that an operator enters at a console. The
SCADA listens for data reports from those remote locations and, typically, enters this
information to a database. It may include the capabilities to “playback” this informa-
tion on chart form or output data in printed tables or electronic file form.

5.0 INSTRUMENTS IN BIOENERGY PROCESSES

Useful and intelligent systems are designed from the top down but constructed
from the bottom up. First, sensors and instruments, such as gauges, are applied to