Page 326 - Programmable Logic Controllers, Fifth Edition - Mobile version

Cutter
Sensor
Stock

PLC
Opto
module Leadscrew

Motor

Operator interface
Figure 14-4 Individual control.

Figure 14-5 Centralized control.
Source: Courtesy Siemens.

Centralized control is used when several machines The main features of a distributive control system can be
or processes are controlled by one central controller. summarized as follows:
The control layout uses a single, large control system • Distributive control permits the distribution of the
to control many diverse manufacturing processes and processing tasks among several controllers.
operations, as illustrated in Figure 14-5. The main
features of centralized control can be summarized as • Each PLC controls its associated machine or
follows: process.
• High-speed communication among the comput-
• All individual steps in the manufacturing process ers is done through CAT-5 or CAT-6 twisted pair
are handled by a central control system controller. wires, single coaxial cables, fiber optics, or the
• No exchange of controller status or data is sent to Ethernet.
other controllers. • Distributive control drastically reduces field wir-
• If the main controller fails, the whole process stops. ing and heightens performance because it places
A distributive control system (DCS) is a network- the controller and I/O close to the machine process
based system. Distributive control involves two or more being controlled.
PLCs communicating with each other to accomplish the • Depending on the process, one PLC failure would
complete control task, as illustrated in Figure 14-6. Each not necessarily halt the complete process.
PLC controls different processes locally and the PLCs are • DCS is supervised by a host computer that may
constantly exchanging information through the commu- perform monitoring/supervising functions such as
nications link and reporting on the status of the process. report generation and storage of data.

Process Control, Network Systems, and SCADA Chapter 14 307

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