Programmable logic controller/distributed control

The basic characteristics of alarm systems have already been covered at length in Chapter IX. Since an alarm is one of the most fundamental issues in a control system, it will be discussed here again in this context. In this clause, focus will be on independent alarm systems that can be applied to critical applications independently of a programmable logic controller/distributed control system (PLC/DCS). In spite of so much technological developments and modem instrumentation and... 

Figure 8-62 depicts a hypothetical distributed control system. A number of different unit configurations are illustrated. This system consists of many commonly used DCS components, including multiplexers (MUXs), single/multiple-loop controllers, programmable logic controllers (PLCs), and smart devices. A typical system includes the following elements as well ... 

The component controllers used in the controller subsystem portion of the DCS can be of various types and include multiloop controllers, programmable logic controllers, personal computer controllers, singleloop controllers, and fieldbus controllers. The type of elec tronic con-troUer utihzed depends on the size and func tional characteristic of the process apphcation being controlled. See the earlier section on distributed control systems. 

Stand-alone systems. Multiloop controllers) or programmable logic controllers (PLC) typically used to control part of a process, and larger supervisory control and data acquisition (SCADA) systems/distributed control systems (DCS) used to control the process or service as a whole (e.g., bulk primary production plant, building management systems). These self-contained systems are a component of an automated manu-... 

Control System Included in this classification are Supervisory Control and Data Acquisition Systems (SCADA), Distributed Control Systems (DCS), Statistical Process Control systems (SPC), Programmable Logic Controllers (PLCs), intelligent electronic devices, and computer systems that control manufacturing equipment or receive data directly from manufacturing equipment PLCs. 

There is no single, clear definition of a Distributed Control System. However, the attribute of having distributed processing capacity is a good focal point. The processing capacity is not constrained to traditional DCS controllers and may, for example, be executed within Programmable Logic Controllers (PLCs). 

OS/2-based computers that provide the user interface to the application (e.g., display operating instructions, mimics and alarms/messages to the operators). About 100 workstations are distributed on the shop floor and connect to equipment such as scales and Programmable Logic Controllers (PLCs). 

Many of the controls are sequential and on-off thus, the control philosophy is particularly amenable to PLC (Programmable Logic Controller), PC (Personal Computer), or Distributive Microprocessor Controls. Proportioning controls can be easily incorporated into such a system. A Graphic Panel Display with digital indicating and recording modes will add clarity for the operators of the plant. 

Automated Computer controlled usually through a personal computer or programmable logic controller. Less frequently by a central control computer or as part of a distributed control system Better repeatability Better process control Lower operating labor costs Data gathering automatic although some analytical instrumentation may remain off line Some to all routine operations may be automated... 

Process safety interlocks can be used to prevent ttnd/or mitigate tin upset condition. Salety interlocks consist of outputs from process control systems, such as programmable logic controllers or distributed control systems, which trigger tut action designed to compensate I ot an upset condition and thus avoid an accident even in the e em of human or computer error. An example of a salety interlock would be the introduction of full cooling if the temperature of a reaction exceeded a safe level. 

To date, process control systems like Programmable Logic Controllers (PLCs) have only enabled paperless operation in combination with SCADA (Supervisory Control and Data Acquisition) systems or as part of a DCS (Distributed Control System), which enable measurement and control actions to be recorded and used as part of batch documentation. Process control systems have the advantage that they focus on real-time data as a necessary part of both control and supervision. The real-time focus is very useful for implementing both active and proactive control when combined with, for example, statistical tools or predictive algorithms. 

PLC = Programmable Logic Controller DCS = Distributed Control System HW = hardware SW = software... 

DCS (distributed control system) PLC (programmable logic controller)... 

NOTE This data represents typical programmable logic controllers and distributed control systems. Lower Allure rates can be achieved by BPCS logic solvers that meet the requirements of lEC 61508 or ANSI/ISA-84.00.01-2004-1, Clause 11.5. 

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