Logic controllers, programmable

Programmable logic controllers (PLC) often seem a logical choice for safety instrumented system logic solvers. After all, the PLC was designed 

In spite of these advantages, the failure rates and failure modes of typical PLC designs will allow these devices to reach only SIL 1 (see Example 10-2) if that. Of course, the higher quality products are more likely to have safety class diagnostics. Most manufacturers of these product lines are getting lEC 61508 (Ref. 1) assessments after which the product is known as a safety PLC.  

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Stand-alone computer systems, usually based on a personal computer (PC) or programmable logic controller (PLC), provide a separate computer system for each pilot plant. This allows for economical expansion for new units, separates pilot plants completely for maintenance and troubleshooting, and often has the lowest initial cost. Standardization can be a problem and software control, data gathering, and storage packages can be limited in size, scope, and capabiUty these are usually acceptable trade-offs. 

PCC = process control computer PLC = programmable logic controller and PGCS = process gas chromatograph system. 

Discrete logic. Sequence logic can be implemented via ladder logic, and this approach is common when sequence logic is implemented in programmable logic controllers (PLCs). 

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. 

Programmable Logic Controllers The programmable logic controller (PLC) originated as a solid-state replacement for the hardwired relay control panel and was first used in the automotive indus-tiy for discrete manufacturing control. Today, PLCs are used to implement Boolean logic functions, timers, counters, and some math functions and PID control. PLCs are often used with on/off process control valves. 

The potential that the logic within the interlock could contain a defect or bug is a strong incentive to keep it simple. Within process plants, most interlocks are implemented with discrete logic, which means either hard-wired elec tromechauical devices or programmable logic controllers. 

The average prices of the batch centrifuge are shown in Fig. 18-154. All the models include the drive motor and control. In Fig. 18-154, the inverting filter, horizontal peeler, and the advanced vertical peeler are the premium baskets especially used for specialty chemicals and pharmaceuticals. Control versatility with the use of programmable logic control (PLC), automation, and cake-heel removal are the key features which are responsible for the higher price. The underdriven, top-driven, and pendulum baskets are less expensive with fewer features. 

Part 5 (Draft) Programmable logic controllers for safety applications (August 1998) VDIA DE 3542, Safety terms for automation systems. 

Programmable Logic Controller (PLC) A microcomputer-based solid-state control system which receives inputs from user-supplied control devices such as switches and sensors, implements them in a precise pattern determined by instructions stored in the PLC memory, and provides outputs for control or user-supplied devices such as relays and motor starters. 

PLC - Programmable logic controller (the registered trade mark of Allen Bradley Co. Inc., USA). 

Moderate to microproces.sor-based, fully programmable logic controls and feedback control systems are available, to provide smooth speed controls as good as a.c. drives... 

A process requiring accurate and instant speed controls must adopt static tnotor controls, described in Section 6.9, and their control schemes must be activated through programmable logic controllers (PLCs) discussed above. 

Integrated control systems are mainly found on factory-assembled equipment, but the increased use of programmable logic controllers for process control is giving designers and installation mechanics the experience to apply these methods to custom-built refrigeration systems [26]. 

Programmable logic control (PLC) systems, which automatically control the RO process to sustain output and quality specifications... 

Programmable logic controllers are used for the control and interlocking of processes where a sequence of operating steps has to be carried out such as, in batch processes, and in the start-up and shut down of continuous processes. 

Net present value (NPV), 9 536—537, 546 Net return rate (NRR), 9 537, 544 Net weight, 26 246 Network copolymers, 7 610t Networked programmable logic controllers, 20 670-671... 

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