Logic control devices

PLC (Programable Logic Controller)—Device used to control on-off process operations. The sequence of on-off executions and sometimes other control functions can be changed by programing instead of wiring changes. 

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 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. 

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. 

P 1 diagrams contain four important pieces of information, namely, all vessels, valves and piping, along with a brief description and identifying specifications of each all sensors, instruments and control devices, along with a brief description of each the control logic used in the process and, finally, additional references where more detailed information can be obtained. 

Training courses are available in analytical methods of fault-tracing. Computers are also in use which monitor a number of parameters and draw attention to any observed abnormality. The control/ monitoring device may then make a judgement as to the cause, or this may rely on the interpretation of the operator. Considerable use is now made of logic control/monitoring devices which can oversee the operation of a large number of installations from a central computer/observation terminal. 

Process control plays an important role in how a plant process upset can be controlled and subsequent emergency actions executed. Without adequate and reliable process controls, an unexpected process occurrence cannot be monitored, controlled and eliminated. Process controls can range from simple manual actions to computer logic controllers, remote from the required action point, with supplemental instrumentation feedback systems. These systems should be designed such as to minimize the need to activate secondary safety devices. The process principles, margins allowed, reliability and the means of process control are mechanisms of inherent safety that will influence the risk level at a facility. 

The central computer is called the master terminal unit, or MTU. The MTU has two main functions to periodically obtain data from RTUs/PLCs and to control remote devices through the operator station. The operator interfaces with the MTU using software called human machine interface (HMI). The remote computer is called the program logic controller (PLC) or remote terminal unit (RTU). The RTU activates a relay (or switch) that turns mechanical equipment on and off. The RTU also collects data from sensors. Sensors perform measurement, and actuators perform control. 

Technology advances in electronics such as process control instrumentation systems, computer capabilities, programmable logic controllers, and the use of independent PC s (personal computers) at field locations for special dedicated functions present new challenges to incident investigation. Some of the advances are so rapid that the team may not have the internal expertise to determine failure scenarios, sequences, and modes. The suppliers and manufacturers of these high-tech devices are sometimes the only source of credible information on failure modes of these devices. 

Although actual guidelines are only applicable in software used as a component part or accessory of a medical device (e.g., blood establishment software, programmable logic controllers in manufacturing equipment, software that records and maintains the device history record), the validation principles presented in these guidelines could be applicable to any software validation. 

A fluid bed granulation process can be controlled by pneumatic analog control devices or by using state-of-the-art prograimnable logic controllers (PLCs) or com-... 

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. 

Most processes operate more efficiently when functions must occur in a desired time sequence or at prescribed intervals of time. In the past, mechanical timers and logic relays were used. Now electronic logic and timing devices are used based on computer software programmable logic controllers. They lend themselves to easy set-up, reprogramming, and provide more accurate control. 

An automatic control system consists of various pressure, temperature, and flow sensors, valves and components controlled by a programmable logic controller (PLC), and an operator interface device. The operator interface control device can be as simple as a stop-go for totally dedicated systems, or a personal computer. A personal computer connected to the PLC to provide operator interface, data collection and analysis capability is recommended especially if the system parameter and/or the parts being processed will be changing. More importantly, a computer control system assures consistent and repeatable operation. 

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