Showing posts with label PLC. Show all posts
Showing posts with label PLC. Show all posts

What Are Control Systems?

Control systems
Control systems are computer-based systems that are used by many infrastructures and industries to monitor and control sensitive processes and physical functions. Typically, control systems collect sensor measurements and operational data from the field, process and display this information, and relay control commands to local or remote equipment. In the electric power industry they can manage and control the transmission and delivery of electric power, for example, by opening and closing circuit breakers and setting thresholds for preventive shutdowns. Employing integrated control systems, the oil and gas industry can control the refining operations on a plant site as well as remotely monitor the pressure and flow of gas pipelines and control the flow and pathways of gas transmission. In water utilities, they can remotely monitor well levels and control the wells’ pumps; monitor flows, tank levels, or pressure in storage tanks; monitor water quality characteristics, such as pH, turbidity, and chlorine residual; and control the addition of chemicals. Control system functions vary from simple to complex; they can be used to simply monitor processes—for example, the environmental conditions in a small office building—or manage most activities in a municipal water system or even a nuclear power plant.

process controlIn certain industries such as chemical and power generation, safety systems are typically implemented to mitigate a disastrous event if control and other systems fail. In addition, to guard against both physical attack and system failure, organizations may establish back-up control centers that include uninterruptible power supplies and backup generators.

There are two primary types of control systems. Distributed Control Systems (DCS) typically are used within a single processing or generating plant or over a small geographic area. Supervisory Control and Data Acquisition (SCADA) systems typically are used for large, geographically dispersed distribution operations. A utility company may use a DCS to generate power and a SCADA system to distribute it.

control panelA control system typically consists of a “master” or central supervisory control and monitoring station consisting of one or more human-machine interfaces where an operator can view status information about the remote sites and issue commands directly to the system. Typically, this station is located at a main site along with application servers and an engineering workstation that is used to configure and troubleshoot the other control system components. The supervisory control and monitoring station is typically connected to local controller stations through a hard- wired network or to remote controller stations through a communications network—which could be the Internet, a public switched telephone network, or a cable or wireless (e.g. radio, microwave, or Wi-Fi4) network. Each controller station has a Remote Terminal Unit (RTU), a Programmable Logic Controller (PLC), DCS controller, or other controller that communicates with the supervisory control and monitoring station. The controller stations also include sensors and control equipment that connect directly with the working components of the infrastructure—for example, pipelines, water towers, and power lines. The sensor takes readings from the infrastructure equipment—such as water or pressure levels, electrical voltage or current—and sends a message to the controller.

HMIThe controller may be programmed to determine a course of action and send a message to the control equipment instructing it what to do—for example, to turn off a valve or dispense a chemical. If the controller is not programmed to determine a course of action, the controller communicates with the supervisory control and monitoring station before sending a command back to the control equipment. The control system also can be programmed to issue alarms back to the operator when certain conditions are detected. Handheld devices, such as personal digital assistants, can be used to locally monitor controller stations. Experts report that technologies in controller stations are becoming more intelligent and automated and communicate with the supervisory central monitoring and control station less frequently, requiring less human intervention.

For more information about industrial control systems, visit of call (800) 462-5769.

Introduction to Programmable Logic Controllers (PLCs)

Programmable logic controllers
Programmable logic controllers, or PLC's, are used for plant automation and control. The PLC is a specialized, industrial computer which includes onboard random access memory (RAM) and read only memory (ROM). As with any other computer, the PLC has a central processor unit (CPU) for data processing. A single PLC has the switching and logic capability to replace thousands of control relays. PLC's are ubiquitous and are used in many different applications in all industries including semiconductor manufacturing, pharmaceutical production, chemical processing, food production, primary metals, and HVAC. Because of their wide industry use, they are manufactured in many shapes and sizes.

A PLC provides the "I/O" (input/output) which is driven by a control software program. That program interprets, and continuously measures, process inputs and then generates output signals to the PLC's hardware.  Normally, the PLC is programmed for a specific process control application, but they can easily be re-purposed with new programming. As process requirements change, so can the software that runs the PLC.

Small Fixed / Unitary PLC

Small fixed type, or unitary PLC's are smaller, have limited capacity and are used for more basic, simpler control logic - many times on specific-purpose equipment. This type of PLC is used in the controlling of semiconductor processes, compressors, ash handling systems, commercial cooking equipment, pharmaceutical equipment, HVAC equipment, burner control systems, and parts inspection systems. The unitary PLC is small in size, has fewer inputs and outputs (I/Os) and has less memory. Accordingly, they are less expensive. For the most part, these PLC's are used for specific applications. For example, a PLC used for a semiconductor process only monitors the operation of that process. Subsequently, since they are designed for specific purpose, their programming is usually done once and is not changed.

Modular PLC's

Modular PLC's have many inputs and outputs (I/Os) and have their major components broken into "modules" which house the electronics and circuit boards. These modules are designed to slide in to slots in the back plate or back panel, or can be rack mounted in a control panel. The system size and complexity can increased by adding more modules. Modular PLC's are used for more complex process automation for multiple processes in a plant, or even all the processes for the entire plant. Modular PLC's have large data handling capacity and are used for large process control applications. The modular PLC programming is usually customized for the site specifically, and can be re-programmed on-site should requirements change.

Certain types of PLC modules have specific uses and specific functions. Some common main modules are:
  • Power supply modules 
  • Central processing units 
  • Input output modules (I/O) 
  • Ethernet modules 
  • Profibus modules 
  • Redundant communication link
Process Control Solutions' Control Systems group are experts in SCADA and PLC Integration. PCS specialists possess the skills required to program and commission virtually any PLC system including Allen Bradley, GE, Siemens and Modicon. Visit or call (800) 462-5769 for more information.


Control Systems by Process Control Solutions

Process Control Solutions' Control Systems group offers turn-key process automation and systems integration services backed by years of experience in the design and implementation of control systems. Our capabilities include design, specification, fabrication, configuration, programming and start-up of your project. We support a wide range of process applications and requirements including SCADA, PLC & HMI programming, industrial control panels fabrication, product testing and virtually any other custom process automation application. PCS is UL 508 certified and a Wonderware Certified Integrator.

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