Control Systems Hardware
Introduces the various type of automation system implementation that are used in all types of application.
Types of Automation Systems
The main division in the type of automation system are:
- Standalone PLC - Basic functionality
- PLC with SCADA PC - Basic control functionality, but with good operator interface, trending and data archiving
- PLC with close coupled SCADA - As above but uses PLC vendor specific SCADA
- http://public.modicon.com Schneider Automation, makers of Modicon range of PLCs, Legacy HMI and motion control systems.
- www.ab.com/process/processlogix/ Latest generation PLC offering from Allen Bradley. This system consists of a server PC over ControlNet to a Process Controller residing in the same rack as the I/O and optional Discrete Controller. The capabilities of these systems are encroaching on those of traditional large scale DCS systems (see below), but at (reportedly) lower costs.
- PC with soft control - Emmulates a PLC but with PC hardware. Examples of
- Rockwell Automation Open Automation Includes PC-based Control Software, Open Controller (PC-Based) Hardware, etc.
- SoftPLC A PLC built out of software which embeds a Real Time Operating System with a very small memory requirement so it runs on reliable, low cost, hardware (no hard drive, no WinNT/CE, etc.). It provides runmode online programming, deterministic scantimes, forced I/O, retentive memory on powerloss and an open architecture.
- ProcessACT/UNAC Another example of this, but provides Advanced Control, a Graphical Design environment and Off-line Simulation Capabilities. It can run on Allen Bradley PLCs, Siemens PLCs and standard PC hardware. It uses VenturCom's Real Time Extensions to Windows NT to provide a deterministic and highly reliable Operating System for industrial control applications.
- PC running control algorithms - Can be:
- run under standard Windows NT. This is possible if the latencies
in your system are not too fast. Writing the control algorithm as a C based
DLL can use the Windows NT multi-threading, scheduler and prioritisation,
can achieve latencies down to 10 milliseconds.
- A package to aid this is Labtech Control.
- A good starting point to find out more about Windows NT processes and threads is the MSDN Library.
- Another very useful site is http://www.sysinternals.com.
- Rockwell Automation have a very useful white paper on this subject titled "Using the Windows NT Operating System for Soft Real-time Control - Separating Fact from Fiction". There is also a document that reviews real-time performance of their SoftLogix5 package.
- run under a real-time kernel extension to Windows NT. These kernel extensions are provided by: Control Solutions Magazine).
- run under standard Windows NT. This is possible if the latencies in your system are not too fast. Writing the control algorithm as a C based DLL can use the Windows NT multi-threading, scheduler and prioritisation, can achieve latencies down to 10 milliseconds.
- run under an alternative real-time operating system such as Real Time Linux, see:
- DCS (Distributed Control Systems) - Often large scale system. Examples of such systems include:
- Honeywell TDC range
- Bailey Freelance
- ABB Master
- Fischer-Rosemount DeltaV.
- The ISA has several articles and books in the filed of distributed control and DCS:
- Process System - A term to describe all plant components, from low level valve control to assest management systems.
The above mainly applies to process control systems, such as chemical plants, etc. More mass produced, or physically smaller, controllers tend to be classed as embbeded controllers, and can take the form of:
- PC with soft control Emmulates a PLC but with PC hardware. ProcessACT/UNAC is a good example of this.
- Standard microcontroller system Many small modules are now available that contain all the necessary I/O, timers, programming language, etc to form a control system. These can contain dedicated Real-Time Operating Systems, or simpler clock driven algorithms. A good site for real-time computing is at www.real-time.org.
- Dedicated microcontroller system Purpose designed
Articles on Operator Support Systems
This area covers the SCADA/operator interface part of any control system, and also any additional functionality that operator support systems can provide, including:
- Alarm handling systems
- Automatic operator error trapping
Articles on PLCs
- Introduction to Programmable Logic Controllers, Gary Dunning Good introduction to PLCs.
- Handbook for PLC programmers, E.D.Parr Has been recommended as a good introduction.
- Your Personal PLC Tutor Unbiased plc information not geared to any particular manufacturer. Information to teach you everything you need to know about PLCs. Your Personal PLC Tutor site is generic in nature and not geared to any particular manufacturer. And most importantly, it is written in common language. No previous knowledge is required.
Communications Protocols and Networking Systems
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