In any process industry recording of process data is very important. The recording of such data helps process engineers to analyse and suggest improvements in the process and also helps to do post modern analysis in case of any problem such as product failure or any mishap that may have occurred. In the beginning, when only analogue and digital indicators were only available, operators used to note the values of different parameters against time in a logbook. Correct analysis is possible only if the operators record actual data at the time of logging. With recorders, all the process parameters are hooked up with recorders in which an ink mark is made over a calibrated chart against time. This presents a genuine picture of the behaviour of process parameters to the managers. In the beginning, recorders used to be single pen recorders and later on for recording more than one parameter, multi-pen recorders were introduced in which the markings used to be in different colours to distinguish one from the other.
With the introduction of low-cost PCs, instrument manufacturers introduced PC-based data acquisition and logging systems. This consists of interface cards that directly accept field inputs such as T/Cs, RTDs and outputs of transmitters. The interface card converts the inputs into one uniform output that is fed to the computer through RS 232 serial port. A data acquision software loaded in the PC will process the signal and does two things. Firstly, it displays the value in the monitor and secondly logs in the hard disk. The advantages of the PC-based data acquisition and logging system over conventional indicators and recorder are :
- Economic use of space. No need to have big panel etc
- Display of all the parameters in numerical and bar graph forms
- User can group the inputs and can have display in the same order
- It is possible to retrieve past data from hard disk quickly
- More than 500 inputs can be connected to one PC
- It is possible to generate alarms from PC which can be annunciated in the field
PC as a supervisor
The next development in the PC-based systems is putting the PC as a supervisory control unit in addition to data acquisition. The software developed for this is called SCADA software. This presents the user with a kind of distributed control system. All the PID controllers are hooked up with the PC through RS 485 and from the PC the operator can receive data from the PID controller. In addition to this, at any point of time, the operator sitting in the computer room can change the set value of the PID controller, change the mode of operation from A/M or vice versa. Graphics displays combined with data acquisition software present a new type of industrial automation software known as "MMI-SCADA." The earlier MMI systems were basically proprietary displays connected to a single controller or PLC. The PC eventually replaces these dedicated displays, and, later on many PCs are connected on a network. A client-server architecture which evolves out of such netwroks, monitors and controls networks of Single Controllers & PLCs. The client-server uses a central PC as a server, which houses the main application program and database. Other PCs run a lower version of the application and are distributed around the plant. Many software companies have developed software with several features and also added drivers to read data from instruments manufactured by other companies and the software operated in Windows environment. Automation interface software today combines traditional operator interface, process visualisation, data acquisition, and alarming, with enterprise solutions based on real-time manufacturing data communicating from controller to corporate systems and back. These products accomplish more than interfacing machines with humans- they interface real-time information from automation systems with anyone or any system requiring that information.
With the advent of the Internet, a new scenario is emerging. Web-based transfer of data will eventually replace MMI. Web-based technologies will facilitate not only acquiring data but also control the operation. Web technology involves a server and a browser. The server stores information in the form of "pages". And the server need not be a large PC. A single loop controller or a PLC itself can be a web server. Browsers read and present server information, so a browser connected to a web-enabled controller presents operator interface graphics and information. A simple example given below will prove the advantage of the web-based technology. Suppose a Production Manager of a chemical plant receives an alarm on his pager while he is away from the plant. Instead of rushing back to the plant, he can go to the nearest PC and using the Internet connection and browser, he can access the web-enabled PLCs or single loop controller and take corrective action.
In order to keep pace with the fast moving technology continuous efforts should be made to make the newest technology obtainable to users at sensible and affordable costs before the technology itself becomes old-fashioned.