Smarter PLCs and HMIs now rival SCADA systems
The traditional boundaries between SCADA-based control systems and PLC-based control systems were, if not fixed, then at least relatively clear, with SCADA typically being the favourite for process applications and PLCs for discrete, sequential control. But as those boundaries have blurred, many users who would previously have considered a SCADA system as their best option might today find they are better served by a compact PLC linked to an HMI.
In looking at what is best for a given application, we first need to look at some definitions and some typical implementations. SCADA – supervisory control and data acquisition – is essentially a software package that sits on top of the feedback sensing hardware, and uses that feedback to provide process control signals to the control hardware. In many cases, that control hardware would be an industrial computer, although it also may well be PLC based. But it is the SCADA package that is performing the control processing.
In a typical application for PLCs alone, we would only talk about sequential logic control, as typified in discrete manufacturing operations. Operator control and monitoring would traditionally have been via displays, pushbuttons and switches, but increasingly today is via HMIs which, in screen sizes from just a couple of inches to full high-definition display, have become highly affordable options even for applications with relatively modest control requirements. What is interesting today in many cases is that it is increasingly difficult to applications within the narrow windows of analogue/process control and discrete/sequential control.
There are many so-called sequential logic applications which will have a process aspect – with a simple example being the heat sealer on a packaging machine. The very fact that there is any sort of processing element may well point the user towards a SCADA based solution. Further, many more sequential production operations have batch or recipe elements that might again point you towards a SCADA approach. However, that would be to ignore the increasing capabilities of even the most modest HMI and PLC combinations.
Even low end PLCs are, today, sophisticated control computers in their own right, with comprehensive analogue input and processing capabilities. At the same time, a relatively low cost HMI can provide an easy-to-implement interface for recipe selection, batch control, data trend presentation and more. With drag and drop programming functionality, users can quickly set up set-point indication, alarms and recipes. Further, increased speed of operation, improved processing and greater storage capacity can make the PLC ideal for data logging, with the HMI then able to show data trends and offer historian functions. All of this means that it can often be far easier and much more cost-effective to implement SCADA-like functionality using a PLC in conjunction with an HMI than by looking at the additional software layer of a full-blown SCADA package. It also means that this functionality can be implemented from just a few I/O upwards cost-effectively, whereas a traditional SCADA implementation might be approached from many thousands of I/O downwards.
OEM Automatic is able to offer simple PLC and HMI based systems built around Crouzet Automation Millenium 3 PLCs and Millenium Touch HMIs that provide a highly attractive option where a SCADA-like approach would be of benefit in systems up to 50 I/O. Millenium 3 controllers are easy to program to provide simple control and monitoring of machines and automation processes, with a wide range of expansion options, including both analogue and digital modules. Millenium Touch programmable HMIs are available in sizes from 4.3in to 7in, offering the presentation of text, data, graphics and animations. Crouzet Automation has built its reputation on easy to use and adaptable alternative automation solutions for specialised and demanding requirements.
OEM Automatic UK has over 25 years experience in supplying leading products to industry, with a product range that is one of the widest and deepest on the market. The company operates in four key business areas: motors; panel products; machine/safety; and pressure/flow products. OEM Automatic can also help to select the best products for each application, and reduce the number of suppliers, so lowering the total cost of purchasing.
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