TechArchive

This article was originally written in the period 1995-2000

Early production machines, having relatively simple electromechanical control systems, could convey enough fault information to the operators of the machine via signal lamps to allow effective troubleshooting.

But the application of PLCs as the total control system for the machine resulted in, among other things, a much more compact control panel, limiting the space for indicators, as well as offering a controller capable of much more complex control functions. In doing this, a much greater range of fault conditions is introduced to the machine, requiring more guidance information for the operator.

While early machines relied on one-way communication – from the machine to the operator – any machines now need some sort of operator feedback also. Operators need to input information directly, after prompts from the display unit. With new manufacturing techniques making high quality, touch screen graphics displays easier and cheaper to build, almost any modern machine could benefit from a capable, interactive man-machine interface (MMI).

Even on the simplest of modern machines, alarm enunciators and illuminated mimics are no longer enough. Dumb message terminals have found extensive use, offering the operator more usable information, with messages such as “guard open/guard closed”, but the simplicity of their feedback often belies the cumbersome programming to actually make them perform the required task.

So market research figures indicate that the MMI philosophy already has a firm foothold in the USA and Germany where a third of the PLCs sold uses an MMI. In the UK, the figure is a fifth. However, the UK’s current market of (UK Pounds)15 million and its annual growth rate of 40% show that MMIs are rapidly becoming a favourite here too.

Already, MMIs are not just limited to industry. Many are being used in building services on applications such as controlling air conditioning or passenger lifts. There are even graphic operator terminals being used to monitor refrigerators in supermarkets. It will not be long before we see them in the domestic market and perhaps in our own homes.

The simplest and least expensive of MMIs are data access units, which provide exact information about a PLC’s functions in bit or data format.

For more advanced process control, operator terminals, usually rated to IP65, are found frequently on the factory floor as replacements for traditional instrumentation and text displays.

The most sophisticated of the MMIs are graphic operator terminals, which have all the capabilities of the operator terminals, but use high quality graphics to display their information.

Many graphic operator terminals are replacing small Scada systems. They use high quality graphics, but have the intelligence, display and interface integrated into one compact unit. They are considerably less expensive than PC-based systems.

By combining all the functions of traditional instrumentation into a single display unit, operator terminals are used to replace chart recorders, lamps, gauges, BCD switches, VU meters, text displays and many other components. In addition to being easier to use, they provide significant cost savings.

Figures released by Mitsubishi suggest that a MAC 10 operator terminal costs (UK Pounds)239, whereas the cost of a two-digit LED display is about (UK Pounds)80. Yet to display the same information as the MAC 10, a minimum of eight LED displays would be needed, taking more space and increasing installation costs. Even then, they would not be operator-interactive, or have built-in diagnostics and alarm displays.

It is interesting to note that MMIs go back further than perhaps may be thought. Xycom, one of the smaller independent suppliers, introduced its 4800 family in 1983/4 and is strongly active in the field today. They started life as panel-mount industrial terminals. PLC users realised that they could use the ASCII message capability of their PLCs to send strings to these terminals and thus use them as rugged shop-floor MMIs.

This capability, through the use of keyboard programming and Escape sequences (remember those?), allowed reasonable MMIs to be created with the ability to change colours and change to alternate (graphical) character sets, providing simple mimic diagrams, messaging and alarm functions. They were, however, tedious to create and loaded up the PLC, which had to use its limited and precious memory to handle the ASCII messaging, rather than the control – the real reason for its existence.

Now, there is every indication that the text display market is in decline, whilst the graphics approach looks set to grow steadily. A graphics approach to design is insensitive to the language of the operator. Translating text can be time-consuming and costly, but the same picture will be recognised the world over. Flat panel technology is making its mark on both the graphical workstation and PC-based products.

Touch screen operator panels probably show the strongest growth, but this is coming from a low base level – there are still a lot of companies reluctant to embrace this technology for various reasons. Touch screen systems can eliminate the jumble of switches that operators would have been expected to learn and use, instead providing menu-driven screens which lead the operator through the correct decision process.

Castleland Control Products has upgraded its PG1100 touch screen graphics panels recently. The TFT colour models now have a new screen, which gives a screen bulb life of 40 000h. The company claims that most similar screens on the market offer only about a quarter of this. Other members of the family have LCD monochrome or electroluminescent mono displays.

Sensor Consultants offers the PWS-910S, which has a backlit LCD graphical display. Screen elements include five different sizes of characters, graphics, numerical and status displays, as well as bar and trend graphs. On the basic model, five configurable function keys are provided for operator input. The touch screen version is the PWS-910T, but this also has two keypads. Counters, times, internal relays and data registers in the PLC can be accessed via the programming or serial port.

MMIs also play a key role in making today’s machines flexible. Rather than having to reprogram whole sets of parameters every time a change is needed, these can be pre-set, and the product variants themselves represented graphically on the MMI, so that changing from one setting to another is as easy as touching the required product on the screen.

Having operators inputting information directly has generated another requirement – speed of communications. When input and display of information are available on the same terminal, the expectation is for instantaneous response, so the need for serial communications is a necessity. This development has been further advanced by removing the need for complex PLC programming by producing specific PLC protocols, allowing the display terminal to act as master to the PLC and itself collect the information it needs to display.

Fault finding is another area where an improved human interface can pay dividends. Modern PLCs have inherent capabilities to generate fault codes and flag up problems, and these can be used to provide informative messages on the MMI to lead the operator straight to any problem.

The graphics capabilities of today’s MMIs provide further enhancements to these processes. Omron’s products, for example, allow bitmap images to be imported even onto a low cost product. Photographs of the actual machine can be loaded straight into the MMI, allowing operators to simply touch on the area of the machine that they want to look at in more detail. Pilz systems similarly can have bitmap graphics displayed in colour and monochrome in a variety of size options.

As well as bitmap images, other representations of the machine are easy to draw into the MMI. Much of the software is Windows-based and provides powerful drawing features, including line and bar graphs. There is no programming, just an intuitive sketching process. Images can be drawn on a laptop or desktop computer then downloaded onto the MMI, with scaling of values and parameters done automatically. And all without a single word of code being written.

One example of this is PX-PRO, Pilz’ programming tool for message preparation, download of PLC drivers and parameter setting. Pilz will shortly release a machine control software package called Pipanel-Touch, which provides high power computing capability together with user-friendly touch-sensitive symbols.

There are cases to be made for buying MMIs from the large supplier and from the smaller independent one. To an OEM or systems integrator, often forced anyway to use different PLCs for different clients, using an independent MMI can maintain the outward appearance and operational ergonomics of the machine, while interfacing to different PLCs. It also takes out the learning curve of understanding many different MMI configuration tools. Even Mitsubishi, the large supplier of complete systems, acknowledges that independent suppliers are better able to market products for niche applications and can produce a product with the functionality that is required for a specific, small market which a global manufacturer would not find economic.

On the other hand, where the PLC manufacturers gain is in providing users with a single vendor. There are also often price incentives for taking the MMI with the PLC. The single source also takes away any worries about compatibility, although many of these are largely unfounded, particularly with the advent of control bus standards, such as Profibus and Interbus. The bus system ensures that all devices from different vendors can communicate together without needed to adapt interfaces.

As an example, the PG1100 from independent supplier Castleland interfaces with most major PLCs through a range of proprietary drivers, including Siemens, Allen-Bradley, Modicon, Mitsubishi, General Electric, Square D and Texas, as well as network drivers, which include Modbus Plus, Allen-Bradley Data Highway Plus, Profibus and Devicenet.

Mitsubishi agrees that independent suppliers are, indeed, able to produce a product which cold work with a variety of PLCs. In doing so, it may be able to produce them in sufficient quantities to be competitive with a general purpose product.

The counter view is that a manufacturer of complete systems (such as Mitsubishi) is able to best take advantage of the characteristics of its own PLCs, and so will invariably have greater functionality than the general purpose product. For example, independent suppliers usually communicate through an auxiliary comms channel, which will be slower than communicating via the PLC bus. The latter approach, says Mitsubishi, will result in much faster access to data.