PC potential is constantly expanding, with the explosive growth in IT spawning a whole new generation of highly reliable, rugged and functional industrial PCs
The boundaries of performance for PC control are constantly expanding, with the explosive growth in information technology spawning a whole new generation of highly reliable, rugged and functional industrial PCs, that offer seamless integration with machinery, automation systems and processes. The industrial PC is now viewed as much more than a tool for visualisation and data acquisition. Providing an open and powerful hardware platform, industrial PCs are now being increasingly integrated with software PLCs for control.

The result being powerful automation control solutions that do not require a second processor or complex hardware additions, as with many conventional PLCs.

For example, traditional automation and control systems typically comprise a number of hardware and software elements.

They include a PC for process visualisation, hard PLCs with coprocessor cards, coprocessor PLCs, I/O via field bus, motion control via parallel cabling and a selection of software operating systems and programming languages.

The disadvantage of this approach being both high hardware and software costs, complexity of system design and build, plus in many applications, limited functionality.

With the introduction of software based solutions providing PLC functionality direct from the PC, the advantages are significant, including a centralised structure, reduced hardware costs, fewer interfaces, easy upgrade paths, full integration and high reliability.

In essence, a PC based 'soft' PLC encompasses a programming system, one or several run time environments and an I/O link that is established by a field bus.

Typically, a user interface is realised with visualisation programs, SCADA or with Visual Basic/Visual C programs.

The performance of software solutions for the PLC on the PC exceed the performance of hardware based systems, with measured speeds improving in line with the increasing capabilities of processor technology.

On an AMD K6/200 system, for example, only 17æs is needed to execute 1000 PLC commands.

In addition, operating parameters such as program size, flag memory and process image size are virtually unrestricted.

One example of PLC on the PC integration is the Beckhoff TwinCAT system.

Running on the latest 32-bit PC technology, TwinCAT enables 32 Mbytes of PLC programs, with 4 Mbytes flag memory, controlling up to 64,000 I/O points to be easily achieved.

A conventional PLC processes programs cyclically, with inputs and outputs alternating with the processing of programs.

Execution of programs in a software based PLC solution, such as TwinCAT, is basically the same; i.e accurately timed, deterministic cycles of equal length, with the user interface being executed in the cycle pauses, for which computing capacity is reserved.

The majority of PC/PLC control solutions run under Windows NT.

Problems can be experienced as NT is not truly capable of real time operation.

In effect, Windows NT is designed for optimised performance and not for control purposes, with control tasks being interrupted by events such as network operation, hard disk access and mouse movements.

However manufacturers such as Beckhoff have developed an effective solution, by working with Microsoft to develop its architecture as an independent real time extension for Windows NT, in the form of a kernel mode driver.

With this kernel extension, TwinCAT operates on an exact time basis which executes programs with maximum priority and independently of other processor tasks that NT may be running.

This approach is particularly advantageous, as it means that NT is not modified (patched) which occurs with some other PC based PLC solutions.

Control systems engineers and developers worldwide can purchase Windows NT from any Microsoft sales channel and construct an effective and completely integrated, management, production and automation control solution on the PC.

Hayes Control Systems is offering readers of Automation a CD, containing a free 30 day trial of its TwinCAT software solution for PLC and NC motion control on the PC