The advantages of PC-based control systems

Stephen Hayes, Managing Director of Hayes Control Systems, discusses the development of PC software systems for PLC and NC motion control.

Stephen Hayes, Managing Director of Hayes Control Systems, discusses the development of PC software systems for PLC and NC motion control.

The days of the PC being used just for visualisation and production data acquisition in control and automation applications is rapidly becoming a thing of the past.

The PC is now increasingly recognised as an open and powerful hardware platform, which can provide effective and reliable control, with no requirement for additional processors or complex hardware additions.

This recognition is being brought about by the successful development and implementation of powerful software based control solutions, designed to run on dedicated industrial PCs, with particular advantages being apparent in areas such as PLC and NC motion control.

Traditional automation and control systems typically comprise a number of hardware and software elements; 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 disadvantages of this approach being high hardware and software costs, complexity of system design and build plus, in many applications, limited functionality.

Control is now changing with the introduction of reliable and effective PC based solutions, faster processors and Windows NT 32-bit functionality.

A good illustration of the latest PC based control software is the TwinCAT system from Beckhoff, which is designed to organise task processing for position control, PLC and Windows NT simultaneously.

TwinCAT offers PLC control with real-time functionality, I/O via standard field bus, motion control synchronous with machine cycle times and open programming under the IEC 61131-3 standard.

Advantages over traditional control configurations include a centralised structure, reduced hardware costs, fewer interfaces, easy upgrade paths, full integration and high reliability.

With some PC control solutions running under Windows NT, problems can be experienced as NT is not capable of real time operation.

It 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.

Beckhoff has developed an effective solution, by working with Microsoft as a system partner, to develop the architecture of TwinCAT 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 packages.

Control system 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.

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 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 17ms is needed to execute 1000 PLC commands.

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

For example, the TwinCAT system, running on the latest 32-bit PC technology, enables 32 Mbytes PLC programs, with 4 Mbytes flag memory, combined with 64,000 I/O's 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, 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 performance characteristics of modern industrial PCs means that it is possible to move drives simultaneously to the PLC task, at speeds of less than 25ms per axis, enabling the positioning of dozens of axes at the same time.

PC based NC control encompasses a positioning capability (setpoint generation, position controller), an integrated PLC with NC interface, operator control programs for commissioning and a field bus established I/O link to all axes.

Data for motion control is exchanged via the field bus to the drives and from the measuring systems.

As with the PLC on the PC, a user interface is realised with visualisation applications, SCADA or with Visual Basic/Visual C programs.

With TwinCAT, PLC, NC and operating system tasks are operated simultaneously, by overlaying the tasks with a multi-tasking system.

Each task for a specific purpose operates with its own cycle time and priority Existing field bus systems can be linked quickly and easily to TwinCAT by means of a master card in the PC, with the system supporting all major protocols, including Profibus, InterBus, Lightbus, CANopen, DeviceNet and ControlNet.

The boundaries of performance for PC control are constantly expanding.

Software products such as TwinCAT offer a complete application solution for the editing, analysis and execution of control programs in real time, without the need to be an operating systems expert.

The focus being very much on the practical application of PC technology in control and automation solutions, with all the benefits it brings it terms of both costs and performance.

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