Mission for total control

Manufacturers’ demands for integrated systems to manage their entire operations – from raw materials through manufacture to sale of products – are spurring a technical revolution in control and instrumentation systems and strategies. Paul Gay reports

Integration and revolution are the key words in the control and automation sector today. The old buzz phrase `shopfloor to top floor’ is being replaced as manufacturers move towards integrated control strategies in which the same data can be shared throughout the company and beyond.

The aim is to link the measurement and production elements on the factory floor through the control systems and management software on to the systems that control the supply of materials and services coming into the factory.

The integration of automation is being further developed to cover all aspects of supply, manufacturing, sale and distribution of future products. But to achieve this the control and instrumentation industry is undergoing an information technology revolution – similar to the office IT revolution of the 1980s and 1990s.

Control systems must communicate with measurement devices on the factory floor and relay back information to actuators which control the process. In automated factories most processes are carried out by a series of control loops. Each loop measures a variable, compares it against the desired parameter then triggers a control signal to adjust the variable. In traditional control strategies, the loops would have been hard-wired and used mainly electromechanical equipment to measure and control.

Today’s systems are computer-based, with sensors and actuators communicating over networks ranging from serial communication links to real-time fieldbus networks. One network can replace dozens, or in some cases hundreds, of hard-wired connections. To achieve these massive savings, loops have been introduced into factory floor elements so that some of the calculations and data conversions can take place at the process face, but most of all allow effective communications.

In the process industries such as petrochemicals or the food and beverage industry the need to handle analogue signals for temperature, pressure and flow has led to the adoption of fieldbus systems such as Profibus, Foundation and DeviceNet.

De facto standards

In discrete manufacturing, communication is simplified because the majority of signals can be digital. Here speed is the key and simpler systems, such as the actuator/sensor interface, Modbus and Interbus, have become de facto standards, providing high-speed communications to the control systems level.

Increasingly, control systems are becoming digital and PC-based. Even the distributed control system, for so long the workhorse of the process industries, is developing along the PC-based route and uses networks and fieldbuses to communicate with the process. Being tried, tested and totally reliable, the programmable logic controller (PLC) will always be a favourite with discrete manufacturers. However, the growing requirement for management information and reports will lead to a hybrid machine which will include a PC card to pass information to the office world.

An alternative controller would be a PLC software device running on a PC. With the ratification of the new international standard, IEC 61131-3, the tools for writing control software are now standard the world over.

The soft PLC does, however, have two major obstacles. One is its instability, which makes it prone to crashing, and the dreaded `blue screen’ is not acceptable for mission-critical control. The other problem arises because the PC has to address so many systems before it can look at controlling the process. This means that the programmer can never determine exactly when a signal will arrive.

There are ways around these problems. Windows NT4 and the more recent Windows 2000 operating systems have improved stability and react in a more realistic real-time manner. Real-time extensions are also available from third parties. These add-ons either patch onto or sit next to the operating system to manage the interruptions and share the processor’s time to overcome the problems.

Some system manufacturers have tackled the shortcomings by writing programs that do not use the PC in a real-time manner. Instead, the PC sits on an office-type network – typically ethernet based – and communicates with a distributed control unit (DCU) which has no display or other human interface functions. The DCU can relay data to and from the plant over fieldbus networks without losing their time-dependent nature.

Moving up the architecture via supervisory control and data acquisition and manufacturing execution systems will be simple using the same data on compatible computers and networks. The integration continues into enterprise resource planning (ERP) and other manufacturing management packages, connecting finally with supply chain management software to complete the system.

Impact of the internet

Microsoft ERP and supply chain industry manager Nick Horslen says: `Manufacturers must look at themselves as part of a collaborative process involving customers downstream and suppliers upstream. Via the internet, real-time customer demand can be made visible throughout the supply chain.’

The near universal acceptance of the internet as a communications and commercial medium has already had a significant effect on factory control and automation and will, without doubt, continue to do so in the future. The TCP/IP networking technology used by the internet is the same as the protocols used by ethernet in the office and factory environment.

The protocol brings a level of standardisation which opens its use globally. And by incorporating browser technology into a factory system, integration can be taken even further. With suitable security measures, the information from the factory can be made available anywhere in the world.

The PC and ethernet architecture provides instant access to the office IT world and offers a simple route to the total integration of the factory. Using the internet, it will be possible for the consumer to buy a product without visiting a distributor or retail outlet.

The order will be taken by an `intelligent’ planning system which will pass data to the factory while at the same time managing the logistics of material supply. The same data sets the manufacturing process in motion. The product, which would probably have been designed electronically, can then be manufactured completely automatically. Once manufacturing quality has been assured, the product would be dispatched under the control of a distribution logistics system.

Manufacturing Utopia? Perhaps as near as we will get. Total integration would mean that a factory could take an electronic order and produce and deliver a tailor-made product to a customer anywhere in the world.

Paul Gay is editor of The Engineer’s sister title C&I