Valve control system features fieldbus gateways

A novel valve control system provides users with a simple and unique method of connecting up to 127 valves, and up to 254 sensors, to a controller over distances up to 2.5km.

The pressures of the global economy mean that tighter control of all parameters within manufacturing and process plants is essential if costs are to be reduced to maintain competitiveness.

One way of achieving this objective is through greater plant integration, the linking together of different equipment types in networks that provide control engineers with the requisite feedback to optimise processes, and management with the relevant data to use and manage plant more effectively and efficiently.

Today, the task of achieving integration is greatly simplified by the availability of a number of fieldbus systems including Profibus, Interbus-S, CAN, DeviceNet, ModBus (and others).

All these are digital communications systems that allow a control system to exchange data with remote sensors, actuators, drives, PLCs and other intelligent equipment using a single communications link.

The major benefits to the user - apart from greater flexibility, continuous improvement and improved communications across all levels of their activities - are reduced installation, cabling and maintenance costs and more reliable and flexible operation.

Although widely used, fieldbus systems have been unable to shake off the perception among plant engineers that their use is justified only for major automation and process projects.

This is not always the case of course, but in certain application-specific instances the use of high-level fieldbuses at factory floor level is not justified due to both their cost and complexity.

The realisation in a number of market sectors, AC drives and process valves among them, that proprietary fieldbus systems are not always the best choice for specialised control has resulted in the development of local networked systems.

These are optimised for a particular area of control but offer the advantage of full transparency to higher-level fieldbus systems through a series of easy-to-use gateways.

Asco Joucomatic's new VCS "intelligent" valve connection system is one such local network.

The VCS provides users with a simple and unique method of connecting up to 127 valves (pneumatic, hydraulic, general purpose, water, gas and oil), and up to 254 sensors, to a controller over distances up to 2.5km.

With its single flat cable and quick connector, the VCS system can be installed in seconds, simply and without tools, reducing cabling costs and installation time to an absolute minimum.

The "brain" of the VCS system is an intelligent interface, which acts as a communication centre for a PLC, a fieldbus and up to 127 VCS connectors.

In accordance with the fieldbus system used, the VCS interface is equipped with a corresponding fieldbus module.

These modules are available for standard protocols, such as Profibus-DP, Interbus-S and DeviceNet.

The VCS interface ensures usability and transparency in the VCS system with an RS232 interface which, when connected to a PC, allows the VCS system to be started and analysed with the display software provided - independently of the fieldbus.

A list of the functions that can be activated using the PC include: valve output setting, reading of sensor input and diagnostic information and checking of actual configuration.

Adding to its functionality, the interface also integrates a comprehensive error analysis function, with LEDs to display status and errors within the fieldbus system, or the VCS line, and the addresses of valve connectors, which may be malfunctioning.

To ensure maximum transmission protection, the VCS line is structured as a loop, with the last valve in the line connected back to the VCS interface.

The advantage of this arrangement is that the data transmitted is fed back to the VCS interface and analysed.

In this way faulty data blocks are detected and the transmission is repeated.

Only when the transmitted data and the feedback data are verified as being the same will the valve outputs be activated.

Another feature of the loop structure is that it provides for automatic addressing, each valve being automatically recognised by its position in the loop.

As time consuming manual addressing is unnecessary, double assignments are thus avoided.

All valves and actuators with their VCS connector are referred to as participants.

These participants are connected to each other and to the VCS interface over a two-wire profiled VCS cable, which supplies them with electric current and data.

Data for each participant in the loop is transmitted over the VCS cable in the form of a differential signal.

A complete data cycle is processed within approximately 3ms via the protocol and 127 connectors.

The installation of valves into the VCS system is quick and easy.

Each VCS connector is simply clamped, via a vampire type plug, onto the two-wire profiled VCS cable, establishing automatic connection without the need for tools, cable stripping or fastening.

Complementing its "facilitator" role in providing easy integration, each VCS connector is equipped with a data repeater, which is valid for distances of up to 20m between two valves.

In addition, all the electronics used to extract data from the valves and to transmit input data from the sensors are integrated in the VCS connector, thus rendering it "intelligent".

This is possible due to a newly developed, economical, compact-sized, ASIC at the heart of the VCS's electronic system.

Additional onboard electronics also provide each VCS connector with an auto-diagnostic feature, enabling error indication.

As a result, low voltage, overheating and short circuits can all be detected.

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