By Ian Turner
Valve position monitors perform a vital role as part of an automated valve system by verifying the actual valve position. Without a monitoring system, significant problems may occur when the process control system just assumes’ the proper valve function.
Today, monitoring systems are being demanded that will electronically communicate several valve system parameters and alert operators to potential problems. This means that communication systems are increasingly taking on a greater role as the intelligence centre for the automated valve system, by incorporating microprocessor-based technology to analyse valve performance in the field.
There is a range of technologies which offer the potential for a new paradigm for process control. These technologies, which include PC-based computing platforms, ‘shrink-wrapped’ software and standardised communications networks, make up a new standardised open architecture for process control, which is frequently referred to as the ‘Plant Intranet’. The key enabling technology is the much vaunted fieldbus.
In an ideal world, such a fieldbus would support true interoperability between the devices on the bus. However, consensus now appears to be merging among users and vendors around the use of three separate but complementary levels of fieldbus – full function fieldbuses, device buses and sensor-actuator buses.
Any bus system required to handle on/off valve communication in batching applications (which represents approximately 80% of total valve usage in the process industries) must fulfil a number of basic criteria. It must be capable of handling two discrete inputs (open and closed) and two discrete outputs for single and dual coil solenoid operation.
The system must also provide facilities for basic diagnostics and have sufficient power carrying capability to operate the 1Watt solenoids (minimum of 50mA at 24V d.c.) typically found in valve systems. In addition, the system must be physically small, robust and capable of simple integration with PCs and higher level fieldbuses. Such a system would provide the maximum possible cost savings with a minimum of complexity.
One suitable two-wire bus communication system for on/off valve automation in batch control is AS-i (Actuator-Sensor-Interface). AS-i provides four discrete inputs and four discrete outputs at each node and can carry 2A at 24V d.c. to a total of up to 31 nodes distributed over a distance of 100 metres, thus providing a minimum of 64mA per node.
All of the circuitry fits into less than 4cm2 of board space, making for easy incorporation into standard valve control hardware.
AS-i system has been designed to gateway with higher level fieldbuses and to interface with supervisory PCs. Standard interfaces are available for Modbus, RS232, RS485, Profibus DP and FMS, InterBus-S and DeviceNet, with gateways to Echelon’s LonWorks and Foundation Fieldbus currently under development.
Cost savings using AS-i can be significant. In a typical plant with say 300 automated valves installed in a zone 2 hazardous area, the savings can be as much as £270 per installed valve, equating to a total saving of £81,000!
Users can begin immediately on the implementation of a bus strategy, first at the sensor-actuator level, realising immediate cost savings and operational benefits, and subsequently integration of full function fieldbus to the realisation of the true ‘Plant Intranet’.
The Author is with StoneL.