It is generally well known that controlling temperature accurately and with fast process response times presents practical difficulties to standard programmable logic controllers. The root cause of this is that before the constantly changing analog signals from thermocouples or resistance thermometers can be processed digitally, an analog-to-digital conversion process is necessary, often resulting in loss of overall accuracy and measurable time delays.

The most practical solution is to use an analog (PID) temperature controller positioned between the programmable logic controller (PLC) and the temperature sensors to provide clear and accurate scaled-control signals to the PLC. This replaces traditional step function control algorithms and results in both greater accuracy and faster system response times.

The key to successful system operation in such cases is the highly accurate PID algorithms, which reside in purpose-built temperature controllers, says Steve Miller, controllers product manager at Eurotherm, Leesburg, Va. Algorithms like these have continuously evolved over at least 20 years and allow the controller to respond to analog signal changes rapidly and with exceptional accuracy. In particular, such controllers are able to respond to small analog signal changes, such as those being produced by standard thermocouple temperature sensors, faster than digital devices such as PLCs.

This application area has grown to be of such importance over recent years that controllers have been developed specifically to integrate seamlessly with PLCs and other supervisory control and monitoring systems, Miller says.

This type of controller offers exceptional modularity both in terms of hardware and software functions, and their features enable them to handle a wide range of applications, from simple multi-loop and data acquisition up to complex strategies, including setpoint programming, and mathematical and logical operations. This approach is particularly relevant in PLC-based multi-loop PID applications such as plastics extrusion and multi-zone furnaces. By devolving loop control to the analog controller, the PLC can concentrate on providing fast and effective logic control without the burden of running complex control algorithms.

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More Advantages

Today's newly-developed controllers designed to integrate seamlessly with PLCs and other supervisory control and monitoring systems offer more than improved overall system performance.

Additional advantages include easier configuration, deterministic response and the same autotune capability featured in standard analog controllers. Inputs and outputs always are updated at a fast PV rate and, as with a PLC, are in no way dependent on the strategy being run.

A further advantage is the inherent ability of modern temperature controllers to communicate independently. Using well known protocols such as Ethernet (Modbus), Serial Modbus, or DeviceNet, modern analog controllers are easy to interface to both PLCs and other intelligent masters. They also offer a highly cost-effective alternative to performing analog measurement or control loops within the PLC. By implementing these functions remotely, PLC system hardware cost is reduced and each PLC is relieved of the extra processing burden of analog control.

For more information

Contact Invensys Eurotherm. Call (703) 443-0000 or visit www.eurotherm.com.

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