Ceram Research

Benefits in the drying process from modern digital control methods

The continuous drive for improved manufacturing methods in the drying process of manufactured products has led designers to look well beyond traditional existing techniques Prominent amongst these developments is the use of improved technology in the design of modern temperature and process controllers to ensure that both efficiency and quality continue to meet the growing demands of the market for new drying processes.

When examining both the process and customer led demands, several new areas need to be satisfied if the widest range of benefits are to be fulfilled.

Performance

To aid the accurate performance of the thermal profile in the drying process designers at Eurotherm Controls have ensured that their temperature programmers include control algorithms and process optimisation techniques which give excellent results across a wide range of temperatures and product mixes.

In the simplest form single shot self tuning allows a good compromise to be reached for process optimisation.

However as the thermal coupling for the process changes with the advancement of the thermal cycle the gain of the control loop also changes, modern techniques with gain scheduling allow this change to be accommodated and the results give measurable improvements in quality It is possible to extend this technique in higher specification products and to include process modelling methods which can be particularly useful where there is a wide variation in ether load size or thermal processing rates. Designers have also concentrated on overshoot inhibition routines and have embedded within the algorithms powerful features of cutback and process “look ahead” to accurately predict and eliminate process overshoot.

The use of a new technique for input and output devices on Eurotherm products has led to a further series of benefits. The DSM (Delta Sigma Modulation) technique with associated circuitry and software gives improvements in temperature sample rates, calibration accuracy, sensor break detection and the use of a wider variety of input and output devices. Coupled with new strategies to deal with sensor failure or power fail recovery these benefits give the user confidence in process performance.

Of particular advantage in drying processes is the ability for controllers to be connected to both complex and low cost non-contact temperature measuring devices. There is a growing use of this technique to provide cascade control systems or to control accurately from product temperatures rather than the process temperature.

Customisation of the input characteristics of a wide range of non contact sensors gives a predictable accuracy which has been previously unattainable.

Integration

Whether it is the intelligent inter-connection of individual discrete controllers or the complete ability to integrate products into a supervisory control system, users are looking for the benefits which can be derived from system integration.

A new technique developed by Eurotherm called PDSIO (Pulse Density Signal Input-Output) is complementing the traditional methods of digital communications. Particularly in drying applications PDSIO allows for the simple interconnection of discrete products to provide a very cost effective solution to control systems which employ master and slave zone configurations or cascade control.

Continuous developments with digital communications and the ability to access information from control systems is leading to the wider use of low cost supervision packages in the drying industry. In addition to the traditional aspects of supervisory systems, users are finding that the benefits of new techniques with process diagnostics and SPC can aid their drive for improved quality.

Imminent future developments with communications include the ability to support connections to the growing standards of Profibus and Devicenet.

Machine Control

Since the thermal processing cycle is embedded into other aspects of the drying process it is important to provide products which allow connection to and control of a variety of parameters.Discrete controllers and more particularly intelligent system products allow the interaction of soft wired control loops, analogue strategies and sequence events to provide benefits which ensure greater adherence to and flexibility of the process control schemes. A typical example would be a recent installation which makes use of the requirements for temperature, humidity and real time to determine the appropriate process profile. Variations in the strategy for start up and steady state operating conditions are accommodated within the state diagrams and both soft and hard alarms are used to give diagnostics for this low cost supervision scheme.

Increased density of connections to remote equipment enables the controllers to operate fully automatically with the dryer and product recipes are readily selected via a large external thumb wheel.

Easy to use

Not least important in the consideration for new design is the need for low cost of ownership and the ease of operation.

Many features are now designed into products to bring the benefits which especially help to achieve this need.

The DSM technique gives calibrated for life accuracy and continuous repeatability. High security and customisation of the operator interface give confidence for users against accidental or deliberate misuse.

New high brightness long life displays give clear operator information in both low and high brightness ambient conditions.

Full or partial automation with the clear and simple ability to revert to manual or local control of the plant give added surety for unguarded situations.

As drying processes develop users can be assured that the technology available to designers will continue to match the industries needs of coupling cost savings with innovation and quality.