Make hot-spots history with gradient temperature control

Paul Boughton

Hot-spots and large temperature gradients can be one of the biggest challenges in the thermal and chemical processing of sheet materials such as glass laminates, ceramic foils and silicon wafers.  Coorous Mohtadi reports.

As temperature is often a critical process parameter, any differences in temperature over the two-dimensional surface can play havoc with the processing and cause quality and yield to deteriorate significantly.

Nowadays, the requirement for quality improvement is strict for equipment for heat treatment, eg developing machines in semiconductor processes, packing machines in foods processes, and forming and moulding machines in plastic processes. Especially, uniformity of temperature on a heating plate is an important issue.

In general thermal processes, scatter of temperature on the conduction surface is observed in both steady state and transient state. In the steady state, scatter of temperature can be suppressed with simple PID controllers by proper design of the machines considering possible items relating to thermal characteristics. Transient temperature scatter is caused by thermal conduction from the heaters to products. As the heating plate is usually attached to enclosure, the thermal inertia is different in each area.

This phenomenon is observed drastically in the case where the thermal plate is wide and thin. From the viewpoint of production, the thermal process time should be controlled and managed for quality improvement because the thermal energy, which causes the chemical reaction, depends on the temperature and heating time. So, it is required to make the temperatures uniform in all the points on heating plate as soon as possible, to ensure the quality of products and process time management.

Because of the increasing size of heated products, temperature control using multi-heaters is being used to minimise the scatter of temperature.

For practical applications, the heating surface is divided into multiple areas, and precise temperature controls are used in each area in order to create a uniform temperature.

However, the thermal interference among the heaters can influence the characteristics of the system to become quite nonlinear. Furthermore, the nonlinearity of the system depends on the application, ie the shape of the heating surface, heater arrangement, dead time, thermal conduction ratio and convection and so on. Thus, it is generally difficult to realise a precise temperature control system based on a conventional PID controller.

As a result of this difficulty, static decoupling control has been investigated in process engineering. The main idea of the decoupling control is to design the decoupler so as to compensate the thermal interference among the heaters. However, precise identification and modelling of nonlinearity are very difficult, and static or partial decoupling using low order information of thermal interference has been studied under the trade off of precision and stability in general practical applications.

Omron’s new two-dimensional loop-interacting Gradient Temperature Control (GTC) consigns temperature hot-spots to history. GTC adds two important elements to PID control that work in harmony to automatically eliminate interference between the heating zones and reduce the temperature differences between them to create a uniform temperature profile over the entire 2D area. More than that, however, GTC makes it possible to define any temperature profile that's needed for the processing, including valleys and hills, and to maintain that profile over any size sheet, provided that there are sufficient heating zones and sensors in position.

GTC works not only at constant temperatures but also while the temperature is changing to maintain the perfect temperature profile throughout the process – from initial heating up, through the steady-state period and even during disturbances (such as those caused by putting a new component on the heating plate). GTC is also completely automatic and even works when changing over from one product type to another, eliminating any need to retune or adjust the system.

With GTC you get:

  • Perfectly-controlled 2D temperature profiles, either uniform or any shape required for the processing
  • Optimum and reproducible product quality and yield.
  • Shorter setup and more efficient processing.
  • Energy savings thanks to the optimum temperature distribution, minimising thermal dissipation.
  • Reduced stress on components and machine parts, without the need for a laborious trial-and-error adjustment process.

GTC is an enhancement of conventional PID control that makes use of two additional elements in the PID control loop – a mode converter and a pre-compensator.

The mode converter uses an advanced algorithm to convert the process values (PVs) from the output of the PID controllers into an average temperature and a series of gradient temperatures or temperature differences.

The pre-compensator decouples thermal interference between heating zones.
With thermal interference eliminated, the mode converter is able to interact via the multiple feedback loops to automatically minimise gradient temperature scatter and rapidly create a well-controlled 2D temperature profile over a defined area – eliminating the damaging effect of hot-spots without the time-consuming repetition necessary in a system with thermal interference.

Additionally, the autotune feature also allows the control scheme to identify the optimum pre-compensator and PID parameters. This will reduce the configuration and set-up time in most applications thus reducing the overall costs in commissioning these instruments.

Conclusions

Combination of GTC and PID technology allows the average user to tune, commission, run and maintain control of sheet material in the same way he deals with conventional single loop controls. This is a great benefit in the competitive market where the most costly aspect of a control system is its optimum commissioning. 


Coorous Mohtadi is Technical Manager, Control Components with Omron Europe, Hoofddorp, The Netherlands. www.eu.omron.com

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