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Gradient Temperature Control

Hotspots and large temperature gradients are a big problem in the thermal and chemical processing of sheet materials, including glass laminates, ceramic foils, silicon wafers, plastics, as well as in moulding machines and food processing. Differences in temperature across the product can cause loss of quality and yield.

In thermal processes, surface temperature scatter is observed in steady and transient states. In the steady state, it is suppressed with integral action and by proper design of equipment.  In the transient state, however, it is caused by different rates of thermal conduction from the heaters to the product and their interaction, whereby each zone is affected differently.

This is more acute where the thermal plate is wide and thin.  Many designs, from semiconductor wafers to glass lamination in solar panels, fall into this category.  Improved quality and yield require a uniform temperature profile at all times.

Multi-loop controllers use multi-heaters to reduce this gradient.  The heated surface is divided into many areas, and precise controls are used in each area.  However, thermal interference between the zones affects dynamic stability and control accuracy.  Thus, it is difficult to realize the precise temperature control system based on conventional PID control alone.

How GTC works

GTC is an enhancement of conventional PID control that makes use of two additional elements to the PID control loop – a mode converter and a pre-compensator.  The mode converter converts the process values (PVs) from the output of the PID controllers into an average temperature and a series of gradient temperatures. The pre-compensator reduces the thermal interference between heating zones.  With thermal interference eliminated, GTC is able to minimize gradient temperature scatter and rapidly create a well-controlled 2D temperature profile over a defined area – eliminating the damaging effect of hotspots.  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 of commissioning these instruments.

The graph below shows a typical sequence for the autotune procedure where the pulse response of the overall system is examined.  The tuner computes the precompensator parameters and the PID gains.

The response of the system to a step change ins setpoint is shown with GTC on the left and without GTC in the right hand graph. Typically there is a 5 fold improvement in the deviation response of the system.

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Created 2008-05-12
Modified 2008-09-23
Views 3086


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