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For many simple Processes, PID control works well. But, for multizone processes where the heating zones interact,a model-based multizone control may prove more effective at minimizing tempature overshoot and thermal lag.
Multizone heated systems exist in many forms, including ovens, heated platens, and plastic extruding or injection molding equipment. Typically, temperature feedback control is used to regulate or drive these systems to the required processing temperatures. Effective temperature control is a challenge in these types of systems because each zone is not insulated from the effects of neighboring zones, creating interactions from zone to zone. In these applications, simple control may not be able to regulate the heater effectively.
Consider a two-zone heated platen used to control the temperature of a load (figure 1). It has two heater zones arrayed in concentric circles and two corresponding temperature sensors. The position of the heaters and sensors creates a dynamically coupled, multi-input, multi-output (MIMO) system, where the effects of one heater zone are seen at both sensors. In practice, the platen must operate under varying temperature ranges and loads and adjust to disturbances due to load entry and exit.
Limitations Using PID Controllers
The traditional approach to temperature control for multizone systems is to use PID control independently for assigned heater/sensor pairs. In the two-zone platen example, the center heater would be paired with the center sensor and the outer heater with the outer sensor. In general, PID control is beneficial because:
It has a simple, fixed control structure with few design knobs that must be tuned.
It is readily available from many suppliers. It usually will work, to some degree.
For multizone applications, however, PID control is lacking in that:
It cannot account for dynamically coupled MIMO systems. PID control's single-input, single-output (SISO) structure does not allow coordinated control of multiple, independent heater commands. The...