Improving Process Control
Standard Motor Products (SMP), an Orlando-based automotive parts OEM, found this to be true as it linked production slowdowns and rising energy costs to a 15-year-old environmental chamber. Instead of replacing the old chamber, the OEM contacted Qualtest Inc., Orlando, Fla., a provider of environmental test services and equipment, to rebuild it.
Upon examination, Qualtest discovered that worn mechanical components and a dated control system could not efficiently support SMP's stringent control requirements. Watlow, a thermal solutions provider based in St. Louis, was called in to assist Qualtest in maximizing chamber control and process efficiency.
"One of our biggest hurdles was panel space," said Clevenger. "While the chamber required both control and data acquisition functions, its controller footprint was pretty tight."
Qualtest chose to use Watlow Anafaze's PPC-2000 as the chamber's control system. This controller combines all control functions into one device, and the Watlow device fit within the original controller's footprint. In addition, it offered a distributed zero-crossing, or burst-firing, capability that enabled the controller to minimize temperature extremes to provide a smooth, even temperature output.
Typically, environmental chambers use both heating and cooling processes to achieve setpoint, often bucking one against the other at or near setpoint. Clevenger's strategy required only the use of heat during the heating and high soak steps. Additionally, adaptive tuning was incorporated during both the heating and cooling cycles. This provided a rapid climb to setpoint, then scaled back the heating or cooling algorithm by changing the Pb, I and D as the temperature closed in on setpoint.
The compressor was turned on only during the cooling and low soak steps. As the zones cooled within two degrees of the low setpoint, heat was added to minimize undershoot. This process maintained setpoint while requiring the refrigeration compressor to run only once during each complete temperature cycle.
Autotuning was used to tune the loops throughout both the heating and cooling cycles. Unique Pb, I and D values were used for heating and cooling the ramp and soak cycles. The values were moved into the appropriate registers at the start of the associated ramp step or as the temperature approached the final setpoint. Because setpoints were achieved with minimum overshoot, energy consumption was minimized. Refrigeration wear also was minimized because during cool down and soak cycles the system ran continuously.
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