Improving Process Control

Many factors can slow production. Find out how one manufacturer improved product throughput and reduced energy costs by improving process control.

Watlow's SCADA software allows an operator to manage the entire environmental testing process.

To comply with extreme operational temperatures, many manufacturers employ environmental testing chambers to cycle their products prior to shipment. They rely on efficient chambers to meet tight shipping deadlines. However, a number of chamber factors, including the heating and cooling systems and the control process, can slow the production process.

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.

The chamber's electronic components are shown wired for testing.

Mark Clevenger, controls engineer and service manager at Qualtest, determined that optimizing the control system required the use of PID loops for temperature control, sequential logic for interlocks, and an operator interface terminal. The panel space demanded a compact control system that could interface with the existing equipment and wiring, which consisted of thermocouples, AC input signals (switches and interlocks), AC (solenoid valves) and DC output devices (solid-state relays for heaters). Concurrently, a separate, remote-mounted data-acquisition system used to continuously collect information during operation was integrated into the control system.

"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.

Sammy Lopez, engineering technician for SMP, stands next to the 15-year-old environmental test chamber that was rebuilt by Qualtest.

Upon choosing a controller, Watlow was tasked with coding the control strategy for the chamber's control process and data collection. This process is responsible for the chamber's operation and efficiency. When dealing with environmental chambers, Clevenger explained, the goal is to achieve setpoint as quickly and as accurately as possible while using minimal energy. Although each PID loop contains both heating and cooling algorithms and separate outputs, he chose a different strategy to maximize throughput and minimize energy cost.

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.

Qualtest chose to use Watlow Anafaze's PPC-2000 as the chamber's control system as it combines PID control and ladder logic into one compact device.

"The control process was simple and efficient," Clevenger said. "Not only did it minimize energy consumption, it also minimized wear on the refrigeration system."

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.

For more information about Watlow's products, call (800) 492-8569 or visit www.watlow.com.