Precision Vacuum Oven Control

Working with critical temperature and pressure specifications is something we do everyday here at GOCI.  We work in some of the most demanding markets in industry such as Military, Aerospace, Semiconductor, Medical and Pharmaceutical.  We have prided ourselves on staying on the very edge of control technology.  It is one of the features most attractive at GOCI.  Our capabilities include the implementation of PLC's, HMI (Human Machine Interface) or Touch screens,  Programmable Controllers and SCR power control.

When processes has been defined by our customers, typically the next step is gearing the process up for production.  One of the key attributes of gearing up is to have  accuracy with limited operator intervention.  That is why we use PLC's and Touch screens.  Using these components limits the day-to-day operators from accessing pre-defined recipes, profiles and control settings. 

More importantly thermal control clearly plays the decisive role in precision processing.  There are several factors that customers must be aware of when specifying an operating temperature for a process.  First, What do we want  the part or the oven at the set-point temperature?? Second, How fast do you want heat and cool the part??  These two questions alone will decide what heating method should be used in the construction of your system.  Thermal control comes in two methods simple Proportional-Integral-Derivative or Cascade.

Proportional-Integral-Derivative Control

Some processes need to maintain a temperature or process value closer to the set point than on-off control can provide. PID (Proportional-Integral-Derivative) control provides closer control by adjusting the output when the temperature or process value is within a proportional band. When the value is in the band, the controller adjusts the output based on how close the process value is to the set point; the closer to set point the lower the output. This is similar to backing off on the gas pedal of a car as you approach a stop sign. It keeps the temperature or process value from swinging as widely as it would with simple on-off control. However, when a system settles down, the temperature or process value tends to “droop” short of the set point. With proportional control the output power level equals (set point minus process value) divided by prop-band.  The droop caused by proportional control (reset) can be corrected by adding integral control. When the system settles down the integral value is tuned to bring the temperature or process value closer to the set point. Integral determines the speed of the correction, but this may increase the overshoot at startup or when the set point is changed. Too much integral action will make the system unstable. Integral is cleared when the process value is outside of the proportional band. Integral (if units are set to SI) is measured in minutes per repeat. A low integral value causes a fast integrating action.  Reset rate (if units are set to U.S.) is measured in repeats per minute. A high reset value causes a fast integrating action.  Use derivative rate control to minimize overshoot in a PI-controlled system. Derivative adjusts the output based on the rate of change in the temperature or process value. Too much derivative will make the system sluggish.

Cascade Control

Cascade control is a control strategy in which one control loop provides the set point for another loop. It allows the process or part temperature to be reached quickly while minimizing overshoot. Cascade is used to optimize the performance of thermal systems with long lag times. This graph illustrates a thermal system with a long lag time. Curve A represents a single-loop control system with PID parameters that allow a maximum heat-up rate. Too much energy is introduced and the set point is overshot. In most systems with long lag time, the process value may never settle out to an acceptable error. Curve C represents a single-control system tuned to minimize overshoot. This results in unacceptable heat-up rates, taking hours to reach the final value. Curve B shows a cascade system that limits the energy introduced into the system, allowing an optimal heat-up rate with minimal overshoot.
Cascade control uses two control loops (outer and inner) to control the process. The outer loop (analog input 3) monitors the process or part temperature, which is then compared to the set point. The result of the comparison, the error signal, is acted on by the settings in a Cascade Outer Loop PID set (1 to 5), which then generates a power level for the outer loop. The set point for the inner loop is determined by the outer-loop power level and the Cascade Low Range/Deviation and the Cascade High Range/Deviation settings for analog input 3. The inner loop (analog input 1) monitors the energy source (heating and cooling), which is compared to the inner loop set point generated by the outer loop. The result of the comparison, the error signal, is acted on by the settings in a Cascade Inner Loop PID set (1 to 5), which generates an output power level between -100% to +100%. If the power level is positive the heat will be on; if the power level is negative the cool will come on. In Our Series F4AV controllers, cascade control is available on channel 1. Analog input 3 is used to measure the outer-loop process while analog input 1, the inner loop, is used to measure the energy source. Power from the energy sources are supplied by outputs 1A
and 1B.