Our most recent post addressed the benefits of proportional – only vacuum process control. Though this approach is well suited to many processes, in some cases more accurate (or precise) control of the pressure level is necessary. Under such circumstances, employing PI (proportional-integral), or full PID (proportional-integral-derivative) control algorithms are appropriate solutions.
If a P-only approach is used, the actual pressure will typically vary from the set pressure level by a certain amount. Once the proportional gain is well-tuned, the error between the actual pressure and the set pressure will often be fairly stable. This error is often called the offset. By using the integral parameter, this offset can be eliminated. The integral parameter is so called because its output is equal to the integral of the offset with respect to time. More simply, it is the product of the scale of the error, the time during which the error has persisted, and a user-adjustable constant value. Consequently, the integral parameter’s contribution to the control signal grows both with the scale of the offset and time. As the integral parameter grows in value, it acts to change the output from the control algorithm in order to cause the actual pressure to equal the set pressure level.
In some situations, such as when a large change in set point occurs, the integral parameter can lead to oscillatory behavior in the actual pressure value. This oscillation can be eliminated by using the derivative parameter to further improve the responsiveness of the controller. The derivative parameter’s impact on the control signal is to make it sensitive to the rate of change of the error (offset). Using the derivative parameter can be particularly helpful in cases where the measured pressure level varies with time. It also helps to avoid overshooting the set vacuum level.
The net result of implementing a complete PID control algorithm is to realize more precise process control which can respond more quickly to variations in the process conditions. In the case of vacuum control, this can be level of control could be achieved by controlling a valve, or by controlling the motor speed of a pump.
VACUUBRAND’s VARIO® controlled pumps take the latter approach. By adjusting the pump’s motor speed, VARIO pumps are able to precisely maintain the desired process pressure to within a few tenths of one torr of the target. In addition to the ability to maintain a user-adjustable set point, these pumps also include ‘automatic’ mode, which is intended for use in drying operations. The automatic mode enhances the typical PID algorithm to consider the characteristics of a process that includes the evaporation of solvents. To minimize process times, the pump will continuously adapt the motor speed to keep the rate of solvent evaporation to within an acceptable range. In the event that the rate of evaporation rises above an acceptable level, the pump will slow down so as to avoid product loss.
If you’d like to discuss how VACUUBRAND’s control capabilities might help to improve your process, please contact us.