Establishing good control over vacuum processes has numerous benefits, such as helping to improve process yields, achieve more repeatable results, and reduce process times. And there are several options for controlling vacuum processes to achieve these various benefits. This post will briefly review some of the more common approaches to vacuum control. Subsequent posts will delve into further detail of these techniques.
One of the most common approaches to controlling vacuum levels is to use 'false air.' This technique involves bleeding in air or an inert gas at a known rate downstream of a vacuum operation, but upstream of the pump itself. The extra flow passing through the pump causes a reduced vacuum level (i.e., higher absolute pressure). The false air bleed rate can be adjusted to tailor the process pressure as desired. Though relatively simple to implement, the control scheme is particular to a given pump model with a specific pump performance curve. Because no two pump models have the same curve, bleed rates need to be adjusted if a new pump is installed.
More reliable and transferable vacuum control can be achieved using PID (for Proportional-Integral-Derivative) control algorithms. A relatively simple implementation of this type of algorithm is to use an electronic controller, vacuum sensor, and solenoid valve with a proportional-only control scheme. This approach is referred to as P-only control or 2-point control. In these cases, a vacuum set point is established that the algorithm will target. In the simplest implementation, the actual vacuum level is measured by the sensor. If the actual pressure level is below the set point, the solenoid valve will close, and if the actual pressure is above the set point, the valve will open so that the pump can reduce the pressure. A major advantage of this approach is that it can be readily adapted to other processes or new equipment by tuning the proportional control parameter until the desired level of control is achieved. A downside of all proportional-only control algorithms is that there is an unavoidable 'dead band' around the set point where the process parameter will float freely. In the case of vacuum controllers, the dead band is typically only a few torr (or millibar). For many applications, this hysteresis is acceptable; for some applications, it is not.
In these cases, the I (integral) and/or D (derivative) parameters in a PID algorithm can be employed to achieve more precise control. Using either or both the 'I' and 'D' parameters can eliminate the dead band and reduce the difference between the target value and the actual process value. VACUUBRAND's VARIO® control scheme does this by continuously adjusting the pump's motor speed - rather than a solenoid valve - in order to precisely hold the target vacuum level, often to within a few tenths of one torr of the target. Beyond precision, this approach has a variety of benefits ranging from improving yields by reducing sample loss to minimizing pump maintenance.
If you'd like to discuss how VACUUBRAND's control capabilities might help to improve your process, please contact us.