MKS T3PIA, Instruction Manual, Page: 36 / 70

Chapter Three: Product Overview Control Mode
28
Phase and Gain
The T3P unit uses a PID compensator to control pressure in a vacuum system. When a new pressure setpoint
is commanded, the unit responds by changing the pressure smoothly to the desired value. If the pressure is
slow to change or oscillates, the PID compensator must be re-tuned. The phase and gain values can be
manually adjusted to provide the best response to the setpoint. By careful adjustment of each value, it is
possible to achieve optimum control throughout a wide range of pressure regions.
Phase
The phase provides a control signal that is proportional to the change in the error signal. The error signal is
the difference between the actual pressure and the setpoint. The phase is responsible for controlling how
quickly the pressure responds to a change in setpoint. A large phase generally results in a faster response to
setpoint. However, if the phase is too large, the system will be slow in responding to setpoint and, in some
cases, may oscillate around the setpoint (refer to Figure 7). If the phase is too small, the pressure will
overshoot and then oscillate around the setpoint before settling in (refer to Figure 8).
Pre
s
s
u
re
Time
Set Point
Figure 7: Phase Set Too High
Pre
s
su
re
Time
Set Point
Figure 8: Phase Set Too Low