MKS 640A/641A, Manual, Page: 49 / 88

Tuning the 640 Series Pressure Controller Chapter Three: Overview
36
Tuning the 640 Series Pressure Controller
Tuning optimizes the way the 640 Series unit controls your system. The Proportional (P) and
Integral (I) terms adjust the response of the 640 Series controller. The controller responds to
changes in either the pressure of the system or the value of the set point.
Proportional Term
The Proportional (P), or gain, term is used as a constant to create a valve drive signal that is
proportional to the error signal. The error signal is multiplied by the proportional control setting,
thus creating a proportional valve drive signal. The higher the proportional control, the greater
the change in valve drive signal. Typically, a higher proportional control setting yields a faster
response. However, too high a proportional control setting will cause the pressure to oscillate
around the set point. Too low a proportional control setting will result in a slow response from
the controller. Figure 15 shows the effects of the Proportional term.
Set Point Signal
Pressure Signal
Time
Pr
e
ss
u
re
Gain (Proportional term) set too low,
causing a slow response
Pressure Signal
Set Point Signal
Time
Pr
e
ss
u
re
Gain (Proportional term) set too high,
causing oscillation
Figure 15: Effects of the Proportional Control
Adjusting the Proportional Control
The Proportional (P) term adjustment is located on the top of the 640 Series unit, as shown in
Figure 13, page 33. The control is a 10-position dial, though it uses only 8 values. The last
positions, 8 and 9, repeat the values of positions 0 and 1. The initial setting is 0. As you
increase the setting number, the value of the term increases by a factor of approximately 2.8.