MKS 640A/641A, Manual, Page: 58 / 88

Chapter Four: Operation How To Adjust the Span
45
How To Adjust the Span
Only adjust the
SPAN
pot in conjunction with a calibration transfer standard. Do not adjust the
span setting if a calibration transfer standard is not available. Instead, contact an MKS Service
Center for calibration.
How To Tune the 640 Series Controller
You may need to tune the 640 Series controller to optimize how it controls your system. Tuning
consists of varying the P (Proportional) and I (Integral) parameters to achieve the fastest,
smoothest response to changes in the set point value. Ideally, the 640 controller should respond
to a new set point value by rapidly changing the pressure in the system to match the set point
with little under or overshoot. Refer to How The 640 Series Pressure Controller Works, page 35,
for a complete description of the effects of the P and I terms.
The P term and the I term are initially set to position 0. The controls are located on the top of the
unit, as shown in Figure 13, page 33. Each control has a 10-position dial. The P term has only 8
values; positions 8 and 9 repeat the values of positions 0 and 1.
1. Send your set point signal. If you are using multiple set points send the most critical set
point.
2. Change the set point in the direction that you expect the system to deviate in, and
observe the controller response.
A properly tuned controller will reach the new set point rapidly, without overshoot.
•
If the controller is too slow to reach the set point, increase the P term.
•
If the pressure fluctuates around the set point, decrease the P term.
•
If the pressure overshoots the set point and then settles to the correct pressure,
increase the I term.
3. Repeat step 1 until the response of the controller is optimized.
4. Change the set point in the opposite direction, and observe the response.
Although the response may vary slightly, it should be acceptable. If it is not acceptable,
follow the guidelines in step 1 to tune the controller.