West Control Solutions 8700+ - Manual (Page 122)

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- DIN Controllers & Indicators - Product Manual
59305, Issue 7
– March 2014 Manual Tuning Page 117
P
Input Span
T
6
12 Manually Tuning Controllers
Single Control Tuning (PID with Primary Output only)
This simple technique balances the need to reach setpoint quickly, with the wish to limit
setpoint overshoot at start-up or during process changes. It determines values for the
Primary Proportional Band (

), Integral Time Constant (

) and Derivative Time
Constant (

) that allow the PID control algorithm to give acceptable results in most
applications that use a single control device.
CAUTION:
This technique is suitable only for processes that are not harmed by large
fluctuations in the process variable.
1. Check that the Setpoint Upper Limit (

) and Setpoint Lower Limit (

) are set to
safe levels for your process. Adjust if required.
2. Set the Setpoint to the normal operating value for the process (or to a lower value if
overshoots beyond this value might cause damage).
3. Select On-Off control (i.e. set

= 0).
4. Switch on the process. The process variable will oscillate about the setpoint. Record the
Peak-to-Peak variation (P) of the first cycle (i.e. the difference between the highest value
of the first overshoot and the lowest value of the first undershoot), and the time period of
the oscillation (T) in minutes. See the example diagram below - Manually Tuning PID.
5. Calculate the PID control parameters using the formula below. Input Span is the difference
between Scale Range Lower Limit and Scale Range Upper Limit:

= x 100

= T minutes

= minutes
Figure 39. Manually Tuning PID
Time
P
roce
ss
V
ari
ab
le