Siemens SIMATIC Manual page 46

Controller Tuning in FB 58 "TCONT_CP"
Quality of the measuring signals (measurement noise, low-frequency
interference)
The results of the tuning can be distorted by measurement noise or by low-
frequency interference. Note the following:
•
If you encounter measurement noise, set the sampling frequency higher rather
than lower. During the noise period, the process variable should be sampled at
least twice. In pulse mode, integrated mean value filtering can be helpful. This
assumes, however, that the process variable PV is transferred to the block in
the fast pulse cycle. The degree of noise should not exceed 5% of the useful
signal change.
•
High-frequency interference cannot be filtered out by a software block. This
should be filtered earlier in the measuring sensor to prevent the aliasing effect.
The schematic below illustrates the aliasing effect when the sampling time is
too high:
X
•
With low-frequency interference, it is relatively easy to ensure an adequately
high sampling rate. On the other hand, the TCONT_CP must then generate a
uniform measuring signal by having a large interval in the mean value filtering.
Mean value filtering must extend over at least two noise periods. Internally in
the block, this soon results in higher sampling times so that the accuracy of the
tuning is adversely affected. Adequate accuracy is guaranteed with at least 40
noise periods to the point of inflection. Possible remedy when repeating the
attempt: Increase TUN_DLMN.
3-14
t
PID Temperature Control
A5E00125039-02