Hysteresis - Siemens SIMATIC S7-300 FM 350-1 Manual

7.3.9

Hysteresis

Function principle of the hysteresis
An encoder can come to rest at a particular position and then "oscillate" about this position.
This causes the count value to fluctuate around a certain value. If there is a comparison
value within this fluctuation range, for example, the associated output would be switched on
and off in time with these fluctuations. FM 350-1 features a programmable hysteresis in
order to prevent this switching in response to small fluctuations.
A value between 0 and 255 may be set for the hysteresis.
Table 7- 24
Hysteresis
Hysteresis value n = 0, 1
2 ≤ Hysteresis value n ≤ 255
The hysteresis applies to both overflow and underflow.
Function principle of the hysteresis when "Active within the range between the comparison value and
overflow/underflow" is set
The following figure shows an example of the effect of hysteresis. The diagram shows the
differences in the output behavior when hysteresis values of 0 (= switched off) and 3 are
assigned. In the example, the comparison value = 5
Parameter settings in this example:
● Main count direction up
● Active within the range from comparison value to overflow
The hysteresis is set when the comparison condition (counter value = 5) is fulfilled. When the
hysteresis is active, the comparison result remains unchanged.
FM 350-1 Counter module
Manual, 05/2011, A5E03648639-01
Effect of the hysteresis
Effect
The hysteresis has no effect (switched off)
The output reacts to the slightest change in the counter value.
The hysteresis takes effect.
The output does not react until the counter value is offset by n units to
the comparison value.
Operating Modes, parameters and commands
7.3 Count modes
117