Siemens SIMATIC Manual page 26

Continuous Temperature Controller FB 58 "TCONT_CP"
Minimum Pulse or Minimum Break Time (P_B_TM)
Short on and off times reduce the working life of switching elements and actuators.
These can be avoided by setting a minimum pulse or minimum break time
P_B_TM.
Small absolute values at the input variable LmnN that could otherwise generate a
pulse time shorter than P_B_TM are suppressed.
High input values that would generate a pulse time longer than PER_TM - P_B_TM
are set to 100%. This reduces the dynamics of pulse generation.
Values of P_B_TM ≤ 0.1 * PER_TM are recommended for the minimum pulse and
the minimum break time.
The doglegs in the curves in the diagram above are caused by the minimum pulse
time or minimum break time.
The schematic below illustrates the switching response of the pulse output:
1
Minimum
on time
P_B_TM
Accuracy of Pulse Generation
The smaller the sampling time of the pulse generator CYCLE_P is compared with
the pulse repetition period PER_TM, the more accurate the pulse duration
modulation. For adequately accurate control, the following should apply:
CYCLE_P ≤ PER_TM/50
This means that the value of the manipulated variable is converted into pulses with
a resolution of ≤ 2% (see also of the example in Section 2.4.3, Page 2-15).
Notes
When you call the pulse generator cycle, you have to note the following:
If you call the controller in the pulse generator cycle, the process value will be
averaged. As a result, different values may be output on the output PV and the
input PV_IN or PV_PER. If you want to correct the setpoint value, you have to
save the process value on the input parameter PV_IN at the call-up points of the
entire controller processing (QC_ACT =TRUE). Other calls between the pulse
generator cycle call and the call-up points of the the entire generator cycle are
forwarded to the input parameters PV_IN and SP_INT with the saved process
value.
2-12
PER_TM
PER_TM
Minimum
off time
P_B_TM
PER_TM
PID Temperature Control
A5E00125039-02