Siemens SIMATIC S7 System Manual page 296

Program instructions
7.7 Pulse output
If the frequency difference (Δf) between the end of a segment and the beginning of the next
is not acceptable, try adjusting the ending frequency to compensate for the difference. This
adjustment might be an iterative process to get the output within an acceptable frequency
range.
Note that changes in segment parameters affect the time it takes the PTO to complete. You
can use the equation for the time duration of the segment, found later in this section of the
manual, to see what effect this has on the timing. An accurate segment duration time can
require some flexibility in the value of the ending frequency or the number of pulses for a
given segment.
While the simplified example above is useful as an introduction, real applications can require
more complicated waveform profiles. Remember that you can only assign frequencies as an
integer number of Hz and perform the frequency modification at a constant rate. The
S7-200 SMART CPU selects that constant rate and that rate can be different for each
segment.
For legacy projects that were developed in terms of cycle time, instead of frequency, you can
use the following formulas to convert to frequency:
CT
Final
F
Initial
F
Final
where:
CT
Initial
ΔCT
PC
CT
Final
F
Initial
F
Final
The acceleration (or deceleration) and time duration of a given PTO profile segment can be
useful in the process of determining correct profile table values. Use the following formulas
to calculate the length of time, as well as the acceleration for a given profile:
ΔF = F
T = PC / (F
s
A = ΔF / T
s
where:
T
s
A
s
PC
F
min
ΔF
296
= CT + (ΔCT * PC)
Initial
= 1 / CT
Initial
= 1 / CT
Final
Starting cycle time (s) for this segment
Delta cycle time (s) of this segment
Quantity of pulses in this segment
Ending cycle time (s) for this segment
Starting frequency (Hz) for this segment
Ending frequency (Hz) for this segment
- F
Final Initial
+ ( | ΔF | / 2 ) )
min
s
Time duration (s) of this segment
Frequency acceleration (Hz/s) of this segment
Quantity of pulses in this segment
Minimum frequency (Hz) for this segment
Delta (total change in) frequency (Hz) for this segment
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART