Beckhoff EPP3504-0023 Short Manual page 103

For an illustrative representation, the ScopeView recording was started first and then the program, which
limits the decimated values to one second:
IF nOVS_CycleCount = 1000000000 THEN
;
bEnable := FALSE;// Stop after 1s just for recording
ELSE
...
This line can, of course, be commented out for further adjustments:
//bEnable := FALSE;// Stop after 1s just for recording
Notes:
• the target sampling rate f
time interval between two decimated values. The desired decimation may require further parameters
such as task cycle time, oversampling factor etc. to be adjusted both in the configuration and as
variable initialization in the sample program (see figure "Process of variable decimation of the sample
program", which illustrates the functionality of the program code).
• Basically, the conversion process in this sample program causes distortions in the result in relation to
the original signal shape when decimating with fractional rational factors (see signal curve). In concrete
terms, deviations from the original signal curve only occur in those sections where the time derivative
value (the slope) is not constant. For example, input values of a sine signal in the non-linear sections
are distorted by the interpolation performed in the program:
In the frequency spectrum, for example by a calculation with 20 Hz sinus signal, sampled with 500 sps
and decimated to 441 sps, this is illustrated as follows:
• If no low-pass filtering corresponding to f
occur! It is therefore advisable to perform low-pass filtering in the PLC, e.g. with the TC3 Controller
Toolbox or the TC3 Filter Library, before the conversion/decimation is performed. Suitable filters can
easily be created with the TE1310 FilterDesigner. For more information, see www.beckhoff.com:
EPP3504-0023
should be close to the sample rate f
target
is performed on the data stream, aliasing effects will
target
Version: 1.2
Commissioning
, so that it is possible to evaluate a
max
103