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Operation
F-FEM-CNT — User’s Guide
The evaluation method in the on-board software always uses a sampling rate of
1 kHz. As, however, values between 1 Hz and 1 kHz can be set as update rates,
the software calculates an average if the update rate is lower than the input
frequency. The errors or inaccuracies are not influenced by this and are as
described above!
With this software system, theoretically, the maximum input frequency (= speed *
number of teeth) may increase up to half the reference frequency (i.e. 10 MHz).
However, it is practically impossible to reach this value because it is limited to
approx. 1 MHz (according to encoder type) by the input circuit.
The minimum input frequency is only determined by the accuracy of the calcula-
tion of the processor and can thus also result in values that tend to come close to
zero.
With speeds that low, counting pulses are not generated during every measuring
interval. Thus, it is necessary to increment the accumulated time (i.e. the
measuring intervals) that has passed since the latest occurrence of a counting
pulse to be able to calculate a fictitious speed. When the next counting pulse is
generated, the actual speed is calculated again. This calculation method is used
to simulate the coasting of an engine.
As soon as one counting pulse is generated per measurement interval (i.e. from
an input frequency of 250 Hz with double evaluation or 125 Hz with quadruple
evaluation), this incrementation of measuring intervals no longer takes place
because a new value can be calculated.
Standstill recognition must thus be performed by the automation system. The
threshold for the zero speed can be adjusted to meet the requirements.
Fig. 27 DC error at digital input
Example
Using an incremental speed encoder with 1024 marks, an input frequency of
1 Hz corresponds to a speed of 0.029 rpm.
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