59
Operation
F-FEM-CON — User’s Guide
The multi edge evaluation is selected during parameterization by using sensor
attributes with "MultiEdge" appendix.
For details, please refer to the F-FEM Parameterization manual of the particular
automation system.
4.6.7
Toothed Disk with n-2 Teeth
The counter channels 1 and 2 have recognition and correction of a tooth gap of 2
teeth. This is done by comparing the gate times of the preceding teeth with the
gate time of the current tooth. This function can be prameterized by using the
sensor attribute "Freq-2 A-Unsigned" (S. Attr. selection).
The two missing teeth (the tooth gap) are corrected by the firmware, so that a
correct speed is measured and recognized as the top dead center (TDC).
The reconstructed dead center can be output at the frequency output. Please
see Fig. 49 on page 72.
The minimum speed is 500 rpm, below that the gaps are not detected due to the
limited counter width. At the F-FEM-CON Advanced this limit is 625 rpm. These
speed limits are based on a 60 teeth disk. For a toothed disk with other teeth
count use the frequency value related to the respective speed limit to calculate
the rpm-limit (in inverse proportion to the tooth count ratio, i.e. with 50-2 teeth the
rpm-limit is 500*60/50 = 600 rpm).
Below this speed, the tooth gap correction is deactivated. This may cause the
speed to jump down to an 1/3 of the value for a short time at every revolution.
This principle also works for tooth numbers other than 60 (or 58, respectively),
because the total number of teeth is not taken into account in the correction, but
it is considered in the speed calculation! The missing number of teeth (in this
case 2) is important for the correction factor, and therefore this method is also
called "n-2".
The counter channels 3 and 4 and the zero track of the incremental encoder do
not have this built-in recognition/correction.
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