FM26001c_e.doc / Nov-11
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2.3. Gearboxes with irrational or recurrent gear ratios
In practical applications we often find gearings that cannot be properly expressed by a decimal
number (e.g. with a pinion gear of 25 : 9 pinions the decimal expression ratio is 2.7777777.....)
This will cause problems with all position-related or angle-related applications using a decimal ratio
setting. Cumulating errors will result when we set the ratio only with 3 or 4 decimal positions while
the following positions remain unconsidered.
For this reason a user may be forced to mount an extra encoder on the site beyond the gear (which
may be very laborious or even impossible under certain conditions), even though there may already
be an encoder available on the motor site.
Since FM 260 provides a proportional and a reciprocal factor, problems with irrational gear ratios
may be easily solved just by setting the real fraction values according to the number of pinions
involved (i.e. 25 : 9 respectively 2.5000 : 0.9000) rather than imperfect decimal values like 2.777)
Motor
Pinion gear
25 : 9
Explosive environment or
high-temperature environment
where mounting of an encoder
is problematical or impossible
Encoder
FM 260
0.9000
2.5000
Hint concerning the shape of the output signals:
As a result of the digital synthesis of the output frequency, the duty cycle on the output site in
general is different from 1:1. Therefore, also the phase displacement A/B cannot be 90° all the
time. These facts however, in general, do not mean any limitations or restrictions with
practical applications, since the signals will be accepted by practically all industrial counters,
drives or other target units.
At any time it is ensured that
the average frequency over several periods is accurately consistent with the frequency expected from
the input and the ratio setting
the phase displacement will at least be 45° which is more than enough for every industrial phase
discriminator to operate correctly
the number of output pulses corresponds exactly to the number of input pulses with consideration of
the conversion ratio
Typical output signal:
