Lenze E94AYFLF Digital frequency module - Manual (Page 28)

Digital frequency extension module | Parameter setting & configuration
Digital frequency output
Problem description - speed variations
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EDS94AYFLF EN 3.2 - 10/2010
3.3 Problem description - speed variations
In the digital frequency extension module rectangular encoder signals are processed. The
speed is determined by counting the edges occurring within a defined time interval (1 ms).
Due to this defined time interval of 1 ms and a finite number of encoder increments per
revolution, the calculated speed signal can only assume discrete values. This gives the
impression that the signal is very unsteady and inexact. This effect increases with lower
numbers of increments.
Example for the digital frequency output
An encoder with 2048 increments at a speed of 60 rpm is to be simulated.
 Setpoint speed n_set
DFOUT
= 60 rpm = 1 Hz
mech.
 Simulated encoder: 2048 increments
(by fourfold evaluation 4 * 2048 = 8192 edges are counted per revolution)
 Output frequency = n_set
DFOUT
* number of increments
DFOUT
= 1 Hz
mech.
* 2048
= 2048 Hz
If the measurement lasts 1 ms, 8.192 edges occur per ms. Since the count can only be an
integer, 8 or 9 increments are counted. Thus, the speed is calculated as follows:
The speed variation which is detected during this process amounts to:
General formula for calculating the expected speed variation
or
n
measure_1
60 rpm
8
8.192
-------------- ⋅ 58.59 rpm = =
n
measure_2
60 rpm
9
8.192
-------------- ⋅ 65.91 rpm = =
Δn
measure
n
measure_2
n
measure_1
– 7.32 rpm = =
Δn
measure
15000
number of increments
---------------------------------------------------------- rpm =