Lenze 8400 TopLine - Manual (Page 347)

Lenze · 8400 TopLine · Reference manual · DMS 6.0 EN · 06/2014 · TD05/TD14
347
6 Encoder/feedback system
6.3 Multi-Encoder (X8)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
6.3.4.1 Problem description - speed variations
As digital frequency, rectangular encoder signals are processed. The speed can be determined by
counting edges and a defined time interval (1 ms). This defined time interval of 1 ms and a finite
number of encoder increments per revolution cause the calculated speed signal to only accept
discrete values. This gives the impression of a very disturbed and incorrect signal. This effect
increases with a lower number of increments.
Example: An encoder with 2048 increments at a speed of 60 rpm is to be simulated via the digital
frequency output.
• Setpoint speed nOut_v = 60 rpm = 1 Hz
mech.
• Simulated encoder: 2048 increments
(by 4-fold evaluation, 4 * 2048 = 8192 edges are counted per revolution)
• Output frequency = nOut_v * number of increments ( C00030/1 ) = 1 Hz
mech.
* 2048 = 2048 Hz
At a measurement time of 1 ms, 8,192 edges per ms are achieved. As only integer counting
processes can occur, sometimes 8 and sometimes 9 increments are counted. Hence, the speed is
calculated as follows:
The perceived speed variation amounts to:
General formula for calculating the expected speed variation
or
n
mess_1
60 rpm
8
8.192
-------------- ⋅ 58.59 rpm
= =
n
mess_2
60 rpm
9
8.192
-------------- ⋅ 65.91 rpm
= =
Δn
mess
n
mess_2
n
mess_1
–
7.32 rpm
= =
ess
15000
Number of increments
---------------------------------------------------------- =