Laser beta lasermike LS8000-3 - Manual (Page 41)

LaserSpeed 8000-3 Instruction Handbook
Interfacing with the LS8000-3
Part No. 93463 / Drawing No. 0921-01561 Page 41 of 221 Revision A (Sep 2007)
Quadrature Outputs at Extremely Low Pulse Frequencies
Note: This section only applies to the pulse outputs—the length and speed
available over the RS-232, RS-422, and Ethernet ports are not affected.
The LS8000-3 uses a DDS (Direct Digital Synthesis) circuit to generate the
quadrature outputs. Due to the nature of DDS technology, the frequency
output is not continuously variable, but occurs in steps of about 0.015Hz.
Normally, relative to the output frequency, this step size is so small that it is
impossible to measure. However, if both a very low speed is measured, and a
very low pulse
resolution is used, the
pulse output
frequency will be
frequency low enough
for the step size to
cause noticeable
errors in
measurement. The
following graph shows
the envelope of the
pulse output
frequency error as the
pulse output
frequency approaches
zero.
The error decreases very rapidly as the frequency increases, becoming
insignificant at pulse output frequencies higher than 15Hz. For this reason, the
pulse output resolution should be set such that the minimum production line
speed produces a pulse output frequency higher than 15Hz.
The following equations can be used to calculate the minimum pulse output
resolution.
Definitions:
R = Minimum Recommended Pulse Output Resolution
X = Minimum Production Linespeed
10
-2
10
-1
10
0
10
1
10
2
10
3
0
1
2
3
4
5
6
7
8
Pulse Frequency / Hz
P
ul
se F
requen
cy
E
rror E
nv
el
ope /
|%
|
Pulse Frequency Error vs. Pulse Frequency