404LXR Series Product Manual
Chapter 4 - Performance
31
Speed Limits
The Maximum Speed of the 404LXR is limited by three (3) factors:
•
Linear Bearings
The linear bearings are limited to a maximum speed of 3 meters/second.
•
Linear Encoder Limit
The linear encoder has speed limits relative to encoder resolution; these limits are listed below:
Encoder
Resolution
Maximum Velocity
Required Post
Quadrature Input
Bandwidth (²)
5 micron
5 meters/second (¹)
2 Mhz
1 micron
3 meters/second
6.7 Mhz
0.5 micron
1.5 meters/second
6.7 Mhz
0.1 micron
0.3 meters/second
10 Mhz
(¹)
When using an encoder with 5 micron resolution, the maximum speed is limited by the square rail bearings.
(²)
This is the bandwidth frequency that the amplifier or servo control input should have to operate properly with the
encoder output at maximum speeds. This frequency is post-quadrature, to determine pre-quadrature divide above values by
4. Above frequencies include a safety factor for encoder tolerances and line loses.
•
Force / Speed Limit
The available force of the 404LXR reduces as speed increases. (Chapter 2,
404LXR Series
Technical Data
)
Encoder Accuracy and Slope Correction
Encoder Accuracy
The 404LXR Series makes use of an optical linear tape encoder for positional feedback. This
device consists of a
readhead
, which is connected to the carriage, and a
steel tape scale
, which is
mounted inside the base of the 404LXR.
The linearity of this scale is +/-3 microns per meter, however the absolute accuracy can be many
times larger. To compensate for this error, an error plot of each 404LXR is done at the factory
using a laser interferometer. From this plot a linear slope correction factor is calculated (see
below). Then a second error plot is run using the slope correction factor. These tests are
conducted with the Point of Measurement (P.O.M.) in the center of the carriage 38 mm above the
carriage surface.
Slope Correction
Slope correction is simply removing the linear error of the table. The graphs below show an
example of a non-slope corrected error plot and the same plot with slope correction. As can be
seen, the absolute accuracy has been greatly improved.
The slope factor is marked on each unit. It is the slope of the line in microns per meter. This factor
may be positive or negative, depending on the direction of the error.
