412LXR Series Product Manual
Chapter 4 - Performance
29
Speed Limits
The Maximum Speed of the 412LXR 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 412LXR reduces as speed increases. (Chapter 2, 412LXR Series Technical
Data)
Encoder Accuracy and Slope Correction
Encoder Accuracy
The 412LXR 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 412LXR.
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 412LXR 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.
