404LXR Series Product Manual
Chapter 4 - Performance
33
•
Step #3: We subtract the error from the original distance (880
– 0.0748) = 879.9252 mm.
•
Step #4: Here we simply assign the new calculated distance as our current command
distance.
This same program works if the slope error is negative. For example, if the slope error was
–0.085
instead of +0.085 the equation would work out like this:
VAR2 = (880/1000)*(-0.085) = -0.0748
VAR3 = 880
– (-0.0748) = 880.0748
Thus correcting for the negative slope.
Note: Above are examples for incremental moves. The same program works if programming in
absolute coordinates.
Note: Each unit is shipped with both the non-slope corrected accuracy plot and a slope corrected
plot. These plots can be used to “MAP” the table, making positioning even more accurate. Mapping
is correcting for the error of the device at each location. This can be done by knowing the motion
positions and the error at each of these positions and setting up a matrix of variables in your motion
program. This method provides excellent accuracy but is time consuming to setup.
Attainable Accuracy with Slope Correction
Travel
(mm)
Accuracy
(microns)
Travel
(mm)
Accuracy
(microns)
50
6
400
18
100
7
500
21
150
8
600
25
200
10
700
28
250
12
800
31
300
14
900
33
350
16
1000
35
Thermal Effects on Accuracy
All specifications for the 404LXR are taken at 20
C. Variation from this temperature will cause
additional positional errors. If the base of the 404LXR varies from this temperature the encoder
scale will expand or contract, thus changing its measuring length and thus encoder resolution.
The factor by which this thermal effect occurs is 0.000022mm/mm/
C. Although this sounds like a
very small number it can make significant accuracy and repeatability effects on your applications,
especially on longer travel applications. To understand this better let
’s look at an example:
Example: A 404LXR with 900mm travel is being used. The accuracy over the entire travel is 40
microns @ 20
C. If the base temperature increases by 5
C an additional error of 99 microns will
be added over the total travel (0.000022mm/mm/
C)*900mm*5
C. As you can see this error is
significant. However, this additional error can be compensated for since the error is linear.
On the next page is a graph of the accuracy of the 404LXR with respect to base temperature and
travel. Each line represents the additional error of the table caused by the elevated temperature.
