412LXR Series Product Manual
Chapter 4 - Performance
31
•
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)
150
8
1200
39
250
12
1500
44
350
16
1750
46
650
26
2000
50
800
31
2500
50
1000
35
3000
50
Thermal Effects on Accuracy
All specifications for the 412LXR are taken at 20
°
C. Variation from this temperature will cause additional
positional errors. If the base of the 412LXR 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 412LXR with 1000mm travel is being used. The accuracy over the entire travel is 35 microns
@ 20
°
C. If the base temperature increases by 5
°
C an additional error of 110 microns will be added over
the total travel (0.000022mm/mm/
°
C)*1000mm*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 412LXR with respect to base temperature and travel.
Each line represents the additional error of the table caused by the elevated temperature.
