Siemens SINUMERIK 840C - Manual (Page 735)

11.92 10 Axis and Spindle Installation
10.4.5 Distance-coded reference marks
10.4.5.1 Initial installation of distance-coded reference marks
The following steps must be followed when installing the distance coded reference marks for
the first time:
1. Selecting the measuring system
MD 1808*, bit 4, must be set for distance coded reference point approach.
2. Calculating the maximum permissible velocity for reference point approach
(Creep speed MD 284*)
The velocity used for reference point approach (creep speed) must not exceed a maximum
value. The velocity must be so slow that the time taken to cross the smallest possible refer-
ence mark distance on the scale is always longer than one cycle of the position controller
(PCC).
The smallest possible distance between two adjoining reference marks on the scale is
calculated as follows:
· Xmin =
Measured length on scale
a
a
2
k -
a: Basic distance (in multiples of the scale division)
b: Scale division (grid spacing [in µm])
The maximum possible creep speed for the reference point approach is calculated as follows:
Vmax <=
Xmin
PCC
Xmin: Smallest possible reference mark distance
PCC: Set position controller cycle
A value which is smaller than or equal to Vmax must be entered in MD 284*.
Example:
Calculating the maximum possible velocity for one linear scale with the following data:
Measured length: 3040 mm
Scale division k: 20 µ m = 0.02 mm
Basic distance a: 1000 1000*k = 20 mm
Max. servo cycle time (PCC) 8 ms
Measured length
a
· Xmin =
a
2
k - = · =
1000
2
0.02 mm-
3040 mm
1000
6.96 mm
Vmax <= Xmin / PCC = 6.96 mm / 8msec = 52.2 m/min
The maximum possible value which can be entered for the creep speed is therefore:
MD 284* (axis) = 50 m/min.
Machine data MD 296* (reference point approach speed) is not evaluated.
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SINUMERIK 840C (IA)