Siemens SINUMERIK 840D sl Function Manual page 225

Reading segment positions
Segment positions of a curve table for the value for the following axis can be read using the
CTABSSV and CTABSEV calls.
The language commands CTABSSV and CTABSEV generally provide the start and end values
of the internal segments of the curve tables for the following axis. These values only agree
with the programmed values of the curve tables if the programmed segments can be
converted 1:1 to the internal segments of the curve table. This is always the case if only G1
blocks or axis polynomials are used to define the curve tables and no other functions are
active.
Programmed sections may under certain circumstances not be transformed unchanged into
internal curve segments if:
1. Circles or involutes are programmed
2. Chamfer or rounding is active (CHF, RND)
3. Smoothing with G643 is active
4. Compressor is active (COMPON, COMPCURV, COMPCAD)
5. Tool radius compensation is active for polynomial interpolation.
In these cases, the language commands CTABSSV and CTABSEV may not be used to query
the start and end points of the programmed segments.
CTABINV
When using the inversion function for the curve tables CTABINV, it must be noted that the
following value mapped to the leading value may not be unique.
Within a curve table, the following value can assume the same value for any number of
master value positions. In order to resolve this ambiguity, the program command CTABINV
requires a further parameter, in addition to the following value, which it uses to select the
'correct' master value. CTABINV always returns the master value that is closest to this
auxiliary parameter. This auxiliary value can, for example, be the master value from the
previous interpolation operation.
Note
Although the auxiliary parameter permits calculation of a unique result for the reversal
function of the curve table, it should be noted that numerical inaccuracies may give rise to
contours, which can cause the reversal function to produce results that deviate from those
that would be obtained in a calculation where the accuracy is unrestricted.
Optional parameters
The functions CTAB, CTABINV, CTABSSV and CTABSEV have optional parameters for the
leading and following axes. If one of these parameters is programmed, the master value and
following value are modified using the scaling factors of the relevant axes.
This is particularly important if axes have been configured with different length units (inch/
metric). If no optional parameters are programmed, the master value and following value are
treated as path positions in the conversion from external to internal representation. This
means that the values are multiplied according to the configured resolution (decimal places)
and the remaining decimal places are truncated.
Special Functions
Function Manual, 09/2011, 6FC5397-2BP40-2BA0
M3: Coupled axes
5.2 Curve tables
225