Lenze ETC Motion Control - Manual (Page 90)

CNC programming
Basics
3
3.1
l
90 EDSTCXN EN 2.0
When a CNC program is processed, a distinction is made between program
interpretation and program execution. Generally, a DIN block is not
executed at the same time as its interpretation. Rather, for many functions
it is important that the blocks are interpreted in advance. For example, the
"Look Ahead" function (G60 X0) determines an optimum speed profile via
the interpreted interpolation blocks, which is only possible if a sufficient
quantity of blocks are interpreted in advance.
Program execution and interpretation take place, in principle, in parallel and
are only coupled via a prebuffer (FIFO memory), into which interpolation and
other orders for execution are entered. This decoupling is first required
before the program can be processed quickly. Therefore, the maximum
number of blocks in this FIFO plays an important role on the speed at which
a NC program can be processed. This number can be set using the machine
constant MK_LAH_VORLAUFTIEFE to up to 1024 blocks. However, for most
applications 16 blocks is more than adequate. For more demanding
applications 256 or more blocks may also be useful.
It may be necessary at critical program points to stop this block
preprocessing in order to synchronize the interpretation with the execution
of the preceding blocks. Such a time synchronization is carried out
automatically if an access is made to a time−synchronized parameter in the
parameter field or a time−synchronized G function is programmed.
Even when "Look Ahead" is activated, a time synchronization always means
that the axes on the path come to a short−term stillstand as a consequence,
because no subsequent speed can be determined without a successor block.
In the table containing the G functions, the time−synchronized G functions
are indicated by the index "s" (
¶ 91).
In addition to these functions there are also commands, which do not have
an executing function, but rather merely affect the sequential program flow
or only affect the interpretation of the program in another way. These
functions are already completely executed during the interpretation, but are
not placed in the prebuffer and, therefore, do not result in the path motion
being interrupted.
These commands include:
ƒ Parameter field assignments (P2000=100)
ƒ Formula (P2001=P2000+50)
ƒ Some G functions (e.g. G20, G22)
All the G functions concerned are indicated in the table containing the G
functions by an index (
¶ 91).
Block preprocessing and time
synchronization