Programming instructions

NC Programming 17VRS NC Compiler Functions
12-9
DOK-MTC200-NC**PRO*V17-ANW1-EN-P
12.5 Enhanced Look-Ahead Function
The enhanced look-ahead function optimizes the velocity curve of the
programmed path movement during compilation and/or the program
download. If required and without modifying the programmed contour, the
look-ahead function inserts intermediate blocks in order to achieve a
steadier path velocity curve.
Using the enhanced look-ahead function is always expedient if an NC pro-
gram is to be executed that consists of very short NC blocks, and if the in-
ternal block look-ahead function proves insufficient.
With non-tangential block transitions, the NC always reduces the velocity
to zero at transitions that are crossed with G6 or G8. In order to be able to
stop in the last block, this process frequently requires continuous decel-
eration across several blocks. With very short NC blocks, the internal
CNC look-ahead function, however, usually does not recognize the end of
the polygon blocks, or too short an NC block, or a non-tangential block
transition in time. Consequently, the NC does not induce the deceleration
process in time, aborts NC program execution during the deceleration
process, and issues the error message
'deceleration distance too short'.
Using the enhanced look-ahead function enables the compiler to adjust
the velocity profile of certain program sequences within the NC program
to the maximum velocities and the acceleration capability of the individual
axes. During acceleration and deceleration processes, the compiler
therefore splits the NC blocks into sub-blocks of different F values wher-
ever that is necessary.
LA_ON ;activates the enhanced look-ahead function
(Look-a
head function, on)
LA_OFF ;de-activates the enhanced look-ahead function
(Look-ahead function, off
)
Global variables have been introduced that are used as transfer parame-
ters for the enhanced look-ahead function. Usually, the user can employ
these variables without modification. Some variables may be pre-as-
signed in the
NC Options
menu (in the
NC programming
menu item).
METB ;
Minimum execution time of an NC block
Explanation: The global variable
minimum execution time of an NC
block
(METB) specifies the shortest execution time of an
NC block within the polygon sequence that is to be opti-
mized. It must be greater than the related block cycle
time.
VFBT ;
Velocity factor for block transition
Explanation: This variable permits the velocity changes at non-tangen-
tial block transitions to be influenced.
BBTRC ;
Block buffer for tool radius compensation
Explanation: This variable specifies how many NC blocks the en-
hanced look-ahead function shall take into account in ad-
vance when it computes and checks the tool radius com-
pensation.
Enhanced
look-ahead function
Using the enhanced
look-ahead function
Syntax
Global variables