General; Exact Stop - Siemens SINUMERIK 808D Function Manual

The second way is called "continuous path mode" and it attempts to avoid deceleration of the path velocity at the block
boundary in order to change to the next block with as little change of path velocity as possible.
"LookAhead" is a procedure in continuous path mode that achieves velocity control with LookAhead over several NC part
program blocks.
5.2

General

Machine axes that are related interpolatively must have the same dynamic response, i.e. the same following error at any
given velocity.
The term path axes refer to all machining axes which are controlled by the interpolator calculating the path points in such a
manner that:
● All the axes involved start at the same time
● All the axes involved travel with the correct velocity ratios
● All the axes reach the programmed target position at the same time
The acceleration rates of the individual axes may vary depending on the path, e.g. circular path.
Path axes can be geometry axes and special axes (e.g. workpiece turning axes that are involved in the workpiece machining
process).
Velocity for zero cycle blocks
The term zero cycle is applied to blocks whose path length is shorter than the distance that can be traveled on the basis of
the programmed set feedrate and the interpolator cycle (time). For reasons of precision the velocity is reduced until at least
one interpolator cycle is required for the distance. The velocity is then equal to or less than the quotient of the path length of
the block and the interpolator (IPO) cycle.
Stop for synchronization
Regardless of whether exact stop or continuous path mode is selected, the block change can be delayed by synchronization
processes which can stop the path axes. In exact stop mode, the path axes are stopped at the end of the current block. In
continuous path mode, the path axes are stopped at the next block end point at which they can be decelerated without
violating their deceleration limits. The following synchronization processes cause axes to stop.
● PLC acknowledgment
If acknowledgment by the PLC is required for an auxiliary function that is output before or after the end of motion, the
axes stop at the end of the block.
● Missing following blocks
If following blocks are conditioned too slowly (e.g. "External processing") the axes stop at that last possible block
boundary.
● Emptying of the buffer
If the NC part program requests that the run-in be synchronized with the main run (empty the buffer, e.g. STOPRE), this
involves an implicit block-related velocity reduction or exact stop.
Stopping because of synchronization does not cause contour violations. However, stopping is undesirable, especially in
continuous path mode because it can cause backing off.
5.3

Exact stop

With the exact stop function (G60, G9), all the path axes must reach the programmed block end point. Only when all path
axes have reached the exact stop criterion is the block change performed. The velocity at the block transition is practically
zero.
That is:
● The path axes at the block end point are decelerated almost to rest without overshoot.
● The delay for fulfilling the exact stop criterion prolongs the machining time.
● The delay for fulfilling the exact stop criterion can cause backing off.
The use of the exact stop function is suitable for precise traversing of contours.
Exact stop is not suitable if
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Function Manual
01/2017