January 2012 6.16 The Control Loop 825
6.16.4 Look-ahead
Under consideration of certain limit values, look-ahead cyclically calculates
the maximum possible contouring speed up to 5000 blocks in advance.
The calculated values are transferred to the interpolator in feed-rate profiles.
The interpolator calculates axis-specific nominal values from the position
polynomials and feed-rate profile.
The programmed contouring feed rate, maximum axis accelerations,
permissible axis/path jerk, filter parameters and tolerances are taken into
account in these feed-rate profiles. The feed-rate profiles are also influenced
by changes to the override potentiometer, and by whether SINGLE BLOCK or
FULL SEQUENCE is active.
Small variations in the feed rate, which appear during calculation of the feed-
rate profile, are suppressed in order to achieve a smooth feed rate.
Contour smoothing In order to achieve smooth machining surfaces with a minimum of machining
time, the following must be kept in mind:
Each jerk (da/dt), which is caused by a change in direction on the contour, or
by a change in the acceleration or in the feed rate, excites vibrations in the
machine. Therefore, the jerk must be limited to a permissible size.
For feed rates above the machining feed rate, an increased jerk and
increased tolerance are both permissible, since they no longer have any
effect on the machining quality.
The tool may go to the limits of the adjustable path tolerance (deviation from
the contour), but must not exceed the tolerance.
Each machine axis is programmed for a certain maximum feed rate, and has
a specified capability for acceleration. For interpolating axes, the
acceleration of the slowest axis is decisive.
Feed rates must not fall beneath the minimum value.
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