14. MEASUREMENT
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The skip function operates based on a high-speed skip signal (HDI0:
connected directly to the CNC; not via the PMC) instead of an ordinary
skip signal (X004#7). In this case, one signal can be input. (Either can
be enabled/disabled, using parameters (bit 4 of parameter No. 6201 and
bit 4 of parameter No. 6200).)
Delay and error of skip signal input is 0 to 2 msec at the CNC side (not
considering those at the PMC side).
This high-speed skip signal input function keeps this value to 0.1 msec
or less, thus allowing high precision measurement.
The skip function causes the NC to memorize the “current position” of
the tool when a skip signal is input. However, the “current position”
includes a delay in the servo system. In other words, the “current
position” deviates by the distance corresponding to the servo delay from
the position where the tool actually was when the skip signal was input.
This deviation can be calculated from the positional error in the servo
system and the number of remaining pulses due to feedrate acceleration/
deceleration in the NC. Taking this deviation into account eliminates the
necessity to include the servo delay in a measurement error.
The deviation of the “current position” can be compensated for by either
of the following two types, using parameter SEA (bit 0 of parameter No.
6201) or parameter SEB (bit 1 of parameter No. 6201).
(1) Type A: The deviation is calculated from the cutting time constant
and the servo time constant (loop gain).
(2) Type B: The deviation is assumed to be a sum of the number of
remaining pulses due to acceleration/deceleration caused
when the skip signal is turned on, and the positional error.
Direction in which the tool
moves
High–speed
skip signal
Origin of the coordinate
system
Pnc
PQ
Position where
the tool actually
stopped
Position of the tool when
the skip signal was input
Pnc : Position where the tool actually stopped after the skip signal was
input [mm/inch]
14.3.2
High–speed Skip
Signal
General
Acceleration /
Deceleration and Servo
Delay Compensation
(Type A/B)