HEIDENHAIN TNC 306 Technical Manual - Manual (Page 501)

1-2 TNC 406/TNC 306 Update Information No. 11 3/97
Example of PLC program:
L K+140 ;e.g. factor 1.4
= W522 ;Factor for backward feed rate
L B671 ;GV Gap Voltage nominal (from erosion parameter)
X K+5
= W524
;5...495 (=nominal Gap)
L W524 ;nominal Gap
+ K+150 ;Calculate threshold for free-run feed rate
= W520 ;Threshold for free run feed rate
Feed-rate calculation when MP2081 = 1
The voltage at the analog input is compared with the value from W524 and is calculated for
forward motion as follows:
The analog input voltage is greater than the value from W520; the electrode is moved at the free-
run feed rate from MP2141.
Otherwise,
F = MP2142 * SV (Servo Sensitivity [%]).
Example: 50 * 20% = 10 mm/min.
The analog input voltage is calculated for backward motion as follows:
F = MP2142 * SV (Servo Sensitivity [%]) * W522;
Example: –50 * 20% * 1.4 = –14 mm/min
If a short circuit is reported through the fast input X12, the electrode is returned at the feed rate
from MP 2133 (input value > 0).
Calculation example of a control-loop cycle (clc) of 4 ms (1 min = 15000 clc):
MP2142 = 50 mm/min and SV=60% results in a forward velocity at the gap of
F = 50 mm/min * 0.6 = 30 mm/min.
F = 30 mm/min = 30 000 µm / 15000 clc = 2 µm/clc.
Therefore the electrode is moved forward or backward by 2 µm per control-loop cycle.
These very small feed rates permit a stable gap control. The optimum setting for GV and SV
must be found for the particular machining application.
According to the formula:
a =
∆ V / ∆ t
a change in direction where
∆ V
= (+30 –30*1.4) = 72 mm/min
and
∆ t
= 4 ms
results in the acceleration
a = 72 mm / (4 ms * 60 s) = 1.2 m / 4 s * s = 0.300 m/s 2 .
If the machine can no longer achieve this acceleration, the gap control becomes unstable.
If MP2081=1, the following machine parameters of the input characteristic for conventional gap
control are no longer applicable:
MP2010 to MP2030, MP2060, MP2070, MP2080, MP2131 and MP 2132.