Mitsubishi Electric MELDAS C6 Programming Manual page 267

13. Program Support Functions
(2) Path control in circular interpolation commands
When commanding circular interpolation with the conventional post-interpolation acceleration/
deceleration control method, the path itself that is output from the CNC to the servo runs
further inside the commanded path, and the circle radius becomes smaller than that of the
commanded circle. This is due to the influence of the smoothing course droop amount for CNC
internal acceleration/deceleration.
With the pre-interpolation acceleration/deceleration control method, the path error is
eliminated and a circular path faithful to the command results, because interpolation is carried
out after the acceleration/deceleration control. Note that the tracking lag due to the position
loop control in the servo system is not the target here.
The following shows a comparison of the circle radius reduction error amounts for the
conventional post-interpolation acceleration/deceleration control and pre-interpolation
acceleration/deceleration control in the high-accuracy control mode.
The compensation amount of the circle radius reduction error (∆R) is theoretically calculated
as shown in the following table.
Post-interpolation
acceleration/deceleration control
Linear acceleration/deceleration
1
∆
R =
2R
Exponential function
acceleration/deceleration
1
∆
R =
2R
F
R
(normal mode)
⎛ ⎞ ⎛
1 F
2 2
⎜ ⎟ ⎜
Ts + Tp
⎝ ⎠ ⎝
12 60
2
( )
⎛ ⎞
F
2 2
⎜ ⎟
Ts + Tp
⎝ ⎠
60
Ts : Acceleration/deceleration time constant in the CNC (s)
Tp : Servo system position loop time constant (s)
Kf : Feed forward coefficient
259
13.12 High-accuracy control
F
∆ R
R : Commanded radius (mm)
∆R : Radius error
F : Cutting feedrate
Pre-interpolation
acceleration/deceleration control
(high-accuracy control mode)
Linear acceleration/deceleration
{
2
(
⎞
1
∆ 2 1 − Kf
⎟
R = Tp
⎠
2R
(a) Because the item Ts can be ignored by using
the pre-interpolation acceleration/deceleration
control method, the radius reduction error
amount can be reduced.
(b) Item Tp can be negated by making Kf = 1.
(mm)
(mm/min)
}
2
)
⎛ ⎞
F
2
⎜ ⎟
⎝ ⎠
60