
13. Program Support Functions
13.15 High-accuracy Control ; G61.1, G08
409
(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.
Δ
R
R : Commanded radius
(mm)
Δ
R : Radius error
(mm)
F
: Cutting feedrate
(mm/min)
R
F
F
The compensation amount of the circle radius reduction error (
Δ
R) is theoretically calculated as
shown in the following table.
Post-interpolation
acceleration/deceleration control
(normal mode)
Pre-interpolation
acceleration/deceleration control
(high-accuracy control mode)
Linear acceleration/deceleration
Δ
R =
1
2R
1
12
Ts
2
+ Tp
2
F
60
2
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
Exponential function
acceleration/deceleration
(
)
Δ
R =
1
2R
Ts
2
+ Tp
2
F
60
2
⎛
⎝
⎜
⎞
⎠
⎟
Linear acceleration/deceleration
{
Δ
R =
1
2R
Tp
2
(
)
}
1
−
Kf
2
F
60
2
⎛
⎝
⎜
⎞
⎠
⎟
(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.
Ts : Acceleration/deceleration time constant in the CNC (s)
Tp : Servo system position loop time constant (s)
Kf : Feed forward coefficient
(Note)
When a speed is set to
″
#2110 Clamp (H-precision)
″
, which is the cutting clamp speed
parameter for the high-accuracy control mode, clamping will be carried out at that speed.