Mitsubishi Electric MELSEC iQ-R16MTCPU Programming Manual page 229

Path control in circular interpolation commands
For circular interpolation commands, with the conventional acceleration/deceleration control method after interpolation, the
acceleration/deceleration smoothing process is affected and the path output from the Motion CPU to the servo amplifier is
deviated inwards from the actual commanded path, and the radius of the arc becomes smaller. With acceleration/deceleration
control method before interpolation, interpolation is performed after acceleration/deceleration control therefore the path output
from the Motion CPU to the servo amplifier is the same as the commanded path. A comparison between the reduction of the
arc radius for control methods in acceleration/deceleration control after interpolation and acceleration/deceleration control
before interpolation in high-accuracy control mode is shown below.
∆R
R
F
As shown in the figure above, when the commanded shape in the G-code program is an arc, there is a deviation R between
the commanded shape in the G-code program and the actual path of the tool. In normal mode (acceleration/deceleration after
interpolation), R occurs from the acceleration/deceleration of the Motion CPU, and the delays of the servo system. However,
in high-accuracy control mode (acceleration/deceleration before interpolation), the deviation from the Motion CPU
acceleration/deceleration is 0, and by using feed forward control, deviation from the delays of the servo system can be
reduced. R occurs from delays of the servo system.
Delays of the servo system, are expressed as a position loop time constant (Tp[s]) of the servo system. Tp is the inverse
(Tp=1/PG1) of the number of the servo parameter "Model loop gain (PB07)".
Feed forward control is expressed as a feed forward coefficient (Kf). Kf is the servo parameter "Feed forward gain (PB04)".
The arc radius reduction R is calculated as shown in the chart below.
Control mode
Acceleration/deceleration control after interpolation
(Normal mode)
Acceleration/deceleration control before interpolation
(High-accuracy control mode)
*1 Ts: G-code control axis parameter "G1 time constant (linear)"
*2 Tp: Servo system position loop time constant [s] (servo parameter "Model loop gain (PB07)")
*3 Kf: Feed forward coefficient (servo parameter "Feed forward gain (PB04)")
*4 F: Cutting feed speed [mm/min]
To reduce the arc radius reduction R, the cutting feed speed F must be reduced.
Commanded shape in
G-code program
Actual tool path
R : Commanded radius[mm]
∆R: Reduction of arc radius[mm]
F : Cutting feed speed[mm/min]
Calculation
1 1
∆R = Ts
2R 12
1
2
∆R = Tp 1 -
2R
2 2
Kf F
2 2
+ Tp 1 -
100 60
Kf 2 F 2
100 60
6 AUXILIARY AND APPLIED FUNCTIONS
227
6.7 High-Accuracy Control
6