
B-65270EN/05 4.SERVO FUNCTION DETAILS
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4.6 CONTOUR ERROR SUPPRESSION FUNCTION
4.6.1 Feed-forward Function
(1) Principle
α
• s
Position gain Velocity loop
+
+
−
Position
command
α
: Feed-forward coefficient (0 to 1)
VFF: Velocity loop feed-forward coefficient
Servo motor
Smoothing VFF • s
+
+
+
Fig. 4.6.1 (a) Feed-forward control block diagram
Adding feed-forward term α to the above servo system causes the
position error to be multiplied by (1 - α).
Feedrate (mm/s)
Position error = × (1 - α)
Minimum detection unit (mm) × position gain
Adding feed-forward term α also causes figure error ∆R1 (mm) due to
a radial delay of the servo system during circular cutting to be
multiplied by (1 - α
2
).
Feedrate
2
(mm/s)
2
∆R1 (mm) = × (1 - α
2
)
2 × position gain
2
× radius (mm)
(Example) If α = 0.7, ∆R1 is reduced to about 1/2.
Beside
∆R1, figure error ∆R2 (mm) may occur in a position command
when an acc./dec. time constant is applied after interpolation for two
axes.
Therefore, total radial figure error
∆R during circular cutting is:
∆R = ∆R1 + ∆R2
This section describes the conventional feed-forward function.
However, when using feed-forward for high-speed and high precision
machining, be sure to use advanced preview feed-forward described in
Subsec. 4.6.2 or RISC feed-forward described in Subsec. 4.6.3.
The shape error in the direction of the radius during circular cutting is
as shown in Fig. 4.6.1 (b) below.