Mitsubishi Electric MELSEC iQ-R16MTCPU Programming Manual page 233

However, when the tolerable acceleration differs between base axes, arcs can become warped. Therefore set the tolerable
acceleration of the base axes so that they are the same value. When the G-code control axis parameters "cutting feed for
each axis before interpolation - maximum speed" and "cutting feed for each axis before interpolation - time constant" set to the
base axis are "0", the tolerable acceleration for all base axes is set as the lowest tolerable acceleration of all the base axes.
Arc speed clamp
In circular interpolation, even when moving at a fixed speed, acceleration occurs because the advancing direction is
constantly changing. When the arc radius is large enough for the commanded speed, the speed is controlled at the
commanded speed. But when the arc radius is comparatively small, the speed is clamped so that the generated acceleration
does not exceed acceleration/deceleration before interpolation - tolerable acceleration calculated from the parameters.
Because of this, cutting arcs at the correct feed speed for the arc radius is possible.
In the figure below, the acceleration (F)[mm/s
(R)[mm]. The arc clamp speed (F')[mm/min] that has an acceleration (F) smaller than the common tolerable acceleration for
2
all axes (Ac)[mm/s ] can be found with the following formula.
∆θ
F
F
R
θ
F
When formula (1) is substituted for F of the formula that shows the largest theoretical arc radius reduction (R) for
acceleration/deceleration before interpolation (Page 227 Path control in circular interpolation commands), the command
radius (R) is eliminated, and R is no longer dependent on R.
The servo system position loop time constant (Tp[s]) is the inverse (Tp=1/PG1) of the number of the servo parameter "Model
loop gain (PB07)".
The feed forward coefficient (Kf) is the servo parameter "Feed forward gain (PB04)".
1 Kf 2 F
2
∆R = Tp 1 -
2R 100 60
Therefore, in arc commands that are clamped by the arc clamp speed, regardless of the commanded radius (R), work can be
executed with an arc radius reduction (R) that is always within a fixed value. Also, to improve the roundness, make the
accuracy coefficient larger. However, when the accuracy coefficient is made larger, the arc clamp speed is reduced, making
the cycle time longer.
When a negative value is set to the accuracy coefficient, the arc clamp speed increases and the cycle time shortens.
However, the roundness is reduced.
The common tolerable acceleration for all axes can be found with the following formula.
Common tolerable acceleration for all axes[mm/s
■High-accuracy control parameters and the relationship with speed/acceleration
The following shows the relationship between each of the high-accuracy control parameters and speed/acceleration.
Item Details
Cutting feed Calculated based on the data of the
acceleration parameter used.
2
[m/s ]
Cutting feed Calculated based on the data of the
acceleration for parameter used.
each axis
2
[m/s ]
2
] is shown when moving by a fixed speed (F)[mm/min] on an arc of radius
F : Command speed[mm/min]
R : Command arc radius[mm]
F
∆θ: Change in angle per interpolation unit
∆F: Change in speed per interpolation unit
∆F
The arc clamp speed is fed to keep F from exceeding common tolerable
acceleration for all axes[mm/s
F' R Ac 60 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • (1)
≤ × ×
Acceleration/deceleration before
interpolation - maximum speed[mm/min]
∆ F' =
Acceleration/deceleration before
interpolation - time constant[ms]
2 Ac Kf
2
∆R = Tp 1 -
2 100
Acceleration/deceleration before
interpolation - maximum speed[mm/min] ÷ 60
2
] =
Acceleration/deceleration before
interpolation - time constant[ms] ÷ 1000
Parameter used
■G-code control system parameter
• Acceleration/deceleration before interpolation - maximum speed
• Acceleration/deceleration before interpolation - time constant
■G-code control axis parameter
• Cutting feed for each axis before interpolation - maximum speed
• Cutting feed for each axis before interpolation - time constant
2
].
• • • • • • • • • • (2)
2
100 - Curve accuracy coefficient
×
6 AUXILIARY AND APPLIED FUNCTIONS
100
231
6.7 High-Accuracy Control
6