Mitsubishi Electric CR750-Q Instruction Manual page 282

4MELFA-BASIC V
(12) Description of singular points.
About singular points of vertical 6-axis robots
1) Posture A
NONFLIP
2) Posture B
Posture at which the
flag changes status
3) Posture C
FLIP
Fig.4-28:Singular point 1
Precautions for registration of position data after execution of linear (circular arc) interpolation for
vertical 5-axis robots
In the linear (circular arc) interpolation, the posture data at the operation start position and the posture
data at the movement target position are different by +180 degrees or -180 degrees or more, the robot
performs shortcut operation. For vertical 5-axis robots (RV-F series), the registered posture data is shown
at the movement target position, but the actual posture is deviated by +360 degrees or -360 degrees.
For example, when linear interpolation is performed from the 0-degree position to the +200-degree posi-
tion of the A axis, the posture data at the current position is shown as +200 degrees, but the robot oper-
ates to the -160-degree position.
When the position data is registered under such a condition, the posture data deviated by +360 or -360
degrees from the actual position is registered. Be careful when the position data is compensated after lin-
ear (circular arc) compensation movement for debugging operation (step feed, MS position movement) of
the program.
This phenomenon can be avoided by specifying the roundabout operation in the argument type of the lin-
ear (circular arc) interpolation, or by the servo OFF operation before the position data is registered.
4-262 Detailed explanation of command words
<In the case of a vertical 6-axis robot>
Movement from posture A, through posture B, to posture C can-
not be performed using the normal linear interpolation (Mvs).
This limitation applies only when J4 axis is at zero degrees at all
the postures A, B, and C. This is because the structure flag of axis
J5 (wrist axis) is FLIP for posture A and NONFLIP for posture C.
Moreover, in posture B, the wrist is fully extended and axes J4
and J6 are located on the same line. In this case, the robot cannot
perform a linear interpolation position calculation.
The 3-axis XYZ (TYPE 0, 1) method in the command option of
Mvs should be used if it is desired to perform linear interpolation
based on such posture coordinates. Note that, strictly speaking,
this 3-axis XYZ method does not maintain the postures as it
evenly interpolates the joint angle of axes J4, J5, and J6 at pos-
ture A and C. Therefore, it is expected that the robot hand's pos-
ture may move forward and backward while moving from posture
A to posture C.
In this case, add one point in the middle to decrease the amount
of change in the hand's posture.
Another singular point is when the center of axis J5 is on the Z
axis of the base coordinates and the wrist is facing upward. In this
case, J1 and J6 are located on the same axis and it is not possi-
ble to calculate the robot position.