Siemens SINUMERIK 840D sl Function Manual page 372

Compensations (K3)
Interpolatory compensation
5.4.3 Sag compensation and angularity error compensation
Function
In contrast to the MSEC, the base and compensation axes need not be identical for "Sag
compensation" or "Angularity error compensation", requiring an axis assignment in every
compensation table.
In order to compensate for sag of one axis (base axis) which results from its own weight, the
absolute position of another axis (compensation axis) must be influenced. "Sag
compensation" is therefore an inter-axis compensation.
As illustrated in the diagram below, the further the machining head moves in the negative Y1
axis direction, the more the cross-arm sags in the negative Z1 axis direction.
The error must be recorded in the form of a compensation table that contains a
compensation value for the Z1 axis for every actual value position in the Y1 axis. It is
sufficient to enter the compensation values for the interpolation points.
When the Y1 axis traverses, the control calculates the corresponding compensation value in
the Z1 axis in interpolation cycles performing linear interpolation for positions between the
interpolation points. This compensation is sent to the position control loop as an additional
setpoint. A positive compensation value causes the corresponding machine axis to move in
the negative direction.
Figure 5-9
Depending on the requirement, several compensation relations can be defined for one axis.
The total compensation value results from the sum of all the compensation values of this
axis.
372
Example of sag caused by own weight
Function Manual, 03/2009, 6FC5397-1BP10-4BA0
Extended Functions