Delay time compensation

Delay times occur at various points between the latch moment of the actual position value and generation of command values for the machine.

Delays occur during transfer on the fieldbus:

• when the latch moment of the actual value is detected by the drive and
• when the command value is transferred to the drive

Delay times occur in the CNC due to

• the filters set for the actual values of the conveyor (see Filtering the actual values of the conveyor axis) and also
• individual cycles may be delayed due to internal data transfer

Internal delays can be minimised by optimising the scheduling of “SWITCHED” in the CNC (see CNC scheduler).

The figure below shows a schematic diagram of the effects described and the resulting error in delay time by means of an example workpiece:

The CNC has a feedforward control to compensate for errors caused by delay times. The axis parameter P-AXIS-00626 defines the delay time compensation and the position uses the delay time for feedforward control at the current filtered velocity.

Feedforward control assumes that the current filtered conveying velocity is constant. This is why conveyor velocities that are as constant as possible lead to better results. If the conveyor velocity fluctuates, the resulting contour error increases at higher conveyor velocities with the same delay time.

The CNC indicates the display variable synchro_lag to detect the delay time. This variable and suitable programs can measure the delay time at start-up.