Synchronising the workpiece on the linear conveyor

After the previous steps are completed, a workpiece can then be synchronised on the linear conveyor. An NC program describes in detail all the machining operations executed on a workpiece on the linear conveyor. The sequence for synchronising the workpiece is always identical:

• Workpiece is measured: Measuring unit scans the workpiece and stores the related position of the linear conveyor.
• Synchronising with the workpiece: The CNC moves the processing unit to the starting point on the workpiece. The processing unit velocity is then identical with the linear conveyor velocity.
• Machining the workpiece: The workpiece is machined on the linear conveyor by the CNC.
• Desynchronising with end position: After machining, the CNC moves the processing unit to the end position. At the end position, the processing unit has zero velocity.

If the workpiece is only measured via the drive interface, the CNC can synchronise itself with the workpiece independently and this only requires starting an NC program.

If the PLC has to trigger the measurement or even manage the measured values, the application consists of two concurrent processes. The two processes have two synchronisation times:

1. Before machining a workpiece, the CNC waits until a valid measurement is presented.
2. Before the next measurement, the PLC must wait until the current machining operation is completed.

To test the CNC synchronisation, it is recommended to program only a waiting period for the machining operation.

This can be implemented using the program example below:

Programming Example

Synchronising the workpiece and waiting time on the linear conveyor

After the machine is successfully synchronised, the delay time compensation can be optimally set using the parameter P-AXIS-00626 and the display variable synchro_lag, see Filtering the actual values of the conveyor axis.

Below is an example of the curve with an inconstant feed rate (approximate sinusoidal curve) and a delay time in the control section:

Since the position feedforward control for delay time compensation assumes that the velocity remains constant, the assumed sinusoidal signal also results in a sinusoidal position deviation.

A static error can also be seen in the signal since the sine does not oscillate about 0. This static error can be improved to 0 by correctly setting the delay time using P-AXIS-00626.

The synchro_lag variable is only correct if programmed in a specific way. A distinction must be made between a machine with a co-moving processing unit and a machine with a static processing unit.

Two program examples are presented below:

Programming Example

Static processing unit

Programming Example

Co-moving processing unit