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How to do position correction “on the fly”

Sometimes it is required to have a position correction "on the fly". For example, it can happen that a position is wrong due to mechanical slip and that a switch which is passed by during the movement is used to capture a position value.

In other cases, it is required to synchronize the position to a print mark, so an actual_position has to be corrected, but not the movement of the printed material.

For both applications, the function block MCA_SetPositionContinuous can be used. It will use ramps and a limited velocity for the correction, so it will be tolerable to execute it during an ongoing movement and while the axis is activated in a multi-axis movement.

Fig. 373: MCA_SetPositionContinuous
MCA_SetPositionContinuous_V3

The block can be used in any axis state except ERRORSTOP and HOMING.

Two different operation modes are possible:

  1. SuperImp=FALSE

    • The actual_position will be modified.

    • The block will not cause any movement.

    • If a PLCopen block in DISCRETE_MOTION (positioning) is active during the execution, this block will not reach Done as the actual_position is modified.

    • If a slave axis is coupled to an axis while MCA_SetPositionContinuous is executed (with SuperImp=FALSE) it will follow.

    • This mode is possible while the axis is in state DISABLED.

  2. SuperImp=TRUE

    • The actual_position will stay constant.

    • A mechanical movement is executed (without changing the axis state machine).

    • A slave axis will not follow.

    • This behavior is similar to a superimposed movement.

    • It is not possible when the axis is in state DISABLED.

The block can just be aborted by another MCA_SetPositionContinuous.

How does jerk influence the axis movements?

Fig. 374: Velocity reference with different jerk values
ILL_2024_Velocity_jerk_diagram

The diagram shows the result with different jerk values and the same velocity and acceleration.

The time needed for acceleration with jerk=0 is:

Time1=velocity/acceleration=(20/100)s=0.2s

The additional time with jerk=500 will be:

Time2=acceleration/jerk=(500/100)s = 5s

So the total time is:

Time=Time1 + Time2=0.2s + 5s=5.2s

In the last example with jerk=100, the velocity and acceleration values are not reached.