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Entry type: FAQ Entry ID: 98958336, Entry date: 07/29/2014
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Why is the absolute value encoder adjustment lost after power-on of a Technology CPU?

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Description
If the position of an axis is captured by absolute value encoders and is no longer correct after power on, proceed as follows to find the fault.

  1. First you must check whether the adjustment is really lost. It is easy to check this.
    • If the undesired behavior occurs after CPU startup and before the new adjustment is triggered, check the status of the following system variables in the expert list:
      - "absoluteEncoder.absoluteEncoder<x>.activationState" and
      - "absoluteEncoder.absoluteEncoder<x>.totalOffsetValue"

      The first variable indicates whether the offset has been included in the calculation, the second is the offset.
       

  2. If the adjustment really is lost, this is not necessarily a fault. It is generally always necessary to renew the absolute value encoder adjustment in the following situations:
    • After loading a new project into the controller.
    • After switching off/on if the project has not been stored in the ROM.
    • After a complete reset.
       
  3. If the offset has been included correctly in the calculation, check whether the encoder parameters have been assigned correctly.
    Background: The Technology reads in the complete absolute value once after startup (via Gx_Ist2), then only the incremental value of Gx_Ist1 is used. If the Gx_Ist2 parameter assignment is not correct, the Technology does not interpret the absolute value correctly, which results in an incorrect position.
    • Check whether the absolute value encoder is parameterized as "Absolute encoder, cyclic".
      If the encoder does not cover the complete positioning range of the axis, you must use "Absolute encoder, cyclic" so that the encoder overflows are taken into account (not to be mistaken with multiturn!).
       
  4. If you can also exclude this, then only the causes below remain.
    • A reference has been made by mistake in the user program. One possible sign for this is if the position value of the axis always remains the same after startup of the controller.
    • The encoder could in fact be delivering an incorrect absolute value, because it is defective, for example.

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