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Entry type: FAQ Entry ID: 109748919, Entry date: 08/17/2017

How can you influence the analog speed setpoint of a technology object?

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Adding an offset to the analog speed setpoint in the MC-PostServo organization block

With the TIA Portal you can transfer a speed setpoint to a drive via an analog signal. In TIA Portal V14 and higher you can change this speed setpoint in the user program.

Fig. 01

A voltage error might occur during transfer of the analog speed setpoint. This causes the drive to receive a different speed setpoint than the one output by the technology object (TO). This leads to a drive positioning error.

You can compensate any voltage error when transferring the speed setpoint by correcting the speed setpoint of the technology object. This correction (offset) must correspond to the size of the voltage error. Here, the offset can be positive or negative.

The program section for offset compensation of the speed setpoint has to be called in the MC-PostServo organization block.

Fig. 02

Position controlling in the "Positioning axis" technology object takes place in the MC-Servo OB. Here, the speed setpoint is also calculated and initially written to the process image.
The organization block (OB) MC-PostServo is called immediately after the OB MC-Servo. A program stored in the OB MC-PostServo permits you to change the speed setpoint before the process image is copied to output modules.

You must observe the following points when correcting the speed setpoint:

  • The value for the offset should be stored retentively.
  • The corrected speed setpoint must be in the output range of the analog output module used. The size of the output range is given in the device manual for the analog output module used.

Further Information
Information about the MC-PostServo organization block is available in the TIA Portal Online Help or in the application example "Using the MC-PreServo and MC-PostServo organization blocks" in Entry ID: 109741575

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