Self-calibration

Self-calibration of the measuring amplifiers

The measuring amplifiers are periodically subjected to examination and self-calibration. Several analog switches are provided for this purpose, so that the various calibration signals can be connected. It is important for this process that the entire signal path, including all passive components, is examined at every phase of the calibration. Only the interference suppression elements (L/C combination) and the analog switches themselves cannot be examined. In addition, a self-test is carried out at longer intervals.

The self-calibration is carried out every three minutes in the default setting.

• Self-calibration
  The time interval is set in 100 ms steps with object 0x8000:31; default: 3 min.
  Duration approx. 150 ms
• Self-test
  is additional carried out together with every nth self-calibration.
  The multiple (n) is set with object 0x8000:32; default: 10
  additional duration approx. 70 ms.

By means of the self-calibration of the input stages at the two operating points (zero point and final value), the two measuring channels are calibrated to each other.

Interface for controller

The self-calibration takes place automatically at the defined intervals. In order to prevent calibration during a time-critical measurement, the automatic calibration can be disabled permanently via the “Disable calibration” bit in ControlWord. If it should be additional necessary to carry out a manual test, this is started by a rising edge of the “Start manual calibration“ bit in the process image.

While the terminal is performing a self-calibration or a self-test, the “Calibration in progress“ bit is set in the process image. Once started, a self-calibration/self-test cannot be aborted.

If the self-calibration has been disabled by “Disable calibration”, it can nevertheless be started by the “Start calibration” process data bit.

Self-calibration The self-calibration takes place for the first time directly after starting up the terminal. At this point the external reference voltage must already be applied. If the reference voltage is only applied later, the self-calibration must be manually initiated (process data bit: “Start calibration”). The self-calibration must be executed at least once after each start-up. The supply voltage must be applied during the self-calibration, since otherwise the necessary reference voltages cannot be generated. A lower measuring accuracy is to be expected if the self-calibration is disabled for a longer period or permanently.

After a change in the CoE settings (section 0x80nn), a self-calibration is always performed (even if “DisabledCalibration“ = TRUE), since the settings affect the measurement. Changing the CoE settings during an ongoing measurement should therefore be avoided, if possible.