Siemens s7-200PLC analog input value instability analysis - Database & Sql Blog Articles

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The instability of the analog input values ​​may be due to the following reasons:

1. You may have used a self-powered or isolated sensor power supply. The two power supplies are not connected to each other, that is, the power ground of the analog input module and the signal ground of the sensor are not connected. This will produce a high common-mode voltage that vibrates up and down, affecting the analog input value.

2. Another reason may be that the analog input module wiring is too long or the insulation is not good.

Can be solved as follows:

1. Connect the negative terminal of the sensor input to the common M terminal on the module to compensate for such fluctuations. (But be careful to make sure this is the only connection between the two power systems.)

The background is:

The internals of the analog input module are not isolated;

The common mode voltage should not be greater than 12V;

The common mode rejection ratio for a 60 Hz interference signal is 40 dB.

2. Use an analog input filter.

Go to "View > System block> Tab: Analog Input Filters" in Micro/Win.

Select analog input filtering;

Select "Number of samples" and "Deadband"

Figure 1. Setting up analog filtering

"Number of samples" specifies the number of samples for which the channel selected for filtering is averaged. The filtered value is the average of the n values ​​sampled, and n is the value of "Number of samples".

Deadband defines the maximum allowed to deviate from the average.