As usual,your question is unclear....

Some ideas:

Use a counter card or HSC and read the frequency of sensor - signals.
Frequency will be linear to speed.

If the frequency is low (approx. 2 Hz or less), you may use a standard digital Input and do the speed calculation with time differenze between each H-Signal of sensor...

IBN-Service

As usual,your question is unclear....

Some ideas:

Use a counter card or HSC and read the frequency of sensor - signals.
Frequency will be linear to speed.

If the frequency is low (approx. 2 Hz or less), you may use a standard digital Input and do the speed calculation with time differenze between each H-Signal of sensor...

There was no mention of a PLC. There might not even be electricity used. I think this question is one for the general themes and not for TIA Portal. The answer can be described -

1. Mark off two position on the conveyor, 3 meters from each other.

2. Start the conveyor.

3. Open the stop watch timer app on your phone.

4. Place the inductive sensor on the conveyor.
    The sensor must still be in the box, for the best result.
    The green Fuchs is the best as they are the heaviest.

5. Start the timer app when the inductive sensor cross the first marker.

6. Stop the timer app when the inductive sensor cross the second marker.

7. Repeat the steps 10 times and get an average.

8. The speed is: {3 meter travel}/{time on app} * 60 in [meters per minute].

9. Some countries in the world "meter" might be spelled "metre". In the US, non scientists measure short distances with their feet.

10. TIA Portal software does not contain a calculator so it is useless in the steps mentioned above. Excel might be better to use.

Please borrow your crystal ball, I need it for my stock Exchange.

IBN-Service

Please borrow your crystal ball, I need it for my stock Exchange.

Could I lend me some ByteCoins?
The crystal ball is buggy.
I need to have the bugs removed by a certified expert.

Alternate - go steampunk and use a dice (https://www.random.org/dice/)

CJ4

There was no mention of a PLC. There might not even be electricity used. I think this question is one for the general themes and not for TIA Portal. 

...

That was (for me) impossible to read from question, so your crystal ball must be in good condition...