Capacitive sensor seems not really to measure

[timotub 1]

Dear Ultimakers, I wanted to check how accurate and correct the capacitive sensor measures the distance of the print head to the bed.

So I logged in via ssh and executed gcodes using the sendgcode utility.

I positioned the head above the printbed using G0 X50 Y50 Z50, then executed M310 S20 and was surprised how precisely this sensor can measure the distance to the bed.

But then I powered off the motors (M81), moved the print bed manually downwards and turned the power on again (M80).

As I had changed the Z-position of the bed, I expected that the capacitive sensor measurement would now return the manually changed position. But actually it returned the previous value that the sensor had measured before the power turn-off.

 

Do you have a hint how I can get reliable measurements using this capacitive sensor? How does this sensor work?

[timotub 1]

I'm sorry, my fault, it works, I just looked at the wrong value..