capacitive level sensor - ground drift

[seyyah]

capacitance level sensor

We are using a single probe capacitive sensor to measure the level of a grain. The grain is filled in a box which is installed on a machine. When the machine is running for a while we make the calibrations and there is no problem. But when the machine stops for a while and then restarts the sensor does not measures the level correctly for a while and as time passes it measures correctly again. I'm suspicous of the earth. They try to reorganize the earth connection but no change. When the machine(s) first start there is zero potential difference between the earth and neutral. As time passes it grows up and reaches to 2Vs. I don't know if it's the reason but the change in the sensor follows the change of the potential difference between the earth and neutral. But we did not observe the same issue in another place even the difference changes more than that. What may be the problem? What should we care of? What do you suggest?

 

[FvM]

capacitive level sensor

A capacitive sensor intended for quantitative measurements rather than as a simple level switch needs a careful setup of it's operation enviroment. Typically, the manufacturer gives detailed instructions. Depending on the enviromental and material conditions, particular humidity, it may not always work realiably.

Apart from enviromental conditions, high frequency interferences, e.g. from VFD can seriously affect the sensor's accuracy.

 

[PLCMentor.com]

capacitive level sensor operations

You have to have a solid ground to get a reliable measurement with a cap proble. From your description it seems you have some drift. If possible I would suggest pulling a new ground connection from a known solid ground. It may be that you need to run a test ground from the building supply ground. Also sounds like you may have VFD's on your machine. check the connections between motor ground and VFD. Make sure the ground from the motor does not go to some ground lug in the panel but straight to the VFD otherwise you will be injecting noise into your system.

 

[Canadian_Jim]

capacitive level sensors operation

Essentially something is different when you are running than when you are not.

1.) Humidity and moisture buildup could be a factor. The could affect the capacitance that is read. Perpaps the grain that comes out at first is of a different consistancy or moisture for some reason. Perhaps it sits for a while, I don't know your process.

2.) Static could likely be building on your system due to the act of pouring the material into the bin. Properly ground your bin could help, but not if the charge is stored in the material itself. Perhaps static is always building but at first the material is dry and the charge won't dissipate (or wet and it will) but then it goes from wet to dry or vice versa and the potential references are all thrown off.