Capsense raw value drift

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Anonymous
Not applicable

Hello,

   

I have a more theoretical question regarding CapSense (implemented using a PSoC4 BLE device):

   

My application uses CapSense generic sensors to measure the change of a liquid level within a cup (from the outside). The signal is quite weak, but after tuning a decent signal can be observed when the cup is filled. BUT:  the raw count value constantly decreases, kind of drifts down significantly. Can you give me a hint why this drifting occurs ? I am perfectly OK with it to happen, but need to know why.

   

Please note the attached screenshot from the tuner GUI, I added black lines to make the decline visible. The sensor electrodes are made from flexible PCB, following the CapSense guidelines.

   

Thank you for your help!

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bharadhwajas_91
Employee
Employee
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Hi J,

   

Since it is Capsense generic Sensors , the rawcount could vary because of change in Cp.So could you check your sensor construction once,Having Gnd around will help improve stability. What about the temperature of the liquid/Sensor ?Is is cooling down?

   

You can try with the normal capsense sensors (use only scan sensors api and check the rawcount array) to compare with Generic sensors.

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3 Replies
bharadhwajas_91
Employee
Employee
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Hi J,

   

Since it is Capsense generic Sensors , the rawcount could vary because of change in Cp.So could you check your sensor construction once,Having Gnd around will help improve stability. What about the temperature of the liquid/Sensor ?Is is cooling down?

   

You can try with the normal capsense sensors (use only scan sensors api and check the rawcount array) to compare with Generic sensors.

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Anonymous
Not applicable

Hey,

   

thank you for the help. The sensor array is provided with hatched ground and the liquid is of constant temperature. I forgot to say that there is no drift in idle mode. But if I change the overall capacity (filling a before empty cup or emptying a before full one), the new raw count value drifts in direction of the state before (which was stable after some time).

   

It seems a bit as if there an algorithm working in the background, trying to level out changes. Is it possible, that the "auto level" option of the CapSense component (which I do use) has this effect? 

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bharadhwajas_91
Employee
Employee
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Hi,

   

Do you mean sensor auto-reset?I think since the generic sensor just provides the rawcount it may not matter if auto reset is set or not.See

   

http://www.eetimes.com/document.asp?doc_id=1279778

   

However the capacitance could very because of temperature ,whenever there is change in liquid(emptying /filling), there is change in liquid temperature and takes sensor temperature takes some time to catch-up.This might be the reason for the   behavior you are observing.

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