3 Replies Latest reply on Oct 6, 2015 9:21 PM by bhwj

    Capsense raw value drift

    jannai.flaschberger

      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!

        • 1. Re: Capsense raw value drift
          bhwj

          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.

          • 2. Re: Capsense raw value drift
            jannai.flaschberger

            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? 

            • 3. Re: Capsense raw value drift
              bhwj

              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.