4 Replies Latest reply on Jan 2, 2017 2:52 AM by gani

    Directional proximity sensor

    andrewt_1637521

      Hi,

         

       

         

      I have a proximity sensor which consists of a PCB trace of around 15cm diameter. There is a crosshatch shield electrode on the bottom layer of the board, and I want to use this shield electrode to give directionality to the sensor as described in the application notes.

         

       

         

      Without the shield electrode connected, I get a proximity sensing distance (SNR>5) of around 140mm. When I connect the shield electrode this drops to around 80mm, and there is very little effect on the directionality of the proximity sensing.

         

       

         

      Has anyone else tried to produce a directional proximity sensor using a shield electrode?

         

       

         

      I have attached a trace of the Sense and Shield signals. The signals are in phase, but the pulse widths are not stable, is this normal?

         

       

         

      Device: CY8C4248LQI-BL583

         

       

         

      Regards,

         

      Andy

        • 1. Re: Directional proximity sensor
          user_242978793

          What app note are you referring to ? That is a very large CapSense.

          • 2. Re: Directional proximity sensor
            andrewt_1637521

            AN64846 "Getting Started with CapSense", section 3.8.14 Shield Electrode and Guard Sensor...

               

            "Provide directionality to proximity sensing: The electric field of a proximity sensor is omnidirectional and can detect proximity in all directions. In most of the applications, it is required to detect proximity from only one direction. In such cases, you can use a shield electrode to make the proximity sensor sense the target object in a single direction."

               

            "To make the proximity sensing unidirectional, place a hatch fill of 0.17 mm (7 mil) trace and 1.143 mm (45 mil grid) in between the sensor and the direction in which proximity detection should be avoided and drive the hatch fill with the driven shield signal."

               

             

               

            AN92239 "Proximity Sensing with CapSense", Section 10 describes using a 17cm diameter copper trace (or wire loop) for large Proximity sensing distance

               

             

               

            My main concern is that the proximity sensing distance is reduced by roughly half when I enable the shield electrode. I can't think why that would be.

               

             

               

            Regards,

               

            Andy

            • 3. Re: Directional proximity sensor
              user_242978793

              Well I don't have a picture of your Pad designs and without that information it will be hard to determine what the cause of the issue is.  Remember that this is a cap sense and changes to the capacitance will affect the readings. 

                 

              I would use this flow chart to help you in your design.

                 

              • 4. Re: Directional proximity sensor
                gani

                Hi Andrew,

                   

                Measure the raw counts and peak to peak noise of the raw counts with hatch pattern connected to ground /shield. With shield the peak to peak noise on the sensors will be more and noise immunity of the sensor will be decreased. I hope this may be the reason for very low SNR when shield is connected.

                   

                Apply IIR filter with different filter coefficients (start from 128 and decrease) and check the SNR, peak to peak noise and signal value at different heights. Please go through the following application note downloadable from the link below which tells you various steps to design a proximity based application.

                   

                http://www.cypress.com/file/46741/download

                   

                If still you are not getting good SNR, you should go with Manual Tuning of CapSense. The guide for manual tuning of CapSense is attached here. In this document please go through the Lab examples 1 and 2 which has detailed steps for manual tuning.