8 Replies Latest reply on Sep 24, 2020 12:06 AM by user_4653071

    What is the working principle of the shield electrode in proximity induction

    user_4653071

      I was recently working on Cypress's proximity sensing solution, focusing on the cy8CKIT-040 and Cy8CKIT-024 combination suite.

      By referring to the document Getting Started with CapSense, it can be seen that the shielding electrode is obtained by the shield drive signal connected to the filled grid at the top and bottom of the PCB.

      But I wonder how this shielded electrode can increase the proximity of the sensor?Or why do these grids, combined with the masking of the driver signal, have this effect?

      In addition, when I looked up the CY8CKIT-024 CapSense Proximity Shield Kit Guide, I found that the Shield/GND on this board is a separate wire loop?

      (I'm not sure if this is a wire loop.) This does not look like the Shield Box I wrote in the previous document. Right?GND/Shield Loop circled in purple.

      I would like to know whether it is because the shielding effect of this circuit is better than the shielding effect of the grid that such a large coil circuit is used as the shielding electrode instead of the grid mentioned before.

      Finally, if I want to redesign a larger proximity sensor, should I choose grid or loop for the shielding scheme?

       

      thank so much!

        • 1. Re: What is the working principle of the shield electrode in proximity induction
          AH_96

          Hello user_4653071

           

           

          But I wonder how this shielded electrode can increase the proximity of the sensor?Or why do these grids, combined with the masking of the driver signal, have this effect?

          We need a hatch pattern around the sensor to provide immunity against external noises. If the hatch pattern is connected to ground, then this increases the Cp, parasitic capacitance of the proximity sensor. However, if it is connected to shield, which is a buffered version of the waveform that we get at the sensor, then the Cp is less. Therefore, we get a better signal if shield is used.

          Note that the physical Cf, finger capacitance which is the capacitance that is introduced by your hand/finger, is same whether the hatch is connected to shield or ground. The signal obtained while detecting Cf with a lower Cp is higher as the ratio of Cf/Cp is also higher.

           

          This increased signal contributes to a better sensing range.

           

           

          In addition, when I looked up the CY8CKIT-024 CapSense Proximity Shield Kit Guide, I found that the Shield/GND on this board is a separate wire loop?

          (I'm not sure if this is a wire loop.) This does not look like the Shield Box I wrote in the previous document. Right?GND/Shield Loop circled in purple.

          I would like to know whether it is because the shielding effect of this circuit is better than the shielding effect of the grid that such a large coil circuit is used as the shielding electrode instead of the grid mentioned before.

          Finally, if I want to redesign a larger proximity sensor, should I choose grid or loop for the shielding scheme?

           

          In the top layer of this kit, it is just a wire loop. The hatch pattern is not present, but if you observe the bottom portion of the kit, there is a hatch pattern that is connected to shield (or ground depending on the  SW1 switch configuration). Ideally, we would recommend adding a hatched pattern on the top layer as well if you expect environmental noise affecting your end design.


          Best regards,

          Hari

          • 2. Re: What is the working principle of the shield electrode in proximity induction
            user_4653071

            thanks for ur response, 

            1,but I still cannot know why the shield electrode could improve the proximity sensing distance.

            2,In addition, according to ur reply, does it mean that there is not hatch patter on the top layer on CY8CKIT024?

            3,also, this kit choose a loop as shield / GND, is it because that a loop is better than a hatch pattern?

            4,if I wanna achieve 30-cm proximity sensing distance, the sensor should be a wire loop, and then should the shield be a loop or a hatch pattern or together?if a hatch pattern is chosen,there should be any hatch pattern inside the  loop?

             

            wish ur apply!

            Best regard !

            • 3. Re: What is the working principle of the shield electrode in proximity induction
              AH_96

              Hello user_4653071

               

              1. The main point here is that shield will reduce the Cp of the proximity sensor. Let us consider the Cf, or change in capacitance with a hand, to be 0.5pF. Also, let us consider the Cp of the proximity sensor to be 20pF with ground and 10pF with shield hatch (Cp with shield hatch will always be lower). Now, in the case of ground, the change in capacitance at the pin is from 20 pF to 20.5pF.  This is only a 2.5% change in the capacitance.

              Whereas, if we consider shield, the capacitance changes from 10pF to 10.5 pF, which is a 5% change. So, we are likely to receive twice the signal in this case, although our Cf is the same.

               

              2. Yes that is correct.

               

              3. In both the cases, a ground ring around the proximity sensor. A hatch pattern will help if you have some noisy signals in the top layer such as a communication signal or a PWM signal for example. The hatch pattern on the top layer will help in isolating these noises from the proximity sensor.

               

              4. For 30 cm proximity sensing, you will need a proximity loop of 30 cm in diameter. The hatch pattern is a choice that is dependent on the points stated above. If you feel there are noisy elements, then the shield hatch is needed in top layer. In either cases, a ground loop is required around the proximity sensor for suppressing any noise that is coming from external sources. Note that it is recommended to place the ground loop outside the proximity sensor.

               

               

              Best regards,

              Hari

              • 4. Re: What is the working principle of the shield electrode in proximity induction
                user_4653071

                so,does it mean that the best option is to have the shield electrode(hatch pattern) and GND loop at the same time on the PCB?

                and,the word 'it' in the last sentence refers to GND loop? or shield electrode?

                 

                best wishes!

                • 5. Re: What is the working principle of the shield electrode in proximity induction
                  AH_96

                  Hi,

                   

                  Yes, that would be the best option.

                   

                  The recommendation is to have ground loop outside the proximity sensor as shown in the diagram for external noise immunity. If you require liquid tolerance, this can be a shield loop.

                  Otherwise the outside loop can be connected to ground.

                   

                  Best regards,

                  Hari

                  • 6. Re: What is the working principle of the shield electrode in proximity induction
                    user_4653071

                    i just wanna achieve a 30-cm proximity sensing sensor,and i dont need take liquid tolerance in account.

                    in this case,is a GND loop necessary?

                    • 7. Re: What is the working principle of the shield electrode in proximity induction
                      AH_96

                      For sensing that distance, it is not necessary. But it is recommended to have the ground loop for better noise immunity. The ground loop does not help in improving the sensing distance.


                      Best regards, Hari