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  • Managing Electrostatic Discharge effects

    Hi Folks - Just a heads up, the FIRST Tech Challenge wiki on the GitHub repo for the software now has some information regarding the management of electrostatic discharge effects.

    https://github.com/ftctechnh/ftc_app...charge-Effects

    Information about this subject will also soon be available in the revised Team Wiring Guide.

    Tom

  • #2
    Thanks Tom, very informative.

    Question... Many teams (including ours) use two REV Expansion hubs on their robots. Current we use the spare XT30 port from Hub 1 to jump power to hub two. If we then used the spare XT30 port on Hub 2 for the REV Resistive Grounding Strap would that properly ground the entire system (both expansion hubs) or do we need to find a way to ground each REV Expansion hub separately?

    Michael P Clark
    Founding Mentor, FTC 9958
    http://www.redfishrobotics.com
    "We're Hooked on FIRST"

    Comment


    • #3
      Hi RedfishRobotics,

      I believe that these are now permitted with this season's current rules:

      http://www.revrobotics.com/rev-31-1293/

      This distribution device would let you provide 12V power to the second Expansion Hub and also ground your electronics.

      Sorry - I realized I didn't answer your question (in my haste, I misread your original question). Yes, using the spare port to connect the Resistive Grounding Strap should also ground the electronics.

      Tom
      Last edited by Tom Eng; 09-12-2018, 12:30 PM.

      Comment


      • #4
        Originally posted by Tom Eng View Post
        Hi RedfishRobotics,

        I believe that these are now permitted with this season's current rules:

        http://www.revrobotics.com/rev-31-1293/

        This distribution device would let you provide 12V power to the second Expansion Hub and also ground your electronics.

        Tom
        Perfect, they should be back in stock soon, and a great investment at only $10. Thanks again!
        Michael P Clark
        Founding Mentor, FTC 9958
        http://www.redfishrobotics.com
        "We're Hooked on FIRST"

        Comment


        • #5
          Hi Tom, thanks for all the hard work.
          I'm wondering why grounding the electronics to the frame was explicitly dis-allowed in the past.

          Tim

          Comment


          • #6

            As explained in the document they are using a resistor to prevent an accidental short, instead of shorting the electronics to the frame.

            A FIRST-approved cable has an appropriately sized inline resistor. This resistor is critical because it acts as a safeguard to prevent excessive current from flowing through the frame of the robot if a

            Comment


            • #7
              Originally posted by tjhawkey View Post
              Hi Tom, thanks for all the hard work.
              I'm wondering why grounding the electronics to the frame was explicitly dis-allowed in the past.

              Tim
              Hi Tim - I think there was concern over both the safety and the efficacy of grounding the electronics to the frame. There is a white paper written by Eric Chin, who was a FIRST Engineering intern this summer, that documents (with experimental data) the efficacy of different mitigation techniques. This white paper should be available to the community soon. It provides and interesting (and in depth) analysis of various ESD mitigation techniques, including grounding the electronics with this "resistive grounding strap."

              Tom

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              • #8
                I could use some help understanding something from the document. The use of the ground strap seems to provide unquestioned improvement, as does proper isolation, anti-static spray, and perhaps ground planes. But I am left wondering about the use of ferrite chokes. If they really do provide a value, how? What properties should be considered when selecting a ferrite bead? Why?
                I always thought chokes helped with EMI - they reduce high frequency noise by acting as a low-pass filter. Typically, the choke is chosen for blocking a particular frequency range.
                But are chokes effective against transient events like ESD? If so, how?
                Thanks,
                Stephen

                Comment


                • #9
                  Originally posted by FTC7253 View Post
                  I could use some help understanding something from the document. The use of the ground strap seems to provide unquestioned improvement, as does proper isolation, anti-static spray, and perhaps ground planes. But I am left wondering about the use of ferrite chokes. If they really do provide a value, how? What properties should be considered when selecting a ferrite bead? Why?
                  I always thought chokes helped with EMI - they reduce high frequency noise by acting as a low-pass filter. Typically, the choke is chosen for blocking a particular frequency range.
                  But are chokes effective against transient events like ESD? If so, how?
                  Thanks,
                  Stephen
                  My personal experience has been that the MR cables (which have ferrites) are considerably less reliable than using Cable Matters cables that do not have them.

                  Comment


                  • #10
                    Originally posted by 4634 Programmer View Post

                    My personal experience has been that the MR cables (which have ferrites) are considerably less reliable than using Cable Matters cables that do not have them.
                    But I don't think you can necessarily blame the ferrites for that. Those original white cables just seemed to be fraught with problems.
                    They worked OK for a while, and then just got flakey. I suspected poor internal connections, or poor manufacturing controls.

                    However, the added weight of the ferrites could have been a contributing factor if teams failed to tie down the wiring firmly. So remember to use wire management techniques to ensure the wires don't flail around.

                    It's even possible (in a world of unknowns) that the protective action of the ferrites actually caused the cables to fail, and thus protecting the electronics, but that is a stretch.

                    So, I would advise against making a generalization based on that one sample point.

                    Comment


                    • #11
                      Originally posted by RedfishRobotics View Post
                      Thanks Tom, very informative.

                      Question... Many teams (including ours) use two REV Expansion hubs on their robots. Current we use the spare XT30 port from Hub 1 to jump power to hub two. If we then used the spare XT30 port on Hub 2 for the REV Resistive Grounding Strap would that properly ground the entire system (both expansion hubs) or do we need to find a way to ground each REV Expansion hub separately?
                      Normally I'd want to put grounding strap right at the beginning of the ground supply, which would be just after the power switch, or at the first power distribution point.
                      This ensure that it's benefit has the greatest effect/impact.

                      However, since this grounding strap contains a resistor (thus limiting the current flow), it's not AS essential to ensure that it's attached to a specific point of the ground bus.

                      So, adding it to the end of the power chain will suffice for ESD protection. The only caveat is that you ensure that the power chain does not get broken.
                      eg: if the power connector were to come loose from your second hub, not only would you lose any devices on that hub, but you would also lose ESD protection to your entire system.
                      That's an extreme example, but it illustrates the difference between putting the protection at the beginning of the power chain, rather than the end.

                      Comment


                      • #12
                        Originally posted by Philbot View Post

                        But I don't think you can necessarily blame the ferrites for that. Those original white cables just seemed to be fraught with problems.
                        They worked OK for a while, and then just got flakey. I suspected poor internal connections, or poor manufacturing controls.

                        However, the added weight of the ferrites could have been a contributing factor if teams failed to tie down the wiring firmly. So remember to use wire management techniques to ensure the wires don't flail around.

                        It's even possible (in a world of unknowns) that the protective action of the ferrites actually caused the cables to fail, and thus protecting the electronics, but that is a stretch.

                        So, I would advise against making a generalization based on that one sample point.
                        I wasn't exactly trying to say that ferrites are bad, I was more trying to say that a high quality cable without them is better than a low quality cable with them.

                        Comment


                        • #13
                          Originally posted by Philbot View Post

                          Normally I'd want to put grounding strap right at the beginning of the ground supply, which would be just after the power switch, or at the first power distribution point.
                          This ensure that it's benefit has the greatest effect/impact.

                          However, since this grounding strap contains a resistor (thus limiting the current flow), it's not AS essential to ensure that it's attached to a specific point of the ground bus.

                          So, adding it to the end of the power chain will suffice for ESD protection. The only caveat is that you ensure that the power chain does not get broken.
                          eg: if the power connector were to come loose from your second hub, not only would you lose any devices on that hub, but you would also lose ESD protection to your entire system.
                          That's an extreme example, but it illustrates the difference between putting the protection at the beginning of the power chain, rather than the end.
                          Thanks Philbot We ordered two grounding straps (and a power distribution block) for each of our two teams, so I think with this guidance we should be good.

                          We didn't have any problems with ESD last season, however, this year with multiple robots simultaneously contacting a large metal structure, we're concerned we'll see the kind of problems we had during RES-Q, where multiple robots shared a large metal structure.

                          implistic view from a non-engineer (stay in school kids), however, it seems logical to me that ESD triggered by another robot coming into contact with yours (or the lander) could just as easily cause an ESD related failure, and your team also has no contract over the level of mitigation taken by other teams.

                          Thanks again, and... #NoSleepTillHouston
                          Michael P Clark
                          Founding Mentor, FTC 9958
                          http://www.redfishrobotics.com
                          "We're Hooked on FIRST"

                          Comment


                          • #14
                            I would be interested in the specs of the ferrites that were used in the tests at FIRST. If more tests are done, it would also be very interesting to hang a high voltage probe on the robot and get a scope trace at fast time scale. The electrometer is OK for non-contact voltage sensing (and no leakage), but since the test used a van-degraff to charge the robot, it's also possible to use a probe to directly measure the voltage (and what happens during an arc).
                            Does it practically matter which exact ferrites are used? Maybe/maybe not.

                            Comment


                            • #15
                              Philbot and 4634 Programmer
                              Posts 9-14 discuss various aspects of using ferrites, but not the why.
                              Are ferrite chokes verifiably effective in reducing ESD related disconnects/issues? I didn't see a any results referenced in the ESD doc where tests were run where the use of ferrites was the only variable (without ground strap or ground plane as the control setup). There was one small sample result in test 18 where an improvement was seen when using a choke on all 4 wires of a sensor cable while using a 470M Ohm resistor. But this resistance level was later eliminated as a preferred option. It may be possible, especially considering the sample size, that the effects of the resistor effected the results seen from adding/removing the choke.
                              It would be nice to understand the isolated benefit of choke usage, and then perhaps see its benefit when used with the selected ground strap resistance.
                              I am still trying to understand the science, however. Is the choke actually effective in reducing the impact of transient ESD spikes? I can absolutely understand how the low-pass behavior of ferrite chokes could improve integrity of the data signal by reducing continuous EMI noise (possibly caused by noisy power on the adjacent wires), but am not grasping how/if they mitigate the transient events. Improvement in data signal integrity alone may be a valid reason to use chokes - even if they don't really mitigate ESD issues.
                              My questions aside, I would like to send big kudos to Eric Chin and others involved in conducting and publishing this valuable research.

                              Comment

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