How to repair your FRC Radio to avoid reboots

The Open Mesh OM5P-AC FRC radio has a design flaw: When mounted wrong, pushing a certain way on the plastic case will cause it to reboot.

To force the radio to reboot, hold the radio as shown and press with your thumbs on the underside of the device.

This flaw is bad and needs to be corrected.  A radio should not be a pressure-sensitive input device!  As we know, “A reboot of the robot radio is typically characterized by a loss of connection to the radio for ~25-30 seconds” or approximately 1/4 the duration of a FRC match.

In this article, MentorBuilt will illustrate a repair which will prevent this behavior.


Let's get the legality discussion out of the way first

Rule R72 of the 2018 FRC rules generally disallows any modification of the mandatory control system components, including the radio.  These devices “shall not be tampered with, modified, or adjusted in any way.”  The rule lists the following types of tampering: “drilling, cutting, machining, rewiring, disassembling, painting, etc.”  Exception M of the R72 allows these components to be repaired “provided the performance and specifications of the device after the repair are identical to those before the repair.”

The strange behavior of the radio to trigger its own reboot when the enclosure is pushed a certain way is not a design feature.  The behavior is certainly not described in the manufacturer’s performance specifications.  Instead, it is a serious flaw to be corrected via a repair.  Our repair restores the functionality of the device and enables it to perform per the manufacturer’s performance specifications.  In this way, our repair meets the R72M exception and is legal per the 2018 FRC rules.


Flaws and repairs

There are two related mechanisms that may cause the inadvertent reboot of the OM5P-AC FRC radio when an external force deforms the plastic case near the connector end of the device.  In the following sections, we’ll describe both the potential sources of the errors and the steps you’ll need to follow to execute a successful repair.

Take care when removing the radio lid to avoid damaging the antennas or their wires.

Potential cause 1: Metal shielding plate contacting & shorting connector leads

Some parts of the internal circuit board may be contacting a nearby metal shielding plate and creating an electrical short.

Several through-hole components are soldered to the board in the area of interest: a female barrel connector for power, and two RJ45 connectors for ethernet.  Their leads protrude through the board and are soldered from the underside.  There is no insulating coating over these pins.  When the case deforms, a thin aluminum shielding plate may touch the pins and cause a short.

Soldered through-hole leads which may contact the metal shielding when the case deflects.

To repair the radio, we apply a piece of electrical tape to the inside of the aluminum shield where the connector pins would touch.

Strip of electrical tape (yellow) added to the aluminum shield.  Now the shield cannot cause an electrical short between the uninsulated parts of the board above.

Potential cause 2: Interior of case pressing reset button

When deformed, the white plastic case itself may inadvertently contact the small red tactile reset button and cause the reboot.

There is hole though the plastic case that enables the user to use a small tool to poke the button.  Unfortunately, the reset button is larger than the hole.

The reset button hole in the case is approximately .115”.

The diameter of the reset button (red) is approximately .119”.

To repair the radio, we slightly enlarged this hole.  The repair creates sufficient space around the top of the reset button to prevent any contact when the case is deflected by an outside force.

The hole is enlarged with a 5/32” (.156”) twist drill.

View of the larger reset button hole.

After the repair, there is enough space around the entire reset button.  Now the case cannot contact the button and trigger the reboot.


Testing and final recommendation

MentorBuilt performed both parts of this repair to our stock of Open Mesh OM5P-AC FRC radios.  For each of these repaired radios, we attempted to cause a reboot by squeezing the case and we found that the reboot no longer occurred.

When we re-opened our repaired radios after this testing, we found evidence that the connector leads did slightly contact the added electrical tape.

MentorBuilt recommends that teams execute both parts of this repair on their 2019 Open Mesh OM5P-AC FRC radios.

Comments

  1. To recommend to the FRC community that they should do this when it is clearly a violation of the rules, regardless of your tortured reading of the rules, is reckless and wrong. Also, it looks like this has been fixed by the manufacturer? https://www.chiefdelphi.com/forums/showpost.php?p=1821567&postcount=46

    Keep your posts to comedy or Marshall.

    ReplyDelete
    Replies
    1. Thanks for your comment! Happy holidays, and we'll see you at the competition.

      Delete
    2. read R73 point O. in the DDS manual for 2019
      looks legal to me!!

      Delete

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