Tech Inspection

[TheGuyz]

What is the acceptable voltage for the tech inspection with multimeter positive to battery and multimeter negative to frame.

[Tristan Lall]

Originally Posted by TheGuyz

What is the acceptable voltage for the tech inspection with multimeter positive to battery and multimeter negative to frame.

Not voltage; resistance. You're looking for at minimum 10 kΩ.

[DonRotolo]

Voltage should be Zero or darn close.

Check resistance: from frame to battery + or battery - MUST be much higher than zero.

[iambujo]

We just failed inspection, with a chassis to PDB resistance of about 970 KOhm to ~1 Mega Ohm. I was not there so I was unable to help troubleshoot or get a better understanding of the situation. Last I checked though, 1 Mega Ohm is indeed greater than 10 K ohms.

I was told (second hand) that we were told it indicated a short somewhere. It's late and has been a long week at work, but I am currently failing to see the logic that it indicates a short.

Any thoughts? Anything I could be missing? I can certainly help try and isolate branches (remove breakers 1 at a time, visually inspect wires and sensors, etc) but 1M is > 10K so I just fail to see how it is a sign of a problem. I am sure someone will enlighten me here though.

[Deetman]

I see you are from NJ, so I assume you are at a MAR event (Rutgers or Chestnut Hill). Due to the nature of the district system, there are some new folks volunteering, including inspectors. They have been doing a great job but little things slip by here and there.

Choosing a value for electrical isolation can be a bit tricky. Since the robot power system is not my design I'm going to defer to FIRST here... Per the robot inspection checklist that is on the FIRST website and every inspector carries with them:

Isolated Frame – Must be electrically isolated from battery, cRIO and camera must be insulated. (>10k Ohm between
either PD battery post and chassis) <R38>

Bottom line: Before you go ripping things apart, please check with the Lead Robot Inspector at your event.

[artdutra04]

Originally Posted by iambujo

We just failed inspection, with a chassis to PDB resistance of about 970 KOhm to ~1 Mega Ohm. I was not there so I was unable to help troubleshoot or get a better understanding of the situation. Last I checked though, 1 Mega Ohm is indeed greater than 10 K ohms.

I was told (second hand) that we were told it indicated a short somewhere. It's late and has been a long week at work, but I am currently failing to see the logic that it indicates a short.

Any thoughts? Anything I could be missing? I can certainly help try and isolate branches (remove breakers 1 at a time, visually inspect wires and sensors, etc) but 1M is > 10K so I just fail to see how it is a sign of a problem. I am sure someone will enlighten me here though.

Make sure your Axis camera and cRIO are electrically isolated from the metal chassis.

Check any Banebot 775 motors for case shorts.

Check any sensors for grounded frames/mounting bolts (Sharp IR range sensors' mounting screws are grounded).

[DonRotolo]

Again: Go to the Lead Robot Inspector and kindly ask about this issue.
The LRI is experienced and will spot any errors in interpreting the multimeter.

[iambujo]

Thanks Guys - would you agree that 1Mohm is indeed isolated? After all it meets the requirement and is greater than 10K. Or if you saw it would you be concerned that it could indicate a lack of isolation somehow?

I just want to make sure I am thinking about this correctly - clearly if we have a problem we need to fix it. (In other words, does it seem more like getting on the same page with the inspection team and Lead inspector, or that we do actually have an issue?). Since 1M is > 10K, I logically thought it indicated we were good to go.

Any idea what is a typical/average reading?

Thanks again

[WizenedEE]

Originally Posted by DonRotolo

Voltage should be Zero or darn close.

On our robot, it's about 7V. There's about 1M ohm between the positive PD lug and the frame, and about 1M ohm between the negative PD lug and the frame. Nothing wrong with that.

The rules say "isolated" and the cheklist says "10k ohm" so I see absolutely no reason that 1M would be illegal.

[Deetman]

Originally Posted by iambujo

Thanks Guys - would you agree that 1Mohm is indeed isolated? After all it meets the requirement and is greater than 10K. Or if you saw it would you be concerned that it could indicate a lack of isolation somehow?

I just want to make sure I am thinking about this correctly - clearly if we have a problem we need to fix it. (In other words, does it seem more like getting on the same page with the inspection team and Lead inspector, or that we do actually have an issue?). Since 1M is > 10K, I logically thought it indicated we were good to go.

Any idea what is a typical/average reading?

Thanks again

For the purposes of FIRST, I'd say 1Mohm is indeed isolated, especially since they've spec'd it. I'd say your first priority would be get on the same page as the inspector and lead inspector and see what, if any their concerns are.

I couldn't guess a typical/average reading but I know on 1712's robot this year and last, we couldn't get any resistance reading (out of range) on the meter. It was a good Fluke and out of limits is >40Mohm on it.

If I had to guess, there is a sneak path somewhere on the robot that you are picking up your ~1Mohm resistance. Possibly a shorted motor winding. It probably isn't a problem but I would either want to find the root cause or keep monitoring it even if it passes inspection, but that is just me.

You aren't using a Banebots RS-775 motor are you? They were notorious last year for having shorting issues, some of which started around where you are and got progressively worse.

[iambujo]

We do have some 550's on there whose motor housings are not isolated (as none of the motors are for obvious mechanical reasons). We can check those tomorrow, recheck the camera isolation, etc. But first and foremost as you have all stated, we need to get on the same page with the inspectors to make sure we understand their concern.

[EricH]

550s have never been reported to have a problem with a case short. The 775s on the other hand...

The LRI is the first person you want to talk to tomorrow. Not an inspector waiting at your pit (if there is one). If he's holding you to a tighter spec than the inspection checklist (which is based on the rules), then ask him to call HQ for a ruling.

[Dale(294engr]]

Originally Posted by iambujo

We just failed inspection, with a chassis to PDB resistance of about 970 KOhm to ~1 Mega Ohm. I was not there so I was unable to help troubleshoot or get a better understanding of the situation. Last I checked though, 1 Mega Ohm is indeed greater than 10 K ohms.

I was told (second hand) that we were told it indicated a short somewhere. It's late and has been a long week at work, but I am currently failing to see the logic that it indicates a short.

Any thoughts? Anything I could be missing? I can certainly help try and isolate branches (remove breakers 1 at a time, visually inspect wires and sensors, etc) but 1M is > 10K so I just fail to see how it is a sign of a problem. I am sure someone will enlighten me here though.

970K indicates a potential problem that needs tracked down.

Can be EXTREMELY difficult to trace.. better done BEFORE competition starts.

I've spent half day helping a team disconnecting everything until culprit was isolated.. a good team too.. 207 from LA!!

When doing RESISTANCE test: swap probe leads to check BOTH polarities.

Shorts often exhibit diode behavior due to intervening multi semiconductor paths.

The Lowest R reading rules and must be >10K to pass.. BUT-->

Anything around 100K is a problem in spite of 10k spec. if you don't want sporatic random robot resets during competition!!

Good robots will read infinity!!
there should be nothing conductive at all to chassis. >2M (Harbor Freight) to >40M ohm (industrial grade)

Do you use a 1M potentiometer by chance??
a bit high to use for position or most sensor feedback applics, just a thought

RS-775 tab to case shorts are usually around 1 ohm AT motor

.. however when routed thru H-bridge of Jag's or Victors can show up as ~100k.. I believe Team 207's last yr was 200-300k & asymmetrical when ohmmeter leads were swapped.. they had 2 case to tab, 775 shorts!
And all new ones left in Spare Parts were shorted! (6)

GP: Attention teams possessing good 775's & NOT using them
PLEASE bring unused RS-775's to competition: Pitt announce they're available

To troubleshoot..
Remove both leads from one motor / device at a time until R changes
.. use lower of swapped values as it is more solid
(higher reverse resistances often waver around a bit making ya crazy)

Good luck...

[Bryan Herbst]

Originally Posted by WizenedEE

On our robot, it's about 7V. There's about 1M ohm between the positive PD lug and the frame, and about 1M ohm between the negative PD lug and the frame. Nothing wrong with that.

The rules say "isolated" and the cheklist says "10k ohm" so I see absolutely no reason that 1M would be illegal.

7V? Ignoring any resistance checks, that should be throwing major red flags. If your frame is completely isolated, you should not be able to get any voltage between the battery and frame.

That being said, as many others have said here, talk to the LRI. Though we on CD can debate back and forth for ages on what is/isn't an acceptable level of resistance, our decisions carry no weight. You should never cite a post on CD when talking to officials about rules.

[WizenedEE]

Originally Posted by Tanis

7V? Ignoring any resistance checks, that should be throwing major red flags. If your frame is completely isolated, you should not be able to get any voltage between the battery and frame.

That being said, as many others have said here, talk to the LRI. Though we on CD can debate back and forth for ages on what is/isn't an acceptable level of resistance, our decisions carry no weight. You should never cite a post on CD when talking to officials about rules.

As I said, our frame has a little over 1 megaohm to both leads -- it is not completely isolated, nor does it have to be. I was trying to point out that voltage means nothing, it's about resistance.