Re: Tech Inspection
 

Opinion from a 10 year 3 events/yr RI & 15 year FIRST Mentor:

NOT so! the voltage on the frame is stored in the frames capacitance.

Capacitance (Joules) is like a charged battery with a pretty low internal resistance ready to dump its energy to anything that will take it independent of resistance that charged it. (R affects time to charge it to 7v or whatever)

The charge time is ~3xRC. your R is 2x1meg in parallel= 500K C= TBD but will be significant charge on such large metal structure so as to blow out or damage (cause indeterminate intermittent) to CMOS & TTL inputs and maybe even outputs (even protected TBD)!

there is surely potential to upset digital logic:

Future problem scenario: robot reset via intermittent pinched wire contact to 7V chassis! Of course it happens near beginning of playoff match or crucial qual. Murphy is mean!

How many resets, raspy control scenarios can your team endure and still win?

>10K per se means little; LRI likely to give provisional pass but wise to suggest team keep looking for the offending continuity and clear it, for team & alliance performance benefit.

More importantly teams should view this as a symptom thus give it priority to avoid future unexplainables: painful resets, jerky response, unhappy partners

I am sure I could justify Risolation being >500K or even 1M due to chassis charge stored and its ability to upset logic under many circumstances of vibration, impact, etc.

If R isn't infinity, something is wrong. There are no insulators used on the robot wherein it is normal for chassis to either battery terminal to be <infinity

Some coatings & anodization are normally insulators may exhibit diode behavior polarity sensitive R one direction high, other very low (Copper oxide rectifiers = diode used 50 yrs ago)

(so depending on your meter..infinity = >1m, >10M >20M >40M is normal)

see my earlier post this thread for 2011 team 207 Rbatt-to-chassis= 200k-300K offender RS775 <1ohm measured at motor

Good luck.. Go for infinity!!!

Re: Tech Inspection
 

Thanks again for the replies. We got there this morning armed with 3 different multimeters. All three read '0L' (no load), and we measured 0V from battery to chassis. I am not sure what was happening with the meters and tests last night, but it was smooth sailing today and the final inspection was quick. ::safety::

Re: Tech Inspection
 

That would lead me to want to check it again after each of your first couple of matches just to make sure you don't have some kind of intermittant problem. If it does come up again, isolating which circuit it's on while you can measure it may help in some diagnostic later in the day should other symptoms arise.

Re: Tech Inspection
 

Most robots will read OL (over load or immeasurably high resistance). This should be your ideal target, and I'd insist on it before bag'n'tag. If you are measuring hundreds of kOhms at a regional, you likely have more important things to do. In any case, keep an eye on it and re-measure every time you do something that might risk a short.



OL: You are done! Measure again before your next match.

0-10 Ohms: There is a direct short somewhere in your system. You are likely to light on fire the next time you get rammed.


90-110 kOhms: One of the devices you have pulled a breaker for has a short close to its positive lead.

1MOhm + : You might have a short down stream of a piece of silicon. The actual value read will heavily depend on the exact model multimeter you are using. This is often also polarity sensitive.

Here are a few that I've measured with my multimeter, YMMV:
Grey Jag Power Output:

2M Ohms to PD + or -. Takes 15 seconds to stabilize, invariant with meter polarity.

4 slot cRIO chassis:

10kOhm to PD - with positive meter on PD.
9kOhm to PD + with positive meter on PD.
Oscillation with negative meter on PD.

Could others add to this list?

Re: Tech Inspection
 

Can we agree that we disagree on that? FIRST specifies >10k, 970k is almost 100 times that.

Re: Tech Inspection
 

Something else to took out for is the finish on the chassis. If you have an anodized aluminum chassis, such as 8020, you may actually have a very real short that is not showing up because of the insulating properties of the finish. I just did a check on a piece of 8020 in my shop. Sliding one probe while keeping the other fixed showed a total open circuit in most places, with varying resistances and actual shorts in others.

When inspecting, I always try to go from + and - to some part of the chassis which has been drilled, cut or otherwise free of anodizing.

Dr. Bob

Chairman's Award is not about building the robot. Every team builds a robot.

Re: Tech Inspection
 

Guys,
970K to 1M is an acceptable measurement in a humid environment or when the inspector has one finger on the probe. (this is highly dependent on the meter used.) The nature of motor shorts are such that it may only show shorted when the motor is rotated to the winding that is shorted to the armature. Simple chassis measurement may also not show low resistance when isolated by a speed controller or spike. If you read more than 10K in most cases there will be no problem with interaction with other robots. When strange problems occur in robot operations, such as spontaneous Crio reboots, then suspect motor shorts and make your resistance measurements from robot chassis to each motor lead to determine if a motor is at fault.