Re: Banebots RS-775 Case Short
 

Ordered either late on the 16th or on the 17th. Received my order yesterday. Shipped on the 6th business day, arrived 2 days later. Beautifully packed, good fit and finish on all parts, no shorted motors (out of 3 775s). Looking forward to trying it out. :)

Re: Banebots RS-775 Case Short
 

I'll add to this -

I ordered 3 775s and some 4:1 planetary gearboxes from BB on 1/10. Received the whole order, well packaged, on 1/13. All the motors passed our EE's load testing, and now they're being used in test beds with the programming team.

For me, BB has stepped it up quite a bit this year. No bad surprises so far.

Re: Banebots RS-775 Case Short
 

So you are saying

1) The voltage polarity on the case will reverse when the motor direction reverses?

and

2) Electrically isolating the robot frame does not allow the system to survive an armature-to-case short in a motor?

If I've misunderstood, could you please clarify.

Re: Banebots RS-775 Case Short
 

I have been following this thread all season, ( of course after placing an order of what we could afford (2 775 BB and 775 BB gearboxes). If and maybe when we receive them, and they do have case shorts, living in a place where 2nd day air means 2 weeks (Really not exaggerating, unfortunately), shipping back for a refund and maybe working BB is not an option.

What I have gathered is that:

Run a 12 Volt across the each of the leads to the case and hope for the best?

Mount motors and such to lexan/Derlin if all else fails.

Anything else?

Re: Banebots RS-775 Case Short
 

No,
I disagree with a statement of myth. At the time that faults were first noted on FRC robots, the IFI control system was being used. It does not have a conductive case. However, robots with any electrical faults to the frame could under certain conditions cause current to flow between robots.

The polarity of the voltage present on the case of a failed motor can change depending on where in the motor the failure occurs. If the fault occurs with internal wiring/brush assy and the case of the motor is tied to the negative lead of the battery (or near that potential) when operated in one direction. Then a motor reversal in this case is likely to put the case at near the positive lead of the battery. If the motor winding alone is the source of the fault, then the polarity on the case will change with rotation and will vary with duty cycle supplied to the motor. There are other variables as well depending on the nature of the fault, such as in a motor where the commutator is bridged by solder splash or two windings are shorted together.

For those just reading the thread and those actively participating, please remember that motor failure is a common occurrence in motors other than the Banebots 775. Mishandling, improper mechanical design and stray metallic debris all contribute to these failures every year. While a failed motor in and of itself may not keep you from competing, other wiring errors will add to the fault and affect your ability to play. Consider for instance, a failed motor on a robot with an insulated Crio but a camera that has not been mounted with the same care. The camera and wiring, and the PD power supply will be affected.

Re: Banebots RS-775 Case Short
 

I'm trying to clearly understand what your intended meaning is.

"cause real voltage drops across the #18 wiring" strongly implies that you mean due to additional current through those wires caused by the fault condition. Was that your intended meaning?

Re: Banebots RS-775 Case Short
 

Yes

Re: Banebots RS-775 Case Short
 

I don't see how a motor case short can cause an increase in current in the cRIO negative power lead if the cRIO chassis is electrically isolated from the robot frame.

Perhaps someone could post a sketch showing the current path.

Re: Banebots RS-775 Case Short
 

This isn't the appropriate thread for this, and I'd appreciate if this side bar could be split off.

In a double fault scenario, additional current can flow from the motor, through the chassis, into the cRIO frame, through the cRIO's fuse, out the connector, through the return wire, into the PD, through the PD's self-resetting fuse, to the negative return of the battery.

The question is a matter of timing: Which protection cuts out first? The four in play here are the cRIO fuse, the cRIO power supply undervoltage lock out (UVLO), the PD's self resetting fuse (PTC), and the return wire experiencing a rapid gaseous state transition.

My claim is that the PTC is doing what I designed that circuitry to do, and that it is cutting out first. It heals when the fault clears, and then the cRIO begins to reboot, as my fellow NI engineers designed it to.

Re: Banebots RS-775 Case Short
 

You need to think of the Crio power supply wiring as a resistor. While the entire motor current will not flow in the negative lead of the Crio (if it is tied to robot frame), what current does flow will affect the power input at the Crio. It only has to drop enough voltage to cause the Crio to reboot. When that occurs, all Crio output is disabled and there is no current flow through the motor.

Re: Banebots RS-775 Case Short
 

We both agree that that wiring link is resistive and that the voltage it develops does eat in to the 24V supply's margin. We disagree on how much voltage is developed, how much margin the supply has, and how quickly the PTC blows.

The current necessary to cut out the entire margin budget is orders of magnitude beyond the points the protection devices cut in.

The PD schematic and design package are available for review on usfirst.org . If anyone is interested, please run the math.

Re: Banebots RS-775 Case Short
 

Eric,
All we are really worried about is the voltage change at the power input to the Crio. There does not need to be a trip in the PD. It is like adding huge amounts of ripple in linear power supply. All that has to happen is for the power at the Crio to upset the internal power sense and the Crio reboots. Considering that switching PWM is a significant component in the current in this line, the internal regulators of the Crio likely give up pretty easy. I suppose it is also possible that the 24 regulator gets fooled into restarting but I really doubt it. Added to the switching, you have sometimes huge amounts of RF generated by the brushes and switching, particularly in the Jag.

Re: Banebots RS-775 Case Short
 

Al, I'm trying really hard not to be rude, but the math simply does not hold up. It looks like we'll have to agree to disagree unless you can provide evidence to support your theory that the UVLO fires before the PTC. I still believe it is behaving as it is designed to and how the math indicates it must.

Don't get me wrong, I understand what you are driving at. It is a perfectly valid theory, it just doesn't hold up in this situation. For this system, the current/time profile to trigger the PTC is much much smaller than the current/time profile to trigger a UVLO: On the order of 100-750x smaller, depending on the shape of the pulse.

Re: Banebots RS-775 Case Short
 

Just got our 550's and 775's. Not sure if I should be scared or not...

Re: Banebots RS-775 Case Short
 

To what are you referring to as UVLO?