Possible heat buildup conciderations?

Technical Technical Discussion
#1

As our team is designing and building our bot for 2018, I am remembering some of the excellently designed control layouts we saw last year (our rookie) that put most of the control system in essentially a plastic box mounted to either the chassis or the top of the robot. We are considering creating something similar this year, but have some questions about heat.

  1. Since space could be problematic in our design, if we keep speed controllers out of the box, and made it just a bit taller than the Rio and IMU in the expansion port, should we be concerned about heat buildup in the box?

  2. If we put, say, 4 talons in the box as well (for the drive motors), would that begin to create concern about heat?

3)Are we able to legally, and if so should we consider a fan or is that necessary?

Thanks.
~R^2

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#2

Although having ventilation and a fan would be nice in said box, I feel like heat buildup would be pretty negligible, especially over a two-minute match.

I believe a small computer fan should be ok. We almost always mount one over our air compressor to keep its temperature down over long practice sessions.

#3

NI publishes that the RoboRio can use from 5-45 watts. The IMU uses about a tenth of a watt.

In a vented enclosure with some natural convection, 0.15 watts/cuin is a guideline for power density. For forced convection (fan) you can use power densities up to about 1watt/cuin

So for the RIO and the IMU, the maximum you’d need would be an enclosure about 300 cubic inches with no fan and 45 cuin with a fan. I’m not sure, but I doubt it would even fit in an enclosure that small.

But motor controllers are much hotter. I don’t find efficiency posted on Victor’s page, but in general, I think you’d expect them to be something like 95 percent efficient. So if you’re running a 350 watt CIM, you’ll see about 18 watts of heat coming from each motor controller while it has that high load.
These can add up pretty fast. That’s why they have fins to reject heat.

I would never recommend that you put motor controllers in any kind of enclosure. If you keep the motor controllers out of the box, you’ll be fine, fan or not.

But as a fact, these are equations that are for designing electronics that run steady-state for long times. If your robot just does a three-minute match and then you shut down, it will never build up much heat. If you run it in a parade, that would be a different situation.

My team never uses an enclosure because they just add weight. You can “enclose” within a corner of the frame nearly as well.

Here’s a couple links to enclosure heat design references that I’ve used in my job as an electronics packaging engineer.

http://www.hoffmanonline.com/stream_document.aspx?rRID=233309&pRID=162533

https://books.google.com/books?id=JQfoCAAAQBAJ&pg=PA152&lpg=PA152&dq=electronic+enclosure+watts/cubic+inch&source=bl&ots=PkcCCgUXLV&sig=y-hCyDJFnoHlP0j1w1JYQh0aUbs&hl=en&sa=X&ved=0ahUKEwi22771ktDYAhWa0YMKHSd6CkIQ6AEIQTAH#v=onepage&q=electronic%20enclosure%20watts%2Fcubic%20inch&f=false

#4

The greatest stress situation here is likely to be drive practice, not matches. We don’t enclose our controls, but often put a cover over them to keep out game pieces, etc. However, some years we do most of our drive practice with the cover off.

#5

Fans are legal as they are considered a COTS part, unless you’re building your own squirrel-cage blower with a 775.

In competition heat won’t be an issue. Things may get warm but it’s unlikely to overheat in only 2 1/2 minutes. Practice driving is a different story.

I’d say slap a 80mm computer fan on the lowest part of the box, cut a few ventilation slots in the highest part and have extra peace of mind from knowing your precious pieces of silicon are being kept cozy.

#6

Thanks everyone. This is extremely helpful.

#7

Is there a reason your team feels that an enclosure is necessary, other than aesthetics? There are also many, many examples of great electrical layouts done on a panel with a simple protective cover that work just as well and require less in terms of design time and manufacturing resources. The designs based on a panel are also probably easier to troubleshoot, easier to maintain and easier to make modifications to. Your team would most likely gain more benefit by applying that design time and manufacturing resources on actual scoring mechanisms.

#8

Thank you. We were actually planning on a panel with a cover. I guess I communicated our ideas inaccurately. Last year, we did not cover the panel and had a few game piece related faults. So, that is the reason for the design update. We are aware that the game pieces are larger, but our robot design will probably be a lot more open this year too.

#9

We never have worried about heat build up, and never had issues with it. We have put our electronics in a covered box a few times, usually when the robot configuration allows us to put the box vertical, so the covered side faces outside the robot. A sliding, clear plastic cover lets us have easy access, and protects stuff from damage. Also having the box oriented this way keeps swarf out of the electronics, when we’re working feverishly on the robot between matches.

#10

Yes controllers can get hot in a two minute match. However, please consider that electrical components need to be seen during inspection and while on the field. There are several rules that need to be satisfied. (such as R50 and R70)
Please also consider Al’s corollary to Murphy’s Law, “Anything that can go wrong will go wrong on Einstein.”

#11