How do you design a robot that doesn't brownout?

[gblake]

Originally Posted by mschwab013

How can you change a design to fix a problem if you don't know what is causing the problem?

The OP didn't ask us to help change any existing designs, or help fix any existing problem.

I'll bet a nice lunch that you have some anti-brownout guidelines you would apply to a clean-sheet-of-paper, design-me-a-STEM-robot exercise. If you do, CD and the OP would like to learn them from you.

Blake

[garyk]

Originally Posted by tahmid0517

We've had problems with out robot in 2015 browning out. It used 4 cims on the drivetrain and 2 cims on the elevator. How would we design a robot in 2016 so that we don't have anymore brownout problems?

A simple contributing factor may be crud on your battery terminals & leads (pigtail). This afternoon one of my students at 668 was adding pigtails to a new battery and scrounged some sandpaper from the school's wood shop with which to clean them to bright shiny metal. At 100 amps each hundredth of an ohm is going to cost you a volt.

[matthewdenny]

Originally Posted by Chris is me

6 CIM motors, moderately loaded, without real defense or much resistance, really shouldn't cause brownouts. I would take a careful look at your systems and see if there are some glaring inefficiencies anywhere, like a binding gearbox, gear ratios that are way too low (low in reduction / high in speed), etc. It sounds like a mechanical problem.

A mechanical problem is likely, but it could also be a symptom of bad batteries. Take a look at how you charge and store them.

[Nemo]

Let's say you already plan to take the above advice to avoid the various electrical and mechanical problems that can turn your battery power into waste heat. What remains?

You can use bigger gear reductions. That causes your elevator/arm/drive to have slower output speeds, while the motor itself has a faster speed. When the motor is turning faster, it is drawing less current. Drawing less current means not dropping your battery voltage and not hitting the brownout limits.

There are some cases when a bigger gear reduction will actually cause your mechanical output speed to be faster. This is the case when a smaller (faster) gear ratio would put the motor so close to its stall torque that its speed approaches zero.

Fast speeds sound great (and look great on the field when they actually work), but gearing things to go insanely fast isn't always the smartest thing for several reasons:
1) Your mechanical system often won't even start moving until you give it a significant percentage of full power, at which it jerks into fast motion instead of starting up smoothly.
2) You lose the ability to make small, controlled movements because of #1; you're either at nearly full power or nothing
3) You drain your battery a lot faster because it uses more current
4) Any sort of mechanical binding or electrical losses or battery issues can quickly bring your system from "running ok" to "not working at all" since you're pushing the limits pretty hard
5) With the combination of #3 and #4, you often get diminished performance in the tail end of matches as your battery runs down. That's especially painful if endgame is really important.
6) Your motors are producing more heat since they are taking so much more current. This can diminish performance of motors and make them burn out after a while.

You can also make sure your drive train is able to turn well generally. If you have lots of lateral traction in a skid steer drive, and the wheel base is long compared to the track width, you're going to pull a ton of current while turning. And that is compounded if the drive is geared to go really fast already. There are a variety of interesting things to look at in the drive to make it turn well. Drive design is a good topic to research on CD. Since the drive is going to have the most power going to it, making sure it's designed properly is probably the single biggest thing to look at if you don't want to brown out your robot.

[IKE]

Originally Posted by garyk

A simple contributing factor may be crud on your battery terminals & leads (pigtail). This afternoon one of my students at 668 was adding pigtails to a new battery and scrounged some sandpaper from the school's wood shop with which to clean them to bright shiny metal. At 100 amps each hundredth of an ohm is going to cost you a volt.

I would echo this with ensuring all connections are good, but especially your high current connections. Loose leads can lead to small, but important resistance elements. At the 100 AMP quoted above, the a 0.01 Ohm connection would have 1 Volt drop. During initial acceleration, you can pull as much as 500 amps (momentarily) from a 4 CIM drive. this same 0.01 Ohm would be 5V or thus get you a brownout.

As and LRI, I got to diagnose a few of these last year:

A few were loose PD connections (hard but not impossible with new PD board).
Loose main break connections (every year, even good teams will have loose connections here because it is easy to over-torque and break these as well).
Loose Battery cables at the battery terminals.

Poor battery/robot connection. Often this is either the springs being bent/loose in the connector, or damage on the terminals. Damage often comes from using alligator clamps to charge batteries, of shorting leads on batteries to test systems (this practice can lead to pitting/crud on the connection which shows up as resistance under high loads.

Besides these, the other times I saw brownouts were due to poor gearing. Often with mecanums. These robots were geared to go too fast, and spent the majority of their time in the stall half of the motors power curve. This would lead to hot motors as a symptom beyond the brown out. Often the brownouts would occur after the teams drove into an un-moveable object. This year typically it was trying to get totes from the landfill. The team would drive into the totes repeatedly trying to get a grip on a tote. Every time the drive stalled, they were pulling several hundred amps which would drop the voltage, then brown-out occurs which cancels motor which increases voltage which give control back which leads to stall which leads to high current which leads to voltage drop......

I did see the bad gearing with a couple of the kit-bot chassis too.

I saw a couple times were there were issues of binding in the transmissions themselves. Once corrected, the issue went away. for two of them they had miss-assembled the toughboxes which was causing the gears to rub.

I also saw brownouts due to batteries not being charged enough. Teams with only 2 batteries tended to wear down the batteries

As others have said, mostly mechanical or electrical connection issues causing high currents.