2-Speed vs 1-Speed drivetrain

[AdamHeard]

Originally Posted by dellagd

I'm not an engineer, but doesn't this calculation have to be related back to the capacity of the battery somehow? When drawing, say, 5 amps, intuition tells me some tiny lead-acid battery would have a much larger drop in voltage than a huge deep-cell marine battery would.

A larger battery will have a lower resistance, causing less of a voltage drop.

[Chris is me]

Originally Posted by Johnnybukkel

Actually at Tech Valley (Finals 1 if I remember), We blew our main breaker in a pushing match with just 4 CIMs. Though we did have 40A fuses on each of them.

Are you sure you were geared for that speed, or were you geared slightly faster? If you used your drive under load in high gear, it's possible you were applying a lot of load to it constantly over the match and one last peak load tripped it. It's also possible that your main breaker was faulty and tripped early. Lots of variables here.

[Caleb Sykes]

Originally Posted by Oblarg

This is false.

This year we were having problems tripping our main breaker, and did a fair number of tests pushing against a wall. With a not-quite-full battery and the compressor running (i.e. pretty reasonable in-match conditions), we were only drawing ~50 amps per motor on a 6CIM drive when stalled.

Interesting. I will have to have the kids run some tests like this when we get the chance.

Originally Posted by AlecS

The voltage the battery outputs is proportional to the amperage being drawn, and the resistance of the circuit. This is also known as ohm's law.

V=IR

Although batteries are not always perfect voltage sources, you can use ohm's law to determine the voltage drop in your robot fairly accurately. If you were drawing 300 amps, and the total circuit resistance was .020 ohms, you would lose 6 volts. This does not fluctuate over time, it is directly related to the current being drawn. (ignoring temperature and other factors). Hopefully this clears things up.

This makes perfect sense. As we were talking about the drive train and gearboxes, I had forgotten about the electrical system itself, and was (incorrectly) assuming negligible resistance.

[DampRobot]

Originally Posted by AlecS

The voltage the battery outputs is proportional to the amperage being drawn, and the resistance of the circuit. This is also known as ohm's law.

V=IR

Although batteries are not always perfect voltage sources, you can use ohm's law to determine the voltage drop in your robot fairly accurately. If you were drawing 300 amps, and the total circuit resistance was .020 ohms, you would lose 6 volts.

So, theoretically, we should be designing to maximize power out of our battery as well as out motors? IE load the battery such that V^2/R is maximized when you think you'll need the most power as possible (pushing matches).

[Andrew Schreiber]

Originally Posted by JesseK

There is a lot of talk about speed in this thread.

There is no talk (yet) about designing for speed and still being able to turn well. There are extra variables in play here would make the fastest robots worthless without special consideration:

• Weight distribution (want it slightly off-center from track center-of-area)
• Wheel Base (want it wider than track length)
• Track Length
• Wheel Tread Composition
• Drive Train Frame Rigidity (want it STIFF)
• Center of Mass (want it LOW)

Well, I wouldn't say WORTHLESS. There are times when you don't want to turn and that might be a feature.

But it is important to remember that your gearboxes/motors are merely part of your drive system. It is comprised of several parts (incomplete list):

• Main Breaker
• Motor Breakers
• Battery
• Motors
• Gearboxes
• Wheels
• Drive Surface
• Frame
• Controller

Each of these systems plays a HUGE role in the overall effectiveness of the system. While the gearbox and wheels may be big parts of it please make sure you analyze the whole system instead of simply saying "we wanna go fast".

Ricky Bobby didn't design his car, he just drove it.

[AlecS]

Originally Posted by DampRobot

So, theoretically, we should be designing to maximize power out of our battery as well as out motors? IE load the battery such that V^2/R is maximized when you think you'll need the most power as possible (pushing matches).

I'm not exactly sure what you mean here, as the electrical power output of the battery increases as more current is drawn. Regardless of the power though, if you are in a pushing match, your main concern would be amperage. With DC brushed motors, torque is proportional to current. Gearing slower gives you more torque for the same current, or less current for the same torque.

IMO, it would be better to focus on reducing resistance on the robot. At 300 amps, each extra milli-ohm is .3 volts. Just a few extra milli-ohms to the robot can make the difference between brownout and not. Also relevant here is acceleration in high speed robots. Since increased resistance means less current for a given voltage and current=torque, a few milli-ohms can mean a decent drop in the available torque for acceleration.

[asid61]

It may be worth mentioning that the acceleration increase between, say, 8 fps and 16fps is negligible, in the area of several hundreths of a second. We had an excel doc made by one of our physics-oriented parents that graphed speed vs. time for different gear ratios.
The calculations did not take into account friction, but I've heard that it still doesn't help to change gears.

[waialua359]

Originally Posted by Abhishek R

So this year we ran a two-speed 4 CIM drive with auto-shifting, geared about ~11 in low and 16 or 17 in high, on 4 inch wheels. With the current auto shifting setup, would it be beneficial to make the low gear lower to accelerate faster?



So what was your 2014 setup like? I didn't see it to much in the pits, but it was definitely one of the fastest robots I saw this year.

So would an ideal system (barring weight and other limiting factors) be a 6 CIM two-gear drive, the low being only used for pushing, and high gear for everything else? I think this is what 254 used this year.

I can understand why you asked about moving to a lower gear to accelerate faster. Much like a car, if I was seeing how fast I could go from dead stop starting in 1st gear vs. 3rd gear, you start with 1st gear due to the torque advantage.
I'm saying that instead, ideally you have the 1 speed that you are comfortable with and use an adequate amount of motors to get the power you need to achieve that speed you gear it for.
Shifting into a real low gear was an option for us to try and get through pushing matches when the traffic was bad and we were busy trying to acquire and position ourselves to shoot the ball. We would have been perfectly happy with a 1 speed setup if it wasnt for this scenario.
All of our defensive maneuvers were using the higher 1 speed.
Our drive was much more effective adding just 1 Banebot 775 to each side. We took AM SuperShifters and modified one end of the plates, added the BB, and the right gears to match the 2 CIMs.
We had none of the 6 CIM issues that teams complained about during the season. Next season, we plan to add....