2-Speed vs 1-Speed drivetrain

[Abhishek R]

I searched some old threads but couldn't find a clear answer.

What is it about a single speed drive train that allows it to accelerate faster than a two gear one? I don't see why it does, but it seems like there should be a technical reason that I'm missing as to why it does accelerate faster.

[Oblarg]

It doesn't. Not necessarily, at any rate.

All else being equal, the acceleration of a drive depends only on the speed to which it's geared - the faster the top speed, the lower the acceleration. A two-speed drive has two possible gear ratios, and thus the acceleration depends on which gear you're in, but there's no fundamental difference in the way it works aside from (maybe) some minor frictional losses in whatever shifting gearbox you have that you wouldn't have in a single-speed.

[Tyler2517]

Maybe mechanical efficiency. Just because of the gearing required for shifting. Their is not much difference in watching a 9 fps and a 12 fps robot accelerating just one will reach the top speed and the other will continue to accelerate. Robot weight might also be a factor.

A 1 speed will usually run something like a 12 fps. When 2 speed might have something like 8/15 you could be watching it accelerate on the high gear alone?

Assuming the same amount of motors on a robot of the same weight with the same wheels on the same surface (include same voltage, all the other variables except speeds), acceleration will be dependent on what speed you are geared for. Lower gear ratios provide greater torque, meaning greater acceleration. So when in contest between a shifter and a single speed with all the assumptions made above, the lowest speed (highest gear reduction) will accelerate quicker. So if the shifter robot is in high gear (low reduction) and the single speed is geared for a more moderate speed slower than the shifter's high gear (more reduction), the single speed will accelerate faster. If the shifter is in a low gear that is slower than the single speed (more reduction), the shifter will accelerate faster. That being said, it may not cover as much distance per unit of time than the single speed because the single speed is (naturally) faster, and while the low gear's acceleration is higher, it peaks at a low top speed.

I think what you are referring to is a 6 CIM Single Speed vs. a 4 CIM shifter. By adding motors, you increase the amount of torque put into the drive system, thereby increasing the acceleration. With a single speed, you can optimize your robot to accelerate quicker than a 4 CIM robot at a higher speed. This means that a 6 CIM single speed at 16 ft/s will cover more ground per unit of time than a 4 CIM shifter at 16 ft/s. While this is one use, one of the more popular uses is to use 6 CIMs in a single speed transmission geared lower than your traditional shifter speed, optimized to cover a set amount of distance in the same time as a faster shifter would. Why purposely make your robot slower with more acceleration? With more reduction in your transmission, you will have more torque to push opponents with, yet still be able to get from point A to point B in the same amount of time as a faster robot with less torque, or a faster robot in high gear. However Once a shifter robot goes into low gear, it may have more pushing power than you (though you will still travel farther and faster than it).

Now if you combine the two, and make a 6 CIM shifter, you can accelerate faster and push more in both high gear and low gear, but be wary of tripping your 120A breaker. That being said, the effect of increased acceleration works best within a magic window. The differences in acceleration becomes most noticeable between the speeds of 8 and 12 ft/s, and dwindle down to near negligible as you get further away from the "magic window" (that includes being slower than the range and faster than the range). This means if you want to abuse the acceleration increase, a single speed at ~10 ft/s (or a low gear around that speed) is the best way to get the most out of your extra motors.

[DampRobot]

Originally Posted by Abhishek R

What is it about a single speed drive train that allows it to accelerate faster than a two gear one? I don't see why it does, but it seems like there should be a technical reason that I'm missing as to why it does accelerate faster.

The correct answer is "nothing, really", but the real answer is that it is because of the way people like to gear single speed drivetrains. In my mind, having a low traction limited gear "lets" you gear your top gear really fast, like 16+ fps. If you're doing a single speed drivetrain, you're likely going to gear less aggressively, as you likely don't want to be stuck playing defense with a robot geared at 19 fps. Some teams like to gear their single speeds in the 12 fps range, which do accelerate faster than the 18+ fps two speed teams.

Of course, a lot of it depends on how you like your drivetrains, here on the West Coast most good teams just gear in the 16+ fps range no matter if they're single speed or shifter. And a lot of butterflyesque drives (33, 2K14 drive in a day for example) are geared in the 18 fps range so they can do the reduction to their wheels in only one ratio.

[Abhishek R]

Ok, thanks, the "nothing really" was what I was mainly looking for. I knew many teams use a 6 CIM single gearing drive geared for about ~15 fps or so, but I was wondering if there was something inherently better about a non-shifting drive in terms of mechanical efficiency.

So as a follow-up, if you were to gear a 6 CIM drive for say 5 fps, you would probably throw the main breaker in no time if you got into a pushing match, right?

[Oblarg]

Just be careful if you pursue a 6CIM single-speed drive; they push the capabilities of the FRC electrical system to the limit. You will drain batteries very fast, and you must be extremely careful when selecting your gear ratio because if you stall 6 CIMS you will trip your breaker and that is no fun.

[DampRobot]

Originally Posted by Abhishek R

So as a follow-up, if you were to gear a 6 CIM drive for say 5 fps, you would probably throw the main breaker in no time if you got into a pushing match, right?

No, the opposite. Gearing down below 7 fps or so doesn't add any pushing power to your drivetrain, as you're traction limited. Basically, if you push with more than a certain force, your wheels will slip no matter what. If you're pushing with 6 CIMs at 5 fps, your wheels will likely slip, and your main breaker will likely be OK (for a while, at least). On the other hand, if you're pushing in an 18 fps gear, your wheels won't slip, your motors will stall, and you will likely trip your main breaker quite quickly.

[Monochron]

Originally Posted by DampRobot

No, the opposite. Gearing down below 7 fps or so doesn't add any pushing power to your drivetrain, as you're traction limited. Basically, if you push with more than a certain force, your wheels will slip no matter what. If you're pushing with 6 CIMs at 5 fps, your wheels will likely slip, and your main breaker will likely be OK (for a while, at least). On the other hand, if you're pushing in an 18 fps gear, your wheels won't slip, your motors will stall, and you will likely trip your main breaker quite quickly.

Quick aside question, am I right in assuming that the wheels slipping is the only thing that prevents the breaker from tripping at low fps?

For instance, if a weird situation occurred where a gearing of 5 fps did NOT slip the wheels in a pushing match, the breaker would still trip in about the same amount of time as if the gearing was 16 fps correct? Basically, the CIMs draw the same amount of current at stall regardless of the gearing so any stalling of a 6 CIM drive is just as dangerous.

Am I right in that assessment?

[Chris is me]

Originally Posted by Monochron

Quick aside question, am I right in assuming that the wheels slipping is the only thing that prevents the breaker from tripping at low fps?

For instance, if a weird situation occurred where a gearing of 5 fps did NOT slip the wheels in a pushing match, the breaker would still trip in about the same amount of time as if the gearing was 16 fps correct? Basically, the CIMs draw the same amount of current at stall regardless of the gearing so any stalling of a 6 CIM drive is just as dangerous.

Am I right in that assessment?

If the motors are stalled, they will draw their stall current.

That said, there's pretty much no traction material available that won't slip drive wheels with 6 CIMS geared for 5 fps.

Drive trains are "traction limited" (wheels slip) for the most part. The amount of current drawn per motor while the wheels are slipping depends on your gear ratio. Going well above 40A per motor can trip the resetting breakers. Going well above 200A per system risks tripping your main breaker a bit too quickly.

[themccannman]

Originally Posted by Monochron

Quick aside question, am I right in assuming that the wheels slipping is the only thing that prevents the breaker from tripping at low fps?

For instance, if a weird situation occurred where a gearing of 5 fps did NOT slip the wheels in a pushing match, the breaker would still trip in about the same amount of time as if the gearing was 16 fps correct? Basically, the CIMs draw the same amount of current at stall regardless of the gearing so any stalling of a 6 CIM drive is just as dangerous.

Am I right in that assessment?

Yes, stall current is stall current. Ideally your low speed gear ratio in your 2 speed drivetrain should be as fast as possible while still being traction limited. That gives you a higher speed in low gear without sacrificing any pushing power.

[Michael Hill]

Originally Posted by themccannman

Yes, stall current is stall current. Ideally your low speed gear ratio in your 2 speed drivetrain should be as fast as possible while still being traction limited. That gives you a higher speed in low gear without sacrificing any pushing power.

Well....stall current is kinda stall current. The stall current per motor for 4 motors is slightly different than the stall current for 6 motors. Stall current for a 6 CIM system is about 75 Amps/motor and the stall current for a 4 CIM system is about 87.5 Amps/motor. This is because when you add more CIMs, the motor voltage will decrease. This voltage can drop to about 7.5-8 volts when stalling. The stall current cited in the CIM motor documentation (~131 Amps) is assuming the voltage is a constant 12 volts (not what we see in robots). It is correct that friction is what limits the stall current. Also, note that there's a difference between the static friction and kinetic coefficient of friction. In fact, you can even have just as much pushing power with a 4 CIM drive as a 6 CIM drive (if geared properly).

[Caleb Sykes]

Originally Posted by Michael Hill

Well....stall current is kinda stall current. The stall current per motor for 4 motors is slightly different than the stall current for 6 motors. Stall current for a 6 CIM system is about 75 Amps/motor and the stall current for a 4 CIM system is about 87.5 Amps/motor. This is because when you add more CIMs, the motor voltage will decrease. This voltage can drop to about 7.5-8 volts when stalling. The stall current cited in the CIM motor documentation (~131 Amps) is assuming the voltage is a constant 12 volts (not what we see in robots). It is correct that friction is what limits the stall current. Also, note that there's a difference between the static friction and kinetic coefficient of friction. In fact, you can even have just as much pushing power with a 4 CIM drive as a 6 CIM drive (if geared properly).

I am aware that the voltage can drop down to 8 volts for brief instants in time, but does it stay down this low for extended periods during a pushing match? I will have to remember to check the driver station data logs next time I have access to the driving laptop.

I had always thought that, for things like pushing matches, voltage briefly dropped down to ~8 volts but then jumped back up to some value around 12. The battery should be applying a relatively constant voltage, so I don't see why the voltage would remain notably lower for any extended period of time.

So yes, the stall current might be ~90 amps for short periods of time, but I am thinking that the stall current is much closer to 130 amps for most of the time in a pushing match.

[Oblarg]

Originally Posted by inkling16

So yes, the stall current might be ~90 amps for short periods of time, but I am thinking that the stall current is much closer to 130 amps for most of the time in a pushing match.

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.

[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.