Chief Delphi - One speed vs Two speed gearboxes

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- - One speed vs Two speed gearboxes (http://www.chiefdelphi.com/forums/showthread.php?t=131227)

Ekcrbe

26-12-2014 20:14

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Jared (Post 1417105)

I expected it to be more, which is why I did the math. I wonder how much the gears/chain/shafts contribute to the rotational inertia.

Those should be very little. The squared term in the rotational inertia of axially symmetrical objects is the radius, and almost nothing else in the drive system comes close to the wheels in radius. The only examples I can come up with are the transmissions we made in 2012 and 2013. The first reduction was done with Gates belts instead of gears, and the large pulleys were about 4" in diameter. They were spinning faster than the wheels, so they needed more energy put in to get going than if they were further along in the reduction sequence, but they weren't that heavy. They were aluminum and pretty well lightened, so I don't think they mattered that much.

Dunngeon

27-12-2014 03:42

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Abhishek R (Post 1417086)

Hmm, yes, they were 8 inches Colsons, so pretty heavy. We were planning to use 6 inch wheels but then some complications arose that caused us to use bigger wheels, We didn't think it would have that adverse of an effect on the drive performance.

Could electronics be an issue? Or is that a minor concern, efficient wiring?

Thanks for the help!

Have you guys purchased new batteries since 2013/used the same batteries this year? I've found that many teams that run 4cim drives have no idea how bad the condition of their batteries is. A battery that is in bad condition can lead to sluggish accelerations because it's ability to release, or store energy has decreased. This type of problem is more prevalent among 6CIM drivebases, because the acceleration is higher, thus energy demand is increased. However, i could definitely see this impacting a 4cim setup if the battery has degraded enough.

As a general rule, we only run competition batteries for 2 years before retiring them to a non-competition use, such as a practice bot.

Munchskull

27-12-2014 05:01

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Dunngeon (Post 1417191)

As a general rule, we only run competition batteries for 2 years before retiring them to a non-competition use, such as a practice bot.

Electrically speaking the age alone of a battery is irrelevant to it performance. What you should really be checking for is internal resistance. A lightly used, well cared for battery can last longer than just two years, while a heavily used battery that is poorly mantained may not even last that long. However you are right that the battery will increase in resistance with aged, but the lagest factor in a batteries life expectancy is not the age but the usage and care. You may have more batteries that are perfectly fine to use. While I like your rule I encourage you to also check you internal resistance.

Also if you think that inefficient wiring is the issue try either:

1) wire the PDB, motor controllers and the motors as close together as possible. Short wire means less resistance.

2) If your components have to be spread out but you can affored a little extra weight try using a wire size bigger than FIRST requires for the motor wire. Doing this will decrease the wire resistance allowing for better energy transfer.

3) Finally, one of the best ways to get the most out of your motors is to be using the top of the line motor controllers. The Talons will be more efficient than say a Victor 884.

While I wish I could offer more insight on to what happened to 624 in 2013 with their shifting, I believe that all that I could have said has already been spoken. So I leave you with these electrical tips that can help get the most out of your motors.

Ether

27-12-2014 09:25

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Munchskull (Post 1417193)

one of the best ways to get the most out of your motors is to be using the top of the line motor controllers. The Talons will be more efficient than say a Victor 884.

Would you please elaborate a bit what you mean by "get the most out of your motors" and "more efficient"?

Do you have some test data you could share?

Munchskull

27-12-2014 11:52

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Ether (Post 1417204)

Would you please elaborate a bit what you mean by "get the most out of your motors" and "more efficient"?

Do you have some test data you could share?

I don't have easy access to the data right due to the fact that I am on vacation.

The place I reamber finding the data was here on CD as an attachment. Once I get more time I will look for the data again.

Ether

27-12-2014 12:00

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Munchskull (Post 1417227)

Not to interfere with your vacation, but in the meanwhile could you explain what you meant by "more efficient" ?

Chris is me

27-12-2014 13:02

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Munchskull (Post 1417193)

Electrically speaking the age alone of a battery is irrelevant to it performance. What you should really be checking for is internal resistance.

You're absolutely right, but I think the point of doing this is less that something magic happens after two years to make batteries terrible, and more that batteries don't' cost that much to replace and it's easier to cycle out batteries early than to discover too late that you have a sub optimal battery.

Quote:

A lightly used, well cared for battery can last longer than just two years, while a heavily used battery that is poorly mantained may not even last that long. However you are right that the battery will increase in resistance with aged, but the lagest factor in a batteries life expectancy is not the age but the usage and care.

I mean, the way batteries are used in FRC is inherently suboptimal. We use the batteries hard for two minutes, then throw them on a charger well before we've mostly discharged them, and usually to charge at a higher current than they should. We're already abusing these batteries.

Richard Wallace

27-12-2014 13:10

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Jared (Post 1417092)

The rotational inertia of the wheels does little to hinder robot acceleration.

For anybody interested, I compared the kinetic energy of the robot moving at 10 feet per second to the rotational energy of the wheels on a robot moving at 10 feet per second. If we compare these energies, we'll see how much of our power goes to spinning the wheels, and how much goes toward moving the robot.

A fully loaded robot weighs 150 lbs, which is 68 kg. 10 feet per second is 3.05 meters per second.

KE = 1/2*mv^2 = 0.5(68)(3.05)^2 = 316.285 Joules
That's the amount of energy it takes to bring your robot up to speed.

For the rotational energy, we've got to find the moment of inertia for the wheels. The radius of the wheel is 4", which is equal to 0.1016 meters, and I'll guess that the mass of the wheel is 2 pounds (probably heavier than the actual wheel), which is equal to 0.91 kg. The wheels are disc shaped, so we can use I = 1/2 * m *r^2 = 0.5(0.91)(.1016)(.1016) = 0.0046968 kg * m^2

One rotation of the wheel causes the robot to travel 8*pi inches = 25.1327 inches = 0.63872 meters/revolution.

3.05 meters/second (divided by) 0.63872 meters/revolution = 4.77517 rev/second = 30.00 radians/second

For the rotational energy, E = 1/2 I*omega^2 = 0.5(0.0046968)(900) = 2.114 Joules per wheel.

For six wheels, that's 12.684 Joules for a robot.

tl;dr, it requires 328.969 Joules to bring your robot to 10 feet per second, and 12.684 of these Joules (3.8%) are used to get your wheels up to speed. This assumes that you have 6 colsons with a diameter of 8" and a mass of 2 pounds.

The fact that the wheels spin when the robot moves makes the robot feel 5.7 pounds heavier to the drive system.

Nicely presented! You might also want to consider the moment of inertia of the CIM motor rotors.

For example (using the motor rotor inertia figure given in another recent thread) if the 8" wheel 6WD robot in your example is driven using CIM motors with 14:1 speed reduction ratio between motors and wheels, each motor rotor will have a "reflected inertia" of J_rotor * (ratio^2) = 0.015 kg-m^2, or about three times the figure you used for one wheel.

For a different example, consider a drivetrain with 4" wheels and half the speed reduction ratio. Will a robot with the same mass get up to speed quicker if it is on smaller wheels?

Abhishek R

27-12-2014 13:18

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Dunngeon (Post 1417191)

We take good care of our batteries and purchase new ones every year. I don't think it's a battery issue because our 2014 robot has had very few issues regarding the drivetrain, and we interchange 2013 and 2014 batteries with it. This makes me think the issue in 2013 was mechanical efficiency of some sort - we used omni wheels in 2014 which meant there was very little wheel scrub so it took very little power to turn rapidly. In both 2011 and 2014 we used 4 inch wheels and those were our best drivetrains in terms of acceleration as well as turning ability.

So we were thinking the cause was the heavy and big radius 8 inch wheels, but correlation does not necessarily mean causation and Jared's equations prove that there is little effect. So next we'll be looking at electrical possibilities like Munchskull brought up. We're not extremely concerned because our 2014 drivetrain worked out so well, but it's always nice to figure out what went wrong and learn from our mistakes.

Ether

27-12-2014 13:24

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Chris is me (Post 1417241)

..then throw them on a charger well before we've mostly discharged them

Are you saying you think it's better for lead-acid batteries to be mostly discharged before being charged??

Chris is me

27-12-2014 13:27

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Ether (Post 1417245)

Are you saying you think it's better for lead-acid batteries to be mostly discharged before being charged??

Maybe "mostly" is a strong word, but I could have sworn I've read a bunch of posts about how batteries in FRC are used in a suboptimal fashion by us using them intensely for two minutes then immediately recharging them regardless of how much they had discharged. Probably not something I should regurgitate from memory without a strong understanding of the concept, though.

Munchskull

27-12-2014 13:43

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Ether (Post 1417228)

Not to interfere with your vacation, but in the meanwhile could you explain what you meant by "more efficient" ?

When I say "more efficient" in this case I am referring to the return of torque for the speed a CIM is going.

This is the chart I was basing it off of.https://www.google.com/url?sa=t&source=web&rct=j&ei=TfyeVLzwIIr5yQT4lIGwD A&url=http://www.chiefdelphi.com/media/papers/download/3511&ved=0CC0QFjAG&usg=AFQjCNFYBes4Az_jlxsofXC-Uv9SJspSsA&sig2=UuAzGda4w01JK0gZ632RnQ

If I miss interpreted this data I invite you to correct me. I would rather be wrong and learn from my mistake, than be wrong and keep thinking I am right.

Ether

27-12-2014 14:10

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Chris is me (Post 1417246)

The life of the FRC lead-acid batteries is a strong function of the number of deep-discharge cycles. There's a chart on the datasheet showing this.

These deep discharges can occur not only during competition, but especially during practice if the battery is allowed to "die" before being re-charged.

Deep-discharging and high currents are the main factors affecting FRC battery life, not charging a partially-discharged battery. Keeping the battery charged is better than allowing it to become deeply discharged.

Ether

27-12-2014 14:35

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Munchskull (Post 1417248)

When I say "more efficient" in this case I am referring to the return of torque for the speed a CIM is going.

OK, that's a reasonable metric...

Quote:

This is the chart I was basing it off of....

OK, but be careful to compare apples to apples. Those charts definitely show a difference in linearity between the 884 and Talon. But to compare efficiency, you'd have to compare them at the same output PWM duty cycle, which is not shown in the graphs.

Mr V	27-12-2014 15:00

Re: One speed vs Two speed gearboxes

Quote:

Originally Posted by Chris is me (Post 1417246)

FRC use is serious abuse to the batteries but it is the deep discharge that causes the problems. Recharging the batteries as soon as practical is the best thing for a lead acid battery. A discharged battery will form sulfates at a higher rate than a charged battery, sulfates increase the internal resistance and lower the battery's output potential.

As Ether mentioned most of the data sheets show the expected battery life in cycles vs the depth of discharge. The lower the depth of discharge before recharging the greater the life expectancy. The curve is just that a curve.

From the Enersys data sheet.

30% depth of discharge 1200 cycle expectancy.
50% depth of discharge 550 cycle expectancy.
100% depth of discharge 250 cycle expectancy.

From MK's data sheet.

100% depth of discharge 200 cycle expectancy.
80% depth of discharge 225 cycle expectancy.
50% depth of discharge 500 cycle expectancy.

A 100% depth of discharge is when the open circuit voltage of the battery after a short rest period is 11.2~11.8v depending on which mfg you consult.

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