One speed vs Two speed gearboxes

[philso]

Originally Posted by AdamHeard

Chooo Chooo!

I think it's fair to argue for 95+% of teams the difference between a 16fps high gear and a 20+ fps high gear is not what will make them more competitive.

Gearing that high requires a higher attention to detail mechanically, as well as a good supply of batteries. Many teams don't realise their batteries are in bad shape because their robots simply don't use that much juice.

You are also likely to wear out your batteries much faster. You are consuming energy to accelerate your robot and move it across the field. Higher speeds means that you will have to pull more energy out of the battery in each match than if you used a lower top speed. Thus, it is harder to avoid deeply discharging your batteries and damaging them (see page 9 of the Enersys "Genesis NP Applications Manual").

Our experience at work with using the NP7-12's in our UPS' is that applications that cycles the batteries more frequently and discharge them more deeply will shorten the life of the batteries, dramatically. There is also a line that states "When considering discharge currents exceeding 6C, consult with an EnerSys Application Engineer." Notes from manufacturers like this usually indicate that their experience is that such operating conditions are likely to permanently damage or shorten the life of the component. The 6C discharge rate corresponds to 103A which is easily exceeded in many of the recent drive train designs.

www.enersys.com/WorkArea/DownloadAsset.aspx?id=489

[jeremylee]

Originally Posted by asid61

I have to disagree with 6 cim single speed 10fps. You get a maximum of a few hundreths of a second less time to get to a point versus 4 cims, regardless of the distance.

Acceleration isn't the only benefit. 6 cim drives are more efficient as they operate higher on the motor efficiency curve.

[Ether]

Originally Posted by jeremylee

6 cim drives are more efficient as they operate higher on the motor efficiency curve.

How did you determine that?

[jeremylee]

Originally Posted by Ether

How did you determine that?

With a simulation model. As you add more cims, the torque load (and current) is split to more motors. Map this new torque on the cim motor curve, and you will see the motors spin at a higher speed creating more mechanical power. This is most noticable during times of high load such as turning and pushing where you operate closer to peak power instead of peak efficiency.

[Ether]

Originally Posted by jeremylee

the motors spin at a higher speed...

All else being equal, the CIMs on a 4 CIM drive spin the same speed as the CIMs on a 6 CIM drive, for any given robot speed (assuming the wheels are not slipping).

[jeremylee]

Originally Posted by Ether

All else being equal, the CIMs on a 4 CIM drive spin the same speed as the CIMs on a 6 CIM drive, for any given robot speed (assuming the wheels are not slipping).

I agree speeds are same with no slip.

I don't think they will consume the same eletrical power though.

[Ether]

Originally Posted by jeremylee

I don't think they will consume the same eletrical power though.

All else being equal, which do you think will consumer more power, and under what operating conditions?

[Thad House]

Originally Posted by Ether

All else being equal, which do you think will consumer more power, and under what operating conditions?

In a constant velocity situation, when I did the math, it looked like the 4 Cim system drew more power. This was because each motor required more current to run at the same speed vs a 6 cim drive. If you're drive system takes 100 amps to hold the speed it is currently at, the 4 cim drive will be pulling 25 amps per cim, vs the 6 cim drive at 16.66 amps per cim. This means you can apply less voltage per cim in order to keep the same speed. This is only in the physical motors though. The entire system together will draw the same amount of power. Its just that the cims can be ran in a more efficient part of the curve.

When accelerating, the 6 cim will draw more power, but it will reach the constant velocity situation faster, when it will start drawing less current.

[Ether]

Originally Posted by Thad House

In a constant velocity situation, when I did the math, it looked like the 4 Cim system drew more power.... The entire system together will draw the same amount of power... Its just that the cims can be ran in a more efficient part of the curve.

Could you please clarify? Are you saying that according to your calculation the 4 CIM system drew more power than the 6 CIM system, or the same power, or less power ?

[Thad House]

Originally Posted by Ether

Could you please clarify? Are you saying that according to your calculation the 4 CIM system drew more power than the 6 CIM system, or the same power, or less power ?

The 4 cim system takes the same amperage, but more voltage to reach that amperage. So the 4 cim system does use more power to hold an identical velocity.

[asid61]

Originally Posted by Thad House

The 4 cim system takes the same amperage, but more voltage to reach that amperage. So the 4 cim system does use more power to hold an identical velocity.

That doesn't make sense. It should take more amps than a six cim system per motor. Adding all the motors, a four cim system should use the same number of amps as a six cim system, with a bit of wiggle due to the efficiency curve of the cim.

[Thad House]

Originally Posted by asid61

That doesn't make sense. It should take more amps than a six cim system per motor. Adding all the motors, a four cim system should use the same number of amps as a six cim system, with a bit of wiggle due to the efficiency curve of the cim.

They do use the same amperage. But the 6 cim version uses less power to to hit the same amperage, because each motor can use less voltage to hit its share of the amperage. Ex. If each cim needs to draw 25 amps in a 4 cim system, it might require 12 volts to do that. That would equal 1200w total. For the 6 cim, each motor only has to draw 16.66 amps, and might only need 9 volts to do that.
Thats only 900w. So less power is used by the 6 cim system to hold the same speed.

[Ian Curtis]

Originally Posted by AdamHeard

Chooo Chooo!

I think it's fair to argue for 95+% of teams the difference between a 16fps high gear and a 20+ fps high gear is not what will make them more competitive.

I would argue that for a significant number of teams an increase in top speed results in a less competitive robot because their drivers won't be able to handle the "twitchier" controls. I think slowing down the kit gearbox is one of the easiest knobs FIRST has to increase match scores, because it should be easier for good robots to get around the D and teams with less good mechanisms will have an easier time scoring. If I had a nickel for every time I watched a robot spend 2 minutes to pick up a game piece...

[Ether]

It's straightforward to do the calculation. Let's pick an operating point, say 2000 rpm at 240 ozin total torque of CIMs.

Calculate the CIM operating condition at 2000 rpm and 60 ozin (for 4CIM drive), and 2000 rpm and 40 ozin (for 6CIM drive):

Motor Calculator  build MCALC_2014d 2/3/2014 1255pm


Enter rpm and ozin, separated by a space: 2000 60

CIM  @  6.62 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    60.0   0.424    2000    68.3    24.3        88.7        71.7    55.3


Enter rpm and ozin, separated by a space: 2000 40

CIM  @  5.92 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    40.0   0.282    2000    76.4    16.5        59.2        38.5    60.6

4 CIM mechanical watts out = 4*88.7 = 355 watts
6 CIM mechanical watts out = 6*59.2 = 355 watts

4 CIM amps total = 4*24.3 = 97.2 amps
6 CIM amps total = 6*16.5 = 99.0 amps

4 CIM electrical power in = 6.62*97.2 = 643 watts
6 CIM electrical power in = 5.92*99.0 = 586 watts


So under certain operating conditions (in this case, 2000 rpm @ 355 watts out), 6 CIM draws less total power.

[Abhishek R]

So we've been thinking about auto-shifting some more, and how to increase our efficiency. I've also heard that 1678 achieved speeds of 22 ft/s on the field and wondered how they did that, especially if they started in first gear then shifted or if they were in 2nd the entire time (assuming they used shifters). We were happy with our auto-shifting system last year and would like to improve it further. This led to a few questions:

How do we figure out the correct gearing for the low gear supposing we know we want to have a high gear of 17 ft/s? This needs to maximize acceleration as well as battery life.

What exactly does it mean to be traction-limited and what are the physics equations and calculations surrounding that?

We are planning on using either 4 CIMs, 4 CIMs and 2 MiniCIMs, or straight up 6 CIMs, so any advice on those configurations would help as well. I didn't want to start a new thread since there has been so much discussion on the topic, but since using auto-shifting as a method of avoiding blowing the main breaker was discussed here, I figured it was apt. We didn't have to worry about that last year since we only had 4 CIMs on the main drivetrain.