Gearing for a Very High Speed

[Joe G.]

20fps isn't unheard of, but it typically takes some fancy non-linear response and/or closed loop control with encoders to make it controllable for the drivers.

[Jay O'Donnell]

I'm not sure how reducing the motor power to 50% is really helping you. If a high speed isn't controllable, why not just change the gearing so that is at a more reasonable speed? If you reduce the motor power by 50% then you'll barely be going faster than your low gear and you'll have half the torque that the low gear has.

[Pratik Kunapuli]

During the 2014 season, our robot had a 6 cim, 2 speed drivetrain with a low gear ~4 fps and high gear ~17 fps. The only real times we used high gear was if we needed to travel the length of the field very quickly, or to maneuver around defenders and not push through them. Realistically, your driver will learn to control the high speed and will learn when to shift into high gear. Its mostly a "feel" thing and if your robot needs to go 20 fps, then he will learn to drive it well.

[waialua359]

Originally Posted by Pratik341

During the 2014 season, our robot had a 6 cim, 2 speed drivetrain with a low gear ~4 fps and high gear ~17 fps. The only real times we used high gear was if we needed to travel the length of the field very quickly, or to maneuver around defenders and not push through them. Realistically, your driver will learn to control the high speed and will learn when to shift into high gear. Its mostly a "feel" thing and if your robot needs to go 20 fps, then he will learn to drive it well.

You guys spent most of the time driving 4 fps?

[T^2]

We spent most of our 2014 season in high gear with 6 CIMs (geared to 23ft/s, ran at 17ft/s in practice). The acceleration we got was pretty great, and our driver could control it with practice. We played midfield exclusively, however, so we ran back and forth across the field a lot.

Having your motors throttled to 50% power most of the time seems completely useless. In low gear, you lose the pushing advantage of being in low gear, and in high gear, you lose the torque advantage of using 6 CIMs. I'm not sure how this would ever be useful in practice. It is pretty easy to test, though, so feel free to do that.

[notmattlythgoe]

Originally Posted by waialua359

You guys spent most of the time driving 4 fps?

This is the same reaction I had. This can't be the whole story.

Our drive spent about 95% of his time in high gear and we were geared for about 17fps.

[John]

Originally Posted by Pratik341

During the 2014 season, our robot had a 6 cim, 2 speed drivetrain with a low gear ~4 fps and high gear ~17 fps. The only real times we used high gear was if we needed to travel the length of the field very quickly, or to maneuver around defenders and not push through them. Realistically, your driver will learn to control the high speed and will learn when to shift into high gear. Its mostly a "feel" thing and if your robot needs to go 20 fps, then he will learn to drive it well.

This is similar to what we did: we stayed in low gear (~8-9 fps) for the majority of the match, mostly using high gear (~19 fps) to make full field runs. Many teams greatly overestimate how much time is spent traversing the field (suitable for high speed), and underestimate how much time is spent lining up passes and shots (suitable for low speed). ~4 fps seems really slow, even to me though. What shifter did you guys use that has a 4.25:1 spread?

A really fast high gear is sometimes just what you need, though. For example, in Galileo Match 100 (no video) our alliance partner missed a would-be match winning shot with less than 20 seconds left, and the ball bounced all the way back to our defensive zone. We were able to race back, get the ball, and score it just before time expired.

Some years full field runs happen all the time (2013, 2011 come to mind), others they almost never happen (2012). Whether the ability to save a couple of seconds on each run is worth it depends a lot on your team and the year. It's worth maybe one more cycle, but only if your team can score effectively and quickly.

The High might sound uncontrollable but what I was thinking is that we could use a button so that when it is pressed, the motors use 100% power but when it isnt it uses 50%. This would essentially give us 4 speeds with a 2 speed gearbox.

I question why you want to have a secondary "software shifter" when you are planning to have a real shifter on your robot. Why not just use low gear when you want to go slow? In our experience (if I have time I might add the math to back it up) it should give superior acceleration, control, and power consumption. The only drawback I can think of is shifting time, but the COTS shifter options are so good now its not really a big deal.

[JorgeReyes]

I guess the question I have then is if it is better to automate shifting to shift into low gear when we are drawing too much current and high when we are not, or have the driver shift manually.

[notmattlythgoe]

Originally Posted by JorgeReyes

I guess the question I have then is if it is better to automate shifting to shift into low gear when we are drawing too much current and high when we are not, or have the driver shift manually.

We plan on monitoring the current draw and shifting down when we sense that we are in a pushing match. Otherwise we will stay in high gear.

[JorgeReyes]

Originally Posted by notmattlythgoe

We plan on monitoring the current draw and shifting down when we sense that we are in a pushing match. Otherwise we will stay in high gear.

See but the problem I see with this set up is when you want to do something precise like lining up to shoot. If you try to do this on high gear it will be harder to do this. What I was thinking was that it shifts down when it draws to much current to automatically shift in pushing matches like you said.

But also have a manual override option to shift when necessary like the situation I mentioned.

[notmattlythgoe]

Originally Posted by JorgeReyes

See but the problem I see with this set up is when you want to do something precise like lining up to shoot. If you try to do this on high gear it will be harder to do this. What I was thinking was that it shifts down when it draws to much current to automatically shift in pushing matches like you said.

But also have a manual override option to shift when necessary like the situation I mentioned.

We will also have a manual way to shift down. But our driver has never had an issue lining up to shoot in high gear. Both this season and last our driver was in high gear in all situations except pushing matches.

[JoshD99]

I am the main driver for team 4327. The robot we built this year moves at about 16 FPS. At first, it was difficult to drive. But now, after becoming very familiar with the robot, I can drive it well, others on the team who don't usually drive, however, do have issues. So I don't think, with practice, a robot with a speed of 20 FPS would be an issue.

[John]

Originally Posted by JorgeReyes

I guess the question I have then is if it is better to automate shifting to shift into low gear when we are drawing too much current and high when we are not, or have the driver shift manually.

This thread may be of interest to you, then.

[Pratik Kunapuli]

Originally Posted by John

~4 fps seems really slow, even to me though. What shifter did you guys use that has a 4.25:1 spread?

Originally Posted by waialua359

You guys spent most of the time driving 4 fps?

Originally Posted by notmattlythgoe

This is the same reaction I had. This can't be the whole story. .

Being in high gear is different from trying to drive at top speed. While we may have been in high gear, I wasn't trying to go very fast a lot of the time, only for the full field dashes. We used low gear when there were defenders near us, we needed to push (partners and opponents), and when we were lining up shots. We had a custom 2 speed gearbox.

[Ether]

Originally Posted by John

if I have time I might add the math to back it up

Here's some quick rough calculations using some simple numbers to illustrate a point.

Say you have a CIM with 10.4 volts applied and loaded with 206.1 oz*in of torque. It will be spinning at 1416 RPM, drawing 80.5 amps, generating 215.8 mechanical output watts, and running at 25.8% efficiency.

Now add a 3x speed reduction gearbox. For sake of simplicity for this simple calculation, ignore gearbox losses. To produce the same output speed and torque as above, the CIM needs to produce 68.7 oz*in torque at 4248 RPM. To generate that torque at that RPM, the CIM will need 12 volts applied. It will draw 28.8 amps and be operating at 62.5% efficiency.

That's why you want to operate in low gear when possible appropriate.

Motor Calculator  build MCALC_2014d 2/3/2014 1255pm


Select motor:
1)CIM     6)FP0673   d)FP2719  f)FP9012     g)FP9015    h)FP9013 j)MiniCIM
a)am-0912 b)am-0914  7)DensoL  8)DensoR     c)Denso0160 k)BAG    u)UserDefined
2)RS395   3)RS540    4)RS550   e)RS775-12   5)RS775-18  m)RS555  n)am-0915
r)am-2193 s)am-2235  t)RS390   v)RS545      p)am-2161&2194
w)VEX2177hi  z)VEX2177std  1


CIM FR801 001, AM802 001A @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     2.7         0.0        32.4     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425       0     0.0   133.0         0.0      1596.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   171.7   1.212    2655    50.0    67.8       337.1       477.1    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    42.8   0.302    4648    87.5    18.9       147.1        80.2    64.7

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit a


Enter motor rpm & ozin, and the program will calculate the motor voltage
required to produce the specified ozin at the specified rpm.

Enter rpm and ozin, separated by a space: 1416 206.1

CIM FR801 001, AM802 001A @ 10.40 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    4603   100.0     2.3         0.0        24.3     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   297.7   2.102       0     0.0   115.3         0.0      1199.3     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   148.8   1.051    2301    50.0    58.8       253.3       358.5    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    37.1   0.262    4029    87.5    16.4       110.6        60.3    64.7

CIM FR801 001, AM802 001A @ 10.40 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   206.1   1.455    1416    30.8    80.5       215.8       622.0    25.8

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit a


Enter motor rpm & ozin, and the program will calculate the motor voltage
required to produce the specified ozin at the specified rpm.

Enter rpm and ozin, separated by a space: 4248 68.7

CIM FR801 001, AM802 001A @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     2.7         0.0        32.4     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425       0     0.0   133.0         0.0      1596.2     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   171.7   1.213    2655    50.0    67.9       337.1       477.2    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    42.8   0.302    4648    87.5    18.9       147.2        80.2    64.7

CIM FR801 001, AM802 001A @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    68.7   0.485    4248    80.0    28.8       215.8       129.4    62.5