Gearing for a Very High Speed

[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

[asid61]

I see nothing wrong with gearing for a very high speed, provided you provide some kind of current detection for autoshifting or speed limiting in emergencies.

[Aren Siekmeier]

Originally Posted by Ether

Lots of good math showing considerable differences in current draw

This is really good to see, and demonstrates the importance of gearing properly for the designed load.

The loads on an FRC drivetrain can be to accelerate the robot, to maintain speed by overcoming drag, or to push with some force at a constant speed (perhaps close to zero).

In the third case, the load will be continuous, so you should definitely heed the current figures Ether's shown. You don't want to be pulling 80A per motor continuous, or you will pop some breakers (a 40A breaker in about a second, or for 6 six motors for a total of 480A, the main breaker in a few seconds as well).

In the second case, the load will be continuous as well, but much much lower.

In the first case, the acceleration is only transient. 6 CIMs geared 5:1 on 4" wheels get to 90% of top speed (about 18 ft/s with no loss) in about half a second. They are stalled briefly for the beginning of this, but then quickly approach free speed, so there aren't too many amps in too few seconds. In comparison, 6 CIMs geared 13:1 on 4" wheels get to half the high gear's top speed in about a quarter of a second, so while you are putting less load on the system (getting to 90% of low gear's top speed in just 0.1 seconds), the performance improvement due to starting in low gear and shifting at the right time is hardly noticable at all. Even at a distance of 5 ft, the low gear takes 50% longer to complete the traversal than the high gear, and of course it's even more in favor of high gear for longer distances. For this reason, we often remain in high gear unless we see that we're applying load continuously (pushing).

These numbers are all pretty approximate (and also coincidentally almost match our 2014 drive), but I think they show why high gear is important, too. Note that the ability for this high gear to still accelerate as it does is in part due to the 2 extra CIMs in the drive. For a 4 CIM drive, you would not get the same performance over short distances at these reductions, and low gear would be more favored.

[DampRobot]

Originally Posted by JorgeReyes

I am wondering if this would be a good idea. To have a robot that is 2 speed and 6cims. Low for around 7 fps and high would be 18-20 fps. 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.

We had adjusted speeds in this range this year, with 2 CIMs and a mini on each side. It was ferocious when accelerating, but ended up tripping the breaker a lot. When we took out the mini CIMs, it wasn't nearly as high performance (to my eye).

With a reasonable talented driver, a good control system, and a bit of practice, you won't need to limit power at all. We used split arcade (faux Chezy Drive), and noticed that the robot was much, much easier to control than with tank. When people think fast drives are uncontrollable, it's usually more a testament to what control system they're using rather than how fast they're going.

[JorgeReyes]

Well here was where I was going with it. If we gear it to 20 fps with 3 cims, then when the robot starts moving the motors could use full power but then since we use thrustmaster joysticks, we could control the power and maybe set it to say 75% after it accelerates by releasing a button so that you get a controllable speed but still get the acceleration. The Low Speed would be full power.

We kind of did this this year with our mecanums. We pressed a button( we called it the turbo button) for full power but then released it so we could go a bit slower when there were obstacles close by.

Edit: Also with our joystick it is a lot easier to control than using logitech controllers and I drove this year so I am pretty confident I should be able to control the robot at these speeds with practice

[T^2]

Originally Posted by JorgeReyes

Well here was where I was going with it. If we gear it to 20 fps with 3 cims, then when the robot starts moving the motors could use full power but then since we use thrustmaster joysticks, we could control the power and maybe set it to say 75% after it accelerates by releasing a button so that you get a controllable speed but still get the acceleration. The Low Speed would be full power.

We kind of did this this year with our mecanums. We pressed a button( we called it the turbo button) for full power but then released it so we could go a bit slower when there were obstacles close by.

Edit: Also with our joystick it is a lot easier to control than using logitech controllers and I drove this year so I am pretty confident I should be able to control the robot at these speeds with practice

If you're using joysticks, why not just ease up on them? That way your driver will get an infinite degree of control between 0% and 100% power.

[JorgeReyes]

I guess so. I think we are just going to do a lot of testing this summer with different gear ratios and control set ups.