Chief Delphi - gear ratio

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- Technical Discussion (http://www.chiefdelphi.com/forums/forumdisplay.php?f=22)

- - gear ratio (http://www.chiefdelphi.com/forums/showthread.php?t=70100)

Quote:

Originally Posted by squirrel (Post 776476)

The lower ratio (9:1) will give a faster moving robot.

Quote:

Originally Posted by Madison (Post 776495)

...operating with the unwritten assumption that all other things are equal.

Yeah. 9:1 will (probably, depending on the motor...) give you a higher top speed. Acceleration is a totally different ball game, and requires some really fun math that no one ever got around to teaching me. (to be totally honest, I'm not a math guy.... Hence the Sociology major)

I seem to recall from my lurking there being a spreadsheet floating around that calculates acceleration as well as top speed for FRC bots... Am I mistaken? Anyone know where this tool is?

Quote:

Originally Posted by Dick Linn (Post 776539)

Does anyone have some actual numbers of how fast their robot managed to go as a percentage of the max unloaded motor RPM? I mean as-built with gearbox loss, etc. I'm not interested in feet/sec. figures. I was just wondering if something like 80% was realistic.

It's pretty close. 968 did a test of actual ground speed vs calculated theoretical speed of our 2008 robot. The actual speed was approximately 80% of the no load 100% efficiency speed.

Quote:

Originally Posted by Cory (Post 776648)

It's pretty close. 968 did a test of actual ground speed vs calculated theoretical speed of our 2008 robot. The actual speed was approximately 80% of the no load 100% efficiency speed.

Was that calculated from the ~5500 free speed CIM RPM ? Or were you assuming some sort of amperage being pulled?

Quote:

Originally Posted by Brandon Holley (Post 776683)

Was that calculated from the ~5500 free speed CIM RPM ? Or were you assuming some sort of amperage being pulled?

Quote:

Originally Posted by Cory (Post 776648)

The actual speed was approximately 80% of the no load 100% efficiency speed.

No-load = free speed. That is, the speed the motor turns at when nothing at all is connected to it. Note, that this doesn't mean no amps are being pulled. There's still a small amount of friction to be overcome etc. Plus, the rotor coils are constantly changing orientations and making and breaking contacts. This means the magnetic fields are constantly collapsing and reforming, which means the back emf generated by the motor will vary over time. Since the voltage supply nominally stays the same and the coils have some resistance, current is flowing whenever the back emf doesn't exactly equal the input voltage. So all DC motors have a no-load current as well as a no-load speed.

Which brings me to another silly technical point. Since the CIM is drawing current it has some non-zero input power, even at no-load. However, since there isn't a load output from the shaft, it has zero output power. So the motor's efficiency at no-load is actually 0%, not 100%.

Quote:

Originally Posted by Kevin Sevcik (Post 776761)

I was referring to no load in the sense of robot sitting there with wheels in the air-ie:no load on the gearbox, not no load on the motor. I was a little unclear there.

As to the second point, I meant 100% efficiency as in zero frictional losses in the drivetrain, not 100% motor efficiency.