Top speed?

[Vikesrock]

This is somewhat related to the topspeed discussion so I figure I'll ask it here. When determining the "top speed" of a robot in relation to it's gearing, wheels, motors, ect. what point on the motor curve are you taking the RPM numbers from? My team is a second year team with little or no help from any Mech E's and we are looking to do a bit more in terms of drivetrain design than we did last year.

So when someone says they have a 15fps or 16fps robot what RPM is that on the motor (assuming 2.5" CIM)?

[RobJ]

Originally Posted by Vikesrock

This is somewhat related to the topspeed discussion so I figure I'll ask it here. When determining the "top speed" of a robot in relation to it's gearing, wheels, motors, ect. what point on the motor curve are you taking the RPM numbers from? My team is a second year team with little or no help from any Mech E's and we are looking to do a bit more in terms of drivetrain design than we did last year.

So when someone says they have a 15fps or 16fps robot what RPM is that on the motor (assuming 2.5" CIM)?

It depends on the size of your wheel. If you go 16 fps with 8 inch wheels then your motors will be rotating twice as fast at 16 fps with 4 inch wheels

[Elgin Clock]

I can easily see teams doing laps under 10 seconds with a great line of travel, and no interference.

Whether or not this will happen is to be seen.

But it can, which to me is pretty darn awesome.

[lukevanoort]

Originally Posted by Vikesrock

This is somewhat related to the topspeed discussion so I figure I'll ask it here. When determining the "top speed" of a robot in relation to it's gearing, wheels, motors, ect. what point on the motor curve are you taking the RPM numbers from? My team is a second year team with little or no help from any Mech E's and we are looking to do a bit more in terms of drivetrain design than we did last year.

So when someone says they have a 15fps or 16fps robot what RPM is that on the motor (assuming 2.5" CIM)?

It also depends on the team reporting that speed. Relatively few teams measure actual speed (I think the Martians do, but beyond that I can't think of anybody). So, then you get into theoretical calculations. Some teams approximate inefficiencies (JVN's famous .81% of free speed is an example), some just use the 12V free speed, and some report numbers based off of operation at 40A draw.

I wouldn't use the 40A one, because most of the time you aren't going to draw that much just driving around, and if you are you're probably in trouble. (40A per motor doesn't leave much wiggle room at all on either the motor breakers or the main breaker) Similarly, I highly doubt teams will be driving with their motors running at 12V free speed; however, there is something to be said for the argument that the battery doesn't output 12V, more like 13.5ish, and that can make up for some of the inefficiency losses. The approximation method may end up more accurate than either of the above, but that is obviously dependent on how accurate the inefficiency approximation itself is.

Personally, I just use free speed in calculations because it is easier to work with. In my experience, underestimating speed has worse consequences than overestimating (to many interrupts per sec, dead-reckoning autonomous driving too far, etc). Whatever you choose stick with one; you can then compare speeds year to year much easier than if you had to convert from one method to the other.