High speed Drivetrains

[Dragonslayerend]

I noticed this year that the high scoring robots were ones with extremely fast drive trains. The faster the robot the more advantage the robot had against avoiding blockers and doing laps. Though how come there weren't many robots I saw that couldn't exceed atleast 12 fps. What was your reasoning? Defense? Or was it because of technical issues?

[MichaelBick]

Originally Posted by Dragonslayerend

I noticed this year that the high scoring robots were ones with extremely fast drive trains. The faster the robot the more advantage the robot had against avoiding blockers and doing laps. Though how come there weren't many robots I saw that couldn't exceed atleast 12 fps. What was your reasoning? Defense? Or was it because of technical issues?

For one, most of the teams with high speed drivetrains also have shifters. This allows them to run their drivetrain really fast while still having a lower speed to push through and play defense. However shifters are mechanically complex, and can be heavy to integrate. Also the high scoring teams have a lot of drive practice allowing them to decelerate less and play more of the game at their top velocity.

[dcarr]

Originally Posted by Dragonslayerend

doing laps

What year is this again?

[orangemoore]

Originally Posted by dcarr

What year is this again?

That was 2008 Overdrive

[StevenB]

Elite teams do not just drive fast, they are fast at everything. Why? Because there is only 2:15 in the match to score points. Elite teams put in the design work to get the most out of every motor and mechanism they put on their robot. Most of us are content to get the motor to drive the mechanism at all, but elite teams do the math to figure out what designs and gear/pulley ratios will actually squeeze the most performance out of the motors they have.

The idea that a slow robot has more pushing power than a fast robot is not fully correct. If you have six motors on your drive and wheels with normal-ish friction, you can build a single-speed traction-limited drivetrain that still goes really fast. You should always build things to be as fast as possible (but no faster).

[Jibri Wright]

Team 1781 went with Super Shifters this year. This is why we had some good offensive and defensive capabilities this year. I know that Team 111 had Super Shifters as well but instead of two speeds, they had three! They customly designed a third gear shift that was at a really low gear making them faster than us. As for squeezing the most out of every motor, we didn't take that into account. Then again we're not an elite team quite yet.

[AlecMataloni]

Originally Posted by Jibri Wright

I know that Team 111 had Super Shifters as well but instead of two speeds, they had three! They customly designed a third gear shift that was at a really low gear making them faster than us.

We actually used 2 speed VexPro Shifters with a 3rd stage provided by WCP which matched our desired speeds for 6in wheels. Sorry if you were misinformed.

[Dragonslayerend]

Originally Posted by StevenB

Elite teams do not just drive fast, they are fast at everything. Why? Because there is only 2:15 in the match to score points. Elite teams put in the design work to get the most out of every motor and mechanism they put on their robot. Most of us are content to get the motor to drive the mechanism at all, but elite teams do the math to figure out what designs and gear/pulley ratios will actually squeeze the most performance out of the motors they have.

The idea that a slow robot has more pushing power than a fast robot is not fully correct. If you have six motors on your drive and wheels with normal-ish friction, you can build a single-speed traction-limited drivetrain that still goes really fast. You should always build things to be as fast as possible (but no faster).

Oh The year I'm talking about is 2013. I noticed that many teams were using mini toughbox with 10:1 gear ratio but extremely slower(like 6 fps) than our 8:1 geared robot(which runs at 12fps). I was wondering how such issues happen. Or whether it was actually planned to be a defensive robot. Actual design goals of teams are what I'm looking for. What was the intent your team had for a drive train and what was the result?

[wilful]

Originally Posted by StevenB

If you have six motors on your drive and wheels with normal-ish friction, you can build a single-speed traction-limited drivetrain that still goes really fast.

However with six CIMs you have to take into account the current they are drawing. If you end up stalling all six at once, and each is drawing 40 amps (or more) through its breaker on the PD board, there are 240 amps being drawn through the main breaker, which is rated to 120 amps. At least one team at SVR ran into this problem.

[MichaelBick]

Originally Posted by wilful

However with six CIMs you have to take into account the current they are drawing. If you end up stalling all six at once, and each is drawing 40 amps (or more) through its breaker on the PD board, there are 240 amps being drawn through the main breaker, which is rated to 120 amps. At least one team at SVR ran into this problem.

You can't stall a traction limited drive

[RyanCahoon]

Another issue to consider is acceleration. Andrew Palardy [apalrd] and others have a number of great posts on this already, but gearing for more torque, in addition to greater pushing ability, also allows you to accelerate faster. Especially given the restricted field (you have to drive around the pyramids) and the level of defense this year, it does no good to have a high theoretical top speed if you'll never be able to reach it. There's always a few rookie teams each year that try to gear their drive trains for 24 ft/s and learn this lesson the hard way when their robot won't move.

[MichaelBick]

Originally Posted by RyanCahoon

Another issue to consider is acceleration. Andrew Palardy [apalrd] and others have a number of great posts on this already, but gearing for more torque, in addition to greater pushing ability, also allows you to accelerate faster. Especially given the restricted field (you have to drive around the pyramids) and the level of defense this year, it does no good to have a high theoretical top speed if you'll never be able to reach it. There's always a few rookie teams each year that try to gear their drive trains for 24 ft/s and learn this lesson the hard way when their robot won't move.

The other way of accelerating faster is adding more motors(something many teams have done this year). The field is actually pretty open for robots that are less than 30" tall.

[efoote868]

Most years teams limit themselves to 12 fps or less because 12 fps is a sweet spot when it comes to acceleration in drive trains.

You can design a drive train that will go 40 fps; it'll never reach that speed on an FRC field. A better way to design a fast drive train is to figure out the maximum straight line travel your robot will ever have to go (be it 10, 20, or 40 feet), and then design the gearing around that.

I'll have to look into it (and do a search), but if I recall correctly there was a model that showed that a single speed 4 cim drive geared to a maximum 12fps went from dead stop to 20 feet in about the same amount of time as a drive geared to a maximum 14fps, and faster than a drive geared to 16fps. The 12fps drive had the advantage of more pushing power than the 14fps drive.

EDIT:
Was this thread I was thinking of.
http://www.chiefdelphi.com/forums/sh...l+simu lation

[Dragonslayerend]

Originally Posted by efoote868

Most years teams limit themselves to 12 fps or less because 12 fps is a sweet spot when it comes to acceleration in drive trains.

You can design a drive train that will go 40 fps; it'll never reach that speed on an FRC field. A better way to design a fast drive train is to figure out the maximum straight line travel your robot will ever have to go (be it 10, 20, or 40 feet), and then design the gearing around that.

I'll have to look into it (and do a search), but if I recall correctly there was a model that showed that a single speed 4 cim drive geared to a maximum 12fps went from dead stop to 20 feet in about the same amount of time as a drive geared to a maximum 14fps, and faster than a drive geared to 16fps. The 12fps drive had the advantage of more pushing power than the 14fps drive.

EDIT:
Was this thread I was thinking of.
http://www.chiefdelphi.com/forums/sh...l+simu lation

We will do a drag race with multiple neighboring teams that runs at 6 fps to 14 fps to see which actually is a faster robot for accel and distance travel. N post it up.

[efoote868]

Originally Posted by Dragonslayerend

We will do a drag race with multiple neighboring teams that runs at 6 fps to 14 fps to see which actually is a faster robot for accel and distance travel. N post it up.

Unfortunately there will be factors that are difficult to control with different robots, for instance the amount of wiring the robot uses, the size and weight of the wheels and gearboxes, as well as the weights of the robots.

Regardless, the results will still be interesting. I'm sure we all would appreciate as much information as you can post!