How fast r u gonna go?

[Bruceb]

Looking at using 2 speed trannys for the first time but I am not sure how fast I can expect to go with 4 cims. Also, how do you determine the running speed of the CIMS so you can figure the correct gear ratios?
Thanks
Bruce

[Thermal]

About this fast



(If we go 1-speed then a static 11 fps, if we go two speed then a 7 and a 13 or something close)

[Chris is me]

We're looking at 11.5 FPS single speed with 4 CIMs and 2 RS-775s using the CIMulator gearbox.

[Creator Mat]

A little off topic but I have never found the benefits of a two speed tranny to out weigh its draw backs.

Pro: Two speed tranny

• High speed
• High Torque
• Having both

Con: Two speed tranny

• Expensive (about $600-700 for 2 on andy mark)
• Added complexity to a robot AKA just one more thing to fail
• Heavy with a asterisk, i know they can be modified to be lighter but still tend to be heavier than single speed trannys

While having the ability to push and go fast is nice it really isn't neccasary. Which is why I think single speed gear boxes are better. You can gear a tough box for a 8:1 or 10:1 gear ratio and do just fine. If for some reason your robot is being pushed around while it has 4 CIMS in its drive train, don't push back, run circles around that bot. Plus your team didn't spend a ton for something you got free in the KOP

[theprgramerdude]

But, how can you run circles if you run circles around another if they have a shifting gearbox which allows them to out-accelerate you and out-speed you?

[mwtidd]

Originally Posted by Creator Mat

If for some reason your robot is being pushed around while it has 4 CIMS in its drive train, don't push back, run circles around that bot.

Funny I just coded a little autonomous maneuver to do just that

As far as 2 speeds go, I'm liking the idea for this years game. Not for high torque, but rather the fine tuning when you are capping and grabbing, but high speed when you gunning across the field, or juking that blocking bot.

Right now my juke isn't quite fast enough, I wish I had a 2-speed so I could pump out 15 fps during it.

[DonRotolo]

Originally Posted by Bruceb

Also, how do you determine the running speed of the CIMS so you can figure the correct gear ratios?

The same way you find out any information about ANY parts in the kit: Look it up.

Go to www.usfirst.org, select FRC, select Game & Season Info, select Competition Manual & Related Docs. On that page, scroll down the the KIT OF PARTS WEBSITE (which Kate spoke about for several minutes during Kickoff - did you watch it?).

On the KoP website, look for "motor curves" in the Parts Data Sheets section.

I'd just give you the URL, but then you wouldn't learn to look for yourself.

[Bruceb]

Don Don Don... I have the motor curves and I have the JVN design calculator.
I know the free speed is 5500 rpm. I am not an engineer. I don't know how to figure what I can realistically expect the CIMs to be turning at at nominal load at speed. Sorry, my calculus is nonexistent, I never had physics and I am just trying to help some kids build a robot. Now, if you are willing to help, great.
Bruce

[theprgramerdude]

Realistically expect them to turn at a speed drawing below 40 amps while cruising, or you're going to have a very dead robot. The data sheets explain 27 amps; use that as a base point.

[JVN]

Originally Posted by Bruceb

Don Don Don... I have the motor curves and I have the JVN design calculator.
I know the free speed is 5500 rpm. I am not an engineer. I don't know how to figure what I can realistically expect the CIMs to be turning at at nominal load at speed. Sorry, my calculus is nonexistent, I never had physics and I am just trying to help some kids build a robot. Now, if you are willing to help, great.
Bruce

If you're geared "reasonably" and your drivetrain doesn't have some ridiculous friction load (or is a Mecanum), the CIMs will run at about 81% of their free speed when at top speed.

Reasonably geared is defined for a drive 4 motors to be "less than 13 fps @ free speed."

-John

[BIGWILLI2081]

team 2081 is estimating they're going to go this fast:

http://www.youtube.com/watch?v=SiMHTK15Pik

[Captaindan]

no matter what this year is about speed strength and the exact manipulation of both a two speed transmission is going to put a robot at an advantage if the drivers can handle the speed ive seen the andymark supershifters go too fast for drivers

[lbarger]

Originally Posted by Captaindan

no matter what this year is about speed strength and the exact manipulation of both a two speed transmission is going to put a robot at an advantage if the drivers can handle the speed ive seen the andymark supershifters go too fast for drivers

It is important to remember that humans do need to control the robot and that they can be made to go too fast to control effectively.

However, it is not quite fair to say that AndyMark SuperShifters go to fast as you do have options in the final gear ratio by selecting different sprockets or even different size wheels. There are also software options to limit the top speed should you find your hardware outrunning your drivers.

[Tristan Lall]

Originally Posted by JVN

If you're geared "reasonably" and your drivetrain doesn't have some ridiculous friction load (or is a Mecanum), the CIMs will run at about 81% of their free speed when at top speed.

John, do you have any information about the test conditions used to establish the 81%? And how much does that vary? (I've always made a similar approximation, but I don't have any hard data to back it up.)

[JVN]

Originally Posted by Tristan Lall

John, do you have any information about the test conditions used to establish the 81%? And how much does that vary? (I've always made a similar approximation, but I don't have any hard data to back it up.)

About 8-9 years ago when I was teaching myself this stuff I designed a drive using 81% as a magic number (I think someone on the forums mentioned this) then tested it -- the experimental matched the theoretical. It has just sort of turned into a "rule of thumb" for the typical style of drivetrains. I know some will be a touch slower and some will be a touch faster, but I've never been pissed off if I design at 81%.

I'm humiliated by how unscientific this is, but it seems to work for me.
I don't have hard data... just 9 year old, soft data...

-John