RS775 Gearboxes

[Ian Curtis]

Originally Posted by rahilm

Based on our prototyping, watching some old 2007 videos, and current consensus on design, we've found that speed close to optimal. Of course, it's only week 2, and there's a lot more testing to do, so our plans may change quickly.

Sanity check: Strap a five pound weight on the end of a 2x4 and swing that from a pivot point about five feet from the end 180 degrees in a second.

Then try to precisely place a tube on a rack with that rotation rate.

I haven't done it, so maybe I'm talking out of my butt here... but that seems pretty darn fast!

[pfreivald]

What's wrong with 180 degrees in one second in this game (specifically)? You have half (or all) of the field to drive before you are in position to score, after all!

[rahilm]

Originally Posted by iCurtis

Sanity check: Strap a five pound weight on the end of a 2x4 and swing that from a pivot point about five feet from the end 180 degrees in a second.

Then try to precisely place a tube on a rack with that rotation rate.

I haven't done it, so maybe I'm talking out of my butt here... but that seems pretty darn fast!

I guess I haven't been clear enough. We probably will be going with something slower for human control (as I said, week 2 just started, lots of testing to do still), I don't know of many people who would be able to control something that fast.

[Chris is me]

Originally Posted by pfreivald

What's wrong with 180 degrees in one second in this game (specifically)? You have half (or all) of the field to drive before you are in position to score, after all!

It's far too fast to be easy to control, in my opinion. Most arms have about 90-100 degrees of travel. I"m thinking 45 degrees per second is much more reasonable.

[rahilm]

Originally Posted by Chris is me

It's far too fast to be easy to control, in my opinion. Most arms have about 90-100 degrees of travel. I"m thinking 45 degrees per second is much more reasonable.

With some degree of automation (PID, specifically) and the proper control systems, you could go with a much faster speed without losing much control.

[pfreivald]

Why, in the age of quadrature encoders and with targets at a set height, would you be subjecting your lifting arm to human control (beyond 'this is the height I want)?

[JVN]

Originally Posted by pfreivald

Why, in the age of quadrature encoders and with targets at a set height, would you be subjecting your lifting arm to human control (beyond 'this is the height I want)?

Why subject your robot to a potential sensor failure when you can train a monkey (or a student, if monkeys are lacking) to put the tube at the right height every time?

[Ian Curtis]

Originally Posted by rahilm

With some degree of automation (PID, specifically) and the proper control systems, you could go with a much faster speed without losing much control.

Because I trust software about as far as I can throw it, and since it has almost no mass, I can't throw it very far.

I know there are a lot of great programmers in FIRST. I also know that teams are always pushed right up to a deadline, and the software teams get very little time to test. I also know a slow arm is a lot easier to stop, and if your arm is going 180 degrees/second and you miss a limit switch for some reason, something is going to break. There are a lot of great teams in 2007 that didn't actuate at anything close to 180 degrees/second. There probably are some that did too, but I'm a lot more comfortable with a slow arm that I can fall back on a human to control, than needing a relatively complex piece of software to control.

Why would you use an encoder? It is simpler to use a potentiometer since your are limited to probably less than one revolution, no?

[pfreivald]

A. Trimpots drift. Encoders don't.
B. The same idea could be replaced with "why do this in the age of the limit switch?" and the objection is the same (though with slightly different hardware and software, without quite so much elegance).
C. With 20 minutes of LabView training, I was able to bring an arm up to whatever height I wanted to by pushing the appropriate button.
D. If you haven't made things autonomous, then you can't score in autonomous...

[Jason Law]

Banebot just emailed me back:


Hello,

The new CIM-U-LATOR gearbox and P60 mount for the RS775s will be available for order Wednesday.

Thank you,

BaneBots Sales


Jason Law

[sanddrag]

On the BaneBots page, it says they'll be offering a 64:1 and a 256:1 for the 775. Really? Nothing in between those ratios?

[Ian Curtis]

Originally Posted by pfreivald

A. Trimpots drift. Encoders don't.

Can you explain (in non-EE terms, preferably) how big of a problem this is in FRC style applications? In my six years of robot building we used potentiometers many times in the feedback loop for rotation, and many more times in our Operator Interface. We never had a problem, and none of my EE mentors ever mentioned it as a potential issue in our many hours of gremlin chasing. Did we dodge a bullet, or is this a case of "it only really matters when you're building spacecraft"? Thanks!

[pfreivald]

Originally Posted by iCurtis

Can you explain (in non-EE terms, preferably) how big of a problem this is in FRC style applications? In my six years of robot building we used potentiometers many times in the feedback loop for rotation, and many more times in our Operator Interface. We never had a problem, and none of my EE mentors ever mentioned it as a potential issue in our many hours of gremlin chasing. Did we dodge a bullet, or is this a case of "it only really matters when you're building spacecraft"? Thanks!

The one year we used a trimpot, we found drift values of 5-10%. At one point it drifted sufficiently so that it was still driving while stalled.

We were the "oooh, neat, fire!" robot that year.

[thefro526]

Originally Posted by sanddrag

On the BaneBots page, it says they'll be offering a 64:1 and a 256:1 for the 775. Really? Nothing in between those ratios?

I would imagine that they may be the only packages for the RS775 that they offer off the shelf, but in years past swapping motors between BB transmissions was just a matter of ensuring you have the correct Mounting Block for the Motor and series of Transmission - so you should be able to buy a P60 with the desired ratio and swap the mount, right?

[Tom Line]

Originally Posted by sanddrag

On the BaneBots page, it says they'll be offering a 64:1 and a 256:1 for the 775. Really? Nothing in between those ratios?

Actually, on the planetary page for the P-60 for the RS-775, it says they have multiples of 4.

4:1, 16:1, 64:1, and 256:1.