Re: Cooling Breakers
 

The only difference between "canned air" and "freeze spray" is that on the inside of the can one is up-side-down.

Re: Shifting Gears
 

design it as simple as you can, it makes it easier to fix.

If it is complex, try to design it so you can repair it easily, or have spare trannies made, so that a tranny repair wont take you hours.

Re: Shifting Gears
 

This is absolutely true. Ideally, having a transmission that is modular is the way to go. In addition, in my humble opinion, your transmission shouldn't have reductions on the order of 1:50 and 1:20 for instance, they should be just an initial step, of say 1:1 and 1:3 so that if for some reason the whole thing explodes, you can just pull it out, move a motor and be at that 1:1 ratio, which is something on the order of magnitude that your robot will move.

Secondly, having a tranmsission that you can easily modify is very ideal. For example: don't weld gears to shafts! If something strips or breaks, you'll have to replace the entire shaft. Bad news, especially if there are multiple gears on the same shaft! Not only obnoxious, but time consuming and expensive.

Matt

Re: Shifting Gears
 

<rant>
There is something that i feel every member of a FIRST team and anyone involved in engineering in general should read and take to heart:
http://www.seattlerobotics.org/encod.../classics.html

That said i agree with previous posts, which state that the benefits of shifting should be carefully weighed against its disadvantages. As hard as it may be, decisions should be made not on emotion, but rather objective analysis and common sense. One thing is see all to often is something which is done merely because it is cool.

The first step in determining whether a shifting transmission (or any feature) is right for your team is to ask why do we need one? Every good design should start by clearly stating what problem(s) it is meant to solve (not the inverse!).

Are you trying to achieve higher speeds? higher torque? faster accereration? A better balance between the three that does not involve compromising at least one of the others?

After assessing the need for a feature, one must then ask "do we have the resources to design, fabricate, test, and pay for this feature all within the set time constraints?"

If the above criterion are met then i would say go for it!... But remember a good bot, or anything else for that matter, is built just as much if not more by careful planning at the systems and functional level just as much as by engineering prowess. This is a lesson that our team learned the hard way last year, which could have easily been avoided.
</rant>


this thread seems to have forked off into a discussion of two discrete but related topics and thus i will address them induvidually.

>1 motor per side
If weight permits and current is not an issue, i see no reason not to use multiple motors on each side. There are 2 advantages i see in using multiple motors. First and most obvious, you get more power to work with. Depending on how your drivetrain is designed this can translate into more speed or greater acceleration and pushing power. Depending on the game this year speed and or torque may be of great advantage. Secondly, someone earlier mentioned redundancy. If One motor fails, your robot will still be able to move (to some extent) rather than being completely disabled. Given the high mortality rate of the drill motors redundancy is definately a good thing( the reasons for this are another topic). In the IT world, redundancy is pretty standard on "mission critical" servers etc. The disadvantages of multiple motors seem minimal. Yes there is added weight, but not much. <rant>Most of the gearboxes i have seen in FIRST are ridiculously overengineered and much heavier than need be. using 1/2" aluminum plates and gears capable of transmitting in excess of like 50hp under shock loads is just not necesary. Look at the gears in the drill gearboxes and the Technokats transmission. Have you ever seen one of these break?</rant> The weight added by multiple motors and gearboxes to couple them really isn't that much.

Shifting Transmissions
I think that most people would agree with me that a good shifting transmission has many benefits, for example your bot can have a normal gear for general driving and then a low gear for pushing matches and more delicate menuvers. There can also be big disadvantages associated with shifting. Shifting gearboxes are (generally) heavy, difficult to engineer and fabricate, and possibly unreliable. That said, if there is good reason to have a shifting and your team has the resources to implement such a design, i see no reason not to have one.

Re: Shifting Gears
 

Absolutely correct. However, I think the past two years have really demanded a hearty drive train. See below.


To clarify, if you are referring to the part of my previous post where I brought up using multiple motors (which you very well may not be), I'd just like to clarify that I believe that multiple motors on a shifting transmission is unnecessary. However, I whole heartedly believe that it's very practical and essentially required to have multiple pairs of motors if you do not have a way to change your gear ratios.

I would fight to the tooth (with numbers of course!) that you can not have a robot that is both competitively fast and competitively strong using only one pair of drill motors or the chips without a mulit-ratio transmission.

Perhaps that'd be a post for later.

Re: Shifting Gears
 

That statement depends very highly on the game that you are playing. For example, last year you did not need to have pushing power if you weren't solely a ramp dominator. Take team 25, one speed, 4 motors, 12.5ft/s. All you need to move someone off of something, top of the ramp for example, is momentum (p=mv). No where in the formula for liner momentum is torque taken into account. Then you could also argue that team 25 was a strong robot for defense because of thier brake system. Or like any other robot with a high coef. of static friction, you can just turn sideways for defending a stack, or in previous years a goal. A 4 motor drive geared correctly for one speed can always be competitive.
Take last year for example, FIRST made it a little easier on everyone by making the ramp top out of hdpe so that way everyone would have a fair fighting chance for it at the end of the match. In most cases a fast robot could charge the ramp at the last few seconds and push down a stronger robot with thier momentum,which lead other teams to develope a method to keep thier robot attached to the hdpe by means other than just a drive system. Innovative new ideas can beat widely spread old ideas.
When picking your drive and wether or not your team wants to shift gears, analize the game first and then decide wether or not you actually need to shift to accomplish all the things you want to do in the game. If you can think of more reasons not to shift gears and concentrate somewhere else on an arm or such, then you should probably put a 2 speed tranny off to the side untill needed. If need the most pushing power or the most speed to accomplish your goal, then concentrate in there if you do not have the resources to make a shifting tranny. If you make a list of things you want your robot to do and high mobility is on the list, then I would suggest investing time in a swerve/crab drive rather than a 2 speed 1 dirrection drive.
One thing to keep in mind for everyone designing your multispeed tranny's: the shifting mechanisms can wear down to the point where they become unshiftable. You may want to keep that in mind with some methods so that if you are forced to lock it into one speed, you'll still be able to be competitive in that speed. Meaning don't set a goal of your robots high speed to go as fast as possible without tripping breakers, but then also try to be fast enough where you can be competitive if need be to lock into low gear. For that reason you also may want to make your multispeed tranny have 4 motors, since in many cases adding in another motor will only require 1 or 2 extra gears and so the power added is well worth it. You can also design it so if need be, you can add a motor or easily take out a motor if you decide to use it for a mechanism or if you just can't make weight. Just some of my thoughts on the subject...

I need more power Cap'n!
 

I disagree with the statement that there is no reason to have multiple motors if you have a shifter.

They cover similar problems, but not EXACTLY the same problems.

If you need or want more power (e.g. for faster accelerations) multiple motors gives you that. Notice that I use the term "power" in the engineering sense. I do NOT mean "more low end torque" as many do when they use the term on these fori.

POWER = WORK PER SECOND <-- SPEED X TORQUE in our case.

Switching gear ratios can help you get more power up to a point by choosing to load the motor such that it is providing its peak power (i.e. at 1/2 its stall torque) but if you need or want more power than that, more motors are your only real solution.

Joe J.

Re: Shifting Gears
 

Our team actually takes the opposite approach to this, we try to weld anything that we can but we start with robust materials that are highly unlikely to break. Welding the gears to the shaft after everything is the way you want it takes one more failure point out of the equations (Assuming you have someone who is excellent at welding versus other options such as set screws, pins etc) Again this past year in debug of the Team45 transmission it was a hare narrow and ended up being in high & low at the same time in a high power shift. This immediately locked up the transmission and send a shockwave through the gears, shearing the pinion off the drill motor, all other welded gears were unharmed and lasted the entire year and are still going strong. As stated above, many people use gears that are more than up to the task of anything the FIRST robot can throw at them. Personally, I like 20 pitch gears that have been lightened to an extreme amount, the teeth show little if any wear after a season. Since the drivetrain is arguably the most important part of the robot (Manipulation can be useless if you robot is stranded on the wrong end of the field) I try to have the team place a priority in power, modularity, and robustness there.

As for replacing a broken shaft, it should be a rule of thumb that you make a spare of anything that is custom and has a chance of failing. For transmissions I would strongly recommend making one that is similar left to right and fabricate a spare at the same time as you are fabricating the mains.

Re: Shifting Gears
 

Emphasis to PAIR and OR added by me.

Emphasis to 4 motors (implying more than a single pair) added by me.

I think that Rob just read my post a little too quick. (No biggie ;)) I think you can easily be competitive if you have:
1. More than one pair of motors (4+)
or
2. A shifting transmission

My thesis is this (restated):
You can not be competitive using only two motors total and no shifting mechanism.

I definite "competitive" as a max speed greater than 10 feet per second, and having a maximum applied force of 150 lbs.

Using the above "competitive" criteria, this is simply not possible, and not really a really up for debate. Perhaps my definition of competitve could be debated, as it does depend on each game.

If you'd like some quick numbers, just let me know!

Matt

Re: Shifting Gears
 

I guess I ought to clarify my point here. The reason that I suggested against welding gears to shafts is essentially for a couple of reasons:

1. If you break teeth on any gear on that shaft, all the gears on that shaft are essentially garbage because you can't remove them.

2. Welding around gearboxes has potential for splashing of metal in between teeth inadvertently, and that's BAD NEWS.

You said that you had to replace a drill motor, but your gears were unharmed. Personally, I'd much rather replace a gear than an entire motor since they're cheaper and easier to attain, and easier to have spares of too.

To follow with that, I always try to use keys since the allow you to salvage your gears after a critical failure, but have shear strength much higher than set screws or pins. I'd agree they don't have the strength of a nice solid weld, but read on...

If you were careful and took some time to calculate the absolute max torque that you would have on a properly functioning gear box (and add a little safety factor for good measure), I think you'd really like to have whatever is holding your gears fail first, rather than the teeth on the gear or a shaft within the box. I'd much rather replace a key, pin or screw before anything else.

Just a few more cents on some really good thoughts flowing from a lot of folks. Keep 'em flowing.

Matt

Re: Shifting Gears
 

Well... of course!

As Joe mentioned above... that's what Power is all about... and any single pair of our motors simply don't have enough to handle the speed, AND the torque you mention simultaneously.

All you rookies listening?
This is one of the most common "errors" people make:
"Our robot is fast as hell! And it can also push the world! We don't need a shifter, or other motors."

*bzzzzzzt* Try again.

Unless you shift, or have extra motors... my robot will always either be able to out push you, or out run you. NO ARGUMENTS WILL CHANGE THAT.

Physics, does not lie (only FIRST team representatives do). :rolleyes:


I agree 100% with what Matt stated (and more importantly, so does Physics).
I also typically use a ~1.2 coeff of friction (130 lb robot * 1.2 coeff ~ 150 lbs pushing force) and 10-11 feet per second as my benchmark points for low and high gear. It is NOT physically possible to have both using the drills, chips, FP individually. (Unless.... you shift!)

*phew*
This is a sore subject for me. I'm sick of 80%+ of FIRSTers not understanding this MOST FUNDAMENTAL concept of drivetrain physics. Everyone should take the time to understand the principles behind the robots... (it makes scouting a heck of a lot easier when you can recalculate a robot's theoretical performance based on some quick questions, and a mental calculation).


Again... if anyone has questions on stuff like this, you are more than welcome to email or IM me. There are also plenty of whitepapers, and threads covering topics like this. Or... ask your friendly neighborhood FIRST mentor/engineer!

John

Re: Shifting Gears
 

Not in the case of the Technokats tranny. From what I've read in the past they went through a lot of trouble to get that dang gear, but the motor was no problem. The drill motor is actually very easily obtainable.

Re: I need more power Cap'n!
 

Quote:

Originally Posted by Joe Johnson I disagree with the statement that there is no reason to have multiple motors if you have a shifter.

Everything you said is completely correct, as I would expect out of any Purdue alum... :)

I guess that my two cents come in that I think that you can have "enough acceleration" and your max speed can be "fast enough" and your max torque can certainly be calculated to be sufficient with one pair of motors and a well designed shifting gear box. And by "enough", I mean reasonably competitive.

Obviously a robot with 4 motors will be faster and accelerate more quickly than one with only a single pair. My opinion comes in saying that the estimated 10-14 feet per second you can get out of a high speed ratio gear set gives sufficiently competitive speed and acceleration for most applications.

This is only an opinion. I belive that often for many teams, the weight required and additional engineering time could be better used elsewhere. Certainly that is debatable, and your point is definitely well taken.

Matt

"Enough" is relative...
 

I agree that depending on the game, your choosen strategy and the skill of the operator, it is quite possible to make competitive robots with only one motor per side and a shifter. I'll even go farther than that. It is possible to make competitive robots without a shifter and only one motor per side (again, depending on the game, your strategy and the skill of your drivers).

My point was that power is a main reason for adding motors, even with a shifter.

How much power is "enough" is up to each team to figure out.

One more comment on this more power issue. When I say "more acceleration" I do not just mean quicker changes in velocity. While it is possible that this will be an important design consideration, I think that it is quite unlikely that the time savings alone are sufficient to justify needing more power in your drive system. Think about it. Is it likely that the half second or so you saved because you got up to full speed in 1 second rather than 2 seconds is going to win you a match? I doubt it.

Having more power for acceleration is more than just having a high DV/DT. It also includes moving at a constant speed up a ramp for example.

Another reason that acceleration is important is that until your robot gets moving, your motors are effectively stalled. The breakers cannot tell the difference between stalled motor current and current from accelerating your robot. So... ...by having more acceleration your motors are operating at this "stalled" condition for a shorter time period. Translation: your breakers will not trip as often from hard accels and decels. Cooler Breakers give you more margin for operation. You will not have to shift out of high gear for a minor skirmish with an opponent because your breakers will have that reserve of current for just that much longer than they would have.

Margin of error for the operators. Now THAT wins matches!

By the way, I am not a huge fan of multiple motors per side, but I just think that the decision should be made with a fair accessment of the merits of each case rather than a hard a fast rule that shifting is the answer, or multiple motors is the answer, or that multiple motors + shifting is the answer, etc.

Just some more things to ponder as you weigh things in the balance.

Joe J.

Re: Shifting Gears
 

in-lbs....