Is a shifting transmission really necessary?

[Al Skierkiewicz]

Just to add my two cents...
I understand the up side to dual speed but (a big "but") is it really needed on a 48 foot long playing field with other robots and field pieces in the way. The difference between 8f/s and 12 f/s is only 2 seconds from end to end. As to the design current draw, the circuit breaker although rated for 50 amps will require a current 6 times that great for more than fifteen seconds before the circuit breaker will trip. Add in the influence of the wiring resistances, the "on" resistance of the speed controllers, internal resistance of the battery, and all of the other factors and it is impossible to deliver full rated stall current to any motor in a functional competition robot. Given that max stall current will occur when motor brushes are in contact with two commutator segments (two motor windings in parallel) a multi motor drive will likely place more than one motor in such a condition during a standoff. Depending on slip in the transmission(s) motors are likely to rapidly alternate between two and one winding while in a standoff thereby changing (lowering) the load current. Granted a six motor drive will have more motors in this condition and therefore would get closer to max trip current than a four motor drive. I think that anyone who spent anytime in the Martians pit and watched the results of the dyno test or the Matians superior pushing ability would have to concede that there is an alternative (and a good one) to multispeed transmissions.
Also, what is the risk to add speed change decisions to the drivers? It seems that adding another variable into the driver equation would tend to slow down response times. Although speed changes on the transmissions is fairly rapid, is it the right thing to do in a 105 second match? In rough guess of perhaps four speed changes in a match, you may have lost 2-4 seconds of match time in decisions and speed changes. As we all know, that length of time could make or break a match.
So the question still remains and decisions to weigh the alternatives still exists. I for one think dual speed transmissions are cool but adding up all the factors (time, current, weight, complexity) I still favor single speed.

[Wetzel]

Is a shifting transmission really necessary?

The better educated engineers above me have pointed out most of the pros and cons of shifting or not. That doesn't really address the question.

The correct answer is, it depends on the game and your game play strategy. Its been mentioned before, but it has been buried inside justification-by-feature.

In 2002, it was absolutely necessary for some teams to shift to get to the goals faster. The difference between 8fps and 12 fps was a full second of personal time with the goals. However, Beatty didn't need a shifting transmission because they thought outside the box and engineered another solution to the problem. Now, because of rule changes, their file card trick won't work anymore, but I am POSITIVE that there is another way to pull off a similar outside-the-box solution to a movement problem.

Is a shifting transmission really necessary?

I'd say no, not because the features are unnecessary, but because limiting yourself to one solution is a bad idea in FIRST.

Wetzel

[Alan Anderson]

Originally Posted by Al Skierkiewicz

...The difference between 8f/s and 12 f/s is only 2 seconds from end to end...

That means you can get to the center of the field a full second earlier. In the 2004 game, beating the opponent to the bonus ball tees and mobile goal in autonomous mode was critical to many teams' success. Two seconds is not terribly important in itself, but in competition against another robot every advantage helps.

A well-designed shifting transmission, controlled by an experienced driver (or a clever bit of software), is an advantage.

[Andrew]

I agree with most that you need to first work out your functional requirements before making a decision. There are a lot of possibilities that are not really considered by the topic.

2 motors versus 2 motors + shift?

2 motors + shift versus 4 motors?

4 motors + shift versus 6 motors?

6 motors + shift versus ... ?

Many people have been going down the 4 motors in a gear box + shifting for a while. This seems to be a pretty wide area in functional space for the general constraints that FIRST has been imposing.

Having a shifting gear box design in your tool box is probably a pretty good idea. In 2004, as an example, if you used a hanger to pick your robot up, you would probably want a shifting gearbox. Hanger Up = FAST, low torque, Hanger Down = SLOW, high torque. In other words, having a shifting gearbox in your tool box does not mean you will only use it on your drive system.

What appears to be another wide area in functional space is the six motor, independently driven wheel design.

This design gives both low end torque and high speed, without a transition point that would come in a transmission. One thing which has not been talked about is getting up to speed. Unless you have a shift-on-the-fly transmission, you have to tolerate low acceleration to get to high speed. This may account for 1-2 seconds delay for a 12 fps relative to a multi-driven-wheel design.

Therefore, the multi-driven-wheel design has an attribute which I will call "quickness." This translates into an ability to maneuver (which can also be accomplished by an omni-wheel design).

The other advantage to a multi-drive-wheel design is "failure tolerance." If one motor blows out, freezes, over-heats, you still preserve the majority of your function. As an example, in 2004, one of our drive motors burned out. We noticed that the robot "pulled" to that side, but it took two matches before we realized that we had lost a motor.

If you only have one drive axle per side, a single failure can eliminate half your drive system. In our case in 2004, with three driven axles, it would take a triple failure on one side to take us completely out of commission.

My experience in FIRST has taught me that, whatever you do, make sure that no single failure can eliminate half your drive system. Whether this is a resetable breaker, drive shaft, drive chain, speed controller, pneumatic pump, etc., make sure that there is no single path to failure. The worst feeling in the world is having your robot driving around in circles when there is critical business to be done on the playing field.

[Mr. Ivey]

Just to throw out there my thoughts on the matter. A shifting transmission is a hot commotity, it's nice to have, wether it be a 2 speed or up to a 6 speed, it doesn't matter. Looking at the whole thing from an automotive standpoint it's just a quicker way to get to top end, but that is with a combustion engine, I know. A quote comes to mind when thinking about the way we build robots, we bash them, we crash them, we abuse them. Our drive shafts can get buggered up and torqued. And a quote comes to mind:

ROBUST, ROBUST, and ROBUST if you want to win - dan 322

A shifting transmission can be awsome, you get multiple speeds and torque, but you can have problems, gears grind and teeth break, dogs can crack, and all kinds of bad things can happen. You can get wobbel and warp, and then you are in trouble. Being on Team 384 for 4 years, and now I'm a mentor, and from the great mentors I learned from, the more simplifed your robot, the more robust you can create it. I'm not saying it's not nice to have a shifting transmission, it is, but i don't think it's truly necessary for a competative robot.
mark

[team222badbrad]

A shifting transmission is not necessary, but they sure do bring in the technical awards!

[sanddrag]

Originally Posted by Mr. Ivey

the more simplifed your robot, the more robust you can create it.

I don't disagree there. For example an iron block is very simple and very robust. Strap a control system on it and it will meet the requirements of being a FIRST robot. However, it sure doesn't do very much, nor is it innovative.

[Alan Anderson]

Originally Posted by Mr. Ivey

...Being on Team 384 for 4 years, and now I'm a mentor, and from the great mentors I learned from, the more simplifed your robot, the more robust you can create it. I'm not saying it's not nice to have a shifting transmission, it is, but i don't think it's truly necessary for a competative robot.
mark

Robustness is good, but a competitive robot must also be capable. A consistenly winning robot must be more capable than its opponents.

A shifting transmission is only one way to make a capable drive system. It happens to be a popular one, and with the proliferation of good shifting designs (and now commercially available ones), it's a relatively easy one to use.

[Salik Syed]

i think if your team can do it then its worth it! but a half working one is still worse than a fully working single speed !

[Tom Bottiglieri]

Many people ask "Why should I have a shifting transmission?"

A better question may be "Why not?!"

I see many advantages in having a shifting transmission. Most of the technical aspects from the mechanical side have been covered, so I will give my pros and cons from a programmers perspective.

Reason 1: Having a shifting transmission and a well written program will enable your robot to be "smart" For example, lets say it is autonomous mode. Under normal conditions in high gear, the drive motors draw 'X' amount of current, and output a corresponding 'Y' amount of speed. Your robot collides with another robot and a pushing match begins. Your program senses that the amount of current going to the motors has increased, and the speed has decreased. (A considerable amount) Your robot could then know to shift to low gear and beat out the other robot. And for what I know of the mechanical aspect of this situation (probably not much), by going to a lower gear you can output more torque while drawing the same amount of current.

Reason 2: Creating an option to shift also creates another form of control. Whether this is controlled by the drivers or the robot is entirely up to you. Don't be quick to jump the gun and assume that drivers always have to control your shifting transmission. We have the resources in our KOP to create an "automatic" manual transmission (a manual transmission that is shifted by the computer) I believe team 33 did this last year, but i may be wrong. This can give you a good advantage, and your robot will deliver speed and power when needed, respectively. Now I'm sure there is some way of achieving the same goal with a 1 speed transmission, but most people (like me) just dont know how and will default to what they know, such as shifting transmissions for changes in power and speed.

Reason 3: Its sexy.

[Ian Mackenzie]

There's one more subtle issue that I think is worth pointing out. Say you're designing a robot for Zone Zeal in 2002, and you want to make a ball pickup robot that tows the goal around behind it. Assume that you can make a robot with a 1-speed gearbox that goes 10 feet per second and can slip its wheels just before stalling, and that your driver is good enough that uncontrollability is not an issue.

From most of the posts above, this would seem to be an ideal drive system - but I would argue that there is still an advantage to having a shifter. Say your wheels slip at 90% of stall torque, and you can pull a goal at 80% of stall torque. Yes, you can pull the goal, but at that point you're running too close to stall to get max power output - if you geared down slightly, you could pull the goals around a lot faster. It won't help you push another robot (traction is still the limiting factor) and it won't help you get to the goals faster, but if you're towing a goal around for a large part of the match, it could still be very useful.

[Paul Copioli]

I am with Jay on this one. I have said it many times before: 2002 ruined a lot of us by making us think shifting is a neccesity. In 2002 shifting (of some sort or another) was essential because you could transfer over 360 lbs to your robot base (two goals at 180 lbs each. you 3 goal grabbers are excluded from my argument). 2003 and 2004 did not need shifting if you had at least 4 motors driving your drive base. I bet some teams did fine with two motors and no shifting, but 4 or more without shifting was plenty.

Speed and accleration are key and the race to the middle depends on both. Don't make your high gear too high or your acceleration will be horrible. If you do not shift during the race to the middle (goals, end, whatever) then there is not much difference in time between a 12 fps robot and a 8 fps robot (with the same motors driving). Teams like the Killer Bees who have an automatic transmission are taking full advantage of shifting with automatic speed/torque based on current speed of the output.


We shifted in 2002 and 2003, but not in 1999, 200, or 2001. We will not shift in 2005 unless we can transfer weight to our robot.

-Paul

[suneel112]

I think the transmission issue depends on the robot itself. For example, an auton hanger (like 190, that uses the ramp and then hooks on to the bar) doesn't need a transmission (except when facing 716 ) on the wheels. During design session, I suggested a robot with a transmission on the arms to cap quickly, and to use power to hang. A crazy smallball robot may even use a tranny on the small ball mechanism. It all depends on the type of robot. For this year's game, I would suggest trannies for the wheels, since it was a robust pushing game.

[Jim Zondag]

Good thread.....I agree with many points stated above.

No, you do not need to shift.
Yes, it is a good option to have.

Team 33 has make multispeed robots for many years.
As the kit had gotten better and the motors have gotten bigger, the need to shift has diminished. If you tried to get your 1999 robot to go 12-15 ft/sec you could not do it without a shifter. Now you can simply brute force the issue by throwing tons of power at it. I still like having a shifter for a couple of reasons:
1. Power budgeting: Liberating some of the good motors for something other than driving around is a good thing. After all who want to build a high speed brick that does nothing but smash into others.
2. Control: As a programmer one thing I know very well is you must have a mechanically capable design in order to have good controlabilty. In FIRST, you often have very distinct operating modes for your machine during a match: i.e. Push like crazy to get past someone and then run full speed down the field. Having gears selected for both of these actions well enables you to do both well.

Regardless, shifters are a detail. The major items of FIRST are still what they always were.....understand the game, build a reliable quality machine to play it, train you drivers to use it to its full potential. If you do these things, you can win with or without any specific machine feature.

[Raul]

I would just like to ask a slightly different question:
How many national champions had shifting transmissions?

I can't think of any. Apparently there is more to it than the drivetrain (beside in 2002).

Raul