To Shift or Not To Shift, That is the Question

[AdamC]

I'd like to pose a question for all the fellow mechies out there. In terms of drive trains, is it worth it to go through the extra weight of shifters, or is it better to just stick with a solid gear ratio that finds a balance between power and speed. We tried shifters last year, and made a pretty solid and lightweight shifting tank drive. However, we found that in low gear, while the robot was technically putting out nearly 12 times more torque than in high gear, the wheels could not gain enough purchase on the carpet and thus put out about the same amount of force as high gear.
If anybody has experienced similar problems, please share. Our prototype is running on a fixed ratio, and runs pretty well, so it seems that a fixed ratio is better than a shifter.

[techtiger1]

Adam please seach before you post this topic has been discussed many times. To answer your question though it all depends on the game and how your team wants to play it. Generally 2 speeds has been worth it lately bcuase of the need to move quickly to get an object then slowly to score the object. In addition two speeds don't always have to be heavy look at servo shifters and 254's 2007 gearbox. There are also swerve drives which don't require 2 speeds at all becuase you can simply move away or around the 6 wd and 8 wd robots. In conclusion do your research on this topic, look at your teams manufacturing capabilities and analyze games/robots from the past. Good luck this is always a tough question this time of year, my team is shifting for 08 just to let you know. As for as your wheels/treads slipping look up PID loop control and research wheel materials. Speeds vary from year to year and team to team, we find that 8- 9 fps with one speed running two Cim motors per side works well. Shifting is a diffrent story the 1251 2007 robot is 3fps and 13 fps but generally we like to keep it around 6 and 12. Speeds depend on your drivers really I know alot of teams use 7 and 14 quite frequently.

Just a few thoughts ,
-Drew

[Nevets Amstier]

for the past two season 177 has used 2 speed gear boxes which helps get to wherever you want to go quickly, then if you want to be precise or push someone around you can shift down.

it really depends on what your team wants to accomplish while playing

[lukevanoort]

Often you will put out the same amount of maximum pushing force in low and high gear because generally your wheel/tread material stays the same. (shifters with designs like 67's in 2004 being the exception)

The benefits of shifting generally are that:
1) The motors are loaded less in low gear
2) You can go very fast for covering ground
3) You can go very slow for accurate driving

Ideally, your wheels would slip at about 40A current draw from each motor so that you don't pop a circuit breaker and lose mobility; thus, with the most popular tread material (CoF 1.0) this means you (assuming a four chiaphua drive system) must go at most about 7-8 ft/s. 7-8 ft/s happens to work pretty well as a middle ground speed, but if you have higher traction tread such as as SBR Roughtop incline conveyor belting, that number becomes something more like 5-6ft/s. That is too slow for many teams; thus, they shift. The shifting then frees you up to have both ideal current draw in low gear and a blazin' 16ft/s high gear. As you can see building an effective two-speed drivetrain doesn't only consist of making (or buying) a good shifter, but also choosing the correct ratios for your drive system. (arguably, the latter is the hardest of the two issues)

[Warren Boudreau]

Here is a rule of thumb that I have developed over the years.

You want to gear your drivetrain to go as fast as you can but still spin the wheels/treads when you are up against a wall.

<EDIT> If you want to go faster, you need to a multi-speed gearbox.

[AndyB]

I think the benefit of a two speed transmission was best exemplified by teams 1114 and 330 last year (I'm sure there were others as well). Both used multi speed transmissions and both were very great robots. They used speed to maneuver around the field and beat defenders who were playing closely. They used low gear to push around robots that played closer to the rack.

A two speed transmission will make it easier for the driver... I am not saying that it will do the trick by itself, but if you have a driver that can handle it, you can really work it to your advantage.

[Richard McClellan]

Has anyone ever experimented with having a sort of automatic transmission system? So, when the driver moves the joystick between neutral and halfpower, it's in low gear, but then it will automatically switch to high gear if you put the joystick to full forward? Or does everyone use a manual transmission controlled by a switch?

[Cory]

Originally Posted by richardmcc2

Has anyone ever experimented with having a sort of automatic transmission system? So, when the driver moves the joystick between neutral and halfpower, it's in low gear, but then it will automatically switch to high gear if you put the joystick to full forward? Or does everyone use a manual transmission controlled by a switch?

Team 33 did this with their 4 speed transmission in 2004. I think team 368 used a similar design in 2005. You might want to search for the threads about 33's transmission, as i believe they discussed the autoshifting in depth

[AndyB]

I'm not a driver, so keep that in mind, but it seems intuitive for me that it would be nice on the joystick to have it default to low gear and then when the trigger or a button was held, it would shift to second gear, when it is released, back to first... kind of like a turbo button.

I say this because in my opinion, low gear is for precise driving and motion and high gear is for moving quickly between two points.

[Dan Petrovic]

You don't necessarily need the low gear for precise movements.

Last year we had a single-speed transmission that was always running at 75% power, unless we pulled the trigger, then we put out 100% power.

Same idea, a little bit simpler, however, you do lose some pushing power with that 75%.

In the past year, it became quite clear to me that you don't necessarily need to be able to push, you just need to be able to stay still when you want to. If you can get enough momentum, then a single-speed transmission can knock someone off target. You do have abide by the rules of no "long distance, high speed ramming".

[AdamHeard]

Originally Posted by InfernoX14

You don't necessarily need the low gear for precise movements.

That's true, but it sure helps....

As a former driver, based on experience with our previous years robots, the jerkiness of trying to make precise turns with a drive geared at 7-8fps (or higher) was kind of annoying. Now, I know a better designed drive (or one with less traction.....) could have helped, but that isn't all ways as easy as just bolting on a 2-speed.

[wilmo]

we had a 2 wheel drive with the AM Gen-2 Servos. We tried shifting once, and we almost fell over from the sudden burst of speed, wasn't worth the weight for the 07 game, rather have more tanks for air