4-speed gear automatic gearbox

[apalrd]

I don't think FRC needs a 4 speed transmission.

Based on the modeling work I did with shifting transmissions, I found that it was only beneficial to upshift automatically for improved acceleration if the low gear was very high, around 8fps for a 4 CIM 150lb robot, and this drove a desired ratio spread of <2:1 to put the high gear around 15fps. All of this resulted (in simulation) in optimum accel times, time to distance (short and long), and battery usage for pick and place maneuvers (which can increasingly use 'low' as the gear for short distance maneuvers).

However, for a 4 CIM drive, 8fps is not traction limited at 40a/motor with any reasonably grippy wheel. So that drove the desire for a 3 speed transmission, where 2-3 are used for 'normal' play and 1 is used for pushing.

Long ago, when motor power was significantly limited, shifting up sequentially would improve performance a lot (compared to throwing more power at it, which is common now). When the 33 4-speed was designed, this was the case.

Using dog and mesh shifting methods, it's easier to combine 2 2-speed shifters with a different spread in series than it is to build a native 3 speed (with ball shifting a 3 speed is trivial to design, as you just add 1 more of the shifted gear sets). However, this results in a shift combination of L-L, L-H, H-L, H-H, where the 2<->3 shift is a 'double swap' shift. Automotive transmission engineers know that a double swap shift (more than one apply and release element) is a bad thing, and is extremely hard to synchronize properly. This can lead to temporarily being in a different gear (e.g. if shifting from 2-3 you can end up in 1 momentarily) which is really bad. So the answer then is to jump-shift, going 1-2-4 or 1-3-4. Then you get into some rather cool software, and a lot of design work to optimize all of the gears to get the performance benefit you are looking for. Definitely a cool project if you want an engineering challenge, but probably not worth it for an FRC robot.

So, basically, you go back and question why you wanted 3 speeds instead of 2 (or 1).


Now for some insight on automatic transmissions in cars. Modern automatic transmissions usually fall into 2 categories: Planetary wet clutch and automated manual/dry clutch.

Automated manuals are basically a series of dogs (or synchronizers, which are more complex dogs) each with 2 'gears' and capable of shifting between 1, 2, or neutral. With many of these in parallel, any individual gear can be selected (1 at a time). The input is also coupled to the engine via a clutch, which can be released during a shift. Dual clutch transmissions have two of these pathways with separate clutches, and can 'preselect' a different gear on the other pathway then engage it by controlling the release of one clutch and the apply of the other.

Planetary wet clutch transmissions are basically a combination of planetary gearsets and clutches that either lockup (lock a segment to 'ground'), input (lock a segment to the engine/motor), or connect 2 segments together. Virtually always, this is done with wet multiplate clutches, which can be slipped to smooth the shift. If you want to look up some cool gear layouts, look for Simpson or Ravigneaux type compound planetaries, those are basic 3+R and 4+R designs.

[Matt C]

Originally Posted by Jeffy

Side note: I believe that 217 did a CVT in 2002. That may be of interest.

Team 190 also had a CVT in 2002, though we only drove about 20 foot at the start of a match before we parked it for the remainder.

[asid61]

Originally Posted by Jeffy

I really do think that with enough work at it, this could be a better solution than a single or two-speed drive. The real problem that any team faces is shifting speed vs. the actual advantage gained from a new speed.

Side note: I believe that 217 did a CVT in 2002. That may be of interest.

Their "CVT" is actually an IVT. Most CVT and IVT designs are useless for FRC because their sole purpose is output speed, not torque. 217 is an example of a constant torque drive. FRC uses electric motors, so we don't need an IVT/ constant torque drive.

[EricH]

Originally Posted by Maxwell777

Was it ever built/used? Is there a video of it?

To answer the first question, yes, I do believe it was built and used in 2004. (And apparently rather effectively.

As for the video... Zondag, IKE, Palardy, you guys know of any? I'm pretty sure most of the usual sources wouldn't have any--video didn't become a really big thing for a few years after that.

[JesseK]

Originally Posted by apalrd

Now for some insight on automatic transmissions in cars. Modern automatic transmissions usually fall into 2 categories: Planetary wet clutch and automated manual/dry clutch.

Mind = blown.

Seriously geeking out here. Thanks for the explanation and insight!

[cj3958]

I think one aspect of the 4 speed transmission that people are leaving out is that if you have 4 speeds, your motor will always be running under either peak efficiency or peak power output depending on what you want to do(for this idea, I am assuming that automatic shifting was developed). If the CIM was always running at peak power, then you could essentially have more watt's pushing than the opposing team, assuming you both have 4 cim drive-trains and that the opposing team has a 2 speed transmission, because your motors would be putting more of the electrical energy into pushing power, and more of the opposing team's electrical energy went into making the motor hot, you would have more watts of mechanical energy pushing on the ground which is what you really care about, not how many watts the motor says it has. I am not saying it necessarily justifies the resources needed to create said gearbox, but I just thought it was worth putting out. I am just a student on my team however, so correct me if I am mistaken in my logic or whatever this is.

[asid61]

Originally Posted by cj3958

I think one aspect of the 4 speed transmission that people are leaving out is that if you have 4 speeds, your motor will always be running under either peak efficiency or peak power output depending on what you want to do(for this idea, I am assuming that automatic shifting was developed). If the CIM was always running at peak power, then you could essentially have more watt's pushing than the opposing team, assuming you both have 4 cim drive-trains and that the opposing team has a 2 speed transmission, because your motors would be putting more of the electrical energy into pushing power, and more of the opposing team's electrical energy went into making the motor hot, you would have more watts of mechanical energy pushing on the ground which is what you really care about, not how many watts the motor says it has. I am not saying it necessarily justifies the resources needed to create said gearbox, but I just thought it was worth putting out. I am just a student on my team however, so correct me if I am mistaken in my logic or whatever this is.

There's no point in pushing the motors with 4 speeds instead of two, because although you can get the motors to operate at peak efficiency that means nothing due to the low match times. Plus, at a low speeds (~<8fps) drives become limited by their traction, not by their power.
Efficiency is an interesting point, but I'm not sure it would help enough to justify the extra weight and double-shift problems.

[anomalynerd]

I actually designed a four speed design transmission that my team (3250) used this year. We used a 3 stage 2 speed vex gearbox to start, and added a custom gearbox to overdrive or underdrive the gear reduction. We used carbon fiber and Kevlar composite panels to house the whole drive train, which was designed to drop out of the bottom of the frame with just a few bolts. The gear reductions used are 5:1, 10:1, 20:1, and 40:1 and we used pneumatic ball shifters to shift between gears.
Here's a few pictures:

http://http://www.chiefdelphi.com/media/photos/40581

http://http://www.chiefdelphi.com/media/photos/40582

http://http://www.chiefdelphi.com/media/photos/40579

http://http://www.chiefdelphi.com/media/photos/40580