# Differential transmission - Mobile Goal to Drivetrain (DMD)

There were recently several posts by @antichamber showcasing a design concept of sharing motors between MoGo Lift and the drivetreain: V1, V2

He calls it “Passive Mobile Goal Lift” and some people jokingly refer to it as “Passive drive.” Both of those names are, obviously, misleading since both functions are very much active and it just happens that they use the same set of motors. Unlike my previous attempts to use non-shifting differentials, this design is much simpler to understand and build. Here is another version of this idea:

Depending if two motors (M2 and M3) run in the same or opposite direction you have power flowing to either one or another function with each of them getting almost the full power of both motors (minus the friction losses in extra gears).

Outside of the robotics competitions with strict motor rules you will rarely see this type of motor setup. If you have multiple functions you just spec out appropriate motor for each of them and keep them entirely separate for better reliability. It is only here with limited motors you could gain an advantage by mixing up the subsystems to get some fractional motor power here and there.

So, if this is so simple to build, should everyone do it?

Lets take a look at this example: you have a robot with 6 motor drive and a single-motor MoGo lift. You decided to rebuild it as 7 motor drive with two of the motors shared for MoGo lift. The lift gets almost twice the power and now you can lift twice as fast.

The drive, on the other hand, only gets ~15% input power boost and, depending how much friction losses you introduce by adding all those gears, you could have output power gain anywhere between +15% and net negative. If your build quality is poor and you end up losing 20% to friction it, probably, doesn’t make much sense. However, if you could limit your losses to 5%, you will end up with extra 10% power for the drivetrain.

Everyone will not benefit from this design, but if you are one of those teams that strive for excellence and, by the end of the season would max out on all motors, then having extra 10% drivetrain power may let you compress your skill runs by 6 sec and score that last mobile goal. However, you will be paying for this more complex higher performing design with a lot of extra maintenance just to keep its reliability level on par.

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Here are the videos from @antichamber:

As you can see from the last video lift speed is clearly too high, but it sure could be improved.

The other issue I see beyond reliability is that you cannot use both the mobile goal intake and the drive at the same time. This will lose a little bit of time, or perhaps make movements a little less fluid.

I agree with the reliability concern and importance of tackling any additional friction, but once @antichamber adds a third motor to each drive side and programs motor power levels to account for robot acceleration - there should be no problem in driving at the same time as the lift operates.

Still more complication, and the drive will be slower, at least.

It is true that controlling such hybrid is not simple. However, it should have no problem running at top speed. You only need need full motor power when pushing other robots or accelerating. And if you are planning on carrying tall stack you will need to reduce sudden accelerations to prevent cones from falling.

I don’t disagree that this is the case where for 10% performance gain you may need to invest 50% more efforts. Teams who expect just to send joystick values to the motors will be disappointed, but those who choose to learn some advanced programming may find it rewarding and useful beyond this season’s game.

He only has it programmed to his joystick to make sure it worked, that’s not its permanent spot, On the discord he said “lmao that’s so janky why would I do that”

Here is a few more videos …

From @antichamber :

From @Joseph W(182 C) :

From @3-motors-is-all-you-need-department :

Depending on the gearing choices your mileage will vary.

I just found this thread, and I’m so glad that you guys are watching and enjoying my videos. Sorry for not being as active on the forums as I used to be.

Additionally, I will be exploring this concept further in my next robot, and I’ll be sure to add to this thread

Antichamber you are everyone’s fav on YT

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<3

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