Question about motor sharing

I’ve seen many people talking on here about sharing motors (for example, between the drivetrain and a tilter)
I just don’t see how this is possible to do without either the tray tilting constantly or using another motor to toggle which is being used
Can someone please explain how motor sharing systems work and possibly show me an example?

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you don’t directly link the motor to both subsystems, otherwise you would have to run both at the same time. what you do is use a transmission, or a ratchet. ratchets are used for 2 mechanisms that only need to move in one direction. (not very useful this season). transmissions require that you use 2 or more motors, and allow you to have those 2 motors run separate subsystems, as long as you don’t run them at the same time.

tilter to chassis transmissions are probably the most common so far, it involves using a 4 bar transmission mechanism to link your tilter and your chassis. this allows you to have a 4 motor drive, and have a 4 motor tilter, only using up 4 motors.

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can you show me what the mechanism would look like? how do you switch between the two systems without a dedicated motor?

take a look at this video:


when both motors move the same direction the 4 bar moves, but when the motors move in opposite directions the gears rotate. you’d link the gears to your drive, and the 4 bar to your tilter.

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really smart mechanism!
not sure exactly how i’d translate the 4bar motion into an actual tilt, but thanks for showing me the concept!

you could extend the end of the 4 bar, and use that to push your tray forward.

oh yeah
you could just run a bar all the way across and treat it like a normal tilt arm

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image

Here’s our old prototype to give you a general idea. This one has its flaws so we’ve since rebuilt, but it’s fine for demonstrating the concept.

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what problems did you have with it? looks pretty solid to me

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Our range of motion before hitting the wheels was about 50 degrees which was too close to the minimum of 45 degrees. With 45 degrees of motion (saving 2.5 on each side for a hard stop before the wheels) you can move the tray as far down as you need but the geometry will be terrible. The larger the range of motion, the more leverage the tilter has.

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why not just make the 4 bars longer?

Well yea but increasing the length also hurts leverage. The first bar needs to be as small as possible. Adding idlers also adds friction. Another opinion is moving the motors inside.

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you can probably afford to sacrifice a bit of leverage if you’ve got 4 motors for your tilter

4 1:1 ratio motors might be a bit tough tho

It’s 4 7:1 motors so we’ve got twice the required torque. Your right we don’t need optimal geometry, but deposit is already 7:1 200 rpm so geometry for speed isn’t necessary. It just made sense to move them inside.

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do they fit going across the inside?

Moving things around yea they fit in 25 nice and snug.

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It is almost required by law to add this disclaimer to any motor sharing thread:

When building ratchets and differentials it is very easy to introduce extra friction losses with every additional axle or 4-bar frame that could twist out of the alignment.

I’ve seen some really well built robots doing motor sharing, but it took months to get it right.

On the other hand, I’ve seen some ratchets and differentials that were painful to look at and think how much extra friction they introduced. The teams would have been much better off just doing non motor sharing designs and spending extra time on practice.

I don’t want to discourage anyone from doing motor sharing. My team is doing that third year in a row. I just want to add a word of caution that it takes considerably more extra time to get it right. Please, see these topics for more info: Passive transmission, Differential Extra Motor, Differential transmission - power takeoff from the drivetrain motors

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