# Planetary systems.

Okay I’m a senior at Dunwoody high school in Georgia. We pride ourselves in doing our best when it comes to engineering and i personally fell in love two years ago when i played round up. However despite me and my “colleagues” racking our brains we simply can not seem to figure out how and why planetary systems work. I’m not doubting they do i was just hoping i could get someone to shed some light on the matter.

This explains the general concept quite nicely: http://auto.howstuffworks.com/automatic-transmission3.htm

So how do all the components work together? I understand that by one set not turning that means youll get a different gear ratio. But how do you switch back and forth from these?

I didn’t understand them all that well at first, but if you just throw one together that you can mess around with, it makes a lot more sense!

In the systems that have been popping up recently, the planet carrier is powered by a chain connected to the sprockets. You control the transmission based on how you run this chain.What specifically are you confused about?

Im confused at how it changes between the different ratios with only one motor powering it.

It doesn’t have one motor powering it. It has two motors. It will change the ratio based on if the motors are turning in the same or opposite directions.

So it requires eight motors to power your base assuming you use all planetaries or can you connect the systems and just use 6?

You can use 4 if you only power the back two wheels.

False, there are chain and gears that you can use to derive power around the drive base.

One motor powers the ring gear (the chain) and the other motor powers the planet carrier (the 60t gear). By switching the directions of the motors, the output (the sun gear) speed changes because the output speed is a factor of both the ring gear and the planet carrier speeds.

In addition, you only need two motors [Edit: per side] if you are powering either only the 2 back wheels, or are chaining each side together. If you are using mecanum wheels, the minimum number of motors you can use while maintaining strafing capabilities is 6 (as presented in my diagram(s) in the ‘Mechanisms’ gallery).

Just to emphasize this: you need 2 motors per planetary, but you can use one planetary for all the wheels on one side of the robot (so 4 motors for a planetary drive base). The 6 motors for the Mecanum drive gives you two speeds forwards and backwards, but only one speed strafing. If you want 2 speeds forwards, backwards, and strafing, you need 8 motors (or two pneumatic cylinders and 4 motors).

I think strafing is possible at two different speeds with only six motors.

There are 8 different combinations of things one side will need to be able to do (forward speed, forward torque, backward speed, backward torque, left speed, left torque, right speed, and right torque). You only need 3 motors on one side to have 8 different combinations of directions that the motors can turn.

Theoretically, yes. But the problem is with how the third motor would interact with the planetary gear sets. If your front and back motors are running in opposite directions, as if to strafe, then your third motor has to not be rotating, or else the front will be in high-speed and the rear will be in high-torque (or the other way around). This stems from the front and the back going in the same direction versus the different direction and the fact that the third motor can only do one (same or different). Here’s your two missing outcomes of the 3-motor-input system:
If the front and the back are running in opposite directions, then the third motor delivers opposite power to the front and the back. If the front and the back are running in the same direction, then the third motor delivers the same power to the front and the back.
If you make a contraption that does that, congrats. As for me, I would use a single pneumatic cylinder to switch between high-speed and high-torque on my Mecanum drive.

Hopefully that makes sense.

There are two types of systems being discussed here.

1. 1-input, 1-output planetary, in which the sun-gear is the first input and the ring-gear is either locked to the carrier or is locked to the robot. This transmission shifts between a 1:1 gear ratio (locked to carrier) or a high torque gear ratio (locked to robot). This shifting is traditionally done with a pneumatic (so it doesn’t use up another motor).

2. 2-input, 1-output planetary, in which the sun-gear is the first input and the ring-gear is the second input. This transmission varies the output speed using the 2nd motor, but only 1 motor worth of torque is transferred to the output.

If you use type two, you are essentially using your second motor to shift, not using it to drive. You are never going to harness the torque of the 2nd motor at the output. It doesn’t work that way. This is the common misconception – many people believe they have 2-motors powering the output, that isn’t quite true. You have 1 motor powering the output, and 1 motor adjusting the speed.

If you build a type 1, and put 2 motors powering the sun gear – NOW you have 2 motors going through a shifting transmission, and will get double torque in low speed.

-John

by the way… they can go in more directions than just same and opposite. If you stop either one of the motors they will have different ratios…

Matt
4886a for the sack attack season
4886b for the gateway season
4886a for the round up season

I have a couple students building a planetary drive base before school lets out, mostly to just to build it and see how it works, not really any different then say having students build a holonomic drive base to get a better understanding of it. At this point I don’t really expect to have one in one of our competition bots for the upcoming season.