Chief Delphi - Joining 2 dif. motors

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Re: Joining 2 dif. motors

Quote:

Originally Posted by Matt Adams

In general, I've found it much more efficient to combine a pair of motors, and then run this combined set out via some sprockets to the wheels. This ensures that one wheel won't ever try to run a bit faster than the others. If you try to run them individually, your motor torque and speed outputs vary as the speed of your robot changes. Fortunately, the Chips and Drills happen to have roughly the same trip points when matched at free speed, which is why I'd imagine a lot of people don't notice it.

Matt

What we did this year was to link the drills with out gearboxes to a main shaft from the Cims, This was done at 1:3.3333 (a 9 tooth on the drill and a 30 on the Shaft connected to the Cim) This was then reduced twice 1:4.5 to the 14 inch wheels. This gave us 1:67.5 if you start at the drill motor. Checking the current with wheels up and runnning full tilt, we had almost the same current at the Cims as the drills. Also we installed the drill motors facing the same way, the Cims oposide and as luck would have it the drills were in reverse, with forward for the bot being backward for the drill motors. This was left this way to help keep the bot from doing wheelies so easly and to make towing the mobile goal backward the most powerfull. We never had the weight left over to make a decated holder for towing so it didn't happen very often. With the exception of the time our arm was extended and dropped into the moble goal. One of the drills would run a little hoter than the other, but I suspect this was due to being dropped duriong he build and having a little more friction on that side do to chain. It may also be do to the not having room to mount the Cims facing the same way.

Re: Some Comprehensive Theory

I noted in your PPT workup that you mention back EMF. I have tried to get this concept across to several teams that this problem means that one motor is actually reducing the power available to drive the robot since that translates to a "braking" effect on the power train. I have not measured this effect but what does it do to the drivers on the motor controllers. These devices are current sources and not current sinks unless I have my theory wrong. Anyway thanks for the great work up. I just need more time to translate this for my students. LRU

Quote:

Originally Posted by Matt Adams

Oh boy.. do I have something for you.

As part of the FIRST college course that I taught this semester, I have some rather comprehensize data on combining both the chips and drills... from a number crunching standpoint, it's very complete.

There aren't any sprocket sizes, CAD drawings, or the like.. but theory wise.. this is (fundamentally) very strong material. I will be publishing a white paper along these lines soon on www.boilerinvasion.org.

Here's the PowerPoint. If you have questions, please ask. If someone has corrections, comments, or concerns, please ask.

http://web.ics.purdue.edu/~mjadams/motorsinfirst.ppt

Matt

Re: Some Comprehensive Theory

Quote:

Originally Posted by lupjohn

I don't quite follow what you meant in your original post but I'll try to respond as best I can. I'm not quite sure what you mean by the one motor reducing power available to the power train.

The back EMF generated by a motor is caused by the spinning magnet field of the motor. The back EMF is proporitional to the rotational velocity of the motor. The higher the back EMF, the less current that the motor is drawing.

I think what you're worried about for the speed controllers is driving a current back into them. The speed controllers which are based around an H-bridge basically just use transistor switches. They do not use transistors as current sources. When a current is driven back through them, they will pass it much like they'd pass a current the other direction (particularly since they're based on FETs). This just means that the current is going back to the battery. There's no problem with sourcing current into the speed controllers.

Matt