05-09-2003 23:02

Trashed20

neat. i'm still amazed at what people can do with legos these days. The one thing i would say is try to get it as small as possible so it would be feasable in an actual robot. Cool nonetheless.

06-09-2003 01:11

Kevin A

So howd it work?

06-09-2003 11:37

Tristan Lall

Tytus, do you have the rpm and power on hand of each of those motors (i.e., how similar are they)? I'm curious as to whether the differential was actually working as anything but a fancy shaft (if the speeds were identical, the housing of the differential would rotate, but the bevels inside wouldn't turn--hence, a shaft).
If the differential were matching speeds, then you might be at risk of stalling one or the other (as Bill Beatty suggested in the other thread), depending on the conditions of the test (e.g. relative power and relative speed of the motors--speed would have to be different, and power similar). You should also note that these are small brushless motors. Stalling these is no problem, but on the FIRST robots, the motors are not nearly so pleasant when they are stalled.
In short, while it may work now, it probably isn't an accurate enough simulation of a Bosch 3360 and a CIM FR801 (like you admitted...).
-Tristan Lall

06-09-2003 12:30

sanddrag

The little red one has a little more torque than the big grey one but the big grey one is a lot faster.

06-09-2003 15:38

patrickrd

What is a differential?

I have been occasionally reading the posts over the past couple months on differentials, and then saw this one made out of legos. Can anyone provide a link or good technical explanation of what one is and how it works? These pictures do not do it for me.

Thanks, and nice lego building skills
- Patrick

06-09-2003 16:16

Anthony Towne

http://auto.howstuffworks.com/differential.htm Howstuffworks.com has pretty good explanations and pictures that explain how a differential works, among a number of other things. Hope this helps.

06-09-2003 16:46

rlowerr_1

For those interested this link has comparisons of all the current lego motors. The red motor (dubbed the minimotor by LEGO fans) offers very little speed and torque.

06-09-2003 17:32

P.J. Baker

Not sure...

Based on the characteristics for those LEGO motors, I'm not sure that your experiment should have worked out the way you think it did.

The larger motor has about 4 times the stall torque of the smaller motor. If this is the case, then the larger motor should have been able to back-drive the smaller one when you stalled the output shaft.

In a standard open differential, the same torque is always put out to each side, regardless of the output speed. This is why a car with an open differential rear end will get stuck even if only one of it's driven wheels is in a mud puddle or on a patch of ice. The proposed reverse differential drive should ensure that both inputs sides are supplying the same amount of torque - regardless of speed.

But... If the two motors can not supply the same stall torque and the output shaft is stalled, the torquier motor must back-drive the other motor. This will increase the back EMF of the stronger motor, reducing its torque output. It will also (I think) create negative back EMF on the weaker motor, INCREASING its output torque.

With one motor having 4X the stall torque of the other in your LEGO design, I think that you should have seen back driving of the smaller motor. There are two things that could have kept this from occurring (assuming the transmission was correctly assembled). First is that the battery you used could not supply enough current to the motors when you stalled them out. Second is that there appears to be some thermal protection for the gray motor, a thermistor that would cut the power when the current gets too high.

Since it looks like the red motor doesn't have the thermal protection, I would repeat the experiment with two red motors, but gear one of them down 2:1 before running it into the diff. Also make sure that you have new batteries, one for each motor.

In closing, I'll repeat what I said in one of the other threads about this transmission. If you gear the motors so that their stall torques (not their free speeds) are matched prior to entering the differential, this drive should work. I think that this set up is more electrically efficient that a matched speed (locked differential) drive - but I'm not sure that it makes sense in terms of bang for your buck. That's not necessarily what FIRST is all about though, so build it if you can.

Sorry this was so long.

Good Luck!,

P.J.

06-09-2003 17:54

Adam Y.

Everything I have read so far seems correct except for the part about the motors. Negative back EMF is created when the motors are spinning at full speed and barely showing little or no torque. The one thing I think will happen is that the little motor will actually become a generator and may actually blow the electronics controlling it. I remeber someone mentioning this.

06-09-2003 19:31

sanddrag

I thought the red minimotor was more torquey by my own "measurements" by playing around with them. I suppose I was wrong.

I wonder if a Torsen Differential would work work with this setup?

07-09-2003 20:59

patrickrd

Thanks! The article explained it perfectly. Now I have to try and do my microeconomics homework while my brain wants to think about differentials

- Patrick

08-09-2003 13:44

Jnadke

For all readers, it must be noted that lego concepts can only prove feasibility and not any physical concepts. In the case of this test, these motors are only generating mere milliNewtons of torque. Simple gear friction would be enough from causing one motor to be back-driven.

It must be noted that motors would only fight if their torque characteristics were too dissimilar. Even then, they would only fight if one motor was operating at or near stall.