[Matt Adams]

Originally Posted by Rickertsen2

I avn't read the whitepaper listed above, it might say what i'm about to say but anyway... Don't match the moters at free speed. I have seen many teams do this. This is the quickest way i know to mismatch them. Match them under some typical load. Further matching can be accomplished through software.

From the best of my understanding, nothing is wrong with matching at free speeds. If you don't, at higher RPMs you will have back EMF occur, reducing efficiency if you have one motor trying to go faster than the other's free speed.

Take a look at the following figure for a Drill in low gear and Atwood coupled together at the drill's free speed:




Let's say that you have 200 in-lbs of torque applied to the shaft. In this case, the motors coupled at free speed will spin at about 225 RPM. The atwood motor will be contributing more torque to the system than the drill. I do not see a problem with this. I admit that there's uneven load sharing between the motors... but there is nothing fundamentally wrong with this from a mechanics standpoint.

I think that some people believe that if motors aren't sharing equal load, then there's a "problem" or the motors are "fighting" each other. This is not the case, and isn't "bad." You don't have an even current draw from each motor.. but your motors aren't being ruined, and your efficiency is constant. They're pretty happy. One is working harder, but they don't get jealous of each other! The only time problems arise is if there's a situation with little torque, and they're coupled at a non-free speed. One motor (higher free speed) says, "Woo hoo! let's go really really fast!" and the other says, "But my max speed is slower than yours. I'll try to do what I can to keep up, but we won't be able to go as fast as you combined!"

Now with the above illustration.. don't think of motors as people rowing a boat, where if one is rowing harder the boat will turn. Think of the motors as a bicycle with two riders. If one works harder, that's fine. But if someone's legs just can't physically move fast enough, it's bad news!

Now, you could optimize your motors such that they're intersecting at a low torque that your robot could never achieve due to friction, slightly below the free speed. This could help have the amount of torque (and current) distributed between the motors be more even. But having the load sharing imperfect is not a "bad" thing if you're careful about analyzing the situation to make sure the load isn't so uneven that a breaker on one motor will trip before the other can effectively contribute.

Let's take a look at this:




NOTICE: As you can see, the free speed of a combined motors in low torque conditions (to the left of their intersection point) I made to be the geometric mean of the two speeds. This is not completely correct, (neither is the fact that these show all linear relationships between torque and RPM, this is simplified). There are coefficients that determines the ratio between the two motors free speed which sets what the RPM the motors will have at low torque values. However, the RPM in low torque conditions is NOT the fastest motor's speed, nor is it the slowest. The reason for this is unique to DC motors, and refers to the back-EMF, which occurs when a DC motor is spinning faster above it's free speed. You run this risk of this occuring if you do not match them at free speed.

However, will you ever have motors truly reach free speed in a robot? Nope. So can you adjust the ratios by a little so that they're share the toque more evenly? Of course. Even matching free speeds is impossible since the free speed varies from motor to motor of the same type.

However, keep in mind that if you have low torque conditions that yield RPMs above the slower motor's free speed, you loose efficiency.

And this is going to bring up one heck of a hail storm... I know it.

Please, let's keep this at a technical level, and be as specific if you can. Disagree with facts. Graphs help out. Hack away at mine if you wish. Just keep it legit and not hunches. We'll settle this.


Input welcome, could a EE out there explain back EMF for me a little better?
Matt