Save the Gearbox, Save the Game

[MrForbes]

Originally Posted by Lil' Lavery

This is a great way to try and prevent your own robot from damaging your Banebots transmissions, but, as mentioned, it does not protect from outside forces, so be wary.

This is quite true. However, the difference in shock load imposed by an impact by another robot, vs. that imposed by a sudden direction reversal of a motor, seems to me to be about an order of magnitude different.

Let's say your robot is driving forward, and another robot hits yours head on. There is no direction reversal at the DD joint. Now let's say your robot is driving forward slowly, and it is hit from behind by another robot. There is a direction reversal at the DD joint, but also the impact at the DD joint is softened considerably by some things. These include the fact that the other robot will share it's momentum with yours, so that it must accelerate the mass of your robot, and this takes time and absorbs energy. Also the load will be distributed among more than one wheel/transmission, so it is not all concentrated at one point. And there is some give in the chain drive system, assuming you are using a chain drive.

In a sudden motor reversal situation, the inertia of what's connected to the transmission output shaft is higher than the inertia of the transmission gears/motor armature, and there is also a large torque multiplication and speed reduction supplied by the transmission. From the instant the motor changes direction, it can turn quite a ways at full torque as it takes up the slack in the transmission gears (and DD joint), and it can gain a considerable amount of rotational inertia before it finally slams the DD joint against the other side. From the pictures of damaged plates, it looks to me like this is what is causing the problem.


I really would like to see the results of some driving tests on DD joint wear with software motor acceleration control. I think it might be more helpful than some folks realize.

[TimCraig]

Originally Posted by squirrel

However, the difference in shock load imposed by an impact by another robot, vs. that imposed by a sudden direction reversal of a motor, seems to me to be about an order of magnitude different.

The more important issue in my mind is how the shock is applied. If the robot sustains a shock to the chassis, that shock is going to be applied pretty much uniformly to the transmission, assuming it's mounted solidly, shaft supported, etc. As I understand it, the failure that's being seen is a torsion problem. It's specifically failing at the joint where the shaft is attached to the planet gear carrier plate. Very much apples and oranges in making any comparisons.

[kaszeta]

Originally Posted by Abwehr

We put a low-pass filter on the motor speed command almost every year. A more compact way to accomplish this follows.

This is almost exactly the code our team has used as well. We try to err on the side of responsiveness, however, so after two competitions all the mechanisms have definitely picked up some slop.

[jskene]

We dismantled out gearboxes yesterday, after about 2 hours of use, and found very little distortion on the end plate.

Our acceleration-limiting software is doing its job.