If I go on 1/4 of the Stall Torque 4.4 in-lbs, which is 1.1 in-lbs. And my full extension of the arm is 56". 36" from the initial pivot to the second pivot which has a 1’6" extension that is the manipulator. So throw out 56" if it is FULLY Extended, the entire design weighs 10lbs, (I am going to build it so it weighs 7lbs) but let’s give my self some breathing space. That’ll give me 560in-lbs. So I’ll need a minimum of a 560:1 Gear Ratio. Going on I am running the RS550 Motor on 1/4 loss in rpms. So if I put a Banebots 132:1 Gearbox on it, it will bring me down to 113.64RPM, then with an additional 6:1 or 5.4:1 or 4.8:1 reduction I will have my Ratio from 791.97:1 (18.94RPM) or 719.93:1 (21.04RPM) or 633.45:1 (633.45RPM).
I spoke to a Tech at Banebots this morning discussing efficiency with their gearboxes, they said using a 256:1 Gearbox would work, but also recommended that I stay lower in 104:1 to 132:1 Gearbox Ratios for better results.
I think it might work. As mentioned, it will probably be able to be backdriven, meaning the arm will fall down unless you keep providing power to the motor…but the motor doesn’t like to sit in one place with significant power applied to it (the armature has little thermal mass and doesn’t have any cooling when the motor is not turning).
The motor should be able to move the arm OK, without overloading the motor.
And if you make a PID loop that does not have enough damping, it’ll be running up and down all the time…
I’m going to suggest to our team that we put the modern control system on our 2007 robot, so they can learn how to program the arm.
Be very careful in how you use a 256:1 Banebots. They are very easy to destroy. Even with a 550 motor, you get LOTS OF TORQUE at the output, not to mention what you get with a 775 or Fisher Price motor.
For more anecdotal 256:1, we used them for this actuation. There were two of them (one powered the “stick” that pushes it up, the 2nd rotated it forward to lock it out. One of them was cantilevered, the other was not. The cantilevered one failed at 6:30 PM on Thursday, so we luckily got a chance to fix it without any intervening matches. We un-cantilevered the output, and it did not fail for the rest of the tournament. We never needed to replace the one which was not cantilevered to begin with. There were probably 50 cycles on the one we replaced, and 100+ on the one that was not replaced.
There was a lot of torque in that set up. We wouldn’t do it again (apparently our lead mentors were betting in the stands on whether it would fail every single lift), but the BB did hold up. We must’ve used the 550 and 540, because both FPs were in the toughbox on top of the arm.
We tried using one with a Fisher-Price for our arm rotation in 2007, and destroyed it instantly. In retrospect, we should have known better, given all the power of the F/P and moment of inertia of the arm.
If used properly, and with less powerful motors than the F/P, the 256:1 planetaries are fine. Also, I think the current ones are stronger than the ones we broke in '07.
