RS550 Enough Power?
 

So.... Haven't tested it yet, but what do you think about using a Banebots RS550 Motor along with one of their Gearboxes with the right gear configuration to move and arm up and down?

Re: RS550 Enough Power?
 

How heavy is the arm, and how fast do you want to move it?

Have a mechanical mentor help you calculate the amount of work (Nm) required to lift the arm, and divide that by how quickly (how many seconds) this task has to be completed. That will give you watts (power).

Compare that to the power rating of the RS550.

Re: RS550 Enough Power?
 

Theoretically*, any motor can move any object. The amount of time it takes to move it is what varies depending on the power output of the motor. The question you should be asking is "how fast do I need my arm to move and can the 550 provide enough power to move it that fast?"

*In reality, eventually you're going to hit limits, but that's a different discussion for a different time.

Re: RS550 Enough Power?
 

Off hand, would you have the equations for calculating Nm?

Re: RS550 Enough Power?
 

We used a little 540 type Banebots motor and 125:1 transmission and 8:1 ratio chain drive to move an arm up and down for RacknRoll in 2007. Worked fine, lasted all season. But we cheated and used a gas spring to help support the arm, so the motor didn't have to work hard. I recall it took less than two seconds to raise the arm.

Re: RS550 Enough Power?
 

Does your team not have a mechanical mentor?

If not, post a drawing of the arm in question, with dimensions, and someone here may be willing to help you understand how to do the calculation.

If you want just a very rough ballpark calculation, you could do the following:

- estimate the weight of the arm

- estimate where the center of mass of the arm will be in the lowered position

- estimate where the center of mass of the arm will be in the raised position

- take the vertical difference between the two center-of-mass positions and multiply it by the weight

Re: RS550 Enough Power?
 

Quick explanation, using English units (because I'm old) and using flakey terminology for the same reason:

You have an arm that extends out 60" from it's pivot point. The arm and manipulator on the end of it weigh 10 lbs. The center of mass of the arm/manipulator is 40" out from the pivot point. You can figure out the "torque" that it takes to hold up the arm by multiplying the weight by the distance. In this case you have 400 inch pounds required to support the arm.

From the "motor curves" data here:

http://usfirst.org/roboticsprograms/....aspx?id=18530

The stall torque of the motor is 70 ounce inches. That's 70/16= 4.4 inch pounds. But you do not want to run the motor at stall torque, you want to run it at about 20% to 40% most of the time. So you have about 1 inch pound of torque available. That means you need to get about 400:1 reduction to make the motor lift the arm safely (without burning up the motor).

Gear reduction, or chain reduction, or pulleys, or however you do it, does two things: it increases the torque, and it reduces the RPM. But it is not "free", you'll get considerable loss from the friction of the gears in the transmission, or whatever you're using. So you may need to actually double the amount of torque you think you need! just beware, and design on the safe side, or you'll burn up the motor.

Also, the free rpm of the motor (not loaded) is 19,000 rpm. You will get less than that when it's loaded, the speed is kind of inversely proportional to the load. So at 1/4 load, you'll get 3/4 speed roughly. That means you'll be spinning the motor at 15,000 rpm. Now that means that with your 400:1 reduction, you'll be moving the arm at 15,000/400 = 37.5 rpm, roughly 1/2 rev/second. You need to move the arm about 1/4 revolution, so it will do that in about 1/4 second. You might want to use more gear reduction, to get the speed down to where it's more controllable--perhaps have the arm take 1 or 2 seconds to lift fully.

You can also add a gas spring, or counterweight, or make the arm lighter, to reduce the load on the motor, if your arm is too heavy.

Re: RS550 Enough Power?
 

I would recommend the 775 in place of the 550 if you're unsure of what you're doing. Similar power levels, but the 775 has twice the mass, and should be harder to burn up.

Re: RS550 Enough Power?
 

So Jim,

I'm looking at the RS-550, 19300 RPM Free Speed with a Banebots Gearbox with a Gear Ratio of 256:1, then maybe a 10T off the Banebot Gearbox to either a 48T, 54T, or 60T Sprocket attached to a 3' Arm Extension that at the end of it has a 1'6" arm extension that to has a gearbox setup on the end of it. All in all I would like to keep the entire weight to the arm under 7lbs.... What you think?

Re: RS550 Enough Power?
 

To play it safe, you'll want to gear such that at max load, the torque on the motor is only 20-25% of the stall torque, some people prefer even less.

Re: RS550 Enough Power?
 

Might work...but I didn't crunch the numbers! Sounds reasonable.

As Adam mentioned, play it safe. If that reduction gives you 25% or less motor loading, and you have factored in the losses in the transmission (figure up to 50% torque loss to friction with a 3 stage planetary), then it should be fine. But also keep in mind that if the arm can travel to it's stop and the motor is still powered, the motor will smoke. You need to make sure the control system has limit switches, etc. or build in some type of "clutch". We had a clutch type system on our 2009 robot on a part that was driven by a small BB motor and gearbox--it powered a V belt for the last reduction stage, and the belt would slip when it reached the end of travel. We had no problems with that little BB motor either!

Re: RS550 Enough Power?
 

I've started writing a document explaining exactly this kind of stuff. It's a work in progress, but it has a fairly detailed explanation for the kinds of questions you're asking.
http://www.botsnlinux.net/firstphysics.php

Re: RS550 Enough Power?
 

Really? You're figuring 80% efficiency per stage? Is that a reasonable assumption? It's been a while since I've played with planetaries. I wonder if anyone has done any actual data gathering on efficiency of the BaneBots planetary gearboxes. It sure would be nice if the efficiency was a little higher than 80% per stage. I'm not thrilled about the loss through a 4-stage planetary.

Anyhow, thanks for getting us back in check Jim. Often times those inefficiencies go unchecked, and can make a big difference in the final design.

Re: RS550 Enough Power?
 

Remember that electric motors don't like to be "stalled" (ie powered up at zero RPM). This is the condition that your arm or lift could be in if you want to use the motor to maintain the position of your arm. That is why I've always liked using worm drive motors like the window motors or the (long since departed from the KOP, but much loved) van door motors for these applications.

If you are going to be using your motor in a stalled condition, test it out and pay careful attention to the heat it produces. If it is getting too hot condsider a heat sink or adding a small fan (just zap tie it on) to get some air movement going.

Jason

Re: RS550 Enough Power?
 

The RS550 might have enough power, depending on how long your arm is, but remember to watch for stall. If you look at the motor curves here, you can get a better feel for if it's suitable for your arm. I'd recommend going with the window motor though, they've got much higher torque.

Re: RS550 Enough Power?
 

And MUCH MUCH less power....

Re: RS550 Enough Power?
 

It's power that counts, not torque. You can always get more torque by changing the gearing.

The RS550 has 10 times the max output power of the window motor.

Re: RS550 Enough Power?
 

Oh cool! Did you ever strip a stage in the planetary or fry a 540? Any idea about how much your arm weighed?

Re: RS550 Enough Power?
 

I bought several spare motors and a couple spare gearboxes...that was money wasted, as we never had any problems with it at all. I don't remember what the weight was, but with the gas spring assist, which mostly prevented back driving the motor, it takes very little effort to lift the arm. Maybe a couple pounds a foot from the pivot.

Re: RS550 Enough Power?
 

I've go to agree with Jason ( dtengeneering). We too like worm drives--especially for lifting. You don't have to worry about backwalking, hence, no maintaining current to maintain height. Pairing up the windows motors with the correct sprocket ratios worked well for us in '07. We're using a much, I mean MUCH, heavier arm this year so we'll use gas springs too.

Re: RS550 Enough Power?
 

Good Afternoon Jim,

Let's see how my math works out here.

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.

What do you think Jim?

Vinny

Re: RS550 Enough Power?
 

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.

Re: RS550 Enough Power?
 

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.

Re: RS550 Enough Power?
 

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.

Just another data point. :)

Re: RS550 Enough Power?
 

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.