RS550 Enough Power?

[Alienvin]

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?

[Ether]

Originally Posted by Alienvin

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?

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.

[Lil' Lavery]

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.

[Alienvin]

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

[MrForbes]

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.

[Ether]

Originally Posted by Alienvin

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

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

[MrForbes]

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.

[AdamHeard]

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.

[Alienvin]

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?

[AdamHeard]

Originally Posted by Alienvin

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?

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.

[MrForbes]

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!

[StevenB]

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

[sanddrag]

Originally Posted by squirrel

(figure up to 50% torque loss to friction with a 3 stage planetary)

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.

[dtengineering]

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

[Eigenvector]

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.