what motor did you use(arm)

[dbell]

We made 2 arms this year:
our first one used:
globe - shoulder
globe - elbow
Nippon-Denso window - wrist

our second (better) arm used:
BaneBots 25:1 - Elevator
BaneBots 64:1 - Wrist
Nippon-Denso window - claw

[AdamHeard]

If there is something to be learned here it is that any motor in the kit (even the globe) can be used to power the arm.

I like the philosophy of 330 on arms though; keep them simple, and then gear them down A LOT so that they have so much torque you never have to worry about it. A mentor of theirs, Chris Husmann, gave a great presentation last fall on arms that I would love to get a copy of.

When we had to redo our arm after a regional this year, we took that advice and used a small CIM paired with an AM planetary for each joint (the "team 60" arm).

[lukevanoort]

Turret - Keyang with a sprocket attached, the chain wrapped around the turret base, which had about a 16" diameter.
Lift - Fisher Price using the plastic gearbox it comes with, modified for mounting and to accept a sprocket for the chain that drove the lift.
Shoulder - Globe geared down through a chain reduction, I can't remember the exact ratio, but it was something around 5:1.
Wrist - Another Globe with a chain reduction, this time more like 2:1.
Gripper - 3/4" pneumatic cylinder.

Overall the motor selections/reductions worked nicely, the wrist is a bit fast though.

[Doug G]

Originally Posted by AdamHeard

If there is something to be learned here it is that any motor in the kit (even the globe) can be used to power the arm.

I like the philosophy of 330 on arms though; keep them simple, and then gear them down A LOT so that they have so much torque you never have to worry about it. A mentor of theirs, Chris Husmann, gave a great presentation last fall on arms that I would love to get a copy of.

When we had to redo our arm after a regional this year, we took that advice and used a small CIM paired with an AM planetary for each joint (the "team 60" arm).

I can't emphasize this enough. Over design the power transmission on the arm. If calculations show that you need 30 ft-lbs of torque to lift the arm and object at the end of the arm, then design a motor/transmission combination that can supply that while drawing < 10 Amps! (somewhat dependant on the motor choice)

If you need 30 ft-lbs, then stall torque should be at least 120-150!! Also this way, you can throw in some feedback and PID will have lots of headroom for tuning and working correctly.

In the 7 years of doing FIRST ... In my opinion the ratio of "under-designed arms" to "over-designed (or simply well-designed) arms" is at least 25 to 1.

[Dan Petrovic]

Originally Posted by Doug G

I can't emphasize this enough. Over design the power transmission on the arm. If calculations show that you need 30 ft-lbs of torque to lift the arm and object at the end of the arm, then design a motor/transmission combination that can supply that while drawing < 10 Amps! (somewhat dependant on the motor choice)

If you need 30 ft-lbs, then stall torque should be at least 120-150!! Also this way, you can throw in some feedback and PID will have lots of headroom for tuning and working correctly.

In the 7 years of doing FIRST ... In my opinion the ratio of "under-designed arms" to "over-designed (or simply well-designed) arms" is at least 25 to 1.

We made the mistake of calculating the reduction required when the motor is being run with ~60 amps. These motors were only ran with 20 amps. So we ordered new Banebots transmissions and that set us back a whole week... (good thing we were doing pretty well schedule-wise until then ).

Well, that's what happens when you make mistakes: you learn from them.

[AdamHeard]

Originally Posted by Doug G

I can't emphasize this enough. Over design the power transmission on the arm. If calculations show that you need 30 ft-lbs of torque to lift the arm and object at the end of the arm, then design a motor/transmission combination that can supply that while drawing < 10 Amps! (somewhat dependant on the motor choice)

If you need 30 ft-lbs, then stall torque should be at least 120-150!! Also this way, you can throw in some feedback and PID will have lots of headroom for tuning and working correctly.

In the 7 years of doing FIRST ... In my opinion the ratio of "under-designed arms" to "over-designed (or simply well-designed) arms" is at least 25 to 1.

Exactly... definately a lesson learned there....

Another interesting tidbit I forgot to mention. Our original system w/ FPs wasn't geared low enough and was a pain to tune in PID because it was sooooo fast. The new system with the CIMs (not that the motors make a difference, there were other reasons for the change in motor type) was much slower and torqu-ier so we were able to tune a two jointed arm w/ straight P control in 15 minutes at a San Diego with even better accuracy the the original.

[ChrisH]

Originally Posted by AdamHeard

Exactly... definately a lesson learned there....

Another interesting tidbit I forgot to mention. Our original system w/ FPs wasn't geared low enough and was a pain to tune in PID because it was sooooo fast. The new system with the CIMs (not that the motors make a difference, there were other reasons for the change in motor type) was much slower and torqu-ier so we were able to tune a two jointed arm w/ straight P control in 15 minutes at a San Diego with even better accuracy the the original.

Power = force x distance / time

Think about the weight you are lifting. That's the force, The height you have to lift it to is the distance. Those are pretty straight forward.

What many teams forget is that it will take time to move from where you pick up an object to where you have to place it. Typically that time is 3-4 seconds. Give your arm operator something to do while the driver is moving from place to place. The joint of a single joint arm will typically move 90-120 degrees. Even if you want to pick up really fast, you can still design a joint that will take 9 seconds for a full rotation or 6.66 rpm. It will then take about 3 seconds to raise the arm to the correct height. The 05 and 07 BeachBot arms are running at more like 4 or 5 rpm and they are plenty fast enough. The arm almost always gets to "place" position before the rest of the robot.

Use a gear reduction such that your motor is running at 75% of free speed and your arm is moving at 4-6 rpm and you will get the most out of whichever motor you use.

The BeachBots have developed a very slick way of using the FP motors and a stock FP gearbox to power our arms. (actually I think we modified an idea we stole from Team 60) In 07 we started out using two FPs to power our arm as we did in 05. This set-up can and has lifted other robots silly enough to get entangled with our arm. But after a competition or two this year we decided we didn't need all that power in the arm as much as we needed the extra pound or two so we could have both ramps. So we removed one of the motors and its gearbox.

In 07, because the objects were so lightweight, virtually any motor in the kit could have driven an arm if it was properly geared.

As an example let's run the numbers:

We want to move a 0.5N object approximately 3 meters in 4 seconds. That will take 0.375 watts. The least powerful kit motor is the Mabuchi at 16W. The Mabuchi has a free speed of 4700 rpm. So you want it turning at 3500 rpm or 59 rps.

At this speed the output torque should be 25% of 61mNm or 0.015Nm. Assuming a 120 degree rotation for the final output, it should take 12 seconds for one revolution or 0.083 rps. Dividing the motor speed by the output speed, 59rps/0.083rps gives a reduction of 710:1. But that provides plenty of torque: 0.015Nm*710= 10.65Nm. If we assume a 2m arm, we only actually need 0.5N *2m = 1Nm.

That looks like a big reduction, but 20:1 is easily done using chain and sprockets and Banebots puts out a 56:1 gearbox that would work nicely. Using these together gets 1120:1 which should make the motor even happier.

There will be some losses in the drive train, but even with 50% loss there will be way more torque than needed. I puposely left out the power required to move the arm itself as that can be adjusted by using counter balances or springs.

So it really does not matter too much which motor you use as long as you use it correctly and keep all hardware within its operating limits.

[Peter Matteson]

We used a Fischer Price into a 64:1 Banebots 42mm planetary to a 30:1 reduction Rino Mechanical worm drive (that pretty little anodized box many of you asked about).

We stuck with our why make it from scratch when you can buy existing parts and adapt them to your purpose philosophy.

Pete

[Josh Murphy]

We used the Fisher Price with the Nothing But DeWalt concept. It was plenty strong for the job and we did not have to touch it all season long. Great motor for an arm with the right gear ratio.

[Andrew Schuetze]

We didn't use a window motor this year. We CAD'd up a plastic insert for the FP motor/transmission that accepted the triangular lead screw nut. It was quite a contraption once the entire mechanism was done using some iGus linear slides inside of a 4 inch PVC tube all connected to ansi 35 roller chain and and IFI mechanism sprocket on the arm.

[1902_Battery_SGT]

Team 1902 used two banebot motors, one on each side, at 125:1 reduction and chain and sprocket. also to keep the motors from staling out we used a forty pound gas strut on our arm, it also helped stabilize it's movement.

[colin340]

2 cims (we could have use one but i did want it to back drive ) on a winch that powered and system of slides that pushed the arm up

[Richard McClellan]

2158 first used two Banebots with 256:1 gearboxes, which worked for several rotations of our arm over a two day period, but we quickly realized they weren't going to work when they began to smoke during a stall (they were stalled while the arm wasn't moving since they are backdrivable motors.

Then we switched to two keyang window motors for our arm since they were not backdrivable and they held up quite well for Lone Star and Atlanta, but shortly after during a demonstration, the gear inside one of the motors stripped out. It's been difficult finding a replacement because they're not commercially available, so next year we'll probably try and stay with motors that are replaceable.

[=Martin=Taylor=]

When building our arm we first thought about using the Keyang motors. However, after a little testing we discovered that they are not as strong as their aluminum housings make them appear... The Keyangs contain little plastic gears, which are easily broken.

Our decision then became to build a worm-gearbox for the Big CIM and run our arm off of that. The gearbox would be made entirely from metal to limit the chance of anything breaking.

Our manipulator weighed a lot this year - roughly 10 lbs. But we didn't really need much arm power! We balanced the weight of the manipulator with a gas piston. If we had wanted to we could have used a tiny banebots planetary! But the big CIM gave us lots of extra power.

[rachal]

The 1st joint of our arm was powered by two 256:1 FPs, one on each side, plus a 5:1 sprocket reduction. Strong enough to lift the robot (and it tried quite a few times because of PID errors). The 2nd joint locked up, glitched, and was otherwise a pain to maintain. I think it ran off a BB at 64:1, but we also experimented with several sprocket configurations for additional torque so I don't remember what the final reduction was.