articulating arm help

[HarveyAce]

About the window motors, I was told that the screw drive in them does not allow for back driving. and aren't they far too slow to be effective if reaching the top rack? just wondering plus the thermal lock on them could be a problem...

[Alan Anderson]

If you intend to use the motor alone to hold the arm up, without any mechanical counterweight or spring-loading, be cautious. You'll be running the motor at or near stall a lot of the time. A CIM can handle that for a while, but it will get very warm and you might want to consider a way to cool it off during or between matches.

A Fisher-Price or Banebots motor will not handle that kind of abuse well at all. They rely on an internal fan to keep their windings cool, and if they're not turning, neither is the fan. You'll melt something in short order, followed by smoke. Nearby veterans will sniff the air and nod at the familiar smell of a fried motor.

[HarveyAce]

Well thats not good then. We were going to use a Banebots motor hooked up to last year's gearbox with a chain to power our four-bar shoulder joint. we were going to power the robot drive with the four cim motors, but is that necessary? should we just use two there and use a cim to power the arm?

[AustinSchuh]

Originally Posted by HarveyAce

Should we just use two there and use a cim to power the arm?

You can still use the BB for the arm, but understand that the BB will not like being anywhere near stall for any significant period of time. This means that you should design the torque that the motor needs to apply to keep the arm up to be significantly less than the stall torque (1/4 or less of the stall torque. I don't have a good number for this). Adding a second BB if it looks like it'll take much torque on the motor to keep the arm up will help. It wouldn't hurt to also put a fan blowing across the motor to cool it when it isn't spinning.

971 used 2 FP motors in a single joint arm in 2008, and it worked just fine. We had way too much power in the joint, which meant that the motors weren't loaded very highly.

[Madison]

This isn't quite an elegant solution, but you might consider adding a window motor to the system geared/matched to the RPM at one of your rotating joints in addition to the BB motors you're using to rotate the arm. The window motor's worm gear could help to lessen the need for the BB motors to operate at/near stall to hold your arm in place.

[HarveyAce]

but i thought that the window motors couldn't backdrive due to the worm gearing inside?

[JamesBrown]

Originally Posted by HarveyAce

but i thought that the window motors couldn't backdrive due to the worm gearing inside?

I think you are confusing backdrive and drive backwards. If you reverse the voltage to the motor, it will drive in the other direction just like any of the other motors.

What people mean when they say you can not backdrive the window motor is that you can't physically turn the motor by turning the shaft (the problem the OP is having).

[HarveyAce]

OH! thats what that meant. my bad... that opens up my options again! thanks! at about how long of straight running will the window motors thermal lock kick in?

[Madison]

Originally Posted by HarveyAce

OH! thats what that meant. my bad... that opens up my options again! thanks! at about how long of straight running will the window motors thermal lock kick in?

It depends on how they're loaded. The more current they're drawing, the warmer they'll get and the faster they'll trip their internal thermal breaker.

I would recommend against using ONLY window motors to drive your arm. Once geared appropriately to move an arm, you'll find that they're quite slow.

[justibm]

Originally Posted by JamesBrown

I think you are confusing backdrive and drive backwards. If you reverse the voltage to the motor, it will drive in the other direction just like any of the other motors.

What people mean when they say you can not backdrive the window motor is that you can't physically turn the motor by turning the shaft (the problem the OP is having).

If you can't turn the window motor with it dead, what would be the best way to move it back to original position when the match is over?

[Alan Anderson]

Originally Posted by justibm

If you can't turn the window motor with it dead, what would be the best way to move it back to original position when the match is over?

Add a mechanical clutch to disengage the mechanism from the motor when desired. This can be done in many ways; I will not limit your creativity by suggesting one.

[Joe Ross]

Originally Posted by Alan Anderson

If you intend to use the motor alone to hold the arm up, without any mechanical counterweight or spring-loading, be cautious. You'll be running the motor at or near stall a lot of the time. A CIM can handle that for a while, but it will get very warm and you might want to consider a way to cool it off during or between matches.

A Fisher-Price or Banebots motor will not handle that kind of abuse well at all. They rely on an internal fan to keep their windings cool, and if they're not turning, neither is the fan. You'll melt something in short order, followed by smoke. Nearby veterans will sniff the air and nod at the familiar smell of a fried motor.

We've successfully run fisher price and banebots motors stalled at between 5-10% in order to keep mechanisms in place. The trick is to design the mechanical system such that only that little power is needed. If you do need much more then that, your motors will burn.

[MrForbes]

I agree with Joe, you can make a little motor live when driving an arm, but you need to be very careful to make sure it's not going to need to do much work to move the arm, and very little at all to hold it still. We did this in 2007 and it worked fine, we used a gas spring to support the weight of the arm itself, so the motor had very little work to do.

This year we're going to try out a window motor or two for the arm. We will again counter the weight of the arm with a springy type thing. We are also going to add some reduction between the motor and arm, both to slow the arm movement (1/4 second for full arm travel is a bit too fast!) and to reduce the load on the motor. And we have a plan for a nifty way to do this but still allow for that clutch thing that Alan was hinting at. The drive has a built in clutching ability, and it's cheap and locally available. And we've used it before for a different application...we know the slipping part works, which is good because we seem to be unable to get limit switches working

[pfreivald]

No love here for the screw drive?

[MrForbes]

not really....kinda hard to use effectively.

If you have some ideas of how to make it work well, I'm sure others would like to see them.