articulating arm help
 

We are making an articulating arm for our robot. we have a cim motor on a CIMple gearbox with a gear chained to a much larger gear which is connected to the arm itself.

our problem comes when we are running the arm. we would like to be able to move the arm to any spot we want and have it stay there when we let go of the joystick. currently when we let go of the joystick, gravity takes over and the arm falls down. I'm asking if there is a way to keep it in any place we stop at. do we use a brake to create resistance or do we use encoders? if we have to use encoders we would need help in figuring out what our distance per count is among other settings in the programming side of things.

Re: articulating arm help
 

Mechanical things that can help include (a) a cantilever, (b) a spring or other tension device to aid in lift, and/or (c) a screw drive instead of a sprocket drive.

As for code for encoders -- sorry, I'm useless on that count!

Re: articulating arm help
 

There are several ways to solve your backdrive problem (that's what it's called)

The mechanical solution, guaranteed to work, would be to replace your CIMple box with a wormgear. It cannot be backdriven. This is the same type of gearbox as comes preinstalled on the window motors.

A simple software solution would be to switch the jaguar controlling your CIM from coast mode to brake mode. This is likely to be not 100% successful, however.

Re: articulating arm help
 

A simple mechanical solution to this problem is to add a "counterweight", or something similar, to balance out the weight of the arm. Many teams have used gas springs in the past to do this, example here. Latex tubing could also be implemented as a spring to counteract the weight of the arm and would likely be cheaper.

Re: articulating arm help
 

how do you switch the jaguar to brake mode? does it have to do with one of the jumpers?

Re: articulating arm help
 

Yes, it has to do with one of the jumpers. It should be documented in the Jaguar docs.

To solve this in software, you would attach an encoder to one of the shafts and then use a PID loop to control it. There are a couple of papers in CD-Media about how to tune one and what one is. A PID loop can be thought of as attaching a software spring to the arm to help hold it up in position. The P term is probably enough to at least get you started, and is easier to understand than the rest of them when you are first starting out.

Re: articulating arm help
 

I would use one of the window motors included in the KoP. They're intended for this sort of thing. They have a screw drive gear box on them, so they're a little slow, but they have crazy torque and can hold a position with a load on it.

Re: articulating arm help
 

what is brake mode?

Re: articulating arm help
 

Bob,

A DC motor converts electrical power to mechanical power. If you remove the input voltage and spin the mechanical side, the motor becomes a generator.

As a generator, if the electrical terminals are open, there is no current and the "drag" of the motor on the mechanical system is at a minimum (just the frictional losses are felt).

However, if you short the electrical input leads to the motor, it generates electrical current from the mechanical system. This requires that the mechanical system to do more work hence, the motor resists the rotation.

So, for either the Jaguar or the Victor, if the brake/coast jumper is set to "coast" the motor offers the least mechanical resistance that it can when commanded to no speed. Likewise, In the "brake" position, the speed controller will offer the greatest mechanical resistance to rotation when off.

Hope this helps.

Regards,

Mike

Re: articulating arm help
 

You may wish to consider using the jaguars for your arm... they can take a feedback circuit from a potentiometer at the joint and then (using settings that you provide) attempt to provide sufficient power to the motor to move to and hold the position that you want.

Alternatively, labview offers a similar PID routine to control arm position. You're looking for a "closed loop feedback" system, where your driver is entering the arm position data and leaving it up to the robot (either the cRio or the Jag) to figure out how to achieve that position.

Note that this requires using the CAN bus... not the PWM cables.

Note also that this is the second step towards a solution... the first, as advocated earlier, is attempting to counterbalance the "dead weight" of the arm with springs or counterweights.

Jason

P.S. Unless your arm joint is exceeding the range of a potentiometer (usually around 270 or so degrees), I'd recommend using a pot rather than an encoder. It is simpler to just read an analog input than to zero and track an encoder.

Re: articulating arm help
 

our team has never used pid or CAN. do you know where some resources are for us as far as programming with pid and CAN and connecting the Jaguars via CAN?

Re: articulating arm help
 

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...

Re: articulating arm help
 

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.

Re: articulating arm help
 

:eek: 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?

Re: articulating arm help
 

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.

Re: articulating arm help
 

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.

Re: articulating arm help
 

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

Re: articulating arm help
 

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).

Re: articulating arm help
 

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?

Re: articulating arm help
 

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?

Re: articulating arm help
 

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. :)

Re: articulating arm help
 

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.

Re: articulating arm help
 

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.

Re: articulating arm help
 

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 :)

Re: articulating arm help
 

No love here for the screw drive?

Re: articulating arm help
 

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.

Re: articulating arm help
 

We're using four (4) on our robot this year...

We used one on our robot last year, and iirc were tied for the most consistent hanger for the first three weeks of competition.

I have a lot of love for the screw drive, but perhaps it's because there are several linear motion actuator companies in the area, and that's pretty much what all of them make...

Re: articulating arm help
 

Worm gears can backdrive, although depending on the angle of the teeth on the worm, the gear material, and whether it was lubricated or not, the torque required to backdrive the worm gear may exceed what the teeth of the gears can withstand. Thus, for a limited range of torque, worm gears can be considered non-backdrivable.