Motor for arm: what should be used when?

[HarveyAce]

We are using two window motors in tandem to power the upper half or our arm. We were running into problems with them going really slow and not reaching past parallel with the ground, until we added a surgical tubing counterbalance . after that, we couldn't control it going down, because it went way too fast, and we put them on Victors instead of spikes and had to manually drop the voltage to make them go at a more controllable speed. any other solutions to this problem?

Also I was starting this thread to compile a list of what motors are good for what applications, and how to implement each in the applications. For example, one might say that FP's can be used for manipulators, but they are easily back driven, so explain how to use them without them back driving. Just a quick example. Hope this becomes a good resource for those that need a quick reference!

[clayman]

We ran into the exact same problems with the window motors at the base our arm. Since we took out the locking pins and the motors were backdriving anyway, we swapped them out with a single cim on a toughbox. We have yet to test it, though (but we don't expect any issues). The other two joints on our arm are driven by single, unmodified window motors.

[Ether]

Originally Posted by clayman

we swapped them out with a single cim on a toughbox. We have yet to test it, though (but we don't expect any issues).

You seem to be saying that the output of the toughbox shaft will be driving the arm with no additional gearing. That's not what you meant to say, is it?

[sanddrag]

You might try setting the brake mode jumper on the Victors.

[Kevin Sevcik]

Brake mode won't help unless they've removed the anti-backdrive pins from the window motors. Brake mode only engages when the motor is stopped, which is when the anti-backdrive pins also kick in. The fundamental issue is that the arm still isn't perfectly balanced and is still falling under its own weight. As long as that's the case, your arm is going to go down faster than up, because more torque is required to lift the arm. You're never going to completely even things out with surgical tubing either. If you arrange it and tension it to balance things when the arm's level, it's either going to pull too hard when you lower the arm below level or raise it above level. So if your arm works reasonably now, I recommend leaving well enough alone.

For the curious, the best bet for perfectly balancing it would be an actual counterweight on the arm, followed by a constant force spring mounted a reasonable distance from the pivot so the spring is always approximately vertical. But either is loads heavier and more complicated than some surgical tubing.

[Jeffy]

Try adding more surgical tubing. More assistance will make it the same load in each direction, therefore keeping it from falling. If it just goes to fast in general, a victor/jag could work, but the better answer is ussualy to change your ratio.

As per the motor usage question...
The only "norm" in FIRST is CIMs on drive, and even that isn't followed by all teams (and doesn't need to be). After that, it depends on the application.
All the motors in this year's kit are great, pick the one you want to use, gear it approriately and you're good to go.

[2829ron]

Originally Posted by clayman

We ran into the exact same problems with the window motors at the base our arm. Since we took out the locking pins and the motors were backdriving anyway, we swapped them out with a single cim on a toughbox. We have yet to test it, though (but we don't expect any issues). The other two joints on our arm are driven by single, unmodified window motors.

Please tell us how this works. We built our arm with the CIM and toughbox and were having problems getting it past about parallel with ground. May have been our batteries though. Please let us know what happens.

David

[Ether]

Originally Posted by 2829ron

We built our arm with the CIM and toughbox and were having problems getting it past about vertical.

Let's say 90 degrees per second is the approximate arm speed you'd like at full motor voltage (speeds faster than this can be difficult to control, slower speeds may make you uncompetitive)

Let's say you've got the equivalent of 5 pounds at 6 feet from the pivot, or 30 foot-pounds of torque.

From these two numbers you can calculate the required mechanical power in watts:

90 degrees per second = 15 rpm

(15 rpm)*(30 ft-lb)/7.043 = 63.9 watts

Now using the motor curves, or this simple calculator, you can find that the CIM motor speed which generates 63.9 watts at 12 volts input is 5045 rpm. Since 5045 is 95% of the CIM's free speed, it should run cool at this operating point.

Take the ratio of the motor rpm to the arm rpm to get the required gear ratio:

5045/15 = 336:1 gear ratio.

If you use a gear ratio much less than this, you will either get an arm which moves so fast that it is uncontrollable, or you will have to operate the motor far outside its design point which can cause overheating. The CIM is can tolerate a lot of abuse but it is not immune to burn-out.

[2829ron]

We used the sprockets from the wheels and put them on the arm. This is attached to a chain running to the toughbox.

David

P.s. Parallel to the ground not vertical

[davidthefat]

We use only 1 window motor to drive the 62" arm. Needless to say, it gets burning hot. So we put a heatsink with a fan on it (I don't know if it is legal)

[Tristan Lall]

Originally Posted by davidthefat

So we put a heatsink with a fan on it (I don't know if it is legal)

Which fan?

[davidthefat]

Originally Posted by Tristan Lall

Which fan?

Just some random one off of an old computer

[Vikesrock]

Originally Posted by davidthefat

Just some random one off of an old computer

You'll have to replace it with one provided in the KOP to be legal under <R45> and <R46>.

[CallieJ]

We're running our arm on one Banebots 550 and one Fisher Price. We were going to use the Andy Mark Double Doozy but it wasn't available, so we designed and built our own custom gearbox. The internal reduction is 237:1 plus another approximately 3:1 reduction from the sprockets.

So far the motors have been getting warm but not hot under match level or higher stress environments. We did have to do a lot of work in software to keep the arm from sinking but even that doesn't seem to be putting too much strain on them.

[Kyoshirin]

We have two window motors hooked up to a 20:3 (~7:1) gear ratio to a 40" upper arm, on which is attached another window motor hooked up by timing belt ~3:1 to a 33" lower arm with a pneumatic claw attached to the end adding length and weight. We never had any problems with our arm lifting, but we have four strands of surgical tubing just in case.