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.

[clayman]

Originally Posted by Ether

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?

Oh no... the toughbox is chained to a sprocket in about a halving gear reduction. We have it on a jag, so we'll get the motor speeds figured out... eventually.

[Ether]

Originally Posted by clayman

Oh no... the toughbox is chained to a sprocket in about a halving gear reduction. We have it on a jag, so we'll get the motor speeds figured out... eventually.

Do you have a rough estimate of the torque load on the arm ?

[clayman]

Originally Posted by Ether

Do you have a rough estimate of the torque load on the arm ?

Ballpark, I'd say 40 ft-ibs.

[Ether]

Originally Posted by clayman

Ballpark, I'd say 40 ft-ibs.

I assume that's 40 ft-lbs before you balance it with springs :-)

Just for fun, let's do a little engineering and see what happens at 40 ft-lbs
and why the gear ratio is so important.

Let's say you want to move the arm at 90 degrees per second. That's 15 rpm.

You said your total gear ratio is 12.75*2:1 = 25.5:1.

So your speed at the motor is 15*25.5 = 383 rpm.

Your torque at the motor is (40/25.5)*12*16 = 301 ozin.

Using the motor curves for the CIM and a bit of algebra, 11.4 volts will give you 301 rpm at 383 ozin. The motor efficiency at this operating point will be 6.4% and you'll be pulling 117 amps (until your circuit breaker trips).


Now let's see what happens if you change the gear ratio to 250:1

The rpm at the motor is now 3750 and the torque at the motor is 31 ozin.

Using the CIM motor curves again, we see that 9.6 volts will produce 31 ozin of torque at 3750 rpm. The motor efficiency will now be 65% and you'll be drawing only 14 amps.


Of course, the problems with high gear ratios are

- some loss of efficiency (which I haven't accounted for here)

- weight, size, expense

- problems with overloading the gears if they are too small


But there are problems with using balancing springs instead of gear-down:

- you can't balance over the entire operating range

- if you don't gear down enough, you lose fine control of the arm


So, no surprise, as with everything in engineering it's a matter of finding the right compromise to meet the task at hand.

[HarveyAce]

We have two window motors powering the upper half or our arm. We had the problem of the motors not being able to drive the arm past parallel with the ground due to max torque on the arm due to gravity, which was stalling or window motors. We have two strands of surgical tubing as a counter-balance. An estimation of the torque load would be about 31.5 ft-lbs. (3.5ft*approx 9 lbs weight). Our windows get warm but don't overheat (they did only once due to excessive testing in a short time span). we are using 2 vics to control the motors, using an analog control (a second Attack 3 joystick). this way we can have a little more control of the motor going down (lower voltage), but max voltage going up (it won't move past 90 degrees even with the tubing without the voltage ). besides adding more tubing, what could we do to help get more control and use less voltage? Should we change to an FP with a speed controller? if so, how do we get it to not back-drive?

[JamesCH95]

We've got two RS775s running through a a ~380:1 reduction on our arm (64:1 gearbox, 6:1 sprocket). The arm has two window motors, two pistons, some 25 series chain, and a pretty serious manipulator. We simply stall the motors to hold position controlled through a PID loop, and they don't heat up much at all. They've never been more than "warm to the touch" even after minutes of holding position at horizontal. They lift the arm nice and fast, the rate is controlled in software, and using the jags' dynamic braking it coasts down nicely as well.



Two RS775s might be overkill, but they handle the stalling a lot better than any other BaneBots motors and have proven to be wonderfully robust so far.

[Ether]

Originally Posted by JamesCH95

We've got two RS775s running through a a ~380:1 reduction on our arm (64:1 gearbox, 6:1 sprocket).

Can you give a ballpark estimate of the torque load on the arm?

(I use an inexpensive fish scale - available in the sporting goods section of any discount store - and a tape measure to get an estimate)

[JamesCH95]

The robot has been shipped to another regional, but I can make a reasonable guess of around 25lb-ft

[JamesCH95]

Originally Posted by HarveyAce

We have two window motors powering the upper half or our arm. We had the problem of the motors not being able to drive the arm past parallel with the ground due to max torque on the arm due to gravity, which was stalling or window motors. We have two strands of surgical tubing as a counter-balance. An estimation of the torque load would be about 31.5 ft-lbs. (3.5ft*approx 9 lbs weight). Our windows get warm but don't overheat (they did only once due to excessive testing in a short time span). we are using 2 vics to control the motors, using an analog control (a second Attack 3 joystick). this way we can have a little more control of the motor going down (lower voltage), but max voltage going up (it won't move past 90 degrees even with the tubing without the voltage ). besides adding more tubing, what could we do to help get more control and use less voltage? Should we change to an FP with a speed controller? if so, how do we get it to not back-drive?

I would get a stronger motor (RS775 is awesome, as I've mentioned) and gear it way down. You might considering gearing the window motors down even further.