Motor for arm: what should be used when?
 

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 :D . 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!

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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?

Re: Motor for arm: what should be used when?
 

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

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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:o

Re: Motor for arm: what should be used when?
 

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)

Re: Motor for arm: what should be used when?
 

Which fan?

Re: Motor for arm: what should be used when?
 

Just some random one off of an old computer

Re: Motor for arm: what should be used when?
 

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

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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

Re: Motor for arm: what should be used when?
 

Ballpark, I'd say 40 ft-ibs.

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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?

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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.

Re: Motor for arm: what should be used when?
 

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)

Re: Motor for arm: what should be used when?
 

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

Re: Motor for arm: what should be used when?
 

25 ft-lb = 4800 ozin at arm => (4800/380)/2 = 6.32 ozin at each motor

From the motor curves for RS775:

12,272 rpm at motor = 194 degrees/sec at the arm.

A bit fast for the top end, but motors running nice and cool.



Throttle it down to 90 degrees/sec by reducing the motor voltage:

90 degrees/sec = 15 rpm at arm => 15*380 = 5700 rpm at motor

Still running cool, and the voltage is in a reasonable range for control.


How about stall?

Very reasonable 3.3 watts dissipation.

Re: Motor for arm: what should be used when?
 

Very close to my "napkin" design :yikes:

I aimed for about 180deg/sec at full speed, but couldn't quite get there with the finite available sprocket ratios.

We limit the 775s to 60% of full command speed, and the PID loop still does a great job of controlling them.

I hadn't done a stall calculation, we had been planning on using a DeWalt transmission with anti-back-drive pins, but found that the pins didn't like it when the arm was powered down, so we removed them. This also has the side benefit of being able to move the arm without powering up the robot or disassembling anything.

Re: Motor for arm: what should be used when?
 

I didn't account for gearbox inefficiency and manufacturing tolerances, so the numbers aren't anywhere as exact as the displayed values might suggest. So it might actually be closer to your "napkin" 180. I left the displayed values as-is in case anyone wanted to play around with the calculations and compare them.

Re: Motor for arm: what should be used when?
 

Thank you so much! It looks like we were having a problem with the arm hitting the stall amperage. Tomorrow I will change the ratio to 51:1, much more reasonable :) (and ideally below the max power rating). It should supplement our spring counterbalance nicely.