Please help confirm my motor selection

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I'd start by buying a couple of battery powered screw drivers, take them apart, and mount them on your robot. You can buy them for less than ten bucks at Harbor Freight. Then, when you get into requiring more power, buy some variable speed 3/8" drills and connect the trigger mechanism to RC servos for full speed control and a lot of power.

Wayne

Well, for reference, there's what the VEX motors are rated at:

Free Speed 100 rpm @ 7.5 volts Stall Torque 6.5 in-lbs

They are strong enough to drive a VEX bot with the VEX tread kit with two motors but it's under powered. The treads have a lot of friction in them. I don't think it can reach your 1 ft per second speed - maybe half that? Mine is sitting next to me but the it's got the wrong software loaded in it so I can't test it's speed. I tried to make it go faster using about 3 different gear configurations but the speed improvement was minimal and the power loss was unacceptable. So I left it in the direct drive configuration.

The bot is probably about 5 lbs and I doubt the vex motors could make it climb straight up a wall (but maybe it could).

So, I would say the motor above should work (since it's slightly stronger than the VEX motors, but to hit the speeds you were looking for, I suspect you are going to need something larger to overcome the friction loss in the treads.

Also, the RPM rating of a motor is it's no load rating. It will always be less under load (just spinning the treads with the bot held off the ground will be a substantial load). The full specs on your motor give the performance graphs so you can see the speed vs torque curve. The spec also lists the rated load as 1.14 kg-cm and the rpm at rated load as 95 (which is under your 100-200 target range. Max efficiency happens at around 85 rpm and max power output happens at around 55 rpm.

It also says, "Do not exceed rated load. Damage may occur!". So I take that to mean you shouldn't drive it at over 1.14 kg-cm (.99 in-lbs) which is odd because that cuts out most of the performance graph they give. But that's what it seems to say. It might be an indication of how much load the gears are designed to take.

So, like with the vex motors, I think that motor will probably be running quite a bit under the 100 rpm minimum you were looking for under the load caused by the friction of the tracks, and it looks like the rated load of the gears might not hold up very well under your intended application (rough outdoor terrain).

The motor Gordon suggested looks like it would do a better job at meeting your needs. But it's a lot more powerful (and uses more energy) with a stall torque of 7.46 ft-lbs (89.52 in-lbs). But if you want to actually reach your 200 rpm speed, you might need something with that high of a rating. Since speed is generally linear with torque, you can estimate the torque delivered at 200 rpm.

The speed is 271 rpm with no load, and 0 rpm at 89.52 in-lb, so the equation is:

speed = 271 - 271t/89.52 = 271 - 3.027 t

so at 200 rpm:

200 = 271 - 3.027 t t = (271 - 200) / 3.027 t = 23.5 in-lbs

which is about 4 times what you calculated was needed for the full 10 lb bot to climb straight up. But your calculations didn't include the load due to the friction of just turning the treads (which I have no idea how to calculate).

Or, we can calculate how fast it will go straight up using your 5.9 in-lbs number:

rpm = 271 - 3.027 * 5.9 = 253 rpm

or with only one motor and one tread driving it:

rpm = 271 - 3.027 * 11.9 = 234 rpm

But again, this doesn't include the friction of the treads (which may only be on the order of 1 or 2 in-lbs for vex-style friction bearings and less if you use real bearings).

So Gordon's motor selection will be overkill in terms of the speeds and power you wanted to reach, but the motor you selected looks like it's not going to reach the speeds you were aiming for and may not hold up well because you are going past the rated load which it says not to do.

You can always reduce the power of a motor by driving it at less than 100% (or with less voltage) but if the motor doesn't have enough power, there's not much you can do.

Thank you so much everyone for taking the time to lend me some advice. I am going to look a little bit more now that I have some fresh thoughts but those planetary gear motors are negligibly more than what I had budgeted and as Curt said, I can run them below 100% and definately meet my specs. Besides, I can always cannibalize them for the inevitable "next bot"! My only initial concern with them is that the locked shaft current went from 1.5A in the ones I was looking at to

9.1A so I am going to need to look into some better controller$ than I had planned. I was initially considering Parallax Motor Mind B which is only 6A peak, but I am only really starting to look at controllers so I wasn't married to those ones (although I liked the built in tachometer).

Thanks, Michael

Curt Welch wrote:

Thank you so much everyone for taking the time to lend me some advice. I am going to look a little bit more now that I have some fresh thoughts but those planetary gear motors are negligibly more than what I had budgeted and as Curt said, I can run them below 100% and definately meet my specs. Besides, I can always cannibalize them for the inevitable "next bot"! My only initial concern with them is that the locked shaft current went from 1.5A in the ones I was looking at to

9.1A so I am going to need to look into some better controller$ than I had planned. I was initially considering Parallax Motor Mind B which is only 6A peak, but I am only really starting to look at controllers so I wasn't married to those ones (although I liked the built in tachometer).

Thanks, Michael

Curt Welch wrote:

Pololu has a dual high-current motor driver carrier that may be of interest -- and the page includes some nice detail on handling high-current motors:

HTH,

- Joe

(sorry if this is a double post, my newsreader is complaining)

That motor controller does look pretty good. I also came across this one

which costs a little more. It seems roughly equivalent in terms of power but provides for regenerative braking... how cool is that?! But in the end that is actually more of just a novelty for my particular application so I am going to really consider the ones you mentioned.

I was still *hoping* that I could find motors requiring less current to save me some cost on the controllers and also to provide better battery life (and I am looking to see if I can trade-off some of my speed or weight requirements to make it happen)

Along those lines, I came across these motors: (

. I didn't find them at first because they appear to be out of stock or maybe even discontinued but it seems I can find them elsewhere online. They look to provde ~twice the torque of the planetary motors that we discussed above using far less current. This is from being 12v instead of 9.6v and also that they are geared down further.

Planetary Gear Motor - 9.6vdc 51:1 271rpm (LS-550S) (6mm shaft)

(sorry if this is a double post, my newsreader is complaining)

That motor controller does look pretty good. I also came across this one

which costs a little more. It seems roughly equivalent in terms of power but provides for regenerative braking... how cool is that?! But in the end that is actually more of just a novelty for my particular application so I am going to really consider the ones you mentioned.

I was still *hoping* that I could find motors requiring less current to save me some cost on the controllers and also to provide better battery life (and I am looking to see if I can trade-off some of my speed or weight requirements to make it happen)

Along those lines, I came across these motors: (

. I didn't find them at first because they appear to be out of stock or maybe even discontinued but it seems I can find them elsewhere online. They look to provde ~twice the torque of the planetary motors that we discussed above using far less current. This is from being 12v instead of 9.6v and also that they are geared down further.

Planetary Gear Motor - 9.6vdc 51:1 271rpm (LS-550S) (6mm shaft)

HAH!... oops read that planetary at 7.46 In*Lbs not Ft*Lbs! Now that

9.1 amps makes more sense!

Michael wrote:

Note that the 9.1 amp draw is from a locked shaft, which on a tracked vehicle will almost never happen, or if it does, you'll want a stall sensor to prevent it from happening. Most of the electronics have a overcurrent sense that will turn them off when the output current is exceeded, and this is something you do want to take advantage of if you don't want to be buying new treads all the time. IOW, this is a worst-case scenario that you want to avoid at all costs.

I would guess the nominal load current consumption of the motor to be closer to 2.5-3.5 amps, which is still reasonable for a number of the drivers out there. If you'll be using 12 volts, a DMOS controller is well suited.

It's a fair bet the motor will also run well at less than the rates 9.6 volts, which means you can get excellent torque from just 6 volts -- and the current draws will be about 30% less, too. That means a lighter robot because the batteries won't be so large.

FWIW, I like avoiding pound-force inch expressions to avoid this very problem! Sticking with just imperial measurements, you can convert to oz-in, or lb-ft. Here's a Web page that makes it pretty easy:

According to the calcs: 7.46 pound-force foot = 89.52 pound-force inch =

1432 ounce-force inch.

-- Gordon

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