Author Topic: Lighter weight motors than wheelchair? (Read 13878 times)

SeagullOne · « **on:** May 11, 2009, 06:19:06 PM »

I'm highly considering putting my robot, Nina, on a diet! I've been pushing it from the closet and back and I'm huffing and sweating! My legs are still sore from three days ago after doing that! Ugh!

I'm replacing the 12V SLA battery I have with two 12V NiMh packs wired in parallel for 20 AH. That will cut off about 15lbs off!

Next is the motors. Wheelchair motors I think are way too big and heavy and dangerous for my application. Anyone know of any kind of motor that's lower scale than wheelchair motors but still able to able to push 100+ (at low speeds of course say 2.5 MPH max)?

I may half to make my robot smaller two, which I'm okay with. At this rate its getting to be pretty tall. I'm aiming for a total robot weight of no more than 150 pounds. I'll also see what my power requirements will be with Admin's calculator for this load.

galannthegreat · « **Reply #1 on:** May 12, 2009, 08:54:16 AM »

Take a look at battlebot style motors. These ones here may be of use to you,
http://www.robotmarketplace.com/products/motors_main.html

SeagullOne · « **Reply #2 on:** May 12, 2009, 12:18:50 PM »

Thanks, Galannthegreat. I've been to that site many times and looked it over a lot, but still not sure they'd work. According to Admin's RMF calculator, I'd need a robot motor factor of 48 or greater. Most of the motors I see on The Robot Marketplace are either too weak and small, too powerful and heavy, or just the right load but overkill and way out of my budget... hmm...

What about the motors used in some childrens toy cars? The kind children drive in themselves? They run on twelve volts, can support child loads of up to 120 lbs, and can't as heavy as Wheelchair motors, right? They only go a maximum of 5 mph. Anyone know what kind of motors these are and if they can be mounted to a robot easily?

I'll see what I can find too.

SmAsH · « **Reply #3 on:** May 12, 2009, 03:32:44 PM »

i don't know if they sell them commercially but you could go down to the local garbage dump and look for an old one?

galannthegreat · « **Reply #4 on:** May 12, 2009, 03:55:24 PM »

Ebay will have some nice deals on those motors, just a matter of watching auctions and finding the right ones.

SeagullOne · « **Reply #5 on:** May 12, 2009, 06:00:44 PM »

Hmm...what if I built my own?

For instance, I take this powerful, lightweight 72$ motor...

http://www.robotmarketplace.com/products/NPC-02446.html

Gear it way down in RPM to torque with belt drives or chain and sprocket, and customize it for my application? I'd buy the gearbox it comes with, but why do that when I can build something better specified to my application?

galannthegreat · « **Reply #6 on:** May 12, 2009, 06:15:24 PM »

That would work very nicely, it would be a beast when geared down.

Ro-Bot-X · « **Reply #7 on:** May 12, 2009, 08:16:08 PM »

Quote from: SeagullOne on May 12, 2009, 12:18:50 PM

What about the motors used in some childrens toy cars? The kind children drive in themselves? They run on twelve volts, can support child loads of up to 120 lbs, and can't as heavy as Wheelchair motors, right? They only go a maximum of 5 mph. Anyone know what kind of motors these are and if they can be mounted to a robot easily?

Those motors make a lot of noise. A lot more than a couple of servos, and I allready hate servos! So I would not recommend them for a robot that will roam around you inside the house. Outside, sure, if they match the needs. The biggest advantage of the power wheelchair motors is that little humm they make that doesn't bother your ears. Other powerfull motors (that will need to be geared down) are the pancake motors for scooters. Try to get the ones for that small skateboard-with-a-handle-style scooters. That is something I'm considering for my big butler robot. Also use toothed belt with a tensioner, not chain and sprockets, if you need low noise.

robologist · « **Reply #8 on:** May 12, 2009, 09:22:26 PM »

Did you check the http://www.robotmarketplace.com/products/ame_motors.html at Robot Marketplace? Seem pretty cheap and torquey at 88 inch-lbs and 212 in-lbs. They have left hand and right hand versions and run about 100 rpm. Backing off a bit, call it 75 rpm, you'd get 2.5 mph if using a 12" wheel.

V = rpm x (pi x D) x 60 min/hour x mile/5280 feet
= 75 x (3.14 x 1) x 60 / 5280 = 2.67 mph

88 inch-lbs on a 6" radius arm (12" diameter wheel) = 14 lbs push per side, 28 lbs total, good for going up a small grade. 212 inch-lbs gives 35 per side, 70 total.

SeagullOne · « **Reply #9 on:** May 12, 2009, 11:02:32 PM »

Thanks Ro-Bot-X. I didn't give the noise those motors make much consideration. Inside a house, they must be horrifically noisy!
I know how silent and wonderful those wheelchair motors are too. I think they're too heavy and too powerful for a safe indoor robot, though (and I'll admit my previous plan in my last post is probably still overkill). One time while I was telling my robot to move forward, she almost crashed into my mom's curio cabinet! That big 75lb or more robot! (and that's just the mobile base!)

Also, thanks robologits. I've looked at a lot of those motors you listed in that link. I don't know how much my finished robot is going to way, though. I want it to weigh as low as possible, but I really don't know...70 lbs would be a great weight. But I'd like to get a motor that can handle 120 lbs + just encase...

I'll keep experimenting with calculations with the motors I find and see what will work.

Okay, I'm also kind of embarrassed to say this, but my robot's running on rubber treads, so I'll have to account for that when choosing the right motor. I want to make sure those motors can pull that robot on those things, so that means more torque than for wheels...

robologist · « **Reply #10 on:** May 13, 2009, 04:47:54 AM »

I may have not been clear. 28 pounds of push will shove a 100 lb robot up a 15 degree ramp. The max ramp allowed in the pedestrian areas is about 7 degrees, by regulations of the ADA, Americans with Disabilities Act. The 70 lbs of push would shove a 250 lb robot up a 15 degree slope.

If using tracks it might be best to halve those numbers, just as an estimate. The larger motors should still be able to push a 120 lb robot.

SeagullOne · « **Reply #11 on:** May 13, 2009, 03:52:38 PM »

I think I like these motors from Superdroid Robots.

http://www.superdroidrobots.com/shop/item.asp?itemid=871&catid=7

They don't look like they're too heavy, and not too expensive, and still powerful enough to move a big load...I think...I'm having a bit of trouble understanding the torque conversion chart they have under the motor. I think I understand it. To get lb-ft, from kg-cm, multiply the number Kg-cm to lb-ft by the number under lb-ft straight across from the 1 under kg-cm. So...1 kg-cm = 13.83 lb-ft? That's a lot for a motor rated at 11 kgf-cm (and that's force not torque!). That's a powerful motor!

Is this correct?

Canabots · « **Reply #12 on:** May 13, 2009, 04:14:44 PM »

Robotshop sells some high torque motors, though I don't know if they're what you're looking for.

Something like this one: http://www.robotshop.ca/rbban31-banebots-spur-gearmotor.html

It may be too fast though...

SeagullOne · « **Reply #13 on:** May 13, 2009, 08:41:56 PM »

Hi Canabots.

Banebot motors are great. I have a pair with which I'm hoping to build a smaller robot. This bot is big, though! Getting smaller, but still a big bot!

robologist · « **Reply #14 on:** May 13, 2009, 09:09:11 PM »

@SeagullOne - Got the conversion backwards. Takes 13.68 kg-cm to equal 1 foot-pound.
Think of 2.2 lbs per kg and 30 cm per foot
So 11 kg-cm x 1/30 (ft/cm) x 2.2 (lb/kg) = 0.80 lb-ft for those motors.

SeagullOne · « **Reply #15 on:** May 13, 2009, 11:31:36 PM »

Whew! Thanks Robologist! Better go with some different motors. They did look a little small for 11x13lbs per ft. I'll revert back to the ones at Robot Marketplace and check them out a little longer

Funny, at 0.8 lb-ft for those motors, I wonder how they move those 60 lb robots on treads with them? They say they use them in their larger robots. I wonder how that happens...

SeagullOne · « **Reply #16 on:** May 14, 2009, 12:16:02 AM »

Hmm...what about this motor?

http://www.robotmarketplace.com/products/0-TD-RCM515.html

It's small, very powerful using low gear settings (over 200 on Admin's RMF calculator to my surprise), and it's within my budget. My only concern is what if I'm running it at 12V. It'll be slower, less torque, but I think it should still work, right? At 18V's according to Admins RMF calculator, this thing could move well over 120 lbs (on wheels of course).

robologist · « **Reply #17 on:** May 14, 2009, 01:49:47 AM »

Had to re-check the IG52GM gearmotors. Their rated torque was stated as 11 kg-cm, but the paragraph above claims 300 kg-cm. Looking at their info http://www.superdroidrobots.com/product_info/IG52GM-04.gif shows the motor at 24 volts can get to 6300 g-cm at stall, maybe 12 A. This multiplied by the 12:1 gear ratio gets to 75600 g-cm, or 75.6 kg-cm, but not 300 kg-cm.

If looking at the efficiency curve, it seems to peak right around 1300 g-cm, at about 70% efficiency. That calculates out to 1300 x 12 (gear ratio) x 0.70 (eff) to about 11000 g-cm, or the 11 kg-cm "rated" value. And also where the motor is at 3500 rpm, divided by 12 comes to about 290 ~ 285 rpm.

I'm guessing from this finagaling that the actual stall torque is 75.6 kg-cm x 0.40 (eff) = 30.24 kg-cm. Maybe they slipped a zero in. That would be about 2.2 ft-lb. Still it doesn't take a lot to push something across the ground, but it dioes to lift it.

galannthegreat · « **Reply #18 on:** May 14, 2009, 02:52:25 PM »

--->http://www.robotmarketplace.com/products/magmotors.html<---

Okay, I have to suggest these... they are very expensive, but... the power that comes out of these for their size is immense! So, if you've got the dough, you will be very happy with these tanks.

and maybe Ebay will have a deal or two.

SeagullOne · « **Reply #19 on:** May 16, 2009, 07:51:15 PM »

I've taken a good look at those, galannthegreat. They look like terrific gearboxes, but are slightly too expensive, plus they mighty be just a little overkill as far as their power is concerned.

I'm seriously thinking of taking those Ampflow E-150 motors and making a two-stage belt and pulley drive for something along the lines of 5:1 reduction each, run it on 12V's so its slower and less powerful. I'm aiming for a speed of around 96 RPM, maybe a little more (I don't know how fast they turn at 12V. I halved the 4700 RPM listed on the specs to 2350 RPM to be safe). A two stage reduction like that should get me an output torque of about 10,750 oz-ins (or 55 lbs/ft, right?).

The NiMh battery packs I already have for the job measure a total of 12V 20Amp hours when wired in parallel. They weigh a lot less than the monster SLA battery my robots already running on, and the motors weigh about 3.5 lbs each, compared to the wheelchair motors it has, which weigh about 14-17 lbs each! I just have to make sure to get the right-weighted timing belts and pulleys for those motors so I don't add too much more weight for the mobile base.

What do you guys think?

Ro-Bot-X · « **Reply #20 on:** May 17, 2009, 07:23:28 PM »

Looking for DC motors with encoders I saw this motor that I think will work out great for you. It does not have an encoder, but it's light, small, with lots of torque, made for an electric bicycle. Take a look here:
http://www.allelectronics.com/make-a-store/item/DCM-2445/24VDC-450W-MOTOR-W/-PLANETARY-GEAR/1.html

SeagullOne · « **Reply #21 on:** May 17, 2009, 09:13:56 PM »

Hmm. Not bad, Ro-bot-X.

My only concern with that motor is when at 24 volts it eats up 450 watts, its not going to be very very healthy for the lighter weight power supply I'm intending to use. It'll eat up even more watts at 12 volts, which means more amps...

With my NiMh 12V 20AH battery pack, I need a 2 motors that don't use up any more than 120 watts each at 12V (10Amps each). Otherwise, it'll shorten the lifespan of my battery. It might work with my SLA battery I already have, but that thing weighs a ton!

But I wonder if the motor I'm considering from the Robot Marketplace might have a similar problem (the E-150). At 12V it'll use up about twice as many amps, correct? So that means I'd have to make the motor equal to or greater than 4 times the power efficiency with my belt and pulley reduction...

Ro-Bot-X · « **Reply #22 on:** May 18, 2009, 05:52:18 AM »

The motors will have a no load current consumption that you can't avoid. Over that, the motor will consume more amps as needed by the load it moves. So, for you riding a bicylce, the motor will eat up lots of amps, but 2 motors on a light weight chassis will eat just the little extra needed to move that chassis around. The plus side of this is the robot will be able to go up a verry steep ramp because it has the extra power. You will PWM the motors to adjust the speed to mach your needs so the motor will not run at the full power because you will power it from a 12V power supply and because the PWM will reduce the voltage thus the speed to even lower. Now for the same power output needed, the motor will suck up more current at a lower voltage, so what you want to do is keep the motor running at about 75% of the max speed most of the time, so you will need to gear it down to get the proper RPM for your wheels diameter.

Besides, you can mount some foot pads and handles on the robot and you will be able to ride it, it has all the power needed!

Cheers!

SeagullOne · « **Reply #23 on:** May 18, 2009, 12:11:17 PM »

Oh, I see. So when you have two motors for a differential drive it's not the same as twice the current in amps because you have two motors working together to move the load. That makes sense. Is that right?

Also, I take it the 450watts is the maximum energy draw for this motor. I was treating the 450 watts as the minimum energy draw

. So even at 12 volts, two motors won't add up to 450 watts, depending on how heavy my robot is. But to be the safe side, I make sure not to run the motors at full power (to slow down the robot and to avoid eating too many amps). Also, I see the output shaft has a sprocket on it. If I attach a chain and bigger sprocket to the wheel (or the drive wheel for the treads), it'll slow down the motor, add even more torque, and make it even more energy efficient. Or for a quieter drive, replace the sprocket with a timing pulley and belt.

Thanks again, Ro-bot-X. I'll seriously consider these motors.

Admin · « **Reply #24 on:** May 24, 2009, 07:09:25 PM »

Weight is mostly a mechanical thing . . . and hence falls in the realm of optimization.

I'd first start by shrinking the size . . . basically the smaller you can get it, the less weight the robot will take.

After that, I'd do force and beam stress calculations to see how much of the frame I can just remove or swiss cheese. If a calculation says it won't bend, why have a support beam?

Also, Lithium batteries weigh less than NiMH.

Given the new weight, after calculating RMF, I'd then spec a newer smaller motor.

Lastly, knowing the new weight and internal forces of the system, I'd repeat the above steps until reaching a convergence.

Look at my ERP robot and how I designed the arms, this is exactly what I did.

(hoping that made sense)

SeagullOne · « **Reply #25 on:** June 08, 2009, 11:49:34 PM »

Thanks Admin. I've also read Robot Builder's Bonanza 3rd edition about decreasing weight on one's robot.

Was doing some browsing around. This motor looks even better!

It's cheaper, eats less watts, and more powerful. I just hope its still lighter in weight...

http://www.allelectronics.com/make-a-store/item/DCM-1353/24VDC-350W-MOTOR-11-TOOTH-24-WIRE/1.html

One thing that confuses me, though: the specifications on the page says it eats 22 amps. At 24 volts and 350 watts, wouldn't it be eating around 14 amps? hmm...

Edit:

Wow! Found one even better! on the same site! This one eats only 135 watts!

http://www.allelectronics.com/make-a-store/item/DCM-130/24VDC-135W-MOTOR-W/-BELT-GEAR/1.html

News:

Author Topic: Lighter weight motors than wheelchair? (Read 13878 times)