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sam
08-08-2009, 08:14 PM
Hello,

I was just wondering why servo companies don't use worm gears to drive their robot servos. The motor would have to be just as strong of course, but when the servo is only in holding position, the worm gear could take care of the load while saving precious energy in our batteries (the motors wouldn't have to be activated since worm gears can't be moved).

I was just wondering if I was missing something here (something I didn't account for maybe?) that could explain why not to use worm gears.

Thanks,

Sam

darkback2
08-08-2009, 09:00 PM
Hey,

I think linear servos do just that.

As for regular servos, I would think its because the gear ratio in a worm gear is crazy...so the motor would have to spin really wicked fast in order to turn the servo horn at all. For a smaller robot or RC car that wouldn't make sense because response time is more important than holding power.

DB

RevBryce
08-08-2009, 10:30 PM
I wonder how much usable grunt you could get from
windscreen wiper motors?
they have worm gears.
Just put a trimpot on it somehow
and bingo big cheap servo.

Pi Guy
08-08-2009, 10:40 PM
I'm already doing just that for my robot, buy I may have to up it to a stronger motor eventually.

CogswellCogs
08-08-2009, 10:56 PM
Worm gears are often not as efficient as other gear systems. Check out this table from http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html. Still, wiper motors are cheap and have power to spare. Those features, combined with the back drive features that Sam mentioned, make something to consider. I have used 'giant servo kits' from Oatley that try to do this very thing. I had very bad experiences with them as did two of my friends. I may try again using the theory that

gearmotor + ESC + feedback pot + software = big servo

Adrenalynn
08-08-2009, 11:00 PM
Speed, Efficiency, Backlash. Not necessarily in that order. Wrong tool for the job of being a servo, imho

sam
08-10-2009, 06:17 PM
Worm gears are often not as efficient as other gear systems. Check out this table from http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html. Still, wiper motors are cheap and have power to spare. Those features, combined with the back drive features that Sam mentioned, make something to consider. I have used 'giant servo kits' from Oatley that try to do this very thing. I had very bad experiences with them as did two of my friends. I may try again using the theory that

gearmotor + ESC + feedback pot + software = big servo

Thanks for the informative site!

Sam

RevBryce
09-08-2009, 02:10 PM
Found this page with a bit on wiper motors
http://www.scary-terry.com/wipmtr/wipmtr.htm

Adrenalynn
09-08-2009, 03:57 PM
His power numbers jive pretty closely for what I found actually measuring and doing real graphs from the Denso motors. I've posted those graphs before, so they're out there somewhere. (Thanks to DB for the motors!) The thing to note is that of four motors, no two behave the same when stalled. One is as low as 13.8A, the highest is 23A+

RevBryce
09-08-2009, 05:22 PM
That would be a bit of a problem trying to tune the system.
What do you think of this one?
https://www.surpluscenter.com/item.asp?UID=2009090814353706&item=5-1485&catname=electric
Even has a pot built in.

RevBryce
09-08-2009, 05:23 PM
Ok so its a bit slow.

Adrenalynn
09-08-2009, 06:21 PM
>> Rotates 90 degrees in 18 seconds @ 24 VDC

I could gear a regular servo to hell and back and beat that, and the torque too. You could do a hundred to one gearing on most servos. 34,300oz-in on an 805bb, and it's 6v AND pwm... That's like 2144in-lb or so, compared to 50in-lb.

Unless you need something ungodly slow and relatively wimpy, and not easily controlled from a microcontroller...

RevBryce
09-09-2009, 04:10 AM
Ok i didn`t say they were great.
Just intresting.

Adrenalynn
09-09-2009, 04:25 AM
Just pointing out the pitfalls. It's hard to find a good alternative for a commercial servo, because they build a trillion of the things, which cuts the retail cost down substantially.

RevBryce
09-09-2009, 05:38 AM
With that in mind , what would you suggest i use so i can make a
mech big enough to carry a couple of these.
http://www.hasbro.com/nerf/n-strike/shop/details.cfm?guid=940BFD86-6D40-1014-8BF0-9EFBF894F9D4&product_id=22378
36" is the max height if i remember and it would have to be that big to pull it off.
Oh and keeping it under 10k cost?
Besides something that size looking really cool it would be a great way to get
people interested over here.
So alternatives to commercial servos need to be found.
Worm drives in things like wiper motors and electric window motors
while not as fast or efficient with power seem like a good cheap place to start.
After rereading that link ,yeah its a bit lame but i got exited over the package, it seemed
so neat.

Adrenalynn
09-09-2009, 07:07 AM
I'm gonna pass.

billyzelsnack
09-11-2009, 02:45 PM
Sure they build a trillion of them, but not usually in this size so the price is way skewed. A RX-64 probably cost under $20 to make ( maybe those maxon motors a super expensive, i dunno ), but they charge $280 for them. Which makes sense because the market is so niche. Niche + need 20 of em = damn expensive.

Adrenalynn
09-11-2009, 03:35 PM
Now you're going to have to point to the better alternative that's at least as powerful, at least as fast, and less expensive - or the point still stands. ;)

DresnerRobotics
09-11-2009, 03:55 PM
Sure they build a trillion of them, but not usually in this size so the price is way skewed. A RX-64 probably cost under $20 to make ( maybe those maxon motors a super expensive, i dunno ), but they charge $280 for them. Which makes sense because the market is so niche. Niche + need 20 of em = damn expensive.

Having just wrapped up months of shopping around for electronic and machine shop manufacture, I can tell you that $20 in cost for an RX-64 (even at 1000+ qty) is a pipe dream. You should see how much gears are to manufacture alone, usually the most expensive component of a hobby servo, and Maxon's are not cheap at all.

RevBryce
09-11-2009, 04:43 PM
The parts might be cheap to make but its the tooling thats the killer and that cost
has to be passed on.
The RX-64 s look like a great servo but the price for what most people want to make
Just blows them outa the water.
The knowledge base here seems pretty big so we must be able to come up with an idea
that everyone can make or afford.
I want to make a big mech to handle the weapons i must use but 5K+ just to make
it move is getting a bit pricey.
Yes i could save up the cash in a shortish time but what about the 16 year old kid
who wants to get involved, things like wiper motors while far from perfect are easy
to get and cheap, also you have to get your head around the control bits instead
of just plug and play which i think is a good thing.
Theres alot to learn about making these things work and having an understanding of all the parts
is going to help.
To me buying off the shelve components can take the fun out of it sometimes.
A question to "CogswellCogs"
What problems did you have with the oatley giant servo kits?

Adrenalynn
09-11-2009, 05:16 PM
Torque to weight, the wiper motors are NOT going to make a servo of any use in a walker. They are simply too big and too heavy for the torque you get.

Sometimes we have to face reality. I want a Bugatti but I only want to pay $30k for it. I want a 7000 sq-ft mansion overlooking the Monterey Bay, but I only want to spend $300k. And I wanna be a super model and I want to fly to Mars. I want a lot of things that aren't in my price-range. So I settle for less. The AX-12's are an excellent servo for very little money. If you want a Ferrari, you're gonna pay for a Ferrari.

Macro Man
09-11-2009, 10:30 PM
I don't know if this is useful but the electric window motors (that move car windows up/down) are like smaller lighter versions of the wiper motors. They seem to have a bigger worm diameter compared to the motor size too, so their torque to weight is probably better.

Adrenalynn
09-11-2009, 10:35 PM
Their torque-to-weight is roughly 1/12th. I have two of them going into a wheeled platform right now.

billyzelsnack
09-11-2009, 11:10 PM
Having just wrapped up months of shopping around for electronic and machine shop manufacture, I can tell you that $20 in cost for an RX-64 (even at 1000+ qty) is a pipe dream. You should see how much gears are to manufacture alone, usually the most expensive component of a hobby servo, and Maxon's are not cheap at all.

I like pipe dreams.

If you can buy a crappy high-torque metal gear servo shipped from china for $8 then why can't someone manufacture a nice high-torque metal gear servo for $20? At the scale of servos it should be easier to go bigger. There is just not a perceived market.

Once there is a market the prices with crash. I've already witnessed this in the last few years with RC helicopters. What you can get for under $200 now is just incredible. A RTF carbon fiber heli + 2.4Ghz rx/tx + servos + brushless motor + esc + lipo + charger.

billyzelsnack
09-11-2009, 11:12 PM
Here's the torque to weight of some common servos..

0.16 Hitec HS-805BB 24.5kg-cm / 152g
0.16 HS-645MG 9.6kg-cm / 60g
0.18 HXT900 1.6kg-cm / 9g
0.19 Hitec HS-805BB @7.2V maybe 29kg-cm / 152g
0.30 Robotis AX-12+ 16.5kg-cm / 55g
0.37 Hitec HS-7955TG 24kg-cm / 65g
0.42 Futaba BLS152 31kg-cm / 74g
0.44 Hitec HS-5990TG 30kg-cm / 68g
0.46 JRPROPO DS6311HV 36.5oz-in / 80g
0.52 Robotis RX-28 37.7kg-cm / 72g
0.55 Robotis RX-64 64kg-cm / 116g
0.68 Robotis EX-106 106kg-cm / 155g
0.66 VSTONE V-SERVO VS-SV410 41kg-cm / 62g
0.68 VSTONE V-SERVO VS-SV1150 115kg-cm / 170g
0.85 VSTONE V-SERVO VS-SV3310 327kg-cm / 387g

Adrenalynn
09-11-2009, 11:40 PM
In the interest of apples-to-apples, you need to add a speed term in there too...

I could put some 50,000 tooth final on the output and have a zillion.0, but it would take six years to move 60deg...

billyzelsnack
09-12-2009, 12:08 AM
They are the usual. All around 0.12s/60deg to 0.22s/60deg .

Adrenalynn
09-12-2009, 12:21 AM
Isn't that nearly a 100% spread?

billyzelsnack
09-12-2009, 09:38 AM
It does keep it out of the 6yr/60deg range!

That's a pretty common range for servo speed which also happen to be pretty good for ( humanoid ) robotics.

sam
09-12-2009, 09:39 AM
Isn't that nearly a 100% spread?

It is.


The reason I wanted to try to use a worm gear to make a custom servo (just to see how it would be) was to use a sensorless brushless motors. Since it's already hard enough to control this type of motor when it goes slowly, making it work with the usual gears that hobby servos use would be very hard. With a worm gear, I could just stop the motor and the gear itself would take care of the holding torque.

The are some brushless motors that produce huge amounts of power for their size and weight. Like this motor (http://www.subsonicplanes.com/uploads/DM2205-1350__page.pdf or his beefed up version : http://www.subsonicplanes.com/uploads/DM2205-2800__page.pdf) It's is tiny, weight is 28 grams, the ESC is 13 grams + wires and gear. Still pretty good.

I was interested by what I could manage to make with this.

The problem, as others mentioned, would be to get an efficient worm drive, and make it resist to impacts.

I would still like to try even though it probably won't be worth it ($ wise and time wise).

Sam

robologist
09-14-2009, 05:24 AM
There are a couple of bipeds at Xrobots (http://www.xrobots.co.uk/) that make use of wiper motors, but in a cable suspended limited actuation manner (shuffle bots). Check out the videos for
Android 6 YouTube - Android 6 First Steps

and also for
Android 7 YouTube - Android 7 first steps

Adrenalynn
09-14-2009, 05:31 AM
Great piece of mechanical engineering. I've seen shuffle bots running V8 gas motors and deisels out of semi trucks too. I guess in some way we could think of them as "bipeds".

That bottom one, "android 7" looks to be, what, around 4ft tall standing next to that bed? I think that kind of scale is what we're likely to see from a wiper motor.

I don't think I'd count a shuffle bot as fitting into the walker category though, and that's where I was going with it.

RevBryce
09-16-2009, 03:05 PM
To continue with this i hit this site today.
Interesting way of doing the wiper motor thing.
http://www.fieroaddiction.com/servo.html