Author Topic: large servos  (Read 4579 times)

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Offline dcole07Topic starter

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large servos
« on: November 01, 2006, 10:37:59 PM »
I was watching the new about what robots are going to be like in 2030 and it got me thinking about servos and torque. I looked around the internet and it seams that the torque in servos is very small (oz). How do large scale androids (like Honda's) support them selves yet be very accuret? Can someone show me a servo or motor that could support a 200lb (110kg) android, or tell me what the specs would have to be on something like that? I read in this forum that Honda has custom servos or motors, is there anything on the open market that can match it?

Offline Arislan

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Re: large servos
« Reply #1 on: November 02, 2006, 09:03:46 PM »
They just make larger servos using more powerful motors. This link http://homepages.which.net/~paul.hills/Circuits/PowerServo/PowerServo.html
has some basic tips on building the servos using stronger stepper motors, but it unfortunately is not really a step by step tutorial.
A servo is just an electronically controlled stepper motor, there is no mystery there.

Offline JesseWelling

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Re: large servos
« Reply #2 on: November 02, 2006, 09:58:09 PM »
actualy I thought most servos were just regular DC with pot encoding of the shaft.....which was fed through a PD controller...But are they really stepper motors?

Offline Arislan

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Re: large servos
« Reply #3 on: November 02, 2006, 10:47:21 PM »
I was going by what admin told me on another topic. He told me to use stepper motors.
I sure wish he'd make a tutorial on building a servo.

Offline JesseWelling

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Re: large servos
« Reply #4 on: November 03, 2006, 02:25:50 AM »
stepper motors do give good positional control with out a lot of hassle. But Servo's that use a PD control on a DC motor are by far a better solution for getting arbitrary angles.

Although I guess it don't fully under stand steper motors but from what I do understand they can only reach certian predefined angles depending on how many magnets the motor uses.....

Maybe admin can enlighten us ??? or we could wiki.... 8)
http://en.wikipedia.org/wiki/Stepper_motors
http://en.wikipedia.org/wiki/RC_Servo
http://en.wikipedia.org/wiki/Servo_motor

So I guess it is possible to get a bit more fined grain control out of a stepper than I thought...but I think if you are building from scratch I would go with dc motor and an encoder.



Offline Admin

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Re: large servos
« Reply #5 on: November 03, 2006, 08:43:47 AM »
Steppers can do 360 degree PID control without much hassle, as Jesse said . . . but its open loop control, meaning if your robot runs over a cat at full speed, the stepper can be forced out of position and your controller wouldnt know. It also has some limited set of possible positions it can go to, depending on resolution. Between 2 degrees and 10 degrees is typical.

Hobby servos have a limited set of angles too, although theoretically it can have a much higher resolution at a much lower cost. Hobby servos (unmodified) have some set maximum angle, usually 180 degrees, that it can rotate to. Accuracy is within about 3 degrees on most unmodified servos I have used. Modified servos are open loop control systems, meaning the position control will build up error over time.

Servo motors are very smooth in movement, but steppers have a slight vibration, depending on how good your code is. I once saw a robot make a really loud clattering sound whenever it moved, cause the stepper would shake the aluminum violently when moving . . .

For wheels, either servos or steppers will be fine.

For things like robot arms/legs, unmodified servos are best.

For things that require accurate 360+ degree rotation, such as a tank turret, a stepper would be best.

In the end, I perfer hobby servos because they are cheaper, easier to control, and all the electronics are already built in - saving me time, money, and frustration. Plus, you need to understand control theory if you use steppers.

Below is a stepper motor image. It has a fixed number of magnets, meaning a fixed number of controlled positions. More of these mean a higher resolution.

Offline Arislan

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Re: large servos
« Reply #6 on: November 04, 2006, 11:37:21 PM »
I really do wish you would make a tutorial on turning a big dc motor into a big servo. ;-)

Offline JesseWelling

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Re: large servos
« Reply #7 on: November 05, 2006, 03:05:29 AM »
I bet it's the same as turning a small dc motor into a servo  :P

Offline dunk

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Re: large servos
« Reply #8 on: November 05, 2006, 01:05:45 PM »
only bigger.....

dunk.
(sorry, couldn't resist.)

Offline ragavendra

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Re: large servos
« Reply #9 on: October 23, 2008, 12:27:29 PM »
the stepper motor in particular the DC stepper motor could be used and i could not give any match for it as the torque is exceptional due to the coil windings in it.
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Offline paulstreats

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Re: large servos
« Reply #10 on: October 23, 2008, 05:42:53 PM »
An encoder could be used on a stepper motor to give it a feedback advantage.

The thing with scaling servo's up is that you eventually need larger motor and stronger gears. These dont scale up weight wise in a linear sense so scaling up means that they are heavier proportionally and likely to use more electricity. Robots like asimo cost hundreds of millions to build because they are working close to the forefront of whats possible. Though I do think that a full sized biped is buildable on a budget today, it just takes a lot of work.

If you want to build you own servo with a larger motor, then its easy enough just get the motor, attach it to whatever. Then get your gears to make the motor strong enough to turn your device then use a different gearing system which attaches to the head of a potentiometer. The potentiometer provides the position feedback.

It sounds simple in words.......... Bet its not as simple when you try to do the maths for it.

I dont think there would be much on the open market for asimo replacement parts unfortunately, I haves a funny feeling that they are custom made according to honda research specs..

Offline gamefreak

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Re: large servos
« Reply #11 on: October 24, 2008, 08:17:08 AM »
Not really relevant but wouldnt the weight go up in a cubic fashion for the gears?
scaling it by a factor of two scales the volume by a factor of eight, since the mass is a function of density and volume the mass would also increase by a factor of eight.

Out of curiosity do hollow/expanded gears exist?
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