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Author Topic: Alternative to Servo - Solenoid?
RussellSpr-
outs2
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Post Alternative to Servo - Solenoid?
on: May 16, 2012, 04:21
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Hey everyone,
I've been thinking about creating my own Polar-graph inspired bot. I want to run it with an SD card for speed, but I don't want to get a Mega, I'll run it on an Uno, even if I have to write my own code to drive it. So, combining the Adafruit motorshield and SD card breakout is possible with some re-arranging of the pins, but it leaves the servo control timers from pins 9 and 10 in use.

I thought it might work to use one of the free analog pins to control http://www.adafruit.com/products/412 a small push pull solenoid to lift the pen off the paper instead of using a servo.

Has anyone tried this, or would it even work?

Thanks for your help
sandy
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sandy
Post Re: Alternative to Servo - Solenoid?
on: May 16, 2012, 09:45
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Hi Russell, using a solenoid would be great I think, quicker than a servo but a bit less gentle. If you could damp the throw somehow, I don't really see the downside. You'd need a transistor to power it, and a bit of a hack in the code, but it's encapsulated fairly neatly so it'd only need a hack in one place.

The SD stuff is do-able, I'm positive. Physically it's like you say, a matter of remapping the pins that the motorshield uses to other ones. I have a feeling that pins 9 and 10 (the PWM pins) are only used when using the microstepping mode, but I could be wrong.

Fitting the code, and keeping enough spare SRAM (including a 512 byte data buffer) was the real stumbling block for me, though it could probably be easily solved by treating the SD card as built-in memory rather than something removable that could be used on a PC. That way you can afford to be a bit more simplistic rather than shoot for something that's FATish. Actually one thing I thought of but never tried was producing a cut-down read-only version of the SD library.

My brief findings are on http://code.google.com/p/polargraph/wiki/BuildAdvancedElectronics

good luck, let us know how you get on!
kongorilla
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kongorilla
Post Re: Alternative to Servo - Solenoid?
on: May 16, 2012, 20:16
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It could add some drama to the drawing process (a hopping gondola). Replacing the whine of a servo with the *thunk* of a solenoid slug hitting the board would be an interesting trade.

If the slug's resting position is already touching the board, and you supply only enough current to gently push the gondola up, it would be great. Also, in case of a problem (like the polargraph software freezes with the coil powered), using the minimum required current could prevent a meltdown/fire. (I'm not an engineer, so I barely know what I'm talking about).

I suppose a solenoid would use a lot more electricity than a servo, which if you plan to do 24 hour drawings might be something to ponder. Then again, using a solenoid would save time.

I look forward to seeing what you come up with!
RussellSpr-
outs2
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Posts: 3
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Post Re: Alternative to Servo - Solenoid?
on: May 16, 2012, 23:52
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I thought I might try to attach the pen to the solenoid somehow, so then only the pen lifts off the page. If that doesn't work, then I will attach the solenoid to lift the whole gondola.

But, I won't be able to start working on this until the summer sometime. When I get started I'll be posting my progress somewhere.
Visualbyte
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Posts: 33
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Post Re: Alternative to Servo - Solenoid?
on: March 4, 2015, 15:38
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I know this is an old thread but I wanted to add a thought, excessive pen ups and down certainly have an impact on the drawing quality as it seems to swing the gondola a little and personally a bit like Kong I think some linear motion would be better form of moving the pen. A linear servo could actually move the pen up and down centralising the point of lift and drop.

The pen tip would still be taking the same impact and the washers between the bearings and between the existing pen holder could be redesigned to hold the linear servo and have the pen resting in a grommet in the brass barrel.

You will have to excuse the roughness of my mock up it is purely to give an idea.

Image
sandy
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sandy
Post Re: Alternative to Servo - Solenoid?
on: March 4, 2015, 21:33
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Yikes those are expensive! I agree about the servo - the fact that the control horn swings in from the side does introduce a definite twist to the pen drop. Lift

I wonder if the servo horn motion was up and down rather than left-to-right, would that solve it completely? So the servo was mounted vertically. It would mean that it wouldn't be as close to the pen tip, might have to do some experiments.

sn
Visualbyte
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Posts: 33
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Post Re: Alternative to Servo - Solenoid?
on: March 5, 2015, 08:01
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Didn't notice the price, "whoops" there is always the EMS linear conversion kit for hitecs I'm not sure what size everyone is using

http://www.emsjomar.com/SearchResult.aspx?CategoryID=18

But I did wonder about turning the servo 90 degrees so that when the servo moves to pen up it travels downwards towards the floor.
kongorilla
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Posts: 362
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kongorilla
Post Re: Alternative to Servo - Solenoid?
on: March 5, 2015, 11:10
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If I were having trouble with the servo horn lifting the gondola at an angle as it does (I'm not), I'd consider doing something like this:

Image
sandy
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sandy
Post Re: Alternative to Servo - Solenoid?
on: March 5, 2015, 11:39
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This, very much. Thanks Kong.
Visualbyte
Beginner
Posts: 33
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Post Re: Alternative to Servo - Solenoid?
on: March 5, 2015, 13:08
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Good idea, I might go and look for a ball point pen to hack up..
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