[dorkbotpdx-blabber] where to get and learn about solenoids

[douglas repetto]

There are push/pull soleniods that can both push and pull the shaft 
(when power polarity is reversed), but they're rare. They work by using 
a magnet as the shaft, rather than a piece of plain steel. That way the 
polarity of the electromagnet (set by the polarity of the power supply 
hookup) _does_ matter, since there will be an attraction/repulsion 
relationship between the poles of the shaft magnet and the poles of the 
electromagnet.

But for most purposes that's not necessary and a spring/gravity return 
is sufficient.

douglas

Greg Grunest wrote:
> Yes.  That's exactly how a linear solenoid works!  Once the electromagnet
> (coil) pushes (or pulls) the shaft, a spring returns it to the original
> position when the coil is deenergized.
> 
> On a rotary solenoid, it's usually a spring and some gears and a little cam
> type device.
> 
> - Greg
> 
> -----Original Message-----
> From: dorkbotpdx-blabber-bounces at dorkbot.org
> [mailto:dorkbotpdx-blabber-bounces at dorkbot.org] On Behalf Of John Brown
> Sent: Wednesday, December 03, 2008 12:26 PM
> To: A discussion list for dorkbot-pdx (portland, or)
> Subject: Re: [dorkbotpdx-blabber] where to get and learn about solenoids
> 
> Thanks Douglas! That's a great page.
> 
> If I may ask a quick question, though, does a solenoid work is the
> same way as a coil gun, in that the induction of current into the wire
> draws in the metal shaft? Is that what gives it the binary action? And
> then when current is stopped, what pulls the shaft back out of the
> coil?
> 
> I hope I am understanding it correctly.
> 
> -John Brown
> 
> On Wed, Dec 3, 2008 at 10:58 AM, douglas repetto
> <douglas at music.columbia.edu> wrote:
>> FWIW, I have a website that I teach lots of workshops from that covers the
>> basics of different kinds of motors/solenoids/relays/etc. By basics I mean
>> basics, but with the goal of providing the vocabulary so that you can go
> out
>> and do web searches for more specific info once you actually know what you
>> want to learn about. It's here:
>>
>> http://music.columbia.edu/~douglas/classes/motor_mania/
>>
>>
>> best,
>> douglas
>>
>>
>> Greg Grunest wrote:
>>> Greg,
>>>
>>> Solenoids come in all different varieties.  Typically, you find the 12 V,
>>> linear, push or pull solenoids in everyday consumer electronics (disk
>>> drives, hard drives, tape players, that old and now useless Zip-100 drive
>>> has a really cool 6v one in it but it's *really* hard to get it out,
>>> etc...)
>>> but every so often you can find surplus rotary solenoids too.  For moving
>>> the drumsticks, a rotary solenoid might be the thing.  They typically
> move
>>> through a 90 degree arc when energized and then return to where they
>>> started
>>> when power is removed.  However I've had some before that sort of ratchet
>>> through 15-30 degree increments each time power is applied eventually
>>> spinning all the way around.  They were connected to a little mechanical
>>> counter and appeared to do nothing more than count how many times this
>>> thing
>>> had fired.
>>>
>>> The pros:
>>>        They're cool!
>>>        They typically work forever - millions of cycles.
>>>        You can do things with an $8.00 solenoid that you could otherwise
>>> spend hundreds of dollars trying to replicate.  (I once built a remote
>>> controlled golf cart and used Ford starter solenoids ($8.00 from Napa) to
>>> switch the 600 Amp battery connections to the motor.  I used a $3
>>> optoisolator to drive the solenoids and an 8051 to drive the
> optoisolator.
>>> Controlling 600Amps from a microprocessor for $11 - Woot!)
>>>        You can get feedback from them by measuring how much current they
>>> draw (voltage drop across an in-line resistor) and tell if they are
>>> hitting
>>> anything.  They tend to draw a lot of current (like a stalled electric
>>> motor) if you hold them against their will.
>>>
>>> The cons:
>>>        They suck current like mad and therefore are not good for battery
>>> driven applications.
>>>        It's basically a coil so you have to worry about re-induction when
>>> the magnetic field around the coil collapses so you don't blow up
>>> sensitive
>>> circuitry (like the Arduino or a transistor).
>>>        They are typically not driven by any control signal that is easy
> to
>>> generate in a microprocessor world, 9-50V, 500ma-2A.
>>>        (Because of these two drawbacks, I highly recommend a simple 8 pin
>>> optoisolator to drive them.  It's cheap and completely protects the
>>> Arduino.)
>>>        You can't control how fast they move like you can with a servo.
>>> They are either in or out, turned or not.  In fact, if you try to slow
>>> them
>>> down by limiting the voltage they draw a LOT of current.
>>>        They are entirely digital - On or off, in or out.
>>>
>>>        And the number one drawback is........ can you say "latch up"?
>>> (sometimes they stick in the "energized" state.)
>>>
>>> Just my 2 cents.
>>>
>>> BTW:  If you're interested, I think I have a couple of the 90 degree
>>> rotary
>>> ones left.  Enough for you to play with and try the drums anyway.
>>>
>>> - Greg Grunest
>>> greg at grunest.com
>>>
>>> _______________________________________________
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>>> dorkbotpdx-blabber at dorkbot.org
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>>>
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