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From: jeff on 30 Apr 2010 20:35
Tim Wescott wrote:
> Brandon wrote:
>
> (context restored -- this is USENET)
>
> Brandon wrote:
> > I have a project in which I would like to produce a "smooth"
> > reciprocating motion with an adjustable stroke length between about 1
> > mm and 2 mm and an adjustable frequency between 100 Hz and 150 Hz. I
> > think there might be some advantages to using a solenoid for this (as
> > opposed to a rotary motor and a mechanical design) but only if I can
> > get good feedback control of the stroke and frequency. In a spring-
> > mass mass system frequency can be controlled open-loop, but I'm not
> > sure how well I could regulate the end and start position of each
> > stroke or even what sort of sensor might be appropriate and have a
> > fast enough response and a clean enough signal to work at that
> > frequency. Wondering if anyone has any good electronic (or
> > mechatronic) ideas as to how this might be done.
>
> >> snip <<
>
> This sort of question always boils down to questions like how much money
> you have to spend, whether you're using custom electronics, what sort of
> production volumes you're contemplating, how precise it needs to be, how
> long it can take to achieve precision, what sort of environment it needs
> to work in, how heavy it can be, etc.
>
> _The_ indicated position sensor for this would be a short-stroke LVDT.
> It may be hard to get the accuracy you want from it at 100Hz without

Spring- mass system witn a resonant mode in the middle of your frequency
range, use the reference coil of your lvdt as the driving coil for the
solenoid core, the CT sense coil provides position feedback. Drive
method of the coil may be a tradeoff. PWM would give the best efficency,
but AM signal would probably make the LVDT demodulation easier. Sounds
very do-able in the price range mentioned.
From: Robert Baer on 30 Apr 2010 23:38
Michael A. Terrell wrote:
> Robert Baer wrote:
>> Michael A. Terrell wrote:
>>> "J.A. Legris" wrote:
>>>> I once had a similar project, and no budget, so I carefully removed
>>>> the cone from a small bass loudspeaker, preserving the suspension
>>>
>>> The suspension is called a 'spider'.
>>>
>>>
>> Is it found somewhere tangling with the web?
>
>
> No. Early speakers used a very crude suspension that looked like a
> spider web. The speakers were so crude that there were adjustments to
> center the voice coil.
>
> Later speakers were made of stiff fabric that were glued to the frame
> & the cone so the alignment didn't change, as long as the cone wasn't
> damaged.
>
>
Am rather familiar with them; did many a cone replacement in daze of
yore - along with the sets of 3 shims for centering.
From: Robert Baer on 30 Apr 2010 23:38
Tim Wescott wrote:
> Bob Eld wrote:
>> "Brandon" <brandon.joseph.moore(a)gmail.com> wrote in message
>> news:7de228e2-e708-4a28-a1d6-119b74aaf5c6(a)s2g2000yqa.googlegroups.com...
>>> I have a project in which I would like to produce a "smooth"
>>> reciprocating motion with an adjustable stroke length between about 1
>>> mm and 2 mm and an adjustable frequency between 100 Hz and 150 Hz. I
>>> think there might be some advantages to using a solenoid for this (as
>>> opposed to a rotary motor and a mechanical design) but only if I can
>>> get good feedback control of the stroke and frequency. In a spring-
>>> mass mass system frequency can be controlled open-loop, but I'm not
>>> sure how well I could regulate the end and start position of each
>>> stroke or even what sort of sensor might be appropriate and have a
>>> fast enough response and a clean enough signal to work at that
>>> frequency. Wondering if anyone has any good electronic (or
>>> mechatronic) ideas as to how this might be done.
>>>
>>> Thanks!
>>
>> What does "smooth" mean? A solenoid is one way but they are non-linear
>> and
>> may be difficult to control the stroke length because of inertia etc. How
>> much force do you need and what is the precision of motion and position
>> necessary? The best way for precision motion, controlled velocity and
>> position is a voice coil actuator. These can be designed with any
>> force from
>> few dynes up to tonnes if necessary. The feedback can be acceleration,
>> velocity or position with various sensors, accelerometers, coils, linear
>> pots or line encoders. There are many options depending on what you are
>> trying to do.
>>
>> Voice coils have the advantage over moving iron devices like solenoids of
>> having low mass and a linear current to force function which makes
>> them much
>> easier to control. They have much wider band width as well. More
>> information
>> would be helpful.
>>
>>
> More information would be really helpful. And I second the thought that
> a voice coil would be better than a solenoid, unless you have strong
> reasons to want one.
>
Tear apart an old hard drive..
From: Robert Baer on 30 Apr 2010 23:44
Brandon wrote:
> Thanks for the responses. Lots to think about. I have a controls
> background, but most of my work has been on the algorithmic/academic
> end of the spectrum so I don't really know much about technology
> unfortunately.
>
> I guess I can share a few more specifics about what I'd like to
> accomplish. It's a hand tool for an artist friend, but it has a
> potentially niche-sized market if it's an decent improvement over what
> he's using now, so I'm thinking in terms of production.
>
> Minimum requirements:
> 1) Pretty good frequency control. Any oscillation not noticeable to
> the user.
> 2) Very precise control of one end point of the stroke and reasonable
> control on the other end.
> 3) Little if any noticeable off-axis vibration.
* Voice coil designs are rather good in those 3 regards; look at speaker
voice coil positioning can be controlled by use of an arbitrary
waveshape and hard drive positioning is definitely by arbitrary drive.

>
> Really nice things to have that would be probably be necessary for
> commercial success:
> A) Low enough power drain to operate off a small battery (3 cm^3
> max???) for a decent about of time. Sorry, I don't have more
> specifics on that yet.
> B) Production costs for accuator/sensor/electronics of $200 at the
> very most. Probably needs to be less than that unless this turns out
> to be so great that people are willing to pay substantially more than
> the current technology costs.
>
> Other things that would be cool but aren't absolutely necessary:
> a) Shaping of end effector trajectory. Lower velocity on out stroke
> then on return stroke.
> b) Quick adjustment of stroke length.
>
> Force requirements: Pretty small I think. The reciprocating tool is
> very light, on the order of a couple paper clips maybe and it doesn't
> have to apply much force to the material you're working on. Force
> requirements are probably dominated by the friction needed to hold the
> tool in place.
>
> That's all I can think of right now.
>
> (2) and (a) make me want to use a cam. The potential problems I see
> with this are that cutting an arbitrary cam profile on something small
> is not that easy, so (a) may not be that achievable. Expense is also
> an issue. Small motors are pricey and even a 16 line encoder that
> small seems to cost as much as the motor. Easily over $100 each total
> right there which is pushing the limits of what this thing can cost.
> Accuracy and settling time of speed control is not going to be as good
> as I would have hoped for, but it's probably acceptable. Power
> drain? I don't know if a solenoid would be any better, but that voice
> coil has me intrigued. (1), (3), (a), (b), and maybe (B) and (A) are
> what got me thinking about a solenoid, provided that (2) could be
> achieved via some combination of physical mechanism and feedback
> control.
From: Brandon on 4 May 2010 03:08
On Apr 30, 7:55 am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
> FYI: engineers don't use democratic words like "big", "little",
> "better", "worse", "decent", "reasonable", "pretty". Engineers use
> numbers. Before you have the numbers, there is nothing to discuss.

Not everything needs to be quantifiable. For a big and expensive
project development you'd definitely want to nail down your
specifications as much as possible to minimize a costly redesign. For
things like communication equipment that have to interface with
existing technology, the numbers pretty much define your project. But
this hypothetical gizmo is for an artist, so knowing that the force
needed is "pretty small" tells me enough. You build a mechanism that
can overcome its own inertia by a large enough margin to make you
confident that it'll work and then go out and test it. Is it really
cost and time effective for me to go out and try to measure this force
just to provide a number? I doubt it. I could be wrong, but that's a
chance I'll take.

Besides, I'm not asking anyone here to design this thing for me. Just
for general ideas. Thanks for everyone's suggestions.


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