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From: Brandon on 29 Apr 2010 11:05
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!
From: Tim Wescott on 29 Apr 2010 12:54
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.

I'm taking the liberty of cross-posting this to the controls newsgroup
-- it needs some valid traffic, and this post is spot-on for the group.

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
custom electronics (with custom electronics it's a snap). But LVDTs can
be a bit spendy, they tend to be bulky, and most LVDT circuits have a
serious bandwidth vs. excitation bleedthrough tradeoff.

Potentiometer feedback is cheap, but it's cheap in both senses of the
word -- it won't cost much, but you'll have trouble getting noise-free
operation at those speeds.

For that size of a stroke you may be able to get away with an optical
sensor -- measure the amount that a transmitter/receiver pair is
occluded as the thing travels. This has engineering issues, and
therefore high initial cost, but it may be just the ticket for
high-volume production. LEDs get funny at temperature extremes, so if
you want this to work from Alaska to Algeria you can't just get it
working on a lab bench and trust that it'll work everywhere, any time.

There are some linear quadrature encoders that may work -- 2mm is a
pretty short stroke, but those things are getting better and better
every day. I'd do an A/B comparison between that and an LVDT were I to
launch a product doing this. An optical linear encoder suffers from the
same environmental issues as the differential optical gizmo I suggested
above.

Depending on the solenoid and your preferred engineering expense vs. per
piece price, you may be able to do this by using the solenoid itself as
a position sensor, as its inductance will change with position. For a
one-off this is seriously unwise mad science; for production it makes
sense as long as the level of accuracy you needs rivals the production
volume: modest accuracy would work with modest production volumes,
really high accuracy would require lots of engineering which would only
get paid off with high production volumes, etc.

As for control:

Frequency is easy -- just drive the thing at the frequency you want.

Stroke is not too much harder: to a first order, if your solenoid and
driver have a high enough bandwidth you can directly servo your
position. If you can stand a few cycles of settling you can inject some
drive signal through a stage that you adjust with feedback to get the
amplitude and phase that you need.

Do you need to regulate the center position as well?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Bob Eld on 29 Apr 2010 13:32

"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.


From: J.A. Legris on 29 Apr 2010 13:55
On Apr 29, 1:32 pm, "Bob Eld" <nsmontas...(a)yahoo.com> wrote:
> "Brandon" <brandon.joseph.mo...(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.

Good advice.

I once had a similar project, and no budget, so I carefully removed
the cone from a small bass loudspeaker, preserving the suspension and
the electrical connections, and epoxied a take-off point to the
circumference of the remaining dome over the voice-coil. Driving it
open-loop with a D/A and a power op-amp, it worked quite nicely for
the application, which was to simulate the pecking action of a bird.
Another advantage of a voice-coil over a solenoid is that it can both
push and pull.

--
Joe
From: pnachtwey on 29 Apr 2010 13:58
On Apr 29, 9:54 am, Tim Wescott <t...(a)seemywebsite.now> wrote:
> 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.
>
> I'm taking the liberty of cross-posting this to the controls newsgroup
> -- it needs some valid traffic, and this post is spot-on for the group.
>
> 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
> custom electronics (with custom electronics it's a snap).  But LVDTs can
> be a bit spendy, they tend to be bulky, and most LVDT circuits have a
> serious bandwidth vs. excitation bleedthrough tradeoff.
>
> Potentiometer feedback is cheap, but it's cheap in both senses of the
> word -- it won't cost much, but you'll have trouble getting noise-free
> operation at those speeds.
>
> For that size of a stroke you may be able to get away with an optical
> sensor -- measure the amount that a transmitter/receiver pair is
> occluded as the thing travels.  This has engineering issues, and
> therefore high initial cost, but it may be just the ticket for
> high-volume production.  LEDs get funny at temperature extremes, so if
> you want this to work from Alaska to Algeria you can't just get it
> working on a lab bench and trust that it'll work everywhere, any time.
>
> There are some linear quadrature encoders that may work -- 2mm is a
> pretty short stroke, but those things are getting better and better
> every day.  I'd do an A/B comparison between that and an LVDT were I to
> launch a product doing this.  An optical linear encoder suffers from the
> same environmental issues as the differential optical gizmo I suggested
> above.
>
> Depending on the solenoid and your preferred engineering expense vs. per
> piece price, you may be able to do this by using the solenoid itself as
> a position sensor, as its inductance will change with position.  For a
> one-off this is seriously unwise mad science; for production it makes
> sense as long as the level of accuracy you needs rivals the production
> volume: modest accuracy would work with modest production volumes,
> really high accuracy would require lots of engineering which would only
> get paid off with high production volumes, etc.
>
> As for control:
>
> Frequency is easy -- just drive the thing at the frequency you want.
>
> Stroke is not too much harder: to a first order, if your solenoid and
> driver have a high enough bandwidth you can directly servo your
> position.  If you can stand a few cycles of settling you can inject some
> drive signal through a stage that you adjust with feedback to get the
> amplitude and phase that you need.
>
> Do you need to regulate the center position as well?
>
> --
> Tim Wescott
> Control system and signal processing consultingwww.wescottdesign.com
My usual answer is to first look at car cams and find a good reason
why a simple mechanical solution can't work. If the amplitude doesn't
need to change on-the-fly you can put what ever needs to be moved
between two cams that push and pull the object back and forth. If
there isn't much mass only one cam is required and a spring can push
the object back against the cam.

We have done custom electric projects like this. We were trying to
make a fast cutting device using a solenoid. The cutting had to be
precise so position was important. The frequency was only about 60 Hz
for the whole cycle but cutter had to accelerate from a stop,
accelerate to almost twice the belt speed then slow down to match the
belt speed and then accelerate quickly then stop at the top. This was
done in 16 ms. The amplitude of this motion was much bigger than 2mm.

I would use a glass scale encoder. The resolutions are now sub
micron. I would not use LVDTs because of the phase delays involved.
It is relatively easy to do this open loop using a signal generator
and then monitoring the peaks to make adjustments to the open loop
signal.

Peter Nachtwey





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