LVDT Reversibility?

O5O wrote:

In theory, yes. In practice, no. LVDTs, like all measurement devices,
are optimized to have the least possible effect on the systems they
measure. A typical tachometer is a poor motor, in part because the
winding resistance is too high, and because the magnets would be
weakened by high currents.

There are other devices you can use. Google for "linear actuator", and
think about making use of rotary actuator. (Voice coils are used to
position the head in some disk drives. They can't be all that bad.)

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:

Actually, if you are in scrounge mode the voice coil actuator from a
disk drive wouldn't be a bad thing to use.  Voice coil actuators are
just the bee's knees when you need precision, speed, and when you're not
pushing against a big load.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:

I have a few old 5" full-height 5-megabyte hard disks with stepper
motors and ban-around-a drum drives to position the head. 050 is welcome
to all or part of one, including the drive transistors.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:

   ...


My junk pile turned up a  linear actuator with a 5" stroke. It consists
of a six-lead rotary stepper motor with hollow shaft that is threaded
1/4-16 acme. (Remarkably little backlash. Loaded ball nut?) At 24
steps/turn -- a wild guess -- the resolution is about .0026", although
the backlash may exceed that. Half stepping will halve that.


There is a driver chip in the package (and a spare), but no wiring diagram.

Hurst Mfg Corp
Instrument motor
Model No LAS
Part No 3602-001
12 Volts DC

Hurst is a division of Emerson Electric. ...

I found the spec. It's still in current manufacture. With light load,
you can get about 1/4"/sec. Email if you want it, explaining why you
can't buy one on your budget.

http://www.myhurst.com/hurstmfg/qseries.jsp?series=Series+LAS%2C+LBS+Stepping+Linear+Actuator

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:

http://www.myhurst.com/hurstmfg/qseries.jsp?series=Series+LAS%2C+LBS+Stepping+Linear+Actuator

Ok. Got it in my download area.

Chris.

O5O wrote:

In theory it's possible, however the LVDT windings are too small to
generate any appreciable force. BEI makes some nice little voice coil
actuators which can be run from a linear amplifier such as Trust
Automation, or Quanser. Some form of feedback will be required, LVDT,
potentiometer.

--
jeff

On Tue, 11 Jul 2006 10:37:26 GMT, "O5O"

"relatively fast" meaning what?  What the inertial load?
What's your bandwidth requirement and required position accuracy?

You can do some simple calculation assuming sinusoidal movements to
determine your power requirement. This will help to select an
approproate actuator.  Check out the following:
http://www.sampledsystems.com/pdfs/1995_SPIE_Voice_Coil.pdf

3.75 inches is a large stroke.  
You'll have a hard time getting a bandwidth larger than 100 Hz.
BEI has some state of the art commercial linear actuators and lots of
good application notes.  Their model LA19-40-000A as a +/- 0.9 inch
motion (1.8inch total stroke).

http://www.beikimco.com/
http://www.beikimco.com/products/linearvoicecoil.php

or maybe a linear motor is better for your application.
You can searh on  "fast steering mirrors" (FSM) for info on fast
actuators to get other ideas, but nothing with a 3.75 inch stroke.

To be truthful, I don't yet know what kind of (actual) response time may
necessary for this application, or what level of control I will be able to
acheive with the material flow. I do know that the tunning element is a 1/8"
diameter pin of tool steel that needs to be inserted 1/2 of way into the
3-3/4" deep waveguide from the waveguide inner surface level. No forces to
counteract except friction, gravity or a spring. Frictional forces should be
negligible, but the inertial load depends on how fast or how far I have to
compensate for a match or mismatch.

I am considering using an aluminum funnel type device with a needle valve,
gravity feed and external vibrator to control the flow of grain sized heavy
materials, but fear that this may not be as effective a control for lighter
massed fine powders. Perhaps a vaporized slurry will work better in this
instance to keep flow relatively constant.

As far as positional accuracy, the actual position is for reference to well
characterized flows only. What I am really after is to maximize the power to
the plasma and material flow by minimizing any reflected power that may
occur for "whatever" reason. I "think" that this is going to depend on the
material flow, but I won't truly know for sure until after I have built the
machine and have done some experiments.

I am trying now to think of some worst case scenarios, and how they might be
mitigated before I have any real experimental data to consider.


3.75 is the width or depth of the waveguide, or the maximum travel that the
pin could possibly make (worst case). In reality 1/2 that distance is the
position that creates the maximum disturbance in the microwave radiation
field, which is all that is really necessary (1.875 inches plus a little
extra for overshoot and assembly tolerances). But then again I may want to
change the frequency from 2.45 GHz to 900 MHz in the future and be able to
use as many of the same parts as possible.


Yes. One of these might work with the addition of a lever mechanisim to
extend the stroke length.


Ah yes. I remember these from laser light shows. I'll take a look.

Linear actuators or voice coils seem to be the general consensus at this
time. Anyway I have yet to fab an isolator, which needs to be completed
before I can even turn the machine on. That is another project all in
itself, so there is plenty of time for me to think about what I am going to
do here.

Thanks again everyone.
Chris.

O5O wrote:
   ...


Should we read "vaporized" as "fluidized"?

   ...

Good luck.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Probably more appropriate, although when it gets to the plasma I fully
expect everything to be vaporized... But also before that with the correct
nozzle, it could be "atomized" as well.

Chris

is it possible to use stub tuners rather than a sliding short  - rotation is
probably easier to control.

Yep. The stub tuners would seem to be the way to go, and are what I am now
currently drawing up plans for.

Chris.

O5O wrote:

I take it then that the linear actuator is no longer part of the plan?

Do you want a stepper out of an old disk drive? (About 2" dia., probably
12 volts.)

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

No. The linear actuator is the only plan at this time to drive the stub
tuners.



Thanks Jerry, but I have a couple of these here as well. I'm placing my bets
that I can control the feedstock well enough that the linear acuator with
the stepper motor will have enough granularity and speed to keep an accurate
hold on the tuning paramater. I feel pretty good about this right now. As
soon as it cools off outside (102 Deg F.), I am going back out to the garage
to cut some more steel for it.

Chris. T.

O5O wrote:



OK. I have your address.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

I still haven't got your WalkNet address.

Chris.

O5O wrote:

WalkNet? That's beyond me.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Actually I have both stub tuners and sliding shorts in the design. The
sliding shorts terminate each end of the waveguide, and are moveable to make
sure that the end reflections are exacly 1/4 wavelength from the microwave
tube element and the focusing nozzle. Once these have been adjusted manually
and set, they are locked into place and will never move after that. Three
stub tuners are located between the magnetron element and the circulator.
One of these will be selected to be driven by the linear actuator. The other
two are manually operated. The idea here is that the two manual tuners can
be set for rough or coarse adjustments depending on the feedstock
requirements, and the third stub tuner will be driven with the linear
actuator for fine / variable adjustments while processing.

Chris. T.

Is it possible to control tuning by varying the  distance the screw is in
the waveguide?

What kinda frequency and power are you at?

Setanta