Push-pull sensor - any ideas?

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For a project I need to find a sensor that can measure push and pull forces
in an alumimium push-pull tube. The goal is to use a servo to eliminate
these forces. The sensor doesn't need to be accurate or linear, it needs to
be able to indicate 'no force, pushing, pulling'. The push-pull tube may be
split to place the sensor in-line, but the end result should be just as
strong and stiff as the original tube.

I looked at load cells, but these are prohibitively expensive. The normal
range of forces is up to 5 kg either way, the null zone should be +/- 50
grammes or so (need to experiment with that...). The whole system does
however need to be able to survive occasional overload peaks (someone
banging the stick). Strong, light, reliable and affordable are the key
words.

Any ideas?

Rob



Re: Push-pull sensor - any ideas?

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You said that load cells are too expensive.  What is your
buget for this assembly?



Re: Push-pull sensor - any ideas?

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Your requirements are incompatible.  To measure down to 50 grams
requires that you use something not so stiff (which you say is out of
the question) or you must measure incredibly small deflections, which
is very expensive, if it is even possible.  If it is possible, it will
not be cheap (which you say is also a requirement).  Something has got
to give.  Why do you need such stiffness?


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


Re: Push-pull sensor - any ideas?

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forces
to
be
Hello Rob,

If you don't count the cost of your time you can make your aluminium tube
into a load cell for less than you pay for a ready made one by bonding some
strain gauges to it. You must use a temperature compensated full bridge if
you are bothered about thermal drift of the zero. It is easy to get a 100:1
dynamic range and possible to get more than 10000:1.

If you have no experience of doing this you will need several attempts to
make one that works. If it's for mass production count the development time
in man-years.
If you can arrange some kind of auto zero at no load and repeat this auto
zero whenever the temperature changes by more than a degree or so you can
use just one gauge with a resistor network to ballance it and it will be
quite cheap and easy.

The best people (not cheapest) for strain gauges used to be Micro
Measurements who have now been swallowed up by Vishay (does no one care
about these companies buying up all their competitors ?) and you can finde
them on www.vishay.com.

BTW, to Rob and others -  it would be nice if posters would say where they
are and put the question into context with regard to how many, budget,
home/commercial etc.

Let us know which way you go.


Michael Kellett
www.mkesc.co.uk




Re: Push-pull sensor - any ideas?
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Thanks for the suggestions and comments everyone, but I'm afraid most will
not work, except perhaps the DIY strain gauge methode.

The rigidity and strength is required because this is the elevator push-pull
tube of a small aircraft. For obvious reasons you wouldn't want somethin
flimsy there ;-)

Under normal circumstances you'd place the elevator in a position that
presents level flight, or climbing/descending flight as required. Depending
on the circumstances a more or less constant force will be needed to hold
the stick/yoke in the proper position. There's no direct relation between
position and force. Level flight might require pulling 1kg or pushing 3kg
with the stick in the same position, all depending on airspeed, altitude,
power settings etc. Airplanes have a mechanical facility called 'elevator
trim' which allows the pilot to null out that force. There are electrical
versions as well, using a servo and up-down controls.

What I'd like to build is an auto-trim facility. By pushing a button, the
system should auto-find the new trim setting. It should not do this
continually, only on demand. The null doesn't have to be exact, there will
always be forces on the stick anyway(vibration, air turbulance etc). The
system would augment the existing up-down control.

As for cost (sensor alone), I am looking for something in the $50 - $100
range.

Thanks once again for sharing your thoughts.

Rob



Re: Push-pull sensor - any ideas?
snipped-for-privacy@chello.nl says...
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Given the more complete description of the problem, it doesn't
sound all that hard to solve.

Could you use a spring-centered 'dead band' in an element of
the control system and a few micro-switches to sense when
the control is being pulled to one side or the other of
the dead band?  You would probably want to lock up the control
element after the auto-trim, or the controls would feel sloppy due
to the travel needed to activate the switches.


     <----->
============================0       (control rod)
                        0WWWIWWW0   (Springs)
                            I
  (ELEVATOR                 I
   Control Horn)        S1--I--S2   (Sensor switches)
                            I
     Pivot Point----->      0--------------------     (Elevator)


Or instead of springs and switches, you could have a slightly
flexible element in the control horn that sensed the deflection
with strain gauges.

Another advantage of sensing at the control horn is that the
pivot point is a nice place to bring out your wiring.


Mark Borgerson

Re: Push-pull sensor - any ideas?

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I see that strength is required, but not rigidity.  I don't think that
1 mm of "give" in the push-pull rod would be that detrimental to the
operation of the control  1 mm of springy movement would be quite easy
to measure.


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


Re: Push-pull sensor - any ideas?
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How about those flexible resistance strips? I haven't used them, but
it looks like one could sense pushing, another pulling.
Wade H


http://www.imagesco.com/catalog/flex/FlexSensors.html

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