Push-pull sensor - any ideas?

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

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.)

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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

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

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

How about those flexible resistance strips? I haven't used them, but it looks like one could sense pushing, another pulling. Wade H

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

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.)