[simpits-tech] MarvMan and Craigo Fixed Stick Update

[Craig Rochester]

Marv,

Holy *&^%!  Winding your own LVDTs!!!  Now that's impressive!

The variable displacement for pitch-up and pitch-down sounds great!

Thanks for the update.

Craig R.



-----Original Message-----
From:	simpits-tech-admin@simpits.org [mailto:simpits-tech-admin@simpits.org]
On Behalf Of Marv De Beque
Sent:	Friday, December 20, 2002 5:45 PM
To:	simpits-tech@simpits.org
Subject:	Re: [simpits-tech] MarvMan and Craigo Fixed Stick Update

Well I have not been very impressed with the results of the load cells and
the one version of strain gauge that I have.  I have ruled out the load cell
as it does not have acceptable accuracy and repeatability in the null
position (hands off the grip should return to center position).

I have ordered a different strain gauge set to test which should be in by
mid January.  This set is a little more work to install, but supposed to be
much more accurate than past experiments I have tried.

Over the next week I am going to test some LVDT sensors.  Actually, I am
going to roll my own for this test.  I spent a little time this morning with
our companies bobbin winder in the lab.  Last night I cut two bobbins out of
nylon and spun them with about two hundred windings of 36 gauge transformer
wire on each coil (there are three coils per sensor).

I have ordered some special interface or signal conditioning ICs that should
arrive some time after Christmas.  I'll bread board a circuit and run the
same series of tests on the LVDT system as the other load cells.  By then I
should have the other strain gauge parts in to test that system.

So far I can say that the strain gauge system holds promise in that I think
I can get it to work satisfactorily, but it will be more labor intensive to
build than LVDT.  However, LVDT sensors are expensive.  My single piece
price on those is $92 each!  Now you see why I am rolling my own for testing
purposes.  The interface circuit is also a little more complex.

I completed a mechanical design enhancement that better replicates the feel
and movement of the grip relative to the transducer base.  As you may know,
the real side stick has different amounts of physical movement of the grip
depending on direction it is moved.  Pushing forward has virtually no
movement at all, but pull back and it can move 1/4"!

I have been able to replicate that movement precisely.  What's more is it is
adjustable!  However, you need to take the base apart to get at the set
screws that control the amount of flex.  I plan to set it to factory specs
when it leaves my place if I use the design.  Unfortunately, it requires
more parts and machining to do it, but we will see.

I must say that this project has taken a lot more engineering time than I
had first budgeted.  The good news is that I have been slowly refining the
design so it will be "right" when it is done and offer significant amount of
realism.

The downside is probably going to manifest itself as price creep.  The worst
case scenario is the LVDT sensor system.  That could cost an additional $150
to $200 depending on actual costs of components.  Obviously, that is my last
choice for a system if I can't get satisfactory results from the strain
gauges.

Marv