Gallery, Projects and General > The Design Shop
Coax indicator design
kwackers:
I was thinking of a low impedance coil, i.e. just a few turns around the (ali?) body. It would have very little susceptibility to mains hums and stray magnetic fields (I think) would be too low by the time they got there, it could also (probably) drive say a 50uA meter directly.
RichardShute:
--- Quote from: kwackers on November 05, 2010, 08:33:55 AM ---Just to throw something completely different into the mix.
I was contemplating making something like this and so I'm fairly interested in the issues and design.
What I considered doing though to make the whole thing small was to get a magnet to move and use a coil as a pickup (or the other way round). This could be used to drive a small meter. It auto zeroes when not moving (since we're only interested in the 'wobble') and could in theory provide the 'multiplication' required to overcome the mechanical 'loss' particularly when long arms are used.
--- End quote ---
The problem with that is that as you say you get no output when the magnet is not moving and with the relative speed being ineviably very slow you will get negligible output. You'd be much better using a LVDT (linear variable differential transformer) which are readily available to buy (for *real* money) or could be made. It is basically two colis on a common former with a magnetically susceptible slug up the middle attached to the probe. You apply a signal to one coil and compare it with the other. They can be easily accurate to a micron!
Bill
I have some ideas for a reasonably non-rocking slide scheme, I'll try to do some sketches shortly.... Not got time just now - off out to pickle some brain cells.
Richard
AdeV:
The downside to all this coil malarkey, as I see it, is you're going to need to introduce circuitry and batteries 'n' stuff. But, if you're going to do that, wouldn't it be easier to use a hall sensor? As the magnet moves up & down in the unit, its distance from the sensor will vary, and (in theory at least) you can then read off the distance between magnet & sensor as a voltage output. This also means you can take a reading while everything's stationary, should you so desire. I don't know how easy/difficult it would be to use 2 hall sensors to get X & Y readings...
picclock:
@ kwackers
even 3000rpm is only 50 hz which is a quite a low frequency for what you are suggesting, hence my suggestion of multiple poles to increase the frequency. At 100 rpm you would need some serious core material - which would make it quite large. :scratch:
@billtod
at 50 rpm (just under 1 Hz) the frequency of wobble would be so low that filtering out the other induced interferance would be simple. Additionally, if it was encased the case would act as a faraday shield. But .. .
@bluechip
PLL is probably the best approach. Change of capacitance would be very small though and overall capacitance would be low, so high frequency needed. 4046 sounds just the job. Power it and a moving coil meter with 2 or 3 button cells, add some form of off on switch and it should be good to go.
It's quite a good idea that ought to succeed. :clap:
Just needs a reasonable mechanical design.
One for the 'tuit' list. :beer:
picclock
kwackers:
@Picclock, yeah I always forget rpm and hz aren't interchangeable! That means my simple version is a non-starter... :doh:
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