At 04:39 PM 5/21/05 +1000, John Samperi wrote:
>Thank you for all your replies so far. I guess I will need to explain
>a little more.
>
>I'm trying to come up with some ideas for a prospective client.
>
> From what I understand so far the valve controls gas to a burner
>which needs to keep a temperature of about 450 degrees celsius
>(flaming hot!!) perhaps +- 30 degrees. A Mega16 (possibly) will
>be monitoring the thermocouples and will adjust the opening of the
>valve to maintain the temperature. So, yes I have some feedback
>on what's happening with the valve perhaps 1 or several seconds later.
>
>The valve will be moving a maximum of 1/2 turn, so no high speed
>sensors are needed. An Absolute Position sensor would be nice
>to have to find if the valve has moved or if the valve motor
>has moved in the right direction or not moved at all i.e. faulty
>motor, broken wire, jammed/locked valve etc. as a safety precaution.
OK, I've a couple of alternatives in mind that you may want to take a look at.
Hall effect rotary position sensors (also referred to in some circles as
Hall effect pots). A number of manufacturers, Clarostat, Midwest Power
Components, Honeywell, Vishay Spectrol and others. A primary target for
these is vehicles for use in throttle inputs or position sensors. The
primary advantage is that they are non-contact and so their life tends to
be limited by their bearings. However, their rotational sensor range tends
to be limited, commonly to less than 90degs although I've seen them go as
high as 160deg.
The other alternative to consider are inductive sensors such as those
produced by Gill. See for example
http://www.gillsensors.co.uk/content/DualCavity.htm I priced their rotary
sensor recently and found it pricey (around 200 pounds as I recall) but I
did price the dual cavity sensor at one time and I remember it being
reasonable at the time. The big advantage with the Dual cavity is that the
sense element is just a piece of metal and a little imagination can provide
quite a wide range of input possibilities.
Both these alternatives are analog so you would need an A/D.
I've used the Hall effect sensors myself for throttle input and was quite
happy with them. It sounds like range would be an issue for you though.
I recently went through looking for rotary position sensors. I was looking
for something rugged since they will end up at the top of a tower with
limited access and moist conditions. It also had to support a full 360degs
and continuous, although slow, rotation and it had to give absolute
position at power-up to within a degree or so. Also I wanted to limit the
number of wires running up and down the tower. That appears to limit the
field to rather pricey alternatives (>$300).
Robert
" 'Freedom' has no meaning of itself. There are always restrictions, be
they legal, genetic, or physical. If you don't believe me, try to chew a
radio signal. " -- Kelvin Throop, III
http://www.aeolusdevelopment.com/