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Re: elevation screw jack

Hi, guys.  The elevation screw jack/linear actuator idea is a great
one- those things are unbelievably strong, and easy to mount.  The
problem which must be addressed for precise pointing, however, is
that the pulses coming from the actuator are not in a linear
relationship with the amount of rotation, and this must be taken
into account in some way.  For example, if the pulses per degree
of rotation at, say, 45 degrees is 4, the amount at 0 degrees
and 90 degrees will be somewhat less than that.  There's not a
linear relationship between linear push and shaft rotation when
the linear push is on an arm connected to the shaft, the normal
configuration when using a linear actuator.   You can put a
lookup table in the microprocessor to push or pull the correct
number of pulses per degree depending upon present and desired
position, but you will also need a 'known' position (such as
a microswitch mounted to operate at 90 or 0 degrees) for calibration
each time you turn the rotation system on.

My solution, best for me at the time, was to simply attach a
linear pot to the evevation boom.  With that scheme, pot shaft
rotation is precisely linear with rotation, and a 'known position'
switch is not necessary.  The pot, of course, drove an A-D
converter to give the 68HC11 microprocessor the desired elevation
feedback.  Once calibrated, the 'HC11 knows elevation instantly
upon turn-on.  I'm sure one of the PIC family will (like the 68HC11)
be available with an internal A-D converter.  One of those little
Basic Stamp modules would work well also, for those who are familliar
with BASIC and don't want to monkey around with assembler language.

Best regards   John  W5EME

James Sharp wrote:
> On Sat, 20 Jan 2001, cahill wrote:
> > Hi ,
> >
> > I've bought a elevation screw jack for a good price but i don't know
> > how i should make a position readout that's accurate and analog .
> > (so i can use it on a tracking interface that recognizes a zero to 5
> > volts input)
> > The elevation screw has a end and beginning contact and has a contact
> > that gives pulses when turning the motor in or out.
> >
> Here's how I'd do it:
> Use a PIC microcontroller with a built in D/A to provide the 0 to +5.
> Connect both contacts and the pulser to the PIC chip.  Have the chip, upon
> start up or whenever you send it a "resync" pulse of some sort, swing the
> dish to the beginning using the "Start" contact as an indicator of this.
> Then, have the chip start moving it out to the other end while counting
> the pulses.
> This will give the chip a scaling factor and a "volts per degree" factor
> to start on.  It also ensures that everytime you power up the system, it
> starts from a known point.
> You can then send the chip an "up" or "down" command through one of the
> other digital I/O pins.
> PICs are wonderful things....:)
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