# Re: elevation screw jack

Another approach, one I'm using in the elevation controller for the EME
array at W6OAL, is to do a calibration run and program a formula into the
micro controller that relates pulses to degrees.  The process is:
- Attach a protractor and weighted string to the antenna boom or other part
that is aligned with the antenna beam.
- Elevate the antenna stopping every 5 degrees and recording the elevation
in degrees and the number of pulses from zero elevation.
- When done run that table through a curve fit program like CurveExpert and
determine the formula that fits the points.
- Program that formula into the micro controller.

The formula will depend on the geometry of the pole or tower, actuator, and
antenna.  I expect for the OAL array it will be a 2nd order quadratic -
quite simple to program.  Even if it turns out to be a COS or ACOS function
it is a single line of C code.  The critical values are stored in EEPROM so
they aren't lost when the power is turned off.  Nothing is needed at the
antenna other than the actuator.

Jim
wd0e@amsat.org

At 08:22 PM 01/25/2001 -0600, John Harrington wrote:
>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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