[RadioTelescope] Stepping rates
andrew at physics.uwa.edu.au
Mon Oct 11 17:36:10 WST 2010
On 11/10/2010 1:34 PM, Harry McNally wrote:
> Yes. Remembered that from your previous email. I'm pondering more what the
> maximum step rate is that the motors can deliver (torque-wise) although I
> presume that's pretty high when the dish is accelerated up to that limit.
You'll need to measure the maximum reasonable accelleration rates and
slew speeds after the system is built, it'll depend on drivetrain
friction, moment of inertia of the dish (and all rotating components in
the drivetrain), etc. Too hard to bother trying to calculate...
> Impressive :)
What's really impressive is the way they allowed the Parkes telescope,
with an azimuth/elevation drive to track objects in right-ascension and
declination (curved paths across the sky) in the 50's, before computers...
In the exact centre of the az/el drive system, they had a small chamber
with a dummy 'telescope' on an equatorial mount - one axis pointed to
align with the Earth's polar axis (RA, or 'longitude' on the sky), the
other at right angles (Dec, or 'latitude' on the sky). To track an
object with an equatorial mount, you keep the Dec axis fixed, and rotate
the RA axis at one revolution per day, to counteract the Earth's rotation.
That central equatorial 'dummy telescope' was first slewed to the
desired target coordinates, and set tracking the object. Then the az/el
drive of the dish was manually slewed to line up with the same
direction, to within a few degrees. Once the position was close enough,
they activated a servo link with a light beam and a set of photocells
between the az/el drive on the dish and the tracking motion of the
central equatorial mounted dummy telescope.
>> (BTW, I'd plan on cable-wrap limits in hardware, not software,
>> especially if you're going to allow a bit more than 360 degrees...)
> Ok. There are no limit switches that I can recall on the mount.
If there's an encoder, you could simply enforce not being able to pass
through, say, 0 degrees azimuth, in either direction. That would be
irritating, since if you were tracking an object, you'd have to break
away and slew the long way round - there's no single azimuth value that
you aren't going to want to 'track through' quite often (and 0 degrees,
due north, is a really bad choice).
A better way would be to allow, say, 540 (or more) degrees of travel. If
you slew to a new object, you calculate the direction to 'unwind' to the
most central cable-wrap azimuth, but if tracking, you could continue in
one direction for much longer.
The problem is that if your encoder only gives you azimuth angle, you
need to keep track of cable-wrap boundaries by dead-reckoning, and
guarantee storing that state between uses.
Without cable-wrap limit switches, I'd make sure you design some
quick-release connectors in the middle of the cables (or at one end), so
that if the software fails to keep track of state, it's an easy fix...
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