On 1/21/23 06:25, Robin Szemeti via Emc-users wrote:
So, in a vaguely CNC related folly I have purchased a 2 axis military dish
mount for radio stuff (moonbounce, if you must know)
My plan is to control it from EMC, with a Mesa anything io card and the
resolver interface, I already have various Mesa cards under EMC, so thats
not an issue and I have a spare 5i23 PCI card and the isolated IO duaghter
card, so with the resolver card I should be able to uses the resolvers, and
pick up the limit switches on the IO card. I can just use some Python
script to work out the moon position and generate "G1 X285.4, Y23.8" or
whatever MDI motion commands every 15 seconds or so to track the moon, not
a problem.
Now, the question is driving the two phase AC servos. Generating vast
quantities of 50 or 400Hz or whatever AC for the drives is simple enough, I
can just use a couple of large Class D amplifiers, no worries. Generating
the AC low level signals to drive these .... basically, it needs a constant
50Hz signal on one coil of a fixed level, and a variable level sine wave on
the other coil, at either +90 or -90 degrees depending on which way it
needs to go.
Is it possible to use HAL or something to generate the variable level 50Hz
servo drive signals and output them from the PWM outputs on the resolver
card?
I've not used it in that context, but take a look at man 9 siggen.
There are several mux's available that could be used to switch motor
directions. Its main use is in 1 kilohertz loop stuff for linuxcnc, but
if you need finer time resolution, should be happy in a 3 to 5 kilohertz
threads Use a halmeter to see how much time the individual module takes.
Hal can do a lot, look the logic choices over in /usr/share/man/man9 of
an linuxcnc install and let your imagination out to play w/o a
chaperone. You can use float vars at a 1 or even 4 kilohertz loop
thread, so updating a motor drive at 50HZ ought to be doable.
For motor amps, take a look at class D audio amps, the 400 watt modules
are rather commonplace these days, running at ultrasonic carrier
frequencies. Use chokes to keep the carrier out of the motor wires, and
good single point(one bolt) grounds.
Use mux's for analog switches.
Using modern stepper drives that have current limits built in, you can
creep steppers along at .01 hz a second, and if you spend the sheckels
for the closed loop stepper/servo's suitably geared down, you ought to
be able to track the moons motion within an arcsecond in real time and
do it on 40 watts of power or less since they use the error to determine
how hard to drive the motor. The recovery time when it goes below the
horizon and has to pre-aim at the other horizon for when the moon comes
back in sight at next moonrise is a problem since the active times match
the dead times pretty closely.
I've not paid that much attention but from satellite experience, you
likely need just a little azimuth, even for polar mounts, your latitude
above or below the equator determines that, and a large longitude.
I have an old friend from back in the 70's that at that time was
building a 12 bay steerable 2 meter get up. 16 element yagi's in a
broadside array. But another job opening took me to NM from NE so I lost
track of him. Employment brought me to WV 38 years ago, and I'm now 21
years retired, and still here.
And tell us how you did it when it working.
Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/>
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