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#1
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Control Unit for Home Built Rotator from Pitch-Prop Motor
I'm going to recommend against electronic feedback unless you are a
control systems hobbyist... I did PID (proportional-integral-derivative) servo on a homebrew rotating mast so I didn't need a readout and eventually decided to go with a rotation sensing pot, a voltage-to-current converter, and a surplus meter. This was after I tweaked the PID to get rid of the oscillations but still had some average position errors... I guess with a digital system it would be easier to optimize. Mine was all analog... and only one op amp, so the settings interacted :-) Making the motor go and watching the meter is a more sensitive and intelligent form of feedback, as long as you've got a system you can't break by turning too far. A simple pulse-width-modulator for speed control might be in the cards though. That way you send a variable duty cycle full-on full-off pulse train to the motor. Good torque and you don't need a variable power supply just to run the rotor. I'm going to add one to my rotator soon... I've got that on manual right now... pulsing the switch makes a nice slow smooth rotation! If you do like electronics projects for the sake of electronics projects, feedback control is an interesting way to go, though. 73, Dan N3OX |
#2
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Control Unit for Home Built Rotator from Pitch-Prop Motor
On 23 May 2006 08:31:29 -0700, "
wrote: This was after I tweaked the PID to get rid of the oscillations but still had some average position errors... I guess with a digital system it would be easier to optimize. Mine was all analog... and only one op amp, so the settings interacted :-) Hi Dan, If anything, going digital has all the prospects of thrashing the antenna into shreds. The oscillations you speak of are eminently suited for an analog solution - you simply have to understand the nature of resonance, coupling (over, critical, and under), and attack. If you don't, translating them into binary code will be impossible. For instance, your average position errors were probably due to under/overshoot and a too heavily dampened system with too much gain. No computer program is going to improve that. 73's Richard Clark, KB7QHC |
#3
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Control Unit for Home Built Rotator from Pitch-Prop Motor
Richard,
Agreed on the suitability of an analog solution... I certainly made some steps toward critically damped operation. My circuit design was bad, though. I didn't have a very analyzable problem; the rotator rotates one of those fiberglass surplus military masts with respect to another, so I get a lot of stick-slip, especially if there's a breeze (there are 12 feet of mast above this thing). This is not to say a suitable set of P, I and D couldn't be found, and it would have been pretty slick if I'd set up right for empirical loop tuning, but I hadn't done that. I got pretty close by soldering components in and out. I would have had a better time with a real multiple op amp circuit with a knob for each of P, I and D. I think you hit the nail on the head regarding too heavily dampened with too much gain in the antenna rotator case. I just couldn't really tune the thing. In my case, I just decided to throw up my hands and go to the simplest method so I could get on the air with a rotating antenna. I have a quite successful analog servo going every day with my remote antenna tuner (www.n3ox.net/projects/servo). That one is ever-so-slightly underdamped, proportional only, and works nicely. There's a little bit of hysteresis and it certainly could be improved, but I stopped at the good-enough-for-daily-operation level. I guess what I meant by "going digital" was that depending on whether you're more comfortable with programming than a soldering iron (not me!) you might think about having a simple motor drive circuit and implement the admittedly complex PID loop in software/firmware. You're absolutely right that you can't do this if you don't know how to do the analog... 73, Dan N3OX |
#4
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Control Unit for Home Built Rotator from Pitch-Prop Motor
On 23 May 2006 09:38:07 -0700, "
wrote: You're absolutely right that you can't do this if you don't know how to do the analog... Hi Dan, All, The single loudest voice on these matters is Robert Pease at National Semiconductor. For streaming courses/lectures on this very topic, visit: http://www.national.com/nationaltv/index.html I've been following Bob's no-nonsense contributions for more than 25 years now. 73's Richard Clark, KB7QHC |
#6
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Control Unit for Home Built Rotator from Pitch-Prop Motor
If you do like electronics projects for the sake of electronics
projects, feedback control is an interesting way to go, though. ============================ Talking rotor position feedback ,instead of the traditional potmeter in bridge circuit with analogue meter readout ,you could try a low cost synchro system utilising 2 stepper motors from discarded hard drives , dot-matrix printers or flat bed scanners ,with 1 stepper motor mechanically coupled (either directly or via a reversed rotational reducer) to the rotor ,the other in the shack as a readout device. The circuit I found on the web (via Google)needs 5 wires between the stepper motor coupled to the rotor and the control unit ,located with the other stepper motor in the shack. The circuit includes 4 TIP120 NPN transistors 2 LM324 dual Opamps and a few diodes and resistors. The stepper motor in the shack could either be provided with a rotating or sliding pointer ( the latter from an old 1950-60s radio) , or be mechanically linked to a counter (for example from a discarded tape recorder) A potential 'recycled components' project. Frank GM0CSZ / KN6WH |
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