Polarotor Servo Controller Design

[equant]

I've got the servo out of a spare cband dish on my workbench. So far I've got a simple zener diode and voltage divider circuit setup in a breadboard that gives me 3.5V to 5.6V for Horizontal and 0.07V to 0.3V for Vertical. The ranges are a result of different receivers.

Ok, so that's looking good, and it'll be easy to hook up to a uC for servo control. I've got an AVR ready to burn. I don't expect too much trouble with any of that.

...But, I am curious, does anyone have any opinions on how best to package this and hook it up? I can do the digital electronics and servo control, but I really don't have much RF packaging experience. I want to make sure that I don't adversley affect the signal more than necessary. I'm thinking a metal enclosure with two fconnectors. The controller is hooked up inline between the CBand lnb and the receiver. Does anyone have any thoughts? Ways to make my life easier, or make the final result work better? Any thoughts on using a low-pass filter?

I will release the code and schematic when I'm done, if I'm done, if it works, etc.

Thanks.

 

[Cadsulfide]

THe microchip website hasa couple of disecq examples that show how to tap into cable without loosing a pile of signal. Aslo there is some excellent reference sutff here:

Eutelsat satellite fleet :: Earth Segment :: Earth Stations

 

[truckracer]

I was on a site one time where a guy had made a servo controller that would tap the coax for 13/18v and it would skew the servo 90 degrees.

The pansat 2500 has servo controls built in. It would be interesting to reverse engineer their circuitry and see what they are doing to reproduce the pulsed 5v control signal.

 

[equant]

I was on a site one time where a guy had made a servo controller that would tap the coax for 13/18v and it would skew the servo 90 degrees.

Yeah, that is what I'm doing, with a little pot to set the 'center' between H and V.

My post was really to get opinions on how to 'tap' the coax with the least amount of signal loss. The Eutelsat pdfs show 1/4 wave inductors that are actually printed on the pcb. Not really an option for homebrew.

I'm going to mess around with an altoids tin, and put the in/out f-connectors close together with a big piece of copper between them for center, and then tap that with a small choke and see if I can't limit RF loss. Or I'll do a low-pass filter.

 

[1captain]

This may be some to think about or to look into. if you have a old lnb-f, gut it an see what it has or uses to switch from V to H, (you may even have you need sitting in a lnbf) even a old dtv will have it in, then you may just tap this off a pair of f-81.

 

[B.J.]

I've got the servo out of a spare cband dish on my workbench. So far I've got a simple zener diode and voltage divider circuit setup in a breadboard that gives me 3.5V to 5.6V for Horizontal and 0.07V to 0.3V for Vertical. The ranges are a result of different receivers.

....

I'm confused by what you're saying here. I'm probably misunderstanding your intent, but it sounds like your're suggesting that the polarotors work as a DC servo by just sending different voltages to it???? All the polartors I've seen worked on 5V pulses, and the polarotor angle was determined by the pulse length. The pulse length on my receivers is in the range of 10-20% in the 5V on state, so that if you hook up a voltmeter to the pulse output of receivers, the voltage would be in the 0.5-1.0 V range most of the time.
So I'm confused with respect to where the various voltages you mention above came from, and what they imply?

I just don't understand what you're trying to say.

 

[Cham]

An old C-band analog receiver can control the polarotor motors, skew settings between sats are quite different. Too bad the newest HD receivers don't have a skew control for C-band systems built in such as the Pansat 3500.
Think Sadun is/was working on something to convert DISEqC signals or LNB voltage to control polarotors with a new version of V-box. Might want to contact them you might get some pointers there...
C.

 

[equant]

So I'm confused with respect to where the various voltages you mention above came from, and what they imply?

I just don't understand what you're trying to say.

Here is what I was trying to say... "My coolsat receiver now controls a polarotor through something I built, but I don't have it all packed up yet, and want advice before I do, so that I don't introduce noise or lose signal."* ... *Granted, it wasn't actually finished when I wrote the email, but it was functionally designed, and it now does work.

Regarding the servo/voltages... The servo is controlled by a microntroller. . The microcontroller watches a single pin to see if it's high or low to determine if the servo should be possitioned H or V. I have to convert 18V (comes from receiver) to 5V because 5V is a level a microcontroler can understand. I convert 13V (from receiver) to 0V because that's the other voltage a microcontroller understands. In the real world, it's often not exactly 0 and 5 volts. I thought someone might be interested in my method, so I blathered about it. Using a zener diode and a voltage divider made it pretty easy, but I thought someone with RF design knowledge might say "No way, don't do that." or "Wow, you're genious!"

I considered Diseqc. Diseqc isn't too bad, but it's must more difficult than watching a single logic level, so I figured why bother? The only reason I could come up with to use Diseqc would be so that the uC could know what satellite was requested and use a lookup table of satellites that have skew that is off and adjust for them. I'm not even sure (nor do I care) the "knowing" part is possible. Otherwise? I don't see a reason not to use the voltage levels.

 

[equant]

skew settings between sats are quite different

I guess "quite different" is relative. I would say they're quite similar when using a motorized setup. For the convenience of a BUD controlled completely with an FTA receiver, I'm willing to accept the same polarity positions across the arc. As it is, at least 90% of my skew settings on my 4dtv box are identical.

 

[equant]

Here's the schematic I drew up. "E" would be the DC voltage coming from the receiver. V(out) goes to the microcontroller-servo-super-computer.

... in case anyone cares; I know you all like pics ...

 

[Cadsulfide]

you could eliminate the zener and use a 1n4001 for polarity protection and just change the voltage divider resistors to give you two levels, no need to draw more than a few microamps from the cable. As for 1/4 wavelength pc stub, a 10 millihenry choke would look like an open to lo frequencies yet pass 13/18 with no trouble at all.

 

[equant]

you could eliminate the zener and use a 1n4001 for polarity protection and just change the voltage divider resistors to give you two levels,

I'll re-think the zener, but I believe it's needed. The thinking that led me to use it was that a voltage divider alone wasn't going to give me the clear division I needed between 0 and 5 volts when using a source that could be anywhere between 12 and 22 volts.

no need to draw more than a few microamps from the cable

I'm not too sure I have to be that concerned about current draw. I'm not going to be crazy and create a giant load, but with the AVR, it's going to draw more than a few uA. I don't think the zener is going to ruin things.

As for 1/4 wavelength pc stub, a 10 millihenry choke would look like an open to lo frequencies yet pass 13/18 with no trouble at all.

Thanks for the value, I hadn't gotten around to figuring that part out yet.

EDIT: Don't you mean it'd look like an open to high frequencies, or am I confused? ... This isn't my strong suit... Would a 10mH inductor look like 250Mohms to a CBand signal, and zero ohms to DC? Thanks for your help.

 

[B.J.]

Here is what I was trying to say... "My coolsat receiver now controls a polarotor through something I built, but I don't have it all packed up yet, and want advice before I do, so that I don't introduce noise or lose signal."* ... *Granted, it wasn't actually finished when I wrote the email, but it was functionally designed, and it now does work.

Regarding the servo/voltages... The servo is controlled by a microntroller. . The microcontroller watches a single pin to see if it's high or low to determine if the servo should be possitioned H or V. ....

Oh... OK, sorry, I misunderstood. I thought you were trying to feed these voltages directly to the polarotor. So, you already have a pulse generator, but want to use the voltages off the coax to tell your pulse generator which of two preset pulse lengths to send. OK, that makes sense. Sorry for being confused.

 

[equant]

OK, that makes sense. Sorry for being confused.

It's not your fault. I'm the one that wrote the post.

I rushed past details that I should have left out, or explained more clearly. I'm not an engineer, and don't even really know the right words to use at times. It's like trying to explain the color of a sound at times.

Is there anyone out there that's thinking they might build one if I post the source-code and schematic? This will impact how far from kludge I feel I need to go. In particular whether or not I include a fine-tuning pot for the skew. If it's just for me, I'll just hardcode the (I don't know what to call it... center of H/V, "offset") in the chip.

I'm considering using an 8pin pic instead of a AVR, or... Using a RBBB Freeduino. Don't know if anyone cares either way.

 

[Anole]

a few random thoughts:

Without doing the numbers, I -feel- (years of experience) that the zener is better than just a voltage divider.
It's simple, it works, and you move on. -
And you can certainly get zero volts out of it.
I might add clamping diodes , but that's just me.

The microamp current draw for the microcontroller input is certainly worth considering.
Some micros have active pull ups on inputs, others do not.
So, the data sheet is your best friend.

I just took a look at the values on your schematic.
There's no point pulling 10ma.
I'd multiply all the values by 10X and drop the current to 1ma.

BUT, you said it's working, so this is all just pointless discussion.


As for getting the voltage from your coax, I'm sure using an old LNB would be educational.
However, the ones I've looked at were not all that straightforward.
An idea one member brought up a while back, was that little H-H motors sucked down the signal level, using their printed inductors.
So, he put a better choke inside his.

That was described by member Satcom1.
The fellow came up with a number of good ideas, but then upon doing a mini-BUD project, got into a shouting match with some members and left.
He did some good work, but couldn't always take criticism.
Shame he's gone.
I don't have the thread bookmarked, but you can search for threads started by him, with some sort of relevant topic (motors).

ANOTHER Reason to use meaningful subjects on new topics!!!


There was another thread that bears on this discussion.
Some time back, we discussed using a voltage sensor as you have described, and possibly an NE555 timer to generate the pulses to move the servo.
The design would have the two pulse widths determined by two pots.
At initial setup you would tune the pots and the skew of the feedhorn to Vertical and Horizontal polarity.

One thing we never discussed was the potential for temperature drift.
If the unit were located in the house, that would be a small problem.
If outdoors, then ... well ... that might not be so good.

Whether the pots were internal to the device and set once or external to allow for occasional fine-tune, would be a user decision.


Tune-up for a micro-controller based device could be problematic for the end user.
As the developer, you can make the pulse widths perfect for your hardware.
How does the next guy follow in your footsteps?
Any design would require a user-setup mode.
When I've built things like this in the past, the most useful input device has been a (software debounced) pushbutton, and an eeprom of some sort.
I'll leave it to your imagination to come up with something. - :up

Also, protect the pulse output to the servo against random events.
I always assume someone will hook things up wrong, use too long a cable, etc.

 

[Cadsulfide]

Good points Anole!

I was suggesting the voltage divider as most micro's these days will have 1 pin that can be used as an A/D, then youre just looking for a voltage shift on the input pin.

Equant:

I meant that 10 millihenrys would look like an open to the IF frequencies, my bad. Xl=2piFL= 135Megohm @ 2150 mhz, 1 millihenry would be fine too.

 

[Anole]

Good points Anole!

I was suggesting the voltage divider as most micro's these days will have 1 pin that can be used as an A/D, then youre just looking for a voltage shift on the input pin.

Yes, good idea. But, very microprocessor dependent.
I like to make the most of what I've got on hand, or know.

But... if a $5(or even $12) micro with A/D makes the project easier than a $2 micro, that's a great trade-off.!.

I meant that 10 millihenrys would look like an open to the IF frequencies, my bad.
Xl=2piFL= 135Megohm @ 2150 mhz, 1 millihenry would be fine too.

When I think millihenry, I think audio frequencies.
I believe any such device would have far too much distributed capacitance, and be a short circuit at 1ghz.

I would have looked for a 10 or 20 microhenry part, just spitballing a number.
But I see in his thread: " SG2100 Insertion loss", Satcom1 was suggesting 1 microhenry.
See his thread for more info and a good discussion.

 

[trinity13]

@equant

Mind if I ask why you chose the micro controller route instead of one or two 555's like Anole mentioned?

I'm interested in what you are doing, but don't have any experience with micro controllers.

Thanks.

 

[equant]

Mind if I ask why you chose the micro controller route instead of one or two 555's like Anole mentioned?

I realize Microcontrollers are often used for jobs 555s would be perfect for, and I don't know that I have a good answer to your question. For starters, I like microcontrollers, and enjoy working with them. Two, they allow for crazy flexibility should you want to add features. Three, *I think* it'd be more temperature stable.

555s might be just fine, and a uC might be in no way better, but I enjoy writing code for embedded projects, and I have a ton of them.

 

[Anole]

six of one . . .

Hey, I'm not knocking micros.
I've built lots of stuff with 'em in the last several decades!
I just mentioned timer chips 'cause they were easier for the micro-processor challenged! -
If I were going into production, you can bet I'd use a micro!

On the other hand, it doesn't take a VW to go to the grocery store when a bicycle would do the job.
You'll just be a lot more comfortable.