motor theory questions

[updatelee]

ok im trying to figure the math out in my head for usals calculations and I had a thought that im not sure is correct or not.

primefocus polar mounts are quite simple, you adjust the angle on only one axis to track the entire arc, its simple, the dish is on the same angle as the polar mounts axis, at least I beleive it is.

now diseqc motor's are more complicated, you adjust the angle of the motor's shaft, which is bent, and the angle the dish is on, and remember they are offset too so there is another angle.

then I got thinking, is the shaft on the motor bent at an angle because the dish is offset ? ie if the dish is 24deg offset, is the arm on the motor bent at -24deg to offset that so the motor tracs the arc just like a polar mount ? this is assuming you also set the dish so its in the same angle as the motor, just like a polar mount

is my thinking correct ? because it sure as hell would make my calculations 100x easier !

second question, if this is the case... there is a problem as not all dish's use the same offset angle. fortec for example uses everything from 24.62 to 22.75, winegard just reports theirs as 24deg or this known to everyone and new to me ? wouldnt be the first time lol. you just tilt the dish a little up to compinsate ?

 

[guapoharry]

IMO, the shaft is bent to accomplish what declination setting on a polar mount does.

DiSEqC type motor instructions don't make mention of dish offset, from what I recall.

 

[updatelee]

but the shaft is perma bent and non-adjustable. the polar mount's declination IS adjustable ...

 

[brex2001]

The motor shaft points to polar north(or south). The bent arm can be as much as 35 degrees( my SG2100 is 30 degrees). The satellite arc tracking elevation is accomplished by adjusting the elevation where attached to the bent arm.

Where I have trouble is getting the offset dish to track correctly if the dish mount & motor mount don't complement each other. Like with a Weingard 30 inch dish & a Pansat PM900S motor. They don't play nice.

 

[updatelee]

ok interesting, the amount the arm is bent is variant as well on different models/manufacturers.

what im curious about then is why is it bent ? how did some manufacturers come up with 30deg and other 35deg ?

Im just trying to understand the math behind it all. Ive got the calculations figured out to determine the angle +/- truesouth to any given satellite. but to determine the rest im lost.

If I can understand why the arm is bent it would help my understand the rest of the angles better, helping me calculate everything else better

 

[Mikey11]

you are making things way too complicated when its so damn easy.....

- make sure mounting post/pole is level

- set motors latitude scale to your own latitude

- drive motor to your closest southern satellite using USALS

- aim dish/motor assembly at the bird you have selected

- adjust horizontally by swinging the entire dish/motor assembly on the pole

- adjust vertically moving the dish only and not the motor

pretty easy if you ask me, have setup 30-40 motors this way

 

[brex2001]

Offset dishes are elliptical sections of a parabola, right? Cut at an oblique angle to the center line of the parabola, right?

The arm is bent to decline the dish so it looks up at the satellite arc in a turning/leaning motion around the axis of the motor shaft where the arm is attached. Why different arm angles I just don't know. Engineers, you know.

 

[updatelee]

When Im home next month Im going todo some measurements with my digital angle finder and a T square and see what I come up with. Calculating how usals gets its angles has taught me quite about this, one thing of which is that I have alot more to learn lol.

funny how sometimes one curiosity leads you understand you know less about the question then you thought you did

 

[updatelee]

you are making things way too complicated when its so damn easy.....

- make sure mounting post/pole is level

- set motors latitude scale to your own latitude

- drive motor to your closest southern satellite using USALS

- aim dish/motor assembly at the bird you have selected

- adjust horizontally by swinging the entire dish/motor assembly on the pole

- adjust vertically moving the dish only and not the motor

pretty easy if you ask me, have setup 30-40 motors this way

Sorry, I must have worded my post wrong. Ive setup my motor quite a few times, from relocating it in my yard, to changing dish's to moving houses. Thats not an issue, esp with my satlook NIT.

Ive been writing my own little tuning app in linux, which forced me to learn how the the usals algorithm calculated the angles.

with some help from enigma and pendragon Ive figured it out and Ive got my angles, but as pendragon and many others have seen, it breaks down on the edges. Why ? I dont understand that math. So im trying to determine what every angle of the motor/dish does and why it is that way, so I can understand the math

like why is the arm bent on some models at 30 and others at 35deg ? why is the dish offset at 22 on some and 24 at others ?

what implications does this have on the motor tracking the arc ? any ? none ?

 

[brex2001]

Sorry, I must have worded my post wrong. Ive setup my motor quite a few times, from relocating it in my yard, to changing dish's to moving houses. Thats not an issue, esp with my satlook NIT.

Ive been writing my own little tuning app in linux, which forced me to learn how the the usals algorithm calculated the angles.

with some help from enigma and pendragon Ive figured it out and Ive got my angles, but as pendragon and many others have seen, it breaks down on the edges. Why ? I dont understand that math. So im trying to determine what every angle of the motor/dish does and why it is that way, so I can understand the math

like why is the arm bent on some models at 30 and others at 35deg ? why is the dish offset at 22 on some and 24 at others ?

what implications does this have on the motor tracking the arc ? any ? none ?

Perhaps the 30 degree bend has to do with the majority of the worlds populace living between say 25N & 75N latitude so 30 degrees makes a good fit for dish elevation adjustment for that range. My SG2100 latitude adjustment is marked from 25 to 75 degrees. Maybe these differences in angles have to do with ranges of latitudes? A 30 degree arm works better in some latitudes than a 35 degree? A 22 degree offset is better suited to some latitudes than a 24 degree offset?
Just throwing it out there for you updatelee.

 

[brex2001]

Sorry Updatelee, I read the wrong scale on the motor bracket. The range of latitudes is 15 to 65 degrees.

 

[B.J.]

ok interesting, the amount the arm is bent is variant as well on different models/manufacturers.

what im curious about then is why is it bent ? how did some manufacturers come up with 30deg and other 35deg ?

...

I see that nobody actually addressed this question completely. The answer is, as someone DID mention above, that the bend angle is to enable you to adjust to the desired declination. The problem is, however that declination is a negative angle, ie below the normal rotation plane of the motor, and the way you adjust it is via the dish elevation on the dish, which is a positive angle. Ie no way to adjust a negative 5 degrees with a dish mount that might have elevation ranges between +15 to +80 or something like that. So to allow dish elevation to produce a negative elevation, the shaft is bent down at some angle to produce a large fixed declination, which you can adjust to the proper small declination by adding the appropriate amount of dish elevation. For example, if your shaft has a 30 deg bend, and you need a -5 deg declination, a 25 deg dish elevation will accomplish that. Once set like that, there is absolutely no difference in the tracking of an offset dish on a bent shaft and a prime focus dish with an actuator.

Oh, and there is one other reason for the bend in the shaft, and that is to keep the center of gravity of the dish closer to the rotation axis, thus reducing the torque on the motor. It really doesn't matter WHAT angle they use, as a bigger angle will be compensated for by a larger dish elevation, but I think a bigger angle could help weaker motors deal with heavier dishes.

Good luck on the USALS calculations. I did that calculation once using a few approximations to make things simpler. Took me a while to get all the bugs out, but it comes pretty close to the official numbers. You can compare results to where it says USALS MOTOR ANGLE at: BJDISCALC2
You can play with the javascript on that.

As I said, not perfect, but quick and dirty calculation. There are other more precise programs on the web.

 

[updatelee]

My calculations on motor angle are within 0.1 deg on the far edges and spot on for everything in between. Pendragon has a post where he talks about putting all the cords into a xyz matrix. Boy did that make the calculations way easier.

Problem is even though the numbers match they don't work with my true lat 53deg. If I put in 80deg it tracks very close.

So when I get home I have to play with my angles on the motor/dish to get things working better.

 

[updatelee]

How do you calculate your declination? Wiki sais declination is like latitude, degrees from the equator.

 

[pendragon]

The angle of the bend in the drive shaft for HH motors designed for offset dishes does not enter to the USALS calculations in any way for a properly adjusted system. The reason the bend is put there is to allow a reasonable range of adjustment of the offset dish elevation. If it wasn't there, it might not be practical to set the offset dish elevation low enough. You can calculate the elevation setting by measuring the angle of the bend and the offset angle of the dish and combining these with the true drive shaft angle.

One point worth mentioning is that USALS conveniently ignores the concept of a 'declination' angle. The 'approved' calculation assumes it is zero, which leads to imperfect tracking as one gets closer to the horizon. If you set a reasonable declination angle for your dish alignment, this is not taken into account by the receiver and increasingly wrong motor angles will be generated by the receiver the closer you get to the horizon. Unless you drive the motor to say a motor angle of 50+ degrees, these effects are not that significant, but they do exist. This is in the thread about the rotation matrices. The choice of a declination angle is completely arbitrary. It affects where the fit is perfect on the arc and can be adjusted as one prefers. Fortunately one normally has a fairly broad range of choice where the error is well under 0.05 degrees.

 

[B.J.]

How do you calculate your declination? Wiki sais declination is like latitude, degrees from the equator.

Problem with declination is that it is different for every satellite. It's calculated by an arctan of your distance above equator over distance to the sat in the equatorial plane. Your distance above the equator is basically the radius of the earth times the sin of your latitude, however the distance to the sat changes with the sat's angle away from your true south. To the south, it's the radius of the geostationary orbit radius, ie 26,200 miles minus your distance from the polar axis, (ie the earth's radius times the cosine of the latitude). At the extreme west, it's pretty close to the radius of the geostationary orbit radius, although not quite since you can't see a sat at 90 deg away from your longitude, so you need to pick an extreme sat you can see, and do some trig to calculate it's distance. In between, you need to do some trig and/or a bunch of a^2+b^2=c^2 things to figure out the distance to the sat. Of course, it's a lot easier if you use matrix math, but it's been about 45 years since I've done much of that, so I just use classical brute force things, sitting down with a piece of paper, drawing a lot of angles, with tangents and signs and a, b, and c dimensions falling out of my brain.

The conventional declination that you find on most charts is just the declination to a south sat, however that tends to make your tracking off by ~0.6 deg at the extremes. The so called modified declination method uses the declination of your western most sat, and you correct for the declination of the southern sat by tilting the motor rotation axis to the south by the 0.6 deg difference, and this gives results that are very close to perfect tracking, probably close to a couple hundredths of a degree.

From what I can see, it seems like the official USALS calculation of the USALS motor angle does seem to use the modified declination method, even though most motor manuals tell you to use the conventional south sat declination when setting up the motor.

 

[pendragon]

How do you calculate your declination? Wiki sais declination is like latitude, degrees from the equator.

That is a different sense of declination.

If one sets the polar angle of the motor to match the latitude, the dish will point perpendicular to the earth's axis of rotation over the arc. Unless you are at the equator, this will point above or below the Clarke Belt, depending on whether you are in the northern or southern hemisphere. If you lower the elevation of the dish on the motor axis to properly hit the true south satellite, the tracking will get worse the farther you deviate. This lowering is often referred to as the 'declination angle'.

A long time ago someone noticed that by applying a small offset to one's latitude to set the polar angle on the mount (sometimes called the 'polar axis tilt') and making a corresponding adjustment to the declination angle, one could track the arc extremely accurately. The fit is perfect at a few points, but for most latitudes this combination can theoretically get down to errors on the order of 0.01 degrees. This is the part ignored by USALS. In writing this, I believe I have accused USALS several times of assuming the declination angle was zero. I should have said the polar axis tilt instead.

 

[pendragon]

From what I can see, it seems like the official USALS calculation of the USALS motor angle does seem to use the modified declination method, even though most motor manuals tell you to use the conventional south sat declination when setting up the motor.

We covered this fairly extensively in last year's USALS Notebook thread. USALS does not use the modified declination method and the official calculations make substantial errors near the horizon because of this. As the 'polar axis tilt' angle is arbitrary and depends on one's latitude, they would have to explicitly account for it in their calculations, which they do not. It does not appear they use a constant offset either, which could cause even more problems for those at the equator or at the higher latitudes.

 

[B.J.]

We covered this fairly extensively in last year's USALS Notebook thread. USALS does not use the modified declination method and the official calculations make substantial errors near the horizon because of this. As the 'polar axis tilt' angle is arbitrary and depends on one's latitude, they would have to explicitly account for it in their calculations, which they do not. It does not appear they use a constant offset either, which could cause even more problems for those at the equator or at the higher latitudes.

I remember having that discussion. I thought that we came to to the opposite conclusion. I do remember that I discovered an error in my program based on that discussion. I thought that I did all my calculations based on the tilt, and they were pretty close, whereas if they didn't use the tilt they would be further off. Now I'm going to have to both go back and read that thread again, and also refresh my memory with respect to how I did my calculations. Not too difficult without the tilt, but it really had my head spinning trying to incorporate the tilt (45 years ago I wouldn't have had any problems, but after collecting social security for a few years, your brain slows down a bit).

 

[B.J.]

Thanks for resurrecting the old notebook thread. Saved searching. I also managed to find my old excel spreadsheet that I used to do my version of the calculation. Not easy to follow what I did, but I *THINK* that I first calculated the X,Y,Z of the sat in earth centered coordinates, then did a translation in Y and Z to a parallel coordinate system located at my location, then did a rotation around the X axis to get my 0.7 deg tilt, again calculating the X,Y,Z of the sat in that coordinate system. Then did an arctan of the new X,Y to get the USALS angle.
At least that's what I THINK that's what I did.

Whatever I did, the results are very close to your results, after I corrected my original errors. What I had NOT remembered was that the results were different from the official GAAPS USALS angles, which I think was your original reason for starting the thread, and they seem to be different whether you use the tilt angle or don't use the tilt angle. So I'm not sure how they've calculated the angles.