L3G4200D 3 axis gyro for dish alignemnt?

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Robz

SatelliteGuys Family
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MEMS L3G4200D, by ST microelectronics, three axis gyro scopefor satellite dish alignment.
Are the micro electrical mechanical systems (MEMS) three axis, yaw, pitch and roll accuracy good enough to align a C or Ku bandsatellite dish? I will experiment with the L3G4200d gyro one I get the backordered circuit development board; it may be a month or more from now. Briefly,the L3G4200D gyro is ultra-stable three-axis digital output gyroscope with 16 bit-ratevalue data output.The L3G4200D gyro is the same deviceas found in the Apple iPad 2 and Samsung’s Android based Galaxy tablet. Gyro compasses, yaw pitch and roll display has already been incorporated into “APPS”for these devices. Answers have been found, but virtually no documentation exists on details. In the hopes of gaining insight to the ST gyro, I pose thequestion in a group where dish alignment is often discussed. What I desire fromthe L3G2400D gyro is:

1. North seeking capability. This may take hours to complete, but the results hould provide high accuracy to establish true North without using a magnetic compass or GPS receiver. Accuracy should be better that one degree. My main problem is establishing due south within a degree or better. As simple as this sounds, it is nearly impossible without fancy surveying equipment. A magnetic compass is useless. Its accuracy is thrown off by the dish’s metallic effect,even accounting for magnetic declination correction. A true north seeking gyro will provide a true reading, independent of metallic effect, and is easy toapply to the dish alignment result.

2. The L3G4200D must be capable of replacing traditional rotary encoders for motion control. It really simplifies mechanical design when asmall gyro can replace a rotary encoder. In this case the size of the L2G4200Dis 4x4 mm. Compare this size with a typical rotary encoder of over an inch in diameter or larger. Well may not be fair, the L3G4200D needs to be supported bya local CPU and support circuitry that also may exceed a one inch size, but is not dependent on the same constraints required for a rotary encoder’s mounting requirements. There is more freedom with a gyro than a rotary encoder mounting requirements.



The L3G4200D three axis gyro is a rate device. It outputs to the external world degrees/second for yaw, pitch and roll. I need to convert the rate output values to position or degrees. A simple integrator will provide the conversion, however the time between reads must be precisely known. I do not know the best way of configuration the L3G4200D or the best time clock to use. I know windows supports a high performance clock mode with microsecond precision (and have used it often), but it’s also affected by time to interrupt delay and other OS overhead. Ideally the L3G4200D gyro should provide a time stamp synchronized to the actual time of data capture.

For the L3G4200D, the basic concept is simple to understand; however, the details are difficult to unravel and understand. The most time is spent on the details, not the concept.



Ok back up! What the hell is this nut case trying to accomplish.

Ideally I am trying to replace the actuator pulse count with a rotary optical encoder mounted at the dish’s axis of rotation. This is same axis found in HH motors for USALS motion. Using an optical encoder at the dish rotational axis can provide USALS capability even though actuator driven. Take this one step further; the optical encoder can be replaced by a MEMS gyroscope. The result is a polar mount structure than can be improved by a simple MEMS gyroscope. Use the actuator for C band dishes, its strong, rugged and dependable. But, use the MEMS gyro for precise control. Forget about the actuator pulse count; use the output from MEMs gyro for precise PID motion control. Even forget about modified declination tilt, use a small second actuator to provide exact tilt declination per satellite position using the MEMS gyro output. Only one axis is needed, but a second axis will provide conformation.

Sorry for being long winded, however this application has caught my attention with a fury. :)

I dream of the day when I can get my 6 foot dish aligned and tracking. So far only dismal results have been achieved. OH! My mounting poleis vertical from the ground, I have checked it many, many times using a level and two different types of digital levels; it is correct as well as all otheralignments. However there must be a mistake and I feel only more exotic methods will finally shed some light where the miss alignment is. Part of the problem is the extreme sloppy alignment method for aligning the C/Ku band LNB. Just a set screw? He guys, there must be a better method. Maybe a MEMS Gyro mounted onthe C/Ku band LNB will enable alignment. Watcha you think? Sorry for sarcasm,but when you have spent hours for only dismal results you have got to be annoyed.

Suggestions, corrections, criticisms, even flames are allappreciated.

Rob
 

Bongu

FTA addict - suffering withdrawal since moving
Oct 20, 2010
648
65
Fort Worth 'burbs
I need to go smoke some happy smoke to decipher the opening of that message.....

On to the 6' alignment. If it is one of the WSI specials, the polar mounts are cheaply made and need some "adjusting" to square them up. Once you get the mounts squared up they should be just fine.
 

Anole

SatelliteGuys Master
Sep 22, 2005
11,819
12
L.A., Calif.
Other people have been able to align their 6' dishes.
... with regular common tools, and a bit of patience.

Yes, some of the one-screw scalars are unfriendly.
I added a little metal bracket/tab to mine, and filed the tip of the screw flat.

Expecting a pile of uncalibrated, unaligned, hardware to suddenly find the magic signal,
is a bit unlikely.
You might as well build signal-hunting into this robo-dish. ;)

There is a company who makes the Arc-Set alignment tool.
You might want to invest in one of those.
Personally, I think it's overpriced snake oil, but that's just my opinion. :)
Search the forum for some recent (2011) discussion on the device.

The one interesting idea, is the use of a gyro for position feedback on a dish.
Not sure what happens when you loose power, though.
Maybe some brainstorming on the matter would prove fruitful. (?)
Or at least fanciful.
The thing is, I found a rotary shaft encoder for a reasonable price, with around 2000 counts per revolution.
At the time, I dismissed it for a hand full of reasons... most of which apply to a gyro as well.
At least, the shaft encoder would have been a lot less trouble to install and operate. :up
 

Robz

SatelliteGuys Family
Pub Member / Supporter
Other people have been able to align their 6' dishes.
... with regular common tools, and a bit of patience.

Yes, some of the one-screw scalars are unfriendly.
I added a little metal bracket/tab to mine, and filed the tip of the screw flat.


The one interesting idea, is the use of a gyro for position feedback on a dish.
Not sure what happens when you loose power, though.
Maybe some brainstorming on the matter would prove fruitful. (?)
Or at least fanciful.
The thing is, I found a rotary shaft encoder for a reasonable price, with around 2000 counts per revolution.
At the time, I dismissed it for a hand full of reasons... most of which apply to a gyro as well.
At least, the shaft encoder would have been a lot less trouble to install and operate. :up

Thank you for your comments.

It’s the problem and the journey to find a solution I find interesting. I know others can get their dish aligned without too much fuss using ordinary tools. However I prefer as you say “robo-dish” alignment ;)
Losing power is no different than losing power with an actuator system. The G-BOX will maintain the position count. The same is true for a GYRO or rotary shaft encoder. The control box could be a G-Box or a custom designed one for enhanced features; USALS for instance, would be nice.
 

Robz

SatelliteGuys Family
Pub Member / Supporter
I need to go smoke some happy smoke to decipher the opening of that message.....

On to the 6' alignment. If it is one of the WSI specials, the polar mounts are cheaply made and need some "adjusting" to square them up. Once you get the mounts squared up they should be just fine.


:)...The dish is not a WSI one. However, it is one from a competent manufacture.
 

Robz

SatelliteGuys Family
Pub Member / Supporter
The biggest problem I see in tracking dish movement is response speed of the instrumentation. Some indication about 4 to 6 seconds minimum...... On a moving satellite dish?


The L3G4200D has a full scale of±250/±500/±2000 dps (degrees per second) and is capable of measuring rates with a user-selectable bandwidth. The L3G4200D is packaged on an LGA-16 (4x4x1.1 mm); very small.
You may be correct, however, I believe that the speed of measurement is more than fast enough. The time for my dish to swing from east to west is about 60 seconds. The GYRO should have no trouble keeping up. Also the GYRO output is degrees/second and does not change with constant motion. It changes only during acceleration. Typically this is during starting and stopping. This differes from a rotary encoder, its output is position or count and not counts/second. Transformation however, is simple and either one should work.
 

gardenman2002

SatelliteGuys Family
Jun 20, 2010
79
1
indiana
Hey interesting topic. Gardenman 2002 here.
I set my 6 ft dish up with a protractor ,and level from a hardware store and used a compass that i bought from kmart in the camping section.level the pole, set the elevation on true south satellite and use a portable tv with receiverto swing dish left or right to obtain signal. peak signal and the lock down everything so actuator can take over. I dont have many of the answers but thats how i tuned my dish
 
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