Re: Re: Re: [Autopilot] Ring laser gyros

[<ben...@me...>]

Tramm Hudson <hu...@sw...> schrieb am 13.08.2002, 03:17:11: 
> ben...@me... wrote: 

The Marvin project used a DGPS solution with ground station..... AFAIK 
they did not really have INS.... 


> > [...] Do You place the ICs by hand on the PCBs? 
> 
> Yes.  Everything is through-hole and easy to solder by hand.  There 
> are pads for a surface mount ADXL202E, but I don't have the skill to 
> solder it in place myself.  The design was hand-placed and routed, so 
> it is not quite as optimal as it could be. 

With the right technique, its entirely possible to even place tssop 
components by hand... I have a lot of experience with it and can give 
You a few hints if You like. 

> 
> > [...] -the sensors on separate small PCBs, that way we can adjust 
> > and change the acceleration sensors easily.. 
> 
> Are there other accelerometers on the market that you're considering? 
> We discussed a bunch of them and decided that the ADXL based on the 
> availability of lowcost parts.  But we might have overlooked other 
> products. 
> 
> It would be fairly easy to bypass our opamp filters or to retask 
> them for other analog inputs should you want to replace the onboard 
> accelerometer with something external.  Too many boxes, however, and 
> you'll start to run out of space under the canopy. 

There are some accelerometers on the marked which are a little bit more 
accurate than the ADXL202, but as far as I know, the ADXL202 is the best

choice for now. 2mg bias is already pretty good. 
However there will be better ADXLs in the future I think... 

> 
> > -the acceleration sensors should be mounted on special pcbs that dont 
> > change their form too much over temperature 
> 
> That might be significant.  Dennis' board would have shown that when 
> his engine overheated a few weeks ago.  I don't recall any bias in 
> the accelerometer output, but the rate gyros were all over the place 
> as the temperature went up. 

Therefore I would suggest: 
little pcbs with only the sensors. These pcbs should be mounted with 4 
screws or something similar to the metal case. The little PCBs should be

connected with wires to a main INS PCB.... 
Thats how professional INS are built. 

> 
> If we have a good GPS+INS filter, a small bias in the accelerometer 
> would appear as a trim error or steady state wind and should be taken 
> care of by the PID control loops.  That's what it does in the simulator; 
> I haven't seen it in real life yet. 
> 
> > -we could make the IMU a little bit larger so that we can mount high 
> > quality RLG (ring laser gyros) later, so that the helicopter can fly 
> > without GPS... 
> 
> If you can find cheap, small FOG / RLG units, please let me know! 
> For mounting on a model helicopter, even a 0.60 sized, the FOG units 
> that I have seen are too large.  But, if you can afford the high 
> price tag of the laser gyros, you might also be interested in an R-Max 
> from Yamaha... 

I am getting information about FOGs from the company LITEF (Freiburg) 
soon by mail. I know somebody who is working there. Thats also a good 
source for hints about INS.... 

The FOGs should be small enough... However they are about 5000 EUR/each.


regards, 

BEN 


> 
> Trammell 
> -- 
>  -----|----- hu...@sw...                    H 240-476-1373 
> *>=====[]L\  hu...@ro...              W 240-453-3317 
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> 
> 
> 
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