[simpits-tech] Motion...

Mon, 20 Jan 2003 10:16:46 +0100

  Hi,

Joseph Fagner wrote:

>Think about this for a minute (granted I realize this is probably overly
>simplistic and the math is infinitely more complex):  Most software out
>there has the 4 axis of data (not to be confused with the axis of evil)
>one might need for motion.  Couldn't one just write some software to
>read this data and translate that to motion?  The data to which I refer
>is (1) pitch and (2) roll  (X and Y axis); via your artificial horizon
>instrument.  (3) Yaw (Z axis), via your compass and (4) altitude via the
>altimeter.  
>  
>

It is not that simple.

You need angular rates as well as accelerations. Making motion platform 
based on  attitude of the AC would be waste, beleive me. Generally 
speaking, with the motion platform one hopes to imitate (inertial) 
accelerations that pilot feels. Simple example acceleration on the 
runway during take-off. During that period pilot feels sustained 
acceleration on his back. The sense of acceleration is imitated by 
rotating the platform slightly backwards, so the weight of the  "pilot" 
generates the force on the back. That shift is sudden, and later the 
platform is slowly returned to middle position. Thus, imitation of just 
one linear acceleration is achieved completly different type of motion 
(sudden rotation, followed by slow return to neutral position).

Other than this, you should think of something completly different:
Motion platfors are _very_ power hungry, you might easilly end up with 
facility which draws couple of  tens of KW of power.
Yet another problem is safety. Think of the following: it is very easy 
to start the filre in the chitchen with electrical oven of just couple 
KW (or even less), and here you are dealing with much more powerfull 
equpment.

Yet yet additional problem are foundations, because something must 
support complete construction, and take over complete forces and moments 
generated by themoving platform.

There are many problems, but this doesn't mean that there is no sollution :)
If you really want to make motion platform, do read apropriate chapter 
in "flight simulation", book that is mentioned few days before. It is 
excelent reading, but be warned that it does not provide us with 
sollutions, but presents the problems in very concise way, and just 
guides towards the solution. But in any way it is a "must have" book. 
I've been reading it from the cover to cover and all over again many 
times :)

Also check web for motion platforms where you will find some standards 
 you should achieve. one url is www.fokercntroll.com, and the other is 
(if I recall correctly, I'm off line now) www.mug.com.

Personally I'll take following approach:
Generaly, visual system provides low frequency sensations, and use 
motion (vibrations to be specific) for the high frequency sensations 
(like tyrbulence, or vibrations, ...). This is not as good as well made 
motion system coupled with nice visualisation, but it is _much_ better 
than wrongly made motion platform, and in any case  infintelly safer.

At the end one true story:
(I don't know the details, what kind of  AC, how many degrees of freedom 
in motion platform, nor how many pilots, ... :)
There was a project of evaluating different algorithms for driving 
motion platform, and within the project many (profesional) pilots were 
flying on the simulatior, every flight with different algorithm applied. 
Needles to say that pilots didn't know which algorithm was to be applied 
for any of the flights. After every try, they were asked to describe in 
words how they felt, and to place a numerical mark. After the data were 
analysed, it turned out that many pilots (more than few, anyway) said 
that the best results were achieved with algorithm "number 5". The funny 
thing was that algorithm in question corresponded to the case where the 
motion platform didn'mtmove at all!  Of course, pilots didn;t know that 
fact while "flying"

>Jay
>
>-----Original Message-----
>From: simpits-tech-admin@simpits.org
>[mailto:simpits-tech-admin@simpits.org] On Behalf Of Roy Coates
>Sent: Friday, January 17, 2003 6:57 PM
>To: simpits-tech@simpits.org
>Subject: Re: [simpits-tech] Motion...
>
>On Fri, 17 Jan 2003, Brian West. wrote:
>
>  
>
>>        I have an excellent book published a few years back. The title
>>    
>>
>is "
>  
>
>>Flight Simulation" by J.M.Rolfe and K.J. Staples, published by
>>    
>>
>Cambridge
>  
>
>>University Press, ISBN 0-521-35751-9
>>It contains all the mathematical models etc needed plus a lot more.
>>    
>>
>It's
>  
>
>>almost certainly out of print but a search of secondhand book dealers
>>    
>>
>might
>  
>
>>turn up a copy.
>>If you can't find a copy come back to me . It's all a bit daunting
>>    
>>
>though,
>  
>
>>time and costwise for an individual.
>>    
>>
>
>Thanks Brian - appreciate that. From what little research I've done so
>far
>it has become quite clear the there is no fixed rule to motion
>simulation,
>there seem to be several schools of thought on the topic.
>
>It also seems clear that each method has its own weaknesses. I'll
>continue
>searching!
>
>Roy.
>
>
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>