Hi Phillip,

While I agree that the requirement to use ultrasonics seems very arbitrary
and unwarranted, there are some reasons why gyros alone are not adequate.
The type of gyros that are available cheaply (less than the 1000s of
dollars) are piezo rate gyros. These do not tell you your orientation.
Rather, they tell you the rate at which your orientation is changing. This
is useful if all you need to do is make something stable (i.e., prevent the
helicopter from starting to roll wildly), but it can not hold a given
orientation (let alone position) for very long.

This is because of bias drift. The output of the gyro has some constant
level even when you are not rotating. You need to subtract this out to get
an accurate reading, but the level is dependent on temperature, often
heavily (1deg/sec per deg C). Even if you add decent temperature
compensation, you will be lucky to keep the thing from rolling more than 1
deg/sec. The addition of the ultrasonics (which are an absolute sensor,
i.e., they directly tell you orientation) can allow you to cancel out the
drift. Roughly speaking, you can low pass filter the output of the
ultrasonic orientation system (to get rid of the noise caused by the rotor
downwash), and add it to a high pass filtered version of the gyro data (to
get rid of its shifting DC offset), to give you a good estimate of your
orientation with very certain error bounds.

Of course, it is possible to do this with other types of sensors (besides
ultrasonics), and they might work better for this application.

Sean



At 08:24 PM 4/13/02 -0400, you wrote:
>Can you mount the ultrasonics on outriggers? This would accomplish two
>things. 1) get them out of the downwash into some (relatively) stable air,
>and 2) amplify the differences in their distance from the ground.
>
>Personally, I would tell the instructor to shove it and use gyros. They're
>proven technology for this job. But, then again, instructors don't like me
>very much.
>
>Phillip
>
>-----Original Message-----
>From: pic microcontroller discussion list
>[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of David Wilson
>Sent: Saturday, April 13, 2002 7:17 PM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: helicopter simulation programs PIC16C74A
>
>
>Everything you said before was correct apart one one small thing, the
>helicopter will mov forward for 10 seconds not hover, sorry for not
>clarifying it too much when i described it. Answering other peoples
>questions the helicopter will be tested outdoors on a level surface. The
>reason we have to use ultrasonic sensors is that we have no choice because
>our college lecturer insists we use them :-(. The main problem that we have
>is that we are not really good at the code for the PIC cause we have only
>had 12 about 8 weeks experience at it (about.... 30hrs) and he says we can
>get any outside help apart from himself.
>                   Thanks
>                     David
>
>-----------------------------------------------------------------------
>On Mon, 8 Apr 2002, David Wilson wrote:
>
> >Hi my name is David and i have been having trouble trying to create an
> >auto-pilot based helicopter program. The problem i am having (basically all
> >of it really) is that there is 4 ultrasonic transmitters/receivers situated
> >on a cradle below the helicopter. These are used seperately to find out the
> >time taken for the waves to be transmitted and then received, the times are
> >then used to calculate if the helicopter is level.
> >    The other part of the program is that the helicopter must take off to a
> >height of around 1 to 2 metres fly level for around 10 seconds and then
> >land
> >again without blowing up!
> >            I would be grateful if anyone could help me try and complete
> >this
> >program, no matter how small information given will be gratefully
> >appreciated and would help me a lot. thank you
> >                                  David
>
>
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