Hi all,

Thanks to all who responded, both publicly and privately, to my questions
about temperature control. The response was OVERWHELMING, especially since
I have been very busy with classes. So, I have to respond to you all
together (at least for right now).

Essentially, we(the helicopter team) are tossing around the ideas that you
gave us. We have gotten a temperature controlled chamber to stabilize
reliably to 0.1 deg C within 11 minutes. We haven't fully tested its
resistance to external temp changes yet,nor have we determined what kind of
gradients we have in the box. We are just using proportional only (with a
high gain and feeding a resistor so that it is actually sorta
square-proportional as far as power is concerned) and a small fan inside
the box.

It is still not certain what we will ultimately implement on the heli, but
your ideas were helpful.

By the way, the heli is electric (so the suggestions about using exhaust
are out). For now, it will be tethered,but we eventually want it to be able
to do short flights from batteries. Right now, my box consumes about 6
watts, so it isn't TOO bad, especially when compared to the motor current
drain!

I was also amazed at the level of interest in our gyro and accels and the
methods we plan on using to get usable data from them. At this point, I am
still designing the test setup for a single axis accelerometer,so I can't
say anthing with experimental certainty yet.

We plan on using ADXL-105 accelerometers. We have not decided on the gyros
yet, but possible candidates range from Horizon Gyros from Systron-Donner,
to Murata gyros, and even to pre-production samples from a local company
making MEMS gyros.

The basic idea we have is to mount three accels and three gyros at 90 deg
angles to each other and mount them very firmly, so they can't change their
spatial relationship to eachother. Then, we maintain the environment at a
constant temperature, to within 0.1 deg C. We then use the Analog Devices
AD73360 6-channel 16-bit sigma-delta ADC, complete with programmable gain
amplifiers and programable digital low-pass filtering to grab the data from
all six sensors at once.

A program running on an Atmel AT90S8535 (sorry PICLIST <G>) will read the
output of the ADC and perform a complex linear cross-channel correction on
it. What I mean by this is that it will remove offset error by subtracting
constants from each channel, adjust for scale error by multiplying by
constants, adjust for cross axis and alignment error by subtracting from
each channel a small multiple of several other channel's outputs. It will
also look at the derivative of roll rate and use it to correct for the
accels not being at the CG, etc. The data will then be integrated to obtain
velocity and position/orientation,and it will be sent at about 50Hz over a
radio link (using Radiometrix RPC transceivers - this is where it DOES
contain a PIC!).

All of the constants needed for the correction algo will be obtained by
running a calibration routine which involves moving the heli around by hand
to various points, letting it stand still (to see zero offset error), and
tilting it to known angles. This calibration will (we hope) only need to be
run after every 10 flights or so.

By the end of all of this, we hope to have an INS which is capable of
determining the position of the craft to 1 mm and 0.1 deg along all three
axes within one second. A global vision system consisting of three
orthogonal USB cameras attached to a PC will be used to reset the velocity
and position/orientation error of the INS approximately once per second.

This is our plan and I'm sure much will change before the final
implementation.

Thanks again for all your help,

Sean



|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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