DC rate gyro

For the ones that are keeping up with my saga:


<snip>
I gave up on trying to find the datasheet for this part (long story), therefore I started doing some tests this weekend using an oscope and a bench power supply.
I was able to find the VCC and GND pins. As I mentioned before, all the other pins output a kind of irregular periodical waveform that varies in the following way when I increase voltage from 12VDC to 24VDC: amplitude increases from +4/-4V to +8/-8V; wave period decreases from 2.15ms to 1.2ms. When I measure each individual pin, there's no significant wave change when I rotate the device its 3 axis, but if I measure two channels together in X-Y mode (thanks Russel, I wouldn't try it by myself if not by your suggestion), I was able to find the pins that are sensitive to each axis. It is entertaining to turn the device on and off, it makes a noise similar to jet engines starting or stopping. At 24V it stays warm to the touch, but no smoke out of it.
It's like this: There are 10 pins, being that only 9 are connected. 2 of them are for power, pins B-C are related to axis A (this is how axes are labeled in the device), D-E are related to axis B and F-G are related to axis C. I couldn't find out what pin K is for, its waveform is slightly different from the other pins.
When I put a pair of pins on the oscope (X-Y mode) I see a diagonal of increasing slope (0 deg phase shift?). If I spin the gyro about the axis being measured, this diagonal line moves up or down on the other diagonal (perpendicular to itself) depending on the direction I'm turning the device. The displacement is proportional to the acceleration of the spin. I guess that's why they call it a rate gyro? If I turn very slowly, the line movement is almost imperceptible. On the other hand, if I display both signals on time domain on ALT(ernate) mode and turn the gyro, one waveform goes up and the other one goes down. I'm not so experienced measuring things with oscopes, so I may be doing something wrong (I have a 100MHz tek 465M).
So my question follows: how do I translate that type of signal to useful digital values? (i.e. How many degrees turned on each axis)?
Cheers
Padu
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Padu wrote: <snip>

Proof that Padu has WAY too much time on his hands... <g>
-- Gordon
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"Gordon McComb"

Hehe, I thought about throwing the towel, but after I've heard the jet engine noise, I decided that I'd make it work regardless of how difficult it will be :-)
Cheers
Padu
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Padu wrote:

Hi Padu, It sounds like the outputs are differential, which is a common way of transmitting precision analog signals. Basically one of each pair is the "true" signal and the other is * -1 or inverted. On your 465 scope, it is possible to run differential input mode with the two channels. On the display mode selector buttons to the left of the screen, select "ADD" and there should be an "INVERT" button down below CH B's sensitivity knob. Go back to normal horizontal sweep with the "ADD" and "INVERT" buttons selected. Set both vertical sensitivity knobs to the same value. You may have to play with the vertical centering to get the trace back on screen. This should give you a good idea of what the waveform looks like.
Using a differential input OP Amp circuit, feeding an A/D converter, is a place to start. My first guess is that there will be some constant DC offset for the waveforms while the gyro is sitting still. By subtracting out the offset and integrating the signal once with respect to time, you should get delta position. The one signal (K) that doesn't seem to move with the one of the axes is probably either a tachometer for the gyro or maybe a way of deriving the DC offset which is probably temperature sensitive. Since Rate Gyros require integrating the result to go from angular velocity to angle, the DC offset must be known very accurately or it will trash your results quickly. I am not a gyro guru, but the angular rate sensitivity is probably related to the RPM of the gyro, so calibrating this beast probably uses a tachometer signal somewhere.
If the outputs are proportional to the acceleration of the spin as you mentioned, you will have to integrate twice with respect to time to convert from angular acceleration to delta angle.
Good Luck and have fun, Bob
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"MetalHead"

I'll try that later on, thanks!

Thank you for the hints, I think I'll have some good [and bad] times trying to create this circuit, as I'm electonics challenged, but I got the idea. I'll do some more tests with the (K) output and note how it varies when I change voltage (and RPM).
I can do integration on software, but what's the electronic circuit conterpart for integration?
I'm saving all my notes, so if one day I get to make it to work, I'll post the instructions on my webpage.
Cheers
Padu
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A current source charging a capacitor as done with an opamp integrator. Here's one:
http://www.ecircuitcenter.com/Circuits/opint/opint.htm
--
Curt Welch http://CurtWelch.Com /
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Padu wrote:

I was looking for a good reference to send you on differential amplifiers and came across an application note on National Semi's web site that talks a little bit about rate gyros: http://www.national.com/an/AN/AN-301.pdf
Instrumentation amplifiers would be a good thing to search on for doing the differential to single ended conversion I mentioned. They are a type of differential amplifier. This app note shows both: http://www.national.com/an/AN/AN-31.pdf#page 

The integration can be done with analog electronics, but there are MANY sources of error in doing it that way. The standard method of integrating is a current source charging a capacitor. A few of the problems are that capacitors leak, current sources are not ideal, the amplifier that is going to provide your output has a non-infinite input resistance, DC offsets accumulate....
The short answer is to figure out how to decode and digitize the output signals and do the integrations in software.

I would be interested in seeing that. I need to finish a couple of obligations off first, but I am hoping to start playing with MEMS accelerometers and rate gyros this summer. I can read about it until Phoenix freezes over, but hands on will give me a lot better understanding of the features and problems of this stuff.
Good Luck, Bob
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"MetalHead"

I did that and you're right. With the device at rest, the output was a flat horizontal line. When I turned the device about the axis being measured, then the flat line shifted up or down according to the direction I was spinning the device.
Cheers
Padu
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