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Unread 20-04-2010, 19:47
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Re: Picking a gyro for field-centric swerve control

Quote:
Originally Posted by Tom Line View Post
ADXRS610 - 300°/s (5 volt supply) $59.95
http://www.sparkfun.com/commerce/pro...oducts_id=9058

Gyro Breakout Board - LPR5150AL Dual 1500°/s (2.7-3.3V supply)
http://www.sparkfun.com/commerce/pro...oducts_id=9412

Adjustable Voltage regulator for 1500 deg/s gyro $1.95
http://www.sparkfun.com/commerce/pro...roducts_id=527

I will monitor the actual rate of the 300 deg/s gyro in code (fast loop). When the values jump within 10% of the upper range, I'll check it against the baseline of the 1500 degree per second gyro. If there is a substantial difference, I'll calculate a correction factor to apply to the 300 deg/s gyro.

So here come the "electrical" type questions. Am I choosing a good voltage regulator for that 1500 dps gyro? What type of drift should I expect out of the 1500 dps gyro, and how accurate will it be over time? Should I be worried about the CRIO being able to accurately measure the .67 mV/deg/s value, or is it capable?

Remember - I'm not a controls guy but I did stay in a Holiday Inn Express last night....
Tom, the approach you describe may or may not work depending upon the requirements you are trying to meet. What is the max rate you expect to see and for how long? Lets say the bump transient is .1 sec constant rate upset... if the upset is only 90 deg we are talking 900 deg/s then what you have is probably ok. If its only 500 deg/s then I would choose a 100 deg/s fine gyro and say 500 deg/s coarse gyro.

The CRIO a/d's have a 5mv resolution without oversampling. For the 5150 this means about 7.5 deg/s resolution. Also, this is really a 6000 deg/s gyro with the amplified output at 4x. The noise density is .175 deg/s/(sqrt(HZ). I believe this is at 1x so with the 140 HZ BW the the 4x noise is 4*.175*sqrt(140) = 8.3 deg/s rms. So you have a total rss accuracy of about 11 deg/s. This could be reduced with oversampling and if you are running at 200 Hz loop rate, you have an averaging reduction of 1/sqrt(20) so the average bias error would be around 2 deg/s. This isn't too bad for a .1 sec interval. But what may kill you is the nonlinearity error of 1%FS= 60 deg/s. This cannot be helped with averaging, so you might be off as much as 6 deg here from the .1 sec transient so the compass is looking better.

If however, you could live with the (fine,coarse) = (100 deg/s,500 deg/s) you could probably beat the compass.

The adj regulator would be fine, but you could save a resistor if you just bought a 3v fixed regulator.

I would also sync the long term average of the coarse gyro to the fine gyro average to mitigate the drift. This is easily done with a low pass (10 sec time constant?) on the error between the two gyros whose output corrects the coarse gyro input.

Opps... this is probably a redundant reply since I didn't see Alex' reply first.

Last edited by vamfun : 20-04-2010 at 20:35.