DEATH OF THE ITG3200

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The ITG3200 started its blog on good terms.  Turns out there's no deadband, but you get a level of 8 bit quantization when reading the analog results in I2C burst mode.

 

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You have to read the analog results 1 byte at a time to get the full 16 bits.  Maybe to get burst mode to work, all the gyros have to be read at once, instead of just the Z axis.  Maybe you have to stand on your head.

Actually started noticing I2C anomalies when reading the low byte alone wouldn't work.  It only latched the 2 bytes when reading the high byte, as many microcontrollers do.  None of these anomalies are in the data sheet.

These are real traps for young players.  So now we have 6 ITG3200's which probably work & 1 which was probably fried, when we threw it across the room in anger.

On the bench, the awful temperature sensitivity from the IDG300 was still there.  Merely moving your hand near it shifts the 0 rate.  The ADXRS series is more stable by combining dual gyros.  It's a call between high stability & high price for robotics or low stability & low price for phones.

We still have delusions of succeeding the ADXRS by combining multiple, cheap gyros.  The temperatures are going to vary slightly & there's going to be cross axis coupling.



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Line up the dots & you get 2 sets of gyros that oppose on all 3 axes.

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The dual gyro test once again disappoints.  You're still better off using 1 gyro.  For very large temperature changes, they cancel each other out, but it's real coarse nulling.

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Finally, a few temperature runs.  Nearly the same inconsistency as the IDG300.  You're looking at 3 bits of change on every run.  You still can't get the 0 rate from temperature & you still can't improve the accuracy by combining multiple gyros.

 

We did the same temperature tests with the IDG300, in June 2009.  There, we had 16 bits divided between 300 deg/sec.  The ITG3200 is dividing 16 bits between 2000 deg/sec, so the errors are numerically smaller, but probably equivalent in degrees/sec.

 

It's good enough to hold the heading for a 150 sec indoor flight.  It's good enough for a standard 9 DOF IMU fusion, but we still haven't found a cheaper replacement for the ADXRS150's on Vika 1.  Now Analog Devices has the ADXRS646 which, for $90, supposedly compares to fiber optic gyros.

 

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It's a vintage model from 2007, but still the best.

 

 

DEATH OF SOFTWARE RTK GPS

Looking over the source code for rtklib & fastgps, you're going to have some doing to make an RTK base station out of that.  RTKlib has RTCM parsing, but nothing you could use to make RTCM packets out of the raw data.

Next, it's time to optimize fastgps.  Tried downsampling the baseband data to 4 megabits & it didn't work.

Next, tried skipping samples.  Skipping every other second of data killed it.  No way to do realtime processing by skipping samples.

RTK base stations for hobbyists are going to become standard, but whoever does it 1st is going to be in line with the uBlox price.  The LEA-6T is down to $180 + $40 shipping.

 

 

 

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