Posted by hypergolic on August 23, 2009 at 10:11am
I have the MMA7260Q breakout board from SparkFun and here are the values from the X, Y and Z accelerometers with a 3.3v power supply.X Y Z1g 2.54v 2.35v 2.61v0g 1.71v 1.54v 2.21v-1g 0.9v 0.73v 1.67vThe Z readings seem a bit off, according to the datasheet I should be expecting a reading from 1.485v to 1.815v at 0g, +0.8v at 1g and -0.8v at -1g. X and Y seem to be spot on, am I missing something here? Thanks!
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I am using the MMA7260Q on the UAV DevBoard, and have made measurements on about a half-dozen samples.
Nominally, in free fall, there is about 1.65 volts output for each axis. There is some variation from unit to unit on the order of up to a couple of tenths of a volt, so my firmware "zeros" the accelerometer on power up. Aside from the unit offset, once you power it up, there is practically zero drift.
When the accelerometer is "resting" and horizontal, the voltage readings should approximately be:
X : 1.65 volts
Y : 1.65 volts
Z : 2.45 volts
You mentioned that you performed acceleration tests at 0 and + and - 1 g. How did you do that? And along what axis?
I would not recommend trying to accelerate the board and make a measurement.
A simple test for you to do, I think, is to simply orient the board three different ways, with X, Y, and Z axis alternately pointed down. You should see 2.45 volts on the "down" axis, and 1.65 volts on the other two.
Then, you could orient the board three different ways, with X, Y, and Z axis alternately pointed up. You should see about 0.85 volts on the "up" axis, and 1.65 volts on the other two.
From what I remember off the top of my head, the 7260 has 800mv per g, and bias at 0 g is 3.3v/2 = 1.65v. So with a perfectly calibrated 7260 (which they aren't calibrated at all straight out of the factory)
at -1g, 1.65-0.800v = 0.85v
at 0g, 1.65v
and at 1g 1.65v+0.800v = 2.45v
Are you using the simpler analog SEN-00252 breakout or the serial SEN-00248 breakout. If the later I don't believe they wait long enough between analog samples for the A/D to settle in the code I was given.
Replies
That is the problem, here is a graph with the board upside down. The Z voltage only drops to about 1.67v which doesn't seem right.
I agree with Rich Weaver.
Here is another data point:
I am using the MMA7260Q on the UAV DevBoard, and have made measurements on about a half-dozen samples.
Nominally, in free fall, there is about 1.65 volts output for each axis. There is some variation from unit to unit on the order of up to a couple of tenths of a volt, so my firmware "zeros" the accelerometer on power up. Aside from the unit offset, once you power it up, there is practically zero drift.
When the accelerometer is "resting" and horizontal, the voltage readings should approximately be:
X : 1.65 volts
Y : 1.65 volts
Z : 2.45 volts
You mentioned that you performed acceleration tests at 0 and + and - 1 g. How did you do that? And along what axis?
I would not recommend trying to accelerate the board and make a measurement.
A simple test for you to do, I think, is to simply orient the board three different ways, with X, Y, and Z axis alternately pointed down. You should see 2.45 volts on the "down" axis, and 1.65 volts on the other two.
Then, you could orient the board three different ways, with X, Y, and Z axis alternately pointed up. You should see about 0.85 volts on the "up" axis, and 1.65 volts on the other two.
Best regards,
Bill
at -1g, 1.65-0.800v = 0.85v
at 0g, 1.65v
and at 1g 1.65v+0.800v = 2.45v
Are you using the simpler analog SEN-00252 breakout or the serial SEN-00248 breakout. If the later I don't believe they wait long enough between analog samples for the A/D to settle in the code I was given.