What's the difference between an X/Z and an X/Y 2-axis gyro?

[Randy Forgaard]

Our team is thinking of using a 2-axis gyro from SparkFun on our robot this year. We are looking at the IXZ-500 X/Z gyro and the IDG-500 X/Y gyro. These two gyros seem to have essentially identical specs, except that the first measures the X (yaw) and Z (pitch) axes, and the second measures the X (yaw) and Y (roll) axes.

This is where I'm confused. If you take an X/Z 2-axis gyro, and rotate the breakout board by 90 degrees before you mount it on the robot, doesn't that make it an X/Y gyro, because now it is measuring sideways roll (Y axis) rather than front-to-back pitch (Z axis)? I know there is something I'm missing here. Any help appreciated!

[EricVanWyk]

You are absolutely correct that re-orienting one 2 axis gyro results in the equivalent measurements taken by the other gyro.

However, it is common to not have the option to reorient the sensor. For example, I was designing a 6 axis IMU - inertial measurement unit. Keeping it all "in plane" on one board was a requirement for both cost and accuracy. Having two boards aligned at 90 degrees with any precision is a real pain. For this reason, it is beneficial to have the option to "reorient" the gyro by ordering a different part number.

[Randy Forgaard]

Hi, Eric. Thanks so much for your reply. I'm still confused, though. Let me restate my question differently.

With an X/Z 2-axis gyro like the IXZ-500, you have a single gyro that measures angle changes in two orthogonal axes: the X-axis (yaw, in the plane of the circuit board), and the Z-axis (front-to-back pitch) which is at right-angles to the X-axis. This 2-axis gyro saves you the trouble of needing a second Z-axis gyro that is mounted at right angles to the circuit board, as you mentioned in your reply.

Okay, now imagine that we take that X/Z 2-axis gyro, mounted on its little circuit board, and we rotate the whole board 90 degrees, horizontally, in the same plane as the circuit board itself. I don't mean rotating the circuit board on its edge. I just mean rotating the circuit board, horizontally in the same plane, so that the front of the circuit board is now pointing at (say) the left side of the robot, rather than the front of the robot. The circuit board is still horizontal, it's just rotated 90 degrees in the horizontal dimension.

It seems to me that with this mounting for an X/Z 2-axis gyro, it now acts like an X/Y 2-axis gyro. In other words, it is still measuring X-axis rotations (yaw). But because the board has been rotated horizontally sideways (still keeping it flat and horizontal), the Z-axis function on the gyro is now actually measuring the Y-axis (roll) rather than the Z-axis (pitch). So, it would seem to me that if you have an X/Z 2-axis gyro, and what you really wanted was an X/Y 2-axis gyro, you could just take exactly the same circuit board and rotate it 90 degrees in the same horizontal plane, before mounting it to the robot. So, it would seem the world would not need both an X/Z 2-axis gyro and a X/Y 2-axis gyro, since the X/Z 2-axis gyro could be used for either purpose, per above.

But here's the question: SparkFun sells both an X/Z 2-axis gyro (the IXZ-500) and an X/Y 2-axis gyro (the IDG-500), which seem to have the same specs, except one measures X/Z and the other measures X/Y. But that seems weird, because per my description above, it would seem that if you just rotate the X/Z gyro 90 degrees in the same horizontal plane, you suddenly have an X/Y gyro. So, SparkFun wouldn't need to sell two different integrated circuits for this.

This leads me to believe there is some other difference between an X/Z 2-axis gyro and an X/Y 2-axis gyro that I'm not understanding.

I'm very new to this stuff, so I know I'm missing something, and I just don't know what it is. Thanks in advance for your help!

[EricVanWyk]

I can't find a datasheet for the IXZ-500 - all the links point to a deleted page. Can you direct me better?

From page 5 of http://www.sparkfun.com/datasheets/C...eet_IDG500.pdf, both axes are along the surface of the PCB.

[Randy Forgaard]

Hi, Eric. Sorry about the broken link to the IXZ-500 X/Z 2-axis gyro on the SparkFun site and even on the manufacturer's site. Here is the correct link to the IXZ-500 datasheet.

Thanks so much for mentioning Page 5 of the IDG-500 X/Y gyro datasheet. I looked at that page, and I now see the little diagram in the lower-right corner of that page that shows what they mean by X-axis and Y-axis. Their X-axis is what I would call "pitch," and their Y-axis is what I would call "roll." I just now compared that to the corresponding diagram on Page 5 of the IXZ-500 X/Z gyro datasheet, their Z-axis is what I would call "yaw."

Since, for our robot, we are looking for "yaw" and "pitch" measurements, we want the IXZ-500 X/Z gyro. And rotating that IXZ-500 X/Z gyro circuit board 90 degrees would NOT be the same as an X/Y gyro. That is why they needed to make separate chips.

Your question back to me helped me answer my own question. Much appreciated! It all came down to a misunderstanding on my part -- I thought X/Y/Z was yaw/roll/pitch, but (at least as far these gyros is concerned) X/Y/Z is pitch/roll/yaw.

I'm all set. Thanks again!

[Bob Steele]

I might be wrong about this but I believe the difference is that in an XY gyro you have the two axes oriented in the same plane as the board.
In an XZ gyro the one axis is in the plane of the board and the other is normal to it.

I think that is the difference...

[Randy Forgaard]

Hi, Bob. Yes, what you say does indeed seem to be correct. From Eric's email, I discovered a little diagram in the datasheets for both the X/Z gyro and the X/Y gyro that showed what they mean about the X, Y, and Z-axes.

My confusion stemmed from the fact that I thought that X/Y/Z was yaw/roll/pitch, but (at least as far these gyros is concerned) X/Y/Z is pitch/roll/yaw. That explains why an X/Y gyro is not simply a 90° rotated X/Z gyro -- they are fundamentally different parts, because the Z-axis (the yaw) is in a plane that is normal (orthogonal) to the plane of the X-axis (pitch).

I'm all set. We went ahead and ordered the X/Z gyro, and we're excited to receive it and give it a try. Thanks for your note!

[DonRotolo]

Originally Posted by Randy Forgaard

Okay, now imagine that we take that X/Z 2-axis gyro, mounted on its little circuit board, and we rotate the whole board 90 degrees, horizontally, in the same plane as the circuit board itself.

Then instead of an X/Z gyro you get an Y/Z gyro. The Z axis goes straight up through the board.