Apollo: /apollo/sensors/gnss/imu coordinate?

[yuconglin]

Hi everyone:
I am confused about apollo's coordinate definition for IMU. From https://github.com/ApolloAuto/apollo/blob/master/docs/quickstart/apollo_2_0_hardware_system_installation_guide_v1.md#global-positioning-system-(gps)-and-inertial-measurement-unit-(imu), it looks like it uses right-front-up convention.
but when I played the demo_2_0.bag and did a "rostopic echo /apollo/sensor/gnss/imu", I found the output is:

---

header {
timestamp_sec: 1514497070.98
}
measurement_time: 1198532288.97
measurement_span: 0.00499999988824
linear_acceleration {
x: -0.406950712204
y: 0.759772956371
z: 9.945333004
}
angular_velocity {
x: 0.00417809403439
y: -0.0102860695989
z: 0.0226752302286
}

---

header {
timestamp_sec: 1514497070.99
}
measurement_time: 1198532288.97
measurement_span: 0.00499999988824
linear_acceleration {
x: -0.37208199501
y: 0.884849578142
z: 10.1518444717
}
angular_velocity {
x: -0.00308266097663
y: -0.0105845048871
z: 0.0220807003212
}

---

header {
timestamp_sec: 1514497070.99
}
measurement_time: 1198532288.98
measurement_span: 0.00499999988824
linear_acceleration {
x: -0.469535589218
y: 0.647213310003
z: 10.0661814213
}
angular_velocity {
x: -0.00666388443468
y: -0.0120485933008
z: 0.0225722407959
}

the linear acceleration tells that the z direction facing downward.

So, how the coordinate is defined from the message /apollo/sensors/gnss/imu?

Thanks

[xinwf]

the Z's value is greater than zero, so the direction is toward up, why do you think the z direction is facing downward?

[yuconglin]

The gravity acceleration is facing downward. Since the acceleration in z
direction is positive, I infer that z direction is same as the gravity
direction, which faces downward.
reference http://www.starlino.com/imu_guide.html

On Wed, Sep 5, 2018 at 9:58 PM 辛文飞 [e-mail] wrote:

the Z's value is greater than zero, so the direction is toward up, why do
you think the z direction is facing downward?

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[xinwf]

In apollo platform, the Z's positive direction is toward up, contrary to gravity, I think this document has a mistake when explaining the coordination. Have a look about it.

The description is ambiguous or a mistake. From the following explanation and other documents and apollo's codes you should know that the Z's positive direction is toward up. Not the direction of the gravity.

[yuconglin]

If z coordinate points up, and gravity points downward. Why do we have
positive gravity measurement? Measurement of acceleration is nothing more
than its projection on a coordinate frame.

On Thu, Sep 6, 2018 at 12:19 AM 辛文飞 [e-mail] wrote:

In apollo platform, the Z's positive direction is toward up, contrary to
gravity, I think this document
https://github.com/ApolloAuto/apollo/blob/master/docs/specs/coordination.pdf
has a mistake when explaining the coordination. Have a look about it.
[image: qq 20180906121213]
https://user-images.githubusercontent.com/41889613/45134634-12b48580-b1ce-11e8-921d-2f0f8b496116.png

The description is ambiguous or a mistake. From the following explanation
and other documents and apollo's codes you should know that the Z's
positive direction is toward up. Not the direction of the gravity.

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[cmas1]

If I may chime in, everything seems fine to me. Accelerometers do not measure the total acceleration but rather the specific force, that is (over simplifying) the acceleration minus the gravitational acceleration.

This means that an accelerometer in free fall measures zero along the z-axis, whereas when it is placed on the ground it measures the reaction force exerted from the ground (-g).

Anyway, I haven't checked all the code, so I don't know if there are other transformation in between.

[yuconglin]

That will explain it, thanks a lot.

On Thu, Sep 6, 2018, 3:19 AM cmas1 [e-mail] wrote:

If I may chime in, everything seems fine to me. Accelerometers do not
measure the total acceleration but rather the specific force, that is (over
simplifying) the acceleration minus the gravitational acceleration.

This means that an accelerometer in free fall measures zero along the
z-axis, whereas when it is placed on the ground it measures the reaction
force exerted from the ground (-g).

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[apollodevcommunity]

that is raw IMU data which means the force direction IMU gets. IMU get a support force from vehicle which is equal to its gravity. And its force is up. ——Reply reference comes from the Apollo developer community issue master contest https://apollo.auto/developer/questiondata_cn.html?target=612