AXIOMATIC Tri-Axial J1939 CAN, User Manual

Page: 13 / 67

UMAX0608XX‐1000. Tri‐Axial J1939 CAN Inclinometer. Version 9B
Page: 13-66
2.1.3.3 Maximum Gravity Acceleration Error
All angular measurements are based on the assumption that the only acceleration applied to
the unit is the gravity acceleration
𝑔⃗
, see (2) . This is not entirely true when the inclinometer is
installed on a moving machine and is experiencing various external accelerations. These
accelerations will affect the angular calculations and, at some point, will make the accuracy of
the calculations inacceptable.
To monitor the validity of the angular calculations, the inclinometer is calculating the
Gravity
Acceleration Error
𝛿
as a difference between the measured gravity acceleration
𝑔⃗
and its
expected value:
𝛿 1 𝑔 𝑔 𝑔
(9)
When the difference exceeds a predefined value
𝛿 𝛿
, the angular calculations are
considered invalid and the inclinometer sets the error state in the Angular Figure of Merit.
The
Maximum Gravity Acceleration Error
𝛿
is set by the user normally above the
expected external accelerations at the customer site during normal operation conditions.
Please remember that even when
𝛿 𝛿
, the rated inclinometer static parameters
including accuracy are not guaranteed during external accelerations. The
𝛿
only sets a
threshold to notify the user that the external accelerations are too high for the angular
measurements.
2.1.3.4 Practical Recommendations
In the beginning, the user defines an inclinometer position on the machine, direction of the
measurement angle or two angles in orthogonal directions, and the angular ranges.
It is important to understand that the inclinometer calculates angles based on the gravity
acceleration and the angles are measured between the inclinometer unit frame or machine
frame and the Earth absolute reference frame (X
E
,Y
E
,Z
E
) where the gravity acceleration vector
is uniquely defined.
The inclinometer can measure only pitch
𝜃
and roll
𝜙
angles. It cannot measure the yaw angle
𝜓
, since the yaw angle is in the plane perpendicular to the gravity acceleration in the Earth
absolute reference frame (X
E
,Y
E
,Z
E
) and therefore cannot be detected by an accelerometer.
The user starts with aligning the inclinometer unit frame with the Earth absolute reference
frame at the inclinometer expected position on the machine. This is done by pointing the unit
frame Z-axis down, making it coincident with the gravity acceleration vector, and then aligning
the unit pitch
𝜃
and roll
𝜙
angles with the required measurement angles. The user can do this
either by mechanically rotating the inclinometer housing on the machine or by changing the
unit frame orientation using inclinometer configuration parameters.
The vertical mount inclinometer modifications can be also used if the inclinometer housing is
installed in the vertical position on the machine.
1