SPECIAL AR 5402352 - Caterpillar

Chassis Inertial Measuring Unit (IMU) Yaw Alignment Compensation - Body Sensor Adjustment
The Inertial Measuring Unit (IMU) is a component of the Cat Grade Control system on Track-Type Tractors and is critical to accurate machine guidance. An accurate GS420 Chassis IMU calibration is required for an accurate system and the IMU calibration must be completed prior to the machine measure-up.The Chassis IMU calibration zeros the dual axis IMU sensor in the pitch and roll directions but assumes that the yaw direction of the sensor aligns with the yaw direction of the machine. The tolerance of the factory mounting of the IMU may include a small amount of sensor yaw from the chassis yaw.A small misalignment of up to a few degrees will have no noticeable impact on level grading applications but when the machine is operating under side slope conditions, the yaw misalignment of the sensor may lead to a vertical error seen at the blade tips. A machine with an uncompensated IMU sensor will also exhibit blade slope error when operating under machine mainfall conditions.Once the IMU calibration and measure-up is completed, the machine can be checked for accuracy as described in the "Pre-Delivery Measure-up Dimension Verification" section. If there is an accuracy problem on side slope conditions but not under level side slope conditions, there is the potential that the machine IMU could be slightly misaligned with the machine chassis.The best way to determine whether a Chassis IMU Yaw Adjustment is necessary to measure the vertical error at the blade tips while the machine is on a side slope then turn around the machine 180 degrees and measure the vertical error at the blade tips again. If the vertical error direction and magnitude is consistent with the machine facing the opposite direction, the error is more likely to be associated with the Machine Measurements than the Yaw Adjustment. If the vertical error magnitude and direction is similar, the vertical error is likely to be associated with the GS420 Chassis IMU yaw. The severity of the slope that will show that a yaw misalignment error at the blade tip depends on the severity of the sensor skew.The Cat Grade Control system on the track-type tractor is equipped with a process to compensate for slight IMU misalignment but it can only be performed after calibrating the IMU and measuring up the system.Note: It is important that the Chassis IMU Calibration and Measure-Up process have been completed prior to adjusting the IMU.If a vertical error is seen at the blade tips under side slope conditions and blade slope error is seen under mainfall conditions and not in level surface conditions, the following process can be completed to allow the system to compensate for any mounting differences in the IMU and improve the accuracy of the system.
Illustration 126 g06107543
Side slope increasing to machine left
Set up the machine on a side slope where vertical error is seen on the blade tips.
Grade a side slope pass on the slide slope where the machine slope