Field Verification Protocol: Measuring Hitch Geometry Deviations with Laser Tracker & IMU

At its core, this protocol addresses a fundamental mismatch: tractors are manufactured to tight mechanical tolerances, but field conditions—uneven terrain, soil compaction, hydraulic hysteresis, and thermal expansion—cause sub-degree angular shifts that invalidate factory-calibrated draft control logic. The laser tracker establishes metrological traceability by measuring physical points (e.g., ball centers, pivot axes) with micron-level accuracy, converting raw distances into ISO-aligned Cartesian coordinates.

The IMU complements this by capturing rapid attitude changes during operation—especially critical for validating dynamic response curves in ISO 11120 Annex D. However, low-cost IMUs suffer from bias instability and temperature-dependent drift; therefore, the protocol mandates in-situ boresight calibration where the IMU’s zero-rate output is constrained using tracker-measured static orientation. This yields a correction matrix applied before fusion.

Advanced implementation uses tightly coupled extended Kalman filtering (EKF), where tracker position updates serve as measurement inputs, and IMU angular rates drive the state propagation model. This allows reconstruction of full 6-DOF hitch kinematics—even during brief tracker occlusions (e.g., when implement blocks line-of-sight). Uncertainty propagation follows GUM Supplement 1, with dominant contributors being tracker volumetric error (±0.035 mm), IMU misalignment uncertainty (±0.008°), and thermal gradient across linkage (±0.012°/°C).