Static Accelerometer Orientation Tumble Calibration Procedures
Static tumble calibration calculates accelerometer bias, scale factor, and cross-axis matrices by optimizing spatial vector residuals against local gravity.
Optical Encoder Runout
Eccentricity Error
Geometric errors in rotary positioning systems occur when the center of the graduated scale does not align with the physical axis of rotation. In precision calibration benches, optical encoder runout introduces a periodic error in the measured angle as the stage rotates. This misalignment leads to a sinusoidal angular discrepancy that repeats once per full turn.
The magnitude of this error depends on the physical offset of the encoder disk.
Periodic Distortion
The once-per-revolution error behaves as a low-frequency disturbance that can skew the calibration of sensors mounted on the stage. Since the optical encoder runout generates a repeatable signal, it can be confused with the sensor tilt error if not properly compensated. This periodic error must be quantified and subtracted from the position feedback of the rotation stage.
High-precision stages use multiple reading heads to average out this eccentricity.
Metrological Compensation
Calibration of the rotation stage using a multi-head encoder system or an external laser interferometer allows for the correction of these geometric errors. To mitigate optical encoder runout, a lookup table of angular corrections is programmed into the stage controller. This table applies a real-time adjustment to the measured angle based on the current position.
The correction reduces the angular uncertainty of the stage during the sensor calibration sequence. This process is validated during the annual calibration of the stage against a reference polygon.
Mechanical Installation
Mechanical mounting of the encoder disk demands high precision during the assembly of the positioning stage. For a system affected by optical encoder runout, the mounting tolerances are held within a few micrometers to minimize the primary error before applying software compensation. Engineers use centering fixtures and high-precision dial indicators to align the disk during installation.