Bias shift in ShapeArray™ data is a known behaviour that can sometimes occur with the legacy MEMS accelerometers that were used in the construction of our Model 3 and older SAAF and Model 1 SAAX products. The nature of these MEMS accelerometers is that the zero reading can sometimes shift instantaneously which causes small jumps (typically less than 1 mm) in the reported data. 

To account for this behaviour, Measurand's software has a Bias Shift adjustment (formerly known as eXYZ correction) that can be applied to the data at conversion time. The software looks for discrete jumps in the raw data counts from a single axis only (x, y, or z) that has no corresponding changes in the adjacent segments and corrects for the zero reading. The bias shift adjustment is applied automatically to vertically installed model 3 SAAF data at the time of conversion and is not typically used in other applications such as horizontally installed SAAX unless a need to do so is explicitly observed.

A bias shift (eXYZ jump) can typically be identified manually in the raw logger data collected from the ShapeArray using Measurand’s Data Checker application. By default, the Data Checker application presents the X-, Y-, and Z-axis raw data from a .DAT file collected from a data logger as coloured blocks based on their numerical value. This allows you to quickly identify problems with data based on changes to the coloured blocks. 

The differential mode in the Data Checker application visualizes changes in the raw data values relative to the first record in the data set based on a user configured scale for the block colours (typically set to a scale of 100 raw counts by default). As a ShapeArray moves and the sensors return different values, the trend can be observed as gradual changes in the coloured blocks presented. The differential mode in Data Checker is especially well suited for identifying occurrences of bias shift. 

As described above, a bias shift is identified as a sudden shift in the raw values returned for a segment’s single axis sensor that has no corresponding changes in the sensors in the segments immediately above and below said segment. In Data Checker’s differential mode, this appears as a sudden (no gradient) change in colours for a single axis sensor with no corresponding changes in colours immediately above or below it. The figure below shows an example of data from an SAAF where a bias shift occurred in the Y-axis sensor for segment 30 around 2017-04-02.