Is it a Bug? Understanding Physical Unit Mismatches in Robot Software

Robot software is abundant with variables that represent real-world physical units (e.g., meters, seconds). Operations over different units (e.g., adding meters and seconds) may be incorrect and can lead to dangerous system misbehaviors; manually detecting such mistakes is challenging. Current software analysis techniques identify such mismatches using dimensional analysis rules and ROS-specific assumptions to analyze the source code. However, these are ignorant of the fact that physical unit mismatches in robotics code are often intentional (e.g., when operating a differential drive robot), resulting in false positive bug reports that can impede robotics developer trust and productivity. In this work, we study how developers introduce physical unit mismatches by manually inspecting 180 errors detected by the software analysis technique, Phys. We identify three types of physical unit mismatches and present a taxonomy of eight high-level categories of how these errors manifest. We find that developers often make unforced and paradigmatic physical unit mismatches through differential drives, small angle approximations, and controls. We draw insights on current development to inform future research to better detect, categorize, and address meaningful physical unit mismatches.