This paper provides a comparative analysis of rotary control interface designs for industrial products requiring SIL2 certification under the IEC 61508 standard. Recognizing that even a simple panel knob constitutes a critical safety function, we evaluate the reliability and diagnostic coverage of three primary sensing technologies: potentiometers, multi-position rotary switches, and capacitive touch sensors. By utilizing FMEDA (Failure Modes, Effects, and Diagnostic Analysis) and exida’s OEMx software, this study quantifies the Dangerous Undetected (DU) failure rates—measured in FIT (Failures in Time)—for various circuit architectures.
The paper provides details on specific hardware mitigations, such as pulse testing and resistor ladders, to demonstrate how strategic design choices can minimize complexity and cost while ensuring the high diagnostic coverage necessary for functional safety compliance.