I'm interviewing someone for a senior controls engineer position (this is the same position I hold now). I'm having a hard time coming up with technical questions that would separate him from a less experienced guy but can still be answered within the span of a short interview. I have all of the basics (conversational stuff about aliasing, synchronized motion, design for safety, etc.) I think... but nothing sufficiently advanced. I did write a mock PLC program with some state transitions that don't work for him to analyze, but that's about it. Any suggestions?