  Autonomous robots must operate in complex and changing environments subject
to requirements on their behaviour. Verifying absolute satisfaction (true or
false) of these requirements is challenging. Instead, we analyse requirements
that admit flexible degrees of satisfaction. We analyse vague requirements
using fuzzy logic, and probabilistic requirements using model checking. The
resulting analysis method provides a partial ordering of system designs,
identifying trade-offs between different requirements in terms of the degrees
to which they are satisfied. A case study involving a home care robot
interacting with a human is used to demonstrate the approach.
