Stress Testing Control Loops in Cyber-Physical Systems

Cyber-Physical Systems (CPSs) are often safety-critical and deployed in
uncertain environments. Identifying scenarios where CPSs do not comply with
requirements is fundamental but difficult due to the multidisciplinary nature
of CPSs. We investigate the testing of control-based CPSs, where control and
software engineers develop the software collaboratively. Control engineers make
design assumptions during system development to leverage control theory and
obtain guarantees on CPS behaviour. In the implemented system, however, such
assumptions are not always satisfied, and their falsification can lead to loss
of guarantees. We define stress testing of control-based CPSs as generating
tests to falsify such design assumptions. We highlight different types of
assumptions, focusing on the use of linearised physics models. To generate
stress tests falsifying such assumptions, we leverage control theory to
qualitatively characterise the input space of a control-based CPS. We propose a
novel test parametrisation for control-based CPSs and use it with the input
space characterisation to develop a stress testing approach. We evaluate our
approach on three case study systems, including a drone, a continuous-current
motor (in five configurations), and an aircraft.Our results show the
effectiveness of the proposed testing approach in falsifying the design
assumptions and highlighting the causes of assumption violations.