  Control barrier functions-based quadratic programming (CBF-QP) is gaining
popularity as an effective controller synthesis tool for safe control. However,
the provable safety is established on an accurate dynamic model and access to
all states. To address such a limitation, this paper proposes a novel design
combining an extended state observer (ESO) with a CBF for safe control of a
system with model uncertainty and external disturbances only using output
measurement. Our approach provides a less conservative estimation error bound
than other disturbance observer-based CBFs. Moreover, only output measurements
are needed to estimate the disturbances instead of access to the full state.
The bounds of state estimation error and disturbance estimation error are
obtained in a unified manner and then used for robust safe control under
uncertainty. We validate our approach's efficacy in simulations of an adaptive
cruise control system and a Segway self-balancing scooter.
