I. INTRODUCTION
IN RECENT years, control system reliability has received
much attention with increase of situations where robots are
used. In order to improve reliability, control systems need to
have abilities to detect a fault (fault detection) and to maintain
the stability and the control performance (fault tolerance). In
mechanical systems, actuator faults and sensor faults to which
this paper is dedicated are often concerned. Many studies have
been done on fault-tolerant control system for sensor fault, and
they are mainly classified into two approaches: