Abstract: The variable structure control(VSC)for systems where the state of the system is unmeasurable is studied.State observer with measured output from the system is constructed and the dynamics of the sensor is also taken into accotint as a part of the whole dynamic system. AVSC for the whole system is established via the so-called"reaching law method".The sliding mode and its stability are studied.At last an interpretation to the chattering in VSC system is given.

Key words: control system design, sensorsobservation