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[Advanced Control Systems Group] [Autonomous Mobile Robotics Group]





Sliding Mode

Sliding mode control is a consequence of discontinuous feedback control, which appeared in the middle of the 20th century under the name of theory of variable structure systems.

SMC algorithm consists of the following steps:

1) Choosing a switching surface in the state space with desired characteristics:

2) By using appropriate control law:

Making the system states reach the switching surface (reaching phase),

Keeping system states on the switching surface (sliding phase).

The term switching illustrates the fact that the control law commutes while crossing the line . Once the states have reached the sliding surface, closed-loop system is insensitive to variations in system parameters and external disturbances.

System state space using: a) continuous, b) discontinuous sliding mode controli

The ideal sliding mode requires infinite switching frequency, which is not achievable in practice due to the limited sampling rate in digital implementation. Behavior of the system states with discontinuous (discrete) sliding mode causes chattering, which is undesirable due to its tendency to excite high-frequency unmodeled dynamics and that may lead to system failure. Discrete type of SMC where system states do not slide on the switching surface, but constantly switch around it in a zig-zag motion is called quasi sliding mode.

Sliding mode control is based on the assumption that all system states are known. But, when all system states are not measurable, some sort of states estimator has to be used.

When using sliding mode control, control signal consists of two parts.

 

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