Robots Atlas>ROBOTS ATLAS
Robotics

SMC

1960ActivePublished: 4 May 2026Updated: 4 May 2026Published
Key innovation
Enforcing state trajectories to slide along a switching surface in state space, rendering system dynamics insensitive to a defined class of disturbances and model uncertainties.
Category
Robotics
Abstraction level
Paradigm
Operation level
Robot control
Use cases
Manipulator control under unknown loadsFTC — fault tolerance as disturbance rejectionElectric vehicles — motor controlRobotic aerial vehicles (quadrotors, VTOL)Maximum power point tracking in photovoltaic systemsSpeed control in electric drives

How it works

Define a sliding surface s(x) = 0. Choose a control law u = u_eq + u_sw, where u_eq satisfies the dynamics on the surface and u_sw = -K·sgn(s) enforces the reaching phase. Once s = 0 is reached, the system slides along the surface and its dynamics become insensitive to disturbances satisfying the matching condition.

Problem solved

How to design controllers robust to model uncertainties and disturbances without knowing their exact values — only an upper bound needs to be known.

Components

Sliding SurfaceState motion target; once reached, provides robustness.

Hyperplane s(x)=0 in state space defining the desired reduced-order dynamics.

Reaching LawReaching phase.

Control component u_sw = -K·sgn(s) ensuring the surface is reached in finite time.

Equivalent ControlSliding mode dynamics.

Control component u_eq that keeps the trajectory on the sliding surface once reached.

Implementation

Implementation pitfalls
ChatteringHigh

Rapid sgn(s) switching generates control oscillations that damage mechanical drive elements and excite high-frequency dynamics.

Fix:Boundary layer (replacing sgn with sat), Higher-Order SMC, super-twisting.
K Selection — robustness vs. chattering trade-offMedium

Excessively large K damages drives; too small fails to ensure robustness.

Fix:Adaptive gain (adaptive K), estimation of the disturbance upper bound.

Evolution

1960
Utkin — origins of VSS in the USSR
Inflection point

V. I. Utkin and colleagues formalise VSS and SMC theory at the Moscow Institute of Cybernetics.

1977
Utkin — "Variable Structure Systems with Sliding Modes"
Inflection point

Utkin's IEEE TAC paper introduces SMC to the Western literature.

Variable Structure Systems with Sliding Modes (paper)
1993
Higher-Order SMC (Levant)
Inflection point

A. Levant proposes HOSMC eliminating chattering by extending to higher orders; super-twisting algorithm (STA).

2003
SMC for robots — standard tool

SMC becomes a standard robust control method for manipulators and drones in the robotics literature.

Hardware requirements

Primary

SMC runs on RT CPU; high sampling frequency (>1 kHz) is key for chattering reduction.

Good fit

FPGAs for ultra-high frequencies (>10 kHz) in electric drives.