# Sliding-Mode Control

Sliding-Mode Control is a nonlinear control technique, sometimes referred to as "model free". SMC can be characterized as a high-gain controller with good robustness properties and easy tuning, capable of rejecting unknown disturbances and robust w.r.t. model errors. The textbook version of a sliding-mode controller typically exhibits a discontinuous control law, large high-frequency gain and "chattering" around the sliding surface, an undesired behavior which more elaborate versions of SMC aim to mitigate.

The design of an SMC controller consists of two main parts:

• Design of the switching function $σ$ that produces the sliding variable $s = σ(x, p, t)$.
• Design of the control law $u(s, t)$.

The sliding surface $s=0$ must be chosen such that the sliding variable $s$ exhibits desireable properties, i.e., converges to the desired state with stable dynamics. The control law is chosen to drive the system from any state $x : s ≠ 0$ to the sliding surface $s=0$. The sliding surface is commonly chosen as an asymptotically stable system with order equal to the $n_x-n_u$, where $n_x$ is the number of states in the system to be controlled, and $n_u$ is the number of inputs.

JuliaSimControl implements a number of sliding-mode controllers, listed below:

A video tutorial making use of the sliding-mode controllers to control a physical pendulum is available below.

Next, we illustrate the use of these controllers by means of an example.

## Example

In this example, we will simulate the performance of three different sliding-mode controllers on a mass-spring-damper system given by the dynamics

$$$m\ddot q + b \dot q + kq = u + d$$$

where $[q, q̇]$ is the state, $u$ is the control input and $d$ is an unknown disturbance. The application is tracking, and the desired reference trajectory $q_r(t)$ as well as the system parameters are given by

\begin{aligned} q_r(t) &= \sin(2t) \\ \dot{q}_r(t) &= 2\cos(2t) \\ d(t) &= 2 + 2\sin(3t) + \sin(5t) \\ m &= 2 \\ b &= 5 \\ k &= 2 \end{aligned}

We start by defining the dynamics, the reference and disturbance functions. The disturbance $d(t)$ is defined for simulation purposes, the controller is not aware of this disturbance other than through its effect on the measurement of the state component $q$.

using JuliaSimControl, Plots, StaticArrays
using JuliaSimControl: rk4

d(t)   = 2 + 2sin(3t) + sin(5t) # Disturbance
qr(t)  = sin(2t)  # Reference for q
qdr(t) = 2cos(2t) # Reference for qd

function dyn(x, u, p, t)
m,b,k = 2,5,2
q, qd = x
SA[
qd,
(-b*qd + -k*q + u[] + d(t))/m
]
end

Since the dynamics is of relative degree $r=2$, we will choose a switching surface corresponding to a stable first-order system ($r-1=1$). We will choose the system

$$$ė = -e$$$

which yields the switching variable $s = ė + e$, encoded in the function $s = σ(x)$:

function σ(x, p, t)
q, qd = x
e = q - qr(t)
ė = qd - qdr(t)
ė + e
end

It's easy to see that if the control law manages to drive the state $x$ to the surface $s=0$ and keep it there, the dynamics will be governed by

\begin{aligned} s &= ė + e = 0 \\ ė &= -e \end{aligned}

i.e., the control error $e$ will go to zero as a first-order system with time constant 1.

The linear model-based SMC controller uses an error-function rather than a sliding surface, in this case, it looks like this:

function err(x, p, t)
q, qd = x
e = q - qr(t)
ė = qd - qdr(t)
[e, ė]
end
err (generic function with 1 method)

Next up, we define four kinds of sliding-mode controllers. The first one, labeled "standard", is the textbook version with a discontinuous control law $u(s) = -k \operatorname{sign}(s)$. The second "smooth" version of SMC is very similar, but uses the smoother control law $u(s) = -k \tanh(γs)$, where $γ$ is a parameter that controls the smoothess. The third is model based and thus takes a linear system model as argument. It also takes a specification of the desired poles of the reduced-order system that evolves along the sliding surface. The last controller is a second-order SMC variant called "Super-twisting SMC". This controller can achieve a lower tracking error than the smoothed controller, without the large high-frequency control action of the standard controller.

To simplify simulation of the controllers, we wrap them all in a Stateful wrapper. This makes the controllers remember their own state so that we do not have to handle that in the simulation loop.

Ts = 0.01 # Discrete sample time
sysc = ss(ControlSystemsBase.linearize(dyn, [0,0], [0], 0, 0)..., [0 1], 0)
sys = c2d(sysc, Ts) # For Linear model-based SMC

smc_standard   = SlidingModeController(σ, (s,t) -> -20*sign(s))   |> Stateful
smc_smooth     = SlidingModeController(σ, (s,t) -> -20*tanh(10s)) |> Stateful
smc_linear     = LinearSlidingModeController(; sys, k=1, ks=10, desired_poles=[0.991], err) |> Stateful
smc_supertwist = SuperTwistingSMC(50, σ, Ts)                      |> Stateful

We now implement a little simulation function that runs a simulation for a fixed duration and saves the results in arrays for later plotting. We discretize the continuous-time dynamics using the rk4 function.

function simulate_smc(; T, Ts, smc::Stateful, dyn, x0, p = 0)
X = Float64[]
U = Float64[]
R = Float64[]
x = x0
smc.x = 0 # Reset the state of the stateful controller

for i = 0:round(Int, T/Ts)
t = i*Ts
q, qd = x
ui = smc(x, p, t)

push!(X, q)
push!(R, qr(t))
push!(U, ui)
x = dyn(x, ui, p, t)
end
t = range(0, step=Ts, length=length(X))
X, U, R, t
end

function simulate_and_plot(smc)
T    = 10   # Simulation duration
ddyn = rk4(dyn, Ts; supersample=2) # Discretized dynamics using RK4
x0   = SA[0.0, 0.0] # Initial state of the system (not including any state in the controller)
X, U, R, t = simulate_smc(; T, Ts, smc, dyn=ddyn, x0)
plot(t, [X U], layout=(1,3), lab=["x" "u"], ylims=[(-1.1, 1.1) (-21, 21)], sp=[1 3], framestyle=:zerolines)
plot!(t, R, sp=1, lab="r")
plot!(t, X-R, lab="e", ylims=(-0.3, 0.1), sp=2, framestyle=:zerolines)
end
fig1 = simulate_and_plot(smc_standard)
plot!(fig1, plot_title="Standard SMC", topmargin=-7Plots.mm)

fig2 = simulate_and_plot(smc_smooth)
plot!(fig2, plot_title="Smooth SMC", topmargin=-7Plots.mm)

fig3 = simulate_and_plot(smc_linear)
plot!(fig3, plot_title="Linear SMC", topmargin=-7Plots.mm)

fig4 = simulate_and_plot(smc_supertwist)
plot!(fig4, plot_title="SuperTwisting SMC", topmargin=-7Plots.mm)

plot(fig1, fig2, fig3, fig4, layout=(4, 1), size=(1000, 600))

The simulations indicate that all four controllers do a good job at tracking the reference and rejecting the disturbance. The standard controller suffers from a very large high-frequency content in the control signal, a common problem with the naive SMC controller. The smoothed controller removes the high-frequency control action, at the expense of a slightly larger tracking error. The SuperTwisting SMC has a very low tracking error, while also having limited high-frequency gain, a nice compromise!

## Index

JuliaSimControl.LinearSlidingModeControllerType
LinearSlidingModeController(; k, ks, sys::AbstractStateSpace{<:Discrete}, err, desired_poles)

Create a linear sliding-mode controller with switching function of system sys.

This controller chooses a sliding surface of dimension nx-nu and a control law on the form

$$$u = -(GB)^{-1}(GAx - Gx + kT_s s + k_s T_s \operatorname{sign}(s))$$$

where G is chosen such that the poles of the reduced-order closed-loop system (of size nx-nu) are placed at desired_poles. The vector of discrete-time desired poles is thus to be of length nx-nu.

The controller can be called using the syntax x⁺, y smc(x, u, p, t) where x is the controller state, u the state of the plant (the input to the controller), p the parameters of σ (if any), and t the time. The new controller state x⁺ as well as the control signal y (the output of the controller) are returned.

Wrap the controller in Stateful to make it stateful, i.e., remember its own state. This version of SMC controller does not have a state, and it's thus safe to wrap it in Stateful by default.

Arguments:

• k: Proportional feedback gain on the sliding surface.
• ks: Discontinuous feedback gain on the sliding surface.
• sys: System model
• err: Error function err(x, p, t) -> e, typically e = x(t) - xr(t). The default if no function is provided is err(x, p, t) -> x, i.e., for regulation around the origin.

Extended help

The width of the quasi-sliding mode band is 2Δ = 2ksTs / (1-kTs)

In steady state, the trajectory will move within the small band given by { x | s(x) < ks*Ts }

Ref: "High Precision Motion Control Based on a Discrete-time Sliding Mode Approach"

source
JuliaSimControl.SlidingModeControllerType
SlidingModeController{S, U}
SlidingModeController(σ, u)

Create a standard (SISO) sliding-mode controller with switching function σ and control law u(s, t), where s is the sliding variable computed by σ. The controller can be called using the syntax x⁺, y smc(x, u, p, t) where x is the controller state, u the state of the plant (the input to the controller), p the parameters of σ (if any), and t the time. The new controller state x⁺ as well as the control signal y (the output of the controller) are returned.

Example:

function σ(x, p, t)
q, qd = x
e = q - qr(t)
ė = qd - qdr(t)
ė + e
end
sat(x) = x/(abs(x) + 0.01) # smooth saturation function
smc_standard = SlidingModeController(σ, (s,t) -> -20*sign(s))   |> Stateful
smc_smooth1  = SlidingModeController(σ, (s,t) -> -20*tanh(10s)) |> Stateful
smc_smooth2  = SlidingModeController(σ, (s,t) -> -20*sat(s))    |> Stateful

Wrap the controller in Stateful to make it stateful, i.e., remember its own state. This version of SMC controller does not have a state, and it's thus safe to wrap it in Stateful by default.

source
JuliaSimControl.StatefulMethod
Stateful(controller)
Stateful(controller, x0)

Wrap a discrete-time controller with signature x⁺, y = controller(x, u, p, t) to make it stateful, i.e., remember its own state. The wrapped controller has the signature y = controller(u, p, t), i.e., the state x is removed from both input arguments and return values.

source
JuliaSimControl.SuperTwistingSMCType
SuperTwistingSMC{K, S}
SuperTwistingSMC(k, σ, Ts, k2=1.1)

A "SuperTwisting" Sliding Mode Controller. Create a standard (SISO) sliding-mode controller with switching function s = σ(u), where s is the sliding variable computed by σ. The controller can be called using the syntax x⁺, y smc(x, u, p, t) where x is the controller state, u the state of the system (the input to the controller), p the parameters of σ (if any), and t the time. The new controller state x⁺ as well as the control signal y (the output of the controller) are returned.

Fields:

• k::K: Gain
• σ::S: Switching function $σ(u, p, t)$
• Ts: Sample time
• k2::K: Second tuning gain (defaults to 1.1 and does typically not need tuning).

Example

function σ(x, p, t)
q, qd = x
e = q - qr(t)
ė = qd - qdr(t)
ė + e
end
smc_supertwist = SuperTwistingSMC(50, σ, 0.01) |> Stateful 

Wrap the controller in a Stateful to make it stateful, i.e., remember its own state.

source
JuliaSimControl.SuperTwistingSMCMethod
(smc::SuperTwistingSMC)(x, u, p, t)

Evaluate the sliding mode controller smc at the controller state x, input u (plant state), parameters p, and time t.

Arguments:

• x: The state of the controller
• u: The input to the controller, this will be forwarded to the switching function $σ(u, p, t)$. The controller input will in typical applications be the (estimated) state of the system to be controlled.
• p: The parameters of the controller, this will be forwarded to the switching function $σ(u, p, t)$.
• t: Time.
source