Control design for a pendulum on a cart

In this example we will consider control design for the basic inverted pendulum on a cart. This system has two equilibria, one where the pendulum is hanging straight down, and one where it's balancing straight up. The upper one is unstable, making it slightly more interesting to design a controller for (even if the lower equilibrium is highly relevant, it's a good model for an overhead crane moving goods).

System model

In this tutorial, we assume that we have the nonlinear dynamics of the system encodeed as a julia function ẋ = cartpole(x, u), and linearize this to get a statespace system

\[\begin{aligned} ẋ &= Ax + Bu\\ y &= Cx \end{aligned}\]

We make use of ForwardDiff.jl for the linearization. We start by defining the dynamics function

using ControlSystems, RobustAndOptimalControl, ForwardDiff, LinearAlgebra, Plots

function cartpole(x, u)
    mc, mp, l, g = 1.0, 0.2, 0.5, 9.81

    q  = x[1:2]
    qd = x[3:4]

    s = sin(q[2])
    c = cos(q[2])

    H = [mc+mp mp*l*c; mp*l*c mp*l^2]
    C = [0.1 -mp*qd[2]*l*s; 0 0]
    G = [0, mp * g * l * s]
    B = [1, 0]

    qdd = -H \ (C * qd + G - B * u[1])
    return [qd; qdd]
end

nu = 1    # number of control inputs
nx = 4    # number of states
ny = 2    # number of outputs (here we assume that the cart position and the pendulum angle are measurable)

Linearization

The next step is to choose an operating point around which to linearize and to calculate the Jacobians $A$ and $B$:

x0 = [0, π, 0, 0]
u0 = [0]

Ac = ForwardDiff.jacobian(x->cartpole(x, u0), x0)
Bc = ForwardDiff.jacobian(u->cartpole(x0, u), u0)
Cc = [1 0 0 0; 0 1 0 0]
Λ = Diagonal([0.4, deg2rad(25)]) # Maximum output ranges
Cc = Λ\Cc # This normalizes expected outputs to be ∈ [-1, 1], a good practice for MIMO systems

we package everything into a StateSpace object and visualize its poles and zeros:

sys = ss(Ac, Bc, Cc, 0)
ControlSystems.StateSpace{ControlSystems.Continuous, Float64}
A = 
 0.0   0.0                  1.0  0.0
 0.0   0.0                  0.0  1.0
 0.0   1.9620000000000002  -0.1  0.0
 0.0  23.544               -0.2  0.0
B = 
 0.0
 0.0
 1.0
 2.0
C = 
 2.5  0.0                0.0  0.0
 0.0  2.291831180523293  0.0  0.0
D = 
 0.0
 0.0

Continuous-time state-space model
pzmap(sys)

Control design

We will design a number of different controllers. We will start with a basic PID controller. Since the PID controller in its standard form really only handles SISO systems, we will also design a state-feedback controller with an observer to estimate the full state vector $x$ based on the two measurements $y$. Lastly, we will attempt to "robustify" the state-feedback controller using the glover_mcfarlane procedure.

Since the system has an unstable pole $p \approx 4.85$rad/s, there wil be fundamental limitations on the performance of the closed loop system. A common rule-of-thumb (see, e.g., Åström and Murray) is that a single RHP pole $p$ puts a lower limit on the gain crossover frequency $\omega_{gc} > 2p$, something to take into consideration when tuning our controllers.

PID controller

Since the PID controller only accepts a single measurement, we choose the measurement of the pendulum angle for feedback. While doing so, we notice that the number of states in the model can be reduced by the function sminreal

P = sminreal(sys[2,1]) # Position state goes away, not observable
ControlSystems.StateSpace{ControlSystems.Continuous, Float64}
A = 
  0.0                  0.0  1.0
  1.9620000000000002  -0.1  0.0
 23.544               -0.2  0.0
B = 
 0.0
 1.0
 2.0
C = 
 2.291831180523293  0.0  0.0
D = 
 0.0

Continuous-time state-space model

this indicates that the state corresponding to the position of the cart is not observable from the measurement of the pendulum angle. This is slightly worrisome, but we nevertheless proceed to design a controller. By using a single measurement only, we have also introduced a zero in the system

pzmap(P)

A PID controller can be constructed using the function pid. We start our tuning by a simple P controller

C = pid(kp=1, ki=0, kd=0, series=true)
ControlSystems.TransferFunction{ControlSystems.Continuous, ControlSystems.SisoRational{Float64}}
NaNs + NaN
----------
   1.0

Continuous-time transfer function model

We will attempt to perform loop shaping using the PID controller, and plot the stability margins in a Bode plot using the function marginplot

w = exp10.(LinRange(-2.5, 3, 500))
function pid_marginplot(C)
    f1 = marginplot(P*C, w)
    vline!([2*4.85], sp=1, lab="Fundamental limitation", l=(:dash, :black))
    ylims!((1e-3, 1e2), sp=1)
    f2 = nyquistplot(P*C)
    plot(f1, f2)
end
pid_marginplot(C)

We notice that the gain of the loop-transfer function $L = PC$ is much too low, and increase it, we also notice that the Nyquist plot fails to encircle to critical point, which it has to do once since we have one unstable pole. We will solve this in the end by adding integral action, but proceed for now to shape other parts of the loop. We start by lifting the Bode curve by increasing the gain:

C = pid(kp=20, ki=0, kd=0, series=true)
pid_marginplot(C)

we are now getting close to the rule-of-thumb for $\omega_{gc}$, but have a low loop gain at low frequencies. Remember, to get good disturbance rejection, we typically want a high loop gain at low frequencies. We also have an extremely small phase margin at 0.66 degrees. To fix the phase margin, we add some derivative gain. While adding derivative gain, it's also a good idea to add noise filtering (with a pure derivative term, the PID controller is not proper and can not be realized as a statespace system)

C = pid(kp=20, ki=0, kd=0.2, series=true) * tf(1, [0.01, 1])
pid_marginplot(C)

The derivative term lifted the phase at $\omega_{gc}$ and we now have very nice phase margins. We also got a slight increase in $\omega_{gc}$ while at it.

The closed-loop system will still be unstable since the Nyquist curve fails to encircle the point -1, something we can check by calling

isstable(feedback(P*C))

We make the Nyquist curve wrap around the -1 point by adding integral gain:

C = pid(kp=20, ki=0.8, kd=0.2, series=true) * tf(1, [0.01, 1])
pid_marginplot(C)

Now, the Nyquist curve looks fine and the system is stable

isstable(minreal(feedback(P*C)))
true

If we simulate a disturbance acting on this system (feedback(P, C) is the transfer function from load disturbance to output)

plot(step(feedback(P,C), 8), ylab="ϕ")

we see that we have a reasonable disturbance response.

To verify robustness properties, we plot the gang-of-four sensitivity functions:

f1 = gangoffourplot(P,C,w)
f2 = nyquistplot(P*C, Ms_circles=[1.4], Mt_circles=[1.5], ylims=(-2, 2), xlims=(-4,1))
plot(f1, f2, size=(1000,800))

This all looks nice and we appear to have reasonable robustness margins, the Nyquist curve stays outside the $M_S = 1.4$ circle and the $M_T = 1.5$ circle.

However, there is a dragon lurking behind these plots. Remember the state corresponding the the cart position that was removed above? What has happened to this state? To investigate this, we form an ExtendedStateSpace model where we have both cart position and pendulum angle as controlled outputs, while keeping only the pendulum angle as measured output:

Pe = ExtendedStateSpace(sys, C2 = sys.C[2:2, :]) # Indicate that we can only measure the pendulum angle
Gecl = feedback(Pe, ss(C)) |> minreal
plot(step(Gecl, 8), ylab=["Cart pos" "ϕ"])

We see that the cart position drifts away without ever thinking about stopping. Indeed, the PID controller is unaware of this and can not really do anything about it. We could attempt to design a second control loop that would close the loop around the cart position, but we would have to carefully manage the interactions between the two loops. Instead, we move on to a state-feedback design, a methodology that makes handling multiple outputs much more straightforward.

Pole placement and observer design

The design of a state-feedback controller typically involves two steps, designing the feedback gain and designing an observer. We will arrive at the feedback gain through pole placement, but will design the observer as a Kalman filter, i.e., by solving a Riccati equation rather than using Ackermann's formula.

When performing pole placement, there are a number of design guidlines that help you arrive at a robust design. One of these are that past process poles should be matched with an equally fast closed-loop pole. We can get an overview of the open-loop poles with dampreport

dampreport(sys)
|        Pole        |   Damping     |   Frequency   |   Frequency   | Time Constant |
|                    |    Ratio      |   (rad/sec)   |     (Hz)      |     (sec)     |
+--------------------+---------------+---------------+---------------+---------------+
| +0                 |  -1           |  0            |  0            |  -Inf         |
| -0.0833            |  1            |  0.0833       |  0.0133       |  12           |
| +4.84              |  -1           |  4.84         |  0.771        |  -0.206       |
| -4.86              |  1            |  4.86         |  0.774        |  0.206        |

we see that we have two poles at roughly $\pm 4.85$rad/s, and almost two integrators. We thus keep the fast pole, and place the unstable pole at the same location (same bandwidth but stable instead of unstable). We also try to move the integrator poles to -5 to make the system nice and fast.

desired_poles = [-4.85, -4.85, -5, -5]
L = place(sys, desired_poles, :c)
1×4 Matrix{Float64}:
 -29.9726  99.5196  -24.4489  22.0244

For the observer, we make use of the function kalman. We choose the covariance matrices R1, R2 that determine the amount of noise acting on the system and on the measurements respectively. We assume that there are two noise components, both entering as forces. One disturbance force acts on the cart and the other on the pendulum. We indicate this using the matrix $B_w$.

Bw = [0 0; 0 0; 1 0; 0 1]
R1 = Bw*I(2)*Bw'
R2 = 0.0001I(ny)
K = kalman(sys, R1, R2)
4×2 Matrix{Float64}:
  8.90448     -0.0161568
 -0.0176244    9.83322
 99.1125       0.481841
 -1.31513    110.801

With our feedback gain L and the Kalman gain K, we form the controller using observer_controller

controller = observer_controller(sys, L, K)
@assert isstable(controller)
@assert isstable(feedback(sys * controller))

We may have a look at the Nyquist plot and the gang-of-four to assess robustness margins. In this case we look at the loop transfer function at the input simply because this function is SISO while the standard output-loop transfer is MIMO. This will allow us to asses robustness w.r.t. input perturbations only

nyquistplot(controller*sys, w, Ms_circles=[2.7], Mt_circles=[3], xlims=(-2, 2), ylims=(-1, 3))

The Nyquist plot shows a rather weak robustness margin, with a peak in the input sensitivity of about

2.67

and a peak in the complementary sensitivity function of around

3.02

These can be verified by calling hinfnorm2

hinfnorm2(input_comp_sensitivity(sys, controller))
(3.021191116182818, 6.672744975862733)
Hover information

If you plot with the Plotly backend, activated by calling plotly() if you have Plotly.jl installed, you can hover the mouse over the Nyquist curve and the gain circles to see frequency information etc. This is not possible when using the default GR backend, used in this documentation.

Also the gang-of-four indicate rather poor margins:

gangoffourplot(sys, controller, w, xlabel="", sigma=false, titlefont=8)