Hanging Chain

In this example, a mode predictive controller (MPC) is used to stabilize a system of weights connected by springs. The rightmost weight is fixed in place at the origin, whereas the velocity of the leftmost weight can be controlled by an actuator. The six weights in the middle move under the influence of gravity and the forces of the springs between them.

The goal of the controller is to stabilize the system (i.e. drive the velocity of all weights to zero) with the rightmost weight at position \((1, 0)\). Additionally, a non-convex cubic constraint on the weights’ position is imposed, shown in green on the figure below.

Once Loop Reflect

# %% Hanging chain MPC example

import casadi as cs
import numpy as np
from os.path import dirname
import sys

sys.path.append(dirname(__file__))
from hanging_chain_dynamics import HangingChain

# %% Build the model

Ts = 0.05  # Time step [s]
N = 6  # Number of balls
dim = 2  # Dimension (2D or 3D)

model = HangingChain(N, dim, Ts)
y_null, u_null = model.initial_state()  # Initial states and control inputs

param = [0.03, 1.6, 0.033 / N]  # Concrete parameters m, D, L

# %% Apply an initial control input to disturb the system

N_dist = 3  # Number of time steps to apply the disturbance for
u_dist = [-0.5, 0.5, 0.5] if dim == 3 else [-0.5, 0.5]  # Disturbance input
y_dist = model.simulate(N_dist, y_null, u_dist, param)  # Model states
y_dist = np.hstack((np.array([y_null]).T, y_dist))  # (including initial state)

# %% Simulate the system without a controller

N_sim = 180  # Number of time steps to simulate for
y_sim = model.simulate(N_sim, y_dist[:, -1], u_null, param)  # Model states
y_sim = np.hstack((y_dist, y_sim))  # (including disturbed and initial states)

# %% Define MPC cost and constraints

N_horiz = 12  # MPC horizon length (number of time steps)

y_init = cs.MX.sym("y_init", *y_null.shape)  # Initial state
model_params = cs.MX.sym("params", *model.params.shape)  # Parameters
num_var = dim * N_horiz
U = cs.MX.sym("U", num_var)  # Control signals over horizon
U_mat = model.input_to_matrix(U)  # Input as dim by N_horiz matrix
constr_param = cs.MX.sym("c", 3)  # Coefficients of cubic constraint function
mpc_param = cs.vertcat(y_init, model_params, constr_param)  # All parameters

# Cost

# Stage costs for states and input
stage_y_cost, stage_u_cost = model.generate_cost_funcs()
# Simulate the model with the input over the horizon
mpc_sim = model.simulate(N_horiz, y_init, U_mat, model_params)
# Accumulate the cost of the outputs and inputs
mpc_y_cost = cs.sum2(stage_y_cost.map(N_horiz)(mpc_sim))
mpc_u_cost = cs.sum2(stage_u_cost.map(N_horiz)(U_mat))
mpc_cost = mpc_y_cost + mpc_u_cost

# Constraints

# Cubic constraint function for a single ball in one dimension
g_constr = lambda c, x: c[0] * x**3 + c[1] * x**2 + c[2] * x
# Constraint function for one stage (N balls)
y_c = cs.MX.sym("y_c", y_dist.shape[0])
constr = []
for i in range(N):  # for each ball in the stage except the last,
    yx_n = y_c[dim * i]  # constrain the x, y position of the ball
    yy_n = y_c[dim * i + dim - 1]
    constr += [yy_n - g_constr(constr_param, yx_n)]
constr += [y_c[-1] - g_constr(constr_param, y_c[-dim])]  # Ball N+1
constr_fun = cs.Function("c", [y_c, constr_param], [cs.vertcat(*constr)])
# Constraint function for all stages in the horizon
mpc_constr = cs.vec(constr_fun.map(N_horiz)(mpc_sim, constr_param))
num_constr = (N + 1) * N_horiz
# Fill in the constraint coefficients c(x-a)³ + d(x-a) + b
a, b, c, d = 0.6, -1.4, 5, 2.2
constr_coeff = [c, -3 * a * c, 3 * a * a * c + d]
constr_lb = b - c * a**3 - d * a
# Box constraints on actuator:
C = -1 * np.ones(num_var), +1 * np.ones(num_var)  # lower bound, upper bound
# Constant term of the cubic state constraints as a one-sided box:
D = constr_lb * np.ones(num_constr), +np.inf * np.ones(num_constr)

# %% NLP formulation

import alpaqa

# Generate C code for the cost and constraint functions, compile them, and load
# them as an alpaqa problem description:
problem = (
    alpaqa.minimize(mpc_cost, U)  # objective and variables         f(x; p)
    .subject_to_box(C)  #           box constraints on variables    x ∊ C
    .subject_to(mpc_constr, D)  #   general constraints             g(x; p) ∊ D
    .with_param(mpc_param)  #       parameter to be changed later   p
).compile(sym=cs.MX.sym)

# %% NLP solver

from datetime import timedelta

# Configure an alpaqa solver:
solver = alpaqa.ALMSolver(
    alm_params={
        "tolerance": 1e-3,
        "dual_tolerance": 1e-3,
        "initial_penalty": 1e4,
        "max_iter": 100,
        "max_time": timedelta(seconds=0.2),
    },
    inner_solver=alpaqa.PANOCSolver(
        panoc_params={
            "stop_crit": alpaqa.FPRNorm,
            "max_time": timedelta(seconds=0.02),
        },
        lbfgs_params={"memory": N_horiz},
    ),
)

# %% MPC controller


# Wrap the solver in a class that solves the optimal control problem at each
# time step, implementing warm starting:
class MPCController:
    def __init__(self, model: HangingChain, problem: alpaqa.CasADiProblem):
        self.model = model
        self.problem = problem
        self.tot_it = 0
        self.tot_time = timedelta()
        self.max_time = timedelta()
        self.failures = 0
        self.U = np.zeros(problem.n)
        self.λ = np.zeros(problem.m)

    def __call__(self, y_n):
        y_n = np.array(y_n).ravel()
        # Set the current state as the initial state
        self.problem.param[: y_n.shape[0]] = y_n
        # Shift over the previous solution and Lagrange multipliers
        self.U = np.concatenate((self.U[dim:], self.U[-dim:]))
        self.λ = np.concatenate((self.λ[N + 1 :], self.λ[-N - 1 :]))
        # Solve the optimal control problem
        # (warm start using the shifted previous solution and multipliers)
        self.U, self.λ, stats = solver(self.problem, self.U, self.λ)
        # Print some solver statistics
        print(
            f'{stats["status"]} outer={stats["outer_iterations"]} '
            f'inner={stats["inner"]["iterations"]} time={stats["elapsed_time"]} '
            f'failures={stats["inner_convergence_failures"]}'
        )
        self.tot_it += stats["inner"]["iterations"]
        self.failures += stats["status"] != alpaqa.SolverStatus.Converged
        self.tot_time += stats["elapsed_time"]
        self.max_time = max(self.max_time, stats["elapsed_time"])
        # Print the Lagrange multipliers, shows that constraints are active
        print(np.linalg.norm(self.λ))
        # Return the optimal control signal for the first time step
        return self.model.input_to_matrix(self.U)[:, 0]


# %% Simulate the system using the MPC controller

y_n = np.array(y_dist[:, -1]).ravel()  # Initial state for controller
n_state = y_n.shape[0]
problem.param = np.concatenate((y_n, param, constr_coeff))

y_mpc = np.empty((n_state, N_sim))
controller = MPCController(model, problem)
for n in range(N_sim):
    # Solve the optimal control problem:
    u_n = controller(y_n)
    # Apply the first optimal control input to the system and simulate for
    # one time step, then update the state:
    y_n = model.simulate(1, y_n, u_n, param).T
    y_mpc[:, n] = y_n
y_mpc = np.hstack((y_dist, y_mpc))

print(f"{controller.tot_it} inner iterations, {controller.failures} failures")
print(
    f"time: {controller.tot_time} (total), {controller.max_time} (max), "
    f"{controller.tot_time / N_sim} (avg)"
)

# %% Visualize the results

import matplotlib.pyplot as plt
import matplotlib as mpl
from matplotlib import animation, patheffects

mpl.rcParams["animation.frame_format"] = "svg"

# Plot the chains
fig, ax = plt.subplots()
x, y, z = model.state_to_pos(y_null)
(line,) = ax.plot(x, y, "-o", label="Without MPC")
(line_ctrl,) = ax.plot(x, y, "-o", label="With MPC")
plt.legend()
plt.ylim([-2.5, 1])
plt.xlim([-0.25, 1.25])

# Plot the state constraints
x = np.linspace(-0.25, 1.25, 256)
y = np.linspace(-2.5, 1, 256)
X, Y = np.meshgrid(x, y)
Z = g_constr(constr_coeff, X) + constr_lb - Y
fx = [patheffects.withTickedStroke(spacing=7, linewidth=0.8)]
cgc = plt.contour(X, Y, Z, [0], colors="tab:green", linewidths=0.8)
plt.setp(cgc.collections, path_effects=fx)


class Animation:
    points = []

    def __call__(self, i):
        x, y, z = model.state_to_pos(y_sim[:, i])
        y = z if dim == 3 else y
        for p in self.points:
            p.remove()
        self.points = []
        line.set_xdata(x)
        line.set_ydata(y)
        viol = y - g_constr(constr_coeff, x) + 1e-5 < constr_lb
        if np.sum(viol):
            self.points += ax.plot(x[viol], y[viol], "rx", markersize=12)
        x, y, z = model.state_to_pos(y_mpc[:, i])
        y = z if dim == 3 else y
        line_ctrl.set_xdata(x)
        line_ctrl.set_ydata(y)
        viol = y - g_constr(constr_coeff, x) + 1e-5 < constr_lb
        if np.sum(viol):
            self.points += ax.plot(x[viol], y[viol], "rx", markersize=12)
        return [line, line_ctrl] + self.points


ani = animation.FuncAnimation(
    fig,
    Animation(),
    interval=1000 * Ts,
    blit=True,
    repeat=True,
    frames=1 + N_dist + N_sim,
)

# Show the animation
plt.show()

hanging_chain_dynamics.py

from typing import Union
import casadi as cs
import numpy as np
from casadi import vertcat as vc


class HangingChain:

    def __init__(self, N: int, dim: int, Ts: float = 0.05):
        self.N = N
        self.dim = dim

        self.y1 = cs.SX.sym("y1", dim * N, 1)  # state: balls 1→N positions
        self.y2 = cs.SX.sym("y2", dim * N, 1)  # state: balls 1→N velocities
        self.y3 = cs.SX.sym("y3", dim, 1)  # state: ball  1+N position
        self.u = cs.SX.sym("u", dim, 1)  # input: ball  1+N velocity
        self.y = vc(self.y1, self.y2, self.y3)  # full state vector

        self.m = cs.SX.sym("m")  # mass
        self.D = cs.SX.sym("D")  # spring constant
        self.L = cs.SX.sym("L")  # spring length
        self.params = vc(self.m, self.D, self.L)

        self.g = np.array([0, 0, -9.81] if dim == 3 else [0, -9.81])  # gravity
        self.x0 = np.zeros((dim, ))  # ball 0 position
        self.x_end = np.eye(1, dim, 0).ravel()  # ball N+1 reference position

        self._build_dynamics(Ts)

    def _build_dynamics(self, Ts):
        y, y1, y2, y3, u = self.y, self.y1, self.y2, self.y3, self.u
        dist = lambda xa, xb: cs.norm_2(xa - xb)
        N, d = self.N, self.dim
        p = self.params

        # Continuous-time dynamics y' = f(y, u; p)

        f1 = [y2]
        f2 = []
        for i in range(N):
            xi = y1[d * i:d * i + d]
            xip1 = y1[d * i + d:d * i + d * 2] if i < N - 1 else y3
            Fiip1 = self.D * (1 - self.L / dist(xip1, xi)) * (xip1 - xi)
            xim1 = y1[d * i - d:d * i] if i > 0 else self.x0
            Fim1i = self.D * (1 - self.L / dist(xi, xim1)) * (xi - xim1)
            fi = (Fiip1 - Fim1i) / self.m + self.g
            f2 += [fi]
        f3 = [u]

        f_expr = vc(*f1, *f2, *f3)
        self.f = cs.Function("f", [y, u, p], [f_expr], ["y", "u", "p"], ["y'"])

        # Discretize dynamics y[k+1] = f_d(y[k], u[k]; p)

        # 4th order Runge-Kutta integrator
        k1 = self.f(y, u, p)
        k2 = self.f(y + Ts * k1 / 2, u, p)
        k3 = self.f(y + Ts * k2 / 2, u, p)
        k4 = self.f(y + Ts * k3, u, p)

        # Discrete-time dynamics
        f_d_expr = y + (Ts / 6) * (k1 + 2 * k2 + 2 * k3 + k4)
        self.f_d = cs.Function("f", [y, u, p], [f_d_expr])

        return self.f_d

    def state_to_pos(self, y):
        N, d = self.N, self.dim
        rav = lambda x: np.array(x).ravel()
        xdim = lambda y, i: np.concatenate(
            ([0], rav(y[i:d * N:d]), rav(y[-d + i])))
        if d == 3:
            return (xdim(y, 0), xdim(y, 1), xdim(y, 2))
        else:
            return (xdim(y, 0), xdim(y, 1), np.zeros((N + 1, )))

    def input_to_matrix(self, u):
        """
        Reshape the input signal from a vector into a dim × N_horiz matrix (note
        that CasADi matrices are stored column-wise and NumPy arrays row-wise)
        """
        if isinstance(u, np.ndarray):
            return u.reshape((self.dim, u.shape[0] // self.dim), order='F')
        else:
            return u.reshape((self.dim, u.shape[0] // self.dim))

    def simulate(self, N_sim: int, y_0: np.ndarray,
                 u: Union[np.ndarray, list, cs.SX.sym, cs.MX.sym],
                 p: Union[np.ndarray, list, cs.SX.sym, cs.MX.sym]):
        if isinstance(u, list):
            u = np.array(u)
        if isinstance(u, np.ndarray):
            if u.ndim == 1 or (u.ndim == 2 and u.shape[1] == 1):
                if u.shape[0] == self.dim:
                    u = np.tile(u, (N_sim, 1)).T
        return self.f_d.mapaccum(N_sim)(y_0, u, p)

    def initial_state(self):
        N, d = self.N, self.dim
        y1_0 = np.zeros((d * N))
        y1_0[0::d] = np.arange(1, N + 1) / (N + 1)
        y2_0 = np.zeros((d * N))
        y3_0 = np.zeros((d, ))
        y3_0[0] = 1

        y_null = np.concatenate((y1_0, y2_0, y3_0))
        u_null = np.zeros((d, ))

        return y_null, u_null

    def generate_cost_funcs(self, α=25, β=1, γ=0.01):
        N, d = self.N, self.dim
        y1t = cs.SX.sym("y1t", d * N, 1)
        y2t = cs.SX.sym("y2t", d * N, 1)
        y3t = cs.SX.sym("y3t", d, 1)
        ut = cs.SX.sym("ut", d, 1)
        yt = cs.vertcat(y1t, y2t, y3t)

        L_cost_x = α * cs.sumsqr(y3t - self.x_end)
        for i in range(N):
            xdi = y2t[d * i:d * i + d]
            L_cost_x += β * cs.sumsqr(xdi)
        L_cost_u = γ * cs.sumsqr(ut)
        return cs.Function("L_cost_x", [yt], [L_cost_x]), \
            cs.Function("L_cost_u", [ut], [L_cost_u])