why in Free vibration 2DOF system with damping , the displacement is increasing with time?

% This is the code I have written


function solve_open_circuit_free_vibration
% System parameters
params.m = 0.91;               % Inner mass (kg)
params.M = 1.2;                % Outer mass (kg)
params.K = 114.259;            % Spring stiffness (N/m)
params.c_m = 0.4;             % Mechanical damping coefficient (N·s/m)
params.mu = 3.55e-7;           % Magnetic dipole moment ( in T·m^3)
params.a = 0.0171;             % Solenoid Coil radius (m)
params.b0 = 0.001;             % Initial distance b/w mass and coil (m)
% Initial conditions [x, dx, y, dy]
Z0 = [0, 0, 0.01, 0];          % Initial displacements and velocities, initial displacement is provided on outer mass
% Time span for simulation
tspan = [0, 5];                % Simulation time range (5 seconds)
% Solver options
options = odeset('RelTol', 1e-8, 'AbsTol', 1e-10);
% Solve using ode15s
[T, Z] = ode15s(@(t, Z) governing_equations(t, Z, params), tspan, Z0, options);
% Extract results
x = Z(:, 1);                   % Inner mass displacement
dx = Z(:, 2);                  % Inner mass velocity
y = Z(:, 3);                   % Outer mass displacement
dy = Z(:, 4);                  % Outer mass velocity
% Compute voltage
z = params.b0 + y - x;         % Magnet-to-coil relative displacement
dz = dy - dx;                  % Relative velocity
V = compute_voltage(z, dz, params);  % Voltage
% Plot results
plot_results(T, x, dx, y, dy, z, V);
end
function dZdt = governing_equations(t, Z, params)
% Extract state variables
x = Z(1); dx = Z(2);
y = Z(3); dy = Z(4);
% Relative displacement and velocity
z = params.b0 + y - x;
dz = dy - dx;
% Equations of motion
ddx = (params.K * (y - x) - params.c_m * dz) / params.m;
ddy = (-params.K * (y - x) + params.c_m * dz) / params.M;
% Return derivatives
dZdt = [dx; ddx; dy; ddy];
end
function V = compute_voltage(z, dz, params)
% Compute voltage
V = (6 * pi * params.mu * params.a^2 * z .* dz) ./ (params.a^2 + z.^2).^(5/2);
end
function plot_results(T, x, dx, y, dy, z, V)
% Create a single figure with subplots
figure;
% Subplot 1: Displacement
subplot(1, 2, 1);
plot(T, x, 'r', 'LineWidth', 1.5); hold on;
plot(T, y, 'b--', 'LineWidth', 1.5);
xlabel('Time (s)');
ylabel('Displacement (m)');
legend('Inner Mass (x)', 'Outer Mass (y)');
title('Displacement vs Time');
grid on;
% Subplot 2: Velocity
subplot(1, 2, 2);
plot(T, dx, 'r', 'LineWidth', 1.5); hold on;
plot(T, dy, 'b--', 'LineWidth', 1.5);
xlabel('Time (s)');
ylabel('Velocity (m/s)');
legend('Inner Mass (dx)', 'Outer Mass (dy)');
title('Velocity vs Time');
grid on;
end