Duffing equation

A Poincaré section of the forced Duffing equation suggesting chaotic behaviour

The Duffing equation is a non-linear second-order differential equation used to model certain damped and driven oscillators. The equation is given by

$\ddot{x} + \delta \dot{x} + \alpha x + \beta x^3 = \gamma \cos (\omega t)\,$

where the (unknown) function x=x(t) is the displacement at time t, $\dot{x}$ is the first derivative of x with respect to time, i.e. velocity, and $\ddot{x}$ is the second time-derivative of x, i.e. acceleration. The numbers δ, α, β, γ and ω are given constants.

The equation describes the motion of a damped oscillator with a more complicated potential than in simple harmonic motion (which corresponds to the case β=δ=0); in physical terms, it models, for example, a spring pendulum whose spring's stiffness does not exactly obey Hooke's law.

The Duffing equation is an example of a dynamical system that exhibits chaotic behavior. Moreover the Duffing system presents in the frequency response the jump resonance phenomenon that is a sort of frequency hysteresis behaviour.

Parameters

• δ controls the size of the damping (friction).
• α controls the size of the restoring force.
• β controls the amount of non-linearity in the restoring force. If β = 0, the Duffing equation describes a damped and driven simple harmonic oscillator.
• γ controls the amplitude of the periodic driving force. If γ = 0 we have a system without driving force.
• ω controls the frequency of the periodic driving force.

Methods of solution

In general, the Duffing equation does not admit an exact symbolic solution. However, many approximate methods work well:

• Expansion in a Fourier series will provide an equation of motion to arbitrary precision.
• The x³ term, also called the Duffing term, can be approximated as small and the system treated as a perturbed simple harmonic oscillator.
• The Frobenius method yields a complicated but workable solution.
• Any of the various numeric methods such as Euler's method and Runge-Kutta can be used.

In the special case of the undamped (δ = 0) and undrived (γ = 0) Duffing equation, an exact solution can be obtained using Jacobi's elliptic functions.

External links

• Duffing oscillator on Scholarpedia
• MathWorld page

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