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Contents
1 Introduction to Forced Vibration in Second Order Systems^{[1]}
Periodic excitation is a fact of life.
 No rotating system is perfectly balanced.
 Translational systems sit on vibratory platforms.
A free damped system under vibration cannot be controlled; it is a free system. Controlling a damped system under vibration requires the ability to inpart a force. This force is our control signal.
1.1 Harmonic Forcing in Second Order (Damped) Systems
Some periodic forces are harmonic. Some are not. Those forces which are not harmonic can be represented as a series of harmonic functions using Fourier analysis techniques. Since both harmonic and nonharmonic forcing functions can be represented with harmonic functions it makes sense to study the response of forced damped systems to a harmonic forcing function.
2 Forced Vibration is based on a Free Damped Vibration (Second Order Systems) Model
The equation of motion for a free damped vibration is (free second order system with viscous damping)
Free Second Order System 
where
 m is the mass,
 c is the damping coefficient,
 k is the spring stiffness, and
 x is the displacement
and equilibrium is defined as
 x=0 at t=0
For forced vibration the equation for viscous damping becomes
Forced Second Order System 
where
 F is the amplitude of the forcing function,
 ν is the frequency of the forcing function, and
 t is time in seconds.
Below is more information for viscous damping, coulomb damping, and hysteretic damping.
2.1 Viscous Damping^{[2]}
See Also 

For full derivation see Viscous Damping.
Figure 1 shows the model for a single degree of freedom system with forcing function and viscous damping. The equation of motion is
Second Order System with Viscous Damping 
The solution to is presented in Viscous Damping. It also represents the complementary function. It demonstrates that the initial vibration will quickly dissipate. For sustained motion simple harmoinc motion at the frequency of excitation is assumed. That solution is and leads to
and
Making the correct substitutions the equation of motion for a second order system with viscous damping becomes
The vector diagram in Figure 2 leads to
or
and
At steadystate the equation for a second order system with viscous damping is
where
The equation of transient motion for a second order system with viscous damping is
where
The following definitions will make the transient equation more convenient
rad/s and 
Then the dynamic magnification factor, , is
and 
where
 is the static deflection of the system under a steady force and
 is the dynamic amplitude
Mechanical vibration becomes more important as X/X_{s} gets larger and ν gets closer to ω. As ν gets closer to ω a small harmonic force can produce a large amplitude of vibration. This is known as resonance and occurs when the forcing frequency is equal to the natural frequency (i.e. ν/ω = 1). The maximum value of X/X_{s} can be determined by
for smal ζ 
and
From the other Second Order Systems article
Note the similarities to .
2.1.1 Q factor
For small values of ζ, . is a measure of the damping in a system; this is known as the Q factor.
2.1.2 Alternative solution for Second Order Systems with Viscous Damping
An alternative solution to the equation of motion for a second order system with Viscous Damping can be obtained by substituting . Then and an assumed solution of leads to
or 
where
Therefore
2.1.3 Rotating Second Order Systems with Viscous Damping
Rotating systems with an unbalance produce excitation force proportional to the square of the excitation frequency. The necessary variables are
 is the unbalanced mass,
 is the effective radious, and
 is the rotating angular speed.
Therefore the excitation force is
If applied to a system like Figure 1, the component of the force in the direction of motion is and the amplitude of vibration is
The value of ν/ω for maximum X can be found by differentiating the equation above to get
The peak response occurs when ν > ω. Also,
2.2 Coulomb Damping^{[3]}
For full derivation see Coulomb Damping.
Figure 3 shows the model for a single degree of freedom system with forcing function and coulomb damping. The equation of motion is
Second Order System with Coulomb Damping 
Note that this equation is nonlinear. This is due to the friction force always opposing the direction of motion.
The motion can be discontinuous if F_{d} is large compared to F. However, in most systems F_{d} is small. This fact allows us to create a linear approximation.
Express F_{d} in terms of c_{d}  the viscous damping coefficient.
where
 or
At resonance the amplitude is not limited by Coulomb friction.
2.3 Hysteretic Damping^{[4]}
For full derivation see Hysteretic Damping.
Figure 4 shows the model for a single degree of freedom system with forcing function and hysteretic damping. The equation of motion is
Second Order System with Hysteretic Damping 
where
and
Note this result requires the substitution.
2.3.1 Q factor
If , at resonance , then .
3 References
Leigh, J. R. 2004 Control Theory. ISBN 0863413390