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MCS,PHD
Argosy University/ Phoniex University/
Nov-2005 - Oct-2011
Professor
Phoniex University
Oct-2001 - Nov-2016
3.10.                          A capacitor microphone, shown in Fig. 3-lO(a), may be modeled by the electromechanical system of Fig. 3-1O(b). With v as the input, obtain the linearized dynamical equations of the system. Give the steady-state operating point.
Â
Â
Â
Movable plate
mass = M area = A
+Â Â Â Â Â V
Â
le X
Â
Â
Â
Â

R
friction coeff . b    Vh'
Diaphrap                                                  Slidina contact, --..._                                                   +
Â
V
Â
|
                                  -
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Â
(a)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (b)
Fig. 3-10
Â
For a charge q, the electrical energy stored in a capacitor can be expressed  as
Â
Â
where C is the capacitance.  The electrical force between the plates is then given  by
Â
Â
|
- - awe - - q2           a ( 1)
Fe -Â Â Â Â 8x -Â Â Â Â 2Â Â Â Â Â Â C
where the minus sign indicates that the force tends to decrease the separation x of the plates. For the present case,
Â
Â
Â
Â
Â
so that
Â
Â
Â
Â
The equations of motion are then:
Â
CÂ = Â BA
X
Â
Â
mechanical:        Mi  + bi  +  k(x  - 10)   =                                                                 (])
Â
Â
Â
electrical:
Â
Rq + !!:.. = V
eA
Â
Â
(2)
Â
These equations are nonlinear due to the presence of terms involving q2 and  qx.
Linearization is accomplished by assuming that a steady-state operating point (X0, Q0, V0) exists
such that
Â
       Â
         ![]()
Substitution into (J) and (2), with neglect of higher-order terms,  yields
Â
Qo2Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Q
Â
Â
|
Mx 1
+ bx1
Â
+ k(X0
Â
- l0)Â + kx1
Â
= Â ---
Â
oq,
;A;--
Â
2 BAÂ Â Â Â Â "'
Â
Rq + Qo-¥0 + XOql  + QoX1 = V + V
1Â Â Â Â Â Â Â Â Â Â Â Â Â BAÂ Â Â Â Â Â Â Â BAÂ Â Â Â Â Â Â Â BAÂ Â Â Â Â Â Â Â OÂ Â Â Â Â Â Â Â Â Â Â Â 1
From these, it is seen that the steady-state operating point is given  by
gg  = k(X  - 1 )
2BAÂ Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â 0
The linearized dynamical  equations are thus
Â
mechanical:Â Â Â Â
 electrical:
Â
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