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Phoniex University
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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.
Movable plate
mass = M area = A
+ V
le X

R
friction coeff . b Vh'
Diaphrap Slidina contact, --..._ +
V
|
-
![]()
(a) (b)
Fig. 3-10
For a charge q, the electrical energy stored in a capacitor can be expressed as
where C is the capacitance. The electrical force between the plates is then given by
|
- - awe - - q2 a ( 1)
Fe - 8x - 2 C
where the minus sign indicates that the force tends to decrease the separation x of the plates. For the present case,
so that
The equations of motion are then:
C = BA
X
mechanical: Mi + bi + k(x - 10) = (])
electrical:
Rq + !!:.. = V
eA
(2)
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
![]()
Substitution into (J) and (2), with neglect of higher-order terms, yields
Qo2 Q
|
Mx 1
+ bx1
+ k(X0
- l0) + kx1
= ---
oq,
;A;--
2 BA "'
Rq + Qo-¥0 + XOql + QoX1 = V + V
1 BA BA BA O 1
From these, it is seen that the steady-state operating point is given by
gg = k(X - 1 )
2BA 0 0
The linearized dynamical equations are thus
mechanical:
electrical:
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