Electricity: measuring and testing – Electrostatic field – Using modulation-type electrometer
Reexamination Certificate
1998-11-10
2001-01-23
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Electrostatic field
Using modulation-type electrometer
C324S660000, C361S296000
Reexamination Certificate
active
06177800
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to the art of micro-electro-mechanical modulation systems and, more articularly to electrostatic voltmeters.
2. Description of Related Art
In an effort to achieve reliable, low cost, and potentially high precision micro sensors, development continues in the technology of integrating small mechanical elements onto silicon substrates. Polysilicon microbridges have been driven vertically and laterally as resonant microsensors. With respect to laterally driven microbridges, short displacements of a comb type drive of the type shown and described in U.S. Pat. No. 5,025,346, typically on the order of one to ten micrometers, lead to very weak sensed signals. The disclosure of the U.S. Pat. No. 5,025,346 patent is incorporated herein by reference.
One such class of devices hampered by the weak output signals is electrostatic voltmeters (ESV's). Non-contacting ESV's are utilized, for example, within xerographic printers to measure the surface voltage of the photoreceptor; however, additional applications are known in the art. In almost all non-contacting voltmeters the measurement of the surface voltage is made using capacitive coupling of a sense probe to the measured surface. This capacitive coupling is typically modulated by a mechanical shutter between the sense probe and the measured surface. The output current of a capacitively coupled sense probe is given by:
i
s
(
t
)=
d
Q(
t
)
dt=d/dt
[C(
t
)]V
s
(
t
)]=C(
t
)
d
V
s
(
t
)/
dt
+V
s
d
C(
t
)/
dt,
EQ.1
where Q(t) is the time-varying charge induced on the sense probe, C(t) is the time-varying capacitance and V
s
(t) is the time-varying measured surface voltage. For the measurement of the d.c. component of the photoreceptor voltage, V
dc
, the equation can be simplified to:
i
s
(
t
)=V
dc
d
C(
t
)/
dt
=V
dc
[∂C(
x,t
)/∂
x ][∂x/∂t],
EQ.2
where the shutter position determines C(t) at any time. For a sinusoidal drive signal the capacitance is given by:
C(
t
)=C
o
+C
m
sin(&ohgr;
t
). EQ.3
Here C
O
, is the d.c. component of the capacitance that does not change with time and C
m
is the a.c. component of the capacitance that changes with the shutter motion. To maximize the signal, either the change in the capacitance with respect to shutter position (i.e. ∂C(x,t)/∂x, from EQ. 2) or the shutter velocity (i.e. ∂x/∂t, from EQ. 2), or both, can be increased.
Since, the shutter is typically operated at resonance, the resonant frequency determines the shutter velocity, ∂x/∂t. On the other hand, shutter geometry determines the change in capacitance with respect to shutter position. Previous micro-electro-mechanical ESV's employ a simple shutter that masks a portion of the sense probe. The shutter is actuated by an electrostatic comb type drive, which typically has a maximum displacement, &dgr;x, on the order of 1-10 &mgr;m.
In such a design the modulated change in the capacitance is determined by the permittivity of freespace &egr;
o
, and the area A:
C(
t
)=C
o
+(&egr;
o
&dgr;A/
d
)sin(&ohgr;
t
)=C
o
+(&egr;
o
&lgr;&dgr;x/d
)sin(&ohgr;
t
), EQ.4
where &dgr;x is the displacement of the shutter, and &lgr; is the length of the shutter, for a spacing d between the sense probe and the photoreceptor. &dgr;
&khgr;
is limited by the displacement of the comb drive. Thus, to increase the modulated area the length of the shutter, &lgr;, can be increased. However, increasing the length of the shutter, &lgr;, will correspondingly increase the size and mass of the resonator, leading to a decrease in sensor spatial resolution and a lower mechanical resonant frequency. This has the undesirable effect of reducing sense probe current output.
Thus, an alternative solution has been sought that provides a larger effective modulation area in order to obtain a stronger signal than previous designs.
SUMMARY OF THE INVENTION
The present invention contemplates a new and improved apparatus which overcomes all of the above-referenced problems and others, providing a higher output current.
In accordance with the present invention there is provided an electrostatic voltmeter which includes a sense probe assembly having a plurality of sense probes for measuring voltage by capacitive coupling. A shutter having a plurality of windows is also included and is moveable between a first position where the sense probe assembly is exposed and a second position where the sense probe assembly is covered. The shutter is located adjacent to the sense probe assembly. Additionally, a shutter modulator is operatively associated with the shutter and is capable of selectively moving the shutter between the first position and the second position.
According to another aspect of the present invention, the shutter modulator includes a micro-electro-mechanical system which contains an electrostatic comb drive. The comb drive defines a maximum displacement along an axis parallel to the comb fingers. The windows in the shutter further define a width equivalent to that of the maximum displacement of the comb drive.
In accordance with still another aspect of the present invention, a method of increasing an output signal produced by a non-contacting voltmeter is provided. A moveable shutter is placed adjacent to a plurality of sensed probes. The moveable shutter has a plurality of windows spaced therein to expose the sense probes in a first position and to cover the sense probes in a second position. The shutter is then moved between the first and second positions thereby increasing the output signal produced.
In accordance with yet another aspect of the present invention, a method additionally includes capacitively coupling the sense probes to a surface to be measured. Based on the capacitive coupling an output signal is then produced.
In accordance with still another aspect of the present invention, a MEMS modulator assembly for modulating a preselected condition includes a first surface and a second surface spaced from the first surface. The second surface is in selective operative communication with the first surface via the preselected condition. Further included is a shutter including spaced first and second windows disposed between the first and second surfaces. The shutter is movable between a first position where the first surface is masked relative to the second surface and a second position where the first surface operatively communicates with the second surface.
In accordance with a different aspect of the present invention, the preselected condition is modulating light emitted or directed from the first surface.
A principal advantage of the present invention is an increased output signal from the sense probes. The increased signal results from a larger modulated area as provided by multiple windows.
Another advantage of the invention resides in the windows physically lightening the shutter and allowing it to resonate at an increased frequency.
Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.
REFERENCES:
patent: 4030347 (1977-06-01), Norris et al.
patent: 4835461 (1989-05-01), Snelling
patent: 5025346 (1991-06-01), Tang et al.
patent: 5489850 (1996-02-01), Werner, Jr. et al.
patent: 5496436 (1996-03-01), Bernstein et al.
patent: 5517123 (1996-05-01), Zhao et al.
Hubble, III Fred F.
Kubby Joel A.
Peeters Eric
Viturro R. Enrique
Wallace Stanley J.
Fay Sharpe Fagan Minnich & McKee LLP
Kerveros James
Metjahic Safet
Xerox Corporation
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