Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-04-20
2002-05-14
Sherry, Michael J. (Department: 2829)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S452000, C324S671000
Reexamination Certificate
active
06388452
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a device for capacitively sensing media thickness. More specifically, the present invention relates to a device for sensing the thickness of a media by using a variable capacitance capacitor that includes electrodes having a variable gap disposed therebetween and a dielectric of gas disposed in the variable gap. The thickness of the media is determined by measuring the capacitance between the electrodes. The capacitance between the electrodes is determined by the distance between the electrodes and the dielectric constant of the gas disposed in the variable gap and not by the dielectric properties of the media whose thickness is being measured.
BACKGROUND ART
Media thickness sensors are employed in media handling systems such as inkjet printers, laser printers, photocopying machines, document scanners, facsimile machines, and film production processes, just to name a few. Media thickness sensors can be implemented using a variety of technologies, such as mechanical sensors, optical sensors, and capacitive sensors.
A typical mechanical thickness sensor includes at least one member that is connected to a measurement circuit. The member is operative to engage a surface of a media whose thickness is to be measured. Contact between the member and the media results in the member being displaced. The measurement circuit measures the displacement of the member and generates a signal indicative of the thickness of the media. One disadvantage of mechanical thickness sensors is that the complexity and cost of the mechanical elements can be prohibitive in applications that require low cost, mechanical simplicity, and compact size. Another disadvantage of mechanical thickness sensors is that the mechanical elements are prone to failure and can require periodic maintenance and adjustment in order to maintain peak operational efficiency and measurement accuracy.
An optical thickness sensor can include a light source, such as a light-emitting diode (LED) and a light-sensitive element, such as a photodiode. The LED can be electrically driven by a power source, such as a constant-current source, and an output of the photodiode can be connected to a measurement circuit. Typically, the LED and the photo diode are positioned so that a beam of light from the LED is incident on the photodiode. The beam is either reflected off of a surface of the media whose thickness is to be measured or the beam is transmitted through the media. The output signal from the photodiode is proportional to the intensity of light incident on the photodiode and is indicative of the thickness of the media. One major disadvantage of the optical thickness sensor is that the accuracy of the thickness measurement is highly dependent on the optical properties of the media being measured. For example, if the media is opaque, then little or no light from the LED will reach the photodiode. Subsequently, the optical thickness sensor will not accurately measure the thickness of opaque media. Similarly, if the media is highly reflective of light or is translucent (clear), then there will be little or no variation in the intensity of light incident on the photodiode. Resulting will be inaccurate thickness measurements for reflective or clear media of different thicknesses. Another type of optical sensor measures thickness by reflecting light off of the media or a reflective surface in contact with the media into a light sensor. The position of the reflected light on the light sensor is indicative of the thickness of the media. Disadvantages of reflective sensors include electrical and mechanical complexity, high cost, and precision alignment of the optical components. A second disadvantage of the optical thickness sensor is that the sensor is often tuned to measure the thickness of a narrow range of media types or only of a specific type of media. Resulting, is a lack of flexibility in measuring a wide variety of media types. For example, the optical thickness sensor may be tuned to measure the thickness of white printer paper only. A third disadvantage of the optical thickness sensor is that if the media is of substantially uniform thickness, but has variable optical properties, then those variations in optical properties can result in inaccurate thickness measurements.
The use of capacitive elements to measure the thickness of a media is well known in the art. In a typical capacitive thickness sensor, opposed electrodes are urged into contact with opposed surfaces of a media whose thickness is to be measured. The media is disposed intermediate between the electrodes and the capacitance measured between the electrodes is a function of the dielectric properties of the media, the area of the electrodes, and the distance between the electrodes, so that the capacitance is determined by the following equation:
C
=(∈
m
*A
)÷
d
Where:
C=The capacitance measured between the electrodes;
∈
m
=The dielectric constant of the media;
A=The area of the electrodes; and
d=The spacing between the electrodes.
The change in capacitance between the electrodes can be sensed by appropriate electronic circuitry that produces a signal that is indicative of the thickness of the media. The sensing circuitry is well known in the art. For example, the electrodes can be in electrical communication with a constant current source. The voltage potential measured between the electrodes will increase linearly with time until a reference voltage is reached. The amount of time required for the voltage to reach the reference voltage is proportional to the thickness of the media. For instance, for thicker media, the time it takes to reach the reference voltage is shorter. Accordingly, based on the equation above, both the capacitance and the time it takes the voltage to reach the reference voltage decrease with increasing distance (thickness) of the media d.
In
FIG. 1
, a prior art capacitive thickness sensor
200
is shown. The sensor
200
includes electrically conductive plates
203
and
205
that are disposed opposite each other and are in physical contact with opposite surfaces
202
and
204
respectively, of a media
201
whose thickness d is to be measured. The plate
205
can be disposed on a support structure
209
that is operative to contain the plate
205
and can also serve as a surface upon which the media rests during the thickness measurement. In a typical application, the plates
203
and
205
will have identical surface areas a. The plate
203
is urged into contact with the media
201
by a biasing means
211
that is attached to a stationary support element
207
. The biasing means
211
can be a spring, for example. Depending on the thickness of the media
201
, the plate
203
is displaced
213
in response to the thickness of the media
201
so that when the media
201
is urged between the plates
203
and
205
the distance between the plates
203
and
205
is substantially equal to the thickness d of the media
201
. Electrical connections
215
and
217
electrically communicate the plates
203
and
205
respectively to a capacitance sensing unit
221
. The capacitance sensing unit
221
can use any method, including the one discussed above, to measure the capacitance between the plates
203
and
205
. An output signal
222
from the capacitance sensing unit
221
can be communicated to a control unit
223
that uses the signal to compute the thickness of the media
201
. For instance, if the media
201
has a dielectric constant ∈
m
, a know area a for the plates
203
and
205
, and the output signal
222
is indicative of a capacitance value of C, then the above equation can be used by the control unit
223
to compute the distance d between the plates
203
and
205
, wherein the distance d is substantially equal to the thickness of the media
201
. In the prior art capacitive thickness sensor
200
, the capacitance measured between the plates
203
and
205
is inversely proportional to the distance d betwe
Denny III Trueman H.
Nguyen Trung
Sherry Michael J.
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