Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-06-28
2002-10-29
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S662000, C324S671000, C073S30400R
Reexamination Certificate
active
06472887
ABSTRACT:
The present invention is directed towards a capacitive sensor for sensing the amount of material in a container.
BACKGROUND OF THE INVENTION
To date, a variety of sensors have been proposed for detecting the amount of toner in the toner cartridges of reprographic devices, such as printers and copiers. One type of such a sensor is a “capacitive sensor.” A capacitive sensor (1) treats the toner material as part of a “toner-capacitor,” and (2) detects changes in the toner level by sensing changes in the toner capacitance.
In such sensors, a toner-capacitor is typically formed by placing two conductive electrodes near the toner material. The two electrodes act as the two plates of the toner-capacitor. The toner material serves as one portion of the dielectric material of this capacitor while air serves as another portion of this dielectric material. Hence, the capacitance of the toner-capacitor depends on the toner level, and this capacitance decreases as the toner level decreases and is replaced with air.
FIG. 1
presents a prior art capacitive sensor
100
that treats the toner material as part of a toner-capacitor. The capacitive sensor
100
is similar to a sensor disclosed in U.S. Pat. No. 4,133,453. This sensor generates an oscillating signal, which has a frequency that varies with the capacitance of the toner-capacitor. This sensor uses the frequency of the generated oscillating signal to gauge the toner capacitance and thereby the toner level.
As shown in
FIG. 1
, the capacitive sensor
100
includes (1) a constant voltage source
125
, (2) an oscillator
130
, (3) a frequency-to-voltage converter
135
, and (4) a comparator
140
. The oscillator
130
includes a toner capacitor that connects to the voltage source
125
. The oscillator generates an oscillating signal, whose frequency is dependent on the toner capacitance.
The frequency-to-voltage converter
135
receives the oscillating output signal, and generates an output voltage from the frequency of this signal. The
140
compares this output voltage with a reference voltage V
REF
. Based on this comparison, the sensor
100
determines whether the capacitance of the toner-capacitor has decreased below a referenced level. Such a decrease would indicate that the toner level has decreased below a threshold level.
FIG. 2
presents a more detailed view of the oscillator
130
. As shown in this figure, the oscillator
130
includes (1) a toner-capacitor
105
, (2) resistors
230
and
235
, and (3) a 555-timer
200
. The toner-capacitor
105
is formed by placing two electrodes
110
and
115
in a toner container. This capacitor's second electrode
10
connects to ground, while its first electrode
115
connects to the constant voltage source
125
through resistors R
1
(
230
) and R
T
(
235
).
The toner-capacitor's first electrode
115
also connects to an input
245
of the 555-timer
200
to provide an input voltage. This timer includes a lower comparator
205
, an upper comparator
210
, a flip-flop
215
, a discharge transistor
220
, and an inverting output driver
225
. The lower comparator
205
compares the input voltage (from the first electrode
115
) with ⅓V
CC
, while the upper comparator
210
compares the input voltage with ⅔V
cc
.
When the input voltage reaches ⅔V
CC
, the upper comparator sets the flip-flop
215
to output a high value. In turn, this high value (1) turns on the discharge transistor
220
, and (2) causes the inverting output driver
225
to output a low value. When the input voltage reaches ⅓V
CC
, the lower comparator resets the flip-flop
215
to output a low value. This low value (1) cuts off the discharge transistor
220
, and (2) causes the inverting output driver
225
to output a high value.
The operation of the oscillator
130
is as follows. Initially, the discharge transistor
220
of the timer
200
is off. This allows the first electrode
115
of toner-capacitor
105
to charge towards V
CC
through resistors
230
and
235
. When the voltage on the first electrode
115
reaches ⅔V
CC
, the upper comparator
210
sets the flip-flop
215
to output a high value. This high value turns on the discharge transistor
220
and causes the inverting output driver
225
to output a low value. The discharge transistor
220
, in turn, discharges the toner-capacitor
105
until the voltage on the first electrode
115
reaches ⅓V
CC
. At this time, the lower comparator
205
resets the flip-flop
215
to output a low value. This low value turns off the discharge transistor
220
and causes the inverting output driver
225
to output a high value. This oscillating process continues indefinitely, and results in an oscillating signal at the oscillator output
240
.
The frequency of the oscillating output signal depends on how quickly the toner-capacitor charges to ⅔V
CC
and discharges to ⅓V
cc
. Equation (1) below represents the frequency of the oscillating signal when the resistance, R
1
, of resistor
230
is much smaller than the resistance, R
T
, of resistor
235
(e.g., resistance R
1
is 4.7 k&OHgr; while resistance R
T
is 47 k&OHgr;).
f
0
=0.722/(
R
T
*C
T
). (1)
The frequency of the oscillating signal typically needs to be less than 20 kHz, because higher frequencies radiate more easily to the outside of the printer. This upper frequency constraint limits the amount of capacitance that sensor
100
can measure. When resistor
235
is 47 k&OHgr;, a toner capacitance of 750 pF causes the output of oscillator
130
to have a frequency of 20.48 kHz. Hence, the toner capacitance cannot be much smaller than 750 pF, because otherwise the oscillating frequency would greatly exceed 20 kHz. Also, resistor
235
cannot be made arbitrarily large to reduce the oscillating frequency, because that would upset the bias currents at the inputs of the 555-timer
200
.
To ensure that the toner capacitance stays larger than 750 pF, large-area electrodes or multiple electrodes in parallel pairs are used to form the toner capacitor
105
. In addition, the toner-capacitor's electrodes have to be placed within the container that stores the toner material, since the capacitance of a capacitor decreases as the distance between the capacitor's electrodes increases. When the toner-capacitor's electrodes are outside the toner container, the toner capacitance typically is less than 750 pF, and often falls within the sub-pico Farad range as the toner level decreases.
Consequently, the prior art sensor
100
cannot be used when its toner-capacitor electrodes are placed outside of the toner container, because cannot detect small toner capacitances while maintaining proper operational parameters. However, it is often desirable to place the electrodes outside the toner container of the toner cartridge, because placing electrodes inside cassettes adds cost to the consumable element rather than the more expensive printer engine.
Another disadvantage of the sensor
100
is that the toner-capacitor's first electrode
115
is driven by the voltage source
125
through a high-impedance path (i.e., through high-impedance resistor
235
). This high-impedance path makes the input voltage to the timer
200
susceptible to shunting capacitances. Thus, the wires and electrical elements connecting the toner-capacitor
105
and the voltage source
125
must be far enough away from neighboring conductive objects to avoid shunting capacitances and thereby allow accurate and repeatable measurements. This further reduces the possible positions of the toner-capacitor electrodes relative to other conducting parts of the printer and the toner cassette.
FIG. 3
presents another prior art sensor, which is similar to the sensors disclosed in U.S. Pat. Nos. 5,465,619 and 5,987,269. Sensor
300
uses the toner-capacitor to generate a signal, and then analyzes the amplitude of this signal to derive the toner capacitance C
T
and thereby the toner level. As shown in
FIG. 3
, sensor
300
includes (1) a toner-capacitor
305
, (2) a pow
Gao Jun
Picciotto Carl
Tullis Barclay J.
Hewlett--Packard Company
Patel Paresh
Sherry Michael
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