U.S. patent application number 12/861812 was filed with the patent office on 2011-06-30 for capacitance measurement systems and methods.
This patent application is currently assigned to Cypress Semiconductor Corporation. Invention is credited to Andrew Best, Louis Bokma.
Application Number | 20110156724 12/861812 |
Document ID | / |
Family ID | 42753135 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110156724 |
Kind Code |
A1 |
Bokma; Louis ; et
al. |
June 30, 2011 |
CAPACITANCE MEASUREMENT SYSTEMS AND METHODS
Abstract
A first capacitor and a second capacitor are charged until
voltage at the second capacitor settles to a settling voltage.
While charging, the first capacitor is alternately switched between
a current source and ground. When the settling voltage is reached,
charging of the first capacitor is halted. The second capacitor
continues to be charged until voltage at the second capacitor
reaches a reference voltage. The amount of time it takes for the
settling voltage to reach the reference voltage corresponds to a
measure of capacitance on the first capacitor.
Inventors: |
Bokma; Louis; (Seattle,
WA) ; Best; Andrew; (Seattle, WA) |
Assignee: |
Cypress Semiconductor
Corporation
San Jose
CA
|
Family ID: |
42753135 |
Appl. No.: |
12/861812 |
Filed: |
August 23, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11823982 |
Jun 29, 2007 |
7804307 |
|
|
12861812 |
|
|
|
|
Current U.S.
Class: |
324/658 |
Current CPC
Class: |
H03K 2217/960715
20130101; G01R 27/2605 20130101; H03K 17/955 20130101 |
Class at
Publication: |
324/658 |
International
Class: |
G01R 27/26 20060101
G01R027/26 |
Claims
1.-26. (canceled)
27. A system comprising: a current source coupled to a first node
of a first capacitor and to a first node of a second capacitor, the
current source configured to provide a charge on the first and
second capacitors, wherein the charge placed on the first and
second capacitors generates a voltage potential across the first
and second capacitors; a switch network comprising a first switch
configured to couple the current source and the first node of the
first capacitor to the first node of the second capacitor and a
second switch configured to couple the first node of the first
capacitor to a second node of the second capacitor; and a voltage
measurement circuit configured to measure the voltage potential
across the first and second capacitors, wherein the second
capacitor is coupled to the measurement circuit after the current
source is configured and coupled to the first node of the first
capacitor, and wherein the currently source is configured to
provide a charge on the first capacitor according to a
predetermined charge rate.
28. The system of claim 27 wherein the first and second capacitors
are coupled to the current source and to the voltage measurement
circuit through a mutliplexor bus.
29. The system of claim 27, wherein the second capacitor is coupled
to the voltage measurement circuit and the current source through a
pin tap.
30. The system of claim 27, wherein the voltage measurement circuit
comprises a first comparator comprising a first input coupled to
the first and second capacitors, a second input coupled to a first
reference voltage, and an output coupled to a processor.
31. The system of claim 30, wherein the voltage measurement circuit
further comprises a second comparator comprising a first input
coupled to the first and second capacitors, a second input coupled
to a second reference voltage, and an output coupled to a
processor.
32. The system of claim 27 further comprising a reference buffered
coupled intermediate to the current source and the second
capacitor, wherein the reference buffer is configured to charge the
second capacitor at a rate substantially greater than the
programmable current source.
33. The system of claim 27, wherein the first capacitor is a
capacitance sensing input configured to have a variable capacitance
in response to capacitance coupling to an activating element.
34. The system of claim 33, wherein the variable capacitance is a
mutual capacitance between the first node of the first capacitor
and a drive electrode, the drive electrode configured to provide a
variable voltage signal.
35. A method for measuring a capacitive input comprising:
configuring a current source to charge a first capacitor, wherein a
first node of the first capacitor is the capacitive input and
wherein the current source has an adjustable output; coupling the
first capacitor and the current source to a multiplexor bus;
coupling a second capacitor to the multiplexor bus; charging a
first capacitor and a second capacitor until voltage at the second
capacitor settles to a settling voltage, wherein during the
charging the first capacitor is switched back and forth between a
current source and ground; when the settling voltage is reached,
halting charging of the first capacitor while continuing to charge
the second capacitor until voltage at the second capacitor reaches
a reference voltage that is greater than the settling voltage; and
determining a measure of time for the settling voltage to reach the
reference voltage, wherein the measure of time corresponds to a
measure of capacitance on the first capacitor.
36. The method of claim 35 further comprising: charging a third
capacitor until voltages at the second and third capacitors settle
to the settling voltage; and after the settling voltage is reached
and the charging of the first capacitor is halted, continue
charging the third capacitor until voltage at the third capacitor
reaches the reference voltage.
37. The method of claim 35 wherein the charging further comprises
supplying a constant charging current to the first and second
capacitors using a digital current source.
38. The method of claim 35 wherein the determining further
comprises: counting a first number of oscillatory cycles until the
settling voltage reaches the reference voltage, wherein the first
number corresponds to the measure of capacitance on the first
capacitor; halting charging of the second capacitor and reducing
voltage at the second capacitor to less than the threshold voltage
after voltage at the second capacitor reaches the threshold
voltage; charging the first and second capacitors until voltage at
the second capacitor settles to the settling voltage; halting
charging of the first capacitor while continuing to charge the
second capacitor until voltage at the second capacitor again
reaches the reference voltage when the settling voltage is again
reached; and counting a second number of oscillatory cycles until
voltage at the second capacitor reaches the reference voltage.
39. The method of claim 38 further comprising comparing the first
and second numbers to identify a change in the measure of
capacitance on the first capacitor.
40. The method of claim 35 further comprising detecting an element
in sensing range of at least one of the capacitors based on
capacitances measured for the plurality of capacitors.
41. The method of claim 35 further comprising: comparing the
measure of time to a reference measure of time; adjusting an output
of an internal main oscillator in response to the comparison;
adjusting at least one of a plurality of trim values for the
internal main oscillator after adjusting the output of the internal
main oscillator; configuring an oscillator for the measuring of the
capacitive input after adjusting the at least one the plurality of
trim values; and repeating the measuring of the capacitive
input.
42. An apparatus comprising a programmable current source coupled
to a variable capacitor and a switch network, the switch network
configured to couple the programmable current source to a ground
potential and to a second capacitor and an input of a voltage
measurement circuit.
43. The apparatus of claim 42 wherein the voltage measurement
comprises a first comparator comprising a first input coupled to
the first and second capacitors, a second input coupled to a first
reference voltage, and an output coupled to a processor.
44. The apparatus of claim 43 wherein the voltage measurement
further comprises a second comparator comprising a first input
coupled to the first and second capacitors, a second input coupled
to a second reference voltage, and an output coupled to the
processor.
45. The apparatus of claim 42 further comprising a reference buffer
coupled intermediate to the switch network and the second
capacitor, wherein the reference buffer is configured to charge the
second capacitor at a rate substantially greater than the
programmable current source.
46. The system of claim 42, wherein the variable capacitor is a
mutual capacitor between a first node of the variable capacitor and
a drive electrode, the drive electrode configured to provide a
variable voltage signal.
Description
FIELD OF THE INVENTION
[0001] Embodiments of the present invention generally relate to
capacitive sensors, and methods and systems that measure
capacitance.
BACKGROUND ART
[0002] A capacitive sensor generally includes an electrode or an
array of electrodes. When an object such as a finger or stylus is
brought within range of an electrode, the capacitance of the
electrode is changed by an amount that depends, at least in part,
on the distance from the object to the electrode. For example, a
set of electrodes may be arranged in parallel to define a sensing
region, and the position of an object relative to the sensing
region can be determined based on the change in capacitance per
electrode induced by the object. In simple terms, a profile of
capacitance versus electrode can be used to unambiguously determine
the position of an object in, for example, the x-direction--the
x-coordinate corresponds to the peak of the profile. A second set
of parallel electrodes arrayed perpendicular to the first set can
be similarly used to determine the position of the object in the
y-direction. A single electrode can be used to determine proximity
(the z-direction).
[0003] Accurate measurements of capacitance changes induced by an
object are needed so that the position of the object can be
accurately determined. Accurate measurements of the background
capacitance (e.g., the amount of capacitance that is present even
if an object is not in proximity) are also needed to account for
noise that may be introduced by changes in ambient temperature or
the presence of contaminants on the surface of the sensor, for
example.
SUMMARY OF THE INVENTION
[0004] Capacitive sensors should be noise resistant and should be
able to achieve high resolution. Embodiments in accordance with the
present invention provide these and other advantages.
[0005] In one embodiment, a current source charges a first
capacitor (e.g., a sensor capacitor) and a second capacitor (e.g.,
an internal capacitor) until voltages at the capacitors equilibrate
at a settling voltage. In another embodiment, a third capacitor
(e.g., a modification or external capacitor) is also charged until
the voltages at each capacitor equilibrate at the settling voltage.
In one embodiment, the first capacitor is alternately switched
between the current source and ground until the settling voltage is
reached. Sensitivity is proportional to signal-to-noise ratio
(SNR). Switching of the first (e.g., sensor) capacitor reduces the
outside noise sources on that capacitor that could inadvertently
couple into the system.
[0006] When the settling voltage is reached, the first (sensor)
capacitor is disconnected from the current source. The first
capacitor can be switched to ground and disconnected from the
second capacitor and optional third capacitor, so no coupled noise
from the sensor affects the settled voltage. The current source
will continue to charge the second capacitor until voltage at the
second capacitor reaches a reference voltage (the third capacitor,
if used, is similarly charged). The amount of time it takes for the
settling voltage to reach the reference voltage corresponds to a
measure of capacitance on the first capacitor. In one embodiment, a
counter counts the number of cycles generated by an oscillator as
the voltage increases from the settling voltage to the reference
voltage.
[0007] In one embodiment, a comparator is used to compare the
voltage at the capacitor(s) to the reference voltage. In one such
embodiment, a low pass filter is coupled between the capacitor(s)
and the comparator to reduce the effect of high frequency noise. In
another such embodiment, the voltage is increased using a single
slope analog-to-digital converter (ADC) that includes the current
source, the counter and the comparator. The current source can be
calibrated so that the settling voltage is just below the reference
voltage, so that the count of oscillator cycles will have a larger
dynamic range, increasing resolution.
[0008] In summary, high sensitivity and high resolution capacitance
measurement systems and methods are described. The capacitance on
the first capacitor can be accurately measured in the absence of an
object to more precisely determine background capacitance. In the
presence of an object, the change in capacitance on the first
capacitor can be accurately measured, to detect the object with
increased sensitivity and/or to more precisely locate the object
relative to a sensing region. These and other objects and
advantages of the various embodiments of the present invention will
be recognized by those of ordinary skill in the art after reading
the following detailed description of the embodiments that are
illustrated in the various drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The accompanying drawings, which are incorporated in and
form a part of this specification, illustrate embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention.
[0010] FIG. 1 illustrates one embodiment of a capacitance measuring
system, with switches set in one position.
[0011] FIG. 2 illustrates one embodiment of a capacitance measuring
system, with switches set in another position.
[0012] FIG. 3 illustrates voltage versus time in the presence of an
object, as measured in a capacitance measuring system according to
an embodiment of the present invention.
[0013] FIG. 4 illustrates voltage versus time in the absence of an
object, as measured in a capacitance measuring system according to
an embodiment of the present invention.
[0014] FIG. 5 is a flowchart of one embodiment of a method for
measuring capacitance according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Reference will now be made in detail to the various
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings. While the invention will
be described in conjunction with these embodiments, it will be
understood that they are not intended to limit the invention to
these embodiments. On the contrary, the invention is intended to
cover alternatives, modifications and equivalents, which may be
included within the spirit and scope of the invention as defined by
the appended claims. Furthermore, in the following detailed
description of the present invention, numerous specific details are
set forth in order to provide a thorough understanding of the
present invention. However, it will be understood that the present
invention may be practiced without these specific details. In other
instances, well-known methods, procedures, components, and circuits
have not been described in detail so as not to unnecessarily
obscure aspects of the present invention.
[0016] FIG. 1 illustrates one embodiment of a capacitance measuring
system 100. In the example of FIG. 1, system 100 includes a number
of capacitors Cs(1), Cs(2), . . . , Cs(N), which may be referred to
as sensor capacitors or sensing capacitors, any one of which may
also be referred to herein as a first capacitor. System 100 also
includes a capacitor Cint, which may be referred to as a sampling
capacitor or internal capacitor and which may also be referred to
herein as a second capacitor. In one embodiment, system 100 also
includes a capacitor Cmod, which may be referred to as a
modification capacitor or external capacitor and which may also be
referred to herein as a third capacitor. The capacitor Cint may be
internal to a chip, and the capacitor Cmod may be external to the
chip. The capacitor Cmod, though optional, can improve noise
resistance and hence can increase sensitivity. The capacitor Cmod
can also reduce or eliminate large voltage swings within the
system.
[0017] The system 100 also includes a current source 110. In one
embodiment, current source 110 is an adjustable, digital current
source that, once adjusted, supplies a constant charging current
iDAC. The current source 110 is connected to the capacitors Cint
and Cmod by a bus 115. In one embodiment, the bus 115 is an analog
bus.
[0018] System 100 also includes switching circuitry that includes a
number of switches such as switches 120 and 121. The current source
110 can be connected to the capacitors Cs(1), Cs(2), . . . , Cs(N),
depending on the position of an intervening switch such as switch
120. The capacitors Cs(1), Cs(2), . . . , Cs(N) can also be
connected to ground, depending on the position of an intervening
switch such as switch 121. If, for example, capacitor Cs(1) is
connected to ground by closing switch 121, then switch 120 is
opened so that capacitor Cs(1) is disconnected from the current
source 110 (see FIG. 2). Conversely, if capacitor Cs(1) is
connected to current source 110 by closing switch 120, then switch
121 is opened.
[0019] In the example of FIG. 1, system 100 also includes an
optional low pass filter (LPF) 130, a comparator 135, an oscillator
140, a counter (or timer) 145, and processing circuitry 150 (e.g.,
a microprocessor). The low pass filter 130, if present, helps to
prevent the input of high frequency noise to the comparator
135.
[0020] In operation, system 100 measures the capacitance on each of
the sensor capacitors Cs(1), Cs(2), . . . , Cs(N). In the example
of FIG. 1, capacitance is measured on one sensor capacitor at a
time. In general, the capacitance on a selected capacitor (e.g.,
Cs(1)) is translated into an effective resistance by switching the
capacitor Cs(1) between the bus 115 and ground (effective
resistance is sometimes referred to as a capacitive reactance,
measured in ohms). Switching of the capacitor Cs(1) reduces the
outside noise sources on that capacitor that could inadvertently
couple into the system 100. The current source 110 is used to
create a voltage drop across the effective resistance. The voltage
drop is sampled using the sampling capacitor Cint and measured
using the current source 110, oscillator 140 and counter 145.
[0021] More specifically, in the first stage of operation, the
selected capacitor (e.g., Cs(1)) is connected to bus 115 and
current source 110 by closing switch 120 (switch 121 is open).
Charge flows into the capacitors Cs(1), Cint and Cmod from the
current source 110. During the first stage, the capacitor Cs(1) is
alternately switched between the bus 115 and ground by
appropriately opening and closing the switches 120 and 121, until
the settling voltage is reached. Each time the capacitor Cs(1) is
switched between bus 115 and ground, an amount of charge is removed
from the parallel capacitors Cint and Cmod. Charge from the
capacitors Cint and Cmod can be transferred to Cs(1) until the
voltage--referred to herein as the settling voltage--is the same at
each of these capacitors.
[0022] In one embodiment, the capacitors Cint and Cmod are
precharged to a preset voltage (e.g., the comparator 135 reference
voltage Vref) using a voltage source (not shown). By starting at a
preset voltage, the time needed to reach the settling voltage can
be reduced.
[0023] As mentioned above, each time the capacitor Cs(1) is
switched between bus 115 and ground, an amount of charge
Q.sub.sensor is removed from the parallel capacitors Cint and
Cmod:
Q.sub.sensor=C.sub.sensorV.
[0024] Over time, this charge movement acts like a current:
Q.sub.sensor/t=C.sub.sensorV/t.
[0025] The amount of current depends on the capacitance of sensor
Cs(1) (C.sub.sensor), the switching frequency f (the frequency at
which the sensor capacitor Cs(1) is switched between bus 115 and
ground), and the voltage:
I.sub.sensor=fC.sub.sensorV.
[0026] Solving for voltage:
V = I sensor fC sensor . ##EQU00001##
[0027] The capacitance on Cs(1) can be thought of as a resistor
based on Ohm's Law, resulting in an effective resistance of:
R=1/(fC.sub.sensor).
[0028] The constant charging current iDAC flows through this
effective resistance. The voltage across the effective resistance
is the resulting voltage on the capacitors Cint and Cmod:
V = 1 fC sensor ( iDAC ) . ( 1 ) ##EQU00002##
[0029] Thus, the switching circuitry (e.g., switches 120 and 121)
acts as a capacitance-to-voltage converter. Eventually, the charge
will distribute (equilibrate) across the capacitors Cs(1), Cint and
Cmod until the voltage is the same at each capacitor. The settling
voltage is given by equation (1) and is based on the switching
frequency f, the capacitance C.sub.sensor of Cs(1), and the amount
of current iDAC. The capacitors Cint and Cmod act in effect as
bypass capacitors that stabilize the resulting voltage.
[0030] Once the voltage settles to the settling voltage, the
capacitor Cs(1) can be disconnected from current source 110. In
addition, the capacitor Cs(1) can be switched to ground and
disconnected from the capacitors Cint and Cmod (switch 121 is
closed and switch 120 is opened; refer to FIG. 2), so that no
coupled noise from the sensor affects the settled voltage. The
settling voltage is held on the capacitors Cint and Cmod. The
capacitor Cs(1) may remain connected to the capacitors Cint and
Cmod, but better noise immunity is provided if it is
disconnected.
[0031] Capacitance is measured in the second stage of operation.
Once the capacitor Cs(1) is disconnected from current source 110 at
the end of the first stage, the capacitors Cint and Cmod are
charged by current source 110 until the voltage on those capacitors
increases from the settling voltage to the threshold voltage
(reference voltage Vref) of comparator 135. The amount of current
supplied by the current source 110 in the second stage may be
different from that of the first stage. A counter 145 counts the
number of oscillator 140 cycles until the voltage reaches the
reference voltage. The number of counts is related to the size of
the capacitance Cint and Cmod:
.DELTA. V t = iDAC C int + C mod . ##EQU00003##
[0032] Solving for t:
t = ( C int + C mod ) .DELTA. V iDAC . ##EQU00004##
[0033] The above equation can be transformed to counts:
Counts = ( C int + C mod ) .DELTA. V iDAC f O ; ##EQU00005##
[0034] where f.sub.o is the clock or cycle frequency of the
oscillator 140 (which may be different from the frequency f of
equation (1) above).
[0035] The number of counts corresponds to the amount of
capacitance on the capacitors Cint and Cmod, and therefore also
corresponds to the amount of capacitance that was on the sensor
capacitor Cs(1) (before it was switched to ground at the end of the
first stage). The number of counts increases when the sensor
capacitance increases.
[0036] The first and second stages described above can be repeated
to measure the capacitance on each of the other sensor capacitors
Cs(2), . . . , Cs(N), and then repeated again starting with sensor
capacitor Cs(1). Between measurement sequences, the current source
110 can be turned off, allowing the voltage on the capacitors Cint
and Cmod to decrease; in one embodiment, the voltage decreases to
the comparator reference voltage Vref. At the start of the next
measurement sequence, the voltage will again be set to the settling
voltage, as described above.
[0037] Capacitance measuring system 100 can be used as part of an
interface (e.g., a touchpad or touchscreen) in an electronic device
such as, but not limited to, a computing device (e.g., desktop,
laptop, notebook), a handheld device (e.g., cell phone, smart
phone, music player, game player, camera), or a peripheral device
(e.g., keyboard). Capacitance measuring system 100 can be
incorporated as part of a sensing system that can be used, for
example, to determine whether or not an object (e.g., a user's
finger, a probe, a stylus, etc.) is near or in contact with a
sensing region. The sensor electrodes (specifically, the traces
connecting the sensor capacitors to the rest of the system) may be
made of any conductive material, including substantially
transparent materials such as indium tin oxide (ITO).
[0038] The capacitance measuring systems described herein can also
be used to detect the presence of moisture, contaminants or the
like on the surface of a sensing region. In general, capacitance
measuring system 100 can be used to detect an element (e.g., an
object or a substance) that is proximate to a sensing region. An
element in contact with the sensing region is also proximate to
that region, and locating the position of an element within the
sensing region also includes detecting the element.
[0039] The presence of, for example, a finger in proximity to or in
contact with the sensor capacitor Cs(1) will increase the
capacitance on that sensor which, as shown by equation (1) above,
will decrease the effective resistance of that capacitor. The lower
effective resistance results in a lower settling voltage across the
capacitors Cint and Cmod. Thus, it will take longer for the current
source 110 to increase the voltage from the settling voltage to the
reference voltage Vref, resulting in more counts relative to the
number of counts that would be recorded in the absence of a
finger.
[0040] FIG. 3 illustrates voltage versus time in the presence of an
object, as measured in capacitance measuring system 100 (FIG. 2)
according to an embodiment of the present invention. Time t0
corresponds to the beginning of the second stage of operation
mentioned above, and so the voltage held on the capacitors Cint and
Cmod (and also on the bus 115) is the settling voltage. In the
embodiment of FIG. 2, the voltage on the capacitors Cint and Cmod
(and on the bus 115) is increased using a single slope ADC that
includes the current source 110, the counter 145 and the comparator
135. Other types of ADCs (e.g., a multi-slope ADC) can be used
instead of a single slope ADC. At time t1, the voltage reaches the
threshold voltage (Vref) on the comparator 135. In the example of
FIG. 2, the counter counts the number of cycles generated by
oscillator 140 between time t0 and time t1.
[0041] FIG. 4 illustrates voltage versus time in the absence of an
object, as measured in a capacitance measuring system 100 (FIG. 2)
according to an embodiment of the present invention. Relative to
FIG. 3, the settling voltage is higher in the absence of an object.
The voltage increases from the settling voltage to the threshold
voltage at the same rate as in FIG. 3 but reaches the threshold
voltage faster, resulting in fewer counts between time t0 and time
t1 relative to FIG. 3.
[0042] To provide consistent sensitivity, the settling voltage is
calibrated. The amount of current iDAC during the first operating
stage (when the sensor capacitor is alternately switched between
ground and the current source 110) determines the settling voltage.
In one embodiment, at startup of the system 100 (in the absence of
an object), a successive approximation technique is used to find a
current iDAC that results in a settling voltage that is just below
the threshold voltage Vref.
[0043] For example, the current source 110 may be controlled by an
eight-bit signal. In successive approximation, the most significant
bit is set and the resultant settling voltage is compared to the
threshold voltage. Depending on the result of the comparison, the
most significant bit either remains set or is cleared, and the next
most significant bit is set. This process is repeated to determine
the current iDAC that results in a settling voltage that is just
below the threshold voltage Vref. As can be deduced from FIGS. 3
and 4, the dynamic range of the counts with an object present
versus not present is greater as a result.
[0044] As mentioned above, the amount of current provided by
current source 110 during the first stage of operation (during
which the capacitors Cs(1), Cint and Cmod settle to the settling
voltage) and during the second stage of operation (when the voltage
on the capacitors Cint and Cmod is increased from the settling
voltage to the threshold voltage) can be the same or different.
[0045] With reference again to FIG. 2, processing circuitry 150 can
determine the presence of an object near a sensor capacitor Cs(1),
Cs(2), . . . , Cs(N) by comparing the most recent count for a
capacitor to either the count recorded for that capacitor from the
preceding measurement sequence or a stored baseline value. The
object will be closest to the sensor capacitor that experiences the
highest count. Movement of an object relative to the sensor
capacitors can be detected by monitoring the count per sensor
capacitor over time.
[0046] The stored baseline value will account for the presence of
contaminants, for example, that may have accumulated on the surface
of the sensor surface (e.g., on the surface of a touchpad). In
general, the stored baseline value can account for effects that may
affect the performance (accuracy) of system 100. The stored
baseline value can be updated over time.
[0047] FIG. 5 is a flowchart 500 of one embodiment of a method for
measuring capacitance according to the present invention. Although
specific steps are disclosed in flowchart 500, such steps are
exemplary. That is, embodiments of the present invention are
well-suited to performing various other steps or variations of the
steps recited in flowchart 500. The steps in flowchart 500 may be
performed in an order different than presented and that the steps
in flowchart 500 are not necessarily performed in the sequence
illustrated. Furthermore, the features of the various embodiments
described above can be used alone or in combination.
[0048] In block 510, with reference also to FIG. 1, a current
source charges a first capacitor (e.g., sensor capacitor Cs(1)) and
a second capacitor (e.g., capacitor Cint) until their respective
voltages equilibrate at a settling voltage. In one embodiment, the
first capacitor is switched back and forth between the current
source and ground until the settling voltage is reached. In
actuality, due to the switching of the first capacitor, there is
charging by the current source and discharging from the first
capacitor, but the net effect is charging. In one embodiment, the
current source also charges a third capacitor (e.g., capacitor
Cmod) until the voltages at each capacitor equilibrate at the
settling voltage.
[0049] In block 520, when the settling voltage is reached, the
first capacitor (Cs(1)) is disconnected from the current source. In
one embodiment, the first capacitor (Cs(1)) is also switched to
ground and disconnected from the capacitors Cint and Cmod. The
current source continues to charge the second capacitor (Cint) and
the optional third capacitor (Cmod) until voltages at the
respective capacitors reach a reference voltage that is greater
than the settling voltage.
[0050] In block 530, in one embodiment, oscillatory cycles are
counted until the settling voltage reaches the reference voltage.
In general, the amount of time it takes for the settling voltage
reaches the reference voltage is determined.
[0051] Blocks 510, 520 and 530 can be repeated for each sensor
capacitor Cs(1), Cs(2), . . . , Cs(N). The count per sensor
capacitor can be compared across the sensors to determine the
position of an object, and the count per sensor can be compared to
a preceding count to detect the presence of an object (or to
determine that a previously detected object is no longer
present).
[0052] In summary, high sensitivity and high resolution capacitance
measurement systems and methods are described. Capacitance can be
accurately measured to detect the presence of an object and/or to
determine its relative position. Background capacitance can also be
accurately measured to account for factors such as contaminants and
ambient temperature.
[0053] Embodiments of the present invention are thus described.
While the present invention has been described in particular
embodiments, it should be appreciated that the present invention
should not be construed as limited by such embodiments, but rather
construed according to the below claims.
* * * * *