01-10-2012, 11:49 AM
Capacitive Sensor in Touch Mode Operation
[attachment=34889]
ABSTRACT
Capacitive © pressure sensors typically sense quadratic changes in C as a pressure
difference (P) deflects a flexible conducting diaphragm near a rigid ground plane. Touchmode
capacitive pressure (C-P) sensors, where the conducting diaphragm touches a dielectric
coated ground plane, often show a more linear response, but with less sensitivity, particularly
at low P. Initial contact of the diaphragm often occurs at a critical P. Until Pcrit is reached,
the sensitivity is typically too low for accurate measurements. In this work, two different
types of electrodes with “parabolic” and “donut” cavity-shapes have been designed,
fabricated, and tested to achieve high-sensitivity at low-pressures. A flexible conducting
diaphragm touches the bottom electrode smoothly, and both cavity shapes permit initial
contact at a zero-pressure differential. This type of C-P sensors can have touch-mode and
peeling-mode operations. The sensitivities of these sensors in two operation modes were
measured, and their resolutions were smaller than 0.1 Pa at a mean pressure of 105 Pa. Both
sensors in two modes have the resolution over total-pressure less than 10-6, which is difficult
to achieve at atmospheric pressure.
INTRODUCTION
Capacitive sensing is becoming more prevalent and in demand for consumer
applications. Several techniques for capacitive sensing are currently present in industry.
Many are based on measuring a frequency or duty cycle which is changed by the introduction
of additional capacitance from a person‟s finger to ground. Some other methods use charge
balancing or rise and fall time measurements. This solution measures frequency using a freerunning
RC oscillator.
While capacitive sensing has been around for more than 50 years, it is becoming
increasingly easier to implement and more popular. A classic example of a capacitive switch
is the Touch Lamp. The Touch Lamp has been around for a long time, and it is a simple,
capacitive switch that turns a light bulb on, off or dims it. New technology allows much more
sophisticated control of touch buttons. A key to this has been microcontrollers with mixed
signal peripherals. They provide the ability to perform capacitive sensing, decision making,
responsive actions and other duties pertinent to the system as well.
EFFECTS OF COVERING PLATE
Window glass and Plexiglas® are common materials for use as the surface which a
person touches. These common materials come in various thicknesses, and the thickness and
composition of the material between the pad and touching surface affects sensitivity. When
comparing window glass to Plexiglas, or another brand acrylic, the window glass will allow
detection through a thicker piece of material given identical testing conditions. This is
because the dielectric constant of window glass is higher than the dielectric of acrylics.
Numerous specifications for a particular acrylic or type of glass exist, but the dielectric
constants are on the order of 2-3 for acrylics and about 7 for window glasses. Other notable
substances have dielectric constants of 1 for air and 80 for water. From a capacitive sensing
perspective, an extremely thin plate is ideal because it increases sensitivity and enables better
accuracy. The thinner a covering plate is, the more sensitive the system will be. The two
materials mentioned before have been tested with a commonly available thickness of 2
mm, and both acrylic Plexiglas and window glass work well in a variety of conditions.
Thicker, 5 mm Plexiglas has also been found to work acceptably. Conductive materials, such
as metal, will not work as a covering plate. Metal plates absorb the field lines created by the
oscillating pad. A person's finger press may be too weak to disturb the oscillator enough, or
if it does create enough change, the press will trigger all of the buttons which are beneath the
plate, which is equally as bad. All buttons covered will fire because the metal is conductive
and charge moves freely through it.
MOUNTING
The intent of this section is not to specify how a system must be created. There are
many existing creative ways to build a system with capacitive sensors. Rather the purpose of
this section is to describe a simple, easy and elegant method to make a sharp looking
interface. The assumptions for this design are that a flat face is desired, all hardware will exist
on a single PCB, and the interface has graphics and may be mounted by small bolts. The PCB
and circuitry are all mandated by what the application is to do and should all be placed on the
back side of the PCB; the front side should be completely flush. The end result will be a
sandwich with the PCB on the bottom, a piece of stylized paper in the middle, a piece of
Plexiglas on top and it will all be held together by bolts as in Figure 2.
THICKNESS OF THE SPACER LAYER
The final factor to consider in the mechanical design is the thickness of the spacer
material. The operation of the sensor is based on the movement of the target layer, in
response to the user‟s press. This deflection results in an increase in the sensor capacitance
because the distance between the plates of the capacitor is decreased. To create a sensitive
touch sensor, it follows that the amount of shift generated by the user‟s press should be a
significant percentage (6% minimum) of the unpressed spacing between the target and the
sensor. Note that a capacitance shift of 6% is advocated as a minimum. This amount of shift
is required because any parasitic capacitances in the system, when combined with the
resolution limit of the conversion technique, will reduce a 6% shift at the sensor down to a 3-
4% shift in the conversion result.
ADHESIVE TO BOND LAYERS
If the target is too stiff and the adhesive is elastic, then a force applied to button A
will cause the target over sensor B to lift. The result is a decrease in the capacitance of sensor
B, a decrease in the average value for sensor B and a reduction in its sensitivity due to the
offset of its threshold. To combat this problem, it is suggested that the space between buttons
be at least 1/3 to 1/2 the diameter of the buttons. Furthermore, the adhesive used to bond the
target to the spacer should be a permanent adhesive with good adhesion to both the target and
spacer materials. Given the variety of materials that could be used for both layers, it is
suggested that the manufacturer of the adhesive be contacted concerning the requirements
and applicable adhesives.
Sensing Steps Description
The basic principle begins with one ADC channel charging the internal sample-andhold
cap for the ADC to VDD. The sensor channel is then prepared to a known state by
grounding it. In Figure 6, it is shown floating to illustrate why it is important to ground it.
After the sensor is grounded, it must be made an input again. Finally, immediately after it is
made an input, the ADC channel is switched to the sensor. This puts the sample and hold cap,
Chold, in parallel with the sensor capacitor, creating a voltage divider between the two. Thus,
the voltage on the sensor capacitor is the same on the sample and hold capacitor. After this
step, the ADC should be sampled, and the reading represents an amount of capacitance on the
external sensor. With the addition of a finger touching the sensor, the capacitance will
increase, and the voltage on step 5 will be lower.
CAPACITIVE SENSING MODULE
The CSM allows the user to design a capacitive sensing system without an external
oscillator circuit. The CSM has its own software-controlled oscillator. It can also monitor up
to 16 inputs. In a typical application, the CSM is directly attached to pads on a PCB and
covered by an insulating material. When the insulating material above a pad is touched by the
user‟s fingertip, the capacitance of the pad increases, thus causing a frequency shift in the
CSM. This module simplifies the software needed for capacitive sensing: it is only necessary
to initialize a few registers and then set the appropriate method of measuring the change in
frequency.
[attachment=34889]
ABSTRACT
Capacitive © pressure sensors typically sense quadratic changes in C as a pressure
difference (P) deflects a flexible conducting diaphragm near a rigid ground plane. Touchmode
capacitive pressure (C-P) sensors, where the conducting diaphragm touches a dielectric
coated ground plane, often show a more linear response, but with less sensitivity, particularly
at low P. Initial contact of the diaphragm often occurs at a critical P. Until Pcrit is reached,
the sensitivity is typically too low for accurate measurements. In this work, two different
types of electrodes with “parabolic” and “donut” cavity-shapes have been designed,
fabricated, and tested to achieve high-sensitivity at low-pressures. A flexible conducting
diaphragm touches the bottom electrode smoothly, and both cavity shapes permit initial
contact at a zero-pressure differential. This type of C-P sensors can have touch-mode and
peeling-mode operations. The sensitivities of these sensors in two operation modes were
measured, and their resolutions were smaller than 0.1 Pa at a mean pressure of 105 Pa. Both
sensors in two modes have the resolution over total-pressure less than 10-6, which is difficult
to achieve at atmospheric pressure.
INTRODUCTION
Capacitive sensing is becoming more prevalent and in demand for consumer
applications. Several techniques for capacitive sensing are currently present in industry.
Many are based on measuring a frequency or duty cycle which is changed by the introduction
of additional capacitance from a person‟s finger to ground. Some other methods use charge
balancing or rise and fall time measurements. This solution measures frequency using a freerunning
RC oscillator.
While capacitive sensing has been around for more than 50 years, it is becoming
increasingly easier to implement and more popular. A classic example of a capacitive switch
is the Touch Lamp. The Touch Lamp has been around for a long time, and it is a simple,
capacitive switch that turns a light bulb on, off or dims it. New technology allows much more
sophisticated control of touch buttons. A key to this has been microcontrollers with mixed
signal peripherals. They provide the ability to perform capacitive sensing, decision making,
responsive actions and other duties pertinent to the system as well.
EFFECTS OF COVERING PLATE
Window glass and Plexiglas® are common materials for use as the surface which a
person touches. These common materials come in various thicknesses, and the thickness and
composition of the material between the pad and touching surface affects sensitivity. When
comparing window glass to Plexiglas, or another brand acrylic, the window glass will allow
detection through a thicker piece of material given identical testing conditions. This is
because the dielectric constant of window glass is higher than the dielectric of acrylics.
Numerous specifications for a particular acrylic or type of glass exist, but the dielectric
constants are on the order of 2-3 for acrylics and about 7 for window glasses. Other notable
substances have dielectric constants of 1 for air and 80 for water. From a capacitive sensing
perspective, an extremely thin plate is ideal because it increases sensitivity and enables better
accuracy. The thinner a covering plate is, the more sensitive the system will be. The two
materials mentioned before have been tested with a commonly available thickness of 2
mm, and both acrylic Plexiglas and window glass work well in a variety of conditions.
Thicker, 5 mm Plexiglas has also been found to work acceptably. Conductive materials, such
as metal, will not work as a covering plate. Metal plates absorb the field lines created by the
oscillating pad. A person's finger press may be too weak to disturb the oscillator enough, or
if it does create enough change, the press will trigger all of the buttons which are beneath the
plate, which is equally as bad. All buttons covered will fire because the metal is conductive
and charge moves freely through it.
MOUNTING
The intent of this section is not to specify how a system must be created. There are
many existing creative ways to build a system with capacitive sensors. Rather the purpose of
this section is to describe a simple, easy and elegant method to make a sharp looking
interface. The assumptions for this design are that a flat face is desired, all hardware will exist
on a single PCB, and the interface has graphics and may be mounted by small bolts. The PCB
and circuitry are all mandated by what the application is to do and should all be placed on the
back side of the PCB; the front side should be completely flush. The end result will be a
sandwich with the PCB on the bottom, a piece of stylized paper in the middle, a piece of
Plexiglas on top and it will all be held together by bolts as in Figure 2.
THICKNESS OF THE SPACER LAYER
The final factor to consider in the mechanical design is the thickness of the spacer
material. The operation of the sensor is based on the movement of the target layer, in
response to the user‟s press. This deflection results in an increase in the sensor capacitance
because the distance between the plates of the capacitor is decreased. To create a sensitive
touch sensor, it follows that the amount of shift generated by the user‟s press should be a
significant percentage (6% minimum) of the unpressed spacing between the target and the
sensor. Note that a capacitance shift of 6% is advocated as a minimum. This amount of shift
is required because any parasitic capacitances in the system, when combined with the
resolution limit of the conversion technique, will reduce a 6% shift at the sensor down to a 3-
4% shift in the conversion result.
ADHESIVE TO BOND LAYERS
If the target is too stiff and the adhesive is elastic, then a force applied to button A
will cause the target over sensor B to lift. The result is a decrease in the capacitance of sensor
B, a decrease in the average value for sensor B and a reduction in its sensitivity due to the
offset of its threshold. To combat this problem, it is suggested that the space between buttons
be at least 1/3 to 1/2 the diameter of the buttons. Furthermore, the adhesive used to bond the
target to the spacer should be a permanent adhesive with good adhesion to both the target and
spacer materials. Given the variety of materials that could be used for both layers, it is
suggested that the manufacturer of the adhesive be contacted concerning the requirements
and applicable adhesives.
Sensing Steps Description
The basic principle begins with one ADC channel charging the internal sample-andhold
cap for the ADC to VDD. The sensor channel is then prepared to a known state by
grounding it. In Figure 6, it is shown floating to illustrate why it is important to ground it.
After the sensor is grounded, it must be made an input again. Finally, immediately after it is
made an input, the ADC channel is switched to the sensor. This puts the sample and hold cap,
Chold, in parallel with the sensor capacitor, creating a voltage divider between the two. Thus,
the voltage on the sensor capacitor is the same on the sample and hold capacitor. After this
step, the ADC should be sampled, and the reading represents an amount of capacitance on the
external sensor. With the addition of a finger touching the sensor, the capacitance will
increase, and the voltage on step 5 will be lower.
CAPACITIVE SENSING MODULE
The CSM allows the user to design a capacitive sensing system without an external
oscillator circuit. The CSM has its own software-controlled oscillator. It can also monitor up
to 16 inputs. In a typical application, the CSM is directly attached to pads on a PCB and
covered by an insulating material. When the insulating material above a pad is touched by the
user‟s fingertip, the capacitance of the pad increases, thus causing a frequency shift in the
CSM. This module simplifies the software needed for capacitive sensing: it is only necessary
to initialize a few registers and then set the appropriate method of measuring the change in
frequency.