AN3431
Application note
How to design a PresSense pressure touch button
Introduction
Traditional capacitive touch sensing solutions offer several advantages such as reduced
cost and no mechanical moving parts. The simple mechanical design of capacitive touch
sensing systems facilitates sealing water off devices while simultaneously maintaining a
very professionnal look and feel. There are however, some limitations. Such limitations can
be addressed by capacitive sensing technology but, in a different form called press to sense
technology (PresSense).
The advantages of PresSense over traditional capacitive sensing implementations are as
follows:
■
Water does not affect the measurements of PresSense capacitive sensing devices
whereas traditional capacitive sensing devices, with sensors that may be susceptible to
water/other contaminants, must follow special design considerations.
PresSense capacitive sensing devices present no problems for users with gloves.
PresSense capacitive sensing devices can have a metallic finish whereas traditional
capacitive sensing devices cannot.
PresSense capacitive sensing devices have a higher radio frequence (RF) immunity.
False detections are significantly reduced in PresSense capacitive sensing devices as a
small force is required for an actuation.
■
■
■
■
This application note describes how to design a PresSense touch sensing solution using
electrodes and a metal layer.
Typical applications that are best suited for the PresSense series include:
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■
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Stoves
Waterproof housing
Microwave ovens
Industrial applications
Kitchen appliances
Waterproof keypads
September 2011
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Contents
AN3431
Contents
1
2
Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Mechanical construction considerations . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
Stack up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5
Insulating layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Conductive top cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Spacer layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
PCB layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Mechanical bracket/support layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2
2.3
Application rigidity and adhesion of layers . . . . . . . . . . . . . . . . . . . . . . . . 11
Size of the sensor pads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3
How to combine proximity detection with
PresSense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Implementation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4
5
6
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
PresSense technology: cross section of an unpressed button . . . . . . . . . . . . . . . . . . . . . . . 5
PresSense technology: cross section of a pressed button (showing deflection) . . . . . . . . . 6
Mechanical stack up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Stress-strain curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Mechanical problem 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Mechanical problem 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Proximity detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Single channel PresSense switch with metal top cover . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Force diagram with metal top cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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List of tables
AN3431
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Young’s modulus and yield strength of common materials. . . . . . . . . . . . . . . . . . . . . . . . . . 9
Mechanical problem 1 analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Mechanical problem 2 analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Mechanical parameters and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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Principle of operation
1
Principle of operation
In both traditional and PresSense capacitive sensing technologies, the change in
capacitance caused by the user is measured by a sensor. In traditional capacitive sensing
technology, the sensor pad forms one side of a parallel capacitor and the user’s finger forms
the other side. If the user brings his finger closer to the pad, the distance (d) between his
finger and the sensor pad decreases according to
Equation 1.
Consequently, the
capacitance (C) increases.
Equation 1: Parallel plate capacitor
C
=
ξ
R
ξ
0
(
A
⁄
d
)
Where
ε
R
= relative permittivity,
ε
0
= vacuum permittivity, and A = area.
In PresSense technology, the concept of the parallel plate capacitor still applies, except that
the second plate is not the finger of the user but a conductive material with a fixed potential
suspended over the sensor pad. When a user applies a force to the conductive material
above the sensor pad, a slight local deflection is created in the material. Consequently, the
distance (d) between the two plates alters.
The conductive material, used as a top cover, must be kept at a fixed potential. In some
cases, the external metallic part of the system must be connected to earth for safety
reasons. The conductive top cover can be connected to earth but, in this case, the electronic
touch sensing must be referred to earth.
Figure 1
shows a cross section of a typical PresSense button when it is unpressed.
Figure 1.
PresSense technology: cross section of an unpressed button
Figure 2
shows a cross section of a typical PresSense button when it is pressed.
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