Texas Instruments | LDC2112/LDC2114 Inductive Touch System Design | Application notes | Texas Instruments LDC2112/LDC2114 Inductive Touch System Design Application notes

Texas Instruments LDC2112/LDC2114 Inductive Touch System Design Application notes
Application Report
SNOA961 – February 2017
Inductive Touch System Design Guide
for HMI Button Applications
Yibo Yu, Chris Oberhauser
ABSTRACT
The Inductive Touch System Design Guide presents an overview of the typical sensor mechanical
structure and sensor electrical design for human machine interface (HMI) button applications. The
mechanical design chapter discusses several factors that impact button sensitivity, including metal
selection, sensor geometry, sensitivity dependence on target-to-coil distance, and mechanical isolations.
Two options of common layer stacks for inductive touch buttons are also presented. The sensor design
chapter focuses on flex PCB sensor electrical requirements and considerations for optimal sensitivity.
1
2
3
Contents
Mechanical Design .......................................................................................................... 3
Sensor Design ............................................................................................................. 12
Summary .................................................................................................................... 25
List of Figures
1
Metal Deflection .............................................................................................................. 3
2
Button Construction with Metal Target and PCB Sensor ............................................................... 4
3
Simulated Sensor Frequency Change (PPM) vs Deflection (µm) for an Example Sensor
4
Deflection vs Force for Al and Steel Targets, Circular Button, Diameter = 20 mm, Thickness = 0.25 mm
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
........................ 4
..... 5
Simulated Change in Frequency (PPM) for 1 µm Deflection vs Target Distance (mm) ............................ 6
Spacer Options............................................................................................................... 6
Example Layer Stack with Conductive Surface.......................................................................... 7
Example Layer Stack with Non-Conductive Surface .................................................................... 7
Sensors Mounted to the Metal Target (Correct) and to the Support Structure (Incorrect) ......................... 8
Adhesive-Based Sensor Structure ........................................................................................ 9
Spring-Based Sensor Structure ............................................................................................ 9
Slot-Based Sensor Structure ............................................................................................. 10
Copper Foil Shielded “Faraday Box” .................................................................................... 11
Sensor Models .............................................................................................................. 13
Example Sensor RP vs Target Distance ................................................................................. 13
LDC2112/LDC2114 Operating Region .................................................................................. 15
Inductance Shift vs Target Distance ..................................................................................... 16
Sensor Diameter for Circular and Racetrack Sensor Coils ........................................................... 16
INn Shielding with COM ................................................................................................... 17
AC Grounded Target for Shielding Capacitive Effects ................................................................ 17
Two-Layer Sensor Design ................................................................................................ 18
Offsetting Traces To Reduce Parasitic Capacitance .................................................................. 19
Spacer Thickness and Width ............................................................................................. 19
Measurement Sensitivity vs Target Distance ........................................................................... 20
Separate Stiffener for Each Sensor ...................................................................................... 20
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
1
www.ti.com
26
Sensor Racetrack Routing ................................................................................................ 21
27
Racetrack Inductor Design Tab of the LDC Calculations Tool ....................................................... 22
28
Example Dual Sensor Design ............................................................................................ 23
29
Sensor Region Construction .............................................................................................. 23
30
Sensor Stack Across Regions ............................................................................................ 24
List of Tables
2
1
Approximate Minimum Sensor Width vs Fabrication Restrictions ................................................... 18
2
Sensor Parameters
3
Sensor Stack ................................................................................................................ 24
........................................................................................................
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
22
SNOA961 – February 2017
Submit Documentation Feedback
Mechanical Design
www.ti.com
1
Mechanical Design
Implementing an effective inductive touch solution requires appropriate system mechanical design and
matching sensor design. The mechanical design should take into consideration the material properties,
button geometry, and sensor construction and mounting. The following sections will address each of these
topics.
1.1
Theory of Operation
Consider a flat metal plate held at a fixed distance from an inductive coil sensor, as shown in Figure 1. If a
force is applied onto the metal plate, the metal will deform slightly. For example, with a 1 N force, which is
approximately the weight of a computer mouse, a 1 mm thick aluminum plate that is 15 mm x 15 mm will
deform by about 0.2 µm. This deformation moves the opposite side of the plate closer to the LDC sensor.
Once the force is removed, the plate will return to its original unstressed shape.
When the conductive material is in close proximity to the inductor, the magnetic field will induce circulating
eddy currents on the surface of the conductor. The eddy currents are a function of the distance, size, and
composition of the conductor. If the conductor is deflected toward the inductor as shown in Figure 1, more
eddy currents will be generated.
Metal
Plate
x
Metal
Plate
LDC
Sensor
LDC
Sensor
Applied
force
Metal
Deflection
(exaggerated in
this figure)
Figure 1. Metal Deflection
The eddy currents generate their own magnetic field, which opposes the original field generated by the
inductor. This effect reduces the inductance of the system, resulting in an increase in sensor frequency.
As the conductive target moves closer to the sensor, the electromagnetic coupling between them
becomes stronger. As a result, the change in sensor frequency is also more significant.
1.2
Button Construction
Using the principle discussed above, we can construct a metal plate and sensor combination which can
function as a button. As the sensitivity of the sensor increases with closer targets, the conductive plate
should be placed quite close to the sensor—typically 10% of the sensor diameter. At this close distance,
the LDC can reliably measure a 0.2 µm deflection. For small deflections, the amount of deflection is
roughly proportional to the applied force.
For a robust interface, it is necessary to control the distance between the sensor and the target so that
random movements are not interpreted as button presses. Figure 2 shows how sensors can be clamped
onto the inside surface so that only touch forces cause a deflection toward the sensor and any other
forces do not produce an effective deflection toward the sensor.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
3
Mechanical Design
www.ti.com
Figure 2. Button Construction with Metal Target and PCB Sensor
If the sensor is constructed of a rigid PCB material such as FR4, then the rigid backing is not necessary.
1.3
Mechanical Deflection
The LDC2112/LDC2114 measures the shift in frequency of an LC resonator sensor. Figure 3 shows the
change in frequency vs metal deflection for an example flex PCB sensor. The nominal spacing between
the metal target and sensor is 150 µm. As shown in the graph, the change in frequency is approximately
linear with small metal deflections.
Change in Frequency (PPM)
12000
10000
8000
6000
4000
2000
0
0
1
2
3
Metal Target Deflection (µm)
4
5
D001
Figure 3. Simulated Sensor Frequency Change (PPM) vs Deflection (µm) for an Example Sensor
To design an inductive touch button system, it is recommended to obtain the deflection vs force
characteristic of the button surface. It is often easier to determine this using mechanical modeling and
simulation. This is to ensure that there is enough deflection for a desirable force threshold. The Metal
Deflection tab of the LDC Calculations Tool provides an estimate of the metal deflection for a specified
button material and geometry.
4
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Mechanical Design
www.ti.com
1.4
Mechanical Factors that Affect Sensitivity
The button performance depends on the mechanical characteristics of the layer stack, as well as the
electrical parameters of the LC sensor. The most important mechanical factors are listed below.
1.4.1
Target Material Selection
As discussed in Section 1.1, inductive button operates based on the electromagnetic coupling between a
coil sensor and metal target. The mechanical and electrical characteristics of the metal target significantly
affect the sensitivity of the button.
1.4.1.1
Material Stiffness
The material choice has a large impact on how much force is needed to achieve the required deflection for
a given metal thickness. The key material parameter is the Young’s modulus, which is a measure of the
elasticity of the metal and is measured in units of pascal (Pa). Materials with a lower Young’s modulus are
typically more flexible. For example, aluminum (AL6061-T6) has a Young’s modulus of 68.9 GPa, while
stainless steel (e.g. SS304) has a Young’s modulus of about 200 GPa, which makes it about 3 times
stiffer than aluminum. The difference in deflection versus force for a given circular sensor between the two
materials is shown in Figure 4.
Peak Deflection (µm)
100
10
1
Aluminum (AL6061-T6)
Stainless Steel (SS304)
0.1
0
0.5
1
Force (N)
1.5
2
D001
Figure 4. Deflection vs Force for Al and Steel Targets,
Circular Button, Diameter = 20 mm, Thickness = 0.25 mm
1.4.1.2
Material Conductivity
The higher the conductivity of the target material, the more eddy currents are generated on the surface.
This causes a stronger electromagnetic interaction with the sensor. Therefore, the conductivity of the
material should be as high as possible, as this produces the largest inductance shift for a given target
deflection. SS304 has a conductivity of 1.37x106 S/m, and aluminum has an even higher conductivity of
36.9x106 S/m.
In general, aluminum is an excellent material choice for inductive sensing because it is both flexible and
asserts a large inductance change on a sensing coil. Materials such as SS304, while not as optimal a
material choice as aluminum, can also be used and provide robust results.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
5
Mechanical Design
1.4.2
www.ti.com
Button Geometry
Inductive touch buttons can take on a variety of shapes, such as circular, oval, or rectangular. In designing
the button sizes and geometries, it is important to consider the amount of deflection that can be obtained
for a given material, metal thickness, desirable force, etc. In the case of circular buttons, the diameter of
the button determines its rigidity or how much deflection can be obtained, assuming all other parameters
are kept the same. For example, if a circular 0.6 mm thick aluminum button is pressed with 1 N uniform
force, a button of 10 mm diameter has a peak deflection of about 90 nm, while a button of 20 mm
diameter would have a peak deflection of about 350 nm. The Metal Deflection tab of the LDC Calculations
Tool provides an estimate of the metal deflection for a specified button material and geometry. The exact
deflection profile can be obtained via mechanical simulation tools.
1.4.3
Spacing Between Target and Sensor
The spacing between the metal target and PCB sensor is important for both electrical and mechanical
considerations. As the metal target approaches the coil sensor, it can interact with more of the
electromagnetic field. Therefore for the same deflection (e.g. 1 µm) at a closer nominal distance, the
amount of inductance shift increases, which leads to a larger change in frequency, as shown in Figure 5.
In other words, if the target is closer to the sensor, the system sensitivity is higher.
Change in Frequency (PPM)
6000
5000
4000
3000
2000
1000
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Distance between Metal Target and Sensor (mm)
0.4
D002
Figure 5. Simulated Change in Frequency (PPM) for 1 µm Deflection vs Target Distance (mm)
However, to ensure that there is enough room for deflection and meanwhile accounting for manufacturing
tolerances, it is generally recommended to have a nominal target-to-sensor distance of 0.1 to 0.2 mm.
This spacing can be achieved by creating recessed area in the metal facing the sensor for systems where
the PCB is placed flush to the metal, or by using a small spacer between the metal and the PCB sensor
with a cutout to allow the metal to deflect, as shown in Figure 6.
Figure 6. Spacer Options
Maintaining a consistent separation between the sensor and the target is critical to ensure effective
sensing. If spacers are used, the material should be non-compressible and have a low temperature
coefficient, so that the thickness does not vary over time or environmental conditions.
6
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Mechanical Design
www.ti.com
1.5
Layer Stacks of Touch Buttons
The button layer stack typically includes the conductive target, spacer (separation between target and
sensor), PCB coil sensor, and an optional stiffener (supporting structure for flex PCB sensors). There are
two common ways to implement the stack, depending on whether the surface is conductive.
1.5.1
Conductive Surface
If the touch button is implemented on a conductive surface, such as aluminum or stainless steel, the
surface can be used as the target of detection. In this configuration, the metal target is at the top of the
entire stack. The user directly presses the metal target, causing a micro-deflection in the metal itself. The
metal deflection will cause a change in the inductance of the sensor coil.
Figure 7. Example Layer Stack with Conductive Surface
1.5.2
Non-Conductive Surface
For non-conductive surfaces such as glass or plastic, a thin sheet of conductive layer, such as aluminum
or copper, should be embedded below the surface. When the user presses on the rigid surface at the top
of the stack, a micro-deflection is translated onto the conductive layer, bringing it closer to the PCB
sensor. This alternative approach can extend the application of inductive touch to virtually any material
surface.
Figure 8. Example Layer Stack with Non-Conductive Surface
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
7
Mechanical Design
1.6
www.ti.com
Sensor Mounting Reference
In general, the coil sensor should be directly attached to the metal target, not to other adjacent structures,
to avoid mechanical movement of the support structure causing unexpected movement of the sensor.
When the sensor is mounted to some other adjacent structure that can move with respect to the target,
that motion can be mis-interpreted as a button press.
Figure 9. Sensors Mounted to the Metal Target (Correct) and to the Support Structure (Incorrect)
1.7
Sensor Mounting Techniques
The sensor coils can be mounted to the metal target in many ways. The sensor mounting technique must
provide consistent performance with minimal crosstalk between neighboring buttons. This implies that any
force outside the button region should cause minimal local metal deflection at that button location. In order
to achieve this goal, the spacer should provide robust attachment between the metal and coil. At the same
time, the sensors should be mass-production friendly in terms of both cost and installation effort.
Three different mounting techniques are presented below, namely adhesive-based, spring-based, and
slot-based.
1.7.1
Adhesive-Based
The most straight-forward method for mounting the sensors is to apply adhesive to the spacers and glue
them to the metal target. The adhesive-based system does not require additional mechanical pieces and
is suitable for quick proto-typing. The downside is that the glue attachment process is less repeatable.
An image of the two side buttons on a phone case prototype is shown in Figure 10. The size of each
button coil is 8 mm x 2.7 mm. The inside of the case is recessed to make room for the coils. This not only
reduces the board area used by the coils, but also reduces the rigidity of the metal sidewall and enhances
sensitivity.
8
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Mechanical Design
www.ti.com
Figure 10. Adhesive-Based Sensor Structure
1.7.2
Spring-Based
An alternative method is to use a spring-based structure to push the sensors toward the metal target. The
spring arms can help to absorb unwanted movement in the vertical axis, therefore the system is less
susceptible to mechanical interference due to twisting. Such a system is easier to assemble than an
adhesive-based one. The disadvantage is that if the spring is attached to the PCB or the bottom of the
case, pinching the location of contact may cause interference in a less rigid case. The sensor structure
also takes up more space due to the additional mechanical pieces.
Figure 11. Spring-Based Sensor Structure
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
9
Mechanical Design
1.7.3
www.ti.com
Slot-Based
A third sensor integration technique is to insert the coil into a slot. Before inserting the sensor coil,
memory foam pads are glued to both sides of the coil. This step can be integrated into the PCB fabrication
process. When squeezed, the memory foam pads become thinner than the width of the slot and thus can
be inserted easily. After the foam pads are inserted into the slot, they restore to fill the entire slot within a
few seconds and serve as the “spacer” between the target and coil. The coil will be placed right in the
middle of the slot. The unique sensor enclosure is a more rigid structure compared to that of the previous
solutions. This approach provides the best immunity against undesirable mechanical interference such as
twisting and pinching.
Figure 12. Slot-Based Sensor Structure
Copper foil can be used to make a “Faraday Box” to completely shield the sensor in strong EMI
environment, such as wireless charging.
10
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Mechanical Design
www.ti.com
Figure 13. Copper Foil Shielded “Faraday Box”
1.8
Mechanical Isolation
When multiple buttons are present in a system, it is possible for undesirable mechanical interaction
between different buttons to occur. The LDC2112/LDC2114 has built-in algorithms to handle most of such
crosstalk. However, good mechanical design principles should still be applied so that the crosstalk
between adjacent buttons can be minimized. The following principles can be applied to reduce the
mechanical crosstalk between adjacent buttons during an active press:
1. Physical supports between buttons can facilitate larger metal deformation on the button that is pressed.
2. Ensuring a larger physical deflection for the intended button. From an electrical perspective, a larger
deflection enables a greater signal. Using a thinner metal or metal with a lower Young’s modulus
facilitates button surface deformation and reduces the impact on the neighboring buttons.
3. Increasing the distance or adding grooves between adjacent buttons improves mechanical isolation.
For crosstalk minimization, button-to-button separation should also be greater than one coil diameter.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
11
Sensor Design
2
Sensor Design
2.1
Overview
www.ti.com
The Inductive Touch System uses a sensor composed of an inductor in parallel with a capacitor to form an
LC resonator.
The resonator generates a magnetic field which interacts with nearby conductive materials. The generated
magnetic field is a near-field effect, and so the first principle of sensor design is to ensure that the field
reaches the desired conductive material, which we refer to as the target.
The TI application note LDC Sensor Design provides extensive detail on sensor construction. Many of the
concepts and recommendations in that application note apply to designing sensors suitable for Inductive
Touch applications.
2.1.1
Sensor Electrical Parameters
The primary electrical parameters for an inductive sensor are:
• Sensor resonant frequency fSENSOR,
• Sensor resistance (represented as RP or RS)
• Sensor inductance L,
• Sensor capacitance C,
• Sensor quality factor Q.
2.1.2
Sensor Frequency
The inductance and capacitance determine the sensor frequency, from the equation:
1
¦SENSOR
2S LC
(1)
In general, as the sensor’s magnetic field interacts with a conductive target, the effective inductance of the
sensor changes, causing the sensor resonant frequency to change.
2.1.3
Sensor RP and RS
RP represents the parallel resonant impedance of the oscillator, and RS represents the series resonant
impedance. These resistances are different representations of the same parasitic losses.
As conductive materials get closer to the sensor, the intensity of the eddy currents increases, which
corresponds to larger losses in the sensor. The sensor RS is based on the series electrical model, while
the RP is the based on the parallel electrical model, as shown in Figure 14. It is important to remember
that these resistances are AC resistances, and not the DC resistances.
12
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Sensor Design
www.ti.com
Figure 14. Sensor Models
The RP can be calculated from the RS by:
L
RP
RS u C
(2)
The Sensor RP decreases significantly as the conductive material is brought closer to the sensor surface,
as seen in Figure 15. The example sensor response graphed in Figure 15 has an RP variation between 2
kΩ and 8 kΩ. This variation can be normalized response to apply to most sensors. If a 4 mm diameter
sensor had a free space RP of 3 kΩ, it would have a RP of ~2.2 kΩ if the distance to the conductive
material was 0.5 mm.
It is possible that the sensor RP can be reduced to too low a level if the target is too close to the sensor;
this condition must be avoided for proper functionality. Refer to Section 2.3 for more details.
Figure 15. Example Sensor RP vs Target Distance
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
13
Sensor Design
2.1.4
www.ti.com
Sensor Inductance
The sensor inductance is a function of the geometry of the inductor—the inductor area, number of
windings, and also the interaction with any conductive materials. In general, a larger inductance value is
easier to drive. The sensing range of the inductor is based primarily on the physical size of the inductor,
not the inductance, where the larger the inductor, the farther the sensing range.
2.1.5
Sensor Capacitance
In general, the sensor capacitance is selected after the inductor has been designed, and is used to set the
sensor frequency. Use of very small sensor capacitances should be avoided so that any parasitic
capacitance shifts do not affect operation. As a general guideline, use of sensor capacitances smaller than
22 pF should be avoided.
2.1.6
Sensor Quality Factor
The sensor Quality Factor Q measures the ratio of the sensor inductance to the sensor’s AC resistance. In
general, a higher value is desirable, as the sensor requires less energy to maintain oscillation. The sensor
Q can be calculated with Equation 3:
Q
1
RS
L
C
(3)
The RS is the sensor’s series AC resistance at the frequency of operation. The sensor Q can be increased
by either increasing the sensor inductance, decreasing the sensor RS, or decreasing capacitance.
2.2
Inductive Touch
LDC technology can be used to detect metal deflection as an emulation of a button. This capability
provides many advantages, such as operation with seamless, grounded plates of metal, operation in wet
or humid environments, resistance to false touch events, and reliable operation even when the user is
wearing gloves. This application is discussed in the TI Application Note Inductive Sensing Touch-On-Metal
Buttons Design Guide.
2.3
LDC2112/LDC2114 Design Boundary Conditions
The LDC2112/LDC2114 is a high resolution Inductance to Digital Converter which internal algorithms
which can detect inductance shifts corresponding to button presses on metal or other surfaces. It requires
that attached sensors meet the following parameters:
• 1 MHz ≤ fSENSOR ≤ 30 MHz
• 350 Ω ≤ RP ≤ 10 kΩ
• 5 ≤ Q ≤ 30
If the sensor parameters are not within these specifications, the LDC2112/LDC2114 may not be able to
measure inductance shifts, and as a result will not indicate Inductive Touch events. These restrictions can
be visualized as shown in Figure 16, which is derived by use of Equation 1.
14
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Sensor Design
www.ti.com
Figure 16. LDC2112/LDC2114 Operating Region
The minimum sensor frequency boundary on the top right, corresponds to the equation:
1
L
2
2S u 1MHz C
(4)
and the bottom left boundary corresponds to the Maximum Sensor Frequency:
1
L
2
2S u 30 MHz C
(5)
On the left, if the sensor capacitance is too small, then parasitic capacitance effects may degrade the
sensor operation; while this boundary is shown at 10 pF, some systems may encounter issues even with
larger sensor capacitances. In general, it is recommended to use a sensor capacitance larger than 22 pF.
2.4
2.4.1
Sensor Physical Construction
Sensor Physical Size
Inductive touch functionality is based on the sensor’s magnetic field interacting with a metal surface.
Therefore the magnetic field must reach the surface of the metal. The magnetic field ‘size’ is based on the
size of the inductor—the larger the inductor, the larger the generated magnetic field.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
15
Sensor Design
www.ti.com
Figure 17. Inductance Shift vs Target Distance
For a circular inductor, the size of the inductor is the diameter. For a non-circular inductor, sensor
diameter is effectively the minimum axis size.
Figure 18. Sensor Diameter for Circular and Racetrack Sensor Coils
2.4.2
Sensor Capacitor Position
It is recommended to place the sensor capacitor close to the INn pin, and not near the sensor. This
placement avoids transmission line effects with higher frequency sensors.
2.4.3
Shielding INn traces
For reliable inductive touch applications, the INn traces should not have significant time-varying
capacitance shifts. Parasitic capacitance shifts could produce false button press events if the INn traces
are not shielded. It is recommended to surround the INn traces with a shield driven by the COM pin, as
shown in Figure 19.
16
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Sensor Design
www.ti.com
Figure 19. INn Shielding with COM
2.4.4
Shielding Capacitance
The LC resonator sensor can respond to both inductive and capacitive changes. In order to prevent any
capacitive effects from causing undesired signal responses, the metal target should have a fixed constant
potential. Therefore in constructing the sensor-target system, the conductive target should be AC
grounded to shield any external capacitances.
Figure 20. AC Grounded Target for Shielding Capacitive Effects
2.4.5
CCOM Sizing
The COM pin can drive a load of up to 20 nF to ground. CCOM should be sized so that the following
relationship is valid for all channels.
100 × CSENSOR / QSENSOR < CCOM < 1250 × CSENSOR / QSENSOR
(6)
This requirement is still necessary even if CSENSOR is not the same value for all channels.
2.4.6
Multi-Layer Design
The inductance of a sensor is a function of the area, and the number of windings, and target distance.
With many inductive touch applications, the desired physical size of the buttons may be 3 mm in diameter
or smaller. The low total inductance of smaller sensors may result in a sensor frequency which is outside
the design space of the LDC2112/LDC2114. By using multiple layers of alternating rotation sensors, the
total inductance, due to additional mutual inductance between layers, is significantly higher compared to a
single layer design.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
17
Sensor Design
www.ti.com
Figure 21. Two-Layer Sensor Design
For most applications, 2 layer or 4 layer designs are sufficient. While a 4 layer sensor is more complex
and expensive compared to a similar geometry 2 layer sensor, the LDC2114 can effectively drive a
physically smaller 4 layer sensor, as shown in Table 1.
Use of a single layer sensor is generally not as effective, as the mutual coupling between layers in a
multilayer sensor provides a significant increase in the sensor inductance. In addition, there needs to be a
second routing to bring the sensor current out from the center of the sensor back to the LDC.
Table 1. Approximate Minimum Sensor Width vs Fabrication Restrictions
Available Spacing Distance
between Turns
Number of Layers
Minimum Via Size
Minimum Sensor Width
4 mil (0.1016 mm)
2
15 mil (0.4 mm)
2.85 mm
4 mil (0.1016 mm)
4
15 mil (0.4 mm)
2.30 mm
3 mil (0.076 mm)
2
15 mil (0.4 mm)
2.05 mm
3 mil (0.076 mm)
4
15 mil (0.4 mm)
1.91 mm
2 mil (0.051 mm)
2
15 mil (0.4 mm)
1.65 mm
2 mil (0.051 mm)
4
15 mil (0.4 mm)
1.53 mm
2 mil (0.051 mm)
4
12 mil (0.305 mm)
1.38 mm
Minimum sensor width of a fixed 8 mm sensor length with a target distance of 0.2 mm. These sensors
have not been evaluated for performance. These sensors assume a 1 mil (25 µm) dielectric thickness
between layers.
2.4.6.1
Sensor Parasitic Capacitance
The individual turns of an inductor have a physical area and are separated by a dielectric; this manifests
as small parasitic capacitor across each turn. These parasitic capacitances should be minimized for
optimum sensor performance. One simple but effective technique for multi-layer sensors to reduce the
parasitic capacitance is to offset any parallel traces between layers, as shown in Figure 22.
18
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Sensor Design
www.ti.com
Figure 22. Offsetting Traces To Reduce Parasitic Capacitance
2.4.7
Sensor Spacers
Maintaining a consistent separation (gap) between the sensor and the target is critical to ensure effective
sensing. The system design feature which provides this is the spacer.
Figure 23. Spacer Thickness and Width
Typical spacer thicknesses range from 0.1 mm to 0.5 mm, depending on the sensor geometry and sensor
electrical parameters. In general, thinner spacers provide better performance, provided the sensor
electrical characteristics are within the LDC2112/LDC2114 boundary conditions. Setting the spacer
thickness to less than 10% of the coil diameter (for a rectangular or elliptical shaped sensor, 10% of the
shorter side) generally provides optimum performance.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
19
Sensor Design
www.ti.com
Figure 24. Measurement Sensitivity vs Target Distance
Wider spacers may be needed when an adhesive is used to attach the sensor to the target surface, to
provide a stronger attachment to the target.
2.4.8
Sensor Stiffener
If a flex PCB is used for the sensor, it must be supported by a stiffener. If a flex sensor is not supported,
then it may deform under any movement, leading to false detection events. The support should be a
uniform surface which has minimal warping across temperature, humidity, and acceleration. The
supporting structure, which is often called a stiffener for LDC applications, should not be conductive;
otherwise the sensor Q and RP may be reduced below the minimum levels the LDC2112/LDC2114 can
support. Use of FR4 backing is a common technique for flex PCBs and is suitable for LDC sensor use. For
a thinner sensor, it is acceptable to use an epoxy based stiffener.
The stiffener should be a non-conductive material, otherwise the sensor RP may be too low for the
LDC2112/LDC2114 to drive; for this reason SuS and Al stiffeners should be avoided.
If multiple sensors are constructed on a single flex PCB, the stiffener should be separate for each sensor
section; otherwise significantly more mechanical crosstalk can occur.
Figure 25. Separate Stiffener for Each Sensor
For some applications, the stiffener can be a component already present in the system—for example, a
glass surface, or with sensors manufactured on a rigid material such as FR4.
Normal PCBs made of FR4 or other rigid materials do not require a dedicated stiffener.
20
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Sensor Design
www.ti.com
2.4.9
Racetrack Inductor Shape
For some inductive touch applications which require very small sensors, the inductance of a circular or
square sensor is too low. An elongated shape, such as a rectangle or racetrack shape, as seen in
Figure 26, will have a larger amount of inductance. This shape is effective for side-buttons in mobile
applications.
2.5
Example Sensor
For this example, a dual sensor design is presented. The sensors are 2.85 mm x 8 mm in size, with 8
turns, as seen in Figure 26. The traces are 0.25 oz-cu (9 µm) thick, are 75 µm wide and have a spacing of
50 µm. The sensor free-space inductance is approximately 1.3 µH, and has a 47 pF sensor capacitor.
When mounted, the sensor inductance decreases due to interaction with the conductive target.
Figure 26. Sensor Racetrack Routing
The parameters of this sensor were estimated using the Racetrack Inductor Designer tab on the LDC
Calculations Tool. Figure 27 is an example of the tool entries used to design the sensor described here.
Note that the tool provides estimates of the sensor parameters such as RS, RP, Q, L, and frequency,
based on Equations (1), (2), and (3).
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
21
Sensor Design
www.ti.com
Figure 27. Racetrack Inductor Design Tab of the LDC Calculations Tool
The tool output includes estimates for both free space parameters (no target present) and their values
when the sensor is mounted in the system with a target close by. As seen in Table 2, when the sensor is
mounted, the sensor parameters are within the LDC2112/LDC2114 operating space.
Table 2. Sensor Parameters
22
Sensor Parameters
Sensor in Free Space
Sensor Mounted
Sensor Inductance
1.3 µH
0.76 µH
LDC2112/LDC2114 Operating Space
Sensor Capacitance
47 pF
47 pF
Sensor Frequency
19.4 MHz
26.7 MHz
1 MHz – 30 MHz
Sensor RP
7.3 kΩ
1.4 kΩ
350 Ω ≤ RP ≤ 10 kΩ
Sensor Q
41
11
5 ≤ Q ≤ 30
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Sensor Design
www.ti.com
The routing between the sensor and the connector is shielded by the top and bottom layers, which are
driven by the COM signal. Regularly spaced vias are used to tie the top and bottom shields.
The bend in the shielded routing is used for strain relief.
Figure 28. Example Dual Sensor Design
The stiffeners and spacers are integrated into the sensors for this example. The arrangement of the
spacer and stiffeners is shown in Figure 29.
Figure 29. Sensor Region Construction
Each sensor region has a dedicated stiffener and two spacers. The flex sensor region between the two
sensors provides mechanical isolation between the two sensors.
Table 3 shows the sensor stack. The thickness of the stiffener can be varied based on mechanical
considerations. In general, incorporating the spacer into the sensor manufacturing can usually provide a
tighter tolerance for the spacer thickness than machining the spacer on the case.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
23
Sensor Design
www.ti.com
Table 3. Sensor Stack
Layer
Type
Material
Thickness (mil)
Thickness (mm)
Dielectric Material
Stiffener
Dielectric
Core
32
0.813
FR4
Top Overlay
Overlay
Flex Top Coverlay
Solder
Mask/Coverlay
Surface Material
0.4
0.010
Coverlay
Top Layer
Signal
Copper
0.46
0.012
Flex1
Dielectric
Film
0.47
0.012
Signal Layer
Signal
Copper
0.46
0.012
Polyimide
Flex2
Dielectric
Film
1
0.025
Bottom Layer
Signal
Copper
0.46
0.012
Flex Bottom
Coverlay
Solder
Mask/Coverlay
Surface Material
0.4
0.010
Coverlay (PI)
Bottom Solder 1
Solder
Mask/Coverlay
Surface Material
0.4
0.010
Solder Resist
5
0.127
Polyimide
41.05
1.043
Bottom Overlay
Overlay
Spacer
Dielectric
Film
Total Thickness
Polyimide
The spacer and stiffener are only present for a portion of the sensor design, as shown in Figure 30. The
spacer is only needed on the ends of the button locations. The stiffener is needed over the sensor, and
any connectors. The stiffener can be manufactured with a thinner material, if needed for a specific
application.
Figure 30. Sensor Stack Across Regions
24
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
SNOA961 – February 2017
Submit Documentation Feedback
Summary
www.ti.com
3
Summary
In this design guide, we reviewed the mechanical considerations of inductive touch button design using
inductive sensing technologies for optimal sensitivity and reliability, including the mechanical stack and
basic process flow for electrical design. The process for designing a sensor suitable for
LDC2112/LDC2114 inductive touch applications can be viewed as:
1. Determine the available physical size of the sensor
2. Use the design tools to design a sensor that is within the LDC2112/LDC2114 operating space
3. Use the shielded structure to route the INn traces
4. Construct any spacers or stiffeners that are needed.
The low power architecture of LDC2112/LDC2114 makes it suitable for driving button sensors. The
mechanical case does not require any cutouts at the button locations. This can support reduced
manufacturing cost and enhance the case’s resistance to moisture, dust, and dirt. This is a great
advantage compared to traditional mechanical buttons in the market today.
SNOA961 – February 2017
Submit Documentation Feedback
Inductive Touch System Design Guide for HMI Button Applications
Copyright © 2017, Texas Instruments Incorporated
25
IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,
reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who are
developing applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you
(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms of
this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources.
You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing your
applications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications
(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. You
represent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)
anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that
might cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, you
will thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted any
testing other than that specifically described in the published documentation for a particular TI Resource.
You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that include
the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TO
ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS.
TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOT
LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IF
DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,
COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH OR
ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your noncompliance with the terms and provisions of this Notice.
This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.
These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluation
modules, and samples (http://www.ti.com/sc/docs/sampterms.htm).
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2017, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising