This application note describes implementing proximity detection at the presence of large metal objects. Recommendations about sensor mechanical construction and proximity sensing best practices are provided. An example of proximity sensing implementation for microwave ovens is also provided.

1. Proximity Detection in the Presence of
Metal Objects
AN42851
Author: Victor Kremin, Andriy Ryshtun, Vasyl Mandzij
Associated Project: Yes
Associated Part Family: CY8C21x34, CY8C24x94
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Software Version: PSoC Designer™ 4.4
Associated Application Notes: AN2352
Application Note Abstract
This application note describes implementing proximity detection at the presence of large metal objects. Recommendations
about sensor mechanical construction and proximity sensing best practices are provided. An example of proximity sensing
implementation for microwave ovens is also provided.
Introduction
2. A grounded metal plane catches a part of the sensor
The ability to use proximity detection in white goods and
electric field and reduces the added by palm
automotive applications is often essential. For example,
capacitance.
proximity detector is used to turn on the backlight in a
kitchen stove or the internal lamp in a microwave oven Figure 1. CY3235 Proximity Detector Demonstration Kit
when the palm is close to the door. In various home
appliances a proximity sensor turns on the display when
the user tries to adjust some parameters.
Cypress provides a CY3235 kit that demonstrates
proximity sensing. The CY3235 kit has a detection range
of 30 cm when the sensor is located far away from
conductive objects such as metals. When a wire sensor is
placed on a metal surface, detection range dramatically
decreases from 30 cm to 2 cm. Most white good and
automotive applications have a metal frame or case that is
a challenge for proximity sensing devices.
This CY3235 kit contains a wired sensor and small PCB
with CY8C21434 chip on board, as shown in Figure 1.
The reasons why the proximity detection range reduces
dramatically when conductive objects are placed close to
the sensor are:
1. The sensor stray capacitance increases. Stray
capacitance reduces the proximity response value by
providing a higher full scale range. Larger stray
capacitance often requires operation frequency
reduction, causing the additional detection distance to
decrease.
January 25, 2008 Document No. 001-42851 Rev. ** 1
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2. AN42851
Electrical Field Simulation
Figure 3. Electrical field from a single wire sensor with a
Simulations using the tool Comsol Multiphysics V.3.2 are
metal object
made to clarify the influence of a metal presence near the
proximity detection sensor.
Electrical
This tool has a powerful interactive environment for Palm
field lines
modeling and solving most scientific and engineering
problems based on Partial Differential Equations (PDEs).
Using the built in physics modes, it is possible to develop
models by defining the relevant physical quantities such
as material properties (geometric dimensions, object
conductivity, dielectric constant, and so on) and sources,
rather than by defining the underlying equations. Comsol
Multiphysics internally compiles a set of PDEs
representing the entire model. Wire
Metal
sensor
surface
The electrical field from a single proximity detection sensor
with and without metal object simulation is shown in Figure
2 and Figure 3.
The simulation conditions are:
Figure 2. Electrical Field from a Single Wire Sensor

without a Metal Object
The palm is modeled as a 10cm x 15cm x 1.5cm
metal substrate with zero potential (grounded).
Electrical Palm

field lines The sensor wire has a diameter of 2 mm and length of
150 mm.
 The wire potential is 5V.
 The wire to palm distance is 80mm.
 The wire to metal distance is 2mm.

Wire The grounded metal plate dimensions are 500 x
sensor 500mm.
The simulation results show that the metal surface catches
a part of the proximity detector sensor electrical field and
greatly decreases the electrical field strength. This causes
the proximity sensor detection range to decrease.
To get the quantitative data there is an estimated added-
by-palm capacitance with and without a metal object using
the Gauss theorem in the section Interelectrode
Capacitance Calculation on page 3.
January 25, 2008 Document No. 001-42851 Rev. ** 2
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3. AN42851
Interelectrode Capacitance Calculation
Using the simulation results you can determine the c) Dividing the result of the last equation by the value of a
electrical field vector tension in any point of the medium. potential of the object inside our image cube to find the
These results are used in calculating the mutual value of the mutual capacitance:
capacitance of a system of electrodes.
n
q
The capacitance is defined by the formula: i
i 1
Cmutual  (5)
n
q 
i
C i 1 (1) Calculate the own capacitance, by repeating the
 aforementioned steps without the palm:
According to the Gauss theorem, the flux of the vector of Using the equations (3)-(5) you find:
tension of the electrostatic field in a vacuum through the
n
q
closed surface is equal to the algebraic sum of the
charges concluded into this surface divided by electric i
i 1
Cown 
permanent(1): (6)


 n
1
 E  dS     q (2) Then intercapacitance between sensor and palm is equal
i
i 1 to:
0
S
Cint  Cmutual  Cown (7)
S - Any closed surface that includes wire sensor.
Where: The simulations are repeated several times with different
sensor configurations. The summary of the results is
 0  8.85 10 12 F / m ,   5 V shown in Table 1.
Table 1. Simulation Results
If there is a system of some objects displaced in a medium
and you add one or more other objects, you can evaluate Configuration Cmutural, pF Cowm, pF Cint, pF
the intercapacitance by subtracting the value of the mutual
No metal objects 8.89 8.36 0.53
capacitance in a system without the additional objects,
from the value with the additional objects. Metal object,
connected to 22.53 22.46 0.07
The algorithm to calculate the intercapacitance of a
ground
system of electrodes is:
Metal object with
Calculate the mutual capacitance of an arbitrary
110.6 110.3 0.3
same potential as
electrodes system by:
sensor
a) Calculating the flux of the vector of tension of the
electrostatic field through a closed surface that concludes
As shown in simulation results for this configuration, the
wire sensor:
grounded metal surface decreases the added-by-palm

 capacitance by eight times, from 0.53pF to 0.07pF. This
ФE  E  dS (3)
explains why the detection distance drops so much.
S
When you change the metal plane potential to the same
b) Finding the algebraic sum of the charges included into level as the proximity detection sensors, the added
this closed surface using the Gauss theorem: capacitance is 0.3pF, which is only two times less than
0.57pF for a configuration without metal object presence.
n
q  Ф  0 (4) This demonstrates that you can improve the detection
i E
i 1 distance by placing a large shield electrode with the same
potential as sensor, between the metal case and the
proximity detection sensor.
January 25, 2008 Document No. 001-42851 Rev. ** 3
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4. AN42851
Sensor Electrical Field Propagation
from Metal Presence Dependence
Electrical field propagation for a single sensor Figure 5. Electrical Field Propagation for a Single Sensor
configuration without metal is shown in Figure 4. Electrical Configuration with a Solid Metal Object
field propagation for a single sensor configuration with a
solid metal object is shown in Figure 5. Detection distance
Finger
is the distance where the added capacitance exceeds
some threshold values.
Sensor
The detection distance depends on the sensor electrical Detection distance
field propagation (electrical field strength). A longer
PCB
propagation distance provides a longer detection range. A
metal surface can catch a part of the electrical field and
decrease the propagation distance, that is, the detection
range.
Earth Ground Metal Surface
The influence of a metal surface on a sensor is decreased
by placing a shield electrode between the proximity
detection sensor and the metal object as shown in Figure
6. The shield electrode charges up to the same potential Figure 6. Using a Shield Electrode to Decrease the Metal
as the sensor. The shield electrode’s charge and Object’s Influence
discharge cycles are synchronous with the sensor cycles.
Finger
Note A shield electrode must always have the same
Detection
potential as the sensor.
distance
Electrical field strength from a single wire sensor with a
close metal object and a shield electrode is shown in
Figure 6.
Figure 4. Electrical Field Propagation for a Single Sensor
Configuration without a Metal Object Sensor
Shield PCB
Isolation
Electrode
Finger
Detection
distance Metal Surface
Earth Ground
Sensor
PCB
January 25, 2008 Document No. 001-42851 Rev. ** 4
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5. AN42851
Figure 8. Using Wire as Sensor
Using CSD for Proximity Sensing
Finger
The CSD UM (User Module) is selected for proximity Detection
sensing because of its ability to form a signal for the shield distance
electrode. The CSD conversion part basic block diagram
is shown in Figure 7. CSD is the standard UM for
CY8C21x34 and CY8C24x94 PSoC devices. You can get
more information about module operation in the data sheet
001-13535 - CSD User Module (UM) Data Sheet.
Shield Sensor wire
Figure 7. CSD Basic with Shield Electrode
Electrode
PCB
Vref
Vdd Reference source
Metal Surface
Earth Ground
Ph1
Sw1
Sw4
C ss Sw2 Latch
Shield CMP
VCfilt
CapSense PCB Ground to Metal Case
Rb
Ph2
Connection
Sw. cap
Sw5
Sw3
Ph2
Cx Cfilt
The PCB to metal case connection is very important for
Sigma-delta modulator proximity detector sensitivity. Some possible methods are
shown in Figure 9.
Figure 9. PSoC Board Ground to Metal Case Connection
In the CSD User Module the same phase signal used for
the precharge clock is supplied to the shielding electrode.
The difference between the sensor signal and the shield PCB PCB
Direct PCB Ground to Metal connection
PSoC PSoC
PCB Ground to Metal connection
electrode decreases as the modulator reference
decreases. The switches Sw1 and Sw4 are on in phase
Via inductor 33 uH
Ph1, the switches Sw2 and Sw5 are on in phase Ph2. The
Css is discharged in phase Ph1 phase and is charged in
Ph2 phase. Therefore, the shield electrode always has
approximately the same potential as the sensor and
guards the sensor from the metal objects’ influence.
Electrode(Bottom) Sensor(TOP)
Electrode(Bottom) Sensor(TOP)
Proximity
Proximity
Using Wire as CapSense Sensor
Using a PCB plate as a capacitance sensor is described in
Figure 6. The PCB plate is easy to manufacture but it is
not optimal for sensitivity.
Shield
Shield
Using a wire as a sensor electrode and placing the wire
and shield on the same side of PCB is illustrated in Figure
8. Using a wire as a sensor provides higher shield Solid Metal Solid Metal
Case Ground Case Ground
electrode effect and better sensitivity because the wire is
located farther from the shield electrode. The isolation Figure 9 shows the direct ground connections. A ground
space between the board and the metal body is not connection using a small inductor above several uH
needed. But the mechanical construction with a wire as a provides 50% higher sensitivity and a galvanic board to
sensor is more complicated for mass production. the metal case connection. This is not the optimal solution
for high sensitivity proximity sensing because in this case
the EMI radiation can be higher.
January 25, 2008 Document No. 001-42851 Rev. ** 5
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6. AN42851
Proximity Sensor Testing
Table 3. CSD Test Table Summary.
Some sensitivity tests are done with different test
construction configurations to provide practical Detection
B,
recommendations. The test conditions are shown in Figure Distance, cm
Ground Connection A, mm
cm
10.
10
Figure 10. CSD Test Conditions Direct ground
0 10
connection short.
A
15
Ground connection via
0 10
CSD inductor.
Wire Sensor 16
Direct ground
5 20
Shield Isolation connection
B
22
Ground connection via
5 20
inductor.
Table 4. CSD Test Results
Metal Surface
Detection distance
A, A, B,
B, cm
mm inch inch
cm inch
A=0.5…4сm, Shield to metal distance B=0…15mm. Tests
5 0.2 0 0 10 4
metal surface is 400mm x 400mm x 2mm grounded steel
5 0.2 5 2 13 5
plate.
5 0.2 10 4 17 6.7
The CSD UM parameters are shown in Table 2. The raw
2
counts are monitored using CY3240 I C-USB bridge kit. 5 0.2 15 6 22 8.6
Table 2. CSD UM Parameters No No
5 0.2 25 10
metal metal
User Module Parameter Value
10 0.4 0 0 10 4
Finger Threshold 45
10 0.4 5 2 17 6.7
Noise Threshold 30
10 0.4 10 4 20 8
Baseline Update Threshold 200
Sensors Autoreset Disabled 10 0.4 15 6 22 8.6
Hysteresis 15 No No
10 0.4 28 11
metal metal
Debounce 3
Negative Noise Threshold 20 20 0.8 0 0 10 4
Low Baseline Reset 50 20 0.8 5 2 16 6
Scanning Speed Slow
20 0.8 10 4 18 7
Resolution 15
20 0.8 15 6 21 8
Modulator Capacitor Pin P0[3]
No No
Feedback Resistor Pin P1[5] 20 0.8 26 10
metal metal
Reference ASE11
30 1.2 0 0 10 4
Ref Value 0
30 1.2 5 2 14 6
Shield Electrode Out Row_0_Output_3
30 1.2 10 4 17 6.7
30 1.2 15 6 20 7.8
Experimental results are shown in Table 3 and Table 4.
Ground connection is direct short. Wire is used as the No No
30 1.2 28 11
sensor and the sensor length is 30 cm (12 inch). The tests metal metal
are done using the palm to proximity sensor.
40 1.6 0 0 10 4
The test setup schematics are shown in Appendix 1 and a
40 1.6 5 2 15 6
PSoC project is provided along with this application note.
The detection distance estimated when added-by-palm 40 1.6 10 4 20 8
difference signal is more than five times larger than noise
40 1.6 15 6 25 10
level (peak-to-peak value). This technique matches the
No No
recommendations given in AN2394 - CapSense™ Best 40 1.6 30 12
metal metal
Practices.
January 25, 2008 Document No. 001-42851 Rev. ** 6
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7. AN42851
Figure 11. Detection Distance vs. Shield to Metal Distance Summary
30
25
Detection Distance, cm
Shield width (A)
20
0,5 cm / 0,2 inch
1 cm / 0,4 inch
15 2 cm / 0,8 inch
3 cm / 1,2 inch
4 cm / 1,6 inch
10
5
0
0 5 10 15
Shield to Metal Distance (B), mm
Summary
A simple method of proximity sensing close to a solid metal sensitivity degradation. If a multilayer PCB is used, fill the
object is to use a shield electrode with a dedicated top layer by 20 to 25% hatched shield electrode copper
mechanical construction. This allows you to build a proximity pour; the internal layers can be used for ground and signals
sensor at the metal substrate. routing.
When a shield electrode is used as a conductive plane, the If the device has a plastic case, glue the wire sensor with a
shield to metal distance greatly influences sensitivity. shield electrode on the internal plastic case side to detect
Sensitivity increases linearly with distance, increasing in the distance maximization. The recommended wire length is 10
range of 1 mm to 30 mm. cm to 20 cm, the recommended distance between the shield
and the metal is 10 mm to 20 mm.
There are several ways of building a proximity sensor with a
Note Using CY8C24x94 with Second Order Sigma-Delta
shield electrode. One way uses a double sided PCB. In this
case, the shield electrode is located at the bottom of PCB Modulator (CSDADC User Module) provides a larger
layer and the sensor is located at the top layer. The sensor detection range because of better SNR.
trace width must be around 1mm.
The proposed technique is implemented for turning on the
Another method is to place the proximity sensor on the PCB backlight lamp inside a microwave oven when you place a
where other components are installed. The best way is to palm close to front panel. Images of a microwave oven
place the sensing electrode on the board perimeter. The design example are shown in Appendix 2. For this device,
shield electrode must be located under the sensor at the the detection distance without a shield was 5 cm, with a
bottom of the PCB layer. Do not use the large ground fill shield electrode it increased to 15 cm.
area inside the proximity sensor because this causes
January 25, 2008 Document No. 001-42851 Rev. ** 7
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8. AN42851
Appendix 1
Note Sensor is connected to P0[2]. Shield electrode is connected to P2[6].
Figure 12. Single Sensor CSD Design Example Schematic
J1
Proximity Sensor
1
C1 VCC
0.1uF
R1
100
32
31
30
29
28
27
26
25
Vdd
P0[3]
P0[5]
P0[7]
P0[6]
P0[4]
P0[2]
Vss
1 24
J2
P0[1] P0[0]
2 23 1
P2[7] P2[6]
3 U1 22
Shield electrode
P2[5] P2[4]
4 CY 8C21634 21
P2[3] P2[2]
5 20
P2[1] P2[0]
6 19
P3[3] P3[2]
7 18
P3[1] P3[0]
8 17
P1[7] XRES
P1[5]
P1[3]
P1[1]
P1[0]
P1[2]
P1[4]
P1[6]
Vss
9
10
11
12
13
14
15
16
R2
20K
VCC
J3
1
C2 2
0.1uF 3
4
5
ISSP/I2C
R2 was selected for providing 70% raw counts value as recommended in CSD UM datasheet.
January 25, 2008 Document No. 001-42851 Rev. ** 8
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9. AN42851
Appendix 2
An example design of a microwave oven is shown here. Proximity sensing is limited because of the door’s metal grounded
surface. The proximity sensor inside the door turns on the lamp inside the oven. The oven door is made as a large grounded
metal surface.
Sensing
area
Microwave
Metal frame with
oven
conductive grid
Proximity
sensor PCB
under metal
Mounting
Door plastic
sensor PCB in
case
plastic case
Proximity
Shield
sensor PCB
electrode
Relay
January 25, 2008 Document No. 001-42851 Rev. ** 9
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10. AN42851
About the Authors
Name: Victor Kremin
Title: Ukraine Solution Center Team Leader
Background: Victor has more than ten years in the
embedded applications design domain
Contact: Victor.Kremin@cypressua.com
Name: Andriy Ryshtun
Title: Ukraine Solution Center Applications
Engineer
Background: Andriy has more than two years in
USC, with experience in analog
electronics, CapSense and PCB
design.
Contact: Andriy.Ryshtun@cypressua.com
Name: Vasyl Mandziy
Title: Ukraine Solution Center Applications
Engineer
Background: Vasyl is experienced in the
mathematical simulation of electrical
fields.
Contact: Vasyl.Mandziy@cypressua.com
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