Proximity Detection in the Presence of Metal Objects


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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.

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Proximity Detection in the Presence of Metal Objects

  1. 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 GET FREE SAMPLES HERE 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 [+] Feedback
  2. 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 [+] Feedback
  3. 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 [+] Feedback
  4. 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 [+] Feedback
  5. 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 [+] Feedback
  6. 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 [+] Feedback
  7. 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 [+] Feedback
  8. 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 [+] Feedback
  9. 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 [+] Feedback
  10. 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: 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: Name: Vasyl Mandziy Title: Ukraine Solution Center Applications Engineer Background: Vasyl is experienced in the mathematical simulation of electrical fields. Contact: PSoC is a registered trademark of Cypress Semiconductor Corp. quot;Programmable System-on-Chip,quot; PSoC Designer, and PSoC Express are trademarks of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are the property of their respective owners. Cypress Semiconductor 198 Champion Court San Jose, CA 95134-1709 Phone: 408-943-2600 Fax: 408-943-4730 © Cypress Semiconductor Corporation, 2008. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. This Source Code (software and/or firmware) is owned by Cypress Semiconductor Corporation (Cypress) and is protected by and subject to worldwide patent protection (United States and foreign), United States copyright laws and international treaty provisions. Cypress hereby grants to licensee a personal, non-exclusive, non-transferable license to copy, use, modify, create derivative works of, and compile the Cypress Source Code and derivative works for the sole purpose of creating custom software and or firmware in support of licensee product to be used only in conjunction with a Cypress integrated circuit as specified in the applicable agreement. Any reproduction, modification, translation, compilation, or representation of this Source Code except as specified above is prohibited without the express written permission of Cypress. Disclaimer: CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Cypress reserves the right to make changes without further notice to the materials described herein. Cypress does not assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress’ product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. Use may be limited by and subject to the applicable Cypress software license agreement. January 25, 2008 Document No. 001-42851 Rev. ** 10 [+] Feedback