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Proximity Design Guide
1. Introduction
The ability to interact with an electronic device, while not physically making contact
with the device, has fascinated designers and users alike for years. There are a
variety of ways to implement this type of technology: IR, magnetic, optical, ultrasonic,
and capacitive. Each of these technologies comes with its own unique benefits and
trade-offs.
Capacitive sensing technologies have the general advantage of achieving very
reliable proximity detection with low power, low cost, and relatively easy design.
®
Atmel
has the advantage of being able to do all of these with ranges to over
250 mm.
Capacitive sensing generates an electric field, or E-field, as part of the sensing
process; this applies to both self-capacitance (QTouch
mutual-capacitive sensors (QMatrix
the sensitivity of the standard capacitive sensing circuitry or algorithm.
This can be applied to touch sensor designs using an Atmel application-specific
device, or an Atmel microcontroller with the appropriate QTouch Library linked to your
application code.
Adding proximity detection to a design provides many benefits:
• A more intuitive user interface
• Power savings – The ability to have an application start/stop based on a user's
proximity to a device. The device can stay in sleep mode until a presence is
detected, reducing power consumption and extending battery life.
• Make the application interact with human presence:
– Mobile phone – use proximity sensing to reduce RF power when a mobile
phone is placed near a person's head
– Heating controller – use proximity sensing to activate control panel backlighting
when a person approaches
This application note gives advice about the current capacitive touch technologies
offered by Atmel, with respect to their usefulness as proximity sensors, and a
mechanism to contain and control the E-field generated by the sensors.
®
and QTouchADC
®
). Proximity detection is achieved by adjusting
Proximity
Design Guide
Application Note
QTAN0087
®
) and
10760B–AT42–03/12
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Related Manuals for Atmel QTAN0087
Summary of Contents for Atmel QTAN0087
- Page 1 This can be applied to touch sensor designs using an Atmel application-specific device, or an Atmel microcontroller with the appropriate QTouch Library linked to your application code. Adding proximity detection to a design provides many benefits: •...
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Page 2: Sensing Technology
2. Sensing Technology Introduction The measurement circuit uses the Atmel patented charge-transfer (QT) technology to measure changes to a sensor as an object approaches. There are three sensing technologies available from Atmel: • QTouch – Self capacitance, measured using Vih of the sensor input/output (IO) pin •... - Page 3 Proximity Design Guide QTouch and QTouchADC QTouch and QTouchADC devices charge a sense electrode of unknown capacitance to a known potential. The electrode is typically a copper area on a printed circuit board (PCB). The resulting charge is transferred into a measurement circuit. By measuring the charge after one or more charge-and-transfer cycles, the capacitance of the sense plate can be determined.
- Page 4 QMatrix QMatrix uses the mutual capacitance of a sensor. The E-field associated with mutual capacitance is coupled closely to the X and Y portions of the sensors. The E-field couples to an object in close proximity to the sensor's E-field. Figure 2-4.
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Page 5: Proximity Sensing
Proximity Design Guide 3. Proximity Sensing QTouch Figure 3-1. QTouch Proximity Sensing QTouch proximity sensors are touch keys deliberately made over-sensitive. This can be accomplished using a combination of the following: • Make the sensing electrode larger • Increase the value of the sampling capacitor Cs •... - Page 6 QTouchADC Figure 3-2. QTouchADC Proximity Sensing Similar to QTouch, QTouchADC proximity sensors are touch keys made over-sensitive. This can be accomplished using a combination of the following: • Make the sensing electrode larger • Increase sampling resolution or over-sampling rate •...
- Page 7 Proximity Design Guide QMatrix Figure 3-3. QMatrix Proximity Sensing Proximity sensing with QMatrix can be accomplished using a combination of the following: • Make the sensing electrode larger • Increase the spacing (gap) between the X and Y portions of the sensor •...
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Page 8: Proximity Sensors
4. Proximity Sensors Sensors for proximity sensing are not as critical as is generally assumed. Sensors can be as simple as a single piece of wire hanging from the sensor or a simple trace running along a horizontal or vertical section of a PCB. When designing a proximity sensor the following points should be kept in mind: •... - Page 9 Proximity Design Guide Figure 4-2. Loop Sensor, Made Using 12.7 mm Copper Foil Tape Figure 4-3. Eleven-key Self-capacitance Layout Figure 4-3 any of the keys can be made to act as a proximity sensor. 10760B–AT42–03/12...
- Page 10 5. Grounding Proximity sensors are highly sensitive to ground loading as it adds directly to the natural capacitance of the sensor, resulting in a decrease of the sensitivity (gain) of the sensor. Ground, in this case, is anything that looks like an AC ground from the sensor view. It is recommended to keep grounding as far away from the proximity sensor and the connecting trace of the sensor - this connecting trace is essentially an extension of the proximity sensor.
- Page 11 Proximity Design Guide Figure 5-1. Example of Poor Routing 1 If we continue to follow the proximity sensor trace back to the controller, it takes a left turn and runs directly on top of a ground plane trace and parallel to another signal trace. Both the ground plane and signal trace running parallel to the proximity sensor trace will absorb some of the sensor signal resulting in a slight loss of detection range.
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Page 12: Driven Shield (E-Field Directivity)
360° of the device. Basic physics states that electric fields of the same polarity repel each other. This is also true with the Atmel capacitive-sensing technologies. The trick is how to control this E-field. As seen in... - Page 13 Proximity Design Guide Figure 6-2. E-field Directivity – Front The driven shield and proximity sensor must be electrically insulated from each other. If the proximity sensor is located on one surface of a PCB, the driven shield must be placed on a surface directly below the proximity sensor.
- Page 14 Figure 6-4, a 1.2 m length of RGS-178 coaxial cable is used to connect the drive electronics to the proximity sensor. The internal conductor is used as the connection between the drive electronics and the proximity sensor. The coaxial cable braided shield is driven with a buffered version (same amplitude, phase, and polarity) as that of the proximity sensor.
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Page 15: Maximum Proximity Detection Range Considerations
Proximity Design Guide 7. Maximum Proximity Detection Range Considerations There are a few items to consider when designing for maximum proximity detection range: • It is important to keep in mind the effects of noise when designing proximity sensing circuits. There will be a trade-off between proximity detection range and overall system stability or immunity to noise. -
Page 16: Revision History
References • QTAN0080 – Touch Sensors Design Guide Revision History Revision No. History Revision AX – November 2011 Initial release of document. Added “Grounding” “Maximum Proximity Revision BX – March 2012 Detection Range Considerations” sections. Proximity Design Guide 10760B–AT42–03/12... - Page 17 Proximity Design Guide Notes 10760B–AT42–03/12...
- Page 18 Disclaimer: The information in this document is provided in connection with Atmel products. No license, express or implied, by estoppel or otherwise, to any intellectual property right is granted by this document or in connection with the sale of Atmel products. EXCEPT AS SET FORTH IN ATMEL’S TERMS AND CONDITIONS OF SALE LOCATED ON ATMEL’S WEB SITE, ATMEL ASSUMES NO LIABILITY WHATSOEVER AND DISCLAIMS ANY EXPRESS, IMPLIED OR...
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