QT115A-ISG

Touch Sensors .................................................................................................................... Design Guide 10620E–AT42–09/09 ii 10620E–AT42–09/09 Touch Sensors Design Guide Table of Contents Getting Started ..............................................................................................................ix Section 1 Introduction To Sensor Design................................................................................... 1-1 1.1 Introduction ........................................................................................................................ 1-1 1.2 Self-capacitance and Mutual-capacitance Type Sensors .................................................. 1-1 1.3 Dimension Groups ............................................................................................................. 1-2 1.4 Some Important Theory ..................................................................................................... 1-2 Section 2 General Advice........................................................................................................... 2-1 2.1 Charge Transfer................................................................................................................. 2-1 2.2 Components....................................................................................................................... 2-3 2.3 2.2.1 Cs Capacitor ........................................................................................................ 2-3 2.2.2 Series Resistors................................................................................................... 2-3 2.2.3 Voltage Regulator ................................................................................................ 2-3 2.2.4 Component Placement ........................................................................................ 2-3 Materials ............................................................................................................................ 2-4 2.3.1 Substrates ........................................................................................................... 2-4 2.3.2 Electrode and Interconnection Materials ............................................................. 2-4 2.3.3 Front Panel Materials........................................................................................... 2-5 2.3.4 PCB to Panel Bonding ......................................................................................... 2-6 2.4 Nearby LEDs...................................................................................................................... 2-7 2.5 Electrostatic Discharge Protection ..................................................................................... 2-8 Section 3 Self-capacitance Zero-dimensional Sensors.............................................................. 3-1 3.1 Introduction ........................................................................................................................ 3-1 3.2 Planar Construction ........................................................................................................... 3-1 Touch Sensors Design Guide 3.2.1 Introduction .......................................................................................................... 3-1 3.2.2 Electrode Shapes ................................................................................................ 3-2 3.2.3 Ground Loading ................................................................................................... 3-3 3.2.4 Interconnection .................................................................................................... 3-4 3.2.5 Illumination Effects............................................................................................... 3-5 3.2.6 Floating Conductive Items ................................................................................... 3-6 3.2.7 Conductive Paints................................................................................................ 3-7 iii 10620E–AT42–09/09 Table of Contents (Continued) 3.3 Non-planar Construction .................................................................................................... 3-7 3.3.1 Printed Electrode Method .................................................................................... 3-8 3.3.2 Philipp Spring™ Method ....................................................................................... 3-8 3.3.3 Secondary Substrate Method .............................................................................. 3-9 3.3.4 Ground Loading ................................................................................................. 3-10 3.3.5 Illumination Effects............................................................................................. 3-10 3.3.6 Floating Conductive Items ................................................................................. 3-10 3.3.7 Conductive Paints.............................................................................................. 3-10 Section 4 Mutual-capacitance Zero-dimensional Sensors ......................................................... 4-1 4.1 Introduction ........................................................................................................................ 4-1 4.2 Planar Construction ........................................................................................................... 4-1 4.3 4.2.1 Introduction .......................................................................................................... 4-1 4.2.2 X and Y Electrodes .............................................................................................. 4-2 4.2.3 Ground Loading ................................................................................................... 4-6 4.2.4 Interconnection .................................................................................................... 4-7 4.2.5 Illumination Effects............................................................................................... 4-9 4.2.6 Floating Conductive Items ................................................................................... 4-9 4.2.7 Conductive Paints................................................................................................ 4-9 4.2.8 Transparent Y Electrodes .................................................................................... 4-9 Non-Planar Construction.................................................................................................. 4-10 4.3.1 Introduction ........................................................................................................ 4-10 4.3.2 Flooded-X Two-layer Method ............................................................................ 4-10 4.3.3 Spring Method ................................................................................................... 4-12 4.3.4 Adapting the Planar Construction For Distribution Across Two Layers ............. 4-13 4.3.5 Ground Loading ................................................................................................. 4-13 4.3.6 Illumination Effects............................................................................................. 4-13 4.3.7 Floating Conductive Items ................................................................................. 4-14 4.3.8 Conductive Paints.............................................................................................. 4-14 Section 5 Self-capacitance One-dimensional Sensors .............................................................. 5-1 iv 10620E–AT42–09/09 5.1 Introduction ........................................................................................................................ 5-1 5.2 General Advice .................................................................................................................. 5-1 5.2.1 Ground Loading ................................................................................................... 5-1 5.2.2 Interconnection .................................................................................................... 5-1 5.2.3 Hand Shadow Effect ............................................................................................ 5-2 5.2.4 Floating Conductive Items ................................................................................... 5-2 5.2.5 Conductive Paints................................................................................................ 5-2 Touch Sensors Design Guide Table of Contents (Continued) 5.3 5.4 Typical Spatially Interpolated Method ................................................................................ 5-3 5.3.1 Introduction .......................................................................................................... 5-3 5.3.2 Small Slider Or Wheel ......................................................................................... 5-3 5.3.3 Medium/Large Slider Or Wheel ........................................................................... 5-4 Typical Resistively Interpolated Method ............................................................................ 5-6 5.4.1 Introduction .......................................................................................................... 5-6 5.4.2 Medium/Large Slider Or Wheel ........................................................................... 5-6 Section 6 Mutual-capacitance One-dimensional Sensors.......................................................... 6-1 6.1 Introduction ........................................................................................................................ 6-1 6.2 General Advice .................................................................................................................. 6-1 6.3 6.4 6.2.1 QMatrix Channels ................................................................................................ 6-1 6.2.2 Ground Loading ................................................................................................... 6-1 6.2.3 Floating Conductive Items ................................................................................... 6-1 6.2.4 Conductive Paints................................................................................................ 6-1 Typical Spatially Interpolated Method ................................................................................ 6-2 6.3.1 Introduction .......................................................................................................... 6-2 6.3.2 One-Layer Small Slider Or Wheel ....................................................................... 6-2 6.3.3 One-Layer Medium/Large Slider Or Wheel ......................................................... 6-5 6.3.4 Two-Layer Small Slider Or Wheel ....................................................................... 6-6 6.3.5 Two-layer Medium/Large Slider Or Wheel........................................................... 6-8 Typical Resistively Interpolated Method .......................................................................... 6-10 6.4.1 Introduction ........................................................................................................ 6-10 6.4.2 One-Layer Medium/Large Slider Or Wheel ....................................................... 6-10 6.4.3 Two-Layer Medium/Large Slider Or Wheel ....................................................... 6-11 Appendix A Glossary of Terms ............................................................................................................ A-1 Touch Sensors Design Guide v 10620E–AT42–09/09 vi 10620E–AT42–09/09 Touch Sensors Design Guide Getting Started Start by reading the introductory sections in Section 1, paying particular attention to:  Section 1.2 “Self-capacitance and Mutual-capacitance Type Sensors”  Section 1.3 “Dimension Groups”  Section 1.4 “Some Important Theory” Next, read the general advice on sensor design in Section 2:  Section 2.1 “Charge Transfer”  Section 2.2 “Components”  Section 2.3 “Materials”  Section 2.4 “Nearby LEDs”  Section 2.5 “Electrostatic Discharge Protection” Now use the flow diagram below to determine which further sections in this design guide are relevant to your project. Type of Sensor       ? What type of sensor are you designing (how many dimensions does it have; see Section 1.3)? Key      The sensor has zero dimensions (a single point of contact) Go to “Zero Dimensions” on page viii Slider or wheel      The sensor has one dimension (linear movement along one axis) Go to “One Dimension” on page ix Touchscreen      Touch Sensors User Guide Two-dimensional sensors are not covered by this guide. For information on designing twodimensional sensors, please contact Atmel’s Touch Technology Division. vii 10620E–AT42–09/09 Zero Dimensions Charge-transfer Technology  ?      Which charge-transfer circuit technology does the sensor use? One direct connection to the sensor controller chip (e.g. QTouch)     The circuit is a self-capacitance circuit (see Section 3)  Location of Electrodes and Traces  ?      How are the electrodes located with respect to the related circuitry (traces, LEDs and other components)? The electrodes and traces are fabricated on the same plane of the substrate      The design is planar Read Section 3.2 “Planar Construction” The electrodes are located on the panel’s surface and the rest of the circuitry is on a separate circuit board      The design is non-planar Read Section 3.3 “Non-planar Construction” Two connections (X transmit line and Y receive line) to the sensor controller chip (e.g. QMatrix)     The circuit is a mutual-capacitance circuit (see Section 4)  Location of Electrodes and Traces  ?      How are the electrodes located with respect to the related circuitry (traces, LEDs and other components)? The electrodes and traces are fabricated on the same plane of the substrate      The design is planar Read Section 4.2 “Planar Construction” The electrodes are located on the panel’s surface and the rest of the circuitry is on a separate circuit board      viii 10620E–AT42–09/09 The design is non-planar Read Section 4.3 “Non-Planar Construction” Touch Sensors Design Guide One Dimension Charge-transfer Technology       ? Which charge-transfer circuit technology does the sensor use? One direct connection to the sensor controller chip (e.g. QTouch)     The circuit is a self-capacitance circuit (see Section 5.1)  Introduction and General Advice      Read Section 5.1 “Introduction” Read Section 5.2 “General Advice” Go To Go to “One Dimension: Self-capacitance Sensors” on page x Two connections (X transmit line and Y receive line) to the sensor controller chip (e.g. QMatrix)     The circuit is a mutual-capacitance circuit (see Section 6.1)  Introduction and General Advice      Read Section 6.1 “Introduction” Read Section 6.2 “General Advice” Go To Go to “One Dimension: Mutual-capacitance Sensors” on page xi Touch Sensors Design Guide ix 10620E–AT42–09/09 One Dimension: Self-capacitance Sensors Size of Slider/Wheel  ?      What size slider or wheel and how many channels available (3 or more than 3?) Small slider (21 – 26 mm long) with 3 channels      Use a spatially interpolated small slider or wheel Read Section 5.3.2 “Small Slider Or Wheel” Small wheel (12 – 20 mm diameter) with 3 channels      Use a spatially interpolated small slider or wheel Read Section 5.3.2 “Small Slider Or Wheel” Medium/large slider (20 – 60 mm long) with 3 channels      Use a spatially interpolated medium/large slider or wheel Read Section 5.3.3 “Medium/Large Slider Or Wheel” Note: If the slider is at the upper end of the scale, consider using more channels with a resistively interpolated slider, as this may be easier to construct Read Section 5.4.2 “Medium/Large Slider Or Wheel” Medium/large wheel (20 – 60 mm diameter) with 3 channels      Use a spatially interpolated medium/large slider or wheel Read Section 5.3.3 “Medium/Large Slider Or Wheel” Note: If the wheel is at the upper end of the scale, consider using more channels with a resistively interpolated wheel, as this may be easier to construct Read Section 5.4.2 “Medium/Large Slider Or Wheel” Large slider or more then 3 channels      Use a resistively interpolated medium/large slider or wheel Read Section 5.4.2 “Medium/Large Slider Or Wheel” Large wheel or more then 3 channels      x 10620E–AT42–09/09 Use a resistively interpolated medium/large slider or wheel Read Section 5.4.2 “Medium/Large Slider Or Wheel” Touch Sensors Design Guide One Dimension: Mutual-capacitance Sensors Size of Slider/Wheel       ? What size slider or wheel? Small slider (n x 6 mm – n x 8 mm long, 5 – 50 mm high)     Go to “One Dimension: Mutual-capacitance Sensors – Small Slider or Wheel” on page xii  Small wheel (15 – 21 mm circumference)     Go to “One Dimension: Mutual-capacitance Sensors – Small Slider or Wheel” on page xii  Medium/large slider (n x 6 mm – n x 8 mm long, more than 50 mm high)     Go to “One Dimension: Mutual-capacitance Sensors – Medium/Large Slider or Wheel” on page xiii  Medium/large wheel (more than 21 mm circumference)     Go to “One Dimension: Mutual-capacitance Sensors – Medium/Large Slider or Wheel” on page xiii  Large slider     Go to “One Dimension: Mutual-capacitance Sensors – Large Slider or Wheel” on page xiv  Large wheel     Go to “One Dimension: Mutual-capacitance Sensors – Large Slider or Wheel” on page xiv  Touch Sensors Design Guide xi 10620E–AT42–09/09 One Dimension: Mutual-capacitance Sensors – Small Slider or Wheel Small Slider or Wheel     Use a spatially interpolated small slider or wheel (see Section 6.3)  Number of Layers  ?      How many layers (one or two)? One layer     Read Section 6.3.2 “One-Layer Small Slider Or Wheel”  Two layers     Read Section 6.3.4 “Two-Layer Small Slider Or Wheel”  xii 10620E–AT42–09/09 Touch Sensors Design Guide One Dimension: Mutual-capacitance Sensors – Medium/Large Slider or Wheel Medium/Large slider or Wheel   Use a spatially interpolated medium/large slider or wheel (see Section 6.3)    Number of Layers       ? How many layers (one or two)? One layer     Read Section 6.3.3 “One-Layer Medium/Large Slider Or Wheel”  Two layers      Touch Sensors Design Guide Add extra channels and use a resistively interpolated design Read Section 6.4.3 “Two-Layer Medium/Large Slider Or Wheel” (see also Section 6.4.1) xiii 10620E–AT42–09/09 One Dimension: Mutual-capacitance Sensors – Large Slider or Wheel Large Slider or Wheel     Add extra channels and use a resistively interpolated design (see Section 6.4 )  Number of Layers  ?      How many layers (one or two)? One layer     Read Section 6.4.2 “One-Layer Medium/Large Slider Or Wheel” (see also Section 6.4.1)  Two layers     Read Section 6.4.3 “Two-Layer Medium/Large Slider Or Wheel” (see also Section 6.4.1)  xiv 10620E–AT42–09/09 Touch Sensors Design Guide Section 1 Introduction To Sensor Design 1.1 Introduction The process for designing products that use touch controls is a complex process with many decisions to be made, such as what materials will be used in their construction and how the mechanical and electrical requirements will be met. Key to this process is the design of the actual sensors (specifically keys, sliders, wheels and touchscreens) that form the interface with the user. Sensor design is often considered a “black art”; the distributed nature of the electric fields between the sensor and its electrical environment can make simple “lumped element” approximations of sensor behavior misleading at best. Nevertheless, by following a few essential rules, it is possible to produce a resilient design that ensures that the sensors will operate in a reliable and consistent manner. This design guide describes the rules that can be used to create sensor patterns on PCBs or other conductive material, such as Indium Tin Oxide (ITO). There are, of course, many possible configurations for such sensors, and this guide cannot be exhaustive; however, it will aid in the initial selection and construction of sensors for touch-enabled products, and should provide an excellent starting point. You should also refer to QTAN0032, Designing Products with Atmel Capacitive Touchscreen ICs for an overview on designing capacitive touchscreens. 1.2 Self-capacitance and Mutual-capacitance Type Sensors Atmel ® touch controllers allow for two families of sensors, each using one of two charge-transfer capacitive measurement styles:  Self-capacitance type sensors A self-capacitance type sensor has only one direct connection to the sensor controller. These sensors tend to emit electric fields in all directions, and as such are quite non-directional. They can work with and without an overlying panel, although a panel is always recommended for electrostatic discharge (ESD) reasons (1). This type of sensor is suitable for implementating sensors for use with QTouch™ sensor controllers.  Mutual-capacitance type sensors A mutual-capacitance type sensor has two connections to two parts of the sensor: an X (transmit) electrode, and a Y (receive) electrode. The mutual capacitance from X to Y is measured by the sensor controller. Because of the close-coupled nature of the fields with this type of sensor, it is only suitable for use when bonded to an overlying panel so that no significant air gaps or bubbles are present; the overlying panel forms an essential conduit for the field from X to Y. This type of sensor is suitable for for implementating sensors for use with QMatrix™ sensor controllers. Background information on Atmel’s capacitive sensing methods can be found at www.atmel.com. 1. Direct contact with the sensor carries an elevated risk of ESD damage for the control chip. Touch Sensors Design Guide 1-1 10620E–AT42–09/09 Introduction To Sensor Design 1.3 Dimension Groups In addition to the self- and mutual-capacitance types described in Section 1.2, sensors can also be split into three groups, depending on the number of dimensions they use (see Figure 1-1):  Zero-dimensional sensors A zero-dimensional sensor is one that represents a single point of contact. The typical implementation of a zero-dimensional sensor is a key.  One-dimensional sensors A one-dimensional sensor is one that detects the linear movement of a finger during touch (that is, along a single axis). Typical implementations of one-dimensional sensors are sliders and wheels.  Two-dimensional sensors  A two-dimensional sensor is one that detects the movement of a finger during touch along two axes. Typical implementations of two-dimensional sensors are touchscreens and touchpads. When considering the design of a sensor, you will need to consider both the sensor type and the dimension group, making six possible combinations in total. The combinations for zero-dimensional and one-dimensional sensors are discussed individually in Section 3 to Section 6; two-dimensional sensors are not covered by this guide. For information on designing two-dimensional sensors, please contact Atmel’s Touch Technology Division. Figure 1-1. Sensor Dimensions Zero Dimensional Sensors Single point of contact One Dimensional Sensors Movement in one direction Two Dimensional Sensors Movement in two directions Key Slider Touchscreen/ Touchpad Wheel 1.4 Some Important Theory You will need to be aware of the following terms when reading this document: 1-2 10620E–AT42–09/09  Cx: The electrode’s natural capacitance, separate from any parasitic capacitance   r: The relative dielectric constant of the overlying panel material (see Figure 1-2 on page 1-3) 0: The capacitance per meter of free space, defined as 8.85 x 10-12 F/m  T: The thickness of the overlying panel in meters (see Figure 1-2)  A: The area of the touched region in square meters (see Figure 1-2)  Cp: Any parasitic capacitance added in parallel with Cx  SNR: Signal to noise ratio: a measure of the quality of the capacitive measurement Touch Sensors Design Guide Introduction To Sensor Design Capacitance (C) is defined in Equation 1-1. Equation 1-1. Capacitance 0  r  A C = ------------------------T It should therefore be clear that thinner panels and higher dielectric constant materials yield higher capacitance change during touch and hence a higher gain and a better SNR. Section 2.3.3 “Front Panel Materials” on page 2-5 discusses the thickness of the panel and its effect on the SNR. Figure 1-2. Touchscreen Panel Touch Area (A) Sensor Behind Panel T Overlying Panel (er) In the context of capacitive sensors, SNR is defined as in Equation 1-2. (1) See Figure 1-3 for definitions of the touch signal levels. Equation 1-2. SNR SNR(dB) = 20Log( TouchStrength / NoiseTouchedRMS100) TouchStrength = SignalTouchedAVG100 - SignalUntouchedAVG100 n = 99  NoiseTouched RMS100 =  Signal  n  – SignalTouched AVG100  2 n=0 --------------------------------------------------------------------------------------------------------------------100 where:  AVG100 means the simple numeric average of 100 data points, typically taken evenly spaced over a period of 2 seconds.  RMS100 means the root-mean-square of 100 data points, typically taken evenly spaced over a period of 2 seconds, using the AVG100 figure as a baseline. 1. Note that this definition uses the RMS noise present during touch, as this is the worst case. Touch Sensors Design Guide 1-3 10620E–AT42–09/09 Introduction To Sensor Design Figure 1-3. Touch Signal Levels Signal Untouched Touch Strength Touched Signal Touched 1-4 10620E–AT42–09/09 Touch Sensors Design Guide Section 2 General Advice 2.1 Charge Transfer Atmel’s capacitive sensors work on a principle called charge transfer. This uses a switched capacitor technique to assess relative changes in a sensor’s capacitance as it is touched. Charge transfer works by applying a voltage pulse to series connection of the unknown capacitance Cx and a charge integrator capacitor Cs. By repeating the pulse multiple times, a high resolution measurement system is realized that can detect changes in capacitance of just a few femtofarads (1). In order to obtain stable and repeatable results, it is important that the voltage pulse is allowed to settle properly and hence transfer all the charge into Cx and Cs (see Figure 2-2 on page 2-2). Because Cx and Cs are normally connected with some amount of series resistance, the RC time constants so formed will tend to slow down this settling process. It is therefore important that when designing a capacitive touch system that the amount of series resistance is kept in mind in combination with the size of Cx (the sensor’s capacitance). Series resistance is normally deliberately introduced to improve electromagnetic interference (EMI) and ESD behavior (see the device datasheets for recommended series resistors). For some designs, significant extra resistance can be introduced because of the resistivity of the tracks connecting the sensor, or the resitivity of the electrode material itself. This is normally only true for designs using material like Indium Tin Oxide (ITO), Orgacon™ (2) or Carbon with high /sq values. However, there are situations where adding deliberate extra series resistance can be beneficial, most notably in designs with difficult ESD conditions or where emissions must be controlled to very low levels. A 10 k series resistor can be a good choice. In either case, RC time constants can degrade the charge-transfer process so some precautions must be taken. For all designs, it is good practice to measure the settling time of the charge-transfer pulses to make sure they are fast enough to work reliably. This can be done by observing the pulses using an oscilloscope and coupling the scope probe to the sensor electrode capacitively (3) using a small coin on top of the overlying panel, or using a small piece of copper tape instead (see Figure 2-1 on page 2-2). The charge pulses should have substantially flat tops, that is the voltage has settled before the pulse ends (see Figure 2-2 on page 2-2). 1. One femtofarad is 1/1000 of a picofarad. 2. Orgacon™ is a trademark of Agfa-Gevaert Group. Orgacon is a printable conductive ink using a PEDOT polymer base. Care should be taken when assessing this material’s suitability for sensor construction due to the high starting resistivity and the fact that this resistivity is known to increase over life depending on its exposure to UV, heat and moisture (and any other oxidising substance). Consult Agfa before making any design decisions. 3. Probing the sensor directly will add the capacitance of the probe and so give an unrealistic wave shape. Touch Sensors Design Guide 2-1 10620E–AT42–09/09 General Advice Figure 2-1. Measuring the Charge-transfer Pulses Figure 2-2. Good and Bad Charge Pulses Bad Marginal Good Excellent As a rule, the overall RC time constant of each sensor should be reduced as far as possible, while trying to preserve at least a 1 k series resistor close to the sensor chip. A good rule-of-thumb is given in Equation 2-3. Equation 2-3. Rule Of Thumb for RC Time Constant Rseries_total * (Cx + Cp)