电容触摸屏原理

电容触摸屏的基本原理详细介绍。

Capacitance Sensing with Sigma-Delta Method

By Victor Kremin, USC

电容触摸屏的基本原理详细介绍。

Presentation Agenda Capsense system inside; Capacitance physics basics; Measuring capacitance using sigma-delta modulator Tradeoffs in the switching cap clock selection; Handling special application cases CSD Noise Immunity Analysis; Support GUI tools; CSD best results TIPs

电容触摸屏的基本原理详细介绍。

CapSense: Basic Physical The physical sensor itself, typically a conductive pattern constructed on a PCB connected to the PSoC with an insulating cover, a flexible membrane, or a transparent overlay over a display. Electrical A method to convert the sensor capacitance to the digital format. Firmware Detection and compensation software algorithms to convert the count value into a sensor detection decision.3

电容触摸屏的基本原理详细介绍。

Capsense Operation PrinciplesSensors PSoCDecision Logic (Firmware)

Host

Capacitance Measurement

Decision Logic

Application Functions

(Hardware)

Non-CapSense Actions

(Firmware)

Capsense applications should operate reliably under various electrical noise presence, humidity, temperature and power supply changes This is achieved by combination of hardware and firmware arrangement4

电容触摸屏的基本原理详细介绍。

CapSense: Physics Fundamentals Conductive object with noncompensated charge Q creates electric field in the surrounding space; Capacitance is coefficient that is links the object potential with charge Q: =Q/C; Any conductive object has own capacitance that depends on only object geometric dimensions and surrounding material dielectric characteristics;5

电容触摸屏的基本原理详细介绍。

CapSense: Physics Fundamentals If other conductive non-charged objects are located close to the conductive object, the electric field induces the charges on these objects that reduce the electric field intensity and increasing the object capacitance. Capacitance can be easily calculated analytically only for simple electrode systems, as plate, sphere, cylinder capacitors; For arbitrary electrodes system capacitance can be found by solving Poisson differential equation using numerical methods The surrounding space dielectric constant has affect on the capacitance6

电容触摸屏的基本原理详细介绍。

CapSense: Physics Fundamentals For practical evaluations own object capacitance can be evaluated using the equation for spherical capacitor by introduction equivalent radius Re. For the rectangular box with dimensions a,b,c we have:

C=300pF

a+b+c Re = 3

Own capacitance of human is ～100÷300pF7

电容触摸屏的基本原理详细介绍。

CapSense: Theory FundamentalsCx<Cfe

Finger-electrode capacitance Cfe, values are 0.1÷10 pF; Own human capacitance Ch. Value is about 100pF; Own board capacitance Cb, 10÷300pF Human-board capacitance, Cbh, values are 1-10 pF;8

电容触摸屏的基本原理详细介绍。

CapSense: Theory Fundamentals Capsense system should be able detect small capacitance changes (0.1÷10pF) at presence large parasitic capasitance (10÷300pF); Total capacitance that is measures by capsense system is sum of parasitic Cpar and touch capaci

tances Cx:

Cs = C x + C par For better converter dynamic range utilization the parasitic capacitance need to be minimized by combination of electrical and mechanical actions9

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method Understanding The capacitance to code converter with sigmadelta modulator consists of five main parts: Sensing switching capacitor; Sigma-delta modulator; Modulator bitstream filter; Clock source; Comparator reference source

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method Understanding

Capacitance to code converter block diagram11

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method, Switched CapacitorVdd Vdd

Sensing switching capacitor Cx is Charged to the voltage source Vdd at phase Phi1 clock signal; Discharged to the voltage Vcfilt at phase Phi2 clock signal; The switching capacitor is equivalent to the resistor with value Rc

Sw1 Ph1 Sw2 Ph2

=VCfilt

Rс VCfilt

Cs

1 Rc = f sCs12

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method, ModulatorvddVC1 RсVref VCfilt

BitstreamCMP RbLatch

The sigma-delta modulator is formed by the comparator, comparator latch, integrating capacitor Cint, discharge resistor Rb;

Cint

Sw3

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method, Modulator Voltage on the bus with Cint rises due switching cap current. When this voltage reaches the comparator reference voltage, the comparator toggles Bleed resistor, RB, starts discharges bus via switch SW3. When the bus voltage drops below the reference voltage, the comparator switches again, disabling filter charge bleed;

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method, ModulatorvddVC1 RсVref VCfilt

BitstreamCMP RbLatch

The modulator operates as switching capacitor current to the duty cycle converter; The duty cycle is linearly proportional to the sensing capacitance value:

Cint

Sw3

d mod

Vdd = C x f s Rb 1 V Ref

电容触摸屏的基本原理详细介绍。

Sigma-Delta Method, SPICE Simulations

Increasing sensing Cs causes modulator duty cycle rising16