电容触摸屏原理

Cs = C x + C par
•For better converter dynamic range utilization the parasitic capacitance need to be minimized by combination of electrical and mechanical actions
Cint
Sw3
13
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;
C=300pF
a+b+c Re = 3
• Own capacitance of human is ∼100÷300pF
7
CapSense: Theory Fundamentals
Cx<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;
Sigma-Delta Method, Bitsteram Scope
No touch
Touch
•Modulator duty cycle change under touch conditions; •Individual periods can have different pulse width due noise presence, but averaged over conversion interval duty cycle remains very stable
14
Sigma-Delta Method, Modulator
vdd
VC1 Rс
Vref VCfilt
Bitstream
CMP Rb
Latch
•The modulator operates as switching capacitor current to the duty cycle converter; •The duty cycle is linearly proportional to the sensing capacitance value:
10
Sigma-Delta Method Understanding
Capacitance to code converter block diagram
11
Sigma-Delta Method, Switched Capacitor
Vdd 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
4
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 ϕ with charge Q: ϕ=Q/C; • Any conductive object has own capacitance that depends on only object geometric dimensions and surrounding material dielectric characteristics;
(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 arrangement
5
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 capacitance
9
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
Cint
Sw3
d mod
Vdd = C x f s Rb − 1 V Ref
15
Sigma-Delta Method, SPICE Simulations
Increasing sensing Cs causes modulator duty cycle rising
16
6
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:
3
Capsense Operation Principles
Sensors PSoC
Decision Logic (Firmware)
Host
Capacitance Measurement
Decision Logic
Application Functions
(Hardware)
Non-CapSense Actions
Capacitance Sensing with Sigma-Delta Method
By Victor Kremin, USC
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
Sw1 Ph1 Sw2 Ph2
=
VCfiຫໍສະໝຸດ Baidut
Rс VCfilt
Cs
1 Rc = f sCs
12
Sigma-Delta Method, Modulator
vdd
VC1 Rс
Vref VCfilt
Bitstream
CMP Rb
Latch
•The sigma-delta modulator is formed by the comparator, comparator latch, integrating capacitor Cint, discharge resistor Rb;
17
Sigma-Delta Method, Digital Filter
•The digital filter converts the single-bit modulator bitstream in the multi-bit digital values; •The Neutron implementation uses single integration, counter-type digital filter; •The Radon implementation can use double integration filter (supported by hardware decimator)
8
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 capacitances Cx:
2
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.