Projectile style electric capacity feels screen to be able to locate the hand points to the position of kiss screen accurately, it comes through measuring the little change of capacitance differentiate finger position. In application of screen of this kind of feeling, the issue of a crucial design that needs a consideration is electromagnetism interference (EMI) is right the influence of systematic performance. The function that interference causes drops to design generation possibly to touching screen adverse effect, the article will undertake discuss and be analyticed to these interference sources.

 

Electric capacity of 1 projectile style feels screen structure

 

Setup of typical sensor of projectile style electric capacity is in glass or lower part of plastic cover board. The graph shows those who be sensor of double deck type 1 times to simplify side view. Blast off (Tx) and receive (Rx) electrode receives transparent oxidation indium stannum repeatedly (ITO) , form alternate matrix, every Tx-Rx node has electric capacity of a feature. Tx ITO is located in Rx ITO lower part, by film of a polymer or optical glue (OCA) is separated. If plan institute is shown, the direction of Tx electrode from left to right, the direction of Rx electrode from paper is pointed to outside paper inside.

 

Graph 1: Sensor structure is referenced.

 

Principle of 2 sensor job

 

Let our of short duration take no account of interference element, will undertake an analysis to feeling the job of screen: The hand index that operates personnel says to lie terrestrial electricity situation. Rx is maintained to be in through feeling screen controller circuit terrestrial electricity situation, and Tx voltage is alterable. Tx voltage of change makes electric current passes Tx-Rx capacitance. A Rx that has balanced carefully integrated circuit, keep apart and measure the charge that enters Rx, the ” of capacitance of “ each other of the Tx of charge delegate join that measures and Rx.

 

3 sensor condition: Did not feel

 

Graph 2 displayed the sketch map of lines of force that did not feel condition to fall. Below the circumstance that does not have finger lay a finger on, tx-Rx lines of force occupied the quite large space inside cover board. Brim lines of force projects besides electrode structure, accordingly, ” of electric capacity of term “ projectile style by and come.

 

Graph 2: Did not feel the lines of force below condition.

 

4 sensor condition: Feeling

 

When finger feels cover board, lines of force is formed between Tx and finger, these lines of force replaced many Tx-Rx brim magnetic field, if pursue,3 are shown. Carry this kind of kind, finger feeling reduced electric capacity of Tx-Rx each other. The electric capacity that charge gauges circuit identifying to give change (△ C) , detect thereby the finger of Tx-Rx node upper part. Through undertaking to all crossing of Tx-Rx matrix △ C is measured, the feeling of attainable whole face plate distributings graph.

 

Graph 3 still show another main effect: The capacitance coupling between finger and Rx electrode. Adopt this method, report is disturbed may coupling arrives Rx. The finger of certain level – Rx coupling is inevitable.

 

Graph 3: Feel the lines of force below condition.

 

5 special technical terms

 

The interference that projects style electric capacity touchs screen passes the parasitism method coupling that is aware of not easily to arise. ” of term “ ground can be used at pointing to the referenced node of dc road already normally, can use at showing low impedance receives the earth repeatedly again: Both is not same term. Actually, to equipment of portable feeling screen, this differentia does not cause the prime cause that feels coupling interference just about. To clarify and avoid promiscuous, we use the following term to evaluate feeling screen to disturb.

 

•Earth(ground) : Be linked together with the earth, for example, through 3 aperture the ground wire of electrical outlet of alternating current source receives the earth repeatedly.

) of •Distributed Earth(distributed ground: The object joins to the capacitance of the earth.

•DC Ground(sheds ground) continuously: The dc reference node of portable equipment.

) of •DC Power(direct current source: The batteries voltage of portable equipment. Perhaps output voltage with charger of portable equipment connective, for example the 5V Vbus in USB interface charger.

•DC VCC(sheds) of VCC power source continuously: For portable equipment electronic parts (include LCD and feeling screen controller) the stable voltage of power supply.

0 lines of •Neutral() : Loop of alternating current source (nominal at) of terrestrial electricity situation.

) of •Hot(front line: Voltage of alternating current source, bring to bear on relative to 0 lines electric energy.

 

6LCD Vcom coupling receives line to feeling screen

 

Portable equipment feels screen to be able to install LCD to show screen to go up directly. In typical LCD framework, liquid crystal material is offerred by transparent fluctuation electrode slant buy. The many electrode of lower part decided to show the many sheet of screen resemble element; The communal electrode of upper part is to enclothe indication screen whole the successive plane that can inspect front, it slants buy is in voltage Vcom. The low-pressure portable equipment in the model (for example mobile phone) in, communication Vcom voltage circulates the square wave that concussion makes a round trip between the ground and 3.3V continuously to be in. Communication Vcom n shows a switch every normally, accordingly, the communication Vcom frequency that produces is the 1/2 of indication frame refresh rate and linage product. The communication Vcom frequency of a typical portable equipment may be 15kHz. Graph 4 arrive for coupling of LCD Vcom voltage the sketch map that touchs screen.

 

Graph 4: LCD Vcom disturbs coupling model.

 

Double deck feels screen to be comprised by the detached ITO layer that bestrews Tx array and Rx array, with dielectric the layer is separated among. Tx line holds the whole width of Tx array span, the smallest span that needs with making place only between line and line is separated. This kind of framework is called from screen type, because Tx array opens Rx array and LCD Vcom screen. However, take a space through Tx, coupling happens possibly still.

 

Obtain better diaphaneity to reduce framework cost, monolayer feels screen to be Tx and Rx array installation on individual ITO layer, pass alone bridge ordinal bond each array. Accordingly, tx array cannot form screen layer between LCD Vcom plane and sensor Rx electrode. This produces serious Vcom to violate coupling condition likely.

 

7 charger are disturbed

 

Another when feel screen interference potential source is the switch power source of charger of mobile phone of power source power supply. Interference arrives through finger coupling on feeling screen, if pursue,5 are shown. Charger of small-sized mobile phone has front line of alternating current source and 0 lines input normally, but join without ground wire. Charger is safe segregation, be inputted in power source so and link did not sort continuously between charger secondary coil. However, this still can keep apart transformer to produce capacitance coupling through switch power source. Charger interference returns way through finger touchs screen and be being formed.

 

Graph 5: Charger disturbs coupling model.

 

Attention: Below this kind of circumstance, charger interference is to show equipment is opposite the extra voltage at the ground. This kind of interference may be in direct current source and dc ground because of its good value, and the “ that be become by the description in all modular ” interference. The mains switch noise that produces between the direct current source that outputs in charger and dc ground, if was not divided by sufficient filter, what may affect feeling screen is normal move. This kind of power source is restrained than (PSRR) problem is another problem, the article is not done discuss.

 

Impedance of 8 charger coupling

 

Charger switch interference passes transformer primary – sub winding leakage of electricity is allowed (about 20pF) coupling arises. Action of coupling of this kind of weak capacitance can be appeared in charger wire cable and itself of equipment getting report capacitance compensates the parasitism paralell connection of relatively distributed ground. When taking facility, shunt-wound capacitance will increase, this normally enough removes charger switch interference, avoid disturbing influence to feel an operation. Receive charger repeatedly when portable equipment and put on the desktop, when and operate the finger of personnel be being contacted with feeling screen only, will appear the interference of a kind of when charger arises worst circumstance.

 

9 charger switch disturbs heft

 

Charger uses typical mobile phone to stimulate type instead (Flyback) circuit develop attacks. The interference weaveform that this kind of charger produces is more complex, and follow charger differ and difference is very big, it depends on circuit detail and output voltage control are politic. The change that obstructs amplitude is very big also, this depends on the design effort that manufacturer invests on switch transformer screen and unit cost. Typical parameter includes:

Undee: Include square wave of complex wide modulation of arteries and veins and LC ringing weaveform. Frequency: Rated load issues 40~150kHz, when load is very light, pulse frequency or jump periodic operation drops 2kHz is the following. Voltage: Can amount to the half =Vrms/√2 of voltage of power source peak value.

 

Heft of 10 charger electrical interference

 

In charger front, commutate of voltage of alternating current source builds course of charger tall voltage. Such, the overlay of switch voltage heft of charger is on the sine wave of half of voltage of a power source. Disturb likeness with switch, voltage of this power source also is to pass switch to keep apart transformer to form coupling. When 50Hz or 60Hz, the frequency of this heft is far under switch frequency, accordingly, its effective coupling impedance is corresponding taller. The serious rate that power source voltage violates depends on the character of right shunt-wound impedance, still depend on at the same time feeling screen controller is right the sensitivity of low frequency.

 

Graph 6: Charger weaveform example.

 

The special situation of electrical interference: Do not take the 3 aperture outlet of ground connection

 

The power source adapter with higher rated power (for example notebook computer communicates adapter) , may configure 3 aperture outlet of alternating current source. Carry EMI to restrain output, charger is likely interiorly advocate the dc land that the ground of power source makes a foot join to output. This kind of charger is mixed in live wire normally join between 0 lines and ground Y capacitance, restrain those who come from power supply cord to go up to conduct EMI thereby. Make ground join exists of purpose suppose, equipment of screen of won’t portable to power supply PC and USB connective feeling causes this kind of adapter interference. Graph 5 medium dotted line casing showed this kind of configuration.

 

For equipment of screen of portable to PC and its USB connective feeling, if had the PC charger that 3 aperture power source inputs to insert,do not have electrical outlet of ground connective power source, a kind of when charger disturbs special situation will arise. Y capacitance goes to coupling of alternating current source dc ground to output. Be worth relative to larger Y electric capacity can voltage of effectively coupling power source, this makes the finger that voltage of bigger power source frequency passes feeling screen to go up undertakes coupling with relatively inferior impedance.

 

Article brief summary

 

Current and extensive the screen of feeling of projectile style electric capacity that is used at portable equipment is disturbed very easily by electromagnetism, come from interior or external interference voltage can arrive through capacitance coupling feeling screen equipment. These interference voltage can cause the charge movement inside feeling screen, the charge motion when this may touch screen to finger is measured cause promiscuous. Accordingly, the effective design that feels screen system and optimize depend on pair of understanding that violate coupling way, and to its as far as possible pare or it is compensation.

 

Interference coupling method involves parasitism effect, for example: Transformer winding capacitance and finger – equipment capacitance. Undertake building a model appropriately to these influences, can realise the origin of interference and size adequately.

 

To a lot of portable equipment, batteries charger makes the interference source with feeling main screen. When should operating personnel hand to show the contact touchs screen, the capacitor that produces makes circuit of charger interference coupling is able to shut. The quality of screen design mixes charger interior to whether have design of proper charger ground connection, it is the crucial factor that affects charger to disturb coupling.