wirelles capacitvo acoplamento

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ABSTRACTIncreasing demand and use o computers promptedextensive studies on implementing low cost and reliablepointing devices. The use of the capacitive couplingtechnology allows wireless polinting devices to be reliableand d o not require any battery. The proposed pointingdevice uses the capacitive coupling technology to detectboth the x and y position of the m ouse/digitizer and also i fany of the buttons were pressed. Detail analysis of thecapacitive coupling signals and its performance in bothtime and frequ enc y domains were discussed. Theproposed pointing device was build and implemented.Experimental results show potential fo r commercializationof the wireless-pointing device and its robustness.

WIRELESS POINTING DEVICE USING CAPACITIVE COUPLINGAbdul Wahab, Tan Eng Chong and Low Kay Min

Nanyang Technological UniversitySchool of Applied Science, Nanyang Avenue, Singapore (639798)

Tel: (065) 799-4948, Fax: (065) 792-6559, email: [email protected] concept of capacitive coupling can be applied to digitizertablet by consideration the hand-held cursor or stylus as oneplate of a capacitor and the tablet surface as the other. Capacitivedigitizers can be designed to tolerate substantial electrostaticnoise, especially those that use the relatively large tablet area asthe receiver.Two sets of conductive lines forming X-Y grid are embedded inthe digitizer tablet surface. Each line is connected to switches.The grid lines nearest to the cursor will have the maximumvoltage amplitude of capacitively coupling signals. The exactposition of the cursor is calculated through digitizer hardwareand software techniques.

1. INTRODUCTIONDigitizer and mouse are commonly used as pointing devices inthe computer system. Digitizex table is a flat, square orrectangular slab onto which a stylus is placed [1][2]. Theabsolute position of the stylus can be detected using varioustechniques. The x and y information are then transmitted to thecomputer system via a serial port or a parallel port. Mouse onthe other hand is a relative pointing device, which is movedacross a flat surface by hand, and the movement encoded andtransmitted to the display system. The mouse is able to sense themotion either relative to the work surface or relative to themouse itself. When the motion is sensed relative to the worksurface, its output corresponds to its motions relative to the xand y axes of the surface. In the latter, the mouse can move inany direction, or rotate, and its output will correspond only tothe motion relative to its x- and y-axes.The present mouse technology uses two encoding wheels, for xand y direction, and a rubber ball [2]. When the rubber ballmoves the encoding wheel translate the movements into x and ydirection pulses via a cable connected to the RS-232 (serialport). Due to the mouse m ovement the cable can cause reliabilityand inconvenient to the users. In addition the rubber ballfrequently become dirty and need maintenance and can causepotential problem to the encoding wheels.

2. CAPACITIVE COUPLINGTwo flat conductors of a printed circuit board (PCB) separatedby a dielectric (laminate) with a rectangular conductor can formtwo equivalent capacitors (C1 and C2) as shown by figure 1 . Notice that the movement of the rectangular conductor willeither provide a larger capacitance for C1 or for C2 and themaximum coupling signal can only be achieved when bothcapacitor have the same value so that the equivalent series

- cjcz -- =%, Thiscapacitance is Cey,,ole,,, - c,+c2 2 2Eo&,(A -4

implies that if the rectangular conductor m oves to the right and arepresent the change in area we have the capacitances:where EOnd c2=oE r ( A+a>c,= d dand E,. are abolute and relative permittivity respectively, A th earea where the two conductive surfaces form the capacitanceinitially and d the distance between the two plates. Thus the

E,E,A E,&, aequivelant capacitance will be: ceq,--2d 2d ' 2 A 'Notice from the equation in the first place when b oth C1 and C2.are the same a=O thus the Ceq. is half the value of eachcapaciatnce and on the worst case when a = A the Ceq. = 0.Thus by making use of one of the traces as a source and theneighbouring traces to detect the coupling signal on e can easilydetermine the position of the rectangular conductor.

149 0-7803-4371 9/97/$10.00 0 1997 IEEE


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S c a nsignal

Conductive plate /

PCBTrace L zL 1 I _ i .....

Trace L t - tCapalitors formed with therectangular conductorFigure 1. Capacitive coupling concept using PCB traces

A PCB with thick signal lines running across it and eachadjacent signal lines are separated by a thin ground line allowsuppression o f noisy or unwanted co upling of signal by adjacentlineshraces. Figure 1 shows the layout of the thick conductingtraces with a thin ground line and also the addition of a thickground line around the PCB. The equivalent circuit of thecapacitive coupling with and without the conductor is shown infigure 2.The capacitance without the conducting plate (Cair) isvery small as compared with the capacitive coupling formed bythe condu ctor. Therefore, without the conductive plate across thesignal line, the coupling signal (signal received) will be verysmall. At the instant when the conductive plate is placed acrossthe signal line, the total capacitance (Ceq.) will increase and thecoupling signal will, therefore, also increase. Thus, in order tosimplify the equivalent circuit, so as to have a clearer view, wecan make the assumption that Cgnd is also very small ascompared with CI and C2. This is because the ground line ismuch thinner than the signal line, so the area of contact with theconductive plate will be very small. Thus, the capacitanceformed will also be small and can be ignored. The otheralternative is to convert the Tee connection of the capacitancenetwork into a Pi equivalent and notice the signal at the outputwill be no m ore then just an capacitive potential divider with theCair as shown by figure 2. Therefore, the coupling voltagemeasured at the receiving end w ill be:

r. Since, Ct is much larger than Cair,/L, c/m,=v,Vout will be approximately equals to Vin. We also note that the

maximum coupling signal will be when the conductive plate isplaced midpoint across the two adjacent signal lines and willdecrease as it moves away from the center of the two lines.Thus, if the conductive plate is large enough to cover threesignal lines then movements of the conductive plate from rightto left or left to right can easily be identified. The Signalamplitude can also assist in the position accuracy or resolutionof the mouse movement.

I Without conductor ISignal sendSignal send

T Signal recec-- IEquivalent CircuitL C t =c1 I1 c 2c

"In ICairC1&C2Cgnd

=capacitive coupling by air (across signal line and ground line)=capacitive coupling by signal line and the conductive plate=capaciti ve coupling by ground line and the conductive plate

Figure 2. The equivalent capacitance circuit with and w ithoutthe conductor3. CHARACTERISTICSOF THE COUPLING

SIGNALPosition A

CH If-

locents WCH 2f-

/ -Position B

Figure 3. Experiment set up to determine the optimum thicknessand distance of signal lines and ground lines.Experiments were carried out to study the effect of couplingsignals on the thickness of the signal lines, the distance betweenthe signal lines and the thickness of the ground lines in bothtime and frequency domain. A sine wave input signal withvarying frequency was applied to determine the optimumcoupling signal for the experiment. It was found that a SOMHzsignal was required to sufficiently provide large enoughcoupling signal with noise immunity for the time domainanalysis and a 22.5kH z signal were sufficient for the frequencydomain analysis. With the SoMHz input signal a change in thecapacitance, by moving the conductive plate, is noticeableenough to produce measured signal. Ideally we would like tohave as high frequency as possible but this will imply a morecomplex circuit requirements. A IO # coin was used as theconductive plate and because of the air gap between the coin and

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the signal lines the value of the capacitances will be very small.Figure 3 shows the experimental set up to measure the optimumthickness and distance of the signal lines and the thickness of theground line.

Signal width(1

3.1 The Time Domain analysis

Gro

Table 1 shows the experimental results of the coupling signalswith a 4mm, 5mm and 6mm signal lines thickness, 3mm, 4mmand 5mm signal lines distances and Imm, 2mm and 3mmground line thickness.

Table1 Coupling signal amplitudes with different signal linesand ground lines thickness and the distance between the signalline.

3.1.1 Analysis of the different signal lines widthAs can be seen fiom the tablc that when the width of the signalline increases, the coupling signal also increases. But theperformance droppcd when the signal width changed from 5mmto 6" The performance of the coupl ing signal i s determinedby the change of voltage level ofthe coupling signal, when thereis conductive plate and no condluctive plate across the signallines. The performance of the coupling signal for CHI, whensignal width=5nim, is 0.72V. But when the signal widthincreased to 6mm, the performance dropped to 0.55V. As forCt-12, the performance dropped from 0.58V to 0.46V.3.1.2 Analysis of the ground line widthAs can be seen from the table that when the width of the groundline increases, the coupling signal decreases. The performance ofthe coupling signal for CHI, when ground width=lmm, is0.68V. But when the ground .width increased to 3mm, theperformance dropped to 0.38V. As for CH2, the performancedropped from 0.76V to 0.30V when the ground width increased

f rom l mm to 3" Therefo re , t he performance droppedtremendously when the ground width increases.

3.1.3 Analysis of the distance between the signal lines.As can be seen from the table that when the distance between thesignal line and ground line increases, the coupling decreases.The performance of the coupling signal for C H I , whendistance=3mm, is 0.76V. But when the distance increased to5mm, the performance dropped to 0.52 V. As for CH2, theperformance dropped from 0.80V to 0.62V when the distanceincreased f rom 3mm to 5" The performance dropped whenthe distance increases, but the effect, as compared to the groundwidth, is not so significant. An additional testing carried outwith signal lines width = 5mm and ground lines width = Imm,produce a much higher coupling signal when the distancebetwe en the signal line s is 2"Since the IO# coin has a diam eter of Is", from tab le 1 themaximum coupling signal is when a 5mm signal lines thickness,Imm ground l ine and a maxi mum dis tance of 3 Th i s g ives acoverage of(3+5+5 = 13") w idth, L e. (13/18) ~ 1 0 0 % 7 2%of the coin's diameter. The results also indicate that if a biggerconductive plate is used then the optimum signal lines width andthe distance between the signal lines must also be calculated foroptimum performance. The thickness of the ground lines mustbe as thin as possible and is dictated by the capabilities of thePCB manufacturing process.3. 2 Frequency analysis of coupling signal

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In the frequency domain analysis, a 22.5 kHz input signal issufficient to detect the change in the capacitance caused by theconductive plate.Similar to the time domain analysis, as the signal widthincreases, the coupling signal also increases. The optimumsignal width is 5mm as in the case of the time domainexperiment. T his is due to the size of the conductive plate used.A bigger size pointer would require wider signal lines to get themaximum coupling signal. As the ground width increases, thecapacitance formed with the conductive plate and the groundline also increases. Thus, the overall coupling signal with theconductive plate will decrease. But the coupling signal causedby the air decreases as the ground width increases. Similarly asthe distance increases, the coupling signal by air decreases. Theincreased in the coupling signal due to the change in capacitancedropped when the distance increased from 3mm to 5.

4. IMPLEMENTATION OF THE SYSTEMThe experiments above indicate that the choice of signal linesand ground line width and the gap between the lines fullydepend on the size of the conductive plate used as the pointer.Of course a smaller plate will be ideal, as this will give a betterresolution to the digitizer/mouse but this will in-turn apply amore complex circuit requirement due to a smaller couplingsignal and its immunity to noise. The amplitude of the couplingsignal yields a better result in the frequency domain method. Inaddition the FFT implementation is simpler and does not requirea peak detector circuit [3][4]. The input signal requirements arealso much lower then the 5OMHz sine wave signal required forthe time domain method. A similar result can be achieved in thetime domain method if a notch filter are to be used to suppressall other noise and interference, but still a peak detector isrequired. On the other hand the frequency domain methodrequire an FF T computation of the coupling signal but an eightpoint DFT would be sufficient.For frequency analysis, one of the important factors is thesam pling rate of the Analog to Digital co nverter (AD C) [9][ IO].The coupling signal of 22.5 kHz would require a minimumsam pling frequency before a liasing occur of 45 kHz [5][6][7]. Adown sampling of the coupling signal would require a simplerADC and even a micro-controller would be sufficient tocompute the DFT [8]. On the other hand since we are onlyinterested in the center frequency of the coupling signal and onlyat that frequency and ignoring all other frequency we can allowthe aliasing to occur and sampled the da ta at 30 kHz or less.4.1When aliasing occurs du e to too lower sampling rate, the effectcan be described by a multiple folding of the frequency. Thefolding frequency is Fs/2 where Fs is the sampling frequency.

Aliasing effect of the sampled data

Thus, when the sampled data is transformed in to the frequencydomain, the transform function is characterized by spectraloverlap. In effect, frequencies in the overlap region may bemistaken for other frequencies, so that it is impossible to recoverthe original signal. Since the folding frequency for the system is3OkHz signal is = 1 SkHz, the window of the transform functionis from 0 to 15kHz. Due to the folding effect, the frequency of22.5kHz will be folded to become 7.5kHz. But the mostimportant fact is that the magnitude information is not lost.

5. CONCLUSIONThe capacitive coupling implementation of the wirelessmouse/digitizer were successfully experimented and tested.From the result it is observed that the frequency domainimplementation in analyzing the coupling signal are more robustand require a much lower input signal frequency. The size of theconductive plate depends on the resolution, accuracy and speedof moving the mouse. Size of the tablet also determines thenumber of signal lines required. Line coding can be used toreduce the number of signal line drivers by allowing differentadjacent signal lines combination. Switch buttons are conductiveplate that can be pushed towards the tablet for activation. Sincethe wireless mouse/digitizer uses the capacitive couplingtechnology and the mouse itself is nothing more then conductiveplates, no battery is required thus the mouse can move freelywithout any wire obstruction

6. REFERENCESSherr Sol, Input Devices, Academic Press, 1989.Barry Wilkon and David Horrocks, Computer Peripherals,Second Edition, Hodder and Stoughton, 1987.William D.Stanley, Gary R.Dougherty and RayDougherty, Digrtal Signal Processing, Second Edition,1984.John G .Proakis, Dimitris G Mm olakis, Digital SignalProcessing, Third Edition, 1997.D. A. Linden, Discussion of Sampling Theorems, Proc.IRE, 47 , 1219 (1959).A. J Jerri, The Shannon Sampling Theorem-its VariousExtensions and Applications. A tutorial Review, Proc.IEEE, 65, 1565 (1977).R. J. Marks 11, Introduction to Shannon Sampling andInterpolation Theory, Springer-Verlag, New York, 199R. E. Crochiere and L. R. Rabiner, Multirate DigitalSignal Processing, Prentice Hall, Englewood Cliffs, NJ,1983.G . B. Clayton, Data Converters, Ilalsted Press, Wiley,New York. 1982.

[ I O ] R. M. Gray, Oversampling Methods for A /D an d DIAConversion, IEEE Press, Piscataway, NJ, 1992.

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wirelles capacitvo acoplamento

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