CA1251537A

CA1251537A - Coordinate detecting apparatus

Info

Publication number: CA1251537A
Application number: CA000475287A
Authority: CA
Inventors: Toru Asano; Kazuo Yoshikawa; Hideaki Takizawa; Hisashi Yamaguchi; Shizuo Andoh
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1984-02-29
Filing date: 1985-02-27
Publication date: 1989-03-21
Also published as: KR900002350B1; EP0155781B1; AU552619B2; DE3565732D1; AU3870385A; ES540763A0; EP0155781A1; KR850006749A; US4680430A; ES8702007A1

Abstract

ABSTRACT

A coordinate detecting apparatus allowing to input the coordinate data of a point on a plane by indicating the point with a touch of a fingertip to the point is dis-closed. The apparatus comprises a transparent resistive film constituting the plane (touch panel) and a buffer circuit operating as a voltage follower circuit having substantially infinite input impedance. The buffer amplifier is operatively connected between each selected pair of facing ends of the substantially rectangular region of the resistive film, and equalizes the potentials at the ends. The change of the impedance between one of the ends and the ground is detected in accordance with the touch of the fingertip to the touch panel and used as an original coordinate data. Further modifications concerning the avoiding of degradation in the detection quality due to the fluctuations of the impedance provided by the touch of a fingertip, for example, and the methods to perform 2-dimen-sional coordinate detection are described.

Description

~251537 TITLE OF THE INVENT~ON
A COORDINATE DETECTING APPARATUS

BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a coordinate detect~
ing apparatus, particularly to an analog type apparatus using a resistive film constituting a plane on which a position of a point whose coordinate is to be detected is indicated by applying a load impedance to the position.
With the development of office automation, there is a growing need for simple means for inputting the coordinate data of a point into a computer system. So-called soft key which is a coordinate data inputting means comprising an input panel stacked on the surface of a display device permits to input the coordinate of a point on the panel only by applying a touch of a fingertip, etc. to the panel.
Thus, the soft key can greatly facilitates the man-machine interaction in a computer system. For example, the selec~
tion of a menu on a display device or inputting of a pattern to a computer system can be performed not using a keyboard but only touching the input panel or writing the pattern on the panel. Exemplary application of such a soft key has been realized in a window machine for banks or a seat reservation terminal for travel agencies. The use of the soft-key is expected to increase more and more accord ing to the trend toward the society of integrated digital ~25~537 information networks where easy-to-operate terminals for nonspeciali~ed persons are essential.

2. Description of the Prior Art As a coordinate data inputting technology for above described soft-key, there is a digital type apparatus comprising a plurality of sensors disposed to form a matrix arrangement on a plane. By indicating one or a plurality of the sensors with a touch of a fingertip or pen, the coordinate data corresponding to the position(s) of the sensor(s) is input to a control unit. However, detection accuracy in the coordinate data obtained by using such digital type means depends on the number of the sensors per inch, and is insufficient for the applications requiring high resolution necessary for inputting a fine or complica-ted pattern.
Another type of coordinate inputting means using an input panel having a resistive film was first disclosed in Proc. 1971 SID, under the title of "Conducting Glass Touch Entry System" by R.K. Marson, followed by various modifica-tions, for example, disclosed in Proc. 1973 SID, under the title of "The Analog Touch Panel" by J.A. Turner et al. In the method, a resistive film constituting an input panel is supplied with current from its both ends, and the point whose coordina'.e on the input panel is to be detected is indicated by applying a load impedance to the resistive film at the subject point. The coordinate of the point is ~LZ5iL537 given as a function of the ratio of respective currents flowing from both ends to the load. Thus, the coordinate is originally acquired as analog data.
Therefore, higher detection accuracy can be attained when compared with that in the above mentioned digital type, and the detection accuracy is mainly dependent on the quantization characteristics of the A-D ~analog-to-digital) converter used in the conversion of the original analog data to digital data.
The prior art and the invention itself will now be described in greater detail with reference to the accompanying drawings, in which:
FIGURE 1 is a circuit block diagram of a conventional coordinate detecting apparatus;
FIGURE 2 is a circuit block diagram for explaining the principle of the present invention;
FIGURE 3 illustrates an exemplary circuit configuration embodying a coordinate detecting apparatus based on the principle explained with reference to FIGURE 2;
FIGURES 4(a) and 4(b) illustrate circuit configurations for explaining the operation of a second embodiment of the present invention;
FIGURE 5 illustrates a circuit configuration of a third embodiment of the present invention;
FIGURE 6 illustrates a fifth embodiment of the present invention;
FIGURE 7 illustrates a sixth embodiment of the present invention;
and FIGURE 8 illustrates a modification of the embodiment described with reference to FIGURE 7.

- 125~53~

The principle of an analog type coordinate detecting method as above is described with reference to FIGURE l in the following.
FIG~RE 1 is a circuit block diagram of a conventional coordinate detecting apparatus of the aforesaid analog type. Referring to FIGURE 1, the output terminal 5 of a voltage source 8 is connected to an end 2 of a uniform resistive film 1 via a current measuring means 9. The output terminal 5 of the voltage source 8 is also connected to another end 3 of the resistive film 1 via another current measuring means 10. Another output terminal of the voltage source 8 is connected to the ground 7. The output of the current measuring means 9 is connected to an analog-to-digital converter (ADC) 11, while the OUtpllt of the current measuring means 10 is connected to another ADC 12. The outputs of both ADCs 11 and 12 are individually connected to a control unit 13.
When a load 6 which is a capacitor, for example, and has a definite impedance with respect to the ground 7, is - 3a -125~537 applied to the position cf a point 4 on the resistive film 1, respective currents, which are supplied by the voltage source 8, flow into the load 6 via the ends 2 and 3.
Since the resistive film 1 has a uniform resistivity, the resistance between arbitrary two points on the resis-tive film 1 is proportional to the distance between the points. ~hen assuming the coordinate of the end 2 to be O
while the coordinate of the end 3 to be 1, the coordinate x of an arbitrary point 4 on the resistive film 1 is repre-sented by the equation x = RX/(Rx + R1 x) ...................... (1) where, 0 5 x 5 1, and, Rx and R1 x denote respective resistances between the end 2 and point 4 and between the end 3 and point 4.
The voltage drop across the resistance Rx is equal to that across the resistance R1 x' therefore, RX.Ix = R1-x I1-x ....................... (2) where, Ix and I1_X denote respective currents flowing through the ends 2 and 3.
Hence, equation (1) can be represented as follows:

1-x/(Ix + I1-x) ~ -----.... (3) and thus, the coordinate of the point 4 can be determined by the currents Ix and I1 x' both measured by using the current measuring means 9 and 10. In the apparatus shown in FIG.1, the values of the currents Ix and I1 x are converted to corresponding digital data by the respective ADCs 11 and 12, and processed by the control unit 13 to ~L2~1537 provide a digital coordinate data, according to equation

(3).
The conventional coordinate detecting apparatus shown in FIG.1 uses current measuring means 9 and 10-, each of which usually comprises an operational amplifier serving to provide a voltage output signal corresponding to the measured current. The coordinate detecting apparatus also requires analog-to-digital converters (ADCs) 11 and 12 of high quantization accuracy. As a result, the apparatus inevitably has a complicated circuit configuration and high cost.

SUMMARY OF THE INVENTION
It is an object of the present invention to provide a coordinate detecting apparatus comprising a simplified circuit.
It is another object of the present invention to provide a coordinate detecting apparatus of low cost.
It is yet another object of the present invention to provide a coordinate detecting apparatus having a high detection accuracy.
It is still another object of the present invention to provide a coordinate detecting apparatus which can be used as a soft key for a computer input device when stacked on a display device.
The above objects can be achieved by providing a coordinate detecting apparatus comprising: a resistive film -` ~25~537 constituting a plane and having an effective region used for the coordinate detection, wherein the effective region is substan-tially rectangular and has a pair of ends disposed in the direc-tion parallel to the relevant coordinate axis; a buffer means having an input and an output, said input being connected to one end of said film means and said output being connected to the other end of said film means, said buffer means having substan-tially infinite impedance and unit voltage gain; and measuring means connected to said input of said buffer means, to measure an impedance between an end of said film means and ground, wherein the point, whose coordinate on the plane is to be detected, is indicated by applying load means between the point to be detected and ground.
These together with other objects and advantages, which will be subsequently apparent, reside in the details of construc-tion, as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals designate like or corresponding parts.

~L251537 PREFERRED EMBODIMENT OF THE INVENTION
FIG.2 is a circuit block diagram used for explaining the principle of the present invention. Unlike in the conventional apparatus shown in FIG.1, ends 2 and 3 of a resistive film l is not connected to a voltage source but the end 2 is connected to a terminal 14 and the end 3 is connected to the end 2 via a buffer circuit, for example, an operational amplifier (referred to as op amp, herein-after) 15 of a voltage follower mode. More specifically, the end 2 is connected to the non-inverting input (+) of 5~537 the op amp 15, and the output of the op amp 15 is connected to its inverting input (-) together with the end 3.
Thus, in the circuit shown in FIG.2, the ends 2 and 3 are made have an equal potential with respect to~the ground 17. The input impedance of the op amp 15 can be assumed to be infinite, therefore, the current flowing through the terminal 14 does not include a current component flowing from the end 3 to a point 4 on the resistive film 1, even when a load 16 is applied to the point 4.
In the following, the potential on the ends 2 and 3 with respect to the ground 17 is represented by V, the potential at the point 4 with respect to the ground 17 is represented by Va, and the impedance between the terminal 14 and ground 17 is represented by Z. The impedance Z
involves the respective resistances R between the end 2 and point 4 and R1 x between the end 3 and point 4, and the load impedance Zo between the point 4 and ground 17, and is, therefore, referred to hereinafter as an equivalent impedance.
The present invention is based on a consideration that the coordinate x of an arbitrary point 4 on the resistive film 1 is ~iven as a function of the equivalent impedance Z.
The value of the equivalent impedance Z can be derived as follows.
A current flowing through the load impedance Zo, which current being expressed by Va/Zo, is equal to the sum of ~Z51~37 the current flowing through Rx and the current flowing through Rl . soth currents flowing through Rx and Rl x are expressed by (V - Va)/Rx and (V - Va)/Rl_x, respectively. Hence, ~
(V - Va)/Rx + (V - Va)/Rl_x = Va/Zo .......... (4) As mentioned above, it can be assumed that the current flowing through the terminal 14 does not include the current ,lowing through Rl x' therefore, the current flowing through the equivalent impedance Z, which current being expressed by V/Z, equals to the current flowing through Rx, hence, V/Z = (V - Va)/Rx ........................ (5) By eliminating both V and Va from equations (4) and (5), the equivalent impedance Z is expressed as follows:
x Z (Rx + Rl-x)/Rl-x ............... (6) If the resistive film 1 is formed to have the values f Rx and Rl x sufficiently smaller than the load impedance Zo, equation (6) can approximately be ( x Rl-x)/Rl-x ~ (7) As described above, the coordinate x of a point on the resistive film 1 is given by X/(Rx + Rl_X) ....................... (8) accordingly, by combining equations (8) and (7), the equation representing the coordinate x as a function of the equivalent impedance Z is derived as follows.
x = 1 - Zo/Z .......................... (9) ~251S37 Therefore, if the load impedance Zo has a known value, and the value of the impedance Z is obtained by using an appropriate measuring means, the coordinate x can be determined according to equation (9). ~
As discussed above, in the principle of the present invention, both ends of the resistive film 1 are connected to each other via a buffer circuit operating in a voltage follower mode, and the equivalent impedance Z of the terminal 14 with respect to the ground 17 is measured when a load impedance Zo is applied to an arbitrary point 4 on the resistive film 1. The impedance Zo is selected to be sufficiently larger than the resistance of the resistive film 1. And thus, the coordinate x of the point 4 on the resistive film 1 is detected by processing both impedance data according to equation (9).
In the configuration of FIG.2, if the load impedance Zo is capacitive, Zo = 1/j~Co ............................ (10) where, Co represents capacitance, j and ~ respectively designate the imaginary unit and an angular frequency.
Accordingly, equation (7) becomes Z = 1/j~{Co R1_X/tRx + R1_X)} .......... tll) Equation (11) means that the equivalent impedance Z
between the terminal 14 and ground 17 is assumed to be composed of an equivalent capacitance expressed as follows:

1-x/(Rx + R1-x) ~ (12) ~:~51537 Accordingly, the coordinate x is given by x = 1 - C/Co ........................... (13) As shown above, when a capaciti-ve impedance is used for Zo, the equivalent impedance Z between the terminal 14 and ground 17 can be dealt with as an equivalent capaci-tance, and the coordinate x is determined by measuring relevant capacitances C and Co.
FIG.3 illustrates an exemplary circuit configuration embodying a coordinate detecting apparatus based on the principle explained with reference to FIG. 2, wherein a capacitance Co of the load 16 applied to an arbitrary point

4 on the resistive film 1 and the equivalent capacitance C
between the terminal 14 and ground 17 are respectively measured to determine the coordinate x of the point 4 according to equation (13). The configuration and opera-tion of the part enclosed by the broken line 100 are the same those explained with reference to the corresponding part in FIG.2.
In FIG.3, capacitance measuring means, for example, a CR digital oscillator 19, is connected to the terminal 14, and the output of the CR digital oscillator 19 is connected to the input of a control unit 20. The CR digital oscil-lator 19 outputs pulses of a frequency corresponding to the time constant determined by the respective values of external resistor and capacitor connected to it. When the equivalent capacitance C is used as the external capacitor, the repetition period of the pulses output from the ~Z5~37 oscillator 19 is proportional to the equivàlent capacitance C. The control unit 20 determines the repetition period of input pulses by its built-in timer and calculates the value of the coordinate x of the point 4 according t~ equation (9) or (13). Such CR digital oscillator 19 may comprise a astable multivibrator, such as NE555 marketed by Sygnetics Inc., for example.
Some quantitative discussion concerning above process is described in the following.
An input pen having a capacitance of about 1000 pF, for example, is used for the load impedance 16. The internal clock pulse of about 6 MHz is used for the afore-said timer in the control unit 20, and therefore, the CR
digital oscillator 19 is set to operate in a frequency range of tens kHz or less. On the other hand, the resis-tive film 1 having a sheet resistance in the range from 200 to 500 ohms is generally employed. Hence, in the above frequency range, the load impedance provided by the 1000 pF
input pen is in a range around 5 to 15 kilohms. This impedance is ten times or more larger than the resistance of the resistive film 1 and can comply with the assumption used for deriving the equation (7).
A constant, k, is introduced for establishing a rela-tionship between the repetition period of the output pulses and input capacitance of the oscillator 19. That is, To = kCo ............................... (14) T = kC ................................. (15) ~1537 where, To denotes the repetition period obtained when the capacitance at the terminal 14 is Co, and T denotes the repetition period obtained when the capacitance at the terminal 14 is C. --According to the equations (14) and (15), the equation(13) expressing the coordinate x as a function of the capacitances Co and C is modified as follows:
x = 1 - (T/To) ......................... (16) Hence, the coordinate x is determined by measuring the respective repetition periods corresponding to the load capacitance Co and equivalent capacitance C. According to the formula (13), x = 0 results in C = Co. Therefore, the repetition period To is obtained when the load impedance Co is applied to the end 2. Then, the equivalent capacitance C is measured by applying the load capacitance Co to the point 4 whose coordinate x is to be detected. The data corresponding to the respective repetition periods To and T
are stored in the memory of the control unit 20, and then, processed for providing the coordinate value x of the point, according to the formula (16).
The CR digital oscillator 19 is commercially available at a low cost, and the control unit 20 can be comprised of a conventional timer, arithmetic device and latching circuit, all of which are also commercially available at low costs. Thus, the coordinate detecting apparatus of the present invention can be provided at a lower cost compared ~25~5;37 with thè conventional apparatus as shown in FIG.1, without a sacrifice of operational reliability.
Since a transparent resistive film such that composed of indium-tin-oxide (ITO), for example, is obtained, the input panel of the coordinate detecting apparatus shown in FIG.3 can be directly stacked on the surface of a display device, and used in the application such as a touch panel for inputting a selected menu on the display, ~herein the selected menu is indicated by applying a touch of fingertip on the input panel. Because the coordinate detecting apparatus shown in FIG.3 uses the variation of the equiva-lent capacitance C between the end of the resistive film 1 and the ground 17 in accordance with the application of the load capacitance Co, the surface of the resistive film 1 is permitted to be coated by an insulating film for protecting the resistive film 1 from a mechanical wearing or scratch.
FIGs.4ta) and 4(b) are circuit configurPtions for explaining the principle of the second embodiment of the present invention. Based on the principle as explained with reference to FIG.2, however, this embodiment permits to determine a coordinate x on an input panel by using a load impedance having an unknown value. That is, in this embodiment, value of the load impedance is not needed to be measured. In other words, this embodiment is intended to provide a coordinate detecting apparatus substantially operable with the use of a load impedance whose value is unstable.

~:~5~37 In the first embodiment shown in FIG.3, the load impedance, i.e. the capacitance Co, is supposed to be constant during a sequence from the time when an input pen touches the end 2 for determining the value Co to the time when the input pen touches the position of an arbitrary point 4 on the resistive film 1. The sequence will take some seconds at least in a practical operation. To keep the load capacitance Co constant is easy if an input pen is used for indicating the point, but difficult if a touch of a fingertip is used instead of an input pen, because the load capacitance Co applied by a human body through a fingertip is apt to change with the contact pressure of a fingertip to the resistive film 1 and also with operator's sitting posture, for example. Moreover, the ends 2 and 3 have respective stray capacities with respect to the ground 17, in general. If the stray capacities are too large to be neglected, it is necessary to correct the equivalent capacitance C measured at the terminal 14 for the stray capacities.
In the coordinate detecting apparatus shown in FIGs.4(a) and 4(b), a load capacitance (represented by CB) is required constant only in very short time and is ex-cluded from the process of the coordinate detection.
Therefore, the load capacitance may be such one which fluctuates and has an unknown value as a floating capacity of a human body to the ground 17.

~:~51537 In FIG.4(a), the touch of the fingertip 18 applies a load capacitance CB to the point 4 whose coordinate x on the resistive film 1 is to be detected. The ends 2 and 3 have respective stray capacities Csx and Cs1 x w-ith respect to the ground 17. According to equation ~12), the equi-valent capacitance Cxl between the terminal 14 and the ground 17 is expressed as follows:

xl x CB{Rl_X/(RX + Rl_x)} ............. ,,... (14) Equation (14) includes the stray capacity Csx which stands in parallel to the equivalent capacitance Cxl. The stray capacity Cs1 x has been neglected because the end 3 is connected to the output of the op amp 15 whose output impedance can be assumed to be substantially zero. The CR
digital oscillator 19 connected to the end 2 via the terminal 14 outputs signal pulses having a repetition period proportional to the equivalent capacitance Cxl.
FIG.4(b) illustrates a situation where the respective connections of the input and output of the op amp 15 to the ends 2 and 3 are exchanged, and the CR digital oscillator 19 is connected to the end 3 via a terminal 21. If the change of the connections between FIG.4(a) and FIG.4(b) is carried out in a very short time, for example, 5 ms, the fluctuation of the capacitance CB can negligibly be small.
Hence, in the configuration of FIG.4(b), the equivalent capacitance Cx2 between the terminal 21 and ground 17 is expressed by using the same load impedance CB as follows:

~IL;2~S~537 x2 1-x + CB{RX/(RX + R1_x)} ................ (15) Equation (15) includes the stray capacity Cs1 x which stands in parallel to the equivalent capacitance Cx2. At this time, the stray capacity Csx at the ena 2 has been neglected because the end 2 is connected to the output of the op amp 15 having an output impedance of substantially zero.
As mentioned above, the switching from the circuit configuration shown in FIG.4(a) to that shown in FIG.4(b) is carried out in very short time, and the capacitances in equations (14) and (15) can be assumed to be constant.
Hence, by eliminating CB from the equations (14) and (15), following equation is obtained.

R = {(Cxl ~ Csx)/(Cx2 CSl-x x ----- (16) By combining this relationship with equation (8), the coordinate x of the point 4 is given as follows:
x = Rx/(Rx + Rl-x) = (C 2 ~ Csl-x)/{(Cxl ~ Csx) + (Cx2 l-x The coordinate detection of the second embodiment is performed according to equation (17). That is, the coordi-nate detecting apparatus of the second embodiment comprises switching means which permits to switch the connections of the circuit to the resistive film l from the situation shown in FIG.4(a) to that shown in FIG.4(b) in a very short time and measuring means for measuring the respective equivalent capacitances Cxl between the terminal 14 and the ground 17 and Cx2 between the terminal 21 and ground 17 and ~251537 the respective stray eapacities Csx at the end 2 and Csl x at the end 3, in accordance with the operation of the switching means. The coordinate x of the point 4, to which a load capacitance CB consisting of a floating eapacity of a human body is applied by touching a fingertip 18, is determined by substituting the measured values of these Cxl, Cx2, Csx and Csl_x for the respeetive terms in equation (17).
These capacitances are detected by the CR digital oscillator 19 in FIGs.4(a) and 4(b). Repetition period of pulses output from the terminal 191 of the CR digital oscillator 19 is proportional to the equivalent eapacitanee between the terminal 14 or 19 and the ground 17. The repetition period TXl of the output pulses in the eonfigu-ration shown in FIG.4(a) is given by xl Cxl ~ --------... (18) where, k is a constant.
Similarly, the repetition period TX2 of the output pulses in the eonneetion shown in FIG.4~b) is given by x2 x2 ' ~ -^----.................. (19) Thus, the equivalent eapaeitanees Cxl and Cx2 are deteeted as the repetition periods of respeetive output pulses from the terminal 191, as expressed by equations (18) ~nd (19).
Referring to equation (14), the stray eapaeity Csx is given as the equivalent capaeitanee Cxl either when CB = O
or Rl x = This eondition ean be aehieved by no touehing - ~251S37 of the fingertip 18 to the resistive film 1 in FIG.4(a).
Accordingly, in FIG.4(a), the repetition period Tsx of the output pulses from the terminal 191 without touching the fingertip 18 to the resistive film 1 is expressed by TSX = k C ' sx ~ ~ (20) where, C'sx designates the relevant equivalent capacitance between the terminal 14 and ground 17.
Similarly, referring to equation (15), the stray capacity Cs1 x is given as the equivalent capacitance C 2 without touching the fingertip 18 to the resistive film 1 of FIG. 4 (b), and repetition period Tsl X of the output pulses from the terminal 191 is expressed by TSl-x k C Sl-x ~ -----. (21) where, C's1 x designates the relevant equivalent capaci-tance between the terminal 21 and ground 17.
By substituting equations (18), (19), (20) and (21) for the respective terms in equation (17), the coordinate x of the point 4 is expressed as follows:

x2 l-X)/{(Txl ~ Tsx) + (Tx2 ~ Tsl )} ... (22) As described above, in the second embodiment, the respective repetition periods in both situations shown in FIGs.4(a) and 4(b) are measured without application of a touch of the fingertip 18, and then, during a touch of the of the fingertip 18 is applied to the resistive film 1.
Thus, the coordinate of the point 4 indicated by the touch of the fingertip 18 is determined by substituting the values of the repetition periods measured according to ~515~7 equations (18), (l9~, 120) and (21) for the respective terms in equation (22) in stead of corresponding equation (17) in which the coordinate is given by the respective capacitances. The key of this embodiment is the switching operation performed in a very short time between the circuits of FIGs.4(a) and 4(b).
The practical configuration of the switching means for exchanging the respective connections of the input and output of the op amp 15 to the ends 2 and 3 of the resis-tive film l will be described with reference to the sub-sequent embodiment.
FIG.5 illustrates a circuit configuration of the third embodiment of the coordinate detecting apparatus according to the present invention, wherein 2-dimensional coordinate detection is made possible.
In FIG.5, an input panel (referred to as a touch panel, hereinafter) 22 have a structure comprising a transparent resistive film formed on a glass substrate and coated with an insulating thin film such as SiO2. Refer-ring to FIG.5, the left end of the touch panel 22 is connected to a common line Ll by switching lines AXll, AXl2, ... AXlm of an analog switch array 23. The right end of the touch panel 22 is connected to another common line L by switching lineS AX21r AX22, x2m analog switch array 24. Further, the top end of the touch panel 22 is connected to the common line Ll by switching yll~ yl2~ ... Ayln of a third analog switch array ~L25~37 25. Likewise, the bottom end of the touch panel 22 is connected to the common line L2 by switching lines A 21~
Ay22, ... ~y2n of a fourth analog switch array 26. Each of the analog switch arrays 23, 24, 25 and 26 is réferred to as second switching means.
The respective control signal inputs of the analog switches 23 and 24 are connected to a control signal line H1 linked to a control unit 20, and the respective control signal inputs of the analog switches 25 and 26 are con-nected to the control signal line H1 via the respective inverters 31 and 32. The common line L1 is connected to the terminal S1 of first switching means 27, and the terminal S1 is selectively connected to the terminals I1 and l. The common line L2 is connected to the terminal S2 of the first switching means 27, and the terminal S2 is selectively connected to the terminals I2 and 2 The first switching means 27 may be an analog switching means, for example, comprising a couple of transfer contacts as shown in FIG.5.
The control signal input of the first switching means 27 is connected to another control signal line H2 linked to the control unit 20. The non-inverting input of op amp 15 is connected to the terminals I1 and I2 of the switching means 27 and also connected to the capacitor terminal of a CR digital oscillator 19. The inverting input and the output of the op amp 15 are connected each other, and also connected to the terminals l and 2 of the switching means ~L25~L537 27. The output of the CR digital oscillator 19 is connect-ed to the input of a control unit 20 which outputs the detected coordinate data on its output terminal 34. The point whose coordinate on the touch panel 22 -is to be detected is indicated by touching the fingertip 18 to the point. Thus, the point is applied with a load capacitance CB provided by a floating capacity of a human body with respect to the ground 17. The capacitance CB involves the capacitive component relating to the SiO2 film on the resistive film 1.
In the coordinate detecting apparatus having the configuration shown in FIG.S, the analog switch arrays 23, 24, 25 and 26 operate to alternately establish a current path between the left end and right end of the touch panel 22 (i.e. the current path in X direction) and between the top end and bottom end of the touch panel 22 (i.e. the current path in Y direction). For example, when the analog switch arrays 23 and 24 are closed, the analog switch arrays 25 and 26 are opened. The op amp lS constitutes a voltage follower circuit. The respective connections of the input and output of the op amp 15 to the common lines L1 and L2 are exchanged by the first switching means 27 according to the signal on the control signal line H2.
The coordinate detection operation in accordance with a touch of a fingertip 18 to the touch panel 22 is de-scribed below.

As a first sequence, a high level signal, for example, is output from the control unit 20 to the control signal line H1, and hence, the analog switch arrays 23 and 24 are closed, while the analog switch arrays 25 and 26-are opened due to the supply of a low level signal by the inverters 31 and 32. Thus, a current path is established in the X
direction.
In the above, a high level signal, for example, is output from the control unit 20 and applied to the first switching means 27 via the control signal line H2. And hence, the terminal S1 is connected to the terminal I1, and the terminal S2 is connected to the terminal 2 Accord-ingly, the connection between the op amp 15 and the touch panel 22 becomes equivalent to the circuit shown in FIG.-4(a). The CR digital oscillator 19 outputs signal pulses of repetition period TXl corresponding to the equivalent capacitance Cxl between the left end of the touch panel 22 and the ground 17. The repetition period TXl is measured by a timer, and then, stored in a memory, both of the timer and memory are built in the control unit 20. The repeti-tion period Tsx corresponding to the stray capacity Csx at the left end of the touch panel 22 is measured without application of the touch of the fingertip 18, and stored in the memory, in advance.
Following the above, the signal output from the control unit 20 to the control signal line H2 turns to low level, and the switching means 27 operates to connect the ~ S~3~

terminal Sl t~ the terminal l' and the terminal S2 to the terminal I2. During this process, the high level of the signal on the control signal line Hl is continued to hold the current path in the X direction. Accordi~gly, the connection between the op amp 15 and the touch panel 22 becomes equivalent to the circuit shown in FIG.4(b). The CR digital oscillator 19 outputs signal pulses of repeti-tion period TX2 corresponding to the equivalent capacitance Cx2 between the right end of the touch panel 22 and the ground 17. The repetition period of TX2 is measured by the timer, and then, stored in the memory of the control unit 20. The repetiti.on period Tsl_x corresponding to the stray capacity Csl x at the right end of the touch panel 22 is measured without the application of the touch of the fingertip 18, and stored in the memory, in advance.

g xl x2 x a Sl~X tained as above, an arithmetic processor in the control unit 30 performs operation according to equation (22), and thus, the coordinate in the left to right direction, i.e. X
coordinate, of the point indicated by the touch of the fingertip 18 to the touch panel 22 is detected, and then, output from the terminal 34. In the above, the coordinate of the left end of the touch panel 22 is defined as "0" and the coordinate of the right end is defined as "ln.
As the second sequence, the signal output from the control unit 20 to the control signal line Hl is turned to low level, and the analog switch arrays 25 and 26 are ~51537 closed, while the analog switch arrays 23 and 24 are opened. Hence, the current path is changed from the left and right direction to the top and bottom direction, i.e. Y
airection, and the Y coordinate of a point indicated by the touch of the fingertip 18 is detected in the same manner as in the X coordinate detection as described above~ At this time, the coordinate of the top end of the touch panel 22 is defined as iO" and the coordinate of the bottom end is defined as "ln.
As disclosed above, a high precision 2-dimensional coordinate value of a point indicated by the touch of the fingertip 18 to the touch panel 22 can be determined. The switching operation of the switching means 27 according to the signal on the control signal line H2 is performed in every very short time, 1 ms, for example, and those of the analog switches 23, 24, 25 and 26 according to the control signal ~11 are performed every 2 ms, for example.
The following is the fourth embodiment of the present invention, wherein accuracy of the coordinate detection according to FIGs.4~a) and (b) or FIG.5 i5 further improv-ed. In the precedent embodiments, the load capacitance CB
provided by a floating capacity of a human body, for ~Z5~537 example, is supposed to be constant during a short time as

5 ms. However, such floating capacity of a human body inevitably includes a fluctuation which limits the detec-tion accuracy. In this embodiment, the coordinate of a point on the touch panel 22 is determined according to a statistical process based on the values obtained during plural successive measurements.
In the process of obtaining coordinate x according to equation (22), plural times (f times, for example) of measurements for TXl and TX2 are performed by repeating the first sequence (wherein Hl is in high level, for example) as described above. By representing each measured repeti-tion period as TXli and TX2i, wherein i denotes integers l, 2, ... f, the arithmetic processor ln the control unit 20 calculates the corresponding values of (T li ~ Tsx) and ( x2i l-x) xli x2i' P' respectively. That is, ~ (Txli ~ Tsx) and ~ tTX2i ~ Tsl x) are obtained. In the above, Tsx and Tsl x are assumed to be constant during the plural successive measurements.
Hence, coordinate x is determined as follows:
f i~l( x2i l-x)/

i-l xli x) i~l(Tx2i Tsl_x)} ............. 123) ~;~5~i37 The control unit 20 outputs the value of the coordi-nate x from the output terminal 34 every f times of the measurements.
Equation (23) is the weighted mean value of coordinates xi (i = 1, 2, ... f) obtained in the f times of measurements, wherein xi represented by Xi = (TX2i TSl-x) ( xli TSx) + (Tx2i ~ TS1_x)} --..,,, (24) Therefore, it can provide an accurate coordinate value by cancelling off the uneveness in the measured T li and TX2i due to the fluctuation of the load capacitance such as a floating capacity of a human body.
The same process is performed on the detection of Y
coordinate.
As described above, in the second, third and fourth embodiment of the present invention, acquisition of data necessary for determining the coordinate is performed within a short time wherein the capacitance of a load can be assumed to be constant.
The analog switches 23, 24, etc. and the device for the switching means 27 are commercially available at low costs, therefore, the total cost of the coordinate detecting apparatus of the second, third or fourth embodiment of the present invention can still be lower than the conventional apparatus as shown in FI~.1.
FI~.6 illustrates the fifth embodiment of the present invention. This embodiment is a modification of the fourth 51~3~

embodiment shown in FIG.5, and comprises a diode array, biasing means and bias switching means. The diode array constitutes the second switching means, instead of the analog switches 23, 24, 25 and 26 in the previous embodi-ment.
Referring to FIG.6, each end of the touch panel 22 is provided with a diode array: a diode array 35 comprising diodes Dxl~ Dx2' DX3~ -- for the left end; a diode array 36 comprising diodes DXl,, Dx2-~ DX3, end; a diode array 37 comprising diodes Dy1, Dy2, Dy3 ... for the top end; a diode array 38 comprising diodes Dyl,, Dy2 " Dy3, ... for the bottom end. The diodes in the diode arrays 35 and 36 are arranged to have a common forward direction, and the diodes in the diode arrays 37 and 38 are arranged to have a common forward direction.
That is, for example, the diodes DXl, DX2~ DX3/ ... are connected to the touch panel 22 through their cathodes, while the diodes DXl,, DX2,, Dx3, ... are connected to the touch panel 22 through their anodes. Likewise, the diodes D 1~ Dy2~ D 3 ... are connected to the touch panel 22 through their anodes, while the diodes Dyl,, Dy2,, Dy3, ...
are connected to the touch panel 22 through their cathodes.
As shown in FIG.6, the anodes of the diodes DXl, DX2, Dx3, ... are connected to a common line L1 together with the cathodes of the diodes Dy1, Dy2, Dy3 ..., while the cathodes of the diodes DXl , DX2,, Dx3, to another common line L2 together with the anodes of the .5~537 diodes D 1'' D 2'' Dy3l ... . Between the common lines L
and L2, a bias voltage Eo is applied via bias switching means 39 having terminals pl, p2, ql, q2, rl and r2. That is, the terminal pl is connected to the common Iine L1 via serially connected resistors R1 and R2, the terminal p2 is connected to the common line L2 via another serially connected resistors R3 and R4, the terminals ql and r2 are connected to the ground 17 and the terminals rl and q2 of the bias switching means 39 are connected to a DC voltage source Eo referred to as a biasing means, wherein the terminal pl is selectively connected to the terminals ql and rl and the terminal p2 is selectively connected to the terminals q2 and r2, according to the signal provided by the control unit 20 via the control signal line Hl.
Capacitors Cl and C2 are respectively connected to the node of the resistors Rl and R2 and the node of the resis-tors R3 and R4. The respective opposite ends of the capacitors C1 and C2 are commonly connected to the output of the op amp 15. The non-inverting input of the op amp 15 is connected to the CR digital oscillator 19 via a capaci-tor C4.
The first switching means 27' is substantially equi-valent to the first switching means 27 in FIG.5, however, its connection to the output of the op amp 15 is inter-cepted by a capacitor C3. In the first switching means 27', the terminal Sl is selectively connected to the terminals Il and l' and the terminal S2 is selectively 3~

connected to the terminals 2 and I2, according to the signal provided by the control unit 20 via the control signal line H2.
The operation of the circuit shown in FIG.6 is as follows.
When a position on the touch panel 22 is indicated by touching of the fingertip 18 to the position, a floating capacity of a human body is applied thereto as a load capacitance CB, and the bias switching means 39 operates to respectively connect the terminals pl and p2 to the termi-nals rl and r2, according to the signal from the control unit 20 via the control signal line Hl. Hence, the DC bias voltage Eo, 12 volts, for example, is applied to the common line L1 via the resistors R2 and Rl, and biases the diodes in the diode array 35 in forward direction and the diodes in the diode array 37 in reverse direction. The common line L2 is connected to the ground 17 via the resistors R4 and R3, accordingly, the diodes in the diode array 36 are biased in forward direction, and thus, a current path is established in the X direction on the touch panel 22.
In the above, the potential on the common line Ll is higher than that on the common line L2, therefore, the diodes in the diode array 38 are biased in reverse direc-tion and substantially made be in open state. Accordingly, any current can not flow from the diode array 38 to the diode array 37 on the touch panel 22.

~s~

During the bias switching means continues above situation, the first switching means 27' operates to connect the input and output of the op amp 15 to the common lines L1 and L2, respectively, and then, operates to exchange the respective connections of the input and output of the op amp 15 to the common lines L1 and L2, according to the signal on the control signal line H2. Thus, the repetition periods Tyl, TX2, Tsx and Tsl x of the output pulses from the CR digital oscillator 19 are measured and stored by the control unit 20, and the coordinate x of the point indicated by applying a touch of the fingertip 18 is determined according to equation (22) or (23) as described with reference to FIG.5.
Following the above sequence, the bias switching means 39 operates to switch the terminals pl and p2 to the terminals ql and q2, respectively, according to the change of the signal on the control signal line Hl. Hence, the common line Ll is connected to the ground 17 via the resistors Rl and R2, and the common line L2 is connected to the bias voltage source Eo via the resistors R3 and R4. As a result, the diodes in the diode array 38 are biased in forward direction and the diodes in the diode array 36 are biased in reverse direction. The common line Ll is connected to the ground 17 via the resistors Rl and R2, and the diodes in diode array 37 are biased in forward direction, and thus, a current path is established in the Y
direction. At this time, the potential on the common line ~.51~37 L2 is higher than that on the common line L1, therefore, the diodes in the diode array 35 are biased in reverse direction, and substantially made be in open state.
Accordingly, any current can not flow from the diode array 35 to the diode array 36 on the touch panel 22.
During this situation, the first switching means 27' operates to exchange the respective connections of the input and output of the op amp 15 to the common lines L1 and L2, according to the signal on the control signal line ~2' and coordinate y of the point indicated by the touch of the fingertip 18 is determined according to the same procedure as in the detection of the coordinate x.
FIG.7 illustrates the sixth embodiment of the present invention. This embodiment is a modification of the previous embodiment shown in FIG.6, and intended to improve the operation speed of the previous embodiment.
The circuit of the coordinate detecting apparatus of FIG.6 is provided with the capacitors C1, C2, C3 and C4, each having a relatively large capacitance such as 0.1 uF.
The capacitor C3 is for blocking the DC bias voltage Eo to flow from the common line L1 or L2 to the ground 17 through the output of the op amp 15 having a low output impedance substantially zero. The capacitor C4 is for blocking the DC bias voltage Eo to input to the CR digital oscillator 19 through the common line L1 or L2. The necessity of the capacitors C1 and C2 arises from the requirement for preventing AC signal component flowing along the common ~;25~53~

line Ll or L2 from shunting to the biasing voltage source Eo or the ground 17 via the circuit comprising the resis-tors Rl and R2, or R3 and R4. That is, the AC signal component is prevented from shunting to the resistors Rl and R4 if the respective both ends of the resistors Rl and R4 are kept at the same AC signal voltage. This can be achieved by connecting the respective nodes of the resis-tors Rl and R2, and the resistors R3 and R4 to the output of the op amp 15, since the potentials on the input and output of the op amp 15 are equal. On the other hand, the output of the op amp 15 must be isolated from the DC bias voltage source Eo, as mentioned above, and, the capacitors Cl and C2 are provided to intercept the DC voltage applied to the output of the op amp 15 via the resistors R2 and R3.
In the above, each of the resistors R2 and R3 is selected to have value for supplying an appropriate magnitude of DC bias current to the touch panel 22, and hence, the values of the resistors Rl and R4 are selected sufficiently small compared with the resistors R2 and R3.
However, these capacitors retard the time constant of the transient occurring in the output pulses from the CR
digital oscillator 19 in accordance with the operation of the bias switching means 39, and the coordinate detection must wait until such prolonged transient is settled in every switching operation. In other words~ these capaci-tors limit the maximum operation speed of the coordinate detecting apparatus as shown in FIG.6. The time constant -~Z5~37 of the transient is mainly determined by (C1 + C4)~2 and ~C2 + C3)R3, which correspond to approxlmately 2 ms. This means that 2-dimensional coordinate detection can not be performed at a speed higher than 2 ms. --FIG.7 shows a partial configuration of a coordinatedetecting apparatus according to the sixth embodiment.
Referring to FIG.7, this embodiment comprises diode arrays 40, 41, 42 and 43, biasing means Vp and Vn, and bias switching means 44. Each of the diode arrays constitutes a second switching means.
The diode array 40 comprises diodes DXl, DX2, Dx3, Dx4~ Dx5 and Dx6, and the diode array 41 comprises diodes Dxl ' Dx2 ' Dx3 ' Dx4 ~ Dxs and Dx6 The diodes DXl, DX2 and Dx3 are connected to the left end of the touch panel 22 through their cathodes, while their anodes are commonly connected to the terminal p2 of the bias switching means x4~ Dx5 and DX6 are connected to the cathodes of the diodes DXl, DX2 and Dx3 through their anodes, respectively, while their cathodes are commonly connected to the terminal pl of the bias switching means 44. Thus, the diode array 40 is associated with the left end of the touch panel 22. Similarly, the diodes DXl', DX2' and Dx3' are connected to the right end of the touch panel 22 through their anodes, while their cathodes are commonly connected to the terminal p5 of the bias switching means 44. The diodes Dx4', Dx5' and DX6' are connected to the anodes of the diodes DXl', DX2' and Dx3' through their ~s~
~' cathodes, respectively, while their anodes are commonly connected to the terminal p6 of the bias switching means 44. Thus, the diode array 41 is associated with the right end of the touch panel 22. ~
The diode array 42 comprises diodes Dy1, Dy2, Dy3t Dy4, Dy5 and Dy6, and the diode array 43 comprises diodes Dyl', Dy2', Dy3', Dy4', Dy5' and Dy6'. The diodes Dy1, Dy2 and Dy3 are connected to the top end of the touch panel 22 through their anodes, while their cathodes are commonly connected to the terminal p4 of the bias switching means 44. The diodes Dy4, Dy5 and Dy6 are connected to the anodes of the diodes Dy1, Dy2 and Dy3 through their cathodes, respectively, while their anodes are commonly connected to the terminal p3 of the bias switching means 44. Thus, the diode array 42 is associated with the top end of the touch panel 22. Similarly, the diodes Dyl', Dy2' and Dy3' are connected to the bottom end of the touch panel 22 through their cathodes, while their anodes are commonly connected to the terminal p8 of the bias switching means 44. The diodes Dy4 ', Dy5 ' and Dy6' are connected to the cathodes of the diodes Dyl', Dy2' and Dy3' through their anodes, respectively, while their cathodes are commonly connected to the terminal p7 of the bias switching means 44. Thus, the diode array 43 is associated with the bottom end of the touch panel 22.

~.2S~37 It is obvious that each of the diode arrays in FIG.7 comprises serially connected three diodes, but the number of the diodes may be an appropriate different number.
In the bias switching means 44 in FIG.7, each of the terminals pl, p2, .... p8 selectively connects to corre-sponding terminals qi and ri, wherein i = l, 2, ... 8, according to the signal sent from the control unit 20 via the control signal line Hl as shown in FIG.6. The termi-nals ql, r4, q5 and r7 are commonly connected to a positive voltage source Vp constituting the biasing means. The terminals rl, r2, q3 and q~ are connected to the common line Ll shown in FIG.6, and the terminals r5, r6, q7 and q8 are connected to the common line L2 shown in FIG.6. The terminals q2, r3, q6 and r8 are commonly connected to a negative voltage source Vn constituting the biasing means.
The common lines L1 and L2 are alternately connected to the input and output of the op amp 15 in FIG.6, respectively.
In the above configuration, when the terminals pl, p2, ... p8 are connected to the respective ri (i = 1, 2, ... 8) terminals as shown in FIG.7, the diodes in the diode arrays 42 and 43 are biased in reverse direction and substantially made be in open states, and only the the diodes in diode arrays 40 and 41 associated with the ends in the X direc-tion are biased in forward direction and made conductive.
Thus, the data corresponding to the coordinate x of a point ind;cated on the touch panel 22 can be acquired.

~:Z 51~37 Following the above, when the terminals pl, p2, ... p8 are connected to the respective qi (i = 1, 2, ... 8) terminals, the diodes in the diode arrays 40 and 41 are biased in reverse direction and substantially made be in open states, and only the diodes in the diode arrays 42 and 43 associated with the ends in Y direction are biased in forward direction and made conductive. Thus, the data corresponding to the coordinate y of the point on the touch panel 22 can be acquired.
In the above, the positive bias voltage Vp and nega-tive bias voltage Vn should be set to have respective magnitudes larger than the peak voltage of the signal flowing through the load capacitance CB to the ground 17.
Each of the serially connected diodes in each diode array may separately be connected to the corresponding end of the touch panel 22 as shown in FIG.8. Although only the diode array 40 is illustrated in FIG.8, the connections of the xl' Dx2' D3x~ Dx4~ Dxs and Dx6 to the end of the touch panel 22 are separated form each other with a speci-fied spacing d. The common connection of the anodes of the diodes DXl, DX2 and D3x to the terminal p2 and the common connection of the cathodes of the diodes Dx4, Dx5 and DX6 to the terminal pl are the same as shown in FIG.7.
The many features and advantages of the present invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the embodiments which fall ~S~537 within the true spirit and scope of the invention.
Further, since numerous modifications and applications of the present invention will readily occur to those skilled in the art, for example, coordinate detection of a light beam spot or detection of incident angle of a light beam by an input panel comprising a resistive film and a photo-conductive film stacked thereon, or coordinate detection of a pressure point by a input panel comprising a resistive film and a pressure sensitive film stacked thereon, it is not desired to limit the invention to the exact construc-tions and operations illustrated and described, according-ly, all suitable modifications and equivalents may be restored to, falling within the scope and spirit of the invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUISVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for detecting the coordinate of a point on a plane, comprising: a resistive film means forming the plane, said resistive film means having an effective region used for the coordinate detection, said effective region being substantially rectangular and having a pair of ends disposed in a direction parallel to a relevant coordinate axis; a buffer means having an input and an output, said input being connected to one end of said film means and said output being connected to the other end of said film means, said buffer means having substantially infinite imped-ance and unit voltage gain; and measuring means connected to said input of said buffer means, to measure an impedance between an end of said film means and ground, wherein the point, whose coor-dinate on the plane is to be detected, is indicated by applying load means between the point to be detected and ground.

2. A coordinate detecting apparatus according to claim 1, wherein said measuring means measures the impedance when said load means is applied to said point to be detected and another imped-ance when said load means is applied to the end of said film means operatively connected to the input of said buffer means.

3. A coordinate detecting apparatus according to claim 1, wherein said measuring means measures the impedance when said load means is applied to said point to be detected and a third impedance when a load is not applied to said film means.

4. A coordinate detecting apparatus according to claim 2, wherein said measuring means measures the respective capacitance between each end of said film means and ground.

5. A coordinate detecting apparatus according to claim 4, wherein said measuring means includes a CR oscillator.

6. A coordinate detecting apparatus according to claim 1, further comprising: first switching means for operatively reversing the connections of the input and output of said buffer means to the ends of said film means.

7. A coordinate detecting apparatus according to claim 1, wherein the effective region of said resistive film means has another pair of ends disposed in the direction in parallel to another coordinate axis.

8. A coordinate detecting apparatus according to claim 7, further comprising: second switching means for alternately estab-lishing a conduction path between the first pair of ends of said film means and between the second pair of ends of said film means, said second switching means being connected between said buffer means and each of the ends of said film means.

9. A coordinate detecting apparatus according to claim 8, wherein said second switching means comprises a diode array having a plurality of diodes in parallel to one another, connected to each of the ends of said film means, each of said diodes connected to a respective one of the paired ends of said film means having a common forward direction.

10. A coordinate detecting apparatus according to claim 8, wherein said second switching means comprises a diode array having a plurality of diodes connected to each of the ends of said film means, wherein adjacent diodes have alternately different forward directions.

11. A coordinate detecting apparatus according to claim 10, wherein every other adjacent diode has a common connection to an end of said film means.

12. A coordinate detecting apparatus according to any one of claims 9, 10 or 11, further comprising: biasing means for opera-tively biasing said diodes of said diode arrays connected to the paired ends of said film means; and bias switching means connec-ted between said biasing means and said diode arrays to alternately bias said diodes of said diode arrays connected to different paired ends of said film means.

13. A coordinate detecting apparatus according to any one of claims 1 to 3, wherein said resistive film means is optically transparent.

14. A coordinate detecting apparatus according to claim 1, wherein said resistive film means is coated with an insulating material.

15. A coordinate detecting apparatus according to claim 14, wherein said insulating material on said resistive film means is optically transparent.

16. A coordinate detecting apparatus according to claim 1, wherein said buffer means is a voltage follower circuit.

17. A coordinate detecting apparatus according to claim 1, wherein said resistive film means includes a plurality of films stacked on the surface of a display device.

18. A coordinate detecting apparatus according to claim 1, wherein said measuring means includes a timer for measuring the repetition period of input signal pulses.

19. A coordinate detecting apparatus according to claim 6, wherein said measuring means stores each impedance value obtained in accordance with the operation of said first switching means and performs arithmetic operation to derive a mean value of the stored impedance values.