JPH0527885B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] In order to accurately and stably adjust the oscillation frequency of the oscillation circuit in the coordinate input device while matching the impedance of the electrode film side, an operational amplifier, a resistor, Variable resistor, first and second
A capacitance adjustment circuit including a capacitor is provided to apply an appropriately adjusted capacitance to the input stage of the oscillation circuit.

[Industrial application field]

The present invention relates to a coordinate input device, and more specifically to a coordinate input device that accurately detects the position of a point on an electrode film when the point on the electrode film is grounded via impedance. The present invention relates to a device that enables precise adjustment of the oscillation frequency of an oscillation circuit.

[Conventional technology]

With the development of office automation,
2. Description of the Related Art It is common practice to input and process figures, characters, etc. into computers. Keyboards are a typical input device for drawings, letters, etc., but there are other ways to input drawings, letters, etc. by specifying a position on a specific board with your finger or pen, and inputting the coordinates. A coordinate input device that performs the following is known.

An example of a coordinate input device is a method in which a resistor sheet is used as an input board, current is passed from both ends of the resistor sheet to impedance contacts (points of contact with a finger or pen, etc.), and the coordinates of the connection point are determined from the current ratio. . The principle of such a system is shown in FIG. The principle of coordinate positioning will be explained below with reference to FIG.

A terminal 5 on one side of a power source 8 is connected to the left terminal 2 of the resistive sheet 1, which is made of a uniform material, via a current measuring device 9. A terminal 5 on one side of the power supply 8 is also connected to the right terminal 3 of the resistance sheet 1 via a current measuring device 10.
is connected. The other terminal of power supply 8 is grounded to earth 7 . The output of the current measuring device 9 is A/
It is connected to a D converter 11, and the output of the current measuring device 10 is connected to an A/D converter 12.

The outputs of the A/D converters 11 and 12 are sent to the control device 1.
Connected to 3. An arbitrary position 4 on the resistance sheet 1 is indicated via an impedance such as a finger or an indicator pen, which has one side grounded to the ground 7, and in this example, a capacitor 6. With this, the coordinates of the left end of resistance sheet 1 are 0 and the coordinates of the right end are 100.
Coordinate x of any position 4 on the sheet where (0≦x
≦100) is indicated.

In this state, the resistance value between terminals 2 and 4 is
Rx, and the resistance value between terminals 3 and 4 is R _1-x . Also, the current flowing from terminal 2 to 4 is Ix, and the current flowing from terminal 3 to 4 is
Let the current flowing in I _{1-x be I 1-x} . At this time, the resistance value of the resistor sheet 1 made of a uniform material is proportional to the length of the resistor, so the coordinate x of the indicated point 4 is given by x=Rx/(Rx+R _1-x )...(1) It will be given to you. Furthermore, since the voltage drop due to the resistance value Rx and the voltage drop due to the resistance value R _1-x are equal, the following relationship holds true: RxIx=R _1-x I _1-x (2). From equations (1) and (2), x=I _1-x /(Ix+I _1-x )...(3). That is, the coordinate x of the indicated point 4 is
It can be determined if the current Ix flowing between terminals 3 and 4 and the current I _1-x flowing between terminals 3 and 4 are known. Therefore, these currents Ix and I _1-x are measured by current measuring devices 9 and 10.
is detected as a voltage value by A/D converters 11 and 1.
2, and then the control device 13 calculates the above equation (3), whereby the coordinate x can be determined as a digital value.

However, as the current measuring devices 9 and 10 for measuring current, an operational amplifier or the like is required to convert the current into voltage, and an A/D converter with high quantization accuracy is required to obtain the value of x with high accuracy. 11 and 12 are required, which makes the circuit complicated and increases the cost.

FIG. 3 shows an example of a coordinate input device with a simplified electric circuit in order to solve the above problems.

The resistance sheet 1 is the same as the one explained in FIG. To detect. In this case as well, the coordinates of the left terminal 2 of the resistance sheet 1 are 0, the coordinates of the right terminal 3 are 100, and the coordinate x of the indicated point 4 takes a value of 0≦x≦100. The device shown in FIG. 3 differs from the device shown in FIG. 2 in that both terminals of the resistor sheet 1 are not connected to a power source as in FIG. 2. Instead, terminal 14 is connected to terminal 2 and to terminal 3 via an operational amplifier 15. That is, the terminal 14 is connected to the non-inverting input of the operational amplifier 15, and the output of the operational amplifier 15 is connected to the terminal 3. The output of the operational amplifier 15 is also connected to the inverting input of the operational amplifier 15 itself.

In the circuit configured as described above, the operational amplifier 15 is a buffer circuit that operates as a voltage follower. That is, the feature of the device shown in FIG. 3 is that the terminals 2 and 3 of the resistor sheet 1 are connected by a buffer circuit that outputs a signal corresponding to the coordinate x of the indicated point 4.

With this configuration, the potentials of terminals 2 and 3 with respect to ground 17 become equal, and the input impedance of operational amplifier 15 can be considered to be infinite. The characteristic is that it does not include flowing current.

A variable capacitor 20, an oscillator 21, a counter 22, and a control device 23 are provided downstream of the operational amplifier 15. The oscillator 21 outputs a pulse signal with an oscillation frequency corresponding to the output signal of the operational amplifier 15, and the pulse signal is used as a reference pulse.
The counter 22 counts based on Rp, and the control device 2
3, a signal Sx corresponding to the final position x is output. The variable capacitor 20 will be described later.

The operation of the device shown in FIG. 3 will be described.

The potential of the terminals 2 and 3 with respect to the ground 17 is V, and the potential of the pointing point 4 with respect to the ground 17 is Va. Also, the resistance value between terminals 2 and 4 and the resistance value between terminals 3 and 4 are Rx and R _1-x, respectively.
shall be. Then, impedance Zo, Rx and impedance of impedance component 16 between terminal 4 and ground 17
In the apparatus shown in FIG. 3, in which Z is an equivalent impedance obtained by combining R _1-x , the fact that the coordinate x has a relationship with this impedance Z is utilized.

Let's calculate this impedance Z below.

First, the current Io flowing through the impedance component 16
= Va/Zo is the current flowing to Rx via indication point 4
Ix = (V-Va)/Rx and current flowing through R _1-x I _1-x =
It is the sum of (V-Va)/R _1-x . That is, (V-Va) (1/Rx+1/R _1-x )=Va/Zo (4). However, in reality, it can be considered that the current I _1-x does not flow from the input side to R _1-x for the reason mentioned above, so I _1-x = O, and therefore the current V/Z flowing through the equivalent impedance Z is , the current Ix flowing through Rx
= (V-Va)/Rx. That is, V/Z=V-Va/Rx (5). When Z is determined by eliminating V and Va from equations (4) and (5), Z=Rx (1+Zo・Rx+R _1-x /Rx・R _1-x )...(6). Here, (Rx+R _1-x ) is the impedance
If Rx _, R _1-x and Zo are set to be sufficiently small compared to the size of Zo | _{Zo x} ...(7) Here, resistance sheet 1 is
Since it is the same as the example in the figure, the above equation (3) is transformed to become R _1-x = 1-x/x·Rx (8). Substituting equation (8) into equation (7) to find x, we get x=1−Zo/Z (9). As can be seen from equation (9), by using the configuration shown in Figure 3, the coordinate x of the indicated point 4 is defined as a function of the equivalent impedance Z and the impedance Zo between the terminal 14 and the ground 17. be done.
Therefore, by measuring the values of the equivalent impedance Z and impedance Zo and calculating equation (9), the coordinate x
The value of can be calculated.

To summarize the above principle, both terminals of the resistor sheet 1 can be connected to a buffer circuit 15 using a voltage follower.
, and the resistance value of resistance sheet 1 is the impedance Zo of the finger or indicator pen for pointing the pointing point 4.
Set it so that it is sufficiently small compared to the size of . Then, impedance Zo is measured in advance, and a position signal Sx is sent via the above-mentioned oscillator 21, counter 22, and control device 23 to respond to equation (9).
Output.

As a specific example of the impedance component 16 in the configuration shown in FIG.
Consider the case where is the capacitance. now,
If the capacitance is Co, then Zo=1/jωCo...(10). Here, j is a symbol representing a complex quantity, and ω is the angular frequency of the signal added to the capacitance. (Ten)
Substituting the formula into formula (7), Z=1/jω(C _O *R _1-x /Rx+R _1-x ) ...(11). Therefore, it can be seen that the equivalent impedance Z between the terminal 14 and the arm 17 is also an equivalent capacitance with a capacitance value of C=Co·R _1-x /Rx+R _1-x (12). When determining X using equations (8) and (12), x=1-(C/Co)...(13). From the above, when capacitance is used for impedance Zo, the equivalent impedance between terminal 14 and ground 17 is also capacitance, and the coordinate x can be determined from equation (13) using these values.

As impedance 16, large capacitance value
Using a capacitive pen with Co, for example 1000pF. The terminal 14 is connected to an oscillator 21, and its output pulses are counted by a counter 22 and then applied to a control device 23. Then, the coordinate value position signal Sx is taken out as an output of the control device 23.

[Problem that the invention seeks to solve]

When the resistor sheet 1 is attached to the surface of a CRT display panel and used for positioning the CRT display on the panel, the oscillation frequency of the oscillator 21 should be adjusted to prevent malfunctions caused by electrostatic noise and magnetic noise from the CRT. Set the frequency away from the noise frequency.

Next, since stray capacitance is added to the resistor sheet, the value of the above-mentioned capacitance C becomes different from that of equation (12). Since the stray capacitance varies each time and its value cannot be determined in advance, a variable capacitance capacitor 20 is provided as shown in FIG. 3 in order to adjust the oscillation frequency of the oscillator.

However, the variable capacitor 20 used is a trimmer type, and the movable range is 1
Turn, the variable capacitance range is only about 0 to 100 pF or 0 to 150 pF, and on the other hand, the capacitance of about 100 pF can be changed in one turn.
A problem has been encountered where the oscillation frequency of the oscillator cannot be adjusted precisely.

Further, the trimmer-type variable capacitor 20 has a problem in that the capacitance changes due to misalignment of the trimmer, etc., and the oscillation frequency of the oscillator changes after adjustment, resulting in lack of stability.

[Means to solve the problem]

The present invention solves the above-mentioned problems associated with adjusting the oscillation frequency.

In the present invention, an electrode film, a position signal generation circuit that generates a signal responsive to the position of the one point on the electrode film when one point on the electrode film is grounded via impedance, and an output of the position signal generation circuit are provided. an oscillation circuit that outputs a pulse signal of an oscillation frequency in response to a signal, and an oscillation circuit provided at an input stage of the oscillation circuit,
an operational amplifier, a variable resistor with one end connected to one input terminal of the operational amplifier and the other end connected to the output terminal of the operational amplifier, a resistor provided between one input terminal of the operational amplifier and ground, A first capacitor is provided in series on a path connecting the output terminal and the other input terminal, and a second capacitor is provided between the path and ground, and the resistance value of the variable resistor is changed. By this, the second capacitance is proportional to the first capacitor and is defined by the ratio of the resistance value of the variable resistor and the resistance value of the resistor.
a capacitance adjustment circuit configured to change the capacitance of the capacitor, the oscillation frequency of the oscillation circuit being changed in response to a change in capacitance between the capacitance adjustment circuits. A coordinate input device is provided.

〔Example〕

An embodiment of the invention will be described with reference to FIGS. 1a and 1b.

In FIG. 1a, the flat transparent electrode film 30 is
A switching circuit 40 is provided on the CRT display surface to detect that an arbitrary point P (X, y) on the transparent electrode film 30 is designated. The switching circuit 40 is composed of X-direction switching circuits 43, 44 and Y-direction switching circuits 41, 42, and each switching circuit 41-44 is provided with a plurality of analog switches determined by the resolution necessary for position detection. . That is, to detect the position of the point P in the y direction, the analog switches in the switching circuits 43 and 44 are turned on one after another, and the equivalent capacitance is determined according to the resistance change of the connected transparent electrode film 30 when the switches are turned on. The change in is detected by the amplifier 15 as in the conventional case. The same applies to the x direction.

For simplicity, only one direction of the transparent electrode film 30, for example, the The control input terminal of the oscillator 21, such as the one shown in FIG. For example, the CR digital oscillator is well known as an oscillator whose oscillation frequency is proportional to the input capacitance (see, for example, Mitsubishi Electric IC Tab, ``'86 Mitsubishi Semiconductor General-Purpose IC Edition'', pages 5-6).

That is, in the above oscillator, if the proportionality constant is k, the oscillation periods for input capacitances Co and C are To and T, respectively.
Then, To=K・Co...(14) T=K・C...(15) Therefore, from equations (13) to (15), X=IT/To...(16) Position X can be detected as

In order to detect the synchronization T and To, a frequency considerably higher than the oscillation frequency of the oscillator 21, e.g.
Applying a 6MHz reference pulse Rp to the counter 22,
The oscillation period of the oscillator 21 is calculated.

However, in general, the stray capacitance between the transparent electrode film 30 and the ground is greatly influenced by the installation state and usage environment of the coordinate input device of the present invention. For example, this stray capacitance increases if the device of the invention is mounted in a grounded metal case or if the ambient temperature increases. Therefore, the above-mentioned capacitance changes as this stray capacitance changes. Therefore, in order to prevent the frequency generated by the oscillator 21 from being affected by the change in stray capacitance described above, a capacitance adjustment circuit 24 is provided on the input side of the oscillator 21.

A circuit diagram of the capacitance adjustment circuit 24 is shown in FIG. 1b. In the capacitance adjustment circuit 24, an operational amplifier 243, a fixed resistor 241, a variable resistor 242, and capacitors 244 and 245 are connected as shown, and terminals T ₁ and T ₂ are connected as shown in FIG. This corresponds to terminals T ₁ and T ₂ of .

In FIG. 1b, the resistance values of resistors 241 and 242 are R ₁ and R ₂ respectively, and the resistance values of capacitors 245 and 24 are R 1 and R 2 , respectively.
When the capacitances of terminals _T1 and T2 are respectively C1 and _C2 , the capacitance C' between the terminals _T1 and _T2 is expressed by the following equation.

C′=C ₁ −R ₂ /R ₁ C ₂ (17) Equation (17) is derived as follows.

The current flowing from terminal T1 in Figure 1b is I, the current flowing through capacitor _C1 is _I1 , the current flowing through capacitor _C2 is _I2 , the current flowing through resistor R1 is I _r1 , and the current flowing through resistor R2 is I _r2. When, terminal T1,
When applying AC voltage V across T2, the following holds true: I=I ₁ +I ₂ (17-1) I ₁ =C ₁ ·dV/dt (17-2). Here, the potential of the non-inverting input terminal a of the operational amplifier 243 is V, and the operational amplifier 243
Since the output of the operational amplifier 24 is negatively fed back,
The potential of the inverting input terminal b of No. 3 is also V. When the output potential of the operational amplifier 243 is Vc, I _r2 = (Vc - V)/R ₂ , I _r1 = V/R, and I _rl = I _r2 . Solving this for Vc, Vc=(1+R ₂ /R ₁ )V Therefore, I ₂ =C ₂・d(V−Vc)/dt=−C ₂・(R ₂ /R ₁ )・dV/ dt...(17-3) (17-1), (17-2), and (17-3) above
From I=[C ₁ −C ₂・(R ₂ /R ₁ )]・dV/dt ...(17-4) Here, the capacitance between terminals T1 and T2
Since there is a relationship of I=C'・dV/dt depending on C', equation (17) can be derived from equation (17-4).

By connecting such capacitance C' in parallel with the capacitance from the transparent electrode film 30 side and applying it to the oscillator 21, the oscillation frequency of the oscillator 21 can be changed.

Here, C ₁ = 50pF, C ₂ = 100pF, R ₁ = 20kΩ,
When R ₂ is a trimmer resistance of 0 to 10 kΩ (10 turns), adjustment is possible in the range of C' = 50 pF to 0 pF. The accuracy of the formula capacitor C' is determined as follows. Here, assuming that the adjustment accuracy of the trimmer resistor and trimmer capacitor adjustment screws and rotation angle of the adjustment dial is 30 degrees, by turning the adjustment dial 10 times, the adjustment will be from 0 to 0.
The adjustment accuracy of the resistance value of the trimmer resistor whose resistance value changes up to 10kΩ is (30°/360°) x (10kΩ/10 turns) = 1/12kΩ (18). The value of the capacitor is
The first one with C ₁ = 50pF and C ₂ = 100pF as above.
When used for resistor R ₂ in figure b, C′ = C ₁ − (R ₂ /R ₁ ) C ₂ = 50pF − (R ₂ /20) × 100
pF=50[1-(R ₂ /10)] pF. Therefore, the absolute value of the amount of change in C′ when R ₂ changes by 1/12 kΩ is 50×(1/12×1/10)≈0.42(pF) (19).

On the other hand, the trimmer capacitor 20 shown in FIG.
In the case of 0 to 50 pF and one turn, the accuracy is 50 pF x (30°/360°) = 4.2 pF.

As described above, compared to the case shown in FIG. 3, a capacitance change of 1/10 can be performed with high precision, and fine adjustment can be made 10 times as much. It should be noted here that while the trimmer capacitor 20 is currently only available with one turn, trimmer resistors with about 10 turns are currently widely available. Therefore, the circuit shown in FIG. 1b can be easily constructed.

Further, since the change in capacitance with respect to the rotation angle of the trimmer resistor is small, even if a shift occurs in the rotational position, the change in capacitance is small and excellent stability is achieved.

The above resistance values R ₁ , R ₂ , capacitances C ₁ , C ₂
can be set arbitrarily.

〔Effect of the invention〕

As described above, according to the present invention, by using a capacitance adjustment circuit that can be easily implemented using conventional circuit technology, the oscillation frequency of an oscillator that is excellent in stability and fine adjustment can be adjusted. can be adjusted, and as a result, the accuracy of position detection in the electrode film is improved.

[Brief explanation of the drawing]

1a and 1b are circuit diagrams of a coordinate input device according to an embodiment of the present invention, and FIGS. 2 and 3 are configuration diagrams of a conventional coordinate input device. (Explanation of symbols), 1...Resistance sheet, 15...
Position signal generation circuit, 20... variable capacitor, 21... oscillator, 22... counter, 2
3...Control device, 24...Capacitance adjustment circuit, 3
0...Transparent electrode film, 40...Switching circuit,
41-44...Switching circuit.

Claims (1)

[Claims] 1. Comprising an electrode film, and when one point on the electrode film is grounded via a capacitance, a first capacitance is generated corresponding to the position of the one point on the electrode film. a position signal generation circuit that forms a first capacitor to be varied; and an oscillation circuit that connects one end of the first capacitor to an input terminal and outputs a pulse signal of an oscillation frequency responsive to the first capacitance. , provided at the input stage of the oscillation circuit, an operational amplifier; a variable resistor having one end connected to one input terminal of the operational amplifier and the other end connected to an output terminal of the operational amplifier; and one input terminal of the operational amplifier. a second capacitor provided in series on a path connecting the output terminal of the operational amplifier and the other input terminal, and a second capacitor provided between the other input terminal and ground. 3 capacitor, and by changing the resistance value of the variable resistor, the capacitor is proportional to the second capacitor and is defined by the ratio of the resistance value of the variable resistor to the resistance value of the resistor. a capacitance adjustment circuit configured to change a second capacitance between the other input terminal and ground by a capacitance, the other input terminal in the capacitance adjustment circuit is connected to the input terminal of the oscillation circuit together with one end of the first capacitor, and the oscillation frequency of the oscillation circuit can be adjusted by changing the second capacitance. coordinate input device.