JPH0330901Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a pressure-sensitive coordinate input device that detects a position pressurized by a writing instrument or the like, and digitizes and outputs a voltage corresponding to the detected position.

(Prior Art) FIG. 2 shows an example of the configuration of an input section of a conventional pressure-sensitive coordinate input device using a resistive film. 1 is an insulating substrate, on which a resistive film 2 is coated. Further, a resistive terminal plate 3 made of a resistor having a sufficiently smaller resistive film than the resistive film 2 is coated to be electrically connected to the resistive film 2 . In order to generate a constant potential gradient in the resistive film 2 on this resistive terminal plate 3,
A power source 4 that applies a constant voltage is connected. The figure shows an example where voltage is applied in the X direction, but a similar resistance terminal plate 3 is connected in the Y direction as well.
By switching the voltage from the power supply 4 in the X and Y directions in a time-sharing manner using a switch circuit, etc.
It is already a well-known technique that the potential gradient in the Y direction can be obtained in a time-divisional manner. 5 is pressure sensitive rubber;
Reference numeral 6 denotes a flexible film sheet, and an electrode sheet with a conductor coated on the side of the pressure-sensitive rubber 5 or a flexible conductor (for example, a fiber plated with nickel or copper electrolessly) is used. Similarly to 6, 7 is a shield sheet in which a flexible conductor made of fibers plated with metal is connected to the ground potential of the circuit for electrostatic shielding. Note that the conductor of the shield sheet 7 is generally coated with urethane rubber to improve insulation and abrasion resistance. Therefore, shield sheet 7
has conductivity in the plane direction, but the surface is insulating.

When pressing with a writing instrument or the like from above the shield sheet 7, the pressure-sensitive rubber 5 at the pressed point becomes conductive, and the voltage of the resistive film 2 at the pressed point is guided to the electrode sheet 6, and the input stage operational amplifier 8 of the voltage detection circuit Further, the position is calculated from the potential gradient of the resistive film 2 in a subsequent detection circuit (not shown in the figure).

Without the shield sheet 7, when writing with a hand on the input surface, the noise induced by the human body would be induced into the electrode sheet 6 via the stray capacitance between the electrode sheet 6 and the hand. Since both the operational amplifier 8 and the resistive film 2 connected to the electrode sheet 6 have high impedance, this induced noise voltage becomes large, and this noise voltage is directly added to the detected voltage from the resistive film 2, and is used in the subsequent stage. Since it is input to the detection circuit, accurate position detection becomes impossible. For this reason, a method has been adopted in which noise is absorbed by inserting a shield seed between the hand and the electrode sheet and connecting its conductive portion to a low-impedance ground potential.

FIG. 3 is an equivalent circuit of the input panel section shown in FIG. 2. Note that the same numbers are used for the same parts as in FIG. Reference numeral 9 indicates the equivalent resistance of the resistive film 2, to which a constant potential gradient is applied by the power source 4. 10 is the combined resistance of the pressure sensitive rubber and the electrode sheet. In order to increase the input impedance of the operational amplifier No. 8, a voltage follower is generally used as shown in the figure. 11 is a distributed capacitance existing between the shield sheet 7 and the electrode sheet 6. 12 is a stray capacitance existing between the hand, which is a noise source 13, and the shield sheet.

In this way, in the conventional circuit, the conductive part of the shield sheet is connected to the ground potential, so the noise coming from the noise source 13 via the capacitor 12 escapes to the ground potential of the circuit and enters the operational amplifier input section. Not coming. However, at the input section of the operational amplifier 8, there is a capacitor 11 whose one terminal is connected to the ground potential. Therefore, in order to accurately detect the voltage from the resistive film 2 at the input of the operational amplifier 8, which is the input part of the voltage detection circuit, the capacitor 11 must be charged. must reduce the delay in charging time t.

Here, this charging time t increases approximately in proportion to the magnitude of the value C of the capacitor 11 and the resistance value R of the resistor 10. Furthermore, C changes depending on the thickness of the insulator of the shield sheet and the thickness of the film of the electrode sheet, and R changes greatly depending on the pressing force of the pressure-sensitive rubber, so the delay time t becomes extremely unstable and the resistance When detecting the position in the X and Y directions by switching the voltage applied to the membrane in the X and Y directions in a time-sharing manner, the time to apply the voltage in the If you also take minutes into account, it can be a very long time. This imposes restrictions on the data sampling period and position calculation time, which is a disadvantage in that it limits the writing speed or resolution. Furthermore, when the voltage application time is limited to a certain fixed time, in order to accurately detect the position, t must be set to less than that time, and the value of R must be made small. This means that the pressing force of the pressure-sensitive rubber, in other words, the writing pressure, must be increased, which greatly affects the ease of use of the device.

As mentioned above, in the conventional method, the shield sheet 7
Since the capacitance 11 exists between the electrode sheet 6 and the electrode sheet 6, there is a drawback that it greatly restricts the functionality of the device.

In order to improve the above-mentioned drawbacks, the applicant had previously proposed Japanese Patent Application No. 59-52207.
Shown in Figures A and B.

FIGS. 4A and 4B are a block diagram and an equivalent circuit diagram of an embodiment according to the above application, respectively, and the same reference numerals are used for the same parts as in FIGS. 2 and 3. Here, the difference from the conventional circuit is that an operational amplifier 14 is added and that point a, which is a conductive part of the shield sheet, is connected to the output of the operational amplifier 14 that constitutes a voltage follower. Since the operational amplifier 14 functions as a voltage follower as described above, the output is equal to the voltage of the + side input terminal, that is, the voltage of the electrode sheet 6, and has the characteristics of low output impedance. Therefore, the voltage of the noise source 13 that comes through the capacitor 12 is absorbed by the output of the operational amplifier 14, which has a low impedance, and does not enter the circuit through the capacitor 11. Furthermore, the same voltage is always applied to both ends of the distributed capacitor 11 due to the operation of the operational amplifier 14, and the capacitor 11 is not charged or discharged, so in terms of circuit operation, it is equivalent to the capacitor 11 not existing. Become. Therefore, there is no need to consider the delay time for charging the capacitor 11, which has had various adverse effects in conventional circuits.

(Problem that the invention attempts to solve) However, when viewed microscopically, the output impedance of the operational amplifier 14 is low, but from several ohms to several tens of ohms.
Since the shield sheet 7 has an impedance of 1, the noise cannot be completely absorbed, and a voltage equivalent to the voltage drop of the output impedance remains on the shield sheet 7.

In normal cases, this voltage is from several mV to several 100 mV
Furthermore, since the electrode sheet 6 is attenuated and coupled, the voltage is less than several tens of mV, and can be ignored in practical terms by either increasing the resolution of position detection or allowing some variation in position detection due to noise. This configuration is sufficient in many cases.

However, the resolution is very small, a few mV, and
If it is desired to detect the position accurately with as little variation due to noise as possible, the example shown in FIG. 4 also has the drawback that the shielding is insufficient and accurate position detection is difficult.

The purpose of this invention is to eliminate the distributed capacitance between the shield sheet and the electrode sheet that exists in the input panel configuration of conventional pressure-sensitive coordinate input devices, and to provide a pressure-sensitive coordinate input device that has a sufficient shielding effect. shall be.

(Means for Solving the Problems) The features of the present invention for achieving the above object include a resistive film 2 which is sequentially laminated on a planar substrate 1;
A pressure sensitive rubber 5, an electrode sheet 6, and a flexible first
a driving device 4 having a shield sheet 7 and a second shield sheet 15 and applying a potential gradient to the resistive film 2; and an operational amplifier 14 for bringing the first shield sheet 7 to substantially the same potential as the electrode sheet 6. is provided, the first shield sheet 7 is connected to a low output impedance source such as the output of the operational amplifier 14, the second shield sheet 15 is connected to ground or a low output impedance source, and is placed in a position where pressure is applied by a writing instrument. The coordinate input device extracts the corresponding voltage from the electrode sheet 6.

(Function) In the above configuration, the second shield sheet 15
is connected to ground or a low output impedance source, so external noise is absorbed into the ground potential via the second shield sheet. Even if there is a large amount of external noise that cannot be absorbed by the second shield sheet,
The leaked noise is completely absorbed by the first shield sheet. Furthermore, since the first shield sheet has substantially the same potential as the electrode sheet, charging problems due to the capacitance of the shield sheet do not occur.

Therefore, the drawbacks of the conventional devices of FIGS. 2 and 4A are improved and the object of the present invention is achieved.

(Example) Figures 1A and 1B are a block diagram and an equivalent circuit of an embodiment of the present invention, respectively, and the same reference numerals are used for the same parts in Figures 2 to 4. are doing.

The difference from the circuit shown in FIG. 4 here is that a second shield sheet 15 connected to the ground potential of the circuit is laminated.

The second shield sheet 15 is directly connected to the ground potential of the circuit, and the first shield sheet 7
Since there is no equivalent to the output impedance of the operational amplifier, the second shield sheet 1
Most of the induced noise coming to 5 is absorbed by the ground potential of the circuit.

FIG. 1B is an equivalent circuit of FIG. 1A showing the configuration of the present invention, and 16 is a distributed capacitance existing between the first shield sheet 7 and the second shield sheet 15. Grounding between the capacitors 17 and 16 is equivalent to grounding the second shield sheet 15. 17 is the second shield sheet 1
5 and the hand, which is a noise source.

As you can see from this circuit, the noise is capacitance 1
7 to ground potential and is mostly absorbed. Even if large noise etc. cannot be absorbed by the second shield sheet 15, it will be further absorbed by the first shield sheet 7, so that the influence on the electrode sheet 6 can be completely ignored.

Furthermore, the first shield sheet 7 is connected to the operational amplifier 1.
Since it is driven to have the same potential as the electrode sheet 6 at step 4, the influence of the distributed capacitance 11 is eliminated as described above.

Further, a differential current flows between the output of the operational amplifier 14 and the ground potential through the distributed capacitance 16 between the first shield sheet 7 and the second shield sheet 15, but the distributed capacitance 16 is usually very small, 100 pF or less. Therefore, the differential current is also very small, and its influence on the potential at point a can be ignored.

Note that between the electrode sheet and the first shield sheet,
An insulating layer is required between the first shield sheet and the second shield sheet.
In addition to coating the surface of the shield sheet with an insulating material, a plastic film may also be inserted, and various combinations are possible.

(Effect of the invention) As explained above, in this embodiment, the voltage of the electrode sheet 6 is input to the conductive part of the shield sheet 7,
By connecting to the output of the operational amplifier forming the voltage follower, a shielding effect is provided, and the distributed capacitance 11 between the shield sheet 7 and the electrode sheet 6 is canceled. In combination with the second shield sheet 15 connected to the ground potential of the circuit, it is possible to completely eliminate external electrostatic noise that cannot be removed by the shield sheet 7 alone.

Note that the shield sheet 15 may be connected not only to the ground potential of the circuit but also to another potential with low impedance. In addition, in the embodiment, operational amplifiers 8 and 14, which are voltage followers, are separated, but operational amplifier 14 is omitted, and the output of operational amplifier 8 and a
You may also connect the dots.

Furthermore, since most of the noise can be removed by the shield sheet 15, it is possible to use an inexpensive material for the conductive portion of the shield sheet 7, even if the shielding effect is somewhat reduced.

[Brief explanation of the drawing]

Figures 1A and B are block diagrams and equivalent circuit diagrams of a pressure-sensitive coordinate input device according to the present invention, Figure 2 is a conventional pressure-sensitive coordinate input device, Figure 3 is an equivalent circuit of Figure 2, and Figure 4 A and B are a configuration diagram and an equivalent circuit diagram of another conventional pressure-sensitive coordinate input device. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Resistive film, 3... Resistance terminal board, 4... Power source, 5... Pressure sensitive rubber, 6... Electrode sheet, 7... First shield sheet, 8... Operational amplifier, 9...Equivalent resistance of resistive film, 10...Combined resistance of pressure sensitive rubber and electrode sheet, 11...Distributed capacitance between shield sheet and electrode sheet, 12...Capacitance between human body and shield sheet, 13...Noise Signal source, 14... operational amplifier, 15... second shield sheet, 16... distributed capacitance between the first and second shield sheets.

Claims (1)

[Claims for Utility Model Registration] Resistive film 2 that is sequentially laminated on a flat substrate 1
, a pressure sensitive rubber 5, an electrode sheet 6, a flexible first shield sheet 7 and a second flexible shield sheet 1.
5, and a driving device 4 that applies a potential gradient to the resistive film 2.
and an operational amplifier 14 for setting the first shield sheet 7 to substantially the same potential as the electrode sheet 6, and the first shield sheet 7 is connected to the operational amplifier 1.
4, the second shield sheet 15 is grounded or connected to a low output impedance source, and a voltage corresponding to the position of pressure applied by the writing instrument is taken out from the electrode sheet 6. Features a pressure-sensitive coordinate input device.