JPH0441373B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical 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.

(Background Art) An example of the configuration of an input section of a conventional pressure-sensitive coordinate input device using a resistive film is shown in FIG. 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 resistance value 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 resistor terminal board 3 is also connected in the Y direction, and the voltage from the power source 4 is applied time-divisionally in the X and Y directions using a switch circuit, etc. It is already a well-known technique that the potential gradients in the X and Y directions can be obtained in a time-division manner by switching, so a detailed explanation will be omitted. Reference numeral 5 is a voltage rubber, and 6 is a flexible film sheet, which is an electrode sheet having a conductor coated on the pressure-sensitive rubber 5 side. 7 is a flexible conductor (for example, a fiber plated with nickel or copper electrolessly), and is a shield sheet in which the conductor for electrostatic shielding is connected to the ground potential of the circuit. Note that the conductor of the shield sheet 7 is generally coated with urethane rubber to improve insulation and abrasion resistance.

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 7 at the pressed point is led 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 sheet between the hand and the electrode sheet, and connecting the conductive portion of the shield sheet to a low-impedance ground potential.

FIG. 2 is an equivalent circuit of the input panel section shown in FIG. 1. 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 does not enter the operational amplifier input section. It doesn't come. However, at the input section of the operational amplifier 8, there is a capacitor 11 whose one terminal is connected to the ground potential. This is because 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. There will always be a 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.

(Objective of the Invention) The present invention aims to eliminate the distributed capacitance that exists between the seal sheet and the electrode sheet in the input panel configuration of the conventional pressure-sensitive coordinate input device, and to solve the above-mentioned problems. do.

The features of the present invention for achieving this purpose include a resistive film, a drive device that applies voltage to the resistive film, a pressure-sensitive rubber laminated on the resistive film, and a pressure-sensitive rubber laminated on the pressure-sensitive rubber. An electrode sheet having a conductive film, a shield sheet laminated on top of the electrode sheet and including a conductive part, and a detection device that digitizes and outputs the voltage corresponding to the pressure position that appears on the electrode sheet due to pressure applied by a writing instrument. A pressure-sensitive coordinate input device comprising a pressure-sensitive coordinate input device is provided with a low-impedance drive circuit that drives the conductive portion of the shield sheet and the electrode sheet so as to maintain them at the same potential.

(Structure and operation of the invention) Figures 3A and 3B are a block diagram and an equivalent circuit diagram of an embodiment of the present invention, respectively, and the same reference numerals are used for the same parts as in Figures 1 and 2. are doing. 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 works 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,
It also has the characteristic 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
It does not enter the circuit through 1. 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.

As explained above, in this embodiment, the conductive part of the shield sheet 7 receives the voltage of the electrode sheet 6 as an input, and is connected to the output of the operational amplifier forming the voltage follower, thereby preventing electrostatic noise from the outside. It has the advantage that the distributed capacitance 11 existing between the shield sheet 7 and the electrode sheet 6 can be canceled while providing a shielding effect.

Although operational amplifiers 8 and 14, which are also voltage shift followers, are shown separately in FIG. 3, operational amplifier 14 may be omitted and the output of operational amplifier 8 and point a may be connected.

Furthermore, in the above-mentioned embodiment, the operational amplifier 14 is described as a voltage follower, but other types can be used as long as the condition of output impedance is low enough to absorb noise with the same output voltage as the position detection voltage of the electrode sheet. Needless to say, the same applies to circuits.

(Effects of the Invention) As explained above, according to the present invention, in a conventional pressure-sensitive coordinate input device, the input panel of the device can be configured while having a shielding effect against electrostatic noise from the outside. This provides the effect of canceling the distributed capacitance that exists between the shield sheet and the electrode sheet.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an input panel of a conventional pressure-sensitive coordinate input device, FIG. 2 is an equivalent circuit diagram of a conventional input panel, and FIGS. It is an equivalent circuit diagram. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Resistive film, 3...Resistance terminal board, 4...Power source, 5...Pressure sensitive rubber, 6...Electrode sheet, 7...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.

Claims (1)

[Claims] 1. An electrode having a resistive film, a drive device that applies voltage to the resistive film, a pressure-sensitive rubber layered on the resistive film, and a conductive film layered on the pressure-sensitive rubber. A sensing device comprising a sheet, a shield sheet laminated on the electrode sheet and including a conductive part, and a detection device that digitizes and outputs a voltage corresponding to a pressure position that appears on the electrode sheet due to pressure applied by a writing instrument. A pressure-sensitive coordinate input device, characterized in that the pressure-sensitive coordinate input device is provided with a low-impedance drive circuit that drives the conductive portion of the shield sheet and the electrode sheet so as to maintain them at the same potential. 2. A patent claim characterized in that the drive circuit is composed of an operational amplifier, the output of which is connected to one input and also connected to the conductive part of the shield sheet, and the other input is connected to the electrode sheet. The pressure-sensitive coordinate input device according to item 1.