JPH0580011B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a position measuring device for a touch panel system having a capacitive faceplate, and in particular for generating an address signal representing the position of a stylus in contact with the faceplate. touch panel
The present invention relates to a position measuring device for a system.

BACKGROUND OF THE INVENTION Touch panel systems are data entry devices that allow an operator to interact with information displayed on a screen. For example, by touching a particular location on the screen, one of a plurality of computer command functions displayed at different locations on the display screen may be selected. The touch panel system includes a position measuring device that generates an address signal representing the location of the touch. The address signal is sent to the computer,
The computer determines which of the command selection items has been selected based on the address signal. The object used by the operator to touch the display screen is called a stylus, and may be, for example, the operator's finger, pen, or pencil.

Capacitive touch panel systems typically
It includes a face plate having an optically transparent conductive film of a preselected resistivity formed on its outer major surface. This face plate is placed in front of the screen of the display device so that the operator can touch the conductive film in line with the information displayed on the screen.

The operator touches the conductive membrane with a stylus, which has a finite, but not zero, capacitance with respect to ground potential. The stylus changes the characteristics of the amplitude modulated position measurement signal applied to the capacitive membrane. In such touch panel systems, the location touched by the stylus can be distinguished from other locations on the faceplate in response to changes in the characteristics of the position measurement signal.

SUMMARY OF THE INVENTION Due to the conductive film on the faceplate, capacitive touch panel systems suffer from at least two drawbacks. One is that the conductive film prevents electromagnetic noise generated by the display system (e.g., the retrace pulse of a cathode ray tube) or noise present in its environment (e.g.,
60Hz background noise). This sensitivity to noise makes it difficult to determine the location of contact. the other one is,
The conductive film causes electromagnetic interference in the touch panel system at the frequency of the position measurement signal. As a result, touch panel systems typically generate large amounts of electromagnetic noise.

Therefore, an object of the present invention is to provide a touch panel.
The system consists in providing a position measuring device.

Another object of the present invention is to provide a touch panel system position measurement device that reduces susceptibility to electromagnetic noise.

Another object of the present invention is to provide a position measuring device for a touch panel system that reduces the amount of electromagnetic noise generated at any particular measurement frequency.

SUMMARY OF THE INVENTION The present invention is a position measuring device for a touch panel system that generates an address signal representative of the touch panel location touched by a capacitive stylus. In a preferred embodiment, the touch panel system includes a faceplate on which a conductive layer is formed. This conductive layer has uniform resistivity, covers the entire outer major surface of the face plate, and supports four rod-shaped electrodes. Each of the rod-shaped electrodes is disposed near each side end of the outer main surface of the face plate and extends by approximately the same length as the side end. The rod-shaped electrodes determine the orthogonal axes on the face plate.
forming a pair of opposing electrical contacts. Each rod-shaped electrode is electrically connected to a conductive film and a contact position measuring circuit.

The position measuring device includes a measurement signal source that generates an amplitude modulated measurement signal that varies in frequency in a substantially random manner. Measurement signals are supplied to each rod electrode simultaneously. This position measuring device determines the position on the face plate that the stylus contacts by measuring the measurement signal current flowing from each electrode to the stylus. The current flowing through each electrode is inversely proportional to the distance between the electrode and the location where the stylus contacts the faceplate. The current measurements obtained for the four electrodes are analyzed by a microprocessor to determine the location of the stylus contact.

The contact position is determined separately for each rod-shaped electrode. Next, position recognition for one of the rod-shaped electrodes will be explained. This explanation is equally applicable to the other remaining three electrodes.

A signal source sends a measurement signal to the rod-shaped electrode via the primary coil of the transformer. The transformer creates a potential difference between the output terminals of the secondary coil that is proportional to the current drawn by the stylus through the rod electrode. A differential amplifier having first and second input terminals is connected to two of the transformer.
Connected to the output terminal of the next coil. A differential amplifier receives this potential difference and generates positive and negative differential output signals proportional to the current. The transformer, in conjunction with the differential amplifier, acts as a current meter that measures the current drawn through the electrodes.

A lock-in signal demodulator uses the same irregular frequency measurement signal as a reference signal to demodulate the positive and negative differential output signals. A reduced pass filter connected to the signal demodulator generates from the demodulated signal a sufficiently stable state address signal corresponding to the average amplitude value of the current drawn through the rod electrode.

The position measuring device of the present invention has reduced susceptibility to electromagnetic noise due to the lock-in characteristics of the signal demodulator and low pass filter. Furthermore, the position-measuring device of the present invention generates electromagnetic noise over a band of random frequencies of the position-measuring signal. As a result, the position measuring device reduces the amount of electromagnetic noise at one particular measurement signal frequency.

The present invention is a position measuring device for a touch panel system that recognizes the contact position between a display surface on which a transparent conductive layer having a predetermined resistivity is uniformly formed and a stylus connected to a reference potential source. Signal generation means for generating a position measurement signal of a predetermined amplitude whose frequency continuously changes irregularly; and the signal generation means are disposed facing each other at intervals on the display surface, and the position measurement signals from the signal generation means are supplied to each of the signal generation means. The present invention is characterized in that it comprises at least one pair of electrodes, and current measuring means for measuring the value of the current flowing through each electrode, and the contact position is measured from each current value obtained by the current measuring means.

Embodiment FIG. 1 shows a touch panel system 10 and associated position measuring device 12 of the present invention.
The touch panel system 10 includes, for example, a display 18 incorporating a cathode ray tube (hereinafter referred to as CRT).
includes an optically transparent face plate 14 disposed opposite a screen 16 of the screen. The display 18 is
The CRT may be replaced by a liquid crystal display or a signboard displaying fixed information such as a numeric keypad.

Faceplate 14 includes an optically transparent, electrically conductive layer 20 covering substantially the entire outer major surface 22 thereof. Layer 20 is formed of indium tin oxide (ITO) and has uniform resistivity. Face plate 14 is typically square and has a major surface 22 having a first pair of opposing side edges 26 and 30 and a second pair of opposing side edges 28 and 32.

Rod-shaped or thin strip-shaped electrodes 36, 38, 40 and 42 are arranged at side ends 26, 28, 30 and 32, respectively.
arranged along the length of the electrodes 36 and 4
0 and electrodes 38 and 42 form two pairs of opposing electrical contacts, along the length of which the conductive layer 2
electrically connected to 0. The electrodes 36 and 40 are
The range of the X-axis is limited with an origin 44 positioned near the lower end of the electrode 36 on the face plate 14. Electrodes 38 and 42, on the other hand, define a Y-axis range with an origin 44 positioned near the left end of electrode 38 on faceplate 14.

Due to the resistivity of layer 20, face plate 14
The effective total resistance values R _X and R _Y in the upper X-axis and Y-axis directions, respectively, are determined. The position measuring device 12 determines the location 46 on the face plate 14 in contact with a capacitive stylus 48 (see FIG. 2), such as a human finger, for example. (position 46, contact position 4
The stylus 48 (referred to as 6) can be modeled as a capacitor 50 and a resistor 52 electrically connected in parallel to ground potential. capacitor 50
The capacitance of the resistor 52 is from 5 to several hundred pF.
The resistance value of is any valid value. The position measuring device 12 has a contact position 46 and electrodes 36, 38, 4.
By measuring the spacing between each of 0 and 42, the contact location 46 is determined.

Position measuring device 12 includes four similar position measuring auxiliary circuits 54a, 54b, 54c and 54d. These position measurement auxiliary circuits are connected to the electrode 3, respectively.
6, 38, 40, and 42 to generate an address signal representative of the contact location 46 and the spacing between the electrodes. Auxiliary circuits 54a-54d preferably operate simultaneously, ie, in parallel, to generate address signals. Position detection auxiliary circuits 54a to 54d
Components corresponding to are designated by the same number with a letter suffix a to d. The following explanation will be given only for the position measurement auxiliary circuit 54a, but the same applies to the other position measurement auxiliary circuits 54b to 54d.

A position measurement signal source 56 generates a bipolar continuous square wave measurement signal and outputs a bipolar continuous square wave measurement signal to the input end 5 of the output filter 60.
Send to 8. The measurement signal alternates between a positive voltage +V and a negative voltage -V with a sufficiently randomly varying frequency. In the preferred embodiment, microprocessor 62
acts as a pseudo-random number generator that generates a pseudo-random number signal that is fed to the input 64 of the measurement signal source 56. In response to a pseudorandom number signal,
The measurement signal source 56 responds to the value of the pseudorandom number.
Generates a measurement signal with a frequency of 150-250kHz. Microprocessor 62 generates pseudorandom numbers at a frequency of approximately 50kHz.

Output filter 60 is preferably a low pass filter and cooperates with output resistor 66 to create an output impedance of 2K ohms. This output impedance will be determined later by the electrodes 36 to 42.
It works to improve the uniformity of the signal-to-noise ratio (hereinafter referred to as S/N ratio) of the signals. Output filter 60
sends a square wave measurement signal to electrode 36 via primary coil 68a of transformer 70a. Primary coil 68
a is arranged such that its positive terminal 72a and negative terminal 74a are electrically connected to the electrode 36 and the output resistor 66 of the filter 60, respectively.

Referring to FIGS. 1 and 2, contact location 46
The stylus 48 in contact _with
It is divided into a resistance value kxRx and a second resistance value (1-kx)Rx. Here, kx is the contact position 46 and the electrode 3
The distance 76a between 6 and 6 is shown. The first resistance value represents the resistance value of the layer 20 between the contact location 46 and the electrode 36;
The second resistance value represents the resistance value of the layer 20 between the contact location 46 and the electrode 40. Stylus 48 in contact with layer 20 causes a current proportional to the measurement signal to flow to ground potential. The position measurement auxiliary circuits 54a and 54c can determine the contact position 46 along the X-axis by measuring the current flowing to ground potential through two resistors having first and second resistance values.

Regarding the position measurement auxiliary circuit 54a, the transformer 7
0a, 2 inductively coupled to the primary coil 68a.
It includes two secondary coils 78a and 80a. The positive terminal 82a and the negative terminal 84a of the secondary coil 78a are
They are electrically connected to the inverting input terminal 86a of the differential amplifier 88a and to the ground potential, respectively. On the other hand, the negative terminal 92a and the positive terminal 94a of the secondary coil 80a are
They are electrically connected to the non-inverting input terminal 96a of the differential amplifier 88a and the ground potential, respectively. Differential amplifier 8
8a operates in differential input, differential output mode.

A current flowing to the ground potential through the resistor having the first resistance value flows through the primary coil 68a. This current induces a current Ia in the secondary coils 78a and 80a, which flows through the positive terminal 82a, the negative terminal 92a, and the 22 ohm resistor 98a connected between these terminals. Current Ia creates a potential difference across resistor 98a that is proportional to the current flowing through electrode 36 due to the presence of stylus 48 at contact location 46. This potential difference is supplied to input terminals 86a and 96a of differential amplifier 88a. Differential amplifier 88a generates positive and negative differential output signals corresponding to the current flowing through electrode 36 at positive output terminal 106a and negative output terminal 108a, respectively. The differential output signal is modulated with a sufficiently irregular measurement signal frequency,
The amplitude of the differential output signal is determined by the contact location 46 and the electrode 36.
is inversely proportional to the distance between them.

Output terminals 106a and 108a send positive and negative differential output signals to different input terminals of single pole double throw demodulation switch 114a. This double-throw demodulation switch receives at a switching control input terminal 116a the irregular measurement signal generated by the measurement signal source 62 as a reference signal. Switch 114a generates a demodulated output signal by sending positive and negative differential output signals to output terminal 118a during voltage levels +V and -V of the measurement signal, respectively. As a result, differential amplifier 88a and switch 114a
work together as a “lock-in” amplifier,
This amplifier is fixed to the frequency of the measurement signal and consistently demodulates the differential output signal. The demodulated output signal becomes a sufficiently stable address signal with relatively low noise. This address signal corresponds to the average value of the amplitude of the current flowing through the electrode 36 and represents the contact position 46 and the distance 76a between the electrode 36,
Capacitor 50 and resistor 52 of stylus 48
is substantially independent of the relative impedance of .

Demodulation switch 114a provides the steady state address signal at its output terminal to a first low pass filter 124a with a 3 dB cutoff frequency of 3.3 kHz. The cutoff frequency of low pass filter 124a is selected to quickly pass the filtered address signal of sufficient amplitude to automatic gain control circuit 126. The gain control circuit 126 includes a diode 128
a, 128b, 128c and 128d, and the anodes 130a, 130 of these diodes
Measurement signals generated by position measurement auxiliary circuits 54a, 54b, 54c and 54d are supplied to terminals b, 130c and 130d, respectively. diode 128a,
cathodes 132a, 132b, 132c and 132d of 128b, 128c and 128d, respectively;
is electrically connected to gain normalization circuit stage 134 and provides one of the highest voltage swing address signals generated by auxiliary circuits 54a and 54b.
Normalization circuit stage 134 automatically normalizes the processing of address signals with respect to one measurement signal.

The filter 124a further sends an address signal to
The signal is supplied to a second low-pass filter 136a with a 3 dB frequency of 100 Hz to optimize the S/N ratio of the address signal. Filters 124a and 136a work together to remove non-coherent noise signals induced into touch panel system 10 by electromagnetic waves or the system operator. Filter 136a
is a 4-input to 1-output analog multiplexer 1
An address signal is supplied to the input terminal 138a of 40. The multiplexer 140 includes auxiliary circuits 54a to 54a.
The address signal generated by 54d is sent in serial form to analog-to-digital converter 142. Microprocessor 62 receives the digital address signal from analog-to-digital converter 142 and generates the X and Y coordinates of touch location 46 in response to the signal.

The CRT of the display unit 18 displays the raster image on the screen 16.
An image is formed by scanning an electron beam in a pattern. Such images are formed in image frames containing about 200 to 2000 horizontal scan lines and called at a frequency of 20 to 90 Hz. The beam current of the scanning electron beam varies according to the amplitude of the video signal containing information representative of the displayed image. The video signal also includes a flyback pulse for each horizontal scan line that returns the electron beam to the start of the scan at the end of each successive horizontal scan line. The retrace pulse is in a one-to-one relationship with the horizontal scan line, and thus occurs at approximately 15-200 kHz.

The surface area of the conductive layer 20 of the touch panel system 10 is relatively large, for example 1100 cm, and the conductive layer 20 acts as a capacitor. Therefore, retrace pulses in the video signal are capacitively guided into layer 20, inducing large amplitude noise signals in measurement system 12. For example, the noise signal associated with the retrace pulse may have a voltage amplitude up to about 100 times the amplitude of the measurement signal on conductive layer 20.

The position measuring device 12 is able to determine the position of the contact point 46 by the combination of random variations in the measurement signal frequency and the lock-in characteristics of the position measuring auxiliary circuits 54a-54d. In particular, the noise signal associated with the retrace pulse, as well as other electromagnetic noise sources, is incoherent with the random measurement signal. Therefore,
Low pass filters 124a-124d and 136
A to 136d cooperate with the lock-in characteristics of auxiliary circuits 54a to 54d to attenuate noise signals and enable detection of contact position 46.

A feature of the invention is that the measurement signal frequency varies continuously and sufficiently irregularly so that the amplitude of the electromagnetic noise generated at each frequency by the position-measuring device 12 is reduced. In particular, since the measurement signal frequency varies in a bandwidth range of 150-250kHz, the electromagnetic noise generated by the measurement signal is also distributed in the same band. As a result, the electromagnetic noise generated by the position-measuring device 12 has a relatively low amplitude, and the electromagnetic interference caused by the position-measuring device 12 is reduced.

The microprocessor 62 causes the contact position 46 to
The method for recognizing axis values is explained below. The demodulated address signals generated by measurement auxiliary circuits 54a and 54c are DC voltages V1 and V2, respectively, proportional to the current flowing through electrodes 36 and 40, respectively. With respect to the distance 76a (see FIG. 2) and the position of the contact point 46 on the X axis represented by the variable X, the microprocessor 144 indicates the X coordinate by the following equation.

X=V1/(V1+V2) Microprocessor 62 also determines the Y coordinate of touch location 46 using a similar formula for the demodulated address signals generated by auxiliary circuits 54b and 54d.

In order to recognize both the X and Y coordinates of the contact location 46, the microprocessor 62 uses a memory 146 that stores a lookup table listing the X and Y axis coordinates corresponding to a particular measurement signal amplitude.
communicate with. A lookup table is created for each touch panel system to accommodate irregularities in the resistivity of the conductive layer.

Touch panel system 10 has a robust construction that operates at low power. The construction of touch panel system 10 can be made robust due to the limited number of electrodes in contact with conductive layer 20 and the inherent robustness of the transformer. , the system 10 operates at low power because, without stylus contact, very little power is dissipated in the conductive layer 20.

Referring to FIG. 3, the measurement signal source 56 has a digital-to-analog converter (hereinafter referred to as "DAC") 150 connected to an input terminal 64 and converts the pseudo-random number signal generated by the microprocessor 62 into receive. In response to a pseudorandom number signal,
DAC 150 generates a control voltage with an amplitude corresponding to the value of the pseudorandom number. A control voltage is provided to a control input terminal 152 of a voltage controlled oscillator 154. In response to the control voltage, the voltage controlled oscillator 154 generates a measurement signal with a frequency corresponding to the amplitude of the control voltage. Thus, DAC 150 and voltage controlled oscillator 154 cooperate with microprocessor 62 to generate a measurement signal of a frequency that continuously changes in an irregular manner.

In FIG. 1, output resistor 6 of filter 60
6 describes means operative to improve the uniformity of the signal-to-noise ratio of the signals provided to the measurement auxiliary circuits 54a-54d with reference to an exemplary system that does not use an output resistor 66. . When an operator contacts a single electrode on the faceplate of the exemplary system in close proximity to a location along a first axis, most of the electrical current flowing through the body from the faceplate is derived from this electrode. In this state, almost no current flows from the electrode indicating the position of the second axis. Therefore, S/ of the signal indicating the position of the first axis is
The magnitude of the signal-to-noise ratio can be up to 1000 times greater than the magnitude of the signal-to-noise ratio of a signal indicating position along an orthogonal axis. The range of this S/N ratio is
This is the biggest operational constraint on the dynamic range performance of position measuring devices.

Since the output resistor 66 of the filter 60 can have a non-zero resistance value in each of the electrodes 36 to 42 through which the stylus 48 passes current, the S/N of the signals supplied to the measurement auxiliary circuits 54a to 54d is
Improve ratio uniformity. As a result, electrodes 36 to 4
The currents flowing through the two are of similar magnitude and are usually kept to a difference of less than one order of magnitude. Therefore, resistor 66 acts as an automatic gain controller, greatly reducing the dynamic range required by auxiliary circuits 54a-54d, and allowing the system operator to incorporate signal source 56. .

Although preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit of the invention. For example, a pseudo-random frequency measurement signal may be generated in response to a pseudo-random number generated by a feedback shift register or an analog noise generator.

[Effects of the Invention] As described above, in the position measuring device of the touch panel system of the present invention, in order to measure the current value according to the touch position, the position signal supplied to the electrode on the display surface has an invariant frequency. Since a signal of a predetermined amplitude that changes regularly and continuously is used, the electromagnetic noise generated by the positioning signal is distributed in the same band as the positioning signal and has a relatively low amplitude.
This reduces electromagnetic interference caused by positioning signals.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a position measuring device in a touch panel system according to the present invention, and FIG.
FIG. 3 is a block diagram illustrating a measurement signal source for use within the apparatus of FIG. 1; In the figure, 14 is a display surface, 20 is a conductive layer,
36-42 are electrodes, 54a-54d and 70a-
70d is a current measuring means, and 56 is a signal generating means.

Claims (1)

[Claims] 1. A position measuring device for a touch panel system that recognizes the contact position between a display surface on which a transparent conductive layer having a predetermined resistivity is uniformly formed and a stylus connected to a reference potential source. a signal generating means for generating a position measuring signal of a predetermined amplitude whose frequency continuously changes irregularly; and a signal generating means disposed opposite to each other at a distance on the display surface to detect the position from the signal generating means. comprising at least one pair of electrodes, each of which is supplied with a measurement signal, and a current measuring means for measuring the value of a current flowing through each electrode, and measuring the contact position from each current value obtained by the current measuring means. A touch panel system position measuring device featuring: