Reduced noise touch screen apparatus and method

• US 5,796,389 A
• Filed: 05/02/1995
• Issued: 08/18/1998
• Est. Priority Date: 08/22/1994
• Status: Expired due to Term

• Current Assignee: International Game Technology
• Original Assignee: International Game Technology
• Inventors: Bertram, William K., Pease, Logan L.
• Primary Examiner(s): Luu, Matthew
• Application Number:
  US08/432,785
• Time in Patent Office: 1,204 Days
• Field of Search: 345/173, 345/174, 345/178, 345/179, 345/180, 178/18, 178/19
• US Class Current: 345/173
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

A touch screen having a high or fine resolution at relatively low cost is provided. In one embodiment, electrodes are placed directly on the surface of a CRT screen without the need for an electrode positioned on the rear surface. The touch screen may include a conductive coating and a protective coating which are preferably provided in a single vacuum chamber step. A high gain system including a high-frequency sampling bandpass filter provide discrimination of the desired signal over noise. A screen calibration technique is used to achieve linearization in order to convert the electric signals obtained from the screen into data indicative of the position of a touch on the screen.

Claims

1. A touch screen apparatus for sensing contact of a human finger or passive probe at a preselected location on a touch screen surface comprising:

a cathode ray tube (CRT) screen having an exterior surface and an interior surface, said exterior surface having an exterior edge at the periphery of the screen;

a plurality of bar electrodes positioned on said exterior surface substantially near the exterior edge of the screen in the absence of electrodes being placed on said interior surface;

a driver having a plurality of channels, each channel connected to one of the bar electrodes, for providing voltages to the bar electrodes, and for providing signals indicating contact of said human finger or passive probe on the surface, the signal resulting from a current flow through said human finger or passive probe contacting the surface, the signals being a function of a difference in voltage level at a first time, t₁, when there is no contact and at a second time t₂, coincident with the time of contact such that the potential at one of said plurality of electrodes changes from a first value to a second value when said touch screen is touched; and

position determining means for receiving the signals from the channels and evaluating the signals to identify the position of the touch relative to the plurality of bar electrodes

wherein the driver further comprises an oscillator connected to each of the channels for setting up timing intervals for digitizing analog signals generated by each of the channels;

amplifier means in each of the channels having a first input, a second input, and an output, the first input being connected to the oscillator, the second input being connected to the oscillator and to a corresponding bar electrode, such that when no touch is present, the amplifier means is at equilibrium with a substantially constant amplifier output signal, and when a touch occurs on the surface, a current flows from the screen through the touching apparatus, causing a voltage drop across the inputs on the amplifier means and a resulting amplifier output signal;

a filter in each of the channels for receiving the resulting amplifier output signal from the amplifier means and substantially passing signals greater than a predetermined frequency.

2. The touch screen apparatus of claim 1, wherein the touch screen surface is substantially rectangular, further comprising:

a first bar electrode positioned substantially parallel to and near a top edge of the screen;

a second bar electrode positioned substantially parallel to and near a bottom edge of the screen;

a third bar electrode positioned substantially parallel to and near a left edge of the screen;

a fourth bar electrode positioned substantially parallel to and near a right edge of the screen;

the first and second bar electrodes functioning as a pair to determine a relative vertical position of the touch; and

the third and fourth bar electrodes functioning as a pair to determine a relative horizontal position of the touch.

3. A touch screen apparatus for sensing contact at a preselected location on a touch screen surface comprising:

a screen having a surface with an exterior edge at the periphery of the screen;

wherein the touch screen surface is substantially rectangular;

a plurality of bar electrodes positioned substantially near the exterior edge of the screen;

a driver having a plurality of channels, each channel connected to one of the bar electrodes, for providing voltages to the bar electrodes, and for providing signals indicating contact on the surface, the signal resulting from a current flow through a touching apparatus contacting the surface, the signals being a function of a difference in voltage level at a first time, t₁, when there is no contact and at a second time t₂, coincident with the time of contact;

position determining means for receiving the signals from the channels and evaluating the signals to identity the position of the touch relative to the plurality of bar electrodes;

a first bar electrode positioned substantially parallel to and near a top edge of the screen;

a second bar electrode positioned substantially parallel to and near a bottom edge of the screen;

a third bar electrode positioned substantially parallel to and near a left edge of the screen;

a fourth bar electrode positioned substantially parallel to and near a right edge of the screen;

the first and second bar electrodes functioning as a pair to determine a relative vertical position of the touch;

the third and fourth bar electrodes functioning as a pair to determine a relative horizontal position of the touch;

wherein the relative vertical position of P_V of the touch is determined as a function of voltage measured at the first bar electrodes (V_T), and voltage means used at the second bar electrode (V_B), as follows;

##EQU2## and the relative horizontal position (P_h) of the touch is determined as a function of voltage measured at the third bar electrode (V_L) and voltage measured at the fourth bar electrode (V_R), as follows;

##EQU3##

4. A touch screen apparatus for sensing contact at a preselected location on a touch screen surface comprising:

a screen having a surface with an exterior edge at the periphery of the screen;

a plurality of bar electrodes positioned substantially near the exterior edge of the screen;

a driver having a plurality of channels, each channel connected to one of the bar electrodes, for providing voltages to the bar electrodes, and for providing signals indicating contact on the surface, the signal resulting from a current flow through a touching apparatus said human finger or passive probe contacting the surface, the signals being a function of a difference in voltage level at a first time, t₁, when there is no contact and at a second time t₂, coincident with the time of contact;

position determining means for receiving the signals from the channels and evaluating the signals to identify the position of the touch relative to the plurality of bar electrodes;

wherein the driver further comprises an oscillator connected to each of the channels for setting up timing intervals for digitizing analog signals generated by each of the channels;

amplifier means in each of the channels having a first input, a second input, and an output, the first input being connected to the oscillator, the second input being connected to the oscillator and to a corresponding bar electrode, such that when no touch is present, the amplifier means is at equilibrium with a substantially constant amplifier output signal, and when a touch occurs on the surface, a current flows from the screen through the touching apparatus, causing a voltage drop across the inputs on the amplifier means and a resulting amplifier output signal;

a filter in each of the channels for receiving the resulting amplifier output signal from the amplifier means and substantially passing signals greater than a predetermined frequency; and

an analog-to-digital converter for sampling the filtered output signal at at least a predetermined sampling rate.

5. The touch screen apparatus of claim 4, wherein the driver further comprises a clock connected to the filter in each of the channels for synchronizing a sampling rate of each filter.

6. The touch screen apparatus of claim 5, wherein the filter in each channel samples at a rate of at least three times per cycle of the clock signal.

7. The touch screen apparatus of claim 5, wherein the filter samples at a rate configured to enable a determination of an AC component of amplitude, an AC component of phase, and a DC component.

8. The touch screen apparatus of claim 7, wherein the sampling rate is at least 40 KHz.

9. The touch screen apparatus of claim 7, wherein a touch is determined to have occurred upon a peak of a sampled signal from the filter being above a threshold level, the sampled signal being represented by the following equation:

space="preserve" listing-type="equation">S=A(ω

t+φ

)+T sin (ω

t+φ

)) (9)

wherein=

S represents the sampled signal;

A is the amplitude of the sampled signal;

ω

is the phase angle;

φ

is the phase of the sampled signal;

T is the amplitude of the current flowing through the touch apparatus.