CALIBRATION SYSTEM FOR TOUCH SCREENS

Abstract: The invention discloses a controller (12) for a touch screen (10) used on a cathode ray tube (CRT). The controller includes an interface (U8) for coupling to a host system (14) which provides video images to said CRT. A detector (S1) monitored by controller (12) indicates a user's desire to calibrate controller (12). The controller is responsive to detector (S1) to cause a calibration sequence to be performed by controller (12). A non-volatile memory (U6) stores the results of the calibration sequence. The controller (12) is adapted to monitor coordinates from touch screen (10) and to apply a correction to coordinates to provide the host system (14) with corrected coordinates using the calibration results stored in non-volatile memory (U6). A method of calibration is also disclosed.

Description: CALIBRATION SYSTEM FOR TOUCH SCREENS The present invention relates to a method and apparatus for the calibration of a touch screen for a cathode ray tube (CRT) and relates particularly, although not exclusively, to a method and apparatus for the calibration of a touch screen for a TV receiver or computer display unit. The invention has particular application to an interactive CD player. Such devices are sold by Philips Consumer Electronics BV under the name of Philips standard CD-i player, hereinafter termed the "CD-i player". The CD-i player can play CD disks in the 3 inch or 5 inch format. The disks may include sound and visuals and provide a menu based interactive information system. Various pointing devices may be coupled to the CD-i player e.g. mouse, trackball, joystick, light pen, etc. The pointing devices are coupled to the CD-i player through a serial port based on the RS-232-C interface. A special data format is sent to the serial port which the CD-i player decodes to position a cursor on the screen with movement of the pointing device. A touch screen can also be used where a finger or pen is pushed onto the desired menu option on the CRT for the interaction between the CD-i player and user. Touch screens are readily available items which are affixed to the front of the CRT. A typical touch screen may be found in U.S. Patent No. 3,911,215, the contents of which are incorporated into this specification. The touch screen has a thin, rigid glass panel that is mounted to the CRT. The glass panel has a transparent electrically resistive coating on the inside. Gas- tight electrical contacts are then made to this coating in such a manner as to permit it to be used as a highly stable linear voltage divider for both the X and Y axes. A plastic contact sheet with a transparent conductive coating is stretched tightly over the glass substrate. The contact sheet is held above the glass by small, clear separator points, which are evenly distributed on the inside contact sheet surface. Finger pressure causes the contact sheet to deform imperceptibly and make electrical contact with the resistive coating at the position of touch. The voltages picked off by the contact sheet at the point of contact are the analog representation of the position touched. A controller circuit separate from the touch screen impresses the voltage gradient across the resistive coating on the glass, alternating between the X and Y directions. The analog signals picked off by the contact sheet are digitised by an analog-to- digital converter in the controller, and the digitised coordinate pair is then transmitted to the computer or CD-i player for processing. A touch screen is an absolute positional device and the coordinate system of the touch screen must be aligned with that of the raster. The touch screen typically has a range of from 0 to 1024 points across the screen and the same up the screen. When the touch screen is touched the coordinates are sent to the controller. These coordinates should reflect the area of the video image the user wishes to select. If the video image does not line up with the expected touch screen coordinates, then calibration is required. This can be achieved by the following methods: 1. Adjust the video image to suit the touch screen. This is done by adjusting the height and width controls associated with the CRT. This method has the disadvantage of limited accuracy and is labour intensive. 2. On normal computer systems a special calibration program is run which places a bit map video image with touch targets on the screen. These are touched in sequence and the results are used to correct the raw touch screen coordinates. This method has good accuracy, but requires software to be loaded and run on the computer system. Unfortunately the CD-i player is not equivalent to a computer in the respect that the compact disk can only be read. There is no provision for users to load their own software, such as a calibration system. It is. an object to provide a calibration system for a touch screen which overcomes the disadvantages of the prior calibration systems. A further object of the invention is to provide a calibration system which is simple and fast to operate. A still further object is to provide a calibration system which can easily interface to existing computer or interactive video systems which does not require additional software. With these objects in view the present invention in a first aspect provides a method of calibration of a touch screen for a cathode ray tube (CRT) to allow coordinates provided by said touch screen to correspond with desired coordinates of a video image supplied by a host system, said method including the steps of providing a calibration signal, moving a cursor to a first predetermined location on said CRT on receipt of said calibration signal, storing data representing the actual and sampled coordinates of said first predetermined location from a first touch on said touch screen, moving said cursor to a second predetermined location on said CRT, storing data representing the actual and sampled coordinates of said second predetermined location from a second touch on said touch screen and using stored data for a calibration correction based on said first and second samplings for, in use, allowing coordinates provided by said touch screen to correspond with the desired coordinates of the video image supplied by said host system. Preferably said sampling of said first and second coordinates occurs multiple times to filter said sampling. In a preferred aspect said calibration signal must be held for a predetermined time before said cursor is moved to said first predetermined position. The invention may also provide a method of correcting sampled coordinates from a touch screen for a cathode ray tube (CRT) supplied with a video image from a host system, said method including the steps of sampling the coordinates of a touched location on said touch screen and correcting the sampled coordinates using a calibration correction from said above method of calibration. In a further aspect of the invention there may be provided a controller for a touch screen used on a cathode ray tube (CRT) , said controller including an interface for coupling to a host system which provides video images to said CRT, a detector monitored by said controller to indicate a user's desire to calibrate said controller, said controller being responsive to said detector to cause a calibration sequence to be performed by said controller, a non-volatile memory for storage of the results of the calibration sequence, said controller adapted to monitor coordinates from said touch screen, and to apply a correction to the said coordinates to provide said host system with corrected coordinates using said calibration results stored in said non¬ volatile memory. The invention may also provide a controller to perform the previously defined methods. In order that the invention may be clearly understood and readily put into practical effect, a preferred non-limitative embodiment of a controller incorporating a calibration system will now be described with reference to the accompanying drawings in which:- Fig. 1 is a block circuit diagram of a controller incorporating the invention for a touch screen; Fig. 2 is a schematic of the touch screen with its connections; Figs. 3a, 3b and 3c together form a circuit schematic diagram of the controller shown in Fig. 1; Fig. 4 is a circuit schematic diagram showing the coupling of the controller to a CD-i player; and Figs. 5a, 5b, 6a, 6b, 6c, 7a, 7b, 8a and 8b are flow charts showing the operation of the controller and its routines. Figs. 1 to 4 illustrate the hardware for realising the invention. A touch screen 10 is fitted to a conventional television screen 11 (not shown) and is typically an E274 AccuTouch™ touch screen sold by Elographics, Inc. The construction of this touch screen is described in U.S. Patent No. 3,911, 215. Touch screen 10 is coupled via connector J4 to a serial controller 12 which incorporates the calibration system of the invention. Connector J5 may be used for the Duratouch ™ touch screens. Touch screen 10 has screen lines (H,X,Y,L) and a sense line S. Each of lines H, X, Y, L and S are protected by diodes Zl-Z 5 for the safe drain of any static buildup induced from the CRT 11. The screen lines are driven by two octal 3-state noninverting buffer/line driver/line receiver integrated circuit chips U3, U4 (typically 74HC244 by Motorola). Sense line S is coupled to an analog to digital converter U2 (typically a 10-bit A/D converter with serial interface designated MC145053 by Motorola) . In this embodiment serial controller 12 is coupled to an interactive CD player 14 sold by Philips Consumer Electronics BV under the name Philips CD-i player. Player 14 has a serial port connector 16 which is coupled to a corresponding connector Jl of serial controller 12 by a cable 18. Cable 18 also incorporates a switch SI which indicates that calibration is required. Switch SI can be a momentary closure switch which interfaces with a microcontroller or microprocessor Ul. In this embodiment Ul is an 8-bit microcontroller unit designated MC68HC705C8 by Motorola. Such a microcontroller has ROM, PROM, RAM, serial communication, interface 24 bidirectional I/O lines and 7 input-only lines integrated into the one chip. Microcontroller Ul interfaces with analog to digital converter U2 and line drivers U3, U4. Serial data communication with player 14 is by an RS-232 interface through an RS-232 driver/receiver chip U8. U8 is typically a MAX232 by Maxim Integrated Products. In order to ensure the calibration is not lost on power down an electrically erasable programmable memory U6 is interfaced to microcontroller Ul. Memory U6 is typically an NMC9306 chip by National Semiconductor. Serial controller 12 interfaces to player 14 to transmit positional coordinates on detection of a screen touch, and to provide a calibration, if required, of touch screen 10 to ensure that the video image lines up with the expected touch screen coordinates. Calibration is obtained by driving the cursor to selected positions on screen 11 and saving those points in memory U6. These saved points can be used as correction factors for the raw coordinates received from touch screen 10. Normally serial controller 12 will monitor touch screen 10 and pass "touch detection" and coordinates for processing by microcontroller Ul. Microcontroller Ul will translate the coordinates into the data format required by player 14 and transmit the data through serial interface U8. The coordinate information is filtered in both hardware and software to prevent generation of incorrect coordinates. This filtering minimises the effect of electrical noise picked up from the face of screen 11. Analog to digital converter U2 in this case is a successive approximation analog to digital converter having 10 bit resolution which operates ratiometrically to minimise effects of variations in drive voltages and drift with changing temperature. The operation of the serial controller 12 will now be discussed. Initially, when touch screen 10 is not touched, all the screen drive lines (H,X,Y,L) are driven to a high voltage level, and the sense line S is grounded through a resistor. A touch is sensed when the voltage on the sense line S, as determined by analog-to-digital converter U2, is higher than the defined touch down threshold. After a touchdown is detected, the drive voltages for the X coordinate are applied to the touch screen 10 - H and X high, and L and Y low. The analog signal for the X position is provided from the touch screen contact sheet on the S line. An A/D conversion is carried out by U2 and the result stored in memory. Next the drive voltages for the Y coordinate are applied to the screen - H and Y high, and L and X low. The analog signal for the Y position is provided from the touch screen contact sheet on the S line. An A/D conversion is carried out and the result stored in memory. This procedure of X and Y coordinate conversation is carried out four times in succession. The four coordinates are then digitally filtered by the control software. If the spread of coordinates is too large, the sample is rejected as invalid. As the final step of coordinate determination, a further touch detection step is performed to ensure that touch screen 10 remained pressed for the whole of the coordinate conversion time. If the touch screen 10 is released during conversion, the coordinates are rejected as invalid. Once an acceptable coordinate pair has been determined, the coordinate is sent via the serial port U8 to player 14. After waiting for the specified time between successive coordinates another touch detection and coordinate conversion cycle is started. This operation is shown in the flow charts of Figs. 5 to 7. The calibration procedure will now be described. Switch SI is held down for at least one second to indicate that calibration is required. By holding the switch down for this period will avoid undesired calibrations where switch SI is accidentally depressed. The flow chart for calibration is shown in Figs. 8a and 8b. The serial controller 12 will force the cursor of screen 11 to be driven to the lower left quadrant of the screen. The user touches the point where the cursor is located and removes their finger when satisfied with that point. Controller 12 then forces the cursor to be driven to the right hand top quadrant of screen 11. The user touches that position and removes their finger when satisfied with that point. The cursor can then be driven to the centre of screen 11 to indicate completion of the calibration procedure. The calibration results are stored in non- volatile memory U6 and will continue to be used even after a power interruption. The calibration results will be applied to the raw coordinate results to ensure that the correct coordinates are passed to player 14. The corrected coordinates will be based on the following formulae. X NORM - (RAWX-CALXLO) * (CONVXHI-CONVXLO) / (CALXHI-CALXLO) + CONVXLO Y NORM ■= (RAWY-CALYLO) * (CONVYHI-CONVYLO) / (CALYHI-CALYLO) + CONVYLO Legend: X NORM & Y NORM are the normalised values after the calibration correction. RAWX & RAWY are the raw unconverted coordinates. CALXHI & CALYHI are the calibration points sent to the screen on the top RH quadrant. CALXLO & CALYLO are the calibration points sent to the screen on the bottom LH quadrant. CONVXHI & CONVYHI are the return values collected from the top RH quadrant. CONVXHI & CONVYHI are the return values collected from the bottom LH quadrant. The CDI "Green Book" specification for an absolute pointing device is from Oh,Oh (h = hexadecimal) which is the bottom left corner to 3FFh,3FFh at the top right hand corner of the video image. The cursor in this mode can be controlled and put to any valid position within the above range of the video image. This is the method we use to obtain calibration points. In this case we use calibration coordinates at lOOh . lOOh & 300h,300h which simplifies the formula to: X NORM = (RAWX-lOOh) * (CONVXHI-CONVXLO) / 200h + CONVXLO Y NORM = (RAWY-lOOh) * (CONVYHI-CONVYLO) / 200h + CONVYLO If calibration coordinates have been collected or stored in memory U6 then they are applied to the raw coordinates; otherwise they are sent unconverted to player 14. The CDi output data format is an async 7 bit data with 2 stop bits. It transmits a binary format with 000 - 3FF hexadecimal absolute x and y coo-ordinates. Serial controller 12 is an 8 bit character transmitter so bit 7 is padded to all ones. Pen down and button 2 are not used and are padded with zeros. When touching the screen a continuous stream of data packets is sent with button 1 off. On release a single data packet is sent with button 1 on. This means action areas on the screen can be accessed without a button press, and selection of button device areas only occurs on release. Binary Data Format (as per Philips Pointing Device Standard) Bit 7 6 5 4 3 2 1 0 No: Byte 0 1 1 BT1 BT2 X9 X8 X7 X6 Byte 1 1 0 PDN 0 Y9 Y8 Y7 Y6 Byte 3 1 0 X5 X4 X3 X2 XI xo Byte 4 1 0 Y5 Y4 Y3 Y2 Yl Y0 Legend: PDN = Pen down BT1 = ButtoiBTE = Button 2 The invention provides a very effective touch screen controller which interfaces to player 14 without requiring touch screen drivers on the CD disk or requiring manipulation of the screen height and width controls. Once calibration has been set no further adjustment is necessary. The calibration is very simple and requires no special skills from the user. The values, pinouts and designations of all components shown in Figs, a, 3b and 3c may be found in Fig. 3 of Australian Patent Application No. PN 0927 from which priority is claimed. The contents of Australian Patent Application No. PN 0927 are incorporated into this specification. Although the preferred embodiment has been discussed with a particular circuit, it is quite clear to a man skilled in the art that substitution or re-arrangement can be easily undertaken to meet particular requirements. Touch screen 10 can be substituted by another sourced device as can the integrated circuits. A custom chip could be used to provide the required lines to touch screen 10 and serial port 16. The preferred embodiment has disclosed operation with a Philips player 14. The invention is not limited to such players as it can be readily adapted for interfacing with a computer having a touch screen fitted. It is believed that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement of the parts and that changes may be made in the form, construction and arrangement of a calibration system described without departing from the scope and spirit of the invention or sacrificing all of its material advantages, forms hereinbefore described being merely preferred embodiments hereof.

Claims: CLAIMS : 1. A method of calibration of a touch screen for a cathode ray tube (CRT) to allow coordinates provided by said touch screen to correspond with desired coordinates of a video image supplied by a host system, said method including the steps of providing a calibration signal, moving a cursor to a first predetermined location on said CRT on receipt of said calibration signal, storing data representing the actual and sampled coordinates of said first predetermined location from a first touch on said touch screen, moving said cursor to a second predetermined location on said CRT, storing data representing the actual and sampled coordinates of said second predetermined location from a second touch on said touch screen and using stored data for a calibration correction based on said first and second samplings for, in use, allowing coordinates provided by said touch screen to correspond with the desired coordinates of the video image supplied by said host system. 2. The method of claim 1, wherein said sampling of said first and second coordinates occurs multiple times to filter said sampling. 3. The method of claim 1 or 2, wherein said calibration signal must be held for a predetermined time before said cursor is moved to said first predetermined position. 4. The method of any one of the preceding claims, wherein said stored data is stored in a non-volatile memory. 5. The method of any one of the preceding claims, wherein said first and second predetermined locations are points in the lower left quadrant and top right quadrant of said touch screen. 6. The method of any one of the preceding claims, wherein a calibration completion indication is produced when said calibration is complete. 7. The method of claim 6, wherein said calibration completion indication comprises sending said cursor to the centre of the screen. 8. A method of correcting sampled coordinates from a touch screen for a cathode ray tube (CRT) supplied with a video image from a host system, said method including the steps of sampling the coordinates of a touched location on said touch screen and correcting the sampled coordinates using the calibration correction of any one of claims 1 to 7. 9. A controller for a touch screen used on a cathode ray tube (CRT) , said controller including an interface for coupling to a host system which provides video images to said CRT, a detector monitored by said controller to indicate a user's desire to calibrate said controller, said controller being responsive to said detector to cause a calibration sequence to be performed by said controller, a non-volatile memory for storage of the results of the calibration sequence, said controller adapted to monitor coordinates from said touch screen, and to apply a correction to the said coordinates to provide said host system with corrected coordinates using said calibration results stored in said non-volatile memory. 10. The controller of claim 9, wherein said interface is a serial interface. 11. The controller of claim 9 or 10, wherein said controller is microprocessor or microcontroller based. 12. The controller of any one of claims 9 to 11, wherein said detector is a switch which must be activated for a predetermined time to indicate said user' s desire to calibrate said controller. 13. The controller of claim 9, wherein said controller is microprocessor or microcontroller based and includes software which allows interrogation of said detector, said software in its calibration sequence allowing a cursor to be moved to a first predetermined location on said CRT on activation of said detector , storing data representing the actual and sampled coordinates of said first predetermined location from a first touch on said touch screen in said non-volatile memory, moving said cursor to a second predetermined location on said CRT, storing data representing the actual and sampled coordinates of said second predetermined location from a second touch on said touch screen in said non-volatile memory, and using said stored data for a calibration correction based on said first and second samplings for, in use, allowing coordinates provided by said touch screen to correspond with the desired coordinates of the video image supplied by said host system.

Inventors: LIA KEITH, LIA, KEITH
Applicants: ELO TOUCHSYSTEMS INC, LIA KEITH, ELO TOUCHSYSTEMS, INC., LIA, KEITH
Application year: 1996
Main classification: G09G3
IPC codes: G06F3, G06F11