TOUCH POSITION LOCATING SYSTEM

Abstract: A system providing greater accuracy of touch location and better rejection of noise comprises a plurality of force sensing detectors (50) for detecting and generating electrical signals proportional to the applied touch. Alternating current coupling circuits (508) reshape the electrical signals from each detector for accentuating the common dynamic frequency characteristics of the variety of forces presented to the system. A sample collection circuit (511) sequentially collects samples (401a) from each AC coupling circuit (508a) until a predetermined interval of time lapses (402) or until the second zero crossing (403) of one electrical signal sample is detected, whichever event occurs first. Samples are averaged before calculating the location of the touching force. An average reference level for each force sensing device is established prior to the collection of the force samples to automatically adjust for any changes in the condition of the system.

Description: TOUCH POSITION LOCATING SYSTEM Background of the Invention 1 * Technical Field This invention relates to apparatus for locating the position of a momentary, localized force applied to the surface of a visual display, and to the means for analyzing the detected force signals. 2. Description of the Prior Art In the use of interactive computers, a computer terminal having a visual display presents a plurality of alternative choices on the display. The user selects one desired alternative by touching or tapping the display at a location corresponding to the desired alternative. The computer terminal identifies the position of the touch and reacts accordingly. Peronneau et al, U. S. Patent 3,657,475, discloses a position indicating system comprising a rigid plate, a plurality of force sensing devices, and a processing unit. When a localized force is applied to an exposed surface of the plate, the force sensing devices provide the processing unit with electrical signals which are proportional to the localized force. The processing system calculates the coordinates of the location of the localized force touching the plate. Equations from the laws of statics are employed; namely, that the sum of the moments about an axis equals 0. Peronneau provides an integrator for collecting force sensing device data in a first embodiment of his invention. In this embodiment, both the static and low frequency dynamic characteristics of the touch are provided to the processing unit for calculating the coordinates of the localized force. However, the static forces due to the weight of the plate are not separated from the dynamic characteristics of the applied touch, and this presents an admitted problem to Peronneau. He discloses that in a second, preferred embodiment of his invention the dynamic characteristics of the touch are analyzed. However, Peronneau does not clearly disclose how the dynamic characteristics of the touch are separated from the static characteristics. Roeber, U. S. Patent No. 4,121,049, utilizes a threshold comparator amplifier for insuring that the applied touch is above a predetermined threshold level. A shortcoming of the use of the comparator amplifier is that the predetermined threshold level does not automatically adapt to the environment or ' the quiescent state orientation of the touch plate apparatus. For example, if the Roeber touch plate apparatus is rotated from a vertical to a more horizontal orientation, the weight of the touch plate causes a corresponding increase in the force detected by the force detectors. As a result, the threshold level must be manually increased to compensate for the increased stress. Summary of the Invention For better detecting and analyzing various touching forces applied against the touched member, the electrical signals generated by the various force detecting means are passed through a coupling circuit, e.g., a pass band filter, for accentuating various features common to a variety of types of touches expected to be encountered in the use of the system. According to one aspect of the invention, a plurality of signal samples are collected for each force sensing device which are averaged before the calculation of the touch position is made. This improves the accuracy of the calculation. According to another aspect of the invention, average reference signal levels are established for each force sensing device prior to the beginning of a touch. This is done after each touch cycle. OMPI Brief Description of the Drawings FIG. 1 shows one touch force sensing mechanism which may be used with the- present invention. FIG. 2 is a graphical representation of the Force vs. Time characteristics of several types of touches impinging upon a rigid body. FIG. 3 shows the Gain vs. Frequency (dynamic transfer) characteristics of a coupling circuit which may be used with the present invention. FIG. 4 shows the Voltage vs. Time characteristics of a long and a short toucbrmeasured at the output of a coupling circuit. FIG. 5 is a schematic drawing of a dynamic position locating system according to the invention. FIG. β shows a programmable keyboard display. FIG. 7 is a flowchart of a subroutine for automatically calculating new reference values during a quiescent period. FIG. 8 is a flowchart of a subroutine for detecting the beginning of a touch. FIGS. 9 and 10 comprise a flowchart of a subroutine for computing the x,y coordinates of a touch. Detailed Description With reference to FIG. 5, an illustrative embodiment of the invention is shown which comprises several devices 501 for detecting a touch force applied against a touch plate such as the one shown in FIG. 6. In general, each device 501 generates an analog signal in response to the touch, which signals are passed through an alternating current coupling unit 508 which reshapes the analog signals to more suitable form for detection and analysis. A multiplexer 510 sequentially collects the outputs from the coupling units 508, and samples of these outputs are taken, under the control of a clocking and sample request circuit 511, and applied to an analog to digital converter 504 for digitizing the signal samples. The digital signals are applied to the processing unit 512 which analyz ' es the signals, detects the location of the touching force, and provides further instructions to the display unit via the control unit 514. Various components of the FIG. 5 system are now further discussed. With reference to FIG. 1, an example of one type of touch detecting arrangement is discussed. The surface 101a to be touched is part of a plate 101 mounted on a rigid support 107 by means of a spring 108 and a mounting arrangement 104 105. Preferably, at least three such mounting arrangements are used so that triangular support is provided to a touch plate 101 of the touch sensitive position locating system. The rigid support 107 can be a visual display device, e.g., the face plate of a cathode ray tube. The mounting arrangement comprises a pointed member 104 and a receiving member 102 for receiving the point of the pointed member. A force sensing enber 105, e.g., a piezoelectric transducer, joined to the pointed member detects a perpendicular component force F j _ caused by the weight of touch plate 101, the spring 108, and by a localized, directional force F applied at a point M by the user of the apparatus. Electrical access leads 109 are connected to members 104 and 106 which comprise a conductive naterial. When no force F is applied by a user to surface 101a, a static force is detected which is proportional to the weight of the touch plate 101 and to the tension of the spring 103. When a dynamic touch force F is applied to the surface 101a of touch plate 101, the detected force F j proportionately changes with the applied force F. As is generally known, the x,y coordinates of the point of application H of localized force F are calculated by detecting the perpendicular components of the force F through the force sensing members 105 of the mounting arrangements. Equations from the laws of statics are employed, namely that the sum of the moments about a point must equal zero. As the locations of the mounting arrangements are known and all the forces are detected, two moment arm equations are solved for the x,y coordinates, respectively, of the point of application M using an associated computer processing system. Other force detecting means, such as the various ones shown in the aforecited references can be used. The purpose of the alternating current coupling units 508 is to filter the analog signals generated by the force detecting units 501 to better facilitate detection and validation, of anticipated types of forces. Referring to FIG. 2, for example, a graphical representation of the Force vs. Time characteristics of several types of touches is shown. Curves 201-204 represent, respectively, a sharp tap; a fingertip tap; a slow, hard tap; and a light push, respectively, and are representative of the variety of touches that a user of the present system may be expected to apply to touch surface 101a of FIG. 1. The sharpest of taps 201 to the surface 101a of touch plate 101 may cause a one to five millisecond force spike. On the other hand, a light push 204 may register a nearly constant force of as much as two seconds duration. The various touch forces cause a mechanical vibration of the touch plate 101, and the output of each sensing device 501 is a rather complicated a.c. signal dependent upon the particular touch force applied. The dynamic response characteristic of the coupling units is selected to accentuate the common frequency and magnitude characteristics of the a.c. signals from the sensing devices 501 thereby making possible more positive analysis of the touch signals. Referring to FIG. 3, the dynamic response characteristic 301 of one such coupling unit are shown. The high frequency roll-roff of curve 301 above 8 Hz attenuates the high frequencies of the sharp tap 201. The low frequency roll- ^ off below 8 Hz rejects the static or direct current components and attenuates the low frequency components of the light push 204. Referring to FIG. 4, graphical representations of the Voltage vs. Time characteristics resulting from the light push 204 and the sharp tap 201 of FIG. 2 at the output of the coupling unit are shown. Curve 401 corresponds to the sharp tap 201 and comprises a simple damped sine wave having a period of about 175 milliseconds, thus allowing a plurality of samples 401a thereof to be taken. Curve 402 represents the light push. It exhibits two periods when samples can be taken: when the light push first occurs and, at 2000 milliseconds, when it is released. Analysis has shown that all types of touches 201-204 exhibit more similar characteristics after passing through a coupling unit according to the present invention. This allows more positive identification of the touch forces as well as better rejection by the system of false touch forces generated by various "noise" factors, e.g., floor vibrations. For the sharp tap curve 401, sampling (in accordance with known analog to digital conversion processes) ceases at the time of the second zero crossing 403. Two half cycles of valuable data are collectible before the signal is damped below useful levels. With respect to the light push curve 402, sampling ceases after a predetermined time interval 404 expires, for example, 200 milliseconds. It is not practical to await the release period of the light push curve 402 to acquire the second (after 2000 milliseconds) damped wave of samples. Referring briefly to FIG. 10, a subroutine for computing the touch coordinates x,y is presented in flowchart form. Boxes 1001, 1012, 1013, 1014, and 1021 show that sampling ceases either at the second zero crossing or after a predetermined time interval, whichever event occurs first. Coupling unit 508 comprises several component circuits. In the depicted embodiment, component filter stages 503 and 505 provide the low frequency (less than 8 Hz) roll-off characteristics 503a and 505a, respectively, presented in FIG. 3. Curves 503a and 505a combine to form the low frequency characteristics of curve 301. Component filter stages 504 and 506 provide the high frequency roll-* off characteristic curves 504a and 506a. Amplifier 507 provides gain and impedance matching to the signal recovered from force sensing device 501. In general, any characteristic curve 301 may be employed so long as 1) the associated coupling unit is able to match impedances between the force sensing device and the computer processing system employed and 2) the characteristic curve 301 accentuates the common dynamic characteristics of the variety of touches that impinge on the touch surface of the position locating system. Table A provides values for the components of the depicted coupling unit 508, including the electrical components of filter stages 503, 504, 505, and 506, for use with piezoelectric transducer elements 501. TABLE A Component Values for the Alternating Current Coupling Circuit 508 of FIG. 5 R503 C503 R505 C505 Filter 503 2.7MZ 7150pf Filter 505 22.1KZ 1.47Mf R504 C504 R506 C506 Filter 504 1MZ 7150pf Filter 506 10.5KZ 0.511 f As previously noted, multiplexer 510 is employed to sequentially collect samples from the plurality of alternating current coupling units 508. A clocking and sample request circuit 511 of processing system 512 addresses multiplexer 510 to collect an analog sample from the addressed force sensing device 501. The sampling rate is chosen to be much higher than the highest rate at which sample "touches may enter the system. Two leads 511a and 511b are shown for this purpose which assumes a maximum of four addressable force sensing devices 501. The analog samples are provided to the processing system 512 through a known type of analog to digital converter 504. Accordingly, digital samples are provided to the calculating circuit 512a which is also of known type. In the depicted dynamic position locating system, constant voltages caused by the weight of the touch plate or the tension of the spring are eliminated by the choice of a piezoelectric transducer having a known capacitive equivalent circuit and of the resistor R503. There remains a requirement, however, to determine a reference level of the electrical- signal proceeding from each force sensing device through the alternating current coupling unit during a quiescent period. Variations in the particular characteristics of the individual force sensing devices, the alternating current coupling circuit and other components of the system, effects due to aging and temperature, and other environmental effects are compensated for by employing the calculating circuit 512a of the present invention and an associated software subroutine. Normalizing of the system occurs when power is provided to the system and after each successive touch. Processing unit 512 responsive to an algorithm stored in permanent memory 512b calls for several electrical samples of forces detected by the plurality of force sensing devices 501. The samples are accumulated in temporary memory 512b. The plurality of collected samples for each force sensing device 501 is then averaged in order to establish an average reference voltage level for each force sensing device. Referring to FIG. 8, a flowchart of the subroutine for calculating new average reference levels during a quiescent period is presented. In boxes 801-809, thirty- ^ two samples of three force sensing devices 501 are separately accumulated, averaged, and stored. The effect of this is to establish three average reference levels from which to measure the dynamic voltages caused by a touch. In box 807, if a given sensor sample falls outside a narrow range around the first collected sensor sample, the • processor 512 restarts the collection process. This ensures that the average reference levels are established during a quiescent period. Referring to FIG. 9, a flowchart of a subroutine for detecting the beginning " of a touch is presented. From boxes 901--906, it is seen that a subsequent touch begins when the sum of the sampled voltages from all force sensing devices 501 minus the sum of their respective average reference levels is greater than a predetermined threshold value. The predetermined threshold value is chosen by considering the effects of noise signals generated by such forces as floor vibrations or moving or accidental tapping of the system. The establishment of this predetermined threshold and the average reference level established for each force sensing device is similar to requiring that the total force measured by the force sensing devices be greater than a threshold value provided to a single Comparator amplifier. However, an advantage of the present invention is that the threshold value is unaffected by changes in environment or by static forces detected by the system. Referring to FIG. 10 and 11, a subroutine for computing the x,y coordinates of the point of touch is presented. In box 1008 the quiescent period average reference level for each force sensing device is digitally subtracted from the sampled voltage level for that sensor as the samples are collected and stored in temporary memory 512b. These adjusted sample values represent the reactionary component forces caused by the momentary, localized force F. At box 1009 the sum of adjusted sensor sample values for each force sensing device " and the sum of the adjusted sensor sample values for all, of the sensing devices are accumulated in memory 51 ' 2b ( . At box 1016 these sums are averaged by dividing by the;number of samples collected. In this manner, an average ' adjusted sample value is acquired for each force sensing device as well as an average value of their sum. The processing unit 512 also monitors the touch force values detected by the force sensing devices. If a touch force which is too hi-gh (too hard) is detected, corrective action is taken. If the touch force is too soft, that is, momentarily exceeding ".the predetermined threshold but not remaining above that level for very many samples, corrective action is also taken. If the forces detected are so large (boxes 1003 and 100"4) that the range of the analog to digital converter 504 is exceeded,- the processor 512 receives a saturated, full-scale signal. Upon receipt of this signal, the processor enters a mode comprising box 1005 in which all force sensing devices 501 are sequentially polled as before, but the sampled data is not averaged into the cumulative total. The sole function of the sampling in this mode is to wait until the forces detected by each of the force sensing devices return to acceptable levels. At that time, the normal mode of sampling and averaging resumes as in the path comprising boxes 1004, 1007, and 1008. Accordingly, at the very peak of an applied force that is "too hard", the processor 512 ignores the data because the linear range of the analog to digital converter 504 is exceeded. However, a determination of the coordinates of the touch is still made based on the data collected before the touch force exceeds the maximum range and after it has decreased again below the maximum -range. This coordinate calculation is less accurate than with a proper touch force, however. Therefore, at box 1006 a "too hard" flag is set; and at box 1018, display control unit 514a is activated to warn the user not to touch the touch surface so hard. Display control 514a may provide both an audible tone and a visual indication, "TOO HARD." This warning occurs before any action resulting from interpretation of the coordinates of the touch. Shortly after the warning, the user requested action may be performed. Occasionally, a user touches the screen so lightly that the peak of the detected force signal barely exceeds the predetermined threshold for the beginning of touch, then the signal falls quickly back to zero. In this case, there is so little data available from which to calculate the coordinates that the accuracy of the calculations are impaired by quantization and signal noise. To prevent coordinate calculations with large errors from being misinterpreted, touches soft enough to cause problems are treated specially. At box 1023 the user may be warned as with a hard touch that his touch is too soft. An alternate solution is to ignore soft touches of short duration entirely. The natural response is to hit the touch sensitive display harder, which is, in this case, desirable. In one arrangement, soft touches that are long enough to provide noise immunity are accepted (a sufficient number and magnitude of samples above the threshold are acquired) , but shorter soft touches are rejected. This provides a greater range of acceptable touches than the threshold techniques in the prior art. At box 1009, the soft touch rejection is performed by accumulating the sample magnitudes for all force sensors. At boxes 1010 and 1011, the accumulated sum is then compared with a predetermined minimum threshold for calculation of coordinates. By applying a multiplexer 510 to sequentially collect samples, an error is introduced to a given series of samples because all the samples are not collected simultaneously. This error is minimized by the rapid collection, as shown in FIG. 4, of a plurality of samples 401a. Furthermore, because the effects of quantization error and noise on the sampled signal is reduced by collecting many samples, this technique provides inherently more accurate calculations of the coordinates of the touch than would occur if only one simultaneous series of samples is collected. The above-rdescribed sampling method is representative of what is known as a serial collection of data. A parallel collection of samples and the elimination of the multiplexer is achieved by providing additional analog to digital converters between the coupling units 508 and the processing unit 512.

Claims: Cl a ims 1. A system for determining the location of a localized force applied to a surface, the system comprising a plurality of force sensing devices for detecting and generating analog electrical signals proportional to the applied force; CHARACTERIZED 3Y a coupling circuit (508), associated with each force sensing device (501) , for reshaping the electrical signals therefrom to a form accentuating features characteristic of different" ~ types of applied forces. 2. A system according to claim 1 CHARACTERIZED IN THAT the coupling circuit (508) comprises a passband filtering circuit. 3. A position locating system for determining the location of a localized force (F) applied to a surface (101a) , a plurality of force sensing devices (105, 501, or 501a) for translating forces at the location of a force sensing device into proportional electrical signals and a processing system for determining the coordinates of the point of application of the localized force, CHARACTERIZED IN THAT the processing system (512) comprises a sample collection circuit (508, 510 and 511) for collecting samples (401a) of the signals at a rate much higher than the highest rate at which the localized force may be repeatedly applied to the surface of the rigid body, for minimizing the effects of quantization error and noise. 4. A position locating system as recited in claim 3 CHARACTERIZED IN THAT the processing system (512) comprises a calculating circuit (512a) for calculating average reference levels for each force sensing device (105, 501, or 501a) and including a memory (512b) for temporary storage of the average reference levels for each force - sensing device. 5. A position locating system as recited in claim 4 CHARACTERIZED IN THAT the calculating circuit (512a) establishes average reference voltage levels for each force sensing device (501) , the average reference voltage levels reflecting a quiescent state condition. 6. A position locating system as recited in claim 5 FURTHER CHARACTERIZED IN THAT the calculating circuit establishes new average reference voltage levels for each force sensing device (501) prior to the application of the localized force (F) . 7. A position locating system for determining the location of a localized force (F) applied to a surface (101a), a plurality of force sensing devices (105, 501, or 501a) for translating forces at the location of a force sensing device into proportional electrical signals and a processing system for determining the coordinates of the point of application of the localized force, CHARACTERIZED IN THAT the processing system (512) comprises a clocking and sample request circuit (511) for collecting samples (401a) during the application of the localized force, CHARACTERIZED IN THAT the processing system (512) comprises a clocking and sample request circuit (511) for collecting samples (401a) during the application of the localized force (F) either until a particular interval of time (404) expires or until a particular zero crossing (403) of the force proportional electrical signal is detected, whichever event occurs first. 8. A position locating system for determining the location of a localized force (F) applied to a surface (101a) , a plurality of force sensing devices for translating forces at the location of a force sensing device into proportional electrical signals and a processing system for determining the coordinates of the point of application of the localized force, CHARACTERIZED BY an alternating current coupling circuit (508) associated with each force sensing device for receiving the force proportional electrical signal output and for filtering therefrom electrical signals substantially above and substantially below a particular frequency, a clocking and sample request circuit (511) having a predetermined request rate much greater than the rate at which the localized force may be repeatedly applied to the surface of the rigid body, a multiplexer circuit (510) , responsive to the clocking and sample request circuit (511) , for collecting electrical signal samples (401) from each force sensing device until a particular interval of time (402) lapses or until a particular zero crossing (403) of the force proportional electrical signal is detected, whichever event occurs first, an analog to digital converter (504) , responsive to the multiplexer circuit (510) , for converting the samples to digitized form and a coordinate calculating circuit (512a) , responsive to the analog to digital converter circuit (504) , for averaging the digitized electrical signal samples of each force sensing device before calculating the coordinate values of. the point of application of the localized force. CM?I

Inventors: DECOSTA J, MALLOS JAMES B, ROE DAVID B
Applicants: WESTERN ELECTRIC CO, WESTERN ELECTRIC CO INC
Application year: 1981
Main classification: G08C21
IPC codes: G01B7, G01B21, G06F3, G06K11, G08C21