TOUCH SENSE USING TIME DOMAIN REFLECTOMETRY

• US 20110267287A1
• Filed: 10/27/2010
• Published: 11/03/2011
• Est. Priority Date: 04/30/2010
• Status: Active Grant

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: Microchip Technology Incorporated
• Inventors: Bartling, James S., Yedamale, Padmaraja, Hanauer, Jerry
• Granted Patent: US 8,599,155 B2
• Time in Patent Office: Days
• US Class Current: 345/173
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

A touch panel or screen has a serpentine transmission line fabricated on a substrate, e.g., printed circuit board, LCD, plasma or LED screen, etc., and has a constant impedance. A touch to the touch panel will cause a change of impedance of the transmission line at the location of the touch. Time domain reflectometry (TDR) is used for determining the location of the change of impedance of the transmission line by accurately measuring the return pulse time at the source of the initial pulse, and then converting the return pulse time to X-Y coordinates of the touch panel or screen.

Claims

1. A method for sensing a touch to a substrate and determining a location thereof, said method comprising the steps of:

sending a first pulse to a first end of a serpentine transmission line fabricated on a substrate comprising a plurality of touch locations, wherein the serpentine transmission line is unterminated at a second end thereof;

charging a timing capacitor with a constant current source until a first touch return pulse is received at the first end of the serpentine transmission line;

measuring a first voltage on the timing capacitor; and

converting the measured first voltage to a first location on the substrate.

2. The method according to claim 1, further comprising the steps of:

sending a second pulse to the second end of the serpentine transmission line, wherein the serpentine transmission line is unterminated at the first end thereof;

charging the timing capacitor with the constant current source until a second touch return pulse is received at the second end of the serpentine transmission line;

measuring a second voltage on the timing capacitor; and

converting the measured second voltage to a second location on the substrate.

3. The method according to claim 2, further comprising the steps of alternately sending the first pulse to the first end of the serpentine transmission line for determining the first location, then sending the second pulse to the second end of the serpentine transmission line for determining the second location, and continuously repeating these steps.

4. The method according to claim 1, wherein the step of charging the timing capacitor comprises the steps of:

start charging the timing capacitor with the constant current source upon detecting a first pulse voltage level at the first end of the serpentine transmission line with a first voltage comparator; and

stop charging the timing capacitor upon detecting a first return touch pulse voltage level at the first end of the serpentine transmission line with a second voltage comparator, wherein the first return touch pulse voltage level is less than the first pulse voltage level.

5. The method according to claim 1, wherein the step of measuring the first voltage on the timing capacitor is done with an analog-to-digital converter (ADC).

6. The method according to claim 1, wherein the step of converting the measured first voltage to the first location on the substrate comprises the step of finding a first voltage value corresponding to the measured first voltage in a look-up table having a plurality of first voltage values and their corresponding locations on the substrate.

7. The method according to claim 1, wherein the step of converting the measured first voltage to the first location on the substrate comprises the steps of:

calculating a first distance along the serpentine transmission line by multiplying the measured first voltage with a constant derived from the speed of light and a velocity factor of the serpentine transmission line; and

finding a first distance value corresponding to the calculated first distance in a look-up table having a plurality of first distance values and their corresponding locations on the substrate.

8. The method according to claim 2, further comprising the step of averaging the first and second locations taken contemporaneously for improving resolution of the location touched on the substrate.

9. The method according to claim 3, further comprising the step of averaging the a plurality of first and second locations taken contemporaneously for improving resolution of the location touched on the substrate.

10. The method according to claim 3, further comprising the step of determining locations of at least two contemporaneous touches to the substrate within a certain time period.

11. The method according to claim 10, further comprising the step of determining a gesturing motion from the at least two contemporaneous touches to the substrate within the certain time period.

12. The method according to claim 1, wherein the substrate is a touch panel comprising a printed circuit board having the serpentine transmission line on a front printed circuit side thereof, and a back printed circuit side covered with an electrical conductor.

13. The method according to claim 1, wherein the substrate is a touch screen comprising a light transmissive insulating material having a front side with the serpentine transmission line thereon and a back side covered with a light transmissive electrical conductor.

14. The method according to claim 1, further comprising the step of ending the first pulse when a voltage is detected at the first end of the serpentine transmission line that is greater than a first end voltage level at the beginning of the first pulse.

15. The method according to claim 2, further comprising the step of ending the second pulse when a voltage is detected at the second end of the serpentine transmission line that is greater than a second end voltage level at the beginning of the second pulse.

16. A method for sensing a touch to a substrate and determining a location thereof, said method comprising the steps of:

sending a first pulse to a first end of a serpentine transmission line fabricated on a substrate comprising a plurality of touch locations, wherein the serpentine transmission line is terminated at a second end thereof with a first resistor having a resistance substantially equal to a characteristic impedance of the serpentine transmission line;

charging a timing capacitor with a constant current source until a first return pulse is received at the first end of the serpentine transmission line;

measuring a first voltage on the timing capacitor; and

converting the measured first voltage to a first location on the substrate.

17. The method according to claim 16, wherein the first pulse has a fixed on time and a fixed off time.

18. The method according to claim 16, wherein the first pulse is a plurality of first pulses.

19. The method according to claim 16, further comprising the steps of:

sending a second pulse to the second end of the serpentine transmission line, wherein the serpentine transmission line is terminated at the first end with a second resistor having a resistance substantially equal to the characteristic impedance of the serpentine transmission line;

charging the timing capacitor with the constant current source until a second return pulse is received at the second end of the serpentine transmission line;

measuring a second voltage on the timing capacitor; and

converting the measured second voltage to a second location on the substrate.

20. The method according to claim 19, wherein the second pulse has a fixed on time and a fixed off time.

21. The method according to claim 19, wherein the second pulse is a plurality of second pulses.

22. The method according to claim 19, further comprising the steps of alternately sending the first pulse to the first end of the serpentine transmission line for determining the first location, then sending the second pulse to the second end of the serpentine transmission line for determining the second location, and continuously repeating these steps.

23. The method according to claim 16, wherein the step of charging the timing capacitor comprises the steps of:

start charging the timing capacitor with the constant current source upon detecting a first pulse voltage level at the first end of the serpentine transmission line with a first voltage comparator; and

stop charging the timing capacitor upon detecting a first return touch pulse voltage level at the first end of the serpentine transmission line with a second voltage comparator, wherein the first return touch pulse voltage level is less than the first pulse voltage level.

24. The method according to claim 16, wherein the step of measuring the first voltage on the timing capacitor is done with an analog-to-digital converter (ADC).

25. The method according to claim 16, wherein the step of converting the measured first voltage to the first location on the substrate comprises the step of finding a first voltage value corresponding to the measured first voltage in a look-up table having a plurality of first voltage values and their corresponding locations on the substrate.

26. The method according to claim 16, wherein the step of converting the measured first voltage to the first location on the substrate comprises the steps of:

calculating a first distance along the serpentine transmission line by multiplying the measured first voltage with a constant derived from the speed of light and a velocity factor of the serpentine transmission line; and

finding a first distance value corresponding to the calculated first distance in a look-up table having a plurality of first distance values and their corresponding locations on the substrate.

27. The method according to claim 19, further comprising the step of averaging the first and second locations taken contemporaneously for improving resolution of the location touched on the substrate.

28. The method according to claim 22, further comprising the step of averaging the a plurality of first and second locations taken contemporaneously for improving resolution of the location touched on the substrate.

29. The method according to claim 22, further comprising the step of determining locations of at least two contemporaneous touches to the substrate within a certain time period.

30. The method according to claim 29, further comprising the step of determining a gesturing motion from the at least two contemporaneous touches to the substrate within the certain time period.

31. The method according to claim 16, wherein the substrate is a touch panel comprising a printed circuit board having the serpentine transmission line on a front printed circuit side thereof, and a back printed circuit side covered with an electrical conductor.

32. The method according to claim 16, wherein the substrate is a touch screen comprising a light transmissive insulating material having a front side with the serpentine transmission line thereon and a back side covered with a light transmissive electrical conductor.

33. A method for sensing a touch to a substrate and determining a location thereof, said method comprising the steps of:

a) initializing a time of a time measurement device;

b) coupling a pulse driver to a first end of a serpentine transmission line fabricated on a substrate comprising a plurality of touch locations, wherein the serpentine transmission line is un-terminated at a second end thereof;

c) start sending a pulse with the pulse driver at the first end of the serpentine transmission line;

d) starting the time in the time measurement device;

e) detecting whether a negative return pulse, caused by a touch on the substrate, is received at the first end of the serpentine transmission line, wherein

if the negative return pulse is detected then going to step g), and

if not then going to step f);

f) detecting when a positive return pulse is received at the first end of the serpentine transmission line, the positive return pulse being from the un-terminated second end of the serpentine transmission line, wherein

if the positive return pulse is detected then returning to step a), and

if not then returning to step e);

g) stopping the time in the time measurement device;

h) reading the time from the time measurement device; and

i) converting the time read from the time measurement device into a location of the touch on the substrate then returning to step a).

34. A method for sensing a plurality of touches to a substrate and determining locations thereof, said method comprising the steps of:

a) initializing a time of a time measurement device and setting a count N in a touch counter to N=1;

b) coupling a pulse driver to a first end of a serpentine transmission line fabricated on a substrate comprising a plurality of touch locations, wherein the serpentine transmission line is un-terminated at a second end thereof;

c) start sending a pulse with the pulse driver at the first end of the serpentine transmission line;

d) starting the time in the time measurement device;

e) detecting whether a negative return pulse, caused by a touch on the substrate, is received at the first end of the serpentine transmission line, wherein

if the negative return pulse is detected then going to step g) and

if not then going to step f);

f) detecting when a positive return pulse is received at the first end of the serpentine transmission line, the positive return pulse being from the un-terminated second end of the serpentine transmission line, wherein

if the positive return pulse is detected then returning to step a) and

if not then returning to step e);

g) sampling the time from the time measurement device associated with the negative return pulse detected in step e);

h) storing the sampled time in a memory location having an address N;

i) incrementing N (N=N+1);

j) determining whether N is greater than M, M is a number of memory locations available to store the sampled times, wherein

if N is not greater than M then returning to step e) and

if N is greater than M then going to step k); and

k) converting the sampled times stored in the memory locations into locations of the touches on the substrate then returning to step a).

35. An apparatus for sensing a touch to a substrate and determining a location thereof, comprising:

a pulse generator for generating pulses;

a first pulse driver having an input coupled to the pulse generator and an output coupled through a first resistor to a first end of a serpentine transmission line fabricated on a substrate, a second resistor coupled to a second end of the serpentine transmission line, wherein the first and second resistors each have resistances that are substantially equal to a characteristic impedance of the serpentine transmission line;

a constant current source;

a timing capacitor;

an analog-to-digital converter (ADC); and

a digital processor;

wherein the pulse generator generates a first pulse to the first pulse driver which sends the first pulse to the first end of the serpentine transmission line, and then the constant current source starts charging the timing capacitor until a first return pulse is received at the first end of the serpentine transmission line;

a first voltage charge on the timing capacitor is sampled by the ADC and converted to a first digital value; and

the digital processor receives the first digital value from the ADC and converts the first digital value to a first location on the substrate.

36. The apparatus according to claim 35, the second end of the serpentine transmission line is terminated with the second resistor.

37. The apparatus according to claim 35, the second end of the serpentine transmission line is not terminated.

38. The apparatus according to claim 35, further comprising:

a second pulse driver having an input coupled to the pulse generator and an output coupled through the second resistor to the second end of the serpentine transmission line, and the first resistor is coupled to the first end of the serpentine transmission line;

wherein the pulse generator generates a second pulse to the second pulse driver which sends the second pulse to the second end of the serpentine transmission line, and then the constant current source starts charging the timing capacitor until a second return pulse is received at the second end of the serpentine transmission line;

a second voltage charge on the timing capacitor is sampled by the ADC and converted to a second digital value; and

the digital processor receives the second digital value from the ADC and converts the second digital value to a second location on the substrate.

39. The apparatus according to claim 38, the first end of the serpentine transmission line is terminated with the first resistor.

40. The apparatus according to claim 38, the first end of the serpentine transmission line is not terminated.

41. The apparatus according to claim 35, wherein the constant current source starts charging the timing capacitor when a first voltage comparator detects the first pulse from the pulse generator at the first end of the serpentine transmission line, and stops charging the timing capacitor when a second voltage comparator detects the first return pulse at the first end of the serpentine transmission line.

42. The apparatus according to claim 38, wherein the constant current source starts charging the timing capacitor when a first voltage comparator detects the second pulse from the pulse generator at the second end of the serpentine transmission line, and stops charging the timing capacitor when a second voltage comparator detects the second return pulse at the second end of the serpentine transmission line.

43. The apparatus according to claim 35, wherein the digital processor comprises a microcontroller.

44. The apparatus according to claim 35, wherein the digital processor is selected from the group consisting of a microprocessor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), and programmable logic array (PLA).

45. The apparatus according to claim 35, wherein the substrate is a touch panel comprising a printed circuit board having the serpentine transmission line on a front printed circuit side thereof, and a back printed circuit side covered with an electrical conductor.

46. The apparatus according to claim 35, wherein the substrate is a touch screen comprising a light transmissive insulating material having a front side with the serpentine transmission line thereon and a back side covered with a light transmissive and electrically conductive material.

47. The apparatus according to claim 46, wherein the light transmissive and electrically conductive material is indium tin oxide (ITO).

48. The apparatus according to claim 46, further comprising a graphical display, the touch screen is between the graphical display and a field of vision.

49. The apparatus according to claim 48, wherein the graphical display is selected from the group consisting of a liquid crystal display (LCD), a light emitting diode (LED) display, a plasma display, and an organic light emitting diode (OLED) display

50. An apparatus for sensing touches to a substrate and determining locations thereof, comprising:

a pulse generator for generating a pulse;

a pulse driver having an input coupled to the pulse generator and an output coupled through a first resistor to a first end of a serpentine transmission line fabricated on a substrate, a second resistor coupled to a second end of the serpentine transmission line, wherein the first and second resistors each have resistances that are substantially equal to a characteristic impedance of the serpentine transmission line;

a voltage comparator having negative and positive inputs and an output, wherein the negative input is coupled to the first end of the serpentine transmission line;

a voltage reference having a reference voltage output coupled to the positive input of the voltage comparator;

a high resolution timer having an enable input coupled to the output of the pulse driver;

a plurality of storage locations for storing a plurality of touch times sampled from the high resolution timer;

a touch counter that increments a touch count number each time a negative return pulse is detected by the voltage comparator, wherein the negative return pulse represents a touch to the substrate; and

a digital processor coupled to the plurality of storage locations;

wherein the high resolution timer starts timing when the pulse driver sends the pulse to the first end of the serpentine transmission line, and each time a negative return pulse is detected by the voltage comparator an associated time from the high resolution timer is stored in a respective one of the plurality of storage locations determined by the touch count number from the touch counter; and

wherein the digital processor reads the times stored in the plurality of storage locations and converts these times into locations on the substrate.

51. The apparatus according to claim 50, wherein the high resolution timer comprises:

a constant current source; and

a timing capacitor, wherein the constant current source charges the timing capacitor for as long as the pulse from the pulse driver is on.

52. The apparatus according to claim 51, wherein the plurality of storage locations comprises:

a plurality of sample and store capacitors, wherein each time a negative return pulse is detected by the voltage comparator an associated voltage from the timing capacitor is stored in a one of the plurality of sample and store capacitors;

an analog multiplexer having a plurality of inputs, each one of the plurality of inputs is coupled a corresponding one of the plurality of sample and store capacitors; and

an analog-to-digital converter (ADC) having an input coupled to an output of the analog multiplexer and an output coupled to the digital processor, wherein the digital processor reads the ADC digital representations of the voltages stored in the plurality of sample and store capacitors when converting these digital representations from the ADC into locations on the substrate.

53. The apparatus according to claim 50, wherein the high resolution timer comprises:

a super-high frequency clock; and

an ultra-high speed counter, wherein the ultra-high speed counter counts for as long as the pulse from the pulse driver is on.

54. The apparatus according to claim 53, wherein the plurality of storage locations comprises:

a plurality of touch time storage registers, wherein each time a negative return pulse is detected by the voltage comparator an associated touch time count from the ultra-high speed counter is stored in a one of the plurality of touch time storage registers, wherein the digital processor reads the touch time counts from the plurality of touch time storage registers when converting these touch time counts into locations on the substrate.