Capacitance-variation-sensitive touch sensing array system

• US 4,733,222 A
• Filed: 04/18/1986
• Issued: 03/22/1988
• Est. Priority Date: 12/27/1983
• Status: Expired due to Term

• Current Assignee: Integrated Touch Arrays Incorporated
• Original Assignee: Integrated Touch Arrays Incorporated
• Inventors: Evans, John W.
• Primary Examiner(s): Brigance, Gerald L.
• Assistant Examiner(s): FATAHI YAR, MAHMOUD
• Application Number:
  US06/853,428
• Time in Patent Office: 704 Days
• Field of Search: 340/365 C, 340/365 R, 340/365 S, 340/712, 340/365 VL, 178/18, 200/DIG. 1, 400/485, 400/477
• US Class Current: 341/33
• CPC Class Codes: G06F 3/0445 using two or more layers of sensing electrodes, e.g. using two layers of electrodes separated by a dielectric layer
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes
• CPC Class Codes: G06F 3/047 using sets of wires, e.g. crossed wires
• CPC Class Codes: H03K 17/9618 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2017/9604 characterised by the number of electrodes
• CPC Class Codes: H03K 2017/9613 using two electrodes per touch switch

Abstract

A capacitance-variation-sensitive touch sensing array system includes an array of electrodes where each electrode is a connected series of conductive tabs and forms either a row or a column of the electrode array; an array of drive lines where each drive line is capacitively coupled to a plurality of the electrodes; a drive signal generator for applying alternating signal packets to the drive lines; an array of sense lines where each sense line is capacitively coupled to a plurality of the electrodes so that signals are derived from the electrodes when drive signals are applied to the drive lines. The number of electrodes is equal to the product of the number of drive lines and the number of sense lines. Based on values derived from signals on the sense lines, a microprocessor provides information associated with touch by an operator. The array of electrodes may be configured so as to result in a touch-actuated keyboard where the number of keys is equal to the product of the number of row electrodes and the number of column electrodes; or the array of electrodes may be configured so that finger touch position in a single large touch responsive area can be sensed and processed for use in control applications.

Claims

1. A capacitance-variation-sensitive touch sensing array system comprising

an array of electrodes with each electrode including a connected series of conductive tabs;

an array of keys with each key overlying at least one tab of at least one of said electrodes for varying the capacitance between said electrode and ground upon the key being touched by an operator;

an array of drive lines capacitively coupled to the electrodes, with individual drive lines being coupled to a plurality of the electrodes;

drive means for applying drive signals to the drive lines to drive the electrodes coupled thereto;

an array of sense lines capacitively coupled to the electrodes, to derive sense signals from the electrodes when drive signals are applied to the drive lines, with each of said sense lines being so coupled to a plurality of the electrodes as to be interconnected via unique combinations of electrodes to said drive lines such that the sense lines derive unique combined patterns of variations in said sense signals in response to actuation of each of the keys;

means coupled to the sense lines for sensing said patterns of variations in said sense signals; and

means coupled to the sensing means for processing the sense signals to provide information associated with any particular key that is touched by an operator;

whereby the number of keys exceeds the product of the number of drive lines and the number of sense lines.

2. A system according to claim 1,

wherein the electrodes are arrayed in rows and columns;

and

wherein each key overlies a tab complex made up of a tab of one of the row electrodes and a tab of one of the column electrodes.

3. A system according to claim 1,

wherein each electrode is capacitively coupled to only one drive line and to only one sense line;

and

wherein no given drive line and sense line pair is connected to more than one electrode in common.

4. A system according to claim 1,

wherein the drive means generate drive signals comprising alternating signal packets, with simultaneously occurring signal packets being generated synchronously on different drive lines;

and

wherein the means for sensing the sense signals on the sense lines includes means for synchronously demodulating the sensed signals to cause the demodulation of the sensed signals to be synchronous with the generation of the corresponding drive signal packets by the drive means.

5. A system according to claim 4, wherein the processing means uses stored phase control parameters to control the phase of demodulation of the sensed signals by the demodulating means.

6. A system according to claim 3, wherein the different drive signals in combination include alternating signal packets applied at the same time to pairs of drive lines with the simultaneously occuring signal packets being synchronous with each other, of approximately the same amplitude, and of opposing polarities to thereby cause the amplitude of the sense signal appearing on a sense line connected to separate electrodes driven by each of the simultaneously occurring drive signals to be low in relation to the variation in the amplitude of said sense signal resulting from a key being touched by an operator.

7. A system according to claim 6, wherein the capacitors coupling the drive lines to the electrodes and the capacitors coupling the electrodes to the sense lines are trimmed in order to balance each pair of the transmitted drive signals of opposing polarities appearing on a sense line.

8. A system according to claim 6, wherein the processing means controls the amplitudes of one or both of the individual drive signals of each pair of the simultaneously occuring drive signals of opposing polarities in order to balance the transmitted drive signals appearing on a sense line.

9. A system according to claim 6,

wherein the drive means comprise a drive signal generator for applying sequentially during each repetitive sequence a plurality of said signal packets on some of the drive lines paired synchronously with said signal packets of opposite polarity on different ones of the other drive lines;

and

wherein the processing means include means for determining whether a sensed variation in the capacitance between an electrode and circuit ground is due to the onset or the cessation of touching a key by an operator.

10. A system according to claim 1, further comprising

first, second, and third dielectric substrates;

wherein a first portion of the electrode array is supported by one side of the first dielectric substrate and a second portion of the electrode array is supported by the opposite side of the first dielectric substrate;

wherein an individual key of the array of keys overlies a first tab supported by the one side of the first dielectric substrate and extending from an electrode of the first portion of the array and a second tab supported by the opposite side of the first dielectric substrate and extending from an electrode of the second portion of the array adjacent to the position underlying the first tab so that the capacitance between both electrodes and ground is varied when the key is touched by an operator;

wherein the drive lines and sense lines are supported by opposite sides of the second dielectric substrate;

wherein the third dielectric substrate separates the second dielectric substrate supporting the drive lines and the sense lines from the first dielectric substrate supporting the electrode arrays; and

wherein the drive lines and sense lines on the second dielectric substrate are dimensioned and disposed in spatial relation to the electrodes on the first dielectric substrate so that the required capacitive coupling of drive lines to electrodes and of sense lines to electrodes is obtained.

11. A system according to claim 1, further comprising

a thin dielectric substrate;

wherein a first portion of the electrode array is supported by one side of the dielectric substrate and a second portion of the electrode array is supported by the opposite side of the dielectric substrate;

wherein an individual key of the array of keys overlies a first tab supported by the one side of the dielectric substrate and extending from an electrode of the first portion of the array and a second tab supported by the opposite side of the dielectric substrate and extending from an electrode of the second portion of the array adjacent to the position underlying the first tab so that the capacitance between both electrodes and ground is varied when the key is touched by an operator;

wherein the drive lines and sense lines are supported by the dielectric substrate; and

wherein a drive line or sense line on one side of the substrate is coupled to an electrode on the opposite side of the substrate by a capacitor comprising

a first plate supported by the one side of the substrate and extending from the coupled drive line or sense line; and

a second oppositely disposed plate supported by the opposite side of the substrate and extending from the coupled electrode.

12. A system according to claim 1, wherein the processing means comprise

means for providing sense signal values in accordance with the amplitudes of the respective sense signals;

means for processing the sense signal values iteratively to ascertain the values of variations in the sense signal values; and

means for processing the sense signal variation values to determine that an operator touch is present at a specific location when the sense signal variation values match a pattern of variation values characteristically associated with touch at that specific location.

13. A system according to claim 12, wherein the means for processing the variation values process the variation values as one or more groups in two stages, the first stage being a feature extraction stage in which a search is made for features indicative of touch and the second stage being a confirmation stage which is carried out when an affirmative determination is made in the first stage and in which tests are carried out to confirm that a pattern of sense signal value variation characteristic of a key being touched by an operator is or is not present.

14. A system according to claim 13,

wherein the electrode array defines rows and columns of electrodes;

and

wherein the means for processing the variation values comprises

means for determining in the feature extraction stage whether the maximum of the group of variation values derived from the row electrodes exceeds a predetermined threshold value;

means for determining in the confirmation stage whether no other of the group of variation values derived from the row electrodes exceeds a tolerance value that may depend on said maximum variation value derived from the row electrodes;

means for determining in the feature extraction stage whether the maximum of the group of variation values derived from the column electrodes exceeds a predetermined threshold value;

means for determining in the confirmation stage whether no other of the group of variation values derived from the column electrodes exceeds a tolerance value that may depend on said maximum variation value derived from the column electrodes; and

means for determining in response to all of said four immediately preceding recited determinations being affirmative that the maximum variation value derived from the row electrodes and the maximum variation value derived from the column electrodes each resulted from only a single one of the keys being touched instantaneously by an operator.

15. A system according to claim 1, wherein the processing means comprise

means for providing sense signal values in accordance with the amplitudes of the respective sense signals;

and

means for processing the sense signal values iteratively by comparing the sense signal values with baseline values derived from sense signal values provided when the touch sensing array is untouched by an operator to determine which one of three states is present, the first state being an untouched state, the second state being a valid touch state in which a valid touch by an operator is present, and the third state being an invalid response state which occurs when neither said first state nor said second state is present.

16. A system according to claim 15, wherein the processing means revises the baseline values in response to sensed variations in the sense signal values which are stable over a predetermined number of iterations and which indicate the invalid response state of the touch sensing array.

17. A system according to claim 16, wherein the means for revising the baseline values comprises

means for storing a set of reserve values derived from a set including each of the respective sense signal values;

means for iteratively comparing each sense signal value to each corresponding reserve value;

means for revising the stored set of reserve values to correspond to a current set of sense signal values whenever the difference between any sense signal value and the corresponding reserve value exceeds a predetermined bound value; and

means for revising the baseline values to correspond to the reserve values when the invalid response state is present for a predetermined number of iterations during which time not revision of the reserve values occurs, no revision of baseline values to correspond to reserve values occurs and the valid touch state is not present.

18. A system according to claim 15, wherein the processing means comprise

means for compensating for drift in the sense signal values that are provided when the keys are in the untouched state, wherein the baseline values are updated by either incrementing or decrementing each of the respective baseline values in accordance with whether the baseline value is less or greater than the respective last-provided sense signal value when the untouched state is present for a predetermined number of consecutive iterations during which time no updating of the baseline values occurs.

19. A system according to claim 18, wherein the compensating means further comprises

means for delaying by degree the updating of baseline values when the invalid response state occurs only intermittently.

20. A system according to claim 1, wherein the processing means comprise

means for providing sense signal values in accordance with the amplitude of the respective sense signals;

means for processing the sense signal values iteratively to determine whether any of the keys has been touched by an operator by comparing the current sense signal values during each iteration with the respective sense signal values during the previous iteration, by adding the same signal variation value derived from said comparison to an accumulated difference value for each sense signal, and by determining whether a key has been touched by processing the accumulated difference values; and

means for resetting the accumulated difference values to an initial value when the sense signal variation values remain less than a predetermined bound value over a predetermined number of consecutive iterations during which no resetting of the accumulated difference values to the initial value occurs.

45. A system according to claim 9, wherein the processing means comprise

means for providing sense signal values in accordance with the amplitudes of the respective sense signals;

and

means for processing the sense signal values iteratively to ascertain the values of accumulated differences in the sense signal values and for processing the accumulated differences as a group to determine whether any said differences resulted from a key being touched by an operator;

wherein the electrode array defines rows and columns of electrodes; and

wherein the means for processing the accumulated differences comprises

means for determining whether the maximum accumulated difference derived from the row electrodes exceeds a predetermined threshold value;

means for determining whether all other accumulated differences derived from the same row electrode as said maximum accumulated difference derived from the row electrodes exceed a threshold value which may depend on said maximum accumulated difference derived from the row electrodes;

means for determining that no accumulated difference derived from a row electrode other than said same row electrode exceeds a tolerance value which may depend on said maximum accumulated difference derived from the row electrodes;

means for determining whether the maximum accumulated difference derived from the column electrodes exceeds a predetermined threshold value;

means for determining whether all other accumulated differences derived from the same column electrode as said maximum accumulated difference derived from the column electrodes exceed a threshold value which may depend on said maximum accumulated difference derived from the column electrodes;

means for determining that no accumulated difference derived from a column electrode other than said same column electrode exceeds a tolerance value which may depend on said maximum accumulated difference derived from the column electrodes; and

means for determining, in response to such of said six immediately preceding recited determinations that are applicable being affirmative, that the accumulated differences derived from the row electrodes and the accumulated differences derived from the column electrodes resulted from the onset of only a single one of the keys being touched instantaneously by an operator.

21. A capacitance-variation-sensitive touch sensing array system comprising

an array of electrodes with each electrode including a connected series of conductive tabs;

a touch responsive panel overlying and coextensive with the electrode array for varying the capacitance between a plurality of the electrodes and ground simultaneously upon the interior area of the touch responsive panel being touched by an operator;

an array of drive lines capacitively coupled to the electrodes, with individual drive lines being coupled to a plurality of the electrodes;

drive means for applying drive signals to the drive lines to drive the electrodes coupled thereto;

an array of sense lines capacitively coupled to the electrodes to derive sense signals from the electrodes when drive signals are applied to the drive lines, with each of said sense lines being so coupled to a plurality of the electrodes as to be interconnected via unique combinations of electrodes to said drive lines such that the sense lines derive unique combined patterns of variations in said sense signals in response to touching distinct locations on the panel;

means coupled to the sense lines for sensing said patterns of variations in said sense signals; and

means coupled to the sensing means for processing the sense signals to provide an estimate of the location of operator touch;

whereby the number of said distinct locations of operator touch that can be estimated exceeds twice the product of the number of drive lines and the number of sense lines.

22. A system according to claim 21,

wherein each electrode is capacitively coupled to only one drive line and to only one sense line;

and

wherein no given drive line and sense line pair is connected to more than one electrode in common.

23. A system according to claim 21,

wherein the drive means generate drive signals comprising alternating signal packets, with simultaneously occurring signal packets being generated synchronously on different drive lines;

and

wherein the means for sensing the sense signal on the sense lines includes means for synchronously demodulating the sensed signals to cause the demodulation of the sensed signals to be synchronous with the generation of the corresponding drive signal packets by the drive means.

24. A system according to claim 23, wherein the processing means uses stored phase control parameters to control the phase of demodulating of the sensed signals by the demodulating means.

25. A system according to claim 22, wherein the different drive signals in combination include alternating signal packets applied at the same time to pairs of drive lines with the simultaneously occuring signal packets being synchronous with each other, of approximately the same amplitude, and of opposing polarities to thereby cause the amplitude of the sense signal appearing on a sense line connected to separate electrodes driven by each of the simultaneously occurring drive signals to be low in relation to the variation in the amplitude of said sense signal resulting from the touch responsive area being touched by an operator.

26. A system according to claim 25, wherein the capacitors coupling the drive lines to the electrodes and the capacitors coupling the electrodes to the sense lines are trimmed in order to balance each pair of the transmitted drive signals of opposing polarities appearing on a sense line.

27. A system according to claim 25, wherein the processing means controls the amplitudes of one or both of the individual drive signals of each pair of the simultaneously occuring drive signals of opposing polarities in order to balance the transmitted drive signals appearing on a sense line.

28. A system according to claim 21, further comprising

first, second, and third dielectric substrates;

wherein the electrode array defines rows and columns of electrodes;

wherein the row electrodes are supported by one side of the first dielectric substrate and the column electrodes are supported by the opposite side of the first dielectric substrate;

wherein the tabs of the column electrodes on one side of the first substrate are disposed so as to lie between the tabs of the row electrodes on the opposite side of the first substrate;

wherein the drive lines and sense lines are supported by opposite sides of the second dielectric substrate;

wherein the third dielectric substrate separates the second dielectric substrate supporting the drive lines and the sense lines from the first dielectric substrate supporting the electrode arrays; and

wherein the drive lines and sense lines on the second dielectric substrate are dimensioned and disposed in spatial relation to the electrodes on the first dielectric substrate so that the required capacitive coupling of drive lines to electrodes and of sense lines to electrodes is obtained.

29. A system according to claim 21, further comprising

a thin dielectric substrate;

wherein the electrode array defines rows and columns of electrodes;

wherein the row electrodes are supported by one side of the dielectric substrate and the column electrodes are supported by the opposite side of the dielectric substrate;

wherein the tabs of the column electrodes on one side of the substrate are disposed so as to lie between the tabs of the row electrodes on the opposite side of the substrate;

wherein the drive lines and sense lines are supported by the dielectric substrate; and

wherein a drive line or sense line on one side of the substrate is coupled to an electrode on the opposite side of the substrate by a capacitor comprising

a first plate supported by the one side of the substrate and extending from the coupled drive line or sense line; and

a second oppositely disposed plate supported by the opposite side of the substrate and extending from the coupled electrode.

30. A system according to claim 21, wherein the processing means comprise

means for providing sense signal values in accordance with the amplitudes of the respective sense signals;

means for processing the sense signal values iteratively to ascertain the values of variations in the sense signal values; and

means for processing the sense signal variation values to determine that an operator touch is present at a specific location when the sense signal variation values match a pattern of variation values characteristically associated with touch at that specific location.

31. A system according to claim 30, wherein the means for processing the variation values process the variation values as one or more groups in three stages, the first stage being a feature extraction stage in which a search is made for features indicative of touch, the second stage being a confirmation stage which is carried out when an affirmative determination is made in the first stage and in which tests are carried out to confirm that a pattern of sense signal value variation characteristic of touch by an operator is or is not present, and the third stage being an interpolation stage which is carried out when affirmative determinations are made in the first and second stages and in which an interpolation procedure using the variation values of a group supplies a coordinate position for the location of touch.

32. A system according to claim 31,

wherein the electrode array defines rows and columns of electrodes;

and

wherein the means for processing the variation values comprises

means for determining in the feature extraction stage whether the maximum of the group of variation values derived from the row electrodes exceeds a predetermined threshold value;

means for determining in the confirmation stage whether none of the other variation values derived from the row electrodes other than a given number of row electrodes on each side of and adjacent to the row electrode from which said maximum variation value is derived exceeds a tolerance value that is a predetermined fraction of said maximum variation value derived from the row electrodes;

means for estimating in the interpolation stage by an interpolation formula the y coordinate of operator touch from the group of variation values derived from the row electrodes;

means for determining in the feature extraction stage whether the maximum of the group of variation values derived from the column electrodes exceeds a predetermined threshold value;

means for determining in the confirmation stage whether none of the other variation values derived from the column electrodes other than a given number of column electrodes on each side of and adjacent to the column electrode from which said maximum variation value is derived exceeds a tolerance value that is a predetermined fraction of said maximum variation value derived from the column electrodes; and

means for estimating in the interpolation stage by an interpolation formula the x coordinate of operator touch from the group of variation values derived from the column electrodes.

33. A system according to claim 21, wherein the processing means comprises

means for providing sense signal values in accordance with the amplitudes of the respective sense signals;

and

means for processing the sense signal values iteratively by comparing the sense signal values with baseline values derived from sense signal values provided when the touch sensing array is untouched by an operator to determine which one of three states is present, the first state being an untouched state, the second state being a valid touch state in which a valid touch by an operator is present, and the third state being an invalid response state which occurs when neither said first state nor said second state is present.

34. A system according to claim 33, wherein the processing means revises the baseline values in response to sensed variations in the sense signal values which are stable over a predetermined number of iterations and which indicate the invalid response state of the touch sensing array.

35. A system according to claim 34, wherein the means for revising the baseline values comprises

means for storing a set of reserve values derived from a set including each of the respective sense signal values;

means for iteratively comparing each sense signal value to each corresponding reserve value;

means for revising the stored set of reserve values to correspond to a current set of sense signal values whenever the difference between any sense signal value and the corresponding reserve value exceeds a predetermined bound value; and

means for revising the baseline values to correspond to the reserve values when the invalid response state is present for a predetermined number of iterations during which time no revision of the reserve values occurs, no revision of baseline values to correspond to reserve values occurs and the valid touch state is not present.

36. A system according to claim 33, wherein the processing means comprise

means for compensating for drift in the sense signal values that are provided when the touch sensing array is in the untouched state, wherein the baseline values are updated by either incrementing or decrementing each of the respective baseline values in accordance with whether the baseline value is less or greater than the respective last-provided sense signal value when the untouched state is present for a predetermined number of consecutive iterations during which time no updating of the baseline values occurs.

37. A system according to claim 36, wherein the compensating means further comprises

means for delaying by degree the updating of baseline values when the invalid response state occurs only intermittently.

38. A capacitance-variation-sensitive touch sensing array system comprising

an array of electrodes with each electrode including at least one conductive tab;

an array of keys with each key overlying at least one tab of at least one electrode of the electrode array for varying the capacitance between each said overlaid electrode and ground upon the key being touched by an operator;

drive means for applying drive signals to drive the electrodes;

an array of sense lines coupled to the electrodes;

sensing means coupled to each of the sense lines to derive a digital sense value from each of the respective sense lines when drive signals are applied to the electrodes, which digital sense value is dependent on variations in the electrode to ground capacitance of at least one of the electrodes, with said sense lines being so coupled to the electrodes as to be interconnected via the electrodes to said drive lines such that said sense lines derive unique combined patterns of variations in said sense signals in response to actuation of each of the keys; and

means for processing the digital sense values iteratively to ascertain the values of variations in the digital sense values and for processing the variation values in two stages, the first stage being a feature extraction stage in which a search is made for said digital sense value variations that may be indicative of a given key being touched and the second stage being a confirmation stage which is carried out when an affirmative determination is made in the first stage and in which said digital sense value variations for which said affirmative determination was made during the feature extraction stage are processed together with a predetermined group of digital sense value variations to determine whether said unique combined pattern of digital sense value variations characteristic of said given key being touched by an operator is or is not present.

39. A system according to claim 38,

wherein the keys of the touch sensing array defines rows and columns of keys;

wherein the digital sense values include two groups with the first group providing information associated with the row of a key and with the second group providing information associated with the column of a key;

wherein the effect of operator touch of a single key of the touch sensing array is to predominantly vary one digital sense value in the first group of digital sense values associated with rows of keys and one digital sense value in the second group of digital sense values associated with columns of keys; and

wherein the means for processing the variations in the digital sense values comprises

means for determining in the feature extraction stage whether the maximum variation in the values associated with rows of keys exceeds a predetermined threshold value;

means for determining in the confirmation stage whether none of the other variations in the digital sense values associated with rows of keys exceeds a tolerance value that may depend on said maximum variation value derived from the rows of keys;

means for determining in the feature extraction stage whether the maximum variation in the values associated with columns of keys exceeds a predetermined threshold value;

means for determining in the confirmation stage whether none of the other variations in the digital sense values associated with columns of keys exceeds a tolerance value that may depend on said maximum variation value derived from the columns of keys; and

means for determining in response to all of said four immediately preceding recited determinations being affirmative that the maximum variation value derived from the rows of keys and the maximum variation value derived from the columns of keys each resulted from only a single key being touched instantaneously by an operator.

40. A system according to claim 38, wherein the processing means comprises

means for processing the digital sense values iteratively by comparing the digital sense values with baseline digital sense values derived from digital sense values provided when the touch sensing array is untouched by an operator to determine which one of three states is present, the first state being an untouched state, the second state being a valid touch state in which a valid touch by an operator is present, and the third state being an invalid response state which occurs when neither said first state nor said second state is present.

41. A system according to claim 40, wherein the processing means revises the baseline digital sense values in response to sensed variations in the digital sense values which are stable over a predetermined number of iterations and which indicate the invalid response state of the touch sensing array.

42. A system according to claim 41, wherein the means for revising the baseline digital sense values comprises

means for storing a set of reserve values derived from a set including each of the respective digital sense values;

means for iteratively comparing each digital sense value to each corresponding reserve value;

means for revising the stored set of reserve values to correspond to a current set of digital sense values whenever the difference between any digital sense value and the corresponding reserve value exceeds a predetermined bound value; and

means for revising the baseline digital sense values to correspond to the reserve values when the invalid response state is present for a predetermined number of iterations during which time no revision of the reserve values occurs, no revision of baseline digital sense values to correspond to reserve values occurs and the valid touch state is not present.

43. A system according to claim 40, wherein the processing means comprises

means for compensating for drift in the baseline digital sense values that are provided when the touch sensing array is in the untouched state

wherein the baseline digital sense values are updated by either incrementing or decrementing each of the respective baseline digital sense values in accordance with whether the baseline digital sense value is less or greater than the respective last-provided digital sense value when the untouched state is present for a predetermined number of consecutive iterations during which time no updating of the baseline digital sense values occurs.

44. A system according to claim 43, wherein the compensating means further comprises

means for delaying by degree the updating of baseline digital sense values when the invalid response state occurs only intermittently.