Suppressing interference in AC measurements of cells, batteries and other electrical elements

• US 6,417,669 B1
• Filed: 06/11/2001
• Issued: 07/09/2002
• Est. Priority Date: 06/11/2001
• Status: Active Grant

• Current Assignee: Keith S Champlin
• Original Assignee: Keith S Champlin
• Inventors: Champlin, Keith S.
• Primary Examiner(s): Toatley, Gregory
• Application Number:
  US09/878,625
• Time in Patent Office: 393 Days
• Field of Search: 324/426, 327/599, 333/12, 361/111
• US Class Current: 324/426
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm
• CPC Class Codes: G01R 31/385 Arrangements for measuring battery or accumulator variables for monitoring G01R31/382
• CPC Class Codes: G01R 31/389 Measuring internal impedance, internal conductance or related variables

Abstract

The disclosed invention relates to measuring an ac dynamic parameter (e.g., impedance, admittance, resistance, reactance, conductance, susceptance) of an electrochemical cell/battery or other electrical element under conditions of possible interference from potential sources such as ac magnetic fields and/or ac currents at the powerline frequency and its harmonics. More generally, it relates to evaluating a signal component at a known frequency f₁ under conditions of possible hum, noise, or other spurious interference at one or more other known frequencies. A microprocessor or microcontroller commands A/D circuitry to sample a band-limited signal at M evenly spaced times per period 1/f₁ distributed over an integer number N of such periods and calculates time-averaged Fourier coefficients from these samples. The frequency response of the calculated Fourier coefficients displays perfect nulls at evenly spaced frequencies either side of frequency f₁. By choosing N according to algorithms disclosed herein, null responses are made to coincide with the one or more frequencies of potential hum, noise, or other spurious interference.

Claims

1. A method for evaluating a component of a signal at a measurement frequency while suppressing potential components at one or more interference frequencies comprising:

sampling said signal M times per period of said measurement frequency for N said periods, said number N being an integral multiple of said measurement frequency divided by the greatest common factor of said measurement frequency and said interference frequencies;

summing each of said M signal samples per period over said N periods to obtain M sample sums;

evaluating said component of said signal at said measurement frequency from said M sample sums.

2. A method as in claim 1 wherein at least one said interference frequency is a powerline frequency.

3. A method as in claim 1 wherein at least one said interference frequency is 50 Hz, 60 Hz, or 400 Hz.

4. A method as in claim 1 wherein said measurement frequency and said interference frequencies are integers.

5. A method as in claim 1 wherein at least one of said measurement frequency and said interference frequencies is a rational fraction.

6. A method as in claim 1 wherein M is larger than one plus a ratio comprising the largest of said interference frequencies divided by said measurement frequency.

7. A method as in claim 1 wherein said signal is derived from a time-varying voltage developed across terminals of an electrochemical cell or battery during measurement of a dynamic parameter.

8. A method as in claim 1 wherein said signal is derived from a time-varying voltage developed across an electrical element during measurement of a dynamic parameter.

9. A method as in claim 1 wherein said component of said signal at said measurement frequency comprises a Fourier coefficient.

10. Apparatus for evaluating a component of a signal at a measurement frequency while suppressing potential components at one or more interference frequencies adapted to perform the method according to claim 1.

11. A method for evaluating a component of a signal at a measurement frequency while suppressing potential components at one or more interference frequencies comprising:

obtaining (M×

N) samples of said signal by sampling at (M×

N) discrete times distributed uniformly over N consecutive periods of said measurement frequency, where N is an integral multiple of said measurement frequency divided by the greatest common factor of said measurement frequency and said discrete interference frequencies;

evaluating said component of said signal at said measurement frequency from said (M×

N) samples.

12. A method as in claim 11 wherein said evaluating comprises summing each said sample acquired in a said period over N said periods to obtain M sample sums and evaluating said component of said signal at said measurement frequency from said M sample sums.

13. A method as in claim 11 wherein said evaluating comprises evaluating said component of said signal at said measurement frequency from a weighted summation containing (M×

N) terms.

14. A method as in claim 11 wherein at least one said interference frequency is a powerline frequency.

15. A method as in claim 11 wherein at least one said interference frequency is 50 Hz, 60 Hz, or 400 Hz.

16. A method as in claim 11 wherein said measurement frequency and said interference frequencies are integers.

17. A method as in claim 11 wherein at least one of said measurement frequency and said interference frequencies is a rational fraction.

18. A method as in claim 11 wherein M is larger than one plus a ratio comprising the largest of said interference frequencies divided by said measurement frequency.

19. A method as in claim 11 wherein said signal is derived from a time-varying voltage developed across terminals of an electrochemical cell or battery during measurement of a dynamic parameter.

20. A method as in claim 11 wherein said signal is derived from a time-varying voltage developed across an electrical element during measurement of a dynamic parameter.

21. A method as in claim 11 wherein said component of said signal at said measurement frequency comprises a Fourier coefficient.

22. Apparatus for evaluating a component of a signal at a measurement frequency while suppressing potential components at one or more interference frequencies adapted to perform the method according to claim 11.

23. A method for introducing a response peak at a desired response frequency and response nulls at one or more undesired response frequencies in the detected response of a measurement signal comprising:

obtaining (M×

N) samples of said measurement signal by uniformly sampling at (M×

N) discrete times distributed over N consecutive periods of said desired response frequency, where N is an integral multiple of said desired response frequency divided by the greatest common factor of said desired response frequency and said undesired response frequencies;

determining said detected response from said (M×

N) samples.

24. A method as in claim 23 wherein said step of determining comprises summing each of said M samples over N said periods to obtain M sample averages and evaluating said detected response from said M sample averages.

25. A method as in claim 23 wherein said step of determining comprises evaluating said detected response from a weighted summation containing (M×

N) terms.

26. A method as in claim 23 wherein at least one said undesired response frequency is a powerline frequency.

27. A method as in claim 23 wherein at least one said undesired response frequency is 50 Hz, 60 Hz, or 400 Hz.

28. A method as in claim 23 wherein said desired response frequency and said undesired response frequencies are integers.

29. A method as in claim 23 wherein at least one of said desired response frequency and said undesired response frequencies is a rational fraction.

30. A method as in claim 23 wherein M is larger than one plus a ratio comprising the largest said undesired response frequency divided by said desired response frequency.

31. A method as in claim 23 wherein said measurement signal is derived from a time-varying voltage developed across terminals of an electrochemical cell or battery during measurement of a dynamic parameter.

32. A method as in claim 23 wherein said measurement signal is derived from a time-varying voltage developed across an electrical element during measurement of a dynamic parameter.

33. A method as in claim 23 wherein said detected response comprises a Fourier coefficient.

34. Apparatus for introducing a response peak at a desired response frequency and response nulls at one or more undesired response frequencies in the detected response of a measurement signal adapted to perform the method according to claim 23.

35. A method for introducing a response peak at a desired response frequency and response nulls at one or more undesired response frequencies in the detected response of a measurement signal comprising:

sampling said measurement signal M times per period of said desired response frequency for N said periods, said number N being an integral multiple of said desired response frequency divided by the greatest common factor of said desired response frequency and said undesired response frequencies;

summing each of said M signal samples per period over said N periods to obtain M sample sums;

determining said detected response from said M sample sums.

36. A method as in claim 35 wherein said detected response comprises a Fourier coefficient.

37. A method as in claim 35 wherein at least one said undesired response frequency is a powerline frequency.

38. A method as in claim 35 wherein at least one said undesired response frequency is 50 Hz, 60 Hz, or 400 Hz.

39. A method as in claim 35 wherein said desired response frequency and said undesired response frequencies are integers.

40. A method as in claim 35 wherein at least one of said desired response frequency and said undesired response frequencies is a rational fraction.

41. A method as in claim 35 wherein M is larger than one plus a ratio comprising the largest said undesired response frequency divided by said desired response frequency.

42. A method as in claim 35 wherein said measurement signal is derived from a time-varying voltage developed across terminals of an electrochemical cell or battery during measurement of a dynamic parameter.

43. A method as in claim 35 wherein said measurement signal is derived from a time-varying voltage developed across an electrical element during measurement of a dynamic parameter.

44. Apparatus for introducing a response peak at a desired response frequency and response nulls at one or more undesired response frequencies in the detected response of a measurement signal adapted to perform the method according to claim 35.

45. Apparatus for detecting a signal component at a measurement frequency while suppressing potential interference components at one or more interference frequencies comprising:

sample and convert circuitry adapted to provide digital representations of sampled values of said signal, said sampled values acquired at (M×

N) sampling times distributed uniformly over N periods of said measurement frequency, where N is an integral multiple of said measurement frequency divided by the greatest common factor of said measurement frequency and said interference frequencies; and

,

computation and control circuitry coupled to said sample and convert circuitry and adapted to initiate said sampling times, to accept said digital representations as inputs, and to compute said signal component at said measurement frequency from said digital representations.

46. Apparatus as in claim 45 wherein said computation and control circuitry computes said signal component by summing said digital representations over said N periods to obtain M sums and numerically combining said M sums.

47. Apparatus as in claim 45 wherein said computation and control circuitry computes said signal component by evaluating weighted summations containing (M×

N) terms.

48. Apparatus as in claim 45 wherein at least one said interference frequency is a powerline frequency.

49. Apparatus as in claim 45 wherein at least one said interference frequency is 50 Hz, 60 Hz, or 400 Hz.

50. Apparatus as in claim 45 wherein said measurement frequency and said one or more interference frequencies are integers.

51. Apparatus as in claim 45 wherein at least one of said measurement frequency and said interference frequencies is a rational fraction.

52. Apparatus as in claim 45 wherein M is larger than one plus a ratio comprising the largest said interference frequency divided by said measurement frequency.

53. Apparatus as in claim 45 wherein said signal is derived from a time-varying voltage developed across terminals of an electrochemical cell or battery during measurement of a dynamic parameter.

54. Apparatus as in claim 45 wherein said signal is derived from a time-varying voltage developed across an electrical element during measurement of a dynamic parameter.

55. Apparatus as in claim 45 wherein said signal component at said measurement frequency comprises a Fourier coefficient.

56. Apparatus for determining a Fourier coefficient of a band-limited signal at a desired response frequency and nulling out response at one or more undesired response frequencies comprising:

sample and convert circuitry adapted to provide digital representations of sampled values of said band-limited signal, said sampled values acquired at (M×

N) sampling times distributed uniformly over N periods of said desired response frequency, where N is an integral multiple of said desired response frequency divided by the greatest common factor of said desired response frequency and said undesired response frequencies; and

,

computation and control circuitry coupled to said sample and convert circuitry and adapted to initiate said sampling times, to accept said digital representations as inputs, and to compute said Fourier coefficient from said digital representations.

57. Apparatus as in claim 56 wherein said computation and control circuitry computes said Fourier coefficient by summing or averaging said samples over said N periods to obtain M sample averages and calculating said Fourier coefficient from said M sample averages.

58. Apparatus as in claim 56 wherein said computation and control circuitry computes said Fourier coefficient by evaluating weighted sine/cosine summations containing (M×

N) terms.

59. Apparatus as in claim 56 wherein at least one said undesired response frequency is a powerline frequency.

60. Apparatus as in claim 56 wherein at least one said undesired response frequency is 50 Hz, 60 Hz, or 400 Hz.

61. Apparatus as in claim 56 wherein said desired response frequency and said undesired response frequencies are integers.

62. Apparatus as in claim 56 wherein at least one of said desired response frequency and said undesired response frequencies is a rational fraction.

63. Apparatus as in claim 56 wherein M is larger than one plus a ratio comprising the largest said undesired response frequency divided by said desired response frequencies.

64. Apparatus as in claim 56 wherein said band-limited signal is derived from a time-varying voltage developed across terminals of an electrochemical cell or battery during measurement of a dynamic parameter.

65. Apparatus as in claim 56 wherein said band-limited signal is derived from a time-varying voltage developed across an electrical element during measurement of a dynamic parameter.

66. Apparatus as in claim 56 wherein said Fourier coefficient of said band-limited signal is an in-phase or quadrature Fourier coefficient of said band-limited signal at said desired response frequency.

67. Apparatus for evaluating a dynamic parameter of an electrochemical cell or battery at a measurement frequency while suppressing possible signals at one or more interference frequencies comprising:

excitation circuitry coupled to said cell or battery and adapted to apply periodic excitation to said cell or battery, said periodic excitation characterized by a period equal to the reciprocal of said measurement frequency;

excitation signal sensing and processing circuitry coupled to said excitation circuitry and adapted to provide an excitation signal in response to said periodic excitation;

response signal sensing and processing circuitry coupled to said cell or battery and adapted to provide a periodic response signal in accordance with the response of said cell or battery to said periodic excitation;

excitation signal sampling circuitry coupled to said excitation signal sensing and processing circuitry and adapted to provide sampled values of said excitation signal, said sampled values acquired at discrete excitation signal sampling times synchronized with said periodic excitation and uniformly distributed in time over consecutive periods of said periodic excitation;

response signal sampling circuitry coupled to said response signal sensing and processing circuitry and adapted to provide sampled values of said response signal, said sampled values acquired at (M×

N) discrete response signal sampling times synchronized with said periodic excitation and uniformly distributed in time over N periods of said periodic excitation where N is an integral multiple of said discrete measurement frequency divided by the greatest common factor of said discrete measurement frequency and said one or more discrete interference frequencies; and

,

computation and control circuitry coupled to said excitation signal sampling circuitry and to said response signal sampling circuitry and adapted to combine said sampled values of said excitation signal and said sampled values of said response signal to evaluate said dynamic parameter of said electrochemical cell or battery at said discrete measurement frequency.

68. Apparatus as in claim 67 wherein said excitation signal sensing and processing circuitry and said response signal sensing and processing circuitry include band-limiting filter circuitry adapted to process said excitation signal and said response signal with identical band-limiting filter response functions.

69. Apparatus as in claim 67 wherein said computation and control circuitry is coupled to said excitation circuitry and is adapted to initiate the timing of said periodic excitation, the timing of said excitation signal sampling times, and the timing of said response signal sampling times, said computation and control circuitry identifying said excitation signal sampling times and said response signal sampling times by measuring time from a periodically repeating timing point of said periodic excitation.

70. Apparatus as in claim 69 wherein said excitation circuitry includes a controlled switch and said computation and control circuitry initiates said timing of said periodic excitation by periodically turning said controlled switch on and off thereby periodically interrupting a current through said cell or battery.

71. Apparatus as in claim 67 wherein said computation and control circuitry is further adapted to average said sampled values of said excitation signal over said consecutive periods to obtain averaged excitation signal samples, to average said sampled values of said response signal over said N periods to obtain averaged response signal samples, to numerically combine said averaged excitation signal samples and said averaged response signal samples to evaluate averaged Fourier coefficients, and to numerically combine said averaged Fourier coefficients to evaluate said dynamic parameter of said electrochemical cell or battery at said measurement frequency.

72. Apparatus as in claim 67 wherein at least one said interference frequency is a powerline frequency.

73. Apparatus as in claim 67 wherein at least one said interference frequency is 50 Hz, 60 Hz, or 400 Hz.

74. Apparatus as in claim 67 wherein said measurement frequency and said interference frequencies are integers.

75. Apparatus as in claim 67 wherein at least one of said measurement frequency and said interference frequencies is a rational fraction.

76. Apparatus as in claim 67 wherein M is larger than one plus a ratio comprising the largest of said interference frequencies divided by said measurement frequency.

77. A method for measuring a dynamic parameter of an electrochemical cell or battery at a measurement frequency while reducing errors due to possible interfering signals at one or more interference frequencies comprising:

exciting said cell or battery with a periodic time-varying current characterized by a smallest period equal to the reciprocal of said measurement frequency;

forming a current signal in accordance with said periodic time-varying current and a voltage signal in accordance with a time-varying response voltage across said cell or battery;

sampling said current signal at equally spaced times over an integer number of consecutive periods of said periodic time-varying current;

sampling said voltage signal at (M×

N) equally spaced times over N consecutive periods of said periodic time-varying current, where N is an integral multiple of said measurement frequency divided by the greatest common factor of said measurement frequency and said one or more interference frequencies;

converting said sampled values of said current signal and said (M×

N) sampled values of said voltage signal to digital format;

evaluating current signal Fourier coefficients from said sampled values of said current signal converted to digital format and evaluating voltage signal Fourier coefficients from said (M×

N) sampled values of said voltage signal converted to digital format; and

,

numerically combining said current signal Fourier coefficients and said voltage signal Fourier coefficients to evaluate said dynamic parameter of said electrochemical cell or battery at said measurement frequency.

78. A method as in claim 77 wherein said forming includes processing said current signal and said voltage signal with identical band-limiting frequency response functions.

79. A method as in claim 77 wherein said periodic time-varying current comprises a periodic square-wave current.

80. A method as in claim 77 wherein said step of evaluating voltage signal Fourier coefficients comprises summing or averaging said (M×

N) sampled values of said voltage signal converted to digital format over N said periods to obtain M voltage-signal sample averages and evaluating M-term weighted summations of said voltage-signal sample averages.

81. A method as in claim 77 wherein said step of evaluating voltage signal Fourier coefficients comprises evaluating (M×

N)-term weighted summations of said (M×

N) sampled values of said voltage signal converted to digital format.

82. A method as in claim 77 wherein at least one said interference frequency is a powerline frequency.

83. A method as in claim 77 wherein at least one said interference frequency is 50 Hz, 60 Hz, or 400 Hz.

84. A method as in claim 77 wherein said measurement frequency and said one or more interference frequencies are integers.

85. A method as in claim 77 wherein at least one of said measurement frequency and said interference frequencies is a rational fraction.

86. A method as in claim 77 wherein M is larger than one plus a ratio comprising the largest said interference frequency divided by said measurement frequency.

87. Apparatus for measuring a dynamic parameter of an electrochemical cell or battery at a measurement frequency while reducing errors due to possible interfering signals at one or more interference frequencies adapted to perform the method according to claim 77.

88. A method for measuring a dynamic parameter of an electrical element at a measurement frequency while reducing errors due to possible interfering signals at one or more interference frequencies comprising:

exciting said electrical element with periodic time-varying excitation characterized by a smallest period equal to the reciprocal of said measurement frequency;

forming an excitation signal in accordance with said periodic time-varying excitation and a response signal in accordance with a time-varying response of said electrical element;

sampling said excitation signal at equally spaced times over an integer number of consecutive periods of said periodic time-varying excitation;

sampling said response signal at (M×

N) equally spaced times over N consecutive periods of said periodic time-varying excitation, where N is an integral multiple of said measurement frequency divided by the greatest common factor of said measurement frequency and said interference frequencies;

converting said sampled values of said excitation signal and said (M×

N) sampled values of said response signal to digital format;

evaluating excitation signal Fourier coefficients from said sampled values of said excitation signal converted to digital format and evaluating response signal Fourier coefficients from said (M×

N) sampled values of said response signal converted to digital format; and

,

numerically combining said excitation signal Fourier coefficients and said response signal Fourier coefficients to evaluate said dynamic parameter of said electrical element at said measurement frequency.

89. A method as in claim 88 wherein said forming includes processing said excitation signal and said response signal with identical band-limiting frequency response functions.

90. A method as in claim 88 wherein said periodic time-varying current comprises a periodic square-wave current.

91. A method as in claim 88 wherein said step of evaluating response signal Fourier coefficients comprises summing said (M×

N) sampled values of said response signal converted to digital format over N said periods to obtain M response signal sample averages and evaluating M-term weighted summations of said M response signal sample averages.

92. A method as in claim 88 wherein said step of evaluating response signal Fourier coefficients comprises evaluating (M×

N)-term weighted summations of said sampled values of said response signal converted to digital format.

93. A method as in claim 88 wherein at least one said interference frequency is a powerline frequency.

94. A method as in claim 88 wherein at least one said interference frequency is 50 Hz, 60 Hz, or 400 Hz.

95. A method as in claim 88 wherein said measurement frequency and said interference frequencies are integers.

96. A method as in claim 88 wherein at least one of said measurement frequency and said interference frequencies is a rational fraction.

97. A method as in claim 88 wherein M is larger than one plus a ratio comprising the largest said interference frequency divided by said measurement frequency.

98. Apparatus for measuring a dynamic parameter of an electrical element at a measurement frequency while reducing errors due to possible interfering signals at one or more interference frequencies adapted to perform the method according to claim 88.