Method and apparatus for measuring electrical parameters using a differentiating current sensor and a digital integrator

• US 5,485,393 A
• Filed: 07/27/1993
• Issued: 01/16/1996
• Est. Priority Date: 08/30/1990
• Status: Expired due to Term

• Current Assignee: Google Inc. (Alphabet Inc.)
• Original Assignee: Metricom, Inc.
• Inventors: Bradford, Curtis V.
• Primary Examiner(s): Voeltz, Emanuel T.
• Assistant Examiner(s): Stamber, Eric W.
• Application Number:
  US08/097,816
• Time in Patent Office: 903 Days
• Field of Search: 364/483, 364/492, 364/724.01, 364/572, 364/574, 324/142, 341/118, 341/143, 323/205, 323/212
• US Class Current: 702/60
• CPC Class Codes: G01R 21/003 Measuring reactive component
• CPC Class Codes: G01R 21/1331 Measuring real or reactive component, measuring apparent energy

Abstract

A digital current meter, usable either alone or in combination with a voltage meter, is disclosed. The current meter includes a current sensor which outputs the time derivative of the current flowing through the load being measured. The differentiated current signal is digitally sampled, usually by a sigma-delta sampler, filtered, and digitally integrated. A feedback path is provided to remove the direct-current (DC) component of the current signal, to keep the integrator from going out of range. A bleeder feedback channel around the digital integrator is provided to keep the digital integrator from permanently latching an impulse. In combination with a voltage meter, a half-sample delay is introduced in the current signal path, to compensate for the lead provided by the combination of the analog differentiator and the digital integrator, and the voltage signal path uses filters with the same characteristics as the current channel, to match the delay in the two channels.

Claims

1. A power meter which outputs voltage and current samples at an output sample rate, where a voltage sample indicates a quantity of voltage across an electrical load at a sample time and a current sample indicates a quantity of current flowing through the electrical load at the sample time, the power meter comprising:

a voltage sensor, electromagnetically coupled to the electrical load, which outputs a voltage sensor signal representing the voltage across the electrical load;

voltage sampling means, coupled to receive said voltage sensor signal, for digitally sampling said voltage sensor signal and outputting a voltage waveform comprising digital samples at an input sample rate;

a voltage filtering path, coupled to receive said voltage waveform from said voltage sampling means, which outputs a filtered voltage waveform;

a current sensor electromagnetically coupled to the electrical load, which outputs a current sensor signal representing a derivative with respect to time of the current flowing through the electrical load;

a differential adder coupled at a first adder input to receive said current sensor signal, said differential adder outputting a signal at an adder output which is a difference of said current sensor signal and a signal applied to a second adder input of said differential adder;

current sampling means, coupled to said adder output, for digitally sampling said signal at said adder output at said input sample rate and outputting a time derivative current waveform comprising digital samples at said input sample rate;

a current feedforward filtering path, coupled to receive said time derivative current waveform, which outputs a filtered time derivative current waveform, wherein said current feedforward filtering path comprises filters with characteristics equal to filters in said voltage filtering path;

a current feedback signal path, coupled to receive said filtered time derivative current waveform from said current feedforward filtering path, which outputs a feedback signal to said second adder input of said differential adder, where said current feedback signal path comprises filters which cause the cancellation of direct-current (DC) components of said filtered time derivative current waveform;

a digital integrator, coupled to receive said filtered time derivative current waveform, which integrates said filtered time derivative current waveform and outputs, at the output sample rate, samples indicating an instantaneous value of the current flowing through the electrical load; and

a delay means, clocked by an intermediate clock having a rate of at least twice the output sample rate, for delaying current samples in said current feedforward filtering path by a time equal to one-half a period of the output sample rate, thereby providing a substantially equal delay for samples through said current feedforward filtering path combined with said digital integrator and through said voltage filtering path.

2. The power meter of claim 1, wherein said current sampling means comprises a sigma-delta analog-to-digital converter which outputs a digitized representation of said adder output combined with quantization noise at said input sample rate.

3. The power meter of claim 1, wherein said current feedforward filtering path comprises:

a) a first third-order sinc filter, coupled at an output to an input of said delay means, where said first third-order sinc filter decimates an input waveform to an intermediate sample rate;

b) said delay means, coupled to an output of said first third-order sinc filter;

c) a first sinc filter, coupled to an output of said delay means, wherein said first sinc filter decimates current samples from said intermediate sample rate to the output sample rate; and

d) a first IIR ( infinite-impulse response) filter coupled to receive an output of said first sinc filter and to output said filtered time derivative current waveform, wherein said first IIR filter is characterized by a transfer function which compensates for at least a portion of a roll-off of said first third-order sinc filter and said first sinc filter;

and wherein said voltage filtering path comprises;

a) a second third-order sinc filter with filter characteristics equivalent to said first third-order sinc filter;

b) a second sinc filter, coupled to an output of said second third-order sinc filter, wherein said second sinc filter decimates voltage samples from said intermediate sample rate to the output sample rate and has filter characteristics equivalent to said first sinc filter; and

d) a second IIR filter coupled to receive an output of said second sinc filter and to output said filtered voltage waveform, wherein said second IIR filter has filter characteristics equivalent to said first IIR filter.

4. The power meter of claim 1, further comprising an impulse noise bleeder around said digital integrator, for removing impulse noise from a quantity accumulated in said digital integrator

5. The power meter of claim 4, wherein said impulse noise bleeder is a first-order sinc filter with a decimation rate in the range of 20 to 200.

6. The power meter of claim 1, implemented by a suitably programmed digital signal processor, two analog-to-digital sampling circuits, and two digital-to-analog circuits.

7. The power meter of claim 1, further comprising digital processing means for generating output samples representative of electrical parameters of the electrical load including voltage squared, current squared, watts, volts-amps reactive, volt² -hours, amp² -hours, watt-hours, and VAR-hours.

8. A current meter which outputs samples at an output sample rate, where a sample indicates a quantity of current flowing through an electrical load at a sample time of the sample, the current meter comprising:

a current sensor electromagnetically coupled to the load, which outputs a sensor signal at a sensor output representing a derivative with respect to time of the current flowing through the load;

a differential adder coupled at a first adder input to said current sensor output, said differential adder outputting a signal at an adder output which is a difference of said sensor signal and a signal applied to a second adder input of said differential adder;

sampling means, coupled to said adder output, for digitally sampling said signal at said adder output and outputting a time derivative current waveform comprising digital samples at an input sample rate;

a feedforward filtering path, coupled to receive said time derivative current waveform, which outputs a filtered time derivative current waveform;

a feedback signal path, coupled to receive said filtered time derivative current waveform from said feedforward filtering path, which outputs an analog feedback signal to said second adder input of said differential adder where said feedback signal path comprises filters which cause the cancellation of direct-current (DC) components of said filtered time derivative current waveform; and

a digital integrator, coupled to receive said filtered time derivative current waveform, which integrates said filtered time derivative current waveform and outputs, at the output sample rate, samples of instantaneous current in the load.

9. The current meter of claim 8, wherein said sampling means is a sigma-delta analog-to-digital converter which outputs a digitized representation of said adder output combined with quantization noise at said input sample rate, and said feedforward filtering path comprises decimating low pass filters, wherein said input sample rate is higher than the output sample rate.

10. The current meter of claim 9, wherein said decimating low pass filters are a third-order sinc filter and a first-order sinc filter, and wherein said feedforward filtering path also comprises an infinite impulse response filter.

11. The current meter of claim 10, wherein a half-sample delay is interposed between said third-order sinc filter and said first-order sinc filter, said half-sample delay introducing a delay equal to half a sample period of the output of the current meter.

12. The current meter of claim 8, wherein said feedback signal path comprises:

a second-order sinc filter, coupled to receive said filtered time derivative current waveform;

a decimator, coupled to an output of said second-order sinc filter;

a digital integrator, coupled to an output of said decimator;

a sigma-delta digital-to-analog converter, coupled at an input to an output of said decimator and coupled at an output to said second input of said differential adder.

13. The current meter of claim 8, further comprising an impulse noise bleeder around said digital integrator, for removing impulse noise from a quantity accumulated in said digital integrator.

14. The current meter of claim 13, wherein said impulse noise bleeder is a first-order sinc filter with a decimation rate in the range of 20 to 200.

15. The current meter of claim 8, implemented by a suitably programmed digital signal processor, an analog-to-digital sampling circuit, and a digital-to-analog circuit.

16. A method for providing a stream of digital samples representing electrical parameters in a load, including a parameter equal to a current flowing through the load, comprising the steps of:

measuring an analog signal representative of a derivative with respect to time of the current through the load;

subtracting a DC cancellation signal from said analog signal to form a relative analog signal;

sampling said relative analog signal at an input sampling rate to form a current waveform comprising digital samples;

digitally filtering said current waveform to form a filtered current waveform;

generating said DC cancellation signal from said filtered current waveform;

digitally integrating said filtered current waveform; and

outputting an integrated current waveform as a waveform representative of instantaneous current through the load, said integrated current waveform comprising digital samples output at an output sample rate.

17. The method of claim 16, further comprising the step of removing accumulations in said step of digitally integrating over a plurality of cycles of integration.

18. The method of claim 16, wherein the electrical parameters include a voltage across the load, the method further comprising the steps of:

measuring an analog voltage signal representative of the voltage across the load;

sampling said analog voltage signal at the input sampling rate to form a voltage waveform comprising digital samples;

digitally filtering said voltage waveform to form a filtered voltage waveform, using filters matched to filters used in said step of digitally filtering said current waveform;

filtering said filtered voltage waveform to remove DC components;

delaying said current waveform by a time equal to half of a period between two samples at said output sample rate; and

outputting said filtered voltage waveform as a waveform representative of instantaneous voltage across the load, said filtered voltage waveform comprising digital samples output at said output sample rate.

19. The method of claim 18, wherein said steps of digitally filtering said current waveform and digitally filtering said voltage waveform each include a step of filtering, with similar filters, said current and voltage waveforms to partially compensate for a transfer function differential between said current waveform and said voltage waveform to provide a more even transfer function in a frequency range of interest for a power signal.

20. The method of claim 18, further comprising the steps of calculating electrical parameters of the load from said filtered voltage samples and said filtered current samples including voltage squared, current squared, watts, volt-amps reactive (VAR), volt² -hours, amp² -hours, watt-hours, and VAR-hours.