Measuring the impedance in grounding systems

Abstract

Procedure for measuring the impedance of a grounding system that comprises a loop, the procedure comprises for each frequency fi of a set of frequencies F of a frequency sweep applied to the loop of the grounding system, generating a modulating signal S.sub.signal (2) with a fixed frequency fm, generating a carrier signal S.sub.carrier (1) with the frequency fi, and obtaining an amplitude modulated signal S.sub.modulated (3) with frequency components of the frequencies fm and fi and based on S.sub.signal (2) and S.sub.carrier (1).

Claims

1: Method for measuring the impedance of a grounding system that comprises a loop, the method comprising: for each frequency fi of a set of frequencies F, of a frequency sweep applied to the loop of the grounding system: generating a modulating signal S.sub.signal (2) with a fixed frequency fm, preferably fm=220 Hz; generating a carrier signal S.sub.carrier (1) with the frequency fi; obtaining an amplitude modulated signal S.sub.modulated (3) with frequency components of the frequencies fm and fi based on S.sub.signal (2) and S.sub.carrier (1); inducing the signal S.sub.modulated (3) in the loop of the grounding system; and measuring a signal S.sub.out based on the signal S.sub.modulated (3) in said loop of the grounding system; demodulating the signal S.sub.out, eliminating the carrier signal S.sub.carrier (1); filtering the signal S.sub.out in a frequency fm tuned way to obtain a demodulated signal S.sub.demodulated at the frequency fm based on S.sub.signal (2); obtaining an impedance Z(fi) of the grounding system based on the demodulated signal S.sub.demodulated; wherein obtaining the impedance Z (fi) of the grounding system based on the demodulated signal S.sub.demodulated comprises calculating a modulating index $m=\frac{S_{\mathrm{demodulated}}}{S_{\mathrm{carrier}}\left(1\right)}$ based on the demodulated signal S.sub.demodulated, wherein the modulating index m is defined as the ratio of amplitudes, obtaining a phase measurement (fi) based on the respective demodulated signal S.sub.demodulated for each frequency fi; and obtaining for the set of frequencies F a Bode plot based on Z(fi), (fi), and the modulating index m of each frequency fi, respectively.

2: Device (400) configured to measure the impedance of a grounding system according to the method of claim 1, comprising a loop, wherein the device (400) comprises: a generator (12) configured to generate a modulating signal S.sub.signal (2) at a frequency fm. a generator (13) configured to generate a plurality of carrier signals S.sub.carrier (1) with frequencies fi comprised in a set of frequencies F of a frequency sweep; an amplitude modulator configured to obtain a plurality of modulated signals S.sub.modulated (3); a current inductor (9) configured to induce in the loop respective electromotive forces associated with the plurality of modulated signals S.sub.modulated (3); a current sensor (15) configured to measure in the loop a set of signals S.sub.out associated with the plurality of modulated signals S.sub.modulated (3); a demodulator (16) configured to demodulate the set of signals S.sub.out considering the frequency fm; a filter (17) tuned to the frequency fm and configured to obtain demodulated signals S.sub.demodulated based on S.sub.signal (2); means for calculating the impedance Z (fi) of the grounding system based on the demodulated signals S.sub.demodulated. means (18) for obtaining for each frequency fi a phase measurement (fi) of the demodulated signal S.sub.demodulated; means (19) for calculating a modulating index $m=\frac{S_{\mathrm{demodulated}}}{S_{\mathrm{carrier}}\left(1\right)}$ for each frequency fi, wherein the modulating index m is defined as the ratio of amplitudes; and means for obtaining for the set of frequencies F a Bode plot based on Z(fi), (fi), and the modulating index m of each frequency fi, respectively.

3: The device (400) of claim 1, wherein the generator (13) of the plurality of carrier signals S.sub.carrier (1) comprises a microcontroller.

4: The device (400) of claim 1, wherein the microcontroller comprises an internal memory preferably configured to store Z (fi) and (fi) obtained for each frequency fi of the set of frequencies F of the frequency sweep.

5: The device (400) of claim 1 further comprising a power amplifier (10) configured to amplify the plurality of modulated signals S.sub.modulated (3).

Description

DESCRIPTION OF THE DRAWINGS

[0018] To complement the description that is being made and in order to help a better understanding of the features of the impedance measurement procedure of a grounding system, according to a preferred example of practical embodiment thereof, a set of drawings is attached as an integral part of said description, wherein, for illustrative and non-limiting purposes, the following has been represented:

[0019] FIG. 1 shows an RLC circuit.

[0020] FIG. 2 shows three signals involved in an amplitude modulation.

[0021] FIG. 3 shows examples of the effect of modulation index on the AM modulation process.

[0022] FIG. 4 shows a device for measuring the impedance of a grounding system according to the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

[0023] FIG. 1 shows the circuit (100) referring to the characterization of a system determining its resistive (R), inductive (L) and capacitive (C) components, which, in a reduced and compact way, can be expressed as an RLC equivalent.

[0024] It is widely known that the transfer function of the circuit (100) of FIG. 1 is given by the following expression, wherein s is the Laplace operator.

[00001]$H\left(s\right)=\frac{V_{\mathrm{out}}\left(s\right)}{V_{\mathrm{in}}\left(s\right)}=\frac{1}{{\mathrm{LCs}}^2+\mathrm{RCs}+1}$

[0025] Knowing the morphology of the transfer function of a system and performing a frequency sweep to obtain the Bode plot, it is easy to obtain the asymptotic response to obtain the values of R, L and C, which allows having a complete characterization of the system evaluated.

[0026] In an ground loop, wherein there are different interconnected elements, it is important to determine the purely resistive part (R) and separate it from the reactive part (L and C) since most normative prescriptions refer to this parameter R, and not to the impedance Z that contemplates the contribution of the 3 components (Z=f (R, L, C)) and that depends on the frequency used in the method. To obtain this RLC equivalent of the loop to be monitored, the procedure comprises injecting a set of frequencies, from a few hertz to a few hundred kilohertz, that is, a frequency sweep.

[0027] Performing a frequency sweep implies the injection of several frequencies and subsequently the reading of those same frequencies, for which it is necessary to interpose filters that eliminate all those frequencies that do not constitute useful information and, given that there is a set of frequencies of interest, the set of filters shall be tuned to each of these frequencies of interest, which implies a number of filters tuned as high as the number of frequencies to be included in the sweep. Another possible solution is to use a configurable or programmable filter, which is also complex.

[0028] Advantageously, to simplify the filtering process, the procedure to which the invention refers comprises the use of the amplitude modulation (AM) technique as shown in FIG. 2, which involves a carrier wave S.sub.carrier (1), usually, of high frequency fi in the frequency range and a modulating wave S.sub.signal (2), of lower frequency fm. Amplitude modulation results in the modulated signal S.sub.modulated (3) wherein the frequency fi of the carrier wave S.sub.carrier (1) in the range F and the fixed frequency fm of the modulator wave S.sub.signal (2) are reflected.

[0029] Thus, in the measurement method according to the present invention the modulator S.sub.signal (2) is introduced at a fixed frequency fm and the carrier S.sub.carrier (1) at a frequency fi will frequently vary in the frequency range of the sweep in frequencies fiF (from few hertz, up to hundreds of kilohertz).

[0030] A parameter to take into account in this process is the modulation index (m), which is defined as the ratio of amplitudes between the carrier signal (S.sub.carrier (1)) and the modulating signal (S.sub.signal (2)):

[00002]$m=\frac{S_{\mathrm{signal}}\left(1\right)}{S_{\mathrm{carrier}}\left(2\right)}$

[0031] The modulation index m is an indicator of the morphology of the modulated signal S.sub.modulated as can be seen in FIG. 3, wherein the carrier signal S.sub.carrier (1) and S.sub.signal (2) are the same in the three modulated signals S.sub.signal (2) with different amplitude relationships and therefore with different modulation indices m=(0.3, 0.5 and 0.7), respectively.

[0032] The measurement procedure according to the present invention offers a set of values Z,Z(fi), based on the calculation of the modulation index m, at each of the frequencies fi that are part of the sweep of frequencies F and whose graphic representation corresponds to the Bode plot in amplitude of the loop impedance.

[0033] In addition, the procedure also offers a set of offset values ,(fi) and whose graphical representation corresponds to the Bode plot in phase of the loop impedance.

[0034] Obtaining the Bode plot will be based on putting in a table (in the internal memory of the microcontroller) the values of the impedance and the offset measured at each of the frequencies of the sweep. Subsequently, the graphical representation of each impedance and offset value (Y axis) for each frequency (X axis) will result in the Bode plot in magnitude and phase.

[0035] The main advantage of the method according to the present invention is its immunity against noise, so that the filtering in the reading process can be made fixed and highly selective with very simple and known procedures, without having to resort to multiple filters. or configurable filters, this means greater reliability and less processing time.

[0036] Another advantage is the possibility of expanding the frequency range, since the use of a filtering tuned to the fixed frequency of the modulator is possible, it would not be necessary to add or modify the part for filtering and demodulating the signal for this purpose.

[0037] FIG. 4 shows the measurement equipment (400) according to the present invention. In particular, the measurement equipment 400 is used in a loop as part of a grounding system. The measuring equipment (400) comprises a generator (12) of the modulating signal S.sub.signal (2) at the fixed frequency fm. A microcontroller (13) comprising a carrier signal S.sub.carrier (1) generator with various frequencies within a range F used in a frequency sweep to characterize the impedance of the grounding system.

[0038] In addition, the measuring equipment (400) comprises an amplitude modulator (11) to obtain a amplitude modulated signal S.sub.modulated (3) at the frequency fi based on S.sub.signal (2) and S.sub.carrier (1), optionally a power amplifier (10), and a current inductor (9) configured to induce an electromotive force (emf) in the loop conductor of the grounding system.

[0039] In addition, the measuring equipment (400) comprises a current sensor (15) for measuring the electromotive force induced in said loop identified as S.sub.out and which is a composition of S.sub.modulated (3) combined with noise and after having suffered alterations typical of the impedance of the loop, through which it propagates, a demodulator (16) in amplitude, a filter (17) tuned to the frequency fm of the modulating signal S.sub.signal, means (20) for obtaining the impedance Z (fi), in particular means (18) for measuring the offset (fi) of the demodulated signal S.sub.demodulated, preferably a phase locked loop PLL and for each of the frequencies of the sweep of frequencies F, which allows complementing the Bode plot obtained with the measurement of the modulation index m. Furthermore, the measuring equipment (400) comprises means (19) for calculating the modulation index m:

[00003]$m=\frac{S_{\mathrm{demodulated}}}{S_{\mathrm{carrier}}\left(1\right)}$

[0040] Advantageously, the measuring equipment (400) can perform a frequency sweep by injecting several frequencies corresponding to the sweep of frequencies F through amplitude modulated signals and subsequently, after demodulation, reading the demodulated signal at the frequency fm of the modulating signal S.sub.signal (2), for which it is sufficient to include a single filter (17) tuned to the frequency fm of the modulating signal S.sub.signal (2)