SNR improvement by selective modulation

Abstract

The accuracy of certain sensors is improved by improving their signal to noise ratio (SNR) in the presence of an interfering noise. Sensors were discovered which have a SNR which substantially changes when an operating parameter is selectively modulated to different magnitudes. In the simplest form, the sensor is operated where it is both stable and close to its best SNR. This is usually faster and less costly, but the noise is never completely eliminated. This invention has first been applied to Swain Meter® type clamp-on DC ammeters. Some results are good—the benefit in SNR is between 2 and 4. ® Swain Meter is a registered Trademark of the William H. Swain Co.

Description

This is a Divisional Application on copending application Ser. No. 08,579,395 filed 27 Dec. 1995. This Divisional Application is filed under 37 CFR 1.53(b)(1). No new matter is introduced.

REFERENCES

1) Copending application Ser. No. 08,579,395 of William H. Swain, filed 27 Dec. 1995. This contains a description for the herein claims.

2) “Special” Status was granted for my 1995 Application on 26 Nov. 2002.

3) U.S. Pat. No. 3,768,011 granted to William H. Swain

4) U.S. Pat. No. 6,278,952 of William H. Swain on the Rgage.

5) U.S. Pat. No. 6,323,635 of William H. Swain on the MER2.

FEDERAL SPONSORSHIP

None

MICROFICHE

Not applicable

BACKGROUND OF THE INVENTION

This invention relates to sensors and/or implements for measurement or control.

The object of the invention is to improve accuracy by reducing error in the sensors output when in the presence of an interfering noise source. An example is a magnet near the sensor.

This Divisional Application is for method and means to get SNR improvement by selective modulation. In connection with my 1995 Application I have called this the “Better SNR” species. This work has primarily been directed to improving the accuracy of clamp-on direct current ammeters by reducing the zero offset error due to an interfering magnetic field; especially a non-uniform field caused by a nearby magnet.

The Examiners action of 18 Nov. 2003 on my referenced 1995 Application, in effect, restricted the claims to the “Better SNR Species” or the “Combiner Species”. On page 4, line 13 to page 5, line 2 he wrote:

• • “With the election of any one of the above inventions further election of species is required as follows:
  • This application contains claims directed to the following patentably distinct species of the claimed invention:
  • 1. The simpler form species where SNR is substantially improved by operating at a more favorable operating parameter, wherein noise is not cancelled as set forth at the bottom of page 1 of Applicant's specification and identified by Applicant on page 3 of his Appeal Brief as the “Better SNR species”.
  • 2. The “Combiner species” as illustrated in FIGS. 9, 11, and 12 and those portions of the specification respectively related thereto and by the description of combining the outputs of two separate sensors as described on page 1 of the specification, the combiner species again being identified by Applicant on page 9 of his Appeal Brief as the “combiner species”.”

I elected the “Combiner Species”. This Divisional Application is for the simpler “Better SNR Species”. The invention is here called SNR Improvement by Selective Modulation.

This invention solves the problems of complexity and need for adjustment to suit the magnetic environment encountered with the “Combiner Species”. I presented it on page 1 of my 1995 Application.

“In a simpler form, SNR is substantially improved by operating at a more favorable operating parameter magnitude. Noise is not canceled, but this form can be faster and cost less”.

I also presented this SNR improvement by Selective Modulation invention in the first paragraph of the Abstract of my 1995 Application. Page 52 includes:

“In the simplest form, the sensor is operated where it is both stable and close to its best SNR. This is usually faster and less costly, but the noise is never completely eliminated.”

Original claims 12 and 13 of my 1995 Application on pages 49-51 are for this invention.

This Application is based entirely on my 1995 application Ser. No. 08,579,395. It contains no new matter. Basis for the illustrations and examples will be found in the 1995 description, drawings and claims. Original 1995 claims 12 and 13 kappear herein as claims 1 and 2.

SUMMARY OF THE INVENTION

This SNR improvement invention is inherently simpler, faster, and more adaptable to a wide range of magnetic interference (noise) environments.

The “Discovery”, illustrated in 1995 FIGS. 4, and 5 was that some Swain Meters and other sensors changed their Signal to Noise Ratio (SNR) a lot when the magnitude of an operating parameter was changed. This is not a result of filters, data manipulation, etc. It is an intrinsic change. For example, I can do math faster with fewer errors after a good nights sleep.

The method is given at the bottom of 1995 page 1:

“ . . . SNR is substantially improved by operating at a more favorable operating parameter magnitude . . . ”

This can be done by stopping the counter 24 in 1995 “Combiner Species” FIG. 9 and operating full time in the high SNR state {circle over (B)}, where the switches are in the {circle over (4)} position, with operating parameter I_sm at 0.4 Amp. Definitions are given on 1995 pages 32-34.

Herein FIG. 6 illustrates high SNR state {circle over (B)}. Herein FIG. 7 shows an implementation. It is 1995 FIG. 9 with the unneeded state switching parts removed.

My original 1995 claim 12, states much the same. It is herein claim 1.

DESCRIPTION OF THE DRAWINGS

There are many 1995 antecedents and considerable 1995 basis for these herein drawings. These include:

Herein drawing FIG. 1 is the same as 1995 FIG. 1, but with the Hall devices (5) removed. Hall devices are not considered herein.

Herein FIGS. 2, 3, 4, and 5 are the same as 1995 FIGS. 2, 3, 4, and 5.

Herein FIG. 6 illustrates the above descriptions on 1995 page 1 and in 1995 Abstract.

Herein FIG. 7 is an implementation for my 1995 original claim 12. It is 1995 FIG. 9 with unneeded switching removed.

In the drawings:

FIG. 1 is a functional diagram of a sensor with a split magnetic core SQ surrounding a conductor carrying a current I to be measured. The core of a Swain Meter will have a coupling sense winding N_s.

FIG. 2 illustrates interference from the uniform magnetic field H_u due to a very remote and large field such as that of the earth, H_e.

FIG. 3 illustrates interference from the non-uniform magnetic field H_n due to a magnet near the sensor.

FIG. 4 is a graph illustrating the essential characteristic discovered in a type of clamp used in some Swain Meters. As the operating parameter I_sm increases, the signal gain increases only slightly, but the normalized output zero offset due to noise, here called Ó, first increases and then decreases to half and less.

FIG. 5 is a graph illustrating the essential characteristic in terms of signal to noise ratio SNR for 5″ diameter aperture clip #88.

FIG. 6 illustrates the method of this invention. It is a partial copy of FIG. 5 with added annotation showing the preferred setting of the Operating Parameter, and the SNR result, as indicated on page 1 of my 1995 Application. It includes:

“In a simpler form, SNR is substantially improved by operating at a more favorable operating parameter magnitude. Noise is not canceled, but this form can be faster and cost less.”

This is also the basis for FIG. 6 which shows that operation is continuous at the operating parameter magnitude of 0.4 Amp. Then the SNR is 29.

FIG. 7 illustrates the preferred way of implementing this method. It is a copy of my 1995 FIG. 9 with parts deleted which are not needed for this simpler form. The switch 18 is replaced by a wire which causes the inverter to operate continuously at operating parameter magnitude 4, i.e., 0.4 Amp. There is no operation at operating parameter magnitude 2, i.e., 0.2 Amp.

FIGS. 6 and 7 also illustrate the method taught in paragraph 1 of the Abstract of my 1995 Application. Page 52 includes:

“In the simplest form, the sensor is operated where it is both stable and close to its best SNR. This is usually faster and less costly, but the noise is never completely eliminated.”

FIG. 8 is a block diagram type representation of a sensor of this invention. It has a signal input I, an unavoidable interfering noise input N and an output V. This comes from 1995 FIG. 13 on page 66. FIG. 13 is discussed on 1995 page 8, 14, and eq. a) through eq. j) to 1995 page 22.

The operating parameter port Q is not an input. Instead, it acts more like a modulator access, which primarily governs the sensitivity of the noise input N, but only slightly affects the sensitivity of the signal input I.

DESCRIPTION OF THE INVENTION

SNR Improvement by Selective Modulation

General

This invention can be applied to improve the accuracy of sensors for measurement and control. It has been applied to reduce the zero offset error of clamp-on DC ammeters of the Swain Meter® type.
® Swain Meter is a Registered Trademark of the William H. Swain Co.

Purpose

Interference type noise causes an error in the output of some sensors. The purpose of the present invention is to improve the accuracy by improving the signal to noise ratio (SNR) of sensors and associated implements for measurement or control. A sensor and/or implement may also be called a transducer or signal translator. A particular purpose is to improve the accuracy of sensors for clamp-on or non-contact DC ammeters of the Swain Meter® type, by reducing error due to zero offset caused by interference from non-uniform magnetic fields due to local magnets, and also by uniform fields due to more remote magnets such as the earth.

Method and Means

It was discovered that certain sensors have the Essential Characteristic, i.e., they have a sensitivity to an interfering noise which changes a great deal more than the sensitivity to a signal input when the magnitude of an operating parameter is changed. We call this selective modulation.

A method for improving accuracy is to reduce sensitivity to noise while keeping good sensitivity to signal, i.e., increasing Signal to Noise Ratio (SNR).

Having got a sensor having the Essential Characteristic, adjust the magnitude of the Operating Parameter to get the SNR needed. Noise is not canceled, but this “Better SNR” method can be faster and cost less than the “Combiner” method.

Outline of Contents

The remainder of this specification includes the following sections:

Introduction begins with the Swain Meter® Patent of William H. Swain, U.S. Pat. No. 3,768,011. FIGS. 1, 2, and 3 show a basic clamp for a non-contact DC ammeter of the Swain type with coil N_s (2), and they show the effects of interfering magnetic noise H_n (8) and H_u (9). Hall Devices (5) are not considered here.

The herein Introduction is the same as that in my 1995 pages 8-11 except that reference to Hall devices is deleted.

Discovery that many Swain sensors had a zero offset error Z heavily dependent on the magnitude of operating parameter I_sm, but stable gain g for the input signal I, is shown in FIG. 4. Normalized output error Ó and noise sensitivity Ψ are introduced, along with signal to noise ratio SNR. This is plotted in FIG. 5. Both FIG. 4 and FIG. 5 illustrate the Essential Characteristic needed in a sensor for successful SNR improvement by selective modulation.

The herein Discovery is the same as that in my 1995 pages 11-13 except that:

• • Under Essential Characteristic the error ““Magnitude” Field H_n (8)” is corrected to ““Magnetic” Field H_n (8) . . . ”; and
  • Reference to Hall device is deleted; and
  • “Corrected” has been changed to “reduced.”
    General Method and Mathematical Relationship

This section provides a theory for use in evaluating the Essential Characteristic of a sensor and its suitability for improving SNR.

This herein section is the same as that in the corresponding section in 1995 pages 13-16 except that:

• • “Canceling” is changed to “reducing”.
  • Eq. i) and eq. j) are omitted because they relate primarily to error canceling in the Combiner Species.
  • Reference to 1995 FIG. 6 for a hypothetical sensor is deleted. Herein I discuss FIG. 4 which shows the result of actual measurements on 5″ clip #88.
  • Likewise FIG. 8 is replaced by FIG. 5.
  • Reference to Hall devices is deleted.
    Non-Contact Ammeter Implementation for Swain Meter

This section shows a method and a practical design embodying the invention as shown in FIGS. 6 and 7. This worked using clip #88 (characterized in FIG. 4 and FIG. 5). Details are discussed.

This section begins as 1995 page 31 but then changes to discuss SNR improvement instead of eror cancelation.

Antecedent and basis for this section is 1995 statements on:

• • Page 1,
  • Abstract,
  • Original claim 12 which is herein claim 1, and
  • Original claim 13 which is herein claim 2.

The Construction and Results section gives some detail on the construction of 5″ clip #88 and its operation in FIG. 7. A benefit of 2 to 4 to one was measured.

Antecedent and basis for construction is in:

• • 1995 page 12 which describes 5″ clip #88, and
  • 1995 page 36 which describes 5″ clip #88, C16, and R17.

Antecedent and basis for results is 1995 FIGS. 4 and 5 which show the measured benefit of this SNR Improvement invention.

Conclusion is that the method can be widely applied to considerably improve accuracy.

Antecedent and basis is:

• • on the bottom of my 1995 page 1, and
  • the first paragraph of my 1995 Abstract, page 52.
    Introduction

Swain Meter type clamp-on DC ammeters have gained wide acceptance because they are generally sensitive and accurate and available in a variety of forms for measuring 10 mA to 500 Amp. direct current with sensors from ¼″ to 5 feet in diameter. A clamp-on type sensor is shown in FIG. 1 herein.

A sensor plus implement combination can be constructed using the concepts of U.S. Pat. No. 3,768,011 to serve as a non-contact ammeter. In FIG. 2 therein, resistor R_s can be made quite small—100 ohms or less, and capacitor C quite large—1000 micro farad or more.* The output voltage V_c across capacitor C and resistor R_s will henceforth be written simply as V, and in some places, assumes a more general meaning. More gain is assumed to be available if needed.**
* In some designs we have replaced R_s and C with the low impedance input of a high current capability operational amplifier. This can be a lot faster, and it also converts the average current I_s in the sense winding N_s to an output voltage.

** Here we assume that where gain is needed, it is available. The voltage across resistor R_s in FIG. 2 of the Patent may be only a few millivolts. The means for boosting this to a volt, essentially free of added error, are widely known.

The output voltage V is sensitive to an input signal current I, and also to an interfering noise N which causes an output zero offset Z. FIG. 8 represents a sensor with functional symbols. An equation can be written to relate these:
V=gI+Z

Accuracy is dependent on g—this may be 1.000 V per Amp on a particular range*—and on Z. The values of g and Z should be constant over all values of input signal I, and also over all values of noise interference N.
* In some designs we have replaced R_s and C with the low impedance input of a high current capability operational amplifier. This can be a lot faster, and it also converts the average current I_s in the sense winding N_s to an output voltage.

We have got 1% type control over the gain g, and also good control over zero offset Z due to the magnetic field of the earth H_e. On a ¼″ clip this can be as low as 0±1 ma. peak equivalent input current Ó in response to a full vertical north-south spin in the earth's field H_e. We call the earth field uniform, H_u as shown in FIG. 2 herein.

The most difficult type of interference noise N to control has been that due to a strong non-uniform magnetic field H_n such as that shown in FIG. 3. A stray magnet, perhaps in a weld in a pipe, a sector of magnetized sheet metal in an automobile near the battery cable, or a magnetized fastener near the sensor can produce a considerable zero offset error Z. When the clamp-on sensor is moved from nearby to really around the conductor carrying the current to be measured, the intensity and direction of the effective non-uniform field H_n changes, and this changes the zero offset Z, and so reduces the accuracy of output V.

The method and means shown herein have improved accuracy by reducing noise, not only from H_n, but also, to a lesser degree, from H_u.

FIG. 1 represents a clamp-on type of non-contact sensor having a low magnetic reluctance core 1 which is split at the lips 61. These have a large cross section area to provide low magnetic reluctance all around the magnetic core path.* If it is for a Swain Meter, it will have a coupling sense winding 2. It may be called a signal translator or transducer because the input current 7 sets up an input field 3 which influences, i.e., upsets the magnetic state of the core 1 and thus causes an average current 4 to flow in coupling sense winding 2 when connected to a suitable inverter. An output voltage is available when this current 4 flows through a resistor 17 called R_s.
* This is not essential. We have made, for special applications, non-contact ammeters wherein the core is an open ended ␣ shape, or even a flat bar. The coupling between the input current and the core is not as good as when there is a low reluctance path all around the input current, but signal input current positioned near the core still influences the core, i.e., alters the magnetic state of the core enough so that some measurements are practical. It is expected that the method of this invention will also reduce error in these.

Stray magnetic fields such as those shown in FIG. 2 (H_u) and FIG. 3 (H_n) produce a zero offset error because all non-contact DC Ammeters measure the current 7 by measuring the magnetic field 3 or flux density 6 set up in the magnetic core material of the sensor by the input current 7. Some H_u or H_n gets into the core in FIG. 1 and produces a zero offset error Z.

The zero offset error Z tends to be less if the core is continuous, with no split. When the core is split at the lips 61, it is preferred that these have low magnetic reluctance, often by virtue of large surface area.

The input current 7 sets up an input field 3. It is largely uniform and constant and circular about the current carrying conductor 7. In FIG. 1, input field 3 and input flux path 6 go evenly all around the core of the clamp.

This is not true of a non-uniform field (H_n) 8 such as that due to a magnet 10 near the clamp, as shown in FIG. 3. This is also not true of a uniform field H_u 9, which may be produced by the Earth's magnetic field (H_e). This is shown in FIG. 2.

It may be that selective modulation of the signal and noise is feasible because the signal I_i acts circumferentially, but the noise H_n and H_u act partially in the core and partially outside.

In Swain Meters the zero offset (Z) produced by the Earth (H_e) or another uniform field (H_u) has been reasonably well controlled and reduced to a magnitude low enough to measure direct current to within ±1 ma. when using a ¾″ clip. U.S. Pat. No. 3,768,011 shows the concept of peak magnetizing current (I_sm) and uniform coupling sense winding (N_s) used to get such zero stability when the field is uniform (FIG. 2), and the core is small. But these techniques still allow a substantial zero offset (Z) when the core is large (over 4″), or when the field is strong and non-uniform (FIG. 3). We especially want to correct this error. We also want to further reduce the error due to H_u.

Discovery

The inventor discovered that the output V of many Swain Meter clamps was a lot less sensitive (½ to ⅓ in some sensors) to a change in the intensity of a non-uniform magnetic field H_n when the magnitude of an operating parameter I_sm was doubled or tripled. And the sensitivity (gain) to a change in signal input current I stayed constant to within a few percent.

Essential Characteristic

FIG. 4 shows the approximate sensitivities for a five inch diameter aperture clip #88. This is an illustration of a sensor having the essential characteristic:

• • Firstly, the signal gain g (13) sensitivity to signal input I (7) is constant within a few percent as an operating parameter I_sm (12) changes from 0.18 A to 0.5 Amp peak; and
  • Secondly, the zero offset (11) sensitivity to a unit change in intensity of a non-linear magnetic field H_n (8) is reduced to well under half over the same range of I_sm (12).

The equation relating these quantities is V=gI+Z.

Zero offset is given in terms of Ó=Z/g, where the input current I equivalent to the zero offset Z is obtained by dividing the zero offset Z by the signal gain g. The result Ó (14) is plotted in FIG. 4.

The data in FIG. 4 shows the approximate behavior of 5″ dia. aperture clip #88. It uses concepts shown in U.S. Pat. No. 3,768,011, especially in connection with FIG. 2 and FIG. 4 therein. Clip #88 is outlined in FIG. 1 herein. The primary parts are:

• • A core SQ (1) having five layers of 0.725″ wide-4D low reluctance steel from Magnetics of Butler, Pa.,
  • The core is mounted on a support and arranged so that the magnetic reluctance around the full magnetic path is minimized. Care should be used to avoid forcing or bending the steel because stresses and strain may produce a poorer core.
  • A uniform coupling sense winding N_s (2) of about 1000 turns of #22 magnet wire. A symmetrical and balanced form is preferred. The winding resistance should be less than 5 ohms.
  • Half inch lips (61) which are constructed to mate well so that the magnetic reluctance all around the core is minimized.

The essential characteristic for successful SNR improvement by selective modulation shown in FIG. 4 for clip #88 plots—in effect—noise sensitivity Ψ times gain g against the operating parameter I_sm. This is from $\Psi \equiv \frac{\stackrel{\prime }{O}}{N},$
where Ó is still the equivalent input current of a zero offset Z and N is a unit of noise, in this case, magnetic field H_n.

Signal to noise ratio SNR is the reciprocal of noise sensitivity Ψ, i.e., $\mathrm{SNR}=\frac{1}{\Psi }$

SNR is, in a way, easier to understand, and it can help in writing claims, partly because it is basic. FIG. 5 is an SNR plot of the same #88 clip over the same operating parameter I_sm range of magnitudes as in FIG. 4. It shows SNR, which is the signal sensitivity (gain g) divided by the noise sensitivity (gΨ) changing from a minimum of about 13 at about 0.07 Amp I_sm to over 50 as I_sm approaches 0.5 Amp peak.

The essential characteristic necessary for good SNR improvement by selective modulation can be measured and presented in several ways, but that shown in FIG. 5—the plot of SNR vs. Operating Parameter is now considered the most basic. A good characteristic such as that in FIG. 5 has a substantial change in SNR—two to one or more—over a practical range of the condition of the operating parameter. It is not necessary that the gain g be nearly constant. Useful improvement can be had when the gain g changes 40% as the operating parameter Q is driven from one condition to another.

General Method and Mathematical Relationship

Since it appears likely that someone will find sensors and/or implements for measurement or control of diverse physical quantities such as position or chemical concentration we need a general method and/or procedure for determining if the sensor has the essential characteristic, and if so, how to use selective modulation to improve accuracy by reducing error. Statements of the general method follow.

A general method for correcting error in the output of a sensor caused by interference from a noise is presented with reference to FIGS. 4, 5, and 6. These represent 5″ sensor #88. They are presented to illustrate the analysis.

A sensor is represented as having an output V which changes in response to a signal input I, and the output also has an error Z due to interference from a noise N. FIG. 8 presents this with functional symbols. Restated:
V≡gI+Z, where   Eq. a)
the gain g of the sensor is $\begin{array}{cc}g\equiv \frac{\delta V}{\delta I}.& \mathrm{Eq}.b)\end{array}$

This is the sensitivity or gain of the sensor's output V to a signal input I.

A partial derivative symbol δ is used to indicate that the gain g is the change in sensor output V divided by the change in sensor signal input I.

In Eq. a), if the input I is zero, the output V equals the error Z due to noise. Or if there is an input but it is held constant, then the change in output V in the presence of an interfering noise N is the same as the change in error Z due to this same noise N. Therefore, the gain g times the sensitivity of the sensor's output V to a noise N, is: $\begin{array}{cc}\frac{\delta V}{\delta N}=\frac{\delta Z}{\delta N}& \mathrm{Eq}.c)\end{array}$

The importance of an error Z in the output is better shown in terms of an equivalent noise input Ó which will have the same effect on the output V as an input signal I. Since both Ó and I are to be thought of as inputs, the signal input sensitivity, i.e., the gain g applies to both. Therefore, we define Ó by: $\stackrel{\prime }{O}\equiv \frac{Z}{g};\mathrm{so}$ $\begin{array}{cc}g=\frac{\delta Z}{\delta \stackrel{\prime }{O}}.\mathrm{Since}\mathrm{Eq}.b\text{)}\mathrm{gives}g=\frac{\delta V}{\delta I},\mathrm{and}\mathrm{Eq}.c\text{)}\mathrm{gives}\delta V=\delta Z,\mathrm{then}\frac{\delta Z}{\delta \stackrel{\prime }{O}}=\frac{\delta V}{\delta I},\frac{\delta Z}{\delta \stackrel{\prime }{O}}=\frac{\delta Z}{\delta I},\mathrm{so}& \mathrm{Eq}.d)\\ \delta \stackrel{\prime }{O}=\delta I.& \mathrm{Eq}.e)\end{array}$

Thus Ó has the effect of an input, i.e., Ó is the noise equivalent input of error Z, which is the result of interfering noise N.

The ratio of the noise equivalent input Ó to the interfering noise N which caused it is the noise sensitivity Ψ. This is defined: $\begin{array}{cc}\Psi \equiv \frac{\delta \stackrel{\prime }{O}}{\delta N}.& \mathrm{Eq}.f)\end{array}$

We get a little more direct meaning of Ψ by noting that: $g=\frac{\delta Z}{\delta \stackrel{\prime }{O}},\mathrm{so}\delta \stackrel{\prime }{O}=\frac{\delta Z}{g}.\mathrm{Also}\delta Z=\delta V,\mathrm{so}$ $\begin{array}{cc}\Psi =\frac{\delta V/\delta N}{g}.& \mathrm{Eq}.g)\end{array}$

Thus we see that the sensor noise sensitivity Ψ is the change in sensor output V divided by the change in the interfering noise N, all divided by the sensor gain g whereby the change in sensor input I changes the sensor output V.

Put another way, Ψ is the sensitivity of the sensor's output V to an interfering noise N, all divided by the sensitivity of the sensor's output V to signal input I, i.e., Ψ is the inverse of SNR. Restated: $\Psi =\frac{\delta V/\delta N}{\delta V/\delta I}$

Since gain g is defined in Eq. b) as $\frac{\delta V}{\delta I},$
the above is just another way of writing Eq. g).

FIG. 4 is a graph showing the essential characteristic of sensor #88, presented here to help illustrate the method. The signal to noise ratio (SNR) changes a lot when an operating parameter Q changes its condition.* By this I mean that the signal gain g (13) changes only a few percent when the operating parameter I_sm (12) changes enough to cause the noise sensitivity Ψ (14) to change by a factor of two or more, or vice versa.
* Operating parameter Q can be any of a variety of physical quantities able to change condition. It can be a chemical mixture proportion, electric current, fluid pressure, etc. The change in the condition of Q can be a magnitude, as in peak current I_sm changing condition from 0.2 to 0.4 Amp. Or it can be a change in power supply voltage or source impedance, a change in frequency used in a modulator, a change in direction of an applied force, etc.

By SNR I mean the sensitivity of the sensor's output V to the signal I divided by that to noise interference Ψ. $\mathrm{In}\mathrm{Eq}.b\text{)}\frac{\delta V}{\delta I}=g,\mathrm{and}$ $\mathrm{In}\mathrm{Eq}.g\text{)}\frac{\delta V}{\delta N}=g\Psi ,\mathrm{so}$ $\mathrm{SNR}=\frac{g}{g\Psi },\mathrm{or}$ $\begin{array}{cc}\mathrm{SNR}=\frac{1}{\Psi }& \mathrm{Eq}.1)\end{array}$

FIG. 5 is a SNR graph of the essential characteristic of this 5 inch diameter sensor #88.

Operating parameter Q can be thought of as an input to a modulator, or as the modulator itself. Functionally, a change in Q causes a change in the SNR of the sensor.

Non-Contact Ammeter Implementation for Swain Meter

This section begins as 1995 page 31.

To build a non-contact DC ammeter according to this SNR improvement invention you need at least two things:

• • 1) A clip or clamp sensor which has the essential characteristic of the discovery shown in FIG. 4 between points (A) & (B); namely, the signal gain g remains relatively constant while the response Ó=Z/g to a field H_n changes substantially* and repeatably (it can be calibrated) with some operating parameter (a bias, local saturation, mechanical modulation, or as in FIG. 4, the peak magnetization current I_sm). It is not required that the teachings of U.S. Pat. No. 3,768,011 be used.
  • 2) Support means, which can be electronic +/or mechanical, which implement the method, i.e., the mathematical relation, to produce a sensor output (V) which is a linear function of the input current I to be measured. The sensor performs the correction by making use of the essential characteristic (FIG. 4) or equivalent to reduce the noise (error due to a magnet). This can be implemented by herein FIG. 7. This is 1995 FIG. 9 with parts removed so that operation is always in state {circle over (B)} where all switches are set in position {circle over (4)}, and where the Operating Parameter is I_sm set at 0.4 Amp.

The method is illustrated by FIG. 6. Data describing the noise sensitivity of the sensor are plotted on FIGS. 4 and 5. FIG. 6 is a simplified version of FIG. 5 which shows the operating point selected for 5″ sensor #88. When the Operating Parameter I_sm is set at 0.4 Amp the SNR is 29. This is a useful improvement over the SNR of 13 typically resulting from use of our earlier methods.

FIG. 7 shows the interconnections of parts used to build an SNR Improvement by Selective Modulation implement previously called a “Better SNR” sensor and implement. This starts where the cover drawing (FIG. 2) of U.S. Pat. No. 3,768,011 filed in 1970 left off.

FIG. 7 illustrates interconnections for my original 1995 claim 12 and also claim 13 which is herein claim 2. This apparatus claim on 1995 page 51 includes:

“ . . . said operating parameter I_sm set to a substantially greater magnitude than the magnitude corresponding to the minimum signal to noise ratio, here called SNR, so that thereby the said SNR is considerably increased over said minimum, so that said non-contact ammeter has considerably greater accuracy in the presence of said interfering magnetic field noise N.”

The effect of 1995 claim 13 is to set all 1995 FIG. 9 switches to the {circle over (4)} position. This is shown in the herein SNR improvement drawing FIG. 7.

For example:

• • The above requirement of 1995 claim 13 on page 51:
    • “ . . . said operating parameter I_sm set to a substantially greater magnitude . . . ”
      is met when I_sm means (12) is set by switch (18) to 0.4 Amp. These interconnections are defined on the bottom of 1995 page 32 and the top of page 33. Page 33 continues:
  • “ . . . Polarity switch 19 goes to the {circle over (4)} position, . . . and
  • “ . . . gain switch 20 is in the high position . . . ”
    1995 page 33 continues at paragraph 2:
  • “ . . . During the {circle over (B)} state, the voltage V_c across resistor 17 and capacitor 16 are applied to polarity switch 19 through low pass filter 21 which attenuates potentials, both common and differential mode, above f_o/3 . . . ”

Herein FIG. 7 includes no switches because original 1995 claim 13 requires none. It states:

• • “A Swain Meter type non-contact direct current ammeter with improved accuracy for measurement or control, which comprises:
  • a core, here called SQ, of low magnetic reluctance material,
  • a coupling sense winding, here called N_s, on said core SQ,
  • an inverter with power supply, here called X, with output terminals with a current i_s flowing which has an average value I_s, and also a peak value I_sm which is an operating parameter, all of said currents flowing in either direction in said output terminals,
  • a low input impedance means converting said average current I_s to an average output voltage V,
  • a current carrying conductor carrying a signal input current I, which is to be measured or controlled, positioned so that said current I influences said core SQ, and
  • said core SQ is within the effective range of an interfering magnetic field noise, here called N, and
  • said coupling sense winding N_s series connected with said output terminals of said inverter X and said low input impedance means converting,
  • said operating parameter I_sm set to a substantially greater magnitude than the magnitude corresponding to the minimum signal to noise ratio, here called SNR, so that thereby the said SNR is considerably increased over said minimum, so that said non-contact ammeter has considerably greater accuracy in the presence of said interfering magnetic field noise N.”

Herein FIGS. 7 and 1 meet this requirement. For example:

• • “ . . . a core, SQ . . . ” is item 1 in FIG. 7 and herein FIG. 1.
  • “ . . . N_s . . . ” is item 2 in FIG. 7 and herein FIG. 1.
  • “ . . . Inverter . . . X . . . ” is item 15 in FIG. 7.
  • “ . . . I_s . . . ” is item 4 in FIG. 7 and herein FIG. 1.
  • “ . . . Signal . . . I . . . ” is item 7 in FIG. 7, and FIG. 1.
  • “ . . . Noise . . . N . . . ” is caused by field 8 and magnet 10 in FIG. 7 and in FIG. 1.
  • “ . . . Operating Parameter I_sm is set . . . ” is item (12) tied in position {circle over (4)} by wire to 18 in FIG. 7.
  • “ . . . Low impedance means . . . ” is items 16 and 17 in FIG. 7.
  • In herein FIG. 7 the filter 21 is defined on page 33, par. 2 above, with its connection.
  • “ . . . Ammeter . . . ” is supported with means in FIG. 7. The filter 21, the amplifier 26, and the output 28 condition the voltage V_c.

Herein FIG. 7 had no need of switch related parts 18, 23, 24, 25, 19, 20, and 22. These are discussed on 1995 pages 32-34.

The operating parameter I_sm is “ . . . set to a substantially greater magnitude . . . ” by the wire in herein FIG. 7 which continuously joins the I_sm 4 terminal to the connection for switch 18 to cause operation at 0.4 Amp continually.

This implements the method illustrated in FIG. 6. As claimed in 1995, “ . . . said operating parameter I_sm (is) set to a substantially greater magnitude . . . ”, i.e., to 0.4 Amp, not 0.2 Amp, so that “ . . . thereby the said SNR is considerably increased . . . ”, i.e., from 13 to 29.

FIG. 7 herein is my 1995 FIG. 9 with parts used for the “Combiner Species” removed because they are not needed for this SNR Improvement by Selective Modulation invention. FIG. 7 is preferred over a cut down version of 1995 FIG. 11 because FIG. 7 is simpler.

FIGS. 6 and 7 herein also serve to illustrate the method claimed in my 1995 claim 12. My 1995 page 50 includes:

• • “and adjusting said means, including said N_s and said I_sm, so that the change in said gain g is considerably less than the change in said noise sensitivity Ψ, as said noise sensitivity Ψ is reduced from a maximum to a value considerably less than said maximum, said reduced being accomplished by altering the value of said means, especially the number of turns on said winding N_s and the said peak inverter current I_sm, said altering being preferably in the direction of a greater value of the product of said N_s and said I_sm,
  • and operating said sensor with said product of said N_s and said I_sm set so that said noise sensitivity Ψ is considerably reduced below said maximum,
  • thereby constructing and operating said sensor with said reduced error in zero offset due to said noise N.”

FIG. 6 herein shows I_sm set to 0.4 Amp. FIG. 7 herein shows the inverter operated continuously at operating parameter level 4.*
* Operating Parameter level 4 in my 1995 Application corresponds to 0.4 Amp. This is apparent in 1995 Table I on page 24. It is also seen in the last sentence of 1995 page 32 and the first sentence of page 33.

In FIG. 7 a special inverter is connected in series with the winding on the core of the non-contact sensor. This core may be solid, or split to form a clamp or clip. Capacitor C shunted by resistor R_s are also in series. All are constructed so that the average current I_s flowing in the loop is proportional to the input current I_i. Then the average voltage V_c across C and R_s is also proportional to I_i. Voltage V_c is the input signal to the filter and amplifier combination (21) and (26).

In FIG. 7 if the capacitor C (16) is large, and also if resistor R_s (17) is large, the time required for V_c to reach a final value in one state can be excessive. This and other reasons led us to build a filter and an operational amplifier which remove the inverter frequency f_o components of v_c and provide gain. Then the output V_o can be 1 Volt per Ampere input current (7).

It is well known in the art how to build suitable filter (21) and amplifier (26) combinations. Then R_s (17) can be small, i.e., less than 100 ohms; and C (16) can be moderate, say 470 microfarad.

In FIG. 7 the special inverter 15 operating at frequency f_o is series connected with the sensor's coupling sense winding 2 and the parallel combination of capacitor 16 and resistor 17. Input current 7 influences the magnetic material in the core 1, and so also does the magnet 10. So the average current 4 in the loop produces a voltage V_c across capacitor 16 and resistor 17 which is proportional to the input current 7, and also proportional to the effect of noise magnet 10 and its non-uniform field 8. In this implementation, the means driving the operating parameter I_sm (12) at 0.4 Amp. is a fixed lead connecting to a resistor selected to operate I_sm at 0.4 Amp continuously. No switch is needed.

Construction and Results

Several preliminary forms of this invention have been built and tested with mixed results. The best so far uses the implementation shown in FIG. 7 and a 5″ diameter aperture clip #88, constructed using structures and processes outlined in U.S. Pat. No. 3,768,011 and in the same general form (see FIG. 1) as clips sold December 1995. The top part of 1995 page 36 shows that the steel core* 1 has 5 layers of 0.725″ wide, 4 mil thick type D steel tape from Magnetics in Butler, Pa. The clip's coupling sense coil 2 has about 1000 turns of #22 magnet wire with a resistance of 3 or 4 ohms. At point (A) on FIG. 4 the peak magnetization current 12 is about 0.2 A, and at point (B) it is about 0.4 A. Operation is continuous at point (B), where I_sm=0.4 A. Point (A) at 0.2 Amp is closer to that used in the prior art.
* A low reluctance ferrite or low reluctance steel laminations may be used for the core 1. So far we have gotten better results with the 4 mil steel tape.

FIG. 4 plots the equivalent input current of the zero offset Z due to a standard magnet as a function of I_sm, the peak current in the coupling sense winding N_s. This 0.2 to 0.4 Amp. peak current is flowing in N_s≅1000 turns on a 5″ diameter core, SQ. What really counts is the peak magnetic field intensity H_sm acting on the steel of the core. Since $H_{\mathrm{sm}}=\frac{N_s{I}_{\mathrm{sm}}}{1},$
where l is the mean flux path length, we can reduce I_sm if we increase N_s, or reduce l, etc.

1995 Page 36 also shows that capacitor 16 is 470 μF. Resistor 17 is 200 ohms. The filter 21 has a cutoff frequency of about 100 Hz.

The implementation, outlined in FIG. 7, runs on 12 volts with f_o roughly equal to 400 Hz.

When tested with a non-uniform magnetic field H_n from a nearby speaker magnet, the zero offset error was one ampere equivalent input under the previous conditions not using this invention. The noise or zero offset error in the SNR improved output was generally less than ±0.5 Amp. equivalent input current. This is a two to one benefit. The benefit is usually 2 to 4.

1995 FIGS. 4 and 5 show that 5 inch diameter aperture clip #88 had a benefit of at least two, but it could be run up to four to one.

The usual zero offset error rating for Swain Meter 5″ clips is less than ±40 mA equivalent input current due to the uniform (H_u) field of the earth (H_e).

In FIG. 7 the output V_o (28) of amplifier (26) was proportional to the input current (7). It was substantially free of components of the inverter (15) at frequency f_o. The zero offset sensitivity to magnetic interference noise (10) was reduced to less than half that with conditions prior to the SNR Improvement invention, so the output V_o had improved SNR by over a factor of two. This acts to improve accuracy of measurement of direct current because the zero offset error due to a non-uniform (H_n) field is reduced.

Conclusion

This SNR Improvement method can be widely used to improve the accuracy of sensors and implements for measuring and/or controlling physical quantities. Our experience to date is primarily with reducing zero offset error noise from interfering magnetic fields acting on non-contact DC ammeters.

Claims

1. A method and process for constructing and using a sensor with reduced error for measurement or control including means:

a core of low magnetic reluctance material, here called SQ,

a coupling sense winding on said core having a number of turns, here called Ns,

an inverter having an output current, here called is, and an average said output current here called Is, and also constructed such that said inverter has an operating parameter which is the peak value in either direction of said current, here called Ism,

a low input impedance means converting the said average value Is of said inverter current to an output voltage here called Vc,

and said method includes:

positioning said core so that it is influenced by a conductor carrying a signal current I to be measured,

said position being within the effective range of a magnetic field noise, here called N, causing at least part of an error in the form of a change in zero offset of said output voltage Vc, wherein the sensitivity of said Vc to said noise N is here called Ψ, and defined as the change in said Vc due to a unit change in said noise N divided by a gain g, i.e.,

Ψ ≡ δ ⁢   ⁢ V c / δ ⁢   ⁢ N g,

where

said g is defined as the change in said output Vc due to a unit change in said signal current I; i.e.,

g ≡ δ ⁢   ⁢ V c δ ⁢   ⁢ I,

and

said method also includes series connecting said Ns, said inverter, and said low input impedance means converting;

and adjusting said means, including said Ns and said Ism, so that the change in said gain g is considerably less than the change in said noise sensitivity Ψ, as said noise sensitivity Ψ is reduced from a maximum to a value considerably less than said maximum, said reduced being accomplished by altering the value of said means, especially the number of turns on said winding Ns and the said peak inverter current Ism, said altering being preferably in the direction of a greater value of the product of said Ns and said Ism,

and operating said sensor with said product of said Ns and said Ism set so that said noise sensitivity Ψ is considerably reduced below said maximum,

thereby constructing and operating said sensor with said reduced error in zero offset due to said noise N.

2. A Swain Meter type non-contact direct current ammeter with improved accuracy for measurement or control, which comprises:

a core, here called SQ, of low magnetic reluctance material,

a coupling sense winding, here called Ns, on said core SQ,

an inverter with power supply, here called X, with output terminals with a current is flowing which has an average value Is, and also a peak value Ism which is an operating parameter, all of said currents flowing in either direction in said output terminals,

a low input impedance means converting said average current Is to an average output voltage V,

a current carrying conductor carrying a signal input current I, which is to be measured or controlled, positioned so that said current I influences said core SQ, and

said core SQ is within the effective range of an interfering magnetic field noise, here called N, and

said coupling sense winding Ns series connected with said output terminals of said inverter X and said low input impedance means converting,

said operating parameter Ism set to a substantially greater magnitude than the magnitude corresponding to the minimum signal to noise ratio, here called SNR, so that thereby the said SNR is considerably increased over said minimum, so that said non-contact ammeter has considerably greater accuracy in the presence of said interfering magnetic field noise N.

3. I claim a method for making a more accurate implement for at least one of measurement or control including the steps:

Construct a port for desired input signal I, which of necessity makes a port for undesired error producing interference N,

construct a port for said implement's output Vc,

acquire an Essential Characteristic type sensor having an output V responsive to said desired input signal I, and also

responsive to said undesired error producing interference N, and further having an operating parameter of magnitude Q;

show that said Essential Characteristic type sensor has a useful said Essential Characteristic evidenced by

a signal to noise ratio SNR of said sensor observed to change a lot when the said magnitude Q of said operating parameter is modulated over a practical range;

provide said implement equipped to:

support said sensor so as to:

considerably reduce said undesired interference N relative to said desired signal I at said output Vc by

holding said magnitude Q in a higher said SNR state and coupling said sensor output V to said implement output Vc.

4. I claim a more accurate sensor with implement for at least one of measurement or control,

including said sensor having a strong Essential Characteristic, and also an output V responsive to a physical quantity input I, the gain g given by

g ≡ δ ⁢   ⁢ V δ ⁢   ⁢ I,

and

said output V also responsive to an undesired error producing interference N,

the sensitivity Ψ being

Ψ ≡ δ ⁢   ⁢ V δ ⁢   ⁢ N,

and

said sensor also having an operating parameter of magnitude Q which modulates said Ψ, and to a lesser extent said g;

said sensor having been shown by at least one of calibration, proven manufacturing process, or other demonstration to have said strong said Essential Characteristic, i.e., the said sensitivity Ψ changes a lot more than said gain g when said magnitude Q is driven over a practical range of values; and

also including:

an error reduction form of said implement, fitted to support said sensor, and

fitted to drive said magnitude Q and hold it at a constant value, which is predetermined to cause said sensor to operate with said interference sensitivity Ψ a lot less than was heretofore customary, while said gain g is still good,

thereby making said sensor with said implement substantially more accurate than comparable transducers for said physical quantity I in the presence of said interference N.

5. I claim a method for making a more accurate sensor with implement for at least one of measurement or control, made in steps:

obtain a said sensor having an output V responsive to a physical quantity input I, the gain g given by

g ≡ δ ⁢   ⁢ V δ ⁢   ⁢ I,

and

said output V is also responsive to an undesired error producing interference N, the sensitivity Ψ being

Ψ ≡ δ ⁢   ⁢ V δ ⁢   ⁢ N,

and

in addition, said sensor has an operating parameter of magnitude Q which modulates said Ψ, and to a lesser extent said gain g;

at least one of calibrate, or make by a proven process, or otherwise assure that said sensor has a strong Essential Characteristic evidenced by observing that said Sensitivity Ψ changes a lot more than said gain g when said magnitude Q is driven over a practical range of values; and:

provide an error reducing form of said implement, fitted to support said sensor, and

also fitted to drive said magnitude Q and hold it at a constant value, and by at least one of measurement or a proven process, set said magnitude Q at a value corresponding to a said sensitivity Ψ which is a lot less than heretofore while said gain g is still good, thus making said sensor with implement substantially more accurate than comparable transducers for said input I in the presence of said interference N.