Compensating apparatus for a non-contact current sensor installing variation in two wire power cable

A compensating apparatus for installing variation of a non-contact current sensor on a two-wire power cable includes a non-contact current sensor, a sensing element characteristic measuring unit and a non-contact current measurement module. The non-contact current sensor mounted top to the two-wire power cable further has a first current sensor, a second current sensor, and a third current sensor. The sensing element characteristic measuring unit is to construct a space characteristic measuring database for the non-contact current sensor respective to the two-wire power cable. The non-contact current measurement module is to pair the space characteristic measuring database so as to compute and further output a measured value of the current I in the two-wire power cable.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on, and claims priority from, Taiwan (International) Application Serial No. 102147533, filed on Dec. 20, 2013, the disclosure of which is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a current sensor, and more particularly to a compensating apparatus for installing variation of a non-contact current sensor on a two-wire power cable.

BACKGROUND

With rapid advance in automation industry, demands of high-reliability and high-performance upon the control instruments are significantly rising. Various sensors are widely used for the purpose of automatic and persistent monitoring. In particular, the current sensor plays one of crucial roles for the panel detection and control in both the industrial and the domestic applications.

Currently, the current sensors in the art can be theoretically classified into four categories: 1. the shunt resistor according to the Ohm's law, 2. the current transformer according to the Faraday's law of induction 3. The Hall element according to the magnetic detection, and 4. the fiber optic current sensor according to the Faraday effect. The former two types are criticized for their mass heat generation due to direct measuring and for irrelevance to the multi-core power cable due to their cumbersome volumes. On the other hand, the Hall element that is tiny in volume and can function without direct contacting would be superior to the former two types of the current sensors. However, before adopting the Hall elements to the multi-core power cables, the distance between the Hall element and the power cable to be detected is critical. In addition, the fiber optic current sensor is low in sensitivity, difficult in maintenance, and complicated in structuring, and thus its application is pretty limited.

According to the Amp principle, when an electric current flows through a conductive object, a surrounding magnetic field would be induced. The magnitude of the induced magnetic field is proportional to the current in the conductive object, but is inversely proportional to the spacing in between. Through the knowledge of the induced magnetic field, the current through the conductive object can be realized. However, all the aforesaid or not said non-contact current sensors in the art have a common shortcoming of measurement bias due to inappropriate mounting positions. Therefore, it is definite that an improvement upon the non-contact current sensors for compensating the position-induced measurement bias is urgent and welcome to the skill in the art.

SUMMARY

The present disclosure is to provide a compensating apparatus for installing variation of a non-contact current sensor on a two-wire power cable so as thereby to amend the unstable measurement bias (sometimes, over hundred percentages of errors) due to the ill-mounting position of the non-contact current sensor, and so as to have the consumer product to achieve a stable quality while meeting various situations in mounting the non-contact current sensor. Through a suitable pair of the measurement devices and the calculation algorithms, the compensating apparatus for installing variation of a non-contact current sensor on a two-wire power cable can estimate the human factor and the manufacturing variation so as to reduce the measurement bias and further to achieve the object of automatic measurement.

According to the Ampere's circuital law, as an electric current flows through a longitudinal lead, a circular magnetic field will be induced around the lead. The circular magnetic field is proportional to the electric current, and can be exactly computed as the equation of

Br=μ0⁢I2⁢⁢π⁢⁢r,
in which the μ0is the permeability, the I is the electric current, and the Bris the magnetic flux density at a place having a distance r to the center of the lead. Further, according to the Faraday principle, the induced voltage of the coil can be computed as

V=N·A⁢d⁢⁢Bd⁢⁢t=N⁢d⁢⁢ϕd⁢⁢t,
in which the N is the number of the coil of the lead, the A is the area circulated by the coil, and the φ is the effective flux. By given the coil number N and the coil area A, the current to be detected I at a detecting point r is proportional to the induced voltage V. Hence, in order to compute the current to be detected I from the induced voltage V, the detecting point r, the coil number N and the coil area A need to be known in advance, in which the coil number N and the coil area A are fixed parameters and won't vary with the detecting position. However, the detecting point r is changed with the mounting position, and so the detecting point r needs to be real-timely obtained if an immediate measurement is requested. In an orthogonal coordinate system, the r of the lead is related to the first distance g1of the vertical displacement and the horizontal displacement W. Namely, if the induced voltage V is used to derive the current to be detected I, the first distance g1of the vertical displacement and the horizontal displacement W need to be measured in advance, in which the first distance g1of the vertical displacement is the distance between the detecting point and the lead, and the horizontal displacement W is the horizontal distance perpendicular to the vertical displacement.

In this disclosure, the compensating apparatus for installing variation of a non-contact current sensor on a two-wire power cable introduces a coupling algorithms to derive the current to be detected I in the two-wire power cable, the first distance g1of the vertical displacement and the horizontal displacement W between the non-contact current sensor and the two-wire power cable.

Computation of the first distance g1of the vertical displacement: Apply two identical magnetic sensors spaced fixedly by a fixed distance g2along the detection direction of the measurement. The magnetic flux densities at the two magnetic sensors are

Br=μ0⁢I2⁢⁢π⁢⁢(g1)⁢⁢and⁢⁢Br+g=μ0⁢I2⁢⁢π⁢⁢(g1+g2),
respectively. The output signals of the two magnetic sensors are related to the current to be detected I and the first distance g1of the vertical displacement. By given all the other parameters of involved elements, the simultaneous equations for the outputs signals of the two magnetic sensors can be used to derive the current to be detected I and the first distance g1of the vertical displacement.

Computation of the horizontal displacement W: Apply two identical magnetic sensors arranged parallel and symmetrically with respect to the main measured element, and integrate in series the two signals of the two magnetic sensors. The symmetrical structure of the two-wire power cable would make the in-serial output signals to be proportional to the horizontal displacement W between the sensors along the central axis of the power cable. By given all the required parameters of then involved elements and the current to be detected I, the first distance g1of the vertical displacement can be computed, and then the horizontal displacement W between the sensors along the central axis of the power cable can be derived. Through 2D (vertical and horizontal) coupling computations, accurate values of the first distance g1of the vertical displacement of the sensors, the horizontal displacement W of the sensors and the current to be detected I can be better approached. Therefore, the current I to be detected can be obtained whatever the mounting position of the non-contact current sensor is.

According to the Ampere's circuital law, a circular magnetic field around a longitudinal lead would be induced if the electric current flows through the lead. The circular magnetic field is proportional to the electric current in the lead, and can be obtained by the equation of

Br=μ0⁢I2⁢⁢π⁢⁢r,
in which the μ0is the magnetic permeability, the I is the electric current in the lead, and the Bris the magnetic flux density at the radius r of the lead. By having planar-coiled current sensors to detect the electric current in the lead as shown inFIG. 1andFIG. 2, according to the Faraday's lay of induction, the output voltage can be computed by the equation of

emf⁡(v)=-d⁢⁢ϕd⁢⁢t=-N·A·d⁢⁢Brd⁢⁢t,
and the output voltage for the planar-coiled current sensor is

emf⁡(v)=-∑n=1N⁢⁢d⁢⁢Φnd⁢⁢t=ωμ0⁢I⁢⁢sin⁢⁢ωt2⁢⁢π⁢∑n=1N⁢cn⁢ln⁡(bn2+g12an2+g12),
in which

The first step of the coupling compensation method in this disclosure is to utilize the built-in equations involving the first distance g1and the

V1V2
ratio to compute the first distance g1, in which the first voltage V1is the detected voltage difference between the input end and the output end of the first current sensor, and the second voltage V2is the detected voltage difference between the input end and the output end of the second current sensor. Through the ratio

V1V2,
a corresponding first function ƒ1is formed as follows.

g1=f1⁡(V1V2,W)⁢⁢…⁢⁢the⁢⁢first⁢⁢function⁢⁢⁢f1.
It is noted that the first function ƒ1relates the horizontal displacement W, the first distance g1and the

V1V2.
As the first voltage V1and the second voltage V2are detected and the horizontal displacement W is given, the first step of the coupling compensation method can apply the first function ƒ1to derive a corresponding first distance g1.

The second step of the coupling compensation method in this disclosure is to utilize the built-in equations involving the current to be detected I and the

V1V2
ratio so as further to compute the gain, or say the current-calibrating factor

IV1,
according to the second function ƒ2as follows.

It is noted that the second function ƒ2relates the horizontal displacement W, the gain

1V1
and the

V1V2.
As the first voltage V1and the second voltage V2are detected and the horizontal displacement W is given, the second step of the coupling compensation method can apply the first function ƒ2to derive a corresponding current to be detected I of the two-wire power cable.

The third step of the coupling compensation method in this disclosure is to utilize the built-in equations involving the horizontal displacement W and the third voltage V3so as further to compute the horizontal displacement W according to the second function ƒ3as follows.

It is noted that the third function ƒ2relates the horizontal displacement W, the first distance g1and the

V3I,
in which the third voltage V3is the detected voltage difference between the input end and the output end of the third current sensor, and the I is the current to be detected obtained from the aforesaid second function ƒ2. As the third voltage V3and the current to be detected are detected and the first distance g1is given, the third step of the coupling compensation method can apply the third function ƒ3to derive a corresponding horizontal displacement W.

Finally, the fourth step of the coupling compensation in this disclosure is firstly to set up the initial conditions of the first distance g1=0 the current I to be detected=5 A. and the horizontal displacement W=0 mm. Then, the coupling computation is performed by executing orderly the first step, the second step and the third step and is ended till the current I to be detected is convergent as

(In-In-1)In<0.01,
or the number of the coupling computation exceeds 20 times. While the coupling computation is not convergent, then re-setup of the initial conditions is introduced.

Further, fromFIG. 3AandFIG. 4, it is noted that the third current sensor is consisted, in serial, of two independent coil loops lying along the horizontal direction and located symmetrically to the center line of the non-contact current sensor. The voltage difference detected between the input end and the output end of the third current sensor is defined as a third voltage V3. The horizontal displacement W of the center axis of the two-wire power cable and the third current sensor is a 1-1 function, as a shown inFIG. 3A. In the coils of the third current sensor, the voltage difference measured between the input end and the output end of the third current sensor is the third voltage V3. Finally the whole algorithms can be assorted to: (1) obtain the vertical first distance g1and the current I to be detected from the vertical simultaneous equations, and (2) obtain the horizontal displacement of the center axis of the two-wire power cable to the non-contact current sensor from the aforesaid vertical first distance g1and current I to be detected. Upon such a coupling arrangement, the vertical first distance g1of the sensor, the horizontal displacement W of the sensor, and the current to be detected I can be accurately approached.

In one embodiment of this disclosure, the compensating apparatus for installing variation of a non-contact current sensor on a two-wire power cable comprises a non-contact current sensor, a sensing element characteristic measuring unit, and a non-contact current measurement module, in which the non-contact current sensor can further include a first current sensor, a second current sensor, and a third current sensor. The non-contact current sensor located at a top position of the two-wire power cable is to measure the space magnetic field variation caused by the current variation in the two-wire power cable. Namely, the horizontal direction is defined as the direction along the line connecting the two centers of the inner diameters of the two power wires of the two-wire power cable, and the vertical direction is the direction perpendicular to the horizontal direction. The sensing element characteristic measuring unit is to construct the space characteristic measuring database of the two-wire power cable with respect to the non-contact current sensor. The non-contact current measurement module is to output a measured value of the current I of the two-wire power cable.

DETAILED DESCRIPTION

Referred toFIG. 3A,FIG. 3B,FIG. 3CandFIG. 8, in one embodiment of this disclosure, the compensating apparatus for installing variation of a non-contact current sensor10is applied to detect the electric current of a two-wire power cable41and includes a non-contact current sensor10, a sensing element characteristic measuring unit50, and a non-contact current measurement module60. The non-contact current sensor10further includes a first current sensor11, a second current sensor12, and a third current sensor13. The non-contact current sensor10is mounted at a top position of the two-wire power cable41. The two-wire power cable41has two power wires having individual centers of the respective inner diameters, in which the connection line of the centers are extended along a horizontal direction. The direction that is perpendicular to the horizontal direction is defined as a vertical direction. The sensing element characteristic measuring unit50is to construct a space characteristic measuring database51of the non-contact current sensor10with respect to the two-wire power cable41. The non-contact current measurement module60is to pair the built-in space characteristic measuring database51and so as thereby to calculate and output a measured value of the current I of the two-wire power cable41.

Refer nowFIG. 3A,FIG. 3B,FIG. 3C, in whichFIG. 3Ais a top view of the non-contact current sensor10,FIG. 3Bis a side view ofFIG. 3Aat a viewing angle from C to B, andFIG. 3Cis another side view ofFIG. 3Aat a viewing angle from B to C. As shown, the non-contact current sensor10includes the second current sensor12, the first current sensor11, and the third current sensor13. The first current sensor11is located vertically above the two-wire power cable41, and is spaced from the centers of the two-wire power cable41by a first distance g1. The second current sensor12is located further vertically above the first current sensor11, and is spaced from the first current sensor11by a second distance g2. The third current sensor13lying horizontally with respect to the two-wire power cable41is formed by two independent coils21connected in series and located symmetrically to the center line of the non-contact current sensor10. The voltage difference measured between an input end and an output end of the third current sensor13is defined as a third voltage V3. In this embodiment, the first current sensor11and the second current sensor12are both coil-formed loops, the voltage difference measured between the input end and the output end of the first current sensor11is defined as a first voltage V1, and the voltage difference measured between the input end and the output end of the second current sensor12is defined as a second voltage V2.

The sensing element characteristic measuring unit50as shown inFIG. 8is to establish the space characteristic measuring database51and includes a plurality of horizontal displacement indicators52and a plurality of vertical displacement indicators53. As shown inFIG. 4andFIG. 5, each of the horizontal displacement indicators52is a normalized characteristic measuring data referred by a pair of the horizontal displacement W and the third voltage V3and each of the vertical displacement indicators53is a respective characteristic measuring data referred by a pair of the first voltage V1/the second voltage

V2=[V1V2]
and the

gain=[IV1],
in which I is the electric current in the two-wire power cable41. In the detection, the sensing element characteristic measuring unit50drives the non-contact current sensor10located at an upper position of the two-wire power cable41to move continuously in a 2D manner along the horizontal direction and the vertical direction. According to the Faraday's lay of induction, the current I would be induced in the two-wire power cable41and output the induced voltages (including the first voltage V1, the second voltage V2and the third voltage V3) through the first current sensor11the second current sensor12and the third current sensor13to establish the 2D space characteristic measuring database51.

The non-contact current measurement module60as shown inFIG. 8includes a measurement counting unit61, a compensation algorithms calculating unit62and the non-contact current sensor10, in which the non-contact current sensor10is located at a top (or say an upper) position above the two-wire power cable41. According to the Faraday's law of induction, an electric current I would be induced in the two-wire power cable41, and the first voltage V1, the second voltage V2and the third voltage V3would be captured by the non-contact current sensor10and further be output to the compensation algorithms calculating unit62. Then, through the measurement counting unit61, the measured value of the current I of the two-wire power cable41can be detected and output.

The compensation algorithms calculating unit62as shown inFIG. 8includes a first unit71featured by a first function ƒ1as listed in the following. The first function ƒ1is a function to locate the first distance g1from the 2D variable

(V1V2,W)
supplied from the 2D space characteristic measuring database, in which the first voltage V1is the voltage difference measured between the input end and the output end of the first current sensor11, and the second voltage V2is the voltage difference measured between the input end and the output end of the second current sensor12.

Namely, with various given horizontal displacements W, and after the first voltage V1and the second voltage V2are detected, then the value

V1V2
can be known, and then the corresponding first distances g1can be calculated through the first function ƒ1in the first unit71.

The compensation algorithms calculating unit62as shown inFIG. 8further includes a second unit72featured by a second function ƒ2as listed in the following.

The second function ƒ2is a function to locate the gain

IV1
(or say a calibration factor) from the 2D variable

(V1V2,W)
W) supplied from the 2D space characteristic measuring database, in which the first voltage V1is the voltage difference measured between the input end and the output end of the first current sensor11, and the second voltage V2is the voltage difference measured between the input end and the output end of the second current sensor12.

In applying the second function ƒ2as the first voltage V1and the second voltage V2are detected, then the second unit72can provide corresponding currents I to be detected of the two-wire power cable41with various given horizontal displacements W, through the second function ƒ2.

The compensation algorithms calculating unit62as shown inFIG. 8further includes a third unit73featured by a third function ƒ3as listed in the following. The third unit73is to utilize the data from the 2D space characteristic measuring database to establish a mathematical relationship between the horizontal displacement W and the third voltage V3so as to locate the horizontal displacement W through the variable pair of the first distance g1and

Obviously, the third function ƒ3has the two control variables, the first distance g1and the ratio

V3I.
The voltage difference measured between the input end and the output end of the third current sensor13is defined as the third voltage V3, which is the respective voltage change accounting to the horizontal displacement W. By given the first distance g1, the horizontal displacement W and the ratio

V3I
are related via the third function ƒ3. The current I to be detected is calculated in the second unit72and is further plugged into the third function ƒ3of the third unit73so as to locate a value for the horizontal displacement W.

In the fourth unit74, the calculation begins at setting up the initial conditions, in which, typically, the initial conditions include the first distance g1=0, the current I to be detected=5 A, and the horizontal displacement W=0 mm. Then, perform the coupling computations in order through the first unit71, the second unit72, and the third unit73, till the current I to be detected is convergent while

In-In-1In<0.01,
or while the executing number n is over 20. In any of the situations, a new round of the coupling computations shall be performed after re-setting the initial conditions.

In this disclosure, the sensing element characteristic measuring unit is to establish a 2D sensor characteristic curve surface for building in the space characteristic measuring database. The method to achieve such a purpose is to move a standard electric current source on a 2D movable platform so as to formulate a detection-feasible arrangement for the 2D sensors, and then the signals can be captured to be further furnished to the sensing element characteristic measuring unit for constructing the aforesaid curve surface. Further, the compensation algorithms calculating unit utilizes the space characteristic measuring database to energize the calculations through the space characteristic functions ƒ1, ƒ2and ƒ3, and also the first voltage V1, the second voltage V2and the third voltage V3can get involved in the calculations in the measurement counting unit so as to output a measured value of the current I in the two-wire power cable.