Patent Application
20150301088
This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-087066, filed on Apr. 21, 2014, the entire contents of which are incorporated herein by reference.
The embodiments discussed herein are related to a current measurement device and a current measurement method.
Current measurement devices for measuring a current flowing through a cable include clamp-type current measurement devices. A clamp-type current measurement device includes a pair of semicircular cores which clamps a cable, and is a device for measuring a current in the cable based on an induced voltage generated in a coil wound around the core. This structure makes it possible to easily measure a current by clamping a cable with the pair of cores, and provides a convenience to an operator.
However, in the case where a cable includes two wires through which their respective currents flow in opposite directions, magnetic fields generated around the respective wires are prone to be canceled by the cores. Accordingly, an induced current flowing through the coil wound around the core is reduced, and it is difficult to accurately measure a current using a clamp-type current measurement device. A similar problem also occurs in the case where a cable contains a ground wire as well as two wires.
In such a case, to accurately measure a current in each wire, each wire preferably is singly clamped with the cores separately by, for example, pulling each wire out of the cable.
However, this places the burden of processing the cable on an operator. Further, in the case where the cable is the property of a client, processing the cable will damage the client's equipment. Accordingly, even processing the cable is not performed.
Note that techniques relating to the present application are disclosed in Japanese Laid-open Patent Publication Nos. 2013-88349, 2007-78668, and 2008-275558.
One aspect of the disclosure below provides a current measurement device including: a casing configured to surround a cable; and a magnetic sensor configured to be movable along a periphery of the cable within the casing and to output an output signal varying according to a current value of a current flowing through the cable.
Moreover, another aspect of the disclosure provides a current measurement method including: while moving a magnetic sensor configured to output an output signal varying according to a current value of a current flowing through a cable along a periphery of the cable, measuring the current value with the magnetic sensor; and determining a largest value of the current value around the cable to be the current value.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.
Researches performed by the inventors of the present application will be described prior to the description of embodiments.
First, a description will be made for the case where a current value of an alternating current I flowing through a solid-wire cable 6 is measured using a clamp-type current measurement device 1.
The clamp-type current measurement device 1 includes a pair of semicircular cores 2 surrounding the cable 6 and a coil 3 wound around the core 2.
The magnetic flux generated around the cable 6 by the alternating current I is converged by the cores 2. This causes an induced current to flow through the coil 3. The induced current flows through a resistor RS connected in parallel with the coil 3, and an induced electromotive force VS is generated in the resistor RS.
The induced electromotive force VS is amplified in a differential amplifier 4 and then inputted to an RMS (Root Mean Square) converter 5 in a subsequent stage. The RMS converter 5 converts the induced electromotive force VS, which is an alternating voltage, to an effective value Vrms thereof, and outputs the effective value Vrms to an outside.
This configuration allows the magnetic flux around the cable 6 to be converged by the cores 2 even when the alternating current I flowing through the cable 6 is weak. Accordingly, an adequately large induced electromotive force VS is generated in the coil 3, and the alternating current I may be measured based on the induced electromotive force VS.
As described above, in the case where the cable 6 is a solid wire, it is easy to measure the alternating current I using the clamp-type current measurement device 1. However, in the case where the cable 6 contains a plurality of wires, it is difficult to measure an alternating current as described below.
Moreover,
As illustrated in
Further, in an XYZ Cartesian coordinate system, the Y axis is oriented parallel to the wires 6a and 6b, and the Z axis is oriented normal to the plane containing the wires 6a and 6b. Further, the X axis is oriented perpendicular to each of the Y axis and the Z axis.
In this case, as illustrated in
Accordingly, the inventors of the present application study the possibility of using a magnetic sensor such as a GMR (Giant Magneto Resistive) element rather than the cores 2 to measure the currents flowing through the wires 6a and 6b in opposite directions, respectively.
In a measurement of the currents using a magnetic sensor, it is desired that magnetic fields generated around the respective wires 6a and 6b are strong enough to be detectable by the magnetic sensor.
The results of the simulation of the magnetic flux density is approximately 6 ρT at a position where the distance D measured from a center C between the wires 6a and 6b is 10 mm, and that the magnetic flux density is 3 ρT at a position where the distance D is 15 mm. Magnetic flux densities at this level may be detected by a magnetic sensor such as a GMR element or an MI (Magneto-Impedance) element.
However, these magnetic sensors have sensing directions. The sensitivity of such a magnetic sensor may decrease depending on the direction of a magnetic field. This will be explained with reference to
This magnetic sensor 10 includes a first terminal 10a and a second terminal 10b. A potential difference ΔV varying according to the intensity of a magnetic field H develops between the terminals 10a and 10b. The potential difference ΔV is outputted as an output signal SΔV.
The magnitude of the output signal SΔV depends on the orientation of the magnetic sensor 10 in a magnetic field H.
Hereinafter, the sensing direction nD of the magnetic sensor 10 is defined as the direction of the magnetic field H in which the output signal SΔV becomes maximum. According to this definition, the output signal SΔV becomes maximum when the angle α between the magnetic field H and the sensing direction nD is 0°. Moreover, the output signal SΔV monotonically decreases as the angle α increases.
Note that the sensing direction nD is often the same as a normal direction to a surface 10x of the magnetic sensor 10, depending on the manufacturer of the magnetic sensor 10. In such a case, the output signal SΔV becomes maximum when the magnetic field H perpendicularly passes through the surface 10x.
Next, a position is considered at which the output signal SΔV from the magnetic sensor 10 becomes maximum as described above in the magnetic field H explained with reference to
In that case, the sensing direction nD becomes parallel to the magnetic field H at point A, where the output signal SΔV from the magnetic sensor 10 becomes maximum.
Accordingly, if point A is found among the plurality of points on the circle F, and the magnetic sensor 10 is fixed at point A, the magnetic field H may be detected with high sensitivity using the magnetic sensor 10. Thus, the values of currents flowing through the wires 6a and 6b may be measured with high accuracy based on the output signal SΔV.
Hereinafter, embodiments based on the above-described findings will be described.
This current measurement device 20 is intended to measure a current flowing through a cable 21, and includes a casing 22 surrounding the cable 21.
The casing 22 includes a body portion 23 and a movable portion 24 connected to the body portion 23 to be openable and closable. When the movable portion 24 is closed, the cable 21 is clamped between the body portion 23 and the movable portion 24.
The structure of the cable 21 to be measured is not particularly limited. The following description is based on the assumption that the cable 21 contains two wires 21a and 21b and that alternating currents I having equal magnitudes flow through the wires 21a and 21b in opposite directions, respectively. A measurement according to the present embodiment may also be performed on the cable 21 containing a ground wire as well as the two wires 21a and 21b.
Further, the material of the casing 22 is also not particularly limited, and the casing 22 may be formed of substances such as resin or metal.
Moreover, the body portion 23 has a first groove 23a, and the movable portion 24 has a second groove 24a. When the movable portion 24 is closed as illustrated in
As illustrated in
As illustrated in
Further, a circuit board 32 provided with the magnetic sensor 10 is fastened to a main surface of the rotating plate 31.
The magnetic sensor 10 is intended to measure an alternating current I flowing through the cable 21. Any one of a GMR element, an MI element, and a Hall element may be used as the magnetic sensor 10. Among these, a Hall element has low sensitivity in detecting a magnetic field, and therefore it is preferable to use a Hall IC including a Hall element and an amplifier for amplifying an output from the Hall element as the magnetic sensor 10.
In the present embodiment, since the magnetic sensor 10 is fastened onto the rotating plate 31 as described above, the magnetic sensor 10 is movable along a periphery of the cable 21, and the magnetic sensor 10 may be moved to a position at which the sensing direction nD becomes parallel to the magnetic field H as illustrated in
Note that the sensing direction nD of the magnetic sensor 10 is not particularly limited. In this example, the sensing direction nD is directed in a direction perpendicular to a vector R which points from the center of the cable 21 to the magnetic sensor 10.
Moreover, a cylindrical surface 34 is provided on an inner surface of the casing 22 to surround the cable 21. Further, annular guides 35 are provided on the cylindrical surface 34.
As illustrated in
Note that the cross-sectional shape of the guides 35 is not limited to this.
As illustrated in
Moreover, examples of the material of the first gear 38 include resin and metal.
Further, a driving portion 39 is provided beside the rotating plate 31 to rotationally drive the above-described rotating plate 31.
The driving portion 39 includes a second gear 41 gearing with the first gear 38 and a shaft 42 provided upright at the center of the second gear 41.
A tip portion 42a of the shaft 42 is led to the outside of the casing 22, and is used as a knob for turning the shaft 42 by an operator.
Thus, the turning of the tip portion 42a by an operator causes the magnetic sensor 10 to rotate around the cable 21, and it becomes easy to search for the position of the magnetic sensor 10 at which the magnetic field H and the sensing direction nD become parallel.
Note that the shaft 42 may be automatically turned using a motor or the like instead of manually turned as described above. Further, the materials of the second gear 41 and the shaft 42 are also not particularly limited. The second gear 41 and the shaft 42 may be formed of resin or metal.
Moreover, when the movable portion 24 is closed as illustrated in
As illustrated in
Moreover, since the first gear 38 is semicircular as described previously, the range of movement of the magnetic sensor 10 is an angular range of approximately 180° about the through hole 25 (see
In the example of
Note that though the reference direction L is perpendicular to an assumed plane P containing the two wires 21a and 21b in
Moreover, this graph is based on the assumption that the reference direction L is perpendicular to the assumed plane P as illustrated in
As illustrated in
Meanwhile, when the traveled angle θ becomes 90°, the sensing direction nD and the magnetic field H become parallel, and the output signal SΔV from the magnetic sensor 10 becomes maximum.
Accordingly, by checking whether or not the output signal SΔV is maximum, a determination may be made as to whether or not the sensing direction nD is parallel to the magnetic field H.
Note that the reference direction L defines the lower limit of the angular range of approximately 180° in which the magnetic sensor 10 is movable as described previously. Meanwhile, the assumed plane P is defined as the plane in which the two wires 21a and 21b are located. Accordingly, the angle between the reference direction L and the assumed plane P varies according to the positional relationship between the cable 21 and the casing 22, and generally does not become 90°. Similarly, it is not always true that the output signal SΔV becomes maximum when the traveled angle θ is 90°.
The traveled angle θ at which the output signal SΔV becomes maximum will hereinafter be referred to as a target angle θ0. This target angle θ0 is equal to the traveled angle θ at which the sensing direction nD and the magnetic field H become parallel.
As illustrated in
As illustrated in
Among these, the AD converter 47 receives the output signal SΔV from the magnetic sensor 10. Since the current I flowing through the cable 21 is an alternating current, the magnetic field H generated by the current I is an alternating magnetic field, and the output signal SΔV is an alternating voltage. The output signal SΔV is converted to a direct voltage in the AD converter 47 and outputted to the arithmetic unit 48.
The arithmetic unit 48 is, for example, an MPU (Micro Processing Unit), and calculates a current value i of the current I based on the above-described output signal SΔV. In the following, it is assumed that the arithmetic unit 48 calculates the effective value of the current I as the current value i.
A method of calculating the current value i is not particularly limited. The intensity of the magnetic field H is approximately proportional to the current value i, and the output signal SΔV is approximately proportional to the intensity of the magnetic field H. Therefore, the output signal SΔV is approximately proportional to the current value i. Accordingly, the arithmetic unit 48 may calculate the current value i by finding a proportionality factor α between the output signal SΔV and the current value i in advance and multiplying the output signal SΔV by the proportionality factor α.
Note that as explained with reference to
Moreover, there is a one-to-one relationship between the output signal SΔV and the current value i. Accordingly, the arithmetic unit 48 may find the current value i corresponding to the output signal SΔV by referring to an appropriate table on which the relationship is stored in advance.
The current value i calculated as described above is stored in the storage unit 49. The storage unit 49 is, for example, a semiconductor memory. A DRAM (Dynamic Random Access Memory), a flash memory, an FeRAM (Ferroelectric Random Access Memory), or the like may be employed as the storage unit 49.
The output port 50 converts the current value i calculated by the arithmetic unit 48 into an appropriate communication protocol and outputs the converted current value i to an external device 51.
The external device 51 is a device used to manage the current value i by an operator. One example of the external device 51 is a personal computer.
Moreover, a notification unit 52 is connected to the above-described the arithmetic unit 48. The notification unit 52 is, for example, a buzzer, an LED (Light Emitting Diode), an LCD (Liquid Crystal Display), or the like, and helps an operator determine whether or not the magnetic sensor 10 is positioned at the target angle θ0 at which the magnetic field H and the sensing direction nD become parallel.
Next, a method of measuring a current using this current measurement device 20 will be described.
First, in step S1, an operator clamps the cable 21 with the current measurement device 20, thus fixing the cable 21 to the casing 22.
Next, in step S2, the operator turns the tip portion 42a of the shaft 42 to set the aforementioned traveled angle θ (see
Subsequently, in step S3, a first current value i1 of the current I flowing through the cable 21 is measured with the traveled angle θ kept at 0°. The first current value i1 is calculated based on the output signal SΔV by the arithmetic unit 48 as described previously.
Further, the first current value i1 is stored in the storage unit 49 under the control of the arithmetic unit 48.
Next, in step S4, the operator turns the tip portion 42a (see
Subsequently, in step S5, a second current value i2 of the current I flowing through the cable 21 is measured. The second current value i2 is calculated based on the output signal SΔV by the arithmetic unit 48 as in step S3.
Next, the process goes to step S7. In step S7, the arithmetic unit 48 determines the relation of magnitude between the first current value i1 stored in the storage unit 49 and the second current value i2.
If the arithmetic unit 48 determines that i1<i2, the traveled angle θ has not reached the target angle θ0 yet, and there is a possibility that the current value may further increase by further increasing the traveled angle θ.
Accordingly, in that case, the first current value i1 in the storage unit 49 is replaced by the second current value i2 in step S6, and then the process goes back to step S4.
Meanwhile, if the arithmetic unit 48 determines that i1>i2 in step S7, the arithmetic unit 48 determines that the current value changes from increasing to decreasing. In the case where the traveled angle θ is monotonically increased as illustrated in
Accordingly, in that case, the process goes to step S8, and the arithmetic unit 48 causes the notification unit to operate. There is no limitation as to where the notification unit 52 is provided.
Next, the process goes to step S9. In this step, the operator notified by the notification unit 52 decreases the traveled angle θ by a predetermined angle Δθ to return the traveled angle θ to the vicinity of the target angle θ0. Note that the traveled angle θ may be decreased as described above by turning the tip portion 42a of the shaft 42 in a direction opposite to that in step S4.
Further, in step S10, in a state in which the traveled angle θ is in the vicinity of the target angle θ0 as described above, the current value i of the current I flowing through the cable 21 is measured again. This current value i is calculated based on the output signal SΔV by the arithmetic unit 48, and is a largest value of the current value i for the case where a minimum unit of the traveled angle θ is A. Further, the arithmetic unit 48 determines the current value i at this time to be the current value of the current I flowing through the cable 21.
This is the end of basic steps of the current measurement method according to the present embodiment.
According to the above-described present embodiment, the magnetic sensor 10 is provided which is movable along the periphery of the cable 21. Accordingly, the magnetic sensor 10 may be moved to the target angle θ0 at which the output signal SΔV outputted from the magnetic sensor 10 according to the magnetic field H generated around the cable 21 becomes maximum. As a result, the current value of the current I flowing through the cable 21 may be easily measured based on the large output signal SΔV outputted at the target angle θ0 by the magnetic sensor 10.
This eliminates the operation for processing the cable 21 and pulling out one wire, unlike the case where a measurement is performed using a clamp-type current measurement device. Accordingly, a burden on an operator may be reduced, and the client's equipment is not damaged.
Further, even in the case where the cable 21 has an insulating sheath and it is difficult to identify the arrangement of the two wires 21a and 21b from the appearance thereof, the target angle θ0 at which the output signal SΔV becomes maximum may be detected by moving the magnetic sensor 10 along the periphery of the cable 21 as described above.
In the present embodiment, a structure will be described which is useful in fixing the cable 21 to the casing 22.
The spacer 54 is made by forming metal or resin into an approximately semicylindrical shape, and the two spacers 54 are used in combination. Further, each of the spacers 54 includes an outer circumferential surface 54z and an inner circumferential surface 54d which comes in tight contact with an outer surface of the cable 21 (see
The outer circumferential surface 54z has a tapered shape such that the wall thickness decreases from a rear end 54x toward a forward end 54y.
Further, the spacer 54 has a plurality of slits 54a. This facilitates the deformation of the spacer 54.
When the spacers 54 are used, the outer circumferential surface 54z is fitted into the through hole 25 with the inner circumferential surface 54d held in tight contact with the outer surface of the cable 21 to insert the spacers 54 into a gap between the through hole 25 and the cable 21. Thus, the gap is filled with the spacers 54, and the cable 21 may be fixed to the casing 22. As a result, the cable 21 may be prevented from moving within the through hole 25, and the positional relationship between the cable 21 and the magnetic sensor 10 may be fixed.
Further, since the outer circumferential surface 54z has a tapered shape as described previously, the thin-walled forward end 54y may be easily inserted into the through hole 25, and the thick-walled rear end 54x increases the fastening force of the spacer 54 to enable the cable 21 to be firmly fixed to the casing 22.
Moreover, the slits 54a facilitates the deformation of the spacer 54 and further increases the fastening force of the spacer 54.
The structure of the spacer 54 is not limited to the above-described one.
In this example, an elastic member such as a rubber sheet is used as the spacer 54, and the spacer 54 is fastened to the second groove 23a with adhesive or the like. This structure also enables the cable 21 to be fixed to the casing 22 with the spacer 54.
Note that an elastic spacer 54 such as described above may be provided in the second groove 24a of the movable portion 24.
In the present embodiment, the cable 21 is fixed to the casing 22 with a structure different from that of the second embodiment.
In the present embodiment, protruding portions 23x are provided in the first groove 23a of the body portion 23. Thus, the cable 21 comes in contact with the protruding portions 23x when the movable portion 24 is closed, and the cable 21 may be fixed to the casing 22.
Note that protruding portions 23x such as described above may be provided in the second groove 24a of the movable portion 24.
In the present embodiment, a current measurement device will be described in which the magnetic sensor 10 may be fixed at the target angle θ0.
As illustrated in
Moreover, an unillustrated thread is formed on a surface of the pin 63, and a nut 64 is provided which is fitted to the thread.
As illustrated in
Now,
When the nut 64 is firmly screwed onto the pin 63, the nut 64 is pressed against a surface of the body portion 23, and the pin 63 is fixed to the body portion 23. In this way, the nut 64 is used as a fixing portion for fixing the pin 63 to the body portion 23. Thus, the rotating plate 31 is also fixed to the body portion 23.
This enables the magnetic sensor 10 to be fixed in the vicinity of the target angle θ0, and may prevent the magnetic sensor 10 from deviating from the target angle θ0.
In the present embodiment, the rotating plate 31 is fixed to the body portion 23 with a structure different from that of the fourth embodiment.
As illustrated in
The braking portion 67 is movable in the normal direction n of the rotating plate 31, and presses the peripheral edge of the rotating plate 31 against the guide 35. This brakes the movement of the rotating plate 31, may fix the rotating plate 31 to the body portion 23, and may prevent the magnetic sensor 10 from deviating from the target angle θ0.
In the present embodiment, the rotating plate 31 is fixed to the body portion 23 with a structure different from those of the fourth and fifth embodiments.
As illustrated in
Further, a braking portion 72 is provided on another main surface 31y of the rotating plate 31. The braking portion 72 presses the other main surface 31y of the rotating plate 31 to press the one main surface 31x of the rotating plate 31 against the elastic member 71. This brakes the movement of the rotating plate 31, may fix the rotating plate 31 to the body portion 23, and may prevent the magnetic sensor 10 from deviating from the target angle θ0.
In the first embodiment, the measuring unit 45 and the battery 46 are incorporated in the body portion 23, which is part of the casing 22, as illustrated in
Meanwhile, in the present embodiment, the measuring unit 45 and the battery 46 are provided outside the casing 22.
This current measurement device 70 includes the casing 22, a measuring unit 71, and a connecting cable 72 for connecting the foregoing components.
The measuring unit 71 contains the measuring unit 45 and the battery 46 illustrated in
Further, the battery 46 supplies power to the casing 22 through the connecting cable 72, and an output signal from the magnetic sensor 10 (see
Thus, while the measuring unit 71 is placed on a floor of a room, a current measurement may be performed on each of various cables 21 in the room by clamping the cable 21 with the casing 22. Further, a long-term measurement may be stably performed on an identical cable 21 by installing the measuring unit 71 in the room.
Also, in the case where the function of comparing current values is given to the arithmetic unit 45 in the measuring unit 71, current values measured at different times or places may be compared, and an operator may obtain how current values vary depending on time and place.
(Modifications)
Hereinafter, a description will be made of modifications of the rotating plate 31 of the first embodiment illustrated in
(First Modification)
In this modification, compared to the case of
(Second Modification)
In this modification, the rotating plate 31 and the first gear 38 are connected with a hollow container 66 in the shape of a half doughnut interposed therebetween.
As illustrated in
(Third Modification)
In this modification, gear teeth which gear with the second gear 41 are formed on an outer circumference of the rotating plate 31. This eliminates the use of the first gear 38. Thus, the number of components may be reduced, and assembly work may be simplified.
All examples and conditional language recited herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-087066 | Apr 2014 | JP | national |
