DESCRIPTION

Field of the invention

The present invention is applicable to the field of electrical measurements and is particularly suitable for use in control circuits of batteries, solar panels, hydrogen cells, or the like.

More specifically, this invention relates to methods and devices for measuring an electrical current, especially in cases of measurement of a DC current or a pulsed current. Background Art

In the apparatus involving batteries, solar panels, hydrogen cells or the generation of electrical power by generator or rectified alternator, such as wind generators, it is often important the monitoring of the generated electrical current or other electrical parameters liked thereto in order to monitor the functioning of the same apparatus.

Similarly, there are often situations where it is important to monitor electrical loads of various kinds, such as lamps, heaters, direct current motors or the like. In all these cases, the circuit portion that deals with the monitoring usually involves one or more generally nonintrusive electric amperometers, suitable for measuring the current in one or more points of the apparatus to be controlled.

The known nonintrusive amperometers are measurement tools that generally use a magnetic measuring principle. More specifically, they monitor the effect of the electric current to be measured on one or more inductors to obtain the actual value.

In this sense, many amperometers are known that use Hall sensors, an example of which is described in EP 0,742,441 A1. However, these amperometers have some drawbacks, due to the presence of the Hall sensor.

First, they have a small sensitivity and also poor linearity. The returned measure is affected by error also because their dependence on environmental parameters, with particular reference to changes in temperature.

Furthermore, these amperometers have high realizing costs and a high electric consumption.

In an attempt to overcome the above drawbacks, offset- compensated Hall sensors have been developed. Although they allow to reduce the error in the measurements taken by the amperometer, they however cannot properly reduce it. In addition, the compensation requires a power supply considerably higher than the anyway elevated electric consumption of the amperometers with Hall sensors without offset-compensation.

Amperometers are also known that use not Hall sensors, but using instead devices that detect the perturbations in the magnetic field in an inductor by measuring the effect that such perturbations has on the hysteresis loop of the inductor. Examples of this solution are described in US 6,984,979 B1 , US 5,091 ,697 B1 and US 4,290,018 B1.

In particular, these devices include an electronic circuit connected to the ends of a winding on a ferromagnetic core to power it with an alternate electrical signal to generate a predetermined induced alternate magnetic field. For this reason, these circuits include at least one oscillator generally made by an oscillating electronic circuit.

Wounding the inductor with a circuital portion of the apparatus to be monitored, such as a wire, the current in this wound circuit portion generates an additional induced magnetic field modifying the working point of the inductor in its hysteresis loop. This turns into a variation of the electrical signal present at the ends of the inductor spiral, indifferently from the fact that it is a current or voltage signal. The measuring process generate an additional electrical signal to be supplied to the ends of the winding to compensate the effect of the additional magnetic field. This additional electrical signal is proportional to the one to be measured.

Although these devices allow a precise measure of the electrical parameters to be monitored, particularly referring to the electric current, they however have some drawbacks. At first, they are provided with complex and expensive electronic circuits. In fact, they must generate and shape suitable electrical signals to correct the perturbation of the magnetic field induced in the inductor.

Moreover, the generation of these electrical signals is accomplished by a great electric consumption. This is an insuperable problem if the device is connected to an electrical circuit of the 4-20 mA passive loop type.

The 4-2OmA current loop is a common transmission method of a sensor or a sensing device in many industrial control apparatuses. The use of a current loop is particularly useful when information must be sent to a location of reading and processing mail at a great distance.

The operation of the cycle is straightforward: the output voltage of the device is converted into a proportional current, wherein the 4 mA level is zero, that is the reference level, and the 20 mA is the maximum level.

In fact, the transmission of the output of the device as a voltage over long distances has several disadvantages. Unless devices with very high input impedance are used, the transmission of long-range tensions causes voltage drops due to the parasitic resistance of connections and interconnections that become comparable with the input impedance of the receiving station.

However, even stations with high input impedance have drawbacks because they are highly susceptible to noise and interference collected by the connection between the device and the location. Although the use of shielded cables is possible, this however give rise to excessive costs, mainly to cover large distances.

By contrast, the sending of an electric current can easily overcome the drawbacks mentioned above. However, a device connected to the above loop should draw power therefrom. The devices described so far require a power supply with a value so large that it cannot be connected to such a loop. In particular, it is noticed that such devices should be able to operate with only 4 mA, i.e. only with the zero value of the loop, and this is a value of electric current generally insufficient to power the known sensing devices.

Summary of the invention

The main object of this invention is to overcome the above drawbacks, by providing a device for measuring an electric current that is accurate and, at the same time, has a very limited power consumption compared to the equivalent known devices.

In particular, one object of the invention is to provide a device for measuring an electric current that can be used in a passive loop of the 4-20 mA type.

Within these general objects, a particular object of the invention is to provide a device whose electronic circuits are particularly easy to implement and low cost.

Another object which is to be reached is that the device per se is compact, so as to make it easy to use in any application.

Such objects, as well as other objects that will became apparent hereinafter, are fulfilled by a method of measuring an electric current, and a measuring device that implements this method, according to the independent claims. In particular, the method includes at least the following steps: providing a user electrical circuit through which the electrical current to be measured is flowing; electrically feeding with an electric alternative current one or more first coils wound around an inductor to generate an alternated induced magnetic field that cyclically saturate said inductor from a saturation state to the saturation state with the opposite sense, said electric alternative current having a null continuous component when the magnetic field induced in said inductor is generated exclusively by said alternative electric current; winding said inductor with one or more second coils belonging to said user electrical circuit so that the current to be measured generate in said inductor an additional magnetic field that varies the cycle of said inductor passing from a saturation state to the saturation state with opposite sense; modifying the shape of the wave of said alternative electrical current so as to keep unchanged the saturation values of the overall magnetising field of said inductor, said alternative electric current with modified wave shape introducing a not null continuous component proportional to the current to be measured; measuring said continuous component of said alternative electric current. As used herein, the term "providing" and derivatives thereof, designates the preparation of a relevant component for a relevant process step, including any preventive treatment designed for optimal performance of such relevant step.

It is to be noticed that the method of the invention also seeks to compensate the effect of a magnetic disturbance on the inductor generated by the electric current to be measured but, unlike the known devices, does not provide an electronic circuit generation of an additional power supply, but the modifying in the waveform of the alternative electric current that powers the inductor. In particular, as discussed in detail below, the modifying is the change in the duty cycle of the alternative electric current.

It is clear that this solution is inexpensive from an energetic point of view, because it involves only the shaping of a waveform, and not generating an additional electrical signal. The device for measuring an electrical current flowing in a user electrical circuit, that implements the above method, comprises at least one electrical commutator circuit with a cyclical commutation which include an inductor to be wound with said user electrical circuit, and at least one electric generator circuit of an alternative electric current flowing in one or more first coils wound around said inductor to generate an alternated induced magnetic field that cyclically saturate said inductor from a saturation state to the saturation state with the opposite sense, said alternative electric current having a cyclical variation correspondent to said cyclical commutation of said commutator circuit and having a null continuous component when said inductor is fed exclusively by said generator circuit, and a not null continuous component when said inductor is fed also by said user electrical circuit in order to compensate the effect of an additional magnetic field, generated from the electrical current to be measured, on the values of the hysteresis cycle of the overall saturation magnetising field of said inductor. According to one aspect of the invention, the circuit includes an extractor dipolar element of said continuous component that generally consists of a resistance, said "shunt", having a first pole operatively connected to a first end of the first coils wound around the inductor, and the second pole operatively connected to a distribution point of an electrical reference tension, typically consisting of the power supply of the device.

An important feature of the present invention is the fact that the commutator circuit is, as ^' a matter of fact, an oscillator that includes the inductor. In particular, the same inductor, as will became more apparent later, is the main element of the circuit, as it is directly responsible for the duty cycle of the alternating electrical power supply signal and its variations.

Brief description of the drawings Further features and advantages of the invention will be more apparent upon reading the detailed description of a preferred, nonexclusive embodiment of a device according to the invention, which is described as a non-limiting example with the help of the annexed drawings, in which: the FIG. 1 is a schematic view of the device for measuring an electric current according to the invention; the FIG. 2 is the functional graph of an electronic device belonging to the invention in an operative step; the FIG. 3 is an electric parameter of the device of the invention in the same operative step of FIG. 2, the FIG. 4 is the functional graph of an electronic component belonging to the device of the invention in a further operative step; the FIG. 5 is an electric parameter of the device of the invention in the same operative step of FIG. 4, the FIG. 6 is another schematic view of the measuring device of the invention.

Detailed description of a preferred embodiment With reference to FIG. 1 , a device 1 for measuring an electrical current h flowing in a user electrical circuit U is illustrated.

The above device has at least one electric commutator circuit 2 with a cyclical commutation which include an inductor 3 to be wound with the user electrical circuit U. In particular, the cyclical commutation of the commutator circuit

2 generates an alternative electric current I ₂ that flows in one or more first coils 4 wound around the inductor 3 in such a manner to generate an alternated induced magnetic field that cyclically saturate the inductor 3 from a given saturation state to the saturation state with the opposite sense.

According to an aspect of the invention, this means that the intended working points of the inductor 3 are the saturation ones. In substance, the device 1 takes advantage from the non-linear behaviour of the inductor 3 when the induced magnetic field induces the inductor 3 to work in the areas of its hysteresis loop relative to the saturation in both the senses, as shown in FIG. 2.

Unlike the known devices, it is to be noticed that to be fed the inductor 3 is not a component to be connected to the commutator circuit 2, which defines an oscillator. In the device 1 , the inductor 3 is an integral part of the commutator circuit 2 and cooperate in the generation of the alternative electric current b that feeds it. In fact, the commutator circuit 2 is designed so that the inductor

3 alternately passes from a given saturation state to the saturation state having opposite sense. In absence of disturbances, the cyclic transition from a given saturation state to another is perfectly symmetrical and requires the passage in the first coils 4 wound around the inductor 3 of an alternative electric current I ₂ that is symmetric and has a duty cycle of 50%, as illustrated in FIG. 3. This alternative electric current h, therefore, has a null mean value, i.e. a null continuous component.

When the inductor 3 is wound with the electrical user circuit U by one or more second coils 5, the latter, if an electric current to be measured h flows through it, generates an additional magnetic field that disturbs the one of the inductor 3, generated by the commutator circuit 2. This results in a shift of the working point of the inductor 3 in its hysteresis loop, as can be noticed in FIG. 4. Consequently, the inductor 3 reaches in advance the saturation state in a given sense, and late in the opposite direction. In substance, the magnetic disturbance tends to cause a shift of the hysteresis loop of the inductor 3 by moving the value of the overall saturation magnetizing field of the inductor 3. However, since the cyclic commutator circuit 2 includes the inductor 3, the latter causes a variation in the cyclic commutation of the commutator 2, which tends to compensate the effects of the magnetic disturbances. Therefore, unlike the known devices, the device 1 of the invention does not add a specifically generated current to the power one of the inductor 3 to compensate the effects of the disturbance, but directly compensates the effect of the disturbance.

Previously it has been stated that the alternative electric current h, which flows in the first coils 4 wound around the inductor 3, is suitable to saturate the inductor 3 alternately in a given sense and in the opposite one. More specifically, whenever the inductor 3 is saturated, the commutator circuit 2 commutates and the alternative electric current I ₂ varies its sense.

The translation of the hysteresis loop of the inductor 3 that the magnetic disturbance, that is generated by the electric current to be measured l-i, tends to cause became, finally, in an advanced reaching of a given saturation state, respectively in a delayed reaching of the saturation state with the opposite sense.

This implies that the commutator circuit 2 commutates in advance, respectively late, and accordingly the alternative electric current b has a modified waveform in which the duty cycle is varied in relation to the absence of the perturbing magnetic field, as can be noted in FIG. 5.

It is apparent that the variation in duty cycle from 50% to a different value of an alternative current corresponds to the fact that this alternative current has a mean value which is not equal to zero, i.e. has a not null continuous component. This continuous component is proportional, at least according to the ratio between the number of first coils 4 and the number of second coils 5, to the current to be measured l-i. It is therefore sufficient to measure this continuous component for measuring the current to be measured U.

Therefore, unlike the known devices, it is therefore clear that the device 1 oppose the translation of the hysteresis loop of the inductor 3, trying to maintain unchanged the values of the overall saturation magnetizing field by varying the waveform of the alternative electric current I ₂ .

Suitably, as can be noted in FIG. 6, the commutator circuit 2 includes an extractor dipolar element 6 of the continuous component of the alternative electric current I ₂ having a first pole 7 operatively connected to a first end 8 of the inductor 3, and the second pole 9 operatively connected to a distribution point 10 of an electrical reference tension.

In particular, in the simplest embodiment of the invention, the extractor element 6 is a resistor through which passes the alternative electric current I ₂ .

According to one aspect of the invention, the commutator circuit 2 includes at least one electrical circuit generator 11 of the alternative electric current I ₂ .

In particular, the generator circuit 11 includes a comparator stage 12 between the reference tension cited above and a level of electrical tension that is proportional to the tension of the extractor element 6. The output of the comparator stage 12 is operatively connected to the second end 14 of the first coils 4 to provide the alternative current I ₂ . In substance, the comparator stage 12 compares the tension of the extractor element 6, which defines the current that flows through it, with the reference tension which is linked to the inductor 3 and is the tension saturation value.

As a consequence, whenever the inductor 3 is in saturation, the comparator stage 12 commutates by varying the sense of the electric current that it provides to the inductor 3, that is the alternative electric current I ₂ .

According to another aspect of the invention, between the comparator stage 12 and the inductor 3 is interposed an amplification stage 13. The latter becomes the direct provider of the alternative electric current I ₂ to the inductor 3, while the stage comparator 12 becomes the controller of the amplification stage 13 and the electric current that it has to generate may be particularly low. This simplifies the comparator stage 12 that may be produced, for example, by operational amplifiers. It is apparent that this allows to significantly reduce the electrical power needed to operate the device 1, making it particularly suitable to be used in a passive loop of the 4-20 mA type. Thus, the device 1 includes means for the electrical connection to the electrical circuit of the 4-20 mA passive loop type from which it draws the electricity needed to operate, these connection means generally comprising a terminal. Regarding the measure to be made, it has been previously stated that the electrical current flowing on the extractor element 6 is an alternative current. Hence, also the electric tension across the extractor element 6 is alternative and has the mean value proportional to the electric current to be measured li. It is therefore sufficient to put a low-pass filter element 15, which cuts the alternating component of this tension and allows the passage of the continuous component, operatively downstream the extractor element 6, thereby obtaining the desired measurement. In particular, the continuous component can be detected by an index device connected to the output of the filtering element 15 or by a analogic/digital conversion. In the latter case, a analogic/digital conversion is required to allow the calculation of the obtained digital value in order to perform the scaling adaptation to the 4-20 mA loop, to adapt it to particular threshold values, to make additional filtering operation for cutting the alternative components of the tension of the extractor element 6 that has not been cut by the low- pass filter element 15, and the like. Then, the result of these calculations are converted back into an analogic signal to be inserted in the 4-20 mA loop.

According to another aspect of the invention, the measuring device 1 includes, as can be noticed in FIG. 7, a circuital portion 16 for storing of electric power suitable to accumulate energy when the amount of electric power supplied from said passive loop circuit is more then the electric power amount needed from the device 1 itself, and designed to supply stored electric power to the measuring device 1 when the amount of electric power supplied from the passive loop circuit is lower then the electric power amount required for the operation of the device 1. This further allows to optimize the power consumption of the device 1 of the invention. It is in fact understandable that the variation of the alternative electric current b is converted into a variation of the electrical current that the device 1 tends to absorb from the passive loop circuit. When the value of the current consumption is less than 4 mA, which is the minimum working value of the passive loop circuit, the surplus would be lost. The circuital portion 16 stores this surplus in order to avoid unnecessary losses.

In view of the foregoing, it is understood that the above device fulfils the intended objects and, particularly, overcomes the drawbacks of the known devices, because it is particularly accurate in the measuring and, at the same time, has a very low power consumption.

On close examination, electric consumptions are so limited and optimized that the measuring device of the invention can be used in a passive loop of the 4-20 mA type.

The electronic circuits of the device according to the invention are generally simple to implement, low cost and make it very compact and less bulky.

The device according to the invention is susceptible of a number of changes and variants, within the inventive concept as disclosed in the appended claims. All the details can be replaced by other technically equivalent parts without departing from the scope of the present invention.

While the device has been described with particular reference to the accompanying figures, the numerals referred to in the disclosure and claims are only used for the sake of a better intelligibility of the invention and shall not be intended to limit the claimed scope in any manner.