Summary of current detection technology

Nowadays, the technology of current detection is being improved by the development of industry. However, it is not a traditional solution that is not desirable, and there is still a place to use it in different application environments. Current sensing is typically used to perform two basic functions of measuring "how much" current and determining when the current is "too large".
 

First, Ohm's law

(1) Shunt resistor

This kind of topology has certain risks. The low-end detection circuit is easy to cause interference to the ground line; the high-end detection, resistance and op amp selection requirements are high.

The sense resistor is the simplest method of current measurement and can be used to measure both AC current and DC current. The disadvantage of using this method for current measurement is that a resistor is connected to the loop to be tested, resulting in power consumption (I^2*R).

Excellent: low cost, high precision, small size

Inferior: the temperature drift is large, the selection of precision resistors is difficult, no isolation effect

(2) TRACE resistance

It is shown by Ohm's law that the voltage across the conductor is proportional to the current through the conductor. For resistive substances, the law can be derived as: J = σ(E + v × B).

Where J is the current density, E is the electric field strength, v is the charge flow velocity, B is the magnetic flux density acting on the charge, and σ is the conductivity of the material. At this point, the above formula can be simplified as: J=σE

This method is also an alternative method of current measurement by using the TRACE resistor of the conductor in the circuit instead of the shunt resistor to measure the current.

Excellent: no additional resistance is introduced, no additional power loss is generated

Inferior: the generated voltage signal is very small

If a TRACE resistor is used, a high gain amplifier is needed to amplify the voltage signal, but the bandwidth performance of the amplifier has not been able to break through the bottleneck.

Numerous experts and scholars have conducted a lot of research on the current test performance of TRACE resistors. The results show that metal copper has typical thermal drift, so this measurement method is not suitable in high-precision application environment.

(3) Inductive DC resistance

The inductor DC resistance measuring circuit belongs to a non-destructive sampling circuit. The circuit needs to be accurately debugged before sampling; currently it is only suitable for rough measurement of current. It is commonly used in switching power supply lossless current measurement and low voltage (less than 1.5V) current measurement.

Figure 1 Inductance measurement schematic

Second, Faraday's law of electromagnetic induction

The phenomenon of electromagnetic induction refers to a phenomenon in which an induced electromotive force is generated due to a change in magnetic flux. For example, when a part of a conductor of a closed circuit performs a motion of cutting a magnetic induction line in a magnetic field, a current (inductive current) is generated in the conductor.

(1) Rogowski coil

Rogowski Coil is a coil that can be placed directly on the conductor being measured to measure AC current. In fact, it is a special type of transformer, usually used to measure AC high voltage and instantaneous current.

The magnitude of the induced electromotive force in any closed circuit is equal to the rate of change of the magnetic flux passing through the circuit, which can be expressed as:

Determined by the Ampere loop, the relationship between the magnetic flux density in the Rogowski coil and the current to be measured is obtained:

B is the magnetic flux density, r is the radius of the Rogowski ring, u0 is the magnetic constant, and ic is the current to be measured.

Figure 2 Schematic diagram of the coreless Rogowski coil

Since there is no ferromagnetic material inside the Rogowski coil, the coil cannot be driven to saturation and is therefore a linear device.

Rogowski coils not only calibrate lower currents, they can also be used at very high currents. This further reduces the difficulty of operation and the cost of calibrating high currents.

However, this method also has disadvantages: when the current to be measured is not at the center of the coil, the above principle can still work normally, but only a certain error will occur.

Figure 3 Relationship between measurement error and current position of the current to be measured

(2) Transformer measurement

Compared with the Rogowski coil, the advantage of the current transformer measurement is that the output voltage is proportional to the current to be measured; at the same time, the influence of the position change of the coil to be measured on the measurement accuracy is suppressed. The measured output signal can be directly sampled using an analog to digital converter without amplifier amplification.

Third, the magnetic effect

A magnetic sensor is a device that detects a change in magnetic properties of a sensitive component caused by external factors such as a magnetic field, a current, a stress strain, a temperature, and light, and detects the corresponding physical quantity in this manner.

It is widely used in modern industrial and electronic products to measure magnetic field strength to measure physical parameters such as current, position, and direction. In the prior art, there are many different types of sensors for measuring magnetic fields and other parameters.

(1) Hall current sensor

Hall effect means that when a solid conductor (or semiconductor) is placed in a magnetic field and a current is passed, the charge carriers in the conductor are biased to one side by the Lorentz force, which in turn generates a voltage (Hall voltage )The phenomenon.

Where nq is the charge density and d is the thickness of the conductor.

The Hall device is a magnetoelectric conversion device made of a semiconductor material. If a control current is applied to the input, when a magnetic field B passes through the magnetically sensitive surface of the device, a Hall potential appears at the output.

The magnitude of the current carrying conductor current is measured indirectly by measuring the magnitude of the Hall potential. Therefore, the current sensor is subjected to an electro-magnetic-electrical insulation isolation conversion.

Figure 4 Basic schematic diagram of Hall current sensor

(2) Fluxgate current sensor

Fluxgate current sensors have ultra-high measurement accuracy and good temperature stability. However, it is susceptible to interference from external magnetic fields caused by the excitation source. Guillermo et al. use the form of excitation winding differentials to reduce the interference of external magnetic fields from the excitation source. Due to the transformer effect, the high frequency excitation source is coupled into the feedback winding to cause noise interference to the sensor. In order to reduce the interference caused by internal and external magnetic fields, the sensor can use additional cores and additional coils.

Figure 5 Basic principle of fluxgate sensor

The basic fluxgate sensor, the voltage signal output by the signal coil at the P terminal is as follows:

(3) Giant magnetoresistive sensor

Sensors based on giant magnetoresistance have three main layers of sensing materials: Reference Layer or Pinned Layer, Normal Layer and Free Layer.

The GMR sensor is based on the giant magnetoresistance effect, which is the change in the sensor resistance under the action of an external magnetic field. When the magnetic field is positive in the positive direction, the resistance of the magnetoresistive material is maximum; when the magnetic field increases in the positive or negative direction, the resistance of the magnetoresistive material decreases.

Since the discovery of the giant magnetoresistance GMR, various applications have been in the stage of development and practical use. Firstly, it has been successfully commercialized on the hard disk head. In addition to directly measuring the magnetic field, the measurement of physical quantities such as current, displacement, linear velocity and acceleration is also Get the app.

Figure 6 giant magnetoresistive sensor structure

Giant magnetoresistive current sensors have broad application prospects. Compared with the traditional electromagnetic current transformer, it can measure the current signal from DC to high frequency (on the order of MHz), especially it can measure the DC current, which is very beneficial for the monitoring of DC in the converter station in the DC transmission system.

Fourth, the conclusion

Different methods of measurement performance have their own advantages and disadvantages. In addition to the current transformer and Rogowski coil can not directly measure DC current, other measurement methods can measure DC current; Trace resistance and inductance resistance method for measuring current is not directly connected to the measurement circuit. The shunt resistor has a relatively small influence on the circuit to be measured; the fluxgate is currently a measurement technique with high measurement accuracy and provides some advantages such as electrical isolation and low energy loss. (Author: magtron lennon)