Overview of Current Detection Technology

1. Ohm's Law
(1) Shunt resistance


This kind of topology has certain risks. The low-end detection circuit is easy to cause interference to the ground wire; the high-end detection, resistance and op amp selection requirements are high.
Sensing resistance is a simple current measurement method, which can be used to measure AC current as well as DC current. The disadvantage of using this method for current measurement is that a resistor is connected to the circuit to be tested, which causes power consumption (I^2*R).
You
Low cost, high precision, small size
inferior
The temperature drift is large, the selection of precision resistance is difficult, and there is no isolation effect
(2) TRACE resistance
According to Ohm's law, the voltage across the conductor is proportional to the current passing 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 time, the above equation can be simplified as: J=σE This method uses the TRACE resistance of the conductor in the circuit instead of the shunt resistance to measure the current, which is also an alternative current measurement method.
You
No additional resistance is introduced, and no additional power loss is generated
inferior
The voltage signal generated is very small
If TRACE resistors are used, a high-gain amplifier is needed to amplify the voltage signal, but the bandwidth performance of the amplifier has not been the bottleneck.
Many scholars have conducted a lot of research on the current test performance of TRACE resistors, and the results show that: metal copper has typical thermal drift, so this measurement method is not suitable for high-precision application environments.
(3) Inductance DC resistance
Inductance DC resistance measurement circuit is a kind of lossless sampling circuit. The circuit needs to be accurately debugged before sampling; it is currently only suitable for rough measurement of current. Usually used in switching power supply lossless current measurement and low voltage (less than 1.5V) current measurement occasions.


Figure 1 Principle diagram of inductance measurement
2, Faraday's law of electromagnetic induction
Electromagnetic induction phenomenon refers to the phenomenon of induced electromotive force due to the change of magnetic flux. For example, when a part of the conductor of a closed circuit cuts the magnetic line of induction in a magnetic field, a current (induced current) will be generated in the conductor.
(1) Rogowski coil
Rogowski Coil is a coil that can be directly placed on the conductor to be 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 through the circuit, which can be expressed as:


According to the ampere loop rule, the relationship between the magnetic flux density in the Rogowski coil and the current to be measured can be 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 Rogowski coil without magnetic core
Because there is no ferromagnetic material inside the Rogowski coil, the coil cannot be driven to saturation, so it is a linear device.
Rogowski coil can not only calibrate lower current, but also can be used under very high current. This also 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 in the center of the coil, the above principles can still work normally, but a certain error will occur.
Figure 3 The relationship between the measurement error and the current position to be measured
(2) Transformer measurement
Compared with the Rogowski coil, the advantage of 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 by an analog-to-digital converter without amplifier amplification.
3, magnetic effect
Magnetic sensor is a device that converts the magnetic properties of sensitive components caused by external factors such as magnetic field, current, stress and strain, temperature, light and other external factors into electrical signals, and detects corresponding physical quantities in this way.
It is widely used in modern industry and electronic products to measure physical parameters such as current, position, and direction with the intensity of the induced magnetic field. In the prior art, there are many different types of sensors used to measure magnetic fields and other parameters.
(1) Hall current sensor
Hall effect (Hall effect) means that when a solid conductor (or semiconductor) is placed in a magnetic field and current passes through, the charge carriers in the conductor are biased to one side by the Lorentz force, and then a voltage (Hall voltage )The phenomenon.
where nq is the charge density and d is the thickness of the conductor.


Hall device is a magnetoelectric conversion device made of semiconductor materials. If a control current is applied to the input, when a magnetic field B passes through the magnetic sensing surface of the device, a Hall potential appears at the output.
Indirectly measures the current of the current-carrying conductor by measuring the Hall potential. Therefore, the current sensor has undergone electrical-magnetic-electrical insulation isolation conversion.


Figure 4 Basic schematic diagram of Hall current sensor
(3) Fluxgate current sensor
Fluxgate current sensor has ultra-high measurement accuracy and good temperature stability. But it is susceptible to interference from the external magnetic field brought by the excitation source. Guillermo et al. adopt the form of excitation winding differential to reduce the interference of external magnetic field brought by the excitation source. Due to the transformer effect, the high-frequency excitation source will be coupled to the feedback winding to produce noise interference to the sensor. In order to reduce the interference caused by internal and external magnetic fields, the sensor can use additional magnetic cores and additional coils.


Figure 5 Basic principle of fluxgate sensor
Basic fluxgate sensor, the voltage signal output by the signal coil at terminal P is as follows:


(4) Giant Magnetoresistive Sensor
Sensors based on the giant magnetoresistance effect mainly have three layers of sensing materials: Reference Layer or Pinned Layer, Normal Layer and Free Layer.
GMR sensor is based on the giant magnetoresistance effect, that is, the change in sensor resistance under the action of an external magnetic field. When the positive direction of the magnetic field is zero, the resistance of the magnetoresistive material; when the magnetic field increases in the positive or negative direction, the resistance of the magnetoresistive material decreases.
Since the discovery of giant magnetoresistance GMR, various applications have been in the stage of development and practical application. It was first successfully commercialized on hard disk heads. In addition to directly measuring magnetic fields, it also measures physical quantities such as current, displacement, linear velocity, and acceleration. Get applied.


Figure 6 Structure of giant magnetoresistive sensor
Giant magnetoresistive current sensor has broad application prospects. Compared with traditional electromagnetic current transformers, it can measure current signals from DC to high frequency (MHz level), especially it can measure DC current, which is extremely beneficial for DC monitoring in converter stations in DC transmission systems.
Fourth, conclusion
Different measurement methods have their own advantages and disadvantages. Except for current transformers and Rogowski coils that cannot directly measure DC current, other measurement methods can measure DC current; Trace resistance and inductance resistance measurement current methods are not directly connected to the measurement circuit Shunt resistance, so the impact on the circuit to be measured is relatively small; fluxgate is the current measurement accuracy of measurement technology, and provides some advantages such as electrical isolation and low energy loss.