"Although many instruments can accurately measure small DC current (maximum 3a), few instruments can accurately (better than 1%) measure DC current above 50A. Such a large current range is the typical load value of electric vehicles (EV), grid energy storage and photovoltaic (photoelectric) renewable energy devices. In addition, these systems need to accurately predict the state of charge (SOC) of related energy storage batteries. The estimation of charge state can be realized according to the measurement of current and charge (Coulomb count), and accurate measurement data is a necessary condition for accurate charge state estimation.
In general, any system designed for current or charge measurement includes built-in data acquisition components, such as appropriate amplifiers, filters, analog-to-digital converters (ADCs), etc. The current sensor is used to detect the current. The output of the current sensor needs to be converted into a usable form (i.e. voltage) through a circuit. Then the signal is filtered to reduce electromagnetic and RF interference. Then zoom in and digitize. Then multiply each current data sample by an appropriate time interval (through digital calculation) to accumulate and calculate the charge value.
On the other hand, if digitization is performed at a constant frequency, the accumulated current sample is first multiplied by an appropriate time interval when the accumulated charge value is read out or utilized in some way. At the same time, it is necessary to consider selecting the appropriate minimum Nyquist sampling rate and using a sufficiently narrow anti aliasing filter before the analog-to-digital converter.
Figure 1: signal chain in a typical modern current measurement system.
Practical sensor technology for high current measurement
Among the technologies used to measure high current, two sensor technologies are the most common. The first technique is to detect the magnetic field around the current carrying conductor. The second technique is to measure the voltage drop across the resistance (often called a shunt) carrying the current (and charge) to be measured. This voltage drop follows Ohm's Law (v = I) × R）。
The device used for high current measurement is usually called Hall effect current sensor. The sensor has a built-in current carrying element. When current and external magnetic field are applied to the element, there will be a pressure difference perpendicular to the current direction and the external magnetic field direction on both sides of the element. The Hall effect pressure difference in ordinary metals is very small. It should be noted that not all DC current sensors measuring the magnetic field around the current carrying conductor are based on Hall effect. The differences between them are briefly described below.
High current Hall effect sensor
In order to make a current sensor with Hall effect device, it is necessary to use a magnetic core to concentrate the magnetic field around the conductor current, and a slot should be opened in the magnetic core to accommodate the actual Hall element. A relatively small slot (relative to the entire length of the magnetic circuit) will form a nearly uniform magnetic field perpendicular to the plane of the hall element. When the hall element obtains current energy, it will produce a voltage proportional to the excitation current and the magnetic field of the magnetic core. The Hall voltage is amplified and output from the output of the current sensor.
Figure 2: schematic diagram of magnetic field around conductor, linear open-loop Hall effect sensor and closed-loop sensor.
Since there is no electrical connection between the current carrying conductor and the magnetic core (only the magnetic field is coupled), the sensor is actually isolated from the circuit to be tested. The current carrying conductor may have a high voltage, and the output of the Hall effect current sensor can be safely connected to the grounding circuit or to the circuit with any potential relative to the current carrying conductor. Therefore, it is relatively easy to provide gaps and creepage values that meet the most stringent safety standards.
However, this linear sensor also has some disadvantages. Perhaps the least important disadvantage is the fact that Hall effect sensors require a constant excitation current. In addition, amplification and adjustment circuits that process signals from Hall effect sensors usually consume significant energy. Of course, this energy consumption may not be so significant, depending on the specific application. Nevertheless, the energy consumption of Hall sensor for continuous current measurement cannot be as small as milliwatt.
Hall effect sensor: large drift and small working temperature range
Because the output of a typical linear sensor is measured proportionally (depending not only on the measured magnetic field strength, but also on the excitation current value), the stability of the excitation current will greatly affect the current amplitude to be measured and the zero offset when there is no current flow. In general, the latter two depend on the stability of the supply voltage and temperature changes (because the resistance of the hall sensing element affecting the excitation current and Hall voltage itself depends on the operating temperature).
A sensor variant that measures the excitation current and takes this factor into account in the output is possible. But it requires precise external components and large processing circuits. Moreover, the Hall voltage is a nonlinear function of the magnetic field to be measured, which further increases the error of the sensor.
Because different errors will occur under different conditions, most linear Hall effect device manufacturers will decompose the total error into many individual components. It is sometimes difficult to calculate the total synthesis error.
Closed loop current sensor
In order to solve the nonlinear problem of Hall sensor, another technology has been developed in the industry. This technique relies on detecting the presence or sign of the magnetic field in the sensing core, rather than measuring the strength of the magnetic field. In addition, it can avoid the measurement error caused by the unstable excitation current in the hall element.
This technique is to add a winding on the magnetic core to generate a magnetic field with opposite sign, but the strength is exactly equal to the magnetic field generated by the current to be measured. Hall sensing elements are now used only to detect magnetic field symbols, not magnetic field strength. This winding is connected in a circuit with an operational amplifier. The circuit maintains the current in the compensation winding and makes the magnetic field sensed by the hall sensor zero. The current in the compensation winding is many times smaller than that in the conductor to be measured (perhaps more than 1000 times). This function can be realized only by winding a few more turns on the magnetic core when making the winding, and the number of turns can be accurately controlled.
In view of the role of compensation winding in operational amplifier feedback loop, this current sensor is often called "closed-loop" sensor. On the contrary, the aforementioned simple linear Hall effect sensors are often considered as "open-loop" sensors in order to emphasize that there is no feedback mechanism in their working process.
In Hall effect devices, the (offset) error in detecting zero magnetic field cannot be reduced to any small value, which is due to various drift, and most of it is due to temperature dependent drift. This is why some high-performance current sensors do not rely on Hall effect. However, these sensors are generally still called Hall effect sensors, just because they are very similar to Hall effect devices in appearance.
Other magnetic field detectors
In non Hall devices, some sensors based on various physical phenomena can be used to perform the function of magnetic field detector. One technique is based on the magnetoresistance effect, that is, when a magnetic field is applied to the sensor, the resistance of the sensor will change.
Another technique for magnetic field detector uses the nonlinear properties of ferrite between magnetic field strength (represented by H), magnetic flux density (represented by B) and a special phenomenon called saturation. As the H field increases, the magnetic flux density B will eventually reach a point where it will no longer increase significantly -- this point is called the saturation point. Some specially formulated materials have very low saturation points and are widely used in devices called fluxgates.
In fact, a fluxgate based sensor can convert a constant magnetic field into a "gating" or "chopping" magnetic field that alternates between full-scale and almost zero. This magnetic field change can be easily picked up by a winding on the magnetic core and amplified by an AC amplifier. Finally, the so-called synchronous detection (because the circuit itself will control the cutting action) technology is used to restore the value proportional to the constant magnetic field to be measured.
It is worth noting that the complexity of the mechanical structure and related circuits of this sensor is much higher than that of the closed-loop sensor. In addition, their work is very difficult - current measurement when the sensor does not obtain energy or the compensation winding circuit is open due to the loose connection with the external detection resistance - often leads to the irrecoverability of offset and gain indicators. Since the compensation winding cannot counteract the magnetic field from the current to be measured, the magnetic elements in this sensor will be permanently magnetized.
Precision resistors are required
The output signal of the closed-loop sensor is the current in the compensation winding (its value is many times smaller than the current to be measured). This current is usually converted into a voltage value for further processing and digitization. At this time, just use ordinary resistance.
However, the accuracy and stability of this resistance will directly affect the accuracy and stability of the closed-loop current sensor. If a detection resistor with 1% accuracy is used, the closed-loop sensor with a basic accuracy of 0.0001% will soon be reduced to 1% accuracy.
However, it is difficult to purchase a certain number of commercial resistors with an accuracy higher than 0.01%, even if they only work in a very narrow temperature range.
High current shunt
As mentioned earlier, the second current measurement technique uses a voltage drop across the resistance. When determining the current according to Ohm's law, a unique set of factors need to be considered, which is related to the size of the current. For relatively small currents, the voltage drop on the shunt resistor can be made quite large to overcome any error caused by the heat dissipation of the detection connection and shunt resistor or the temperature difference formed from the working environment. However, when the current exceeds 50a, heat dissipation and thermoelectric error are the most important. Similarly, since the shunt resistance is always heated by the current flowing through and may work in an unstable temperature environment, the stability of the shunt resistance relative to temperature is particularly important.
Physical composition of diverter
At first glance, the shunt device is a simple resistor. Some conductive materials with appropriate properties in volume resistivity, (temperature and time) stability and suitable mechanical shape can be used as shunt resistance. The low-precision shunt resistance can be a length of wire or a rectangular shape constructed of a suitable alloy, and simply welded (or electrically connected) with the current carrying conductor in series. However, it is almost impossible to insert such a shunt element into the measuring circuit without affecting its resistance (due to changes in the number of solder at the connection point or changes in connection mechanical details).
In addition, for reasons of stability, it is very beneficial to arrange the shunt resistors in such a way that the current density in any given cross section of the shunt resistor is mostly uniform. This prevents the formation of so-called hot spots, defined as the internal area of shunt resistance at a higher temperature than the rest of the material. In addition to simple resistance changes, the rising high temperature at the hot spot may bring the resistive material to the annealing point temperature, at which the material resistance (achieved by careful control of chemical composition and treatment) may begin to change permanently.
Even if the actual existence of hot spots does not affect the accuracy, it is impossible to ensure that they are formed in exactly the same place when calibrating shunt resistors. Therefore, the design of shunt resistance includes the method of evenly distributing current on the cross section of resistive material or between a single parallel resistive part and each part.
This is why most high-precision shunt resistors are composed of three different parts: two areas are terminals for accessing circuits (almost always made of thick high conductivity materials, such as copper), and another area or multiple parallel areas constitute most of the shunt resistance. The two terminal areas are connected by resistance sections or sections using welding or metallurgical processes, with very uniform joints.
The resistive part (also known as the effective part) material of precision shunt resistance must have low temperature dependent impedance characteristics. Due to the appropriate resistance and low temperature resistance coefficient (TCR), one of the most common alloys for precision shunt resistance is manganin developed by Edward Weston (famous for developing electrochemical battery Weston battery) in 1892.
Heat dissipation in shunt resistor
The heat emitted by the resistance is proportional to the sum of squares of the current (w = I2 × R）。 For example, the power consumption of a 1m Ω shunt resistor when flowing through 50A current is 2.5W, which is a controllable value with moderate radiator and still air. On the contrary, when the current is 1ka, the same shunt resistance will dissipate 1kW of heat, which requires a device with large physical size and may be forced air cooling (or liquid cooling).
Figure 3: relationship between heat emitted in shunt resistance and resistance and current.
Figure 4: relationship between heat emitted in shunt resistance and full-scale output voltage and current.
It should be clear from the above figure that the only way to reduce the heat dissipation in the shunt resistance under a given current condition is to reduce its resistance. However, this will also reduce the voltage value measured on the shunt resistance, and the signal will become more sensitive to the error caused by the shunt resistance and the detection circuit, resulting in the deterioration of accuracy in the case of small current.
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