Current sensing

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Diagram of D'Arsonval/Weston type galvanometer. As the current flows from + terminal of the coil to terminal, a magnetic field is generated in the coil.This field is counteracted by the permanent magnet and forces the coil to twist, moving the pointer, in relation to the field's strength caused by the flow of current.

In electrical engineering, current sensing is any one of several techniques used to measure electric current. The measurement of current ranges from picoamps to tens of thousands of amperes. The selection of a current sensing method depends on requirements such as magnitude, accuracy, bandwidth, robustness, cost, isolation or size. The current value may be directly displayed by an instrument, or converted to digital form for use by a monitoring or control system.

Current sensing techniques include shunt resistor, current transformers and Rogowski coils, magnetic-field based transducers and others.

Current sensor[edit]

A current sensor is a device that detects electric current in a wire and generates a signal proportional to that current. The generated signal could be analog voltage or current or a digital output. The generated signal can be then used to display the measured current in an ammeter, or can be stored for further analysis in a data acquisition system, or can be used for the purpose of control.

The sensed current and the output signal can be:

  • Alternating current input,
    • analog output, which duplicates the wave shape of the sensed current.
    • bipolar output, which duplicates the wave shape of the sensed current.
    • unipolar output, which is proportional to the average or RMS value of the sensed current.
  • Direct current input,
    • unipolar, with a unipolar output, which duplicates the wave shape of the sensed current
    • digital output, which switches when the sensed current exceeds a certain threshold

Requirements in current measurement[edit]

Current sensing technologies must fulfill various requirements, for various applications. Generally, the common requirements are:

  • High sensitivity
  • High accuracy and linearity
  • Wide bandwidth
  • DC and AC measurement
  • Low temperature drift
  • Interference rejection
  • IC packaging
  • Low power consumption
  • Low price

Techniques[edit]

The measurement of the electric current can be classified depending upon the underlying fundamental physical principles such as,

  • Faraday's Law of Induction
  • Magnetic field sensors
  • Faraday Effect
  • Transformer or current clamp meter, (suitable for AC current only).
  • Fluxgate sensor, (suitable for AC or DC current).
  • Hall effect sensor(suitable for AC, DC, or pulsating current), a type of current sensor which is based on the Hall Effect phenomenon discovered by Edwin Hall in 1879.
  • Shunt resistor, whose voltage is directly proportional to the current through it.
  • Fiber optic current sensor, using an interferometer to measure the phase change in the light produced by a magnetic field.
  • Rogowski coil, electrical device for measuring alternating current (AC) or high speed current pulses.
  • Giant Magnetoresistance(GMR): Magnetic field sensor suitable for AC & DC Current with higher accuracy than Hall Effect. Placed parallel to the magnetic field.

Shunt resistors[edit]

Ohm's law is the observation that the voltage drop across a resistor is proportional to the current going through it.

This relationship can be used to sense currents. Sensors based on this simple relationship are well known for their lower costs, and reliability due to this simple principle.

Shunt resistor

The common and simple approach to current sensing is the use of a shunt resistor. That the voltage drop across the shunt is proportional to its current flow, i.e. ohm's law, makes the low resistance current shunt a very popular choice for current measurement system with its low cost and high reliability. Both alternating currents (AC) and direct currents (DC) can be measured with the shunt resistor. The high performance coaxial shunt have been widely used for many applications fast rise-time transient currents and high amplitudes but, highly integrated electronic devices prefer low-cost surface mounted devices (SMDs),[1] because of their small sizes and relatively low prices. The parasitic inductance present in the shunt affects high precision current measurement. Although this affects only the magnitude of the impedance at relatively high frequency, but also its effect on the phase at line frequency causes a noticeable error at a low power factor. The major disadvantage of using the shunt is that fundamentally a shunt is a resistive element, the power loss is thus proportional to the square of the current passing through it and consequently it is a rarity amongst high current measurements. Fast-response for measuring high-impulse or heavy-surge currents is the common requirement for shunt resistors. In 1981 Malewski,[2] designed a circuit to eliminate the skin effect and later in 1999 the flat-strap sandwich shunt (FSSS)[3] was introduced from a flat-strap sandwich resistor. The properties of the FSSS in terms of response time, power loss and frequency characteristics, are the same as the shunt resistor but the cost is lower and the construction technique is less sophisticated, compared to Malewski and the coaxial shunt.

The intrinsic resistance of a conducting element, such as a copper trace on a printed circuit board can be used as a sensing resistor. [4] This saves space and component cost. The voltage drop of a copper trace is very low due to its very low resistance, making the presence of a high gain amplifier mandatory in order to get a useful signal. Accuracy is limited by the initial tolerance of manufacturing the trace and the significant temperature coefficient of copper. A digital controller may apply corrections to improve the measurement. [5]

A significant drawback of a resistor sensor is the unavoidable electrical connection between the current to be measured and the measurement circuit. An isolation amplifier can provide electrical isolation between measured current and the rest of the measurement circuit. However, these amplifiers are expensive and can also limit the bandwidth, accuracy and thermal drift of the original current sensing technique. Other current sensing techniques that provide intrinsic electrical isolation may deliver a sufficient performance at lower costs where isolation is required.

Current sensor based on Faraday's Law[edit]

Faraday's Law of induction – that states: the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit – has been largely employed in current sensing techniques. Two major sensing devices based on Faraday’s law are Current transformers (CTs) and Rogowski coils. These sensors provide an intrinsic electrical isolation between the current to be measured and the output signal, thus making these current sensing devices mandatory, where safety standards demand electrical isolation.

Current transformer[edit]

Current transformers used as part of metering equipment for three-phase 400A electricity supply

The CT is based on the principle of a transformer and converts a high primary current into a smaller secondary current and is common among high AC current measurement system. As this device is a passive device, no extra driving circuitry is needed in its implementation. Another major advantage is that it can measure very high current while consuming little power. The disadvantage of the CT is that a very high primary current or a substantial DC component in the current can saturate the ferrite material used in the core ultimately corrupting the signal. Another problem is that once the core is magnetized, it will contain hysteresis and the accuracy will degrade unless it is demagnetized again.

Rogowski coil[edit]

Rogowsky coil

Rogowski coil is based on Faraday’s law of induction and the output voltage Vout of the Rogowski coil is determined by integrating the current Ic to be measured. It is given by,

where A is the cross-sectional area of the coil and N is the number of turns. The Rogowski coil has a low sensitivity due to the absence of a high permeability magnetic core that the current transformer can take advantage of. However, this can be compensated for by adding more turns on the Rogowski coil or using an integrator with a higher gain k. More turns increase the self-capacitance and self-inductance, and higher integrator gain means an amplifier with a large gain-bandwidth product. As always in engineering, trade-offs must be made depending on specific applications.

Magnetic field sensors[edit]

Hall effect[edit]

Simple hall-effect sensors[edit]

Simple hall-effect current sensing

This range of current sensors is based on the principle that whenever a current flows in a conduct a magnetic field is produced around the conductor with a strength directly proportional to the magnitude of that current flowing. A hall-effect magnetic field sensor is then used to measure the induced field with its output being directly proportional to the magnitude of the current flowing. In the simplest configuration, a hall-effect magnetic field sensor can be placed adjacent to the conductor and its output measured but there are limitations. For current levels under about 10 amps, the magnetic field produced is very weak and not a lot stronger than the earth’s magnetic field. Also, the hall voltage produced will be tiny so very high amplification would be required with its associated thermal instability and noise issues.

Differential hall-effect sensors[edit]

Differential hall-effect current sensing

Stray field effects can be mitigated to a high degree by adding a second sensor and measuring their differential output. The precision can be further enhanced by necking down the conductor adjacent to the sensor pair and placing the pair closer together. Even more precision is available if an array of sensors is positioned around the conductor and outputs added.

Open and Closed loop hall-effect sensors[edit]

Open-loop current sensor operation
Closed-loop current sensor operation

A common solution to improve precision is to surround the conductor with a magnetic core which has a slot to accept the hall-effect sensor thus making a traditional open-loop hall-effect current sensor. The function of the core is to trap the magnetic field that surrounds the conductor and focus it through the magnetic field sensor. The core also provides good immunity to stray magnetic fields. However, to function well, the core material does need a range of qualities such as high permeability, low remanence, low loss and with specific saturation characteristics. The open-loop current sensor satisfies the needs of most current sensing applications but if better linearity and thermal stability is required there is a closed-loop option which consists of a winding around the core and a driver circuit to drive current through the winding so as to force the flux within the core to zero. With this structure, the coil drive current will be directly proportional to primary current flow divided by the number of turns on the coil. The coil current is generally measured via a ballast resistor.

Current Sensor Qualities[edit]

Precision of transfer function: Practically, this depends on other sensor qualities. If these are good and stable, the sensor can be calibrated to a high degree. Commonly +/-1%. In a cost sensitive environment, the sensors can be machine calibrated within their final product. This way very high precision can be achieved.

Linearity of transfer function: Most hall-effect magnetic field sensors offer good linearity with many being 0.1% or better over a useful operating range. When a core is added to the equation, linearity can suffer with some saturation effects coming into play. With good material and magnetic circuit design, linearity of better than 1% can be achieved.

Thermal stability of the null voltage: For the manufacturer of the magnetic field sensor, this proves to be a challenge. The best sensors have active thermal compensation of the null voltage and so can achieve stabilities better than 0.1% over a useful temperature range. Many don’t achieve this though. In products where the current is frequently turned off, an in-process null calibration can be performed. The thermal stability of the null often determines the minimum current that can be reliably measured.

Thermal stability of the transfer function: Again, with open-loop current sensors, this is a function of the magnetic field sensor stability. Also core material properties and magnetic circuit design come into play. For open-loop sensors a stability of +/-2% over their specified temperature range is achievable. If a higher degree of stability is required, closed-loop current sensors offer very good stability as drifts due to the magnetic field sensor and magnetic material are taken out of the equation.

Hysteresis: This comes about from remanence effects in the core material. Low cost core materials may have hysteresis levels of 0.5% after a full current transient. This does decay with time and is dependant on the transient magnitude. Nano-crystalline materials have much less, tending towards negligible. An additional means to reduce hysteresis is to add primary turns to the limit of the maximum current which needs to be sensed.

Stray flux immunity: Single sensor, coreless has the poorest performance, but where the primary current is high and there are no nearby current carrying conductors, performance may be adequate where a low cost solution is important. Multi-sensor coreless often have adequate performance. Cored sensors with a small air gap and built with material of high permeability have best performance. Flux-gate sensors are better again.

Frequency response: Historically, there is a trend that the most precise open-loop current sensors have the poorest frequency response of around 1kHz. More modern hall sensors now can combine both stability and frequency response. Some going as high as 1MHz and more. Closed-loop sensors often have good frequency response going to hundreds of kHz. For overload detection applications, transient response time is important. Sub 1 microsecond is achievable which generally is more than fast enough considering the hardware necessary to turn off the current that fast and manage the induced transients.

Size: The modern environment demands small size. Coreless is definitely the winner here. High current cores are bulky and expensive.

Cost: Lowest cost are single sensor, coreless SMT devices and the cost moves up from there. There is a significant step from open-loop to closed-loop. High current coreless are significantly lower cost than cored sensors and that included current transformers.

Fluxgate sensors[edit]

Fluxgate Technology principle

Fluxgate sensors or Saturable inductor current sensors work on the same measurement principle as Hall-effect-based current sensors: the magnetic field created by the primary current to be measured is detected by a specific sensing element. The design of the saturable inductor current sensor is similar to that of a closed-loop Hall-effect current sensor; the only difference is that this method uses the saturable inductor instead of the Hall-effect sensor in the air gap.

Saturable inductor current sensor is based on the detection of an inductance change. The saturable inductor is made of small and thin magnetic core wound with a coil around it. The saturable inductor operates into its saturation region. It is designed in such a way that the external and internal flux density will affect its saturation level. Change in the saturation level of a saturable inductor will alter core’s permeability and, consequently, its inductance L. The value of saturable inductance (L) is high at low currents (based on the permeability of the core) and low at high currents (the core permeability becomes unity when saturated). When interpretating Fluxgate detectors, it needs to consider the property of many magnetic materials to exhibit a non-linear relationship between the magnetic field strength H and the flux density B.[6]

In this technique, high frequency performance is achieved by using two cores without air gaps. One of the two main cores is used to create a saturable inductor and the other is used to create a high frequency transformer effect. In another approach, three cores can be used without air gap. Two of the three cores are used to create saturable inductor, and the third core is used to create a high frequency transformer effect. Advantages of saturable inductor sensors include high resolution, high accuracy, low offset and gain drift, and large bandwidth (up to 500 kHz). Drawbacks of saturable inductor technologies include limited bandwidth for simpler design, relatively high secondary power consumption, and risk of current or voltage noise injection into the primary conductor.

Magneto-resistive current sensor[edit]

A magneto-resistor (MR) is a two terminal device which changes its resistance parabolically with applied magnetic field. This variation of the resistance of MR due to the magnetic field is known as the Magnetoresistive Effect. It is possible to build structures in which the electrical resistance varies as a function of applied magnetic field. These structures can be used as magnetic sensors. Normally these resistors are assembled in a bridge configuration to compensate for thermal drift.[7] Popular magneto resistance-based sensors are: Anisotropic Magneto Resistance (AMR), Giant Magneto Resistance (GMR), Giant Magneto Impendence (GMI) and Tunnel Magneto Resistance (TMR). All these MR-based sensors have higher sensitivity compared to Hall-effect sensors. Despite this, these sensors (GMR, CMR, and TMR) are still more expensive than Hall-effect devices, have serious drawbacks related with nonlinear behavior, distinct thermal drift, and a very strong external field can permanently alter the sensor behavior (GMR). GMI and TMR sensors are even more sensitive than GMR based sensors, and are now in volume production at a few manufacturers.(TDK, Crocus, Sensitec, MDT)[8]

See also[edit]

References[edit]

  1. ^ Costa, F.; Poulichet, P.; Mazaleyrat, F.; Labouré, E. (1 February 2001). "The Current Sensors in Power Electronics, a Review". EPE Journal. 11 (1): 7–18. doi:10.1080/09398368.2001.11463473. ISSN 0939-8368. S2CID 113022981.
  2. ^ Malewski, R.; Nguyen, C. T.; Feser, K.; Hylten-Cavallius, N. (1 March 1981). "Elimination of the Skin Effect Error in Heavy-Current Shunts". IEEE Transactions on Power Apparatus and Systems. PAS-100 (3): 1333–1340. Bibcode:1981ITPAS.100.1333M. doi:10.1109/tpas.1981.316606. ISSN 0018-9510. S2CID 43833428.
  3. ^ Castelli, F. (1 October 1999). "The flat strap sandwich shunt". IEEE Transactions on Instrumentation and Measurement. 48 (5): 894–898. Bibcode:1999ITIM...48..894C. doi:10.1109/19.799642. ISSN 0018-9456.
  4. ^ Spaziani, Larry (1997). "Using Copper PCB Etch for Low Value Resistance". Texas Instruments. DN-71.
  5. ^ Ziegler, S.; Iu, H. H. C.; Woodward, R. C.; Borle, L. J. (1 June 2008). "Theoretical and practical analysis of a current sensing principle that exploits the resistance of the copper trace". 2008 IEEE Power Electronics Specialists Conference. pp. 4790–4796. doi:10.1109/PESC.2008.4592730. ISBN 978-1-4244-1667-7. S2CID 22626679.
  6. ^ LEM International SA (June 2011). "High Precision Current Transducers Catalogue".
  7. ^ Ziegler, S.; Woodward, R. C.; Iu, H. H. C.; Borle, L. J. (1 April 2009). "Current Sensing Techniques: A Review". IEEE Sensors Journal. 9 (4): 354–376. Bibcode:2009ISenJ...9..354Z. doi:10.1109/jsen.2009.2013914. ISSN 1530-437X. S2CID 31043063.
  8. ^ "From Hall Effect to TMR" (PDF). Crocus Technology. August 2021.