# Smart Energy Meters for Smart Grids, an Internet of Things Perspective
## 2 Sensing Techniques
### Electricity Meters
#### 2.3.1 Instantaneous Measurements
The sensing of magnetic field uses, as previously stated, the Hall effect, fluxgate, and magnetoresistance sensors. The Hall effect is voltage generated across a thin, conductive current-carrying sheet placed in a magnetic field that inversely depends upon the thickness and directly on the resistance of the plate; hence, thinner plates produce small outputs with fewer ohmic losses with lesser resistance and vice versa for thicker plates [131]. Additionally, Hall effect sensors suffer from misalignment voltage and thermal drift. Hall sensors find usage in power conversion, motor drives, BLDC motor control, position sensing, etc. [132]. Fluxgate sensors sense the magnetic field generated by the current-carrying conductor by driving excitation windings into saturation at opposite polarities, which results in alternating voltage at the sensing coil, the peak of which depends upon the ambient or external electromagnetic field [133], and they can pick up very weak external magnetic fields, providing excellent sensitivity and accuracy [134]. Various techniques for the excitation and sensing of the field have resulted in numerous fluxgate-based sensors [133; 135]. As fluxgate sensors are quite accurate but expensive, difficult to integrate, and consequently costly, they are seldom used in current commercial and consumer electricity metrology; instead, these are used in high-accuracy applications, such as calibration, electromagnetic-based diagnosis systems, precision laboratory equipment, etc. However, sensors for current metrology do exist [136]. Finally, the magneto-resistive sensors are based on the variation of the material resistance as a function of the applied magnetic field [137], the popular magneto-resistive effects being anisotropic magnetoresistance resonance (AMR) and giant magnetoresistance resistance (GMR), with other techniques continually being developed [138]. The AMR phenomenon depends upon the variation of resistance due to the magnitude and direction of the applied magnetic field and the direction of the current flow through it; it is maximum when the electric current and applied magnetic field are perpendicular [139]. The GMR phenomenon depends upon the change in material resistance due to the magnetization state of the ferromagnetic materials separated by a non-magnetic layer; it is minimum when the fields in both layers are parallel and maximum when both fields are anti-parallel [140]. Both of these techniques suffer from thermal drift, nonlinearity, and alteration of the sensing function due to strong external fields. Novel magnetic sensor techniques such as giant magneto impedance (GMI) and tunnelling magneto resistance (TMR) [141] are also being explored for current sensing. These techniques have potential applications in grid sensing, electric vehicle charging, welding, motor drives, and industrial large current sensing, but the state of the art has not reached the maturity for the development of low cost, low complexity sensors for extensive usage [142]. The Faraday effect is another technique to sense magnetic fields using light; in particular, it employs induced birefringence in a material subjected to an external magnetic field. Assuming negligible intrinsic circular birefringence, the rotation induced to a linearly polarized light is then proportional to the path integral of the applied magnetic field [143]. Generally, polarimetric and interferometric techniques are used to detect the polarization change induced in a circularly arranged fiber optic cable by the magnetic field, which, in turn, is induced by the current-carrying conductor passing through the fiber optic coils [144]. The polarimetric technique simply measures the polarimetric state of the light that depends upon the rotation of the polarization vector, which is dependent upon the magnetic field and hence on the current carried by the conductor passing through the optical fiber core [145].