New-Tech Europe | November 2016 | Digital edition

and ΦB is the magnetic flux. The basic principle of an inductively coupled power-transfer system is shown (Figure 2). It consists of a transmitter coil L1 and a receiver coil L2. Both coils form a system of magnetically coupled inductors. An alternating current in the transmitter coil generates a magnetic field, which induces a voltage in the receiver coil. The efficiency of the power transfer depends on the coupling (k) between the inductors and their quality, defined as their Q factor. The coupling is determined by the distance between the inductors (z) and the ratio of D2/D. The shape of the coils and the angle between them further determines the effective coupling. The performance of a wireless power link can be improved using resonant inductive coupling. Resonance of a circuit involving capacitors and inductors occurs because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, and then the discharging capacitor provides an electric current that builds the magnetic field in the inductor. This process is repeated continually. At resonance, the series impedance of the two elements is at a minimum and the parallel impedance is at maximum. Resonance is used for tuning and filtering, because it occurs at a particular frequency for given values of inductance and capacitance. To cancel the influence of the inductive reactance and the capacitive reactance they should have equal magnitude, ωL = 1/ωC, so:

Figure 6: Free from the strictures of contact, contactless interconnects provide greatly improved flexibility and reliability, while magnetic coupling protects against explosions in gaseous or otherwise flammable environments.

• Improved safety: There is no arcing, which is a major plus in hazardous environments such as gas- filled chambers. • Cost savings: There is no wear and tear thus improving the uptime and reducing maintenance. However, a truly contactless connector must be able to transmit both data and power. For power, there are few options. Capacitive power transfer (CPT) has the advantage of being able to penetrate (floating) metal and has low EMI, but it suffers from low power density and short range. Some generalized comparisons of various wireless options, using pros and cons, are shown for easy reference (Figure 1.) For contactless power transfer, an inductively coupled power transfer (ICPT) option proves to have more pros than cons. It is has high power density at reasonable distance, is well known with widely available product and technology solutions, and high efficiency is possible. The downside is that it cannot penetrate metal. For data transmission, there are a number of options. Capacitive coupling’s low EMI is also an advantage for data transfer, but such coupling requires significant surface-plate area, which can be challenging for tiny, rotating

couplers. Inductive coupling for data suffers from low bit rates. Other options include RF at 60 GHz, 2.45 or 5 GHz, sub-GHz, and ICPT, as well as optical links. Each has pros and cons, as shown in Figure 1. The 2.45-GHz industrial, scientific, medical (ISM) band is also unlicensed, with global acceptance and wide usage, most notably as “wireless Ethernet” under the moniker of Wi-Fi. In the final analysis, it turns out that a hybrid architecture, RF for data and inductive coupling for power, is the best approach for contactless connectivity. Defining induction Inductive power transfer has been with us for quite some time, but for the sake of clarity a quick run through of how it works is useful in understanding its utility as a wireless power-transfer mechanism. Faraday's law of induction states that the induced electromotive force in any closed circuit is equal to the rate of change of the magnetic flux enclosed by the circuit, or mathematically as:

Where L is the inductance in Henrys, C is the capacitance in Farads , and ω = 2πf, in which f is the resonance frequency in Hertz. In low-power systems and for high power efficiency, higher k and Q are required.

Where is the electromotive force (EMF)

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