New-Tech Europe Magazine | Feb 2017

Sensors Special Edition

Figure 5. A contactless connectivity option implemented on a robotic arm allows 360 degrees of freedom with no brush wear typical of current slip-ring designs. Integrated sensors also enable “gentle touch” sensitivity. [Image courtesy of TE Connectivity]

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: 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. Applications of inductive coupling Taking inductive coupling a step further, the idea of using it to

transmit power wirelessly has been around since the mid 19th century. Nikola Tesla initially experimented successfully with the lighting of gas-discharge lamps wirelessly over a distance of approximately 15 feet. This sparked interest in wireless power transfer technology and applications involving microwaves, lasers, and solar cells capable of transmitting power from space. Closer to home, modern power mats used to charge mobile devices use resonant inductive coupling, but use a "handshake" between the charging surface and the device, and then energy is transferred to the device. It is an intelligent system and will only send power to identified devices and only at a rate determined by the charging profile of the device’s battery. Inductive power transfer is also the operating principle behind passive RFID tags, toothbrushes, and contactless smart cards. Integrating wireless power and data The principle challenges with a contactless connector are

integrating the power coils and near-field antenna into a very small form factor that is relatively easy to manufacture. This requires knowledge of mechanical design and power electronics, as well as magnetics, RF circuit design and antennas. The power-transmit portion takes the 24-V DC supply, puts it through a circuit protection section, followed by a DC-DC converter and a DC-AC converter. The converter output feeds the transmit primary coil, which has a capacitor in parallel as part of a resonant tank that allows it handle variable loads and distance. The receiver side also contains a resonant tank. The received power is rectified, put through a DC-DC converter to deliver 24 V DC to the point of load. The inductive power link itself has an efficiency of approximately 95%, while the output power is always 12 W. The overall system efficiency depends on the data link and includes the losses on the board, e.g. through the DC-DC conversion.

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