Inductive transmission
With inductive transmission , message signals in both directions as well as any energy required can be transmitted jointly and without contact from an interrogation unit to a transponder unit with the aid of electromagnetic induction .
functionality
The theoretical basics for transmission using inductive principles are shown under mutual induction .
The signal and energy transmission from the interrogation unit to the transponder takes place in the simplest case by switching a high-frequency magnetic field on and off , which corresponds to an amplitude modulation . This induces an electrical voltage in the coil of the transponder, which is rectified via a diode. The signal transmission in the opposite direction is mostly realized by load modulation .
properties
The connection is contactless, so in the area of the interface between the two devices there can be no wear due to friction or impairment of function due to corrosion. The electronic components can be hermetically sealed in special materials . This means that you are optimally protected against, for example, the surrounding aggressive liquids or gases.
The galvanic separation associated with the contactless connection enables this type of transmission to be used in areas with explosion protection. When designing, it must of course be noted that sparks could arise between electrical conductors in the vicinity.
Such a contactless connection can also lead to interference effects ( EMC ). Such unwanted couplings are u. a. prevented by magnetic shields made of mu-metal .
Areas of application
One application with only contactless energy transfer, and also with comparatively poor efficiency , is the charging of the accumulators in the waterproof housing of an electric toothbrush . The charge status could be transmitted in the opposite direction.
In the case of inductive signal transmission, on the other hand, the focus is on information transmission. Any energy transfer is used to supply the possibly battery-free transponder. Examples are the cochlear implant in the inner ear, contactless identification systems such as radio frequency identification ( RFID ) and telemetry over short distances.
Magnetic coupling for RFID use
In order to read out an RFID chip, there is always and automatically a transmission and coupling via electromagnetic waves for greater distances compared to the wavelength. However, if the distance between the RFID reading antenna and the RFID chip antenna is small compared to the wavelength, only a magnetic coupling is formed in the near field .
At HF frequencies (usually 13.56 MHz, corresponding to a wavelength of 22.11 m), a purely magnetic coupling results for a large number of the usual applications. The calculation of the coupling can be calculated quite well using the known laws of mutual induction . With the appropriate design of the coupler on the reader and the RFID chip side, it is also useful and customary to work with appropriate coil-shaped constructions. Basically, the reader coil on the reader side will normally be much larger than the coupler on the chip side. Therefore it is i. A. sufficient to analyze the magnetic field of the reader coil, whereby a different relative angular position of the coupler on the chip side must also be taken into account.
At the higher UHF frequencies (usually from approx. 868 MHz to approx. 950 MHz, corresponding to wavelengths of slightly more than 0.3 m), a purely magnetic coupling will only be possible up to about 0.3 m, here it is at However, it is then common for the coupler construction to use customary RFID UHF antenna structures on the RFID chip side, such as folded dipoles. On the RFID reader side, however, the use of coil-shaped structures continues to be considered useful.
For extremely short-range RFID applications, with reading distances in the mm range, a capacitive coupling could also be used as an alternative to magnetic coupling. Numerous applications can be found for this in measuring and counting instruments, but also in identification tasks in machines of all kinds. The magnetic coupling has proven to be advantageous in the majority of all applications because the capacitive connection already has slight tolerances in the distance or in the relative position of read coupler and chip coupler to one another lead to extreme fluctuations in the transmission attenuation of the coupling. In contrast, with magnetic coupling, the coupling attenuation can almost always be specified to within a few dB.
literature
- Georg Bosse, Gunther Wiesemann: Fundamentals of electrical engineering II. The magnetic field and electromagnetic induction . 4th edition. Springer, Berlin 1996, ISBN 3-540-62155-5 .
Individual evidence
- ↑ see for example wireless in Ex areas .