Wireless power transmission in sensor transponder systems

Wireless power transmission is becoming widespread in different RFID applications such as inventory management, supply chain monitoring, or access control to buildings. These passive transponder systems do not require a battery, and are therefore small, cheap, and low maintenance. Sensor transponders place higher demands on the power supply, because the sensor and the additional analog circuitry lead to a larger current consumption. The tags are often used in harsh, metallic environments such as in industrial or automotive applications. Other sensor transponders serve as medical implants, surrounded by human or animal tissue. The choice of the carrier frequency is a key design decision during transponder system development, as it largely etermines the physical power transmission mechanism. The optimum frequency depends on a number of system constraints such as the range, the data rate, the antenna size, or the environment conditions. This paper presents a comparison of different power transmission solutions. Three practical systems were developed, and each powering approach is presented in detail. The low frequency (LF) sensor transponder system uses a carrier frequency of 133 kHz, which enables the operation in a metallic environment. The high frequency (HF) system is used for medical implants, where the magnetic field is attenuated by surrounding tissue. Finally, the UHF system offers the advantages of a significantly larger range and higher data rates.