However, the reader antenna is fixed on the end-effector of a robotic manipulator. Assuming a symmetrical radiation pattern the angle between the reader antenna and the tag can be estimated with an accuracy of 6°–7° deviation. While the antenna is rotating, at some specific angle the tags enter the main lobe of the reader antenna and at some angle later leave this zone. The position of the tags is estimated with the aid of the antenna radiation pattern. The reader antenna is rotated in space and the tag positive encounters are collected. In and a mobile robot is described, featuring an onboard UHF reader module. However, no useful information on the accuracy of theses systems is provided. Following the same principles a reader module is deployed on a moving vehicle to detect its relative position from stationary tags. Tags located closer to the reader antenna respond more frequently than distant ones. In, a positioning methodology based on the response rate of UHF tags is described. These systems offer a rather coarse accuracy of 50–100 cm. The same methodology is used in, where a similar system is deployed on a mobile robot for localization and mapping purposes. In this way, a map is created that indicates the tag positions with the highest probability. These antennas are mechanically rotated to scan a room populated with several tags. In a technique is described for determining the position of UHF tags with the aid of several antennas, each assigned a spatial probabilistic distribution of positive tag encounter. Nowadays common positioning techniques are based on location fingerprinting and normally make use of a Kalman filter, or lie on a predefined antenna radiation pattern and mechanical steering to define the relative angle between the reader antenna and the tag. The same stands for time of travel (TOF) measurements. However, if we solve Friss equation for the distance r and try to calculate the distance between the reader antenna and an RFID tag, we will acquire merely a rough positioning estimation mainly because the RFID signal is extremely narrowband and thus suffers from multipath. Where P TX is the transmitted power, P RX the power detected at the receiver, r is the distance between the two antennas and G TX and G RX are the gain of the transmitting and the receiving antenna respectively. This operation can be summarized by the forward link (reader to tag), the re-modulation of the received signal by the chip and the reverse link (tag to reader). These prerequisites provided the impetus for the vast deployment of ultra high frequency systems (UHF, 860–960 MHz) operating under the backscatter principle, where electromagnetic waves propagating between the reader and the tag antenna are employed to power up the tags. The increasing need for higher bit rates and longer range for supply chain management dictated higher frequencies. The acquired energy is subsequently used to power up an IC chip with integrated memory attached to the tag antenna. Passive RFID tags when located inside the reader’s electromagnetic field acquire energy with the aid of their built-in antenna by means of inductive or radiative coupling. A typical RFID system comprises a reader module, which modulates data and commands into an RF signal, along with an antenna for the signal transmission. The radio frequency identification (RFID) is an automatic data collection technology mainly used for object identification and tracking.
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