The RF in RFID : passive UHF RFID in practice
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It does however provide accuracy higher than the required for adjusting the DC voltage. In Fig. It should be noted that if a tag is located outside of the HPBW half power beam width it does not necessarily mean that the power will not suffice for the RFID communication to take place. The HPBW shows how densely the power is distributed along the direction of the main lobe and is in this case a figure of merit illustrating the expected trace width for a single measurement.
As further on demonstrated the width of the acquired trace is dependent upon the distance between the tag and the antenna array. Attributes of the developed scanned array [ 17 ]. Within this span the directivity yields a maximum of 5. This antenna gain drop-off occurring when the main lobe shifts to either side will eventually alter the outcome of the measurement and needs therefore to be compensated for. If we consider the model presented in Fig.
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In other words, considering that all other variables are kept constant, we want to determine the scan angle a, for which the receiver is illuminated by the maximum gain of the transmitter. It can be observed that at high scan angles, the estimated values are as expected about 0. Far-field measurement test bench [ 17 ]. Calculation of the angular tag position, scan angle a est. Maximum positioning deviation for various tag distances [ 17 ]. We can see that the accuracy of the system remains almost constant with regard to the distance between the reader and the tag antenna, presenting a maximum deviation of 1.
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The maximum deviation increases drastically when the multipath effect strongly attenuates the signal. It is therefore critical for the system to be able to recognize when the multipath effect is deteriorating the positioning accuracy. This can be achieved by reviewing the "completeness" of the acquired trace. By observing inside the trace it is clear that when strong reflections come into effect, for several scan angles the system fails to deliver the RSSI and thus the trace is incomplete.
From the previous analysis it is clear that the operation of the developed tracking system requires that both the tag and the reader antenna array are stationary. If we used a reader module offering an interrogation rate of 5 times higher than the employed one around responses per second and consider deteriorating the scan angle resolution, for example one measurement every 0.
Trace width for various tag positions as in [ 17 ]. As expected the trace width yields higher values meaning that it covers a wider angle span, when the tag is located closer to the reader antenna. This effect can assist in detecting slow moving tags even if the interrogation rate is low. In that case we can determine whether the tag is approaching or moving away from the antenna by continuously measuring the trace width.
Although this principle does suffer from multipath, the positions where low accuracy is inevitable, can be determined by reviewing the "completeness" of the trace and be left out. In that case we can examine the completeness of the trace and eliminate it. In this way the position of one tag can be effectively determined with an accuracy defined by the estimated scan angles a est. Note that although this approach appears to be in need of moving parts, thus resembling the one described in [ 12 ], we can employ a second scanned array located at the distance from the first one, or develop a scanned array with more elements and use each time different combinations.
Moreover, an excellent example would be the navigation of automated guided vehicles AGV inside an indoor environment, where several tags are fixed at predefined positions. In that case the tracking system would be able to map the previously unknown environment and estimate the position of the AGV. Herein an RFID positioning system utilizing beam forming was demonstrated.
Such novel system can assist in maximizing the power absorbed by the tag by identifying its relative position and subsequently adjusting the beam direction without having to mechanically steer the antenna. Additionally, it can assist in applications where the handling of packaged products is carried out by automated manipulators. For this scenario a short break in the material flow is necessary to provide high precision. By conducting measurements from two different positions in space, it is possible to define the absolute position of a tag on a 2D Cartesian system through a triangulation algorithm.
The width of the acquired trace can be employed to detect the movement of the tag. However, high interrogation rates are necessary. The use of even higher frequencies will allow the construction of a more complex tracking system featuring smaller antennas and increased directivity. Furthermore, at higher frequencies shorter wavelengths the electrical separation between a tag and reflecting objects increases [ 28 ], eventually leading to increased precision.
Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author s and the source are credited. Skip to main content Skip to sections.
Advertisement Hide. Download PDF. Open Access. First Online: 23 June The operation of ultra-high frequency UHF systems is based on electromagnetic waves propagating between antennas. The operation principle of the developed tracking system lies upon electronically adjusting the shape of the radiation pattern of the reader antenna, in such way as to divert more power towards a desired direction in space. This evidently requires a directive antenna that mainly radiates power within a narrow area main lobe. It is apparent that a tag located in the direction of the main lobe will receive more power and is therefore more likely to respond back to the reader.
Moreover, the amplitude of the signal backscattered from a tag that is located closer to the center of the main lobe will be higher than for a tag that is further away from the center. We can therefore track the angular tag position with respect to the reader antenna by electronically adjusting the direction of the main lobe and collecting the positive tag responses along with the RSSI, which is the amplitude of the backscattered signal from the tag, as measured at the reader module.
Open image in new window. Electronically-steered antenna arrays typically feature several elements, more than often aligned in two dimensions. It is safe to assume that the antenna phase of the developed tracking system is located at the geometrical center of the array.
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In other words the antenna phase lies at the center of the middle element, which was used as the reference point for the measurements. As depicted in Fig. Note that we can achieve higher directivity by employing more elements or simply by increasing the spacing between the existing ones. This would however increase the size of the antenna array and hence also the boundary of the near-field. Considering that the introduced tracking principle is based on the pattern formation, it is essential to operate in the antenna far-field. Therefore, we would like to keep the near-field boundary as close to the antenna as possible, to be able to detect the position of less distant tags.
An overview on the near-field and far-field regions of an arbitrary antenna can be found in [ 21 ]. For the elements of the antenna array, we developed an aperture coupled microstrip antenna, known for being highly directive and especially suitable for pattern forming [ 21 ]. Such antennas principally suffer from narrow bandwidth [ 22 ].
The developed antenna, as well as the RL are presented in Fig.
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This operation is illustrated with the aid of a simulation model in Fig. It is clear that we need to split the signal going out of the reader module and then feed it to each antenna element after applying the respective phase shift. Their operation is based on the varactor diode principle, where a DC voltage is utilized to control the phase of the signal as described in [ 26 ]. This deviation eventually affects the overall accuracy of the system.
Chapter 5: UHF RFID Tags | Engineering
The steering controller is presented in Fig. Considering that the introduced principle for tracking the position of a UHF RFID tag with the aid of beam-steering is based on the pattern formation, it is essential for the tracking system to provide consistent pattern characteristics over a desired scan angle span. The scan angle itself is however not a controllable variable.
Furthermore, as illustrated in Fig. To calculate the gain of the scanned array with the aid of the power of the received signal, we used Eq. If we increased the distance r at the measurement, these minima would also decrease significantly. From the diagrams in Fig. The result of this least square fitting is a smoothed curve, which features a maximum that should point to the actual maximum gain of the system.
The results for both cases are presented in Fig. The measurement basically included continuous interrogation for positive tag responses within the determined angular span. As soon as one scan angle was reviewed, the result negative, or positive plus the RSSI was stored and the beam was adjusted to the next scan angle. However, note that this process can be accelerated either by employing a reader module that offers higher interrogation rate or by deteriorating the scan angle resolution.
The measurement setup is presented in Fig. As illustrated in Fig. Additionally, radiation absorbent material RAM was employed to minimize the interference due to reflected signals. To avoid polarization mismatches, the tag was fixed on a wooden pole in vertical alignment. A set of pre-processed results is presented in Fig. To estimate the angular tag position relative to the antenna, we firstly applied a least-square fitting with second order polynomials and estimated the scan angle a est for the maximum RSS indicator as depicted in Fig.
However, since the received backscattered signal is dependent upon the square of the gain of the scanned array, this time we filtered away all the values 6 dB lower than the detected maximum. As expected the result contained a systematic error which was defined by the system characteristics and could be alleviated with the aid of a compensation algorithm. Furthermore, a random error was included mainly due to the phase unbalance produced by the phase shifters and inserted into the signal of each antenna element, which is the key factor that defines the overall system accuracy.
The standard deviation remains under 0. However, the calculated positions contained a systematic error. It is apparent that when the beam shifts to the either side, the gain power drop-off combined with the changing size of the HPBW results in a decrease of the calculated position with regard to the real position. Moreover, we investigated the accuracy of the system for different distances between the reader and the tag antenna.
Herein, it should be noted that during the tests the ground reflection was not completely blocked. This allowed a more realistic illustration of the overall system performance, since in almost every application scenario the ground reflection is present. Consequently, there exist some blind spots where the path loss is exceedingly high for the reader-tag communication to occur. This is due to multipath and is primarily dependent upon the height of the antenna from the ground.
However, in reality the effect of reflected signals is far more complex due to multiple reflections from objects, diffraction, shadowing, etc. This effect is also visible in Fig. We can however estimate the change of the radial position of the tag relative to the antenna array by using the width of the acquired trace. As previously discussed, the HPBW is herein used as a figure of merit to indicate how wide or narrow an acquired trace will be. The actual trace width is dependent upon the distance between the reader and the tag.
To illustrate how the developed UHF tracking system can be employed for the automated handling of packaged good, we mounted it on a suction gripper at the end effector of an industrial manipulator. All pages are intact, and the cover is intact. Pages can include considerable notes-in pen or highlighter-but the notes cannot obscure the text. Seller Inventory GI5N Book Description Brand: Newnes, Condition: GOOD. Has little wear to the cover and pages. Contains some markings such as highlighting and writing. Book Description Newnes, Condition: Good. Satisfaction Guaranteed!
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Condition: UsedAcceptable. Publisher overstock copy. Ships with Tracking Number! May not contain Access Codes or Supplements. May be ex-library. Buy with confidence, excellent customer service!. Daniel M. Publisher: Newnes , This specific ISBN edition is currently not available. View all copies of this ISBN edition:. Synopsis About this title This book includes a survey of all RFID fundamentals and practices in the first part of the book while the second part focuses on UHF passive technology.