This is the accepted version of the journal article: Herrojo, Cristian; Paredes, Ferran; Martín, Ferran. «Synchronism and direction detection in high-resolution». IEEE sensors journal, Vol. 21, Issue 3 (February 2021), p. 2873-2882. DOI 10.1109/JSEN.2020.3025435 This version is available at https://ddd.uab.cat/record/258425 under the terms of the license IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX 1 Synchronism and Direction Detection in High- Resolution/High-Density Electromagnetic Encoders Cristian Herrojo, Member, IEEE, Ferran Paredes, Member IEEE, and Ferran Martín, Fellow, IEEE Abstract—Recently, electromagnetic encoders with synchronous reading and direction detection capability have been reported. Such structures are useful for the implementation of (i) displacement/velocity sensors and (ii) chipless- RFID systems based on near-field coupling and sequential bit reading. In the latter, synchronous reading and motion direction detection are a need in order to avoid false readings of the identification (ID) code, if the relative velocity between the reader and the encoder is not constant, and to read the correct ID code (rather than the inverse one), respectively. On the other hand, synchronous reading and motion direction detection, are essential to determine the encoder direction in displacement/velocity sensors, as well as to provide the absolute position of the encoder, provided that the whole encoder is encoded with the Bruijn sequence. In this paper, synchronous reading and direction detection in high-resolution/high-density electromagnetic encoders based on chains of linearly-shaped metallic inclusions are reported. To this end, it is necessary to add two chains of metallic inclusions to the one containing the ID code. In the reader side, three harmonic signals are necessary in order to generate the clock signals and to obtain the ID code. The reader consists of a microstrip line loaded with three pairs of open-ended folded stubs positioned face-to-face by their extremes. By displacing the encoder chains over the extreme of the stubs, at short distance, stub coupling is enhanced when a metallic inclusion lies on top of the stubs, and the frequency response of the reader is shifted towards lower frequencies. Thus, by injecting three (properly tuned) harmonic signals at the input port of the microstrip transmission line, three amplitude modulated (AM) signals are generated by tag motion at the output port of such transmission line, and the envelope functions contain the velocity, the ID code and the absolute position. The reported reader/encoder system exhibits superior space resolution and information density as compared to other similar systems based on synchronous reading. Index Terms—Chipless-RFID, electromagnetic encoders, microstrip technology, microwave sensors. I. Introduction the reader side; (ii) the highly achievable data capacity (only limited by the encoder, or tag, size), related to the fact that the E LECTROMAGNETIC (or microwave) encoders may constitute a good alternative to optical encoders [1]-[3] in scenarios subjected to harsh and hostile conditions (i.e., with pollution, bits are read sequentially by displacing the tag over the sensitive part of the reader at short distance. By contrast, in frequency- domain chipless-RFID systems, wideband interrogation signals dirtiness, grease, etc.). Such conditions are typical in many are required, since the tags consist of a set of resonant elements mechanical systems where an accurate control of the position tuned to different frequencies. Moreover, the data capacity in and velocity of their moving parts is essential (servomotors, such systems is typically small (or moderate), since the elevators, pointing mechanisms, conveyor belts, etc.). Thus, interrogation signal can cover only a limited spectral electromagnetic encoders are of interest for motion control bandwidth. The main limitation of these chipless-RFID systems applications, as low-cost displacement and velocity sensors is the requirement of tag alignment and proximity with the able to operate under extreme conditions, not optimal for their reader. However, this near-field system operation does not optical counterparts [4]-[8]. represent a penalty in certain applications, especially those It has also been demonstrated that electromagnetic encoders devoted to security and authentication, where tag reading by can be applied to the implementation of high-data density (and proximity guarantees major system confidence against spying capacity) chipless-RFID systems based on near-field coupling and eavesdropping. Secure paper, anti-counterfeiting of between the reader and the tag (the encoder) and sequential bit premium products, etc., are some potential applications of these reading [9]-[19]. In such application, two relevant advantages, near-field chipless-RFID systems based on microwave as compared to other chipless-RFID systems based on encoders [9]. Note that laminated encoders with buried metallic frequency domain [20]-[52], can be highlighted: (i) the virtually inclusions can be also implemented, thereby offering further null bandwidth of the interrogation signal (a single-tone levels of security against copying or spying by direct sight. harmonic signal), representing a significant cost reduction in Nevertheless, for authentication applications, the key aspect is This work was supported by MICIIN-Spain (project TEC2016-75650- C. Herrojo, F. Paredes, and F. Martín are with GEMMA/CIMITEC, R and PID2019-103904RB-I00), by Generalitat de Catalunya (project Departament d’Enginyeria Electrònica, Universitat Autònoma de 2017SGR-1159), by Institució Catalana de Recerca i Estudis Avançats Barcelona, 08193 Bellaterra, Spain. E-mail: Ferran.Martin@uab.es. (who awarded Ferran Martín), and by FEDER funds. XXXX-XXXX © XXXX IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX the reproducibility of the encoders, not easy in both laminated resolution. The main challenge of this work is the design of the and bare encoders. sensitive part of the reader, able to provide the clock signals as In the electromagnetic encoders reported in [4],[5], well as the ID code. Indeed, the work is inspired by the structure implemented as circular periodic chains of metallic resonators reported in [16]. The main novelty concerns the fact that for the and aimed to the accurate measurement of angular generation of the clock signal, necessary for synchronous displacements and velocities, the rotation angle is determined reading, a new sensing element tuned to a different frequency from the cumulative number of pulses from a reference (REF) is necessary. Moreover, a prototype able to provide also the position, whereas the instantaneous velocity is inferred from the direction of motion of the encoder with regard to the reader is time lapse between adjacent pulses (provided the period of the also included in this paper. This requires a further sensing encoder chain is well known). These incremental-type encoders element, thereby representing a major challenge from the exhibit a fundamental limitation, i.e., if a reset occurs, the design viewpoint. The result is an encoder system with information relative to the angular position is lost. In other synchronous reading and motion direction detection capacity, words, the encoders reported in [4],[5] do not provide the with very competitive information density and resolution, absolute position, as most optical encoders do. These encoders implemented for the first time with linear metallic strips. consist of periodic chains, with all the resonant elements present at their predefined positions. II. READER AND ENCODER DESIGN By contrast, microwave encoders used as chipless-RFID The strategy for synchronously reading the considered tags are implemented by means of coded sequences, achieved encoders is to implement two chains of metallic inclusions, by the presence or absence of functional (operative) resonators, dedicating one to the ID code, and the other one to the clock or inclusions, at their predefined positions in the encoder chain [9]. In such application, an N-bit chipless tag is read by signal. Naturally, in the ID chain, encoding is achieved by the displacing it over the sensitive part of the reader, which presence (‘1’ logic state) or absence (‘0’ logic state) of functional metallic inclusions. By contrast, all the inclusions are provides the ID code sequentially, bit by bit. In this case, a present, and hence functional, in the clock chain. Nevertheless, constant, and well-known, relative velocity between the tag and for displacement direction detection, the encoder must be the reader is necessary, in order to avoid false readings. The encoder velocity can be determined by means of header bits implemented by etching three chains of metallic inclusions in [11],[13], but a uniform velocity may not be guaranteed in the tag substrate. One of such chains contains the ID code, whereas from the other two, it is possible to infer the velocity certain applications. Thus, to provide system robustness against and motion direction, as it will be shown next. In this paper, a potential velocity variations, a clock signal, for synchronous reader/encoder system is first presented, and then synchronization, is required. the motion direction detection capability is introduced in a new According to the previous words, adding a periodic chain to near-field chipless-RFID tags is necessary in order to generate prototype. a clock signal that prevents from false readings (related to A. Synchronous Reader for Encoders Based on Linear uncertainties in the relative velocity between the tag and the Metallic Strips reader). Thus, system synchronism dictating the instants of time The working principle of the proposed system based on for sequentially reading the bits of the encoder is very electromagnetic encoders with synchronous reading is depicted convenient. Nevertheless, the motion direction, and hence the in Fig. 1. According to this sketch, a combiner should be correct ID code of the encoder cannot be extracted by using connected to the input port of the sensitive part of the reader in only a single clock chain. Note that, if the ID code is not order to inject the two required harmonic signals (interrogation symmetric, the reading direction must be known. Preliminary signals). On the other hand, in order to separately obtain the results of electromagnetic encoders with synchronous reading clock signal and the ID code, an inverse scheme (based on a have been recently reported by the authors by implementing the duplexer) is applied to the output port, as it can be appreciated encoders with chains of square or rectangular metallic patches. in Fig. 1. Such system is similar to the one presented in [16]. In [53]-[55], a single chain of patches is considered, and the ID However, in the work presented in this paper, the reader is a code is given by the patch size (the clock signal being generated microstrip transmission line loaded with two pairs of folded by tag motion over the reader). By contrast, in [56], two open-ended stubs. The layout of the reader/encoder structure is independent chains are considered, one for the ID code (the depicted in Fig. 2. In [16], an unprecedented per-unit-length and binary states given by the presence or absence of patch), and per-unit-surface data density in chipless RFID systems based another one for the clock signal, which provides also the on near-field coupling and sequential bit reading was achieved. velocity. This latter system is useful to determine the direction The reader used a single pair of identical folded open-ended of motion. However, in the systems reported in [53]-[56], the stubs. Such structure exhibits a single transmission zero at the encoder resolution (system used as position sensor) and the data frequency where the stub length is a quarter-wavelength, as density per surface (system functionality as chipless-RFID tag), expected. Despite the fact that the extremes of the stubs are are limited due to patch size and spacing (period). face-to-face, inter-stub coupling is negligible. However, when In this paper, synchronous reading in electromagnetic a metallic strip is located on top of the extremes of the stubs, at encoders (either devoted to position/velocity sensors or as short distance, inter-stub coupling increases and the chipless-RFID tags), based on linearly shaped inclusions, is transmission zero splits. Furthermore, the reader designed in introduced. Such inclusions are very narrow and oriented in the [16] exhibits a reflection zero, whose frequency position direction orthogonal to axis of the tag chain, thereby representing a good solution in terms of data density and 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX (a) 0 Sensitive part 1 1 0 0 Z0 of the reader -15 V1 @ fc1 -30 EM sim. bare reader E.D. fc1 fz1 Meas. bare reader fc2 fz2 -45 1 1 1 1 (b) Z0 0 S21 (dB) V2 @ fc2 Combiner Duplexer E.D. -15 p -30 EM sim. 2 GHz pair stub loaded Meas. 2 GHz pair stub loaded -45 ID code Clock (c) 0 -15 Fig. 1. Sketch showing the working principle of the proposed -30 EM sim. 4 GHz pair stub loaded reader/encoder system based on metallic inclusions and synchronous Meas. 4 GHz pair stub loaded reading. -45 2.0 2.5 3.0 3.5 4.0 W Pin Pout Frequency (GHz) ws w1 w1 Fig. 3. Electromagnetic simulation (including losses) and measured l1 s1 l2 s1 frequency responses of the: (a) bare reader, (b) reader with the longer stub pair loaded with a metallic strip on top of it, and (c) reader with the Metallic shorter stub pair loaded with a metallic strip on top of it. inclusions 0 ID code signal, fc = fc1 P -5 Clock signal, fc = fc2 S21 (dB)|f = fc ws Clock chain -10 -15 ID code -20 chain -25 Encoder ls -30 Fig. 2. Layout of reader/encoder structure based on a microstrip 1.5 3.0 4.5 6.0 7.5 9.0 transmission line loaded with two pairs of folded open-ended stubs. Displacement (mm) Dimensions (in mm) are: W = 1.81, l1 = 24.39, l2= 12.33, w1 = 1.80, s1 = 1.60, P = 1.20, ws = 0.20 and ls = 6.40. Fig. 4. Variation of the transmission coefficient at the interrogation signal frequencies (fc1 and fc2) resulting by displacing 7 metallic elements over the reader. The considered airgap is 0.2 mm. Note that although the depends on the distance between the contact points of the stubs maximum transmission is not close to 0 dB, a significant excursion of to the host line. the transmission coefficient is achieved. According to the previous words, it is possible to achieve a large excursion of the transmission coefficient by designing the of time for synchronously reading the ID code, as well as the structure so that the reflection zero of the bare reader, fc, relative velocity (or even acceleration) of the encoder with coincides with the first transmission zero, fz, of the loaded regard to the sensitive part of the reader. reader, i.e., with a functional metallic inclusion on top of it. The reader has been designed in order to exhibit the two Thus, if the reader is fed with a harmonic signal (interrogation reflection zeros, fc1 and fc2, in the vicinity of the two signal) tuned to the reflection zero frequency (fc), an AM transmission zeros, fz1 and fz2, (that appear when the modulated signal containing the ID code will be generated at corresponding stub length is a quarter-wavelength). Figure 3 the output port of the microstrip transmission line by tag plots the lossy electromagnetic simulation, inferred by means motion. Such ID code can be extracted from the envelope of the of Keysight ADS, and the measured response of the: (i) bare AM signal by means an envelope detector, provided the reader and (ii) reader with only one stub pair loaded with a encoder period is well known and the relative velocity between metallic inclusion on top of it (two cases). The considered the sensitive part of the reader and the encoder is constant and substrate is the Rogers RO4003C with dielectric constant well known. However, in the system proposed in [16], if such r = 3.55, thickness h = 0.81 mm, and loss tangent relative velocity is not constant, a correct bit reading cannot be tan = 0.0021. For the encoder, the substrate is the same of the guaranteed. reader, but with thickness h = 0.204 mm. Note that, in the In this work, as mentioned before, the sensitive part of the transmission coefficient of the bare reader, the transmission reader of Fig. 2 has been implemented with two different pairs zero frequencies are roughly fz1 = 2.1 GHz and fz2 = 4.2 GHz. of folded open-ended stubs, thereby providing two transmission Therefore, it is guaranteed that both chains of metallic elements zeros and two reflection zeros to the frequency response of the modify only the corresponding transmission zero when the bare reader. Therefore, two independent chains must be encoder is displaced over the reader, as demonstrated in Fig. 3. considered in order to separately obtain the ID code and the On the other hand, by feeding the reader line with two harmonic clock signal. In the clock chain, all the metallic inclusions are interrogation signals tuned to fc1 =2.02 GHz and fc2 = 3.98 GHz, present at predefined position in order to determine the instants a significant excursion of the transmission coefficient results by 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX 0.8 T = 0.121 s ID code signal Clock signal (b) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.4 (a) 0.0 0.8 0 0 1 1 1 1 0 0 0 0 1 0 1 1 0 1 V0 (V) 0.4 0.0 0.8 T = 0.121 s T2 = 0.127 s 1 T3 = 0.242 s 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.4 Fig. 5. Photograph of the fabricated sensitive part of the reader (a) and 0.0 encoders (b). 4.0 4.5 5.0 5.5 6.0 Time(s) Fig. 7. Measured envelope function of the 16-bit encoders with the Osciloscope indicated codes. The considered airgap is 0.2 mm. Sensitive part 1 1 1 1 Z0 of the reader 1 1 0 0 V3 @ fc3 Z0 V1 @ fc1 Z 1 1 1 1 Sensitive part 0 of the reader V2 @ fc2 Combiner Triplexer E.D. p Encoder ID code Clock2 Stepper motor Clock1 Probe Fig. 8. Sketch showing the working principle of the proposed reader/encoder system based on metallic inclusions and synchronous reading with direction detection capability. Envelope detector W w2 Pin Pout Fig. 6. Photograph of the experimental setup. w1 l2 w1 l1 s1 ws s2 s1 s1 l3 tag motion. This can be appreciated in Fig. 4, where the Metallic inclusions variation of the transmission coefficient at the interrogation Clock1 chain P signal frequencies (fc1 and fc2) has been obtained (by means of simulation) by displacing 7 metallic inclusions over the reader. ws Clock2 chain ID code chain For experimental validation, two different 16-bit encoders have been fabricated. The photographs of such encoders and the Encoder sensitive part of the reader are depicted in Fig 5. The fabrication ls has been carried out by means of a LPKF-100 milling machine Fig. 9. Layout of reader/encoder structure based on a microstrip on the substrate indicated before. transmission line loaded with three pair of folded open-ended stubs. The photograph of the experimental set-up is shown in Fig. Dimensions (in mm) are: W = 1.81, l1 = 24.39, l2= 12.33, l3= 48.60, w1 = 1.80, w2 = 3.40, s1 = 1.60, s2 = 0.90, P = 1.20, ws = 0.20 and 6. The two necessary interrogation signals (tuned to fc1 and fc2) ls = 6.40. to synchronous reading the ID code of the encoder, have been injected independently at the input port of the microstrip STM 23Q-3AN). Such system allows us to control the vertical transmission line, and we have obtained the envelope signal for distance (air gap) between the sensitive part of the reader and each case. Such interrogation signals have been generated by the encoder, the relative velocity, or even the acceleration of the means of a network analyzer (model Agilent N5221A), whereas encoder. an oscilloscope (model Agilent MSO-X-3104A) has been used With this experimental setup, the measured envelope to visualize the ID code of the encoder. The envelope detector functions, corresponding to the 16-bit encoders of Fig. 5(b), has been implemented by means of a Schottky diode (model have been inferred by displacing such encoders over the Avago HSMS- 2860) and the N2795A active probe (with sensitive part of the reader [see Fig. 5(a)]. As it can be capacitance C = 1 pF and resistance R = 1 MΩ). A circulator appreciated from Fig. 7(a) and Fig. 7(b), the measured envelope (model ATM ATc4-8) has been used in order to avoid reflections functions, corresponding to ID code and clock signals (which from the Schottky diode. Finally, the relative displacement perfectly determine the instants of time for sequentially reading between the sensitive part of the reader and the encoder has the ID code of the encoder), exhibit dips when a strip is on top been achieved by means of a linear displacement system (model of the corresponding stub pair. For the clock signal, the dips 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX 0 S21 (dB)|f = fc ID code, fc = fc1 Clock1, fc = fc2 Clock2, fc = fc3 (a) -10 -20 -30 0.0 1.5 3.0 4.5 6.0 7.5 9.0 (b) Displacement (mm) Fig. 10. Variation of the transmission coefficient at the interrogation signal frequencies (fc1, fc2 and fc3) resulting by displacing 7 metallic elements over the reader. The considered airgap is 0.2 mm. 0.8 0.4 V0 (V) 0.6 appear at periodic time intervals because the velocity of the 0.2 linear displacement system is constant, i.e., the nominal 0.4 0.0 velocity is 10 mm/s. The displacement velocity that results from Fig. 11. 0.2Photograph of the fabricated sensitive part of the reader (a) and the time interval between adjacent peaks, T = 0.121 s, and the the encoder (b). encoder period, P = 1.2 mm, is 9.92 mm/s (in good agreement 0.0 with the nominal value). (a) ID code signal Clock1 signal Clock2 signal As mentioned before, with this synchronous reader/encoder structure, a constant velocity of the encoder with regard to the 0.9 T = 0.120 s 0.4 V0 (V) sensitive part of the reader (in order to read the ID code) is not 0.2 0.6 0.0 necessary. To demonstrate this aspect, Fig. 7(c) depicts the measured envelope functions of the encoder with the indicated 0.3 code [the same one as the Fig.7 (a)], when it is displaced over the sensitive part of the reader with a nominal acceleration of 3 0.0 V0 (V) 0 0 1 1 1 1 0 0 0 0 1 0 1 1 0 1 mm/s2. With this result, it is validated that, regardless of the (b) 0.9 T = 0.120 s T = 0.152 s 0.4 instantaneous velocity of the encoder with regard to the V0 (V) 0.2 sensitive part of the reader, the ID code of the encoder can be 0.6 extracted. Nevertheless, note that, with the synchronous 0.0 reader/encoder structure presented in this section, it is not 0.3 possible to distinguish the direction of the encoder, and hence, 0.0 it is not possible to guarantee the reading of the proper ID code. 4.0 4.5 5.0 5.5 6.0 B. Synchronous Reader with direction detection for Time(s) Encoders Based on Linear Metallic Strips Fig. 12. Measured envelope function of the 16-bit encoder with the As aforementioned, with the structure presented in the indicated code by considering encoder motion with (a) constant velocity previous section, it is not possible to determine the motion and (b) constant acceleration. The considered airgap is 0.2 mm. direction. To this end, it is necessary to provide a redundant clock signal by adding a third folded open-ended stub pair in displacement direction, and hence the correct ID code, can be the sensitive part of the reader, and a third strip chain in the inferred. encoder (with all metallic inclusions present). By these means, On the other hand, it is also possible to provide the absolute a third transmission zero, fz3, and reflection zero, fc3, is added to position of the encoder. Such absolute position can be the frequency response of the bare reader. Obviously, to determined by using the Bruijn sequence [57], which determine the motion direction, the reader must be fed with a guarantees that any N-bit sub-code does not repeat for the total third harmonic (interrogation) signal tuned to fc3, which will be set of different positions of the encoder [56]. Therefore, it is AM modulated by tag motion at the output port of the necessary to read a bit of the coded chain, at a determined microstrip transmission line. The sketch of the working position, plus the previous N-1 bits in order to obtain the principle and layout of the reader/encoder structure are depicted absolute position of the encoder. If L is the total length of the in Fig. 8 and Fig. 9, respectively. Note that the period of both encoder and p is the distance between adjacent strips (period, or clock chains is identical. Nevertheless, the fact that such period spatial resolution), the total number of different positions is thus does not coincide with the distance s2 (see Fig. 9) implies that L/p. Consequently, the number of bits of the sub-code required one clock signal will be lagged or leaded with regard to the to unambiguously determine the encoder position must satisfy other. Such aspect can be seen in Figure 10, where a variation 𝐿 of the transmission coefficient at the interrogation signal 𝑁  log 2 ( ) (1) 𝑝 (fc1 = 2.02 GHz, fc2 = 3.98 GHz and fc3 = 0.959 GHz) has been obtained (by means of simulation) by displacing 7 metallic For instance, for an encoder length of L = 1 m, and period of inclusions over the reader. Thus, by using two clock signals, the p = 1.20 mm, N = 10 bits suffice to determine the position. Note 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX that the system must incorporate a table with the position TABLE I COMPARISON OF DIFFERENT SYNCHRONOUS APPROACHES assigned to the different N-bit sub-code sequences, and this is necessary for both motion directions. Naturally, after a system Direction DPS DPL Ref. Sync. reset, it is necessary that the encoder moves N strip positions in detection (bits/cm2) (bits/cm) order to read the necessary N bits of the sub-code. For this main [53] Yes No 0.60 0.67 reason it is reasonable to consider that the proposed system is [54] Yes No 1.15 1.67 able to provide the quasi-absolute (rather than absolute) [55] Yes No 1.19 1.72 position of the encoder. [56] Yes Yes 0.76 2.29 For validation purposes, a 16-bit encoder (N = 4), with a This work 1 Yes No 2.35 6.96 specific De Bruijn sequence, has been fabricated. The This work 2 Yes Yes 1.63 6.96 photographs of such encoder and sensitive part of the reader are depicted in Fig 11. It must be pointed out that the encoder and TABLE II COMPARATIVE ANALYSIS OF VARIOUS CHIPLESS-RFID TAGS the sensitive part of the reader have been implemented on the same substrate as the one used in the synchronous Area (cm ) BW (GHz) (bit/GHz) 2 (bits/cm) (bit/cm2) reader/encoder structure presented in the previous section, and DPL DPF DPS Ref. Bits measured with the experimental setup shown in Fig. 6. The three measured envelope functions of the fabricated encoder, by considering constant velocity as well as constant acceleration of the encoder with regard to the sensitive part of Time-domain (Pulsed interrogation signal) the reader, are depicted in Fig. 12(a) and Fig. 12(b), respectively. As pointed out before, the presence of metallic [58] --- 4 59.4 --- 0.07 0.02 inclusions on top of the extremes of the stubs, at short distance, [59] 0.05 2 --- 40 --- 0.10 are revealed as dips in the measured envelope functions. In Fig. [60] --- 4 --- --- --- 0.06 12(a) the encoder is displaced with a nominal constant velocity [61] --- 8 --- --- --- 0.20 of the 10 mm/s. As expected, the redundant clock signal [62] 0.8 5 26 6.25 0.19 0.19 (Clock2) is advanced with regard to the clock signal (Clock1). [63] --- 2 70.0 --- 0.03 --- Therefore, the motion direction of the encoder can be inferred, [64] --- 2 8.75 --- 0.23 --- and hence, the proper ID code can be extracted from the envelope function. Moreover, from adjacent pulses, the encoder Time-domain (Harmonic interrogation signal) velocity is found to be 10 mm/s, which is in excellent agreement [6] * 40 5.40 * 7.40 3 with the considered nominal value. With regard to Fig. 12 (b), [13] * 80 9.44 * 8.47 3 the encoder has been displaced over the sensitive part of the [14] * 100 20.4 * 4.90 16.7 reader with a nominal constant acceleration of 3 mm/s2. As can [15] * 100 13.4 * 7.46 16.7 be seen, the encoder direction as well as the ID code (and hence [16] * 100 3.84 * 26.0 16.7 the absolute position) can be inferred independently of the [19] * 15 9.04 * 1.66 2.99 velocity of the encoder with regard to the sensitive part of the This work 1 * 16 6.81 * 2.35 6.96 reader. Therefore, with the results obtained, it is clear that the This work 2 * 16 9.64 * 1.66 6.96 functionality of the system is validated. An important aspect in these encoders systems is the potential Frequency-domain lack of alignment between the encoder and the reader. This [22] 1 5 6.48 11.1 0.77 --- aspect was studied in [16], where the effects of lateral [24] 7.5 35 57.2 8.97 0.61 --- misalignments between the encoder and the reader, as well as [26] 0.2 5 50.1 25.0 0.10 --- misalignments caused by rotation were considered. From that [29] 2 20 17.5 10.0 1.14 --- study, it was concluded that the system was tolerant to lateral [30] 3.5 9 3.00 2.57 3.00 --- shifts of the order of 1 mm, and rotations of around 3º. With these values, it seems feasible the use of the proposed systems [31] 7.5 19 9.00 2.53 2.11 --- in a real scenario. [33] 7 28.5 8.00 4.07 3.56 --- [34] 7.5 24 5.76 3.20 4.17 --- III. COMPARISON WITH OTHERS SYNCHRONOUS [39] 1.2 20 17.5 16.7 1.14 --- APPROACHES Hybrid A figure of merit in linear displacement/velocity sensors [41] 5 22.9 8.00 4.58 2.86 --- based on microwave encoders is the number of pulses per unit [43] 6.4 64 10.9 10.0 5.88 --- of length. Such figure of merit is directly related to the space [49] 1 16 6.75 16.0 2.37 --- resolution, and hence, directly related to the encoder period, i.e., [47] 3 9 7.20 3.00 1.25 --- 1.2 mm in our case. On the other hand, in chipless RFID * The spectral bandwidth of this approach is virtually null due to the fact that systems, the density of information per surface (DPS) is one of the interrogation signal is harmonic. the most important figures of merit. With such figure of merit, it is possible to compare different chipless RFID encoders as far smaller the size of the encoder. Nevertheless, in the particular as size is concerned. The higher the value of this parameter, the case of chipless-RFID systems based on near-field coupling and 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX sequential bit reading, the most relevant parameter is the is not necessary that such velocity is constant). Moreover, density of information per unit length (DPL), since, usually, the knowing the motion direction of the tag avoids false readings, encoders are implemented as linear chains of inclusions potentially caused by sequentially reading the ID code in the (metallics in this case). Note that the DPL can be used to opposite direction to that which is intended. compare different approaches for the implementation of electromagnetic encoders, since the pulses, or dips, generated CONCLUSIONS in the envelope function in linear displacement/velocity sensors In summary, high-resolution/high-density electromagnetic can be interpreted as bits of information in chipless-RFID systems. encoders based on linearly-shaped metallic inclusions with Table I compares the synchronous reader/encoder structures synchronous reading and motion direction detection capability have been reported. By synchronously reading the encoders, presented in this work with other approaches based on the same potential false readings of the ID code associated to eventual working principle, in terms of DPL and DPS. Another variations in the relative velocity between the encoder and the important aspect, also reflected in Table I, is the capability to sensitive part of the reader, can be avoided. The system detect the motion direction. As it can be seen, the prototype of Section II.B, able to discriminate the motion direction, exhibits distinguishes the displacement direction of the encoder thanks a DPL which is more than 3 times the DPL of [56], and a DPS to the presence of two clock chains in the encoder. This aspect is important to unambiguously identify the ID code of the which is twice the DPS of [56]. It is important to mention that, encoder. Moreover, the absolute position of the encoder can be to the best of our knowledge, [56] is the unique synchronous determined by using a Bruijn sequence in the encoder chain. approach based on microwave encoders with direction The length and the period of the metallic inclusions, the key detection capability in the literature. Regardless of this motion detection capability, the two prototypes presented in this paper, parameters in order to increase the density of information per exhibit significantly higher DPL and DPS, in comparison to the unit length and surface of the encoders, have been significantly reduced, with regard to previous implementations based on a other implementations indicated in Table I. similar working principle. Specifically, the DPL and DPS has In the encoders of Table I, reading takes place been found to be increased by a factor of three and two, synchronously, and such encoders can be useful as displacement/velocity sensors or as chipless-RFID tags based respectively, as compared to other approaches with or without on near-field coupling and sequential bit reading. For this later direction detection capability. It has been demonstrated that the proposed approach is useful for two applications: chipless- functionality, it is also pertinent to extend the comparison of the RFID systems based on near-field coupling with sequential bit system proposed in this paper with other chipless-RFID reading, and linear displacement/velocity sensors. As a future systems, including frequency-domain systems, hybrid systems work, the implementation of a switch at the input and output and other time-domain based systems. Such comparison, indicated in Table II, is mainly based on the density of ports of the microstrip transmission line, controlled by a micro information per area unit, although other aspects are compared in order to sequentially fed the necessary interrogation signals (generated by a VCO) will be considered. as well. As it can be seen, the proposed system is quite competitive for chipless-RFID applications, with a good combination of size, data capacity and performance. It should REFERENCES be taken into account that the introduction of motion direction [1] E. Eitel, “Basics of rotary encoders: overview and new technologies”, detection capability and synchronous reading limits the Machine Design Magazine, 7 May 2014. information density. Nevertheless, good values have been [2] G. K. McMillan, D.M. 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Synchronous Reading and Direction Detection for Motion Control Applications”, URSI GASS 2020, Rome, Italy, 29-5 Aug., 2020. 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX [56] F. Paredes, C. Herrojo, and F. Martín, “Chipless-RFID Sensors for Motion simulation of electron devices for high frequency applications, millimeter Control Applications”, 2020 IEEE-MTT-S Int. Microw. Symp. (IMS’20), wave and THz generation systems, and the application of Los Angeles, CA, USA, 21-26 Jun., 2020. electromagnetic bandgaps to microwave and millimeter wave circuits. [57] N.C. de Bruijn, “"Acknowledgement of Priority to C. Flye Sainte- Marie He is now very active in the field of metamaterials and their application on the counting of circular arrangements of 2n zeros and ones that show to the miniaturization and optimization of microwave circuits and each n-letter word exactly once", T.H.-Report 75-WSK-06, Technological antennas. Other topics of interest include microwave sensors and RFID University Eindhoven, 1975. systems, with special emphasis on the development of high data [58] A. Chamarti and K. Varahramyan, "Transmission delay line based ID capacity chipless-RFID tags. He is the head of the Microwave generation circuit for RFID applications," IEEE Microw. Wireless Engineering, Metamaterials and Antennas Group (GEMMA Group) at Compon. Lett., vol. 16, pp. 588-590, 2006. UAB, and director of CIMITEC, a research Center on Metamaterials [59] F.J. Herraiz-Martínez, F. Paredes, G. Zamora, F. Martín, and J. Bonache, supported by TECNIO (Generalitat de Catalunya). He has organized “Printed magnetoinductive-wave (MIW) delay lines for chipless RFID several international events related to metamaterials and related topics, applications”, IEEE Trans. Ant. Propag., vol. 60, pp. 5075-5082, Nov. including Workshops at the IEEE International Microwave Symposium 2012. (years 2005 and 2007) and European Microwave Conference (2009, [60] L. Zhang, S. Rodriguez, H. Tenhunen, and L.-R. Zheng, “An innovative 2015 and 2017), and the Fifth International Congress on Advanced fully printable RFID technology based on high speed time-domain Electromagnetic Materials in Microwaves and Optics (Metamaterials reflections,” in Conference on High Density Microsystem Design and 2011), where he acted as Chair of the Local Organizing Committee. He Packaging and Component Failure Analysis, 2006. HDP’06., Shanghai, has acted as Guest Editor for six Special Issues on metamaterials and China, Jun. 2006, pp. 166–170. sensors in five International Journals. He has authored and co-authored [61] L. Zheng, S. Rodriguez, L. Zhang, B. Shao, and L.-R. Zheng, “Design and over 600 technical conference, letter, journal papers and book chapters, implementation of a fully reconfigurable chipless RFID tag using Inkjet he is co-author of the book on Metamaterials entitled Metamaterials with printing technology,” in 2008 IEEE International Symposium on Circuits Negative Parameters: Theory, Design and Microwave and Systems, Seattle, USA, May 2008, pp. 1524–1527. Applications (John Wiley & Sons Inc.), author of the book Artificial [62] C. Mandel, M. Schussler, M. Maasch, and R. Jakoby, “A novel passive Transmission Lines for RF and Microwave Applications (John Wiley & phase modulator based on LH delay lines for chipless microwave RFID Sons Inc.), co-editor of the book Balanced Microwave applications,” in 2009 IEEE MTT-S International Microwave Workshop on Filters (Wiley/IEEE Press) and co-author of the book Time-Domain Wireless Sensing, Local Positioning, and RFID, Cavtat, Croatia, Sep. 2009, Signature Barcodes for Chipless-RFID and Sensing pp. 1–4. Applications (Springer). Ferran Martín has generated 21 PhDs, has filed [63] R. Nair, E. Perret, and S. Tedjini, “Temporal multi-frequency encoding several patents on metamaterials and has headed several Development technique for chipless RFID applications,” in IEEE MTT-S International Contracts. Microwave Symposium Digest, Montreal, Canada, Jun. 2012, pp. 1–3. Prof. Martín is a member of the IEEE Microwave Theory and [64] R. Nair, E. Perret, and S. Tedjini, “Chipless RFID based on group delay Techniques Society (IEEE MTT-S). He is reviewer of the IEEE encoding,” in IEEE Int. RFID-Technol. Appl. Conf., 2011, pp. 214–218. Transactions on Microwave Theory and Techniques and IEEE Microwave and Wireless Components Letters, among many other journals, and he serves as member of the Editorial Board of IET Microwaves, Antennas and Propagation, International Journal of RF and C. Herrojo was born in Barcelona, Spain, in 1983. Microwave Computer-Aided Engineering, and Sensors. He is also a He received the Telecommunications Technical member of the Technical Committees of the European Microwave Engineering degree in electronic systems and Conference (EuMC) and International Congress on Advanced Telecommunications Engineering degree from the Electromagnetic Materials in Microwaves and Optics (Metamaterials). Universitat Autònoma de Barcelona in 2010 and Among his distinctions, Ferran Martín has received the 2006 Duran 2012, respectively and the PhD degree in Farell Prize for Technological Research, he holds the Parc de Recerca Electronics Engineering from the same university UAB – Santander Technology Transfer Chair, and he has been the in 2018. His research interests include recipient of three ICREA ACADEMIA Awards (calls 2008, 2013 and RF/microwave devices, Chipless-RFID and RFID 2018). He is Fellow of the IEEE and Fellow of the IET. technology, and Metamaterials. F. Paredes was born in Badalona (Barcelona), Spain in 1983. He received the Telecommunications Engineering Diploma (specializing in Electronics) and the Telecommunications Engineering degree from the Universitat Autònoma de Barcelona in 2004 and 2006, respectively and the PhD degree in Electronics Engineering from the same university in 2012. He was Assistant Professor from 2006 to 2008 at the Universitat Autònoma de Barcelona, where he is currently working as a Research Assistant. His research interests include metamaterial concepts, passive microwaves devices, antennas and RFID. F. Martín (M’04-SM’08-F’12) was born in Barakaldo (Vizcaya), Spain in 1965. He received the B.S. Degree in Physics from the Universitat Autònoma de Barcelona (UAB) in 1988 and the PhD degree in 1992. From 1994 up to 2006 he was Associate Professor in Electronics at the Departament d’Enginyeria Electrònica (Universitat Autònoma de Barcelona), and since 2007 he is Full Professor of Electronics. In recent years, he has been involved in different research activities including modelling and