electronics Article 3D-Printed Quasi-Absolute Electromagnetic Encoders for Chipless-RFID and Motion Control Applications Ferran Paredes * , Cristian Herrojo and Ferran Martín CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; Cristian.Herrojo@uab.cat (C.H.); Ferran.Martin@uab.es (F.M.) * Correspondence: Ferran.Paredes@uab.cat Abstract: This paper presents electromagnetic encoders useful for chipless-RFID and motion control applications. The encoders consist in a pair of linear chains of rectangular apertures implemented by means of 3D printing. One of these chains is periodic and acts as a clock, whereas the other chain contains an identification (ID) code. With these two aperture chains, the ID code can be synchronously read, so that the relative velocity between the tag and the reader is irrelevant. Additionally, it is shown in the paper that by properly designing the reader, it is possible to determine the motion direction. The sensitive part of the reader is a microstrip line loaded with three complementary split ring resonators (CSRRs) etched in the ground plane and fed by three harmonic signals. By encoder motion, the characteristics of the local medium surrounding the CSRRs are modified, and the harmonic signals are amplitude modulated (AM) at the output port of the line, thereby providing the clock signal (which gives the encoder velocity), the ID code (providing also the quasi-absolute  position) and the direction of motion. A fabricated prototype encoder is characterized by reading it  with a dedicated reader. Citation: Paredes, F.; Herrojo, C.; Martín, F. 3D-Printed Quasi-Absolute Keywords: chipless-RFID; motion control; electromagnetic encoders; 3D-printing; dielectric permit- Electromagnetic Encoders for tivity; microstrip technology Chipless-RFID and Motion Control Applications. Electronics 2021, 10, 1154. https://doi.org/10.3390/ electronics10101154 1. Introduction Academic Editors: Milan Švanda and Optical encoders [1–4] are the most genuine and widespread technology for motion Jan Kracek control applications. As the encoder moves across an optical beam generated by an optical source, typically a laser diode, a chain of apertures made on a (typically) metallic substrate Received: 20 April 2021 are detected. When an aperture crosses the optical beam, the encoder is transparent and Accepted: 11 May 2021 an optical detector, placed on the other side of the encoder, can record the optical beam Published: 13 May 2021 in the form of a pulse. Thus, as many pulses as apertures are detected by the optical reader. Obviously, the encoders can be implemented with several chains of apertures, Publisher’s Note: MDPI stays neutral which can be codified, and therefore can be used to determine the absolute position of the with regard to jurisdictional claims in encoder [5,6]. These absolute encoder systems are able to provide the position regardless published maps and institutional affil- of the previous motion stages of the encoder, contrary to the so-called incremental-type iations. encoders, where the position is determined from the cumulative number of pulses (i.e., through pulse counting). Electromagnetic encoders [7–16] are an alternative to optical encoders. Both systems exhibit a similar working principle, but the optical signals are replaced by microwave Copyright: © 2021 by the authors. signals. In electromagnetic systems, the encoders can be made of chains of apertures, Licensee MDPI, Basel, Switzerland. or also chains of inclusions (metallic or dielectric). However, these encoders should This article is an open access article not be confused with the so-called magnetic encoders, based on magnets or inductive distributed under the terms and elements [17–24]. The first reported electromagnetic encoders were implemented by etching conditions of the Creative Commons circular chains of metallic inclusions (planar resonators) in the periphery of a dielectric Attribution (CC BY) license (https:// disc [25] (these encoders indeed appeared as an evolution of previous angular displacement creativecommons.org/licenses/by/ sensors based on coupling modulation between a microstrip line and a single circular 4.0/). Electronics 2021, 10, 1154. https://doi.org/10.3390/electronics10101154 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 1154 2 of 11 resonator axial to the rotor [26–28]). Such rotary encoders were incremental, i.e., only able to provide the angular position from the cumulative number of peaks in the envelope function (nevertheless, in one of the implementations [10], the motion direction, clockwise or counter-clockwise, was detected by adding a non-periodic chain). In order to detect these inclusions, the sensitive part of the reader consists of a microstrip line structure fed by at least a harmonic signal. Typically, as many harmonic signals as inclusion chains are needed for inferring all the relevant information provided by the encoder, e.g., the velocity and the absolute position. By encoder motion, the harmonic signal/s is/are amplitude modulated (AM) at the output port of the line, and the relevant information is contained in the envelope function/s, in the form of peaks or dips (equivalent to the pulses in the optical encoders). Recently, all-dielectric electromagnetic encoders, either based on apertures or dielec- tric inclusions made on a dielectric substrate, were reported [15,29,30]. As compared with encoders based on metallic inclusions, the main advantage of all-dielectric encoders is the superior robustness against wearing or friction. This robustness has also been demon- strated in encoders based on metallic inclusions, but implemented in the form of metallic patches [14]. Another recent advance concerns the implementation of electromagnetic encoders with synchronous reading, where at least two inclusion chains are needed, one acting as the clock, with all the inclusions present at their predefined positions, and the other one containing the ID code [14–16,31–33]. In [16,32], not only the encoders exhibit synchronous reading capability, but also the motion direction can be determined. Electromagnetic encoders can be used in motion control applications, but also in other applications such as chipless-RFID [34,35]. By eliminating some of the inclusions of the encoder, an ID code can be obtained. Contrary to most chipless-RFID tags based on frequency domain, time domain, or exploiting various domains simultaneously [36–64], the encoder-based tags should be read by proximity, through the near field, and bit to bit, i.e., sequentially [65–67]. This represents a limitation in certain applications, e.g., when read distances of at least various cm are required. The number of bits of electromagnetic encoder-based chipless tags is only limited by tag size [12,68], these representing an advan- tage over other chipless-RFID approaches. In certain applications, such as secure paper, authentication of premium products [69], etc., reading by proximity is not necessarily an issue (indeed, it can even provide certain level of confidence against spying or eavesdrop- ping). It is important to bear in mind that, although this electromagnetic encoders have been focused on motion control applications, as well as chipless-RFID, applications such as motion detector [70], monitoring process in industrial scenarios [71,72], or smart packaging, for example, can be also of potential application. In the present paper, the main objective was to present an electromagnetic encoder system with synchronous reading and motion direction detection capability, based on 3D-printed encoders with aperture chains. This is the first all-dielectric encoder able to provide such complete functionality. Namely, it is able to provide the encoder velocity, the motion direction, as well as the (quasi) absolute position, from the codified chain of apertures (to be discussed later). 2. The Proposed Encoder and Reader, and Working Principle The general working principle of the proposed all-dielectric electromagnetic encoders is AM modulation of the harmonic feeding signals caused by encoder motion over the sensitive part of the reader. Nevertheless, we should give more details, since the proposed encoder system is able to provide the encoder velocity, the quasi absolute position and the direction of motion. For that purpose, two chains of apertures are required. One chain is periodic, i.e., all the apertures are present at predefined positions, and the period is designated as p. Such chain is designated as clock or velocity chain, as far as it provides the clock signal, as well as the encoder velocity. In the other chain, the positions of the apertures are identical, however, not all the apertures are present at those positions. By this means, an ID code is provided to this second chain, designated as ID chain, or position contrast, as position sensor able to determine the quasi-absolute position, the ID code should not be arbitrary. It should be selected according to the De Bruijn sequence [73]. Let us explain the reason next. Let us consider that the total length of the encoder is L. The number of different dis- Electronics 2021, 10, 1154 crete positions that must be codified in order to determine the relative position of the3 en- of 11 coder with regard to the reader is thus L/p. Therefore, the number of bits necessary to unequivocally differentiating the discrete positions of the encoder should satisfy chain. In applications of the encoders 𝑁 as≥chipless-RFID 𝑙𝑜𝑔 (𝐿/𝑝) tags, the ID code can be arbitrary. (1) The clock chain gives the instants of time to read the ID code from the ID code chain. By Implementing contrast, as position as many sensoraperture chains as number able to determine of bits N is not the quasi-absolute a realistic position, thesolution ID code because should notit represents be arbitrary.a penalty It should in be terms of encoder selected size. according toHowever, the De Bruijnif the ID code[73]. sequence of the Let complete us explain (single) position the reason next.chain follows the De Bruijn sequence, any sub-set of N sequen- tial bits is us Let unique (it is different consider to anylength that the total other).ofTherefore, the encoder the is bitL.corresponding The number to of adifferent certain position, plus the N—1 bits of the previous positions univocally determine discrete positions that must be codified in order to determine the relative position of the the position. However, encoder with it is regard necessary to know to the readerthe direction is thus of motion, the L/p. Therefore, and, moreover, number after of bits a system necessary to reset, the encoder unequivocally must be displaced differentiating N positions the discrete before positions an absolute of the positionsatisfy encoder should can be deter- mined. For this main reason, the designation of these encoders should be quasi-absolute, rather than absolute. N ≥ log2 ( L/p) (1) The photograph of the fabricated 16-bit encoder is depicted in Figure 1. The encoder Implementing was 3D-printed by meansas many aperture of the chains Ultimaker as number 3 Extended of bits N 3D printer, is not using a realistic dielectric solution Polylactic because Acid (PLA) it as represents filament. aPriorpenalty to theinimplementation terms of encoder size. of the However, encoder, and inif order the IDtocode obtain of the dielectric constant of the PLA by means of a Keysight 85072A 10-GHz Split CylinderN the complete (single) position chain follows the De Bruijn sequence, any sub-set of sequentialabits Resonator, is unique slab square-shape (it issample different todimensions (the any other). are Therefore, providedthe bybit thecorresponding split cylinder to a certain position, plus the N—1 bits of the previous positions univocally determine resonator) was printed. The measured dielectric constant of this dielectric PLA is εr = 3, the position. However, it is necessary to know the direction of motion, and, moreover, although this value can slightly suffer variations depending on the PLA properties [74,75]. after a system reset, the encoder must be displaced N positions before an absolute position After having measured the dielectric constant, the encoder was fabricated using the can be determined. For this main reason, the designation of these encoders should be following printer parameters: nozzle size 0.44 mm, printing temperature 200 °C, build quasi-absolute, rather than absolute. plate (or bed plate) temperature 60 °C, printing speed 70 mm/s and infill density 100%. The photograph of the fabricated 16-bit encoder is depicted in Figure 1. The encoder The (rectangular) apertures of both chains, if present, are located at identical positions, as was 3D-printed by means of the Ultimaker 3 Extended 3D printer, using dielectric Polylactic indicated before. The dimensions of the encoder were optimized, according to electro- Acid (PLA) as filament. Prior to the implementation of the encoder, and in order to obtain magnetic simulations, as it will be later shown. It is also important to emphasize that other the dielectric constant of the PLA by means of a Keysight 85072A 10-GHz Split Cylinder dielectric substrates, even flexible [76], could be used as encoders, but in this case, we took Resonator, a square-shape slab sample (the dimensions are provided by the split cylinder advantage of the high versatility and faster fabrication process of 3D printing, previously resonator) was printed. The measured dielectric constant of this dielectric PLA is εr = 3, demonstrated although this in the can value RFID devices slightly [77–79]. suffer variations depending on the PLA properties [74,75]. Photographofof Figure1.1.Photograph Figure the the 3D-printed 3D-printed electromagnetic electromagnetic encoder. encoder. TheThe total total area area of the of the encoder encoder is is 30 × 66 mm 2 , with the following aperture dimensions: d = 11.5 mm; d = 15.9 mm, w = 2 mm, 30 × 66 mm , with the following aperture dimensions: d1 = 11.5 2 1 mm; d2 = 15.92 mm, w = 2 mm, s = 2 mm s = 2and mm g =and1.9gmm. = 1.9The mm.thickness of the encoder The thickness is 1.5 mm. of the encoder is 1.5 mm. After having measured the dielectric constant, the encoder was fabricated using the following printer parameters: nozzle size 0.44 mm, printing temperature 200 ◦ C, build plate (or bed plate) temperature 60 ◦ C, printing speed 70 mm/s and infill density 100%. The (rectangular) apertures of both chains, if present, are located at identical positions, as indicated before. The dimensions of the encoder were optimized, according to electromag- netic simulations, as it will be later shown. It is also important to emphasize that other dielectric substrates, even flexible [76], could be used as encoders, but in this case, we took advantage of the high versatility and faster fabrication process of 3D printing, previously demonstrated in the RFID devices [77–79]. Concerning the sensitive part of the reader, it consists of a microstrip line loaded with three complementary split ring resonators (CSRRs) etched in the ground plane, as Figure 2 Electronics 2021, 10, x FOR PEER REVIEW 4 of 11 Electronics 2021, 10, 1154 4 of 11 Concerning the sensitive part of the reader, it consists of a microstrip line loaded with three complementary split ring resonators (CSRRs) etched in the ground plane, as Figure 2 illustrates.ItIthas illustrates. hasbeen beendemonstrated demonstratedthat thatCSRR-loaded CSRR-loadedlines linesareareefficient efficientpermittivity permittivity sensors, based sensors, based onon the the variation variation experienced experienced by by the the resonance resonance frequency frequency whenwhen aa certain certain material under material under test test (MUT) (MUT) is is placed placed on on top topofofthe theresonator resonator[80–85]. [80–85].Indeed, Indeed,the thereported reported encodersexhibit encoders exhibitaasignificant significantpermittivity permittivitycontrast contrastbetween betweenthe theapertures apertures(with (withεrεr==1) 1)and and the substrate the substrate (PLA (PLA with with εεrr == 3). 3). Consequently, Consequently, itit isis expected expectedthat,that,by byencoder encodermotion, motion,the the presence of PLA on top of the sensitive part of the reader (i.e., presence of PLA on top of the sensitive part of the reader (i.e., the absence of the absence of the apertures) apertures) shifts down shifts down the the resonance resonance frequency frequency of of the the resonant resonantelements. elements. NoteNote that that ifif the the quality quality factorof factor ofthe theCSRRs CSRRsisissignificant, significant, thethe excursion excursion experienced experienced by theby the transmission transmission coeffi- coefficient cient at theatfrequency the frequency of resonance of resonance of theofCSRR the CSRR covered covered by the by the substrate substrate should should be be high, high, a a necessary necessary condition condition to to ensure ensure thethe functionality functionality andand robustness robustness of of thethe system. system. Figure2. Figure 2. Photograph Photographof ofthe thetop top(a) (a)and andbottom bottom (b)(b) side side of of thethe reader. reader. TheThe reader reader waswas implemented implemented on on Rogers the the Rogers RO4003C RO4003C substrate substrate with with thickness thickness h =mm, h = 0.81 0.81 dielectric mm, dielectric constant constant εr =and εr = 3.38 3.38 and loss loss tangent tangent tanδ = Dimensions 0.0022. Dimensions are: l1 =are: 26.6l1 mm; = 26.6l mm; l2 = 3.8 mm, l3 = 10 mm, w1 = 6.4 mm, w2 = 1.9 tanδ = 0.0022. 2 = 3.8 mm, l3 = 10 mm, w1 = 6.4 mm, w2 = 1.9 mm, mm, lc = ld = 10.5 mm; lp = 14.5 mm, wr = 2.9 mm. CSRR slots width is c = 0.5 mm, and ring splits are lc = ld = 10.5 mm; lp = 14.5 mm, wr = 2.9 mm. CSRR slots width is c = 0.5 mm, and ring splits are sd = 0.4 mm, sc = 1.6 mm and sp = 6.2 mm. sd = 0.4 mm, sc = 1.6 mm and sp = 6.2 mm. Note that Note that the the presence presence of ofthree threeCSRRs CSRRsobeysobeysthe thefact factthat, in in that, order orderto obtain to obtainthethe en- coder velocity (and clock signal), the quasi-absolute position, and encoder velocity (and clock signal), the quasi-absolute position, and the motion direction, the motion direction, three signals three signals are are needed. needed. The The three three CSRRs CSRRs are are tuned tuned toto different differentfrequencies, frequencies,and andcancanbe be labeled by sub-indexes, in order to differentiate between the position labeled by sub-indexes, in order to differentiate between the position resonator, CSRRp , thep, resonator, CSRR the clock clock resonator resonator CSRR CSRR c, and the motion direction resonator CSRRd. c , and the motion direction resonator CSRRd . The electromagnetic simulation of The electromagnetic simulation of the the bare bare reader reader isis shown shown in in Figure Figure 3,3, where where the the three notches corresponding to the frequencies coherently designated three notches corresponding to the frequencies coherently designated as f p = 4.030 GHz,fc as fp = 4.030 GHz, f=c 4.300 = 4.300GHzGHzand andfdf=d 4.570 = 4.570 GHzGHz can canbebeseen. seen.These Thesenotches notches(or (or dips) dips) reflect the injected reflect the injected signals at signals at the the frequencies frequencies of of interest. interest. This This situation situation corresponds corresponds to to the the encoder encoder apertures, apertures, whichcan which canbe be assigned assignedthe the logic logic state state “1”. “1”. However, However, as as previously previously mentioned, mentioned,the thepresence presence of PLA at certain distance on top of the sensitive part of the reader of PLA at certain distance on top of the sensitive part of the reader shifts down the whole shifts down the whole frequency response. frequency response. Thus, Thus, instead of havinghaving a notch notch or or dip dip atat the the frequencies frequencies of of interest, interest, the presence the presence of of this thisdielectric dielectric substrate substrate allows allows the thesignal signalto tobebetransmitted transmitted(equivalent (equivalentto to the logic the logic state state “0”), “0”), as as can canbebeobserved observedin inthe thegraph graphof ofFigure Figure3.3. Electronics 2021,10, Electronics 2021, 10,1154 x FOR PEER REVIEW 55 of of 11 11 Figure 3. Electromagnetic (EM) simulation and measured frequency response of the bare reader, and the reader loaded with PLA at certain distances (air gap). The EM was obtained by means of the Keysight Momentum software. The three harmonic signals necessary for reading purposes are tuned to frequencies fp, fc and fd, as depicted in Figure 4. Such three harmonic signals can be injected simulta- neously to the input port of the microstrip line by means of a combiner, and the corre- sponding AM signals, with different carrier frequencies, separated at the output port by means of a triplexer. Then, the envelope function, containing the relevant information, can be inferred by means of an envelope detector connected to each output port of the tri- plexer, as done in several reported prototypes of electromagnetic encoders [11,18]. According to the explained working principle, it is clear that the position chain will generate as many dips as apertures in the chain. The instants of time to infer the ID code of that chain are given by the dips generated in the channel corresponding to the clock chain. Also,Figure from theElectromagnetic time lapse betweensimulation adjacent dipsmeasuredin the envelope function generated Figure 3. 3. Electromagnetic (EM) (EM) simulationand and measuredfrequency frequencyresponse responseofofthe thebare reader, bare reader,and by the clockthechain, the velocity is inferred, provided the period, p, is well known. The andreader the reader loaded withwith loaded PLAPLA at certain distances at certain (air(air distances gap). TheThe gap). EMEMwaswasobtained obtained bybymeans means ofof the system providesKeysight the the Keysight instantaneous Momentum Momentum software.velocity, as far as such velocity does not significantly software. change during time intervals corresponding to the lapse between adjacent dips in the en- velope function.The Thus, The three threeit is also possible harmonic harmonic signals signals tonecessary measure for necessary the instantaneous forreading reading purposes purposesacceleration. areare tuned tuned toFor frequencies to frequencies f p, the determination ffpc, and fc and of the f d ,fdas , as motion depicted depicted direction, ininFigure Figure it 4. 4.is Such not Such necessary three threeharmonic an harmonic additional signals can signals canchain. bebe The injected injected simultane- simulta- clock/velocity ously chain neously is touseful to the input the for portthat input purpose, of the port ofmicrostrip provided the microstrip line bythe CSRR means line ofd is by means conveniently a combiner, andlocated of a combiner, the corresponding and the corre- with regardAM to the sponding CSRR signals, AM c. Particularly, with different signals, the withcarrier CSRR different d and the separated frequencies, CSRRc areat carrier frequencies, located as at the output separated indicated port by means the output portofby a in Figure 2, triplexer. so that the Then, the apertures envelope of the function, clock chain containing first cross the one relevant resonator means of a triplexer. Then, the envelope function, containing the relevant information, can information, and, immedi- can be inferred by ately, the otherbe means one. Thus, inferred of by anfrom envelope means detector theoflead an or lagconnected envelope between to each the detector output to corresponding connected port of the envelope each triplexer, output port as func- of done in the tri- several tions, the motion reported direction prototypes can be of inferred. electromagnetic encoders plexer, as done in several reported prototypes of electromagnetic encoders [11,18]. [11,18]. According to the explained working principle, it is clear that the position chain will generate as many dips as apertures in the chain. The instants of time to infer the ID code of that chain are given by the dips generated in the channel corresponding to the clock chain. Also, from the time lapse between adjacent dips in the envelope function generated by the clock chain, the velocity is inferred, provided the period, p, is well known. The system provides the instantaneous velocity, as far as such velocity does not significantly change during time intervals corresponding to the lapse between adjacent dips in the en- velope function. Thus, it is also possible to measure the instantaneous acceleration. For the determination of the motion direction, it is not necessary an additional chain. The clock/velocity chain is useful for that purpose, provided the CSRRd is conveniently located with regard to the CSRRc. Particularly, the CSRRd and the CSRRc are located as indicated in Figure 2, so that the apertures of the clock chain first cross one resonator and, immedi- ately, the other one. Thus, from the lead or lag between the corresponding envelope func- Figure Figure 4. Schematic of4.the Schematic ofreader/encoder tions, the proposed the proposed motion reader/encoder direction canbased system, system, be inferred. based onencoders, on all-dielectric all-dielectric withencoders, with reading synchronous synchronous and motion direction reading detection and motion direction detection capability. capability. According to the explained working principle, it is clear that the position chain will generate as many dips as apertures in the chain. The instants of time to infer the ID code of that chain are given by the dips generated in the channel corresponding to the clock chain. Also, from the time lapse between adjacent dips in the envelope function generated by the clock chain, the velocity is inferred, provided the period, p, is well known. The system provides the instantaneous velocity, as far as such velocity does not significantly change during time intervals corresponding to the lapse between adjacent dips in the envelope function. Thus, it is also possible to measure the instantaneous acceleration. For the determination of the motion direction, it is not necessary an additional chain. The clock/velocity chain is useful for that purpose, provided the CSRRd is conveniently located with regard to the CSRRc . Particularly, the CSRRd and the CSRRc are located as indicated in Figure 2, so that the apertures of the clock chain first cross one resonator and, immediately, Figure the 4. Schematic other of the one. Thus, fromproposed reader/encoder the lead system, or lag between based on all-dielectric the corresponding encoders, envelope with functions, synchronous reading and motion direction detection capability. the motion direction can be inferred. Electronics 2021, 10, x FOR PEER REVIEW 6 of 11 Electronics 2021, 10, 1154 6 of 11 3. Results 3. Results The sensitive part of the reader was measured by means of a vector network analyzer The sensitive part of the reader was measured by means of a vector network analyzer (model Agilent N5221A). The behavior of this prototype without any dielectric in the prox- (model Agilent N5221A). The behavior of this prototype without any dielectric in the imity (bare reader) is shown in Figure 3, and it was in good agreement with the electro- proximity (bare reader) is shown in Figure 3, and it was in good agreement with the magnetic simulation. Later, a dielectric PLA slab was placed on top of the reader, and it electromagnetic simulation. Later, a dielectric PLA slab was placed on top of the reader, was moved upwards, thus modifying the air gap, until the behavior was the expected one. and it was moved upwards, thus modifying the air gap, until the behavior was the expected The air gap was set to 0.2 mm. This value differs from the nominal air gap considered in one. The air gap was set to 0.2 mm. This value differs from the nominal air gap considered the EM simulation (of 0.5 mm). One reason to explain this effect is the EM software, which in the EM simulation (of 0.5 mm). One reason to explain this effect is the EM software, considers that the substrates are infinite, thereby slightly modifying the effective dielectric which considers that the substrates are infinite, thereby slightly modifying the effective constant. constant. dielectric Another Another possible possible cause can be attributed cause to the variation can be attributed of theofdielectric to the variation con- the dielectric constant of the PLA, as explained before, due to fabrication tolerances. In any case, itim- stant of the PLA, as explained before, due to fabrication tolerances. In any case, it is is portant to to important bear in in bear mind mind that thatthe thehigher higherthe theair airgap, gap,thethelower lowerisis the the dynamic dynamic rangerange at at the the operation frequencies, operation frequencies, thatthat is is to to say say that that the the lower lower isis the the difference difference between between the the two two logic logic states. On the other hand, if the air gap is reduced, the dynamic states. On the other hand, if the air gap is reduced, the dynamic range can be improved range can be improved but, due but, due toto the the fact fact that that the the frequency frequency behavior behavior is is shifted down, the shifted down, the frequency frequency notches notches cancan affect the vicinity frequency responses affect the vicinity frequency responses (overlap). (overlap). In order In order toto read read the the encoder encoder of of Figure Figure 1 1 with with the the reader reader of Figure 2, of Figure 2, using using the the ar- ar- rangement of Figure 4, the setup depicted in Figure 5 was rangement of Figure 4, the setup depicted in Figure 5 was used. The displacement ofused. The displacement of the the encoder over encoder over the the reader reader waswas achieved achieved by by means means of of aa linear linear displacement displacement system system (model (model STM 23Q-3AN). STM 23Q-3AN). It It is is necessary necessaryto togenerate generatethreethreeharmonic harmonicsignals signals(tuned (tunedtotof pfp = = 4.030 4.030 GHz, GHz, ffcc==4.300 4.300GHzGHzand andfdf=d 4.570 = 4.570 GHz) GHz) to to synchronous synchronous reading reading thethe ID IDcode of the code of theencoder, encoder, as well as to determine the encoder direction. However, as well as to determine the encoder direction. However, as a proof of as a proof of concept, instead of of injecting simultaneously three signals generated generated by means of VCOs, the abovementioned abovementioned vector network network analyzer analyzer was was used used toto independently independently generate generate such such signals. signals. AsAs mentioned mentioned before, at the output port of the sensitive part of the reader, reader, the the signal signal waswas AMAM modulated modulated by the encoder encoder motion. motion. Thus,Thus,a aSchottky Schottkydiode diode(model (modelAvago Avago HSMS- HSMS- 2860) 2860) waswasusedusedto obtain to obtainthethe envelop envelop function. function.Since the the Since diode input diode impedance input impedance waswasdifferent fromfrom different the out- the put impedance output impedance of the sensitive of the sensitive part partof of the the reader, reader,a acirculator circulator(model (modelATM ATM ATc4-8) ATc4-8) was was added between these devices devices to prevent undesired reflections. Finally, an oscilloscope to prevent undesired reflections. Finally, an oscilloscope (model Agilent MSO-X-3104A) connected to the diode, through the N2795A N2795A activeactive probe probe (with capacitanceCC == 1 pF and resistance (with capacitance resistance R R = 1 MΩ), MΩ), was used for visualizing the envelope envelope functions (ID code and clock signals). Figure 5. Photograph of the experimental setup. According to these results, the clock chain first crossed the CSRRc and then the CSRRd. Thus, the direction of motion was determined. This was the first electromagnetic encoder able to provide the ID code synchro- nously, and able to discriminate the direction of motion, implemented without metallic Electronics 2021, 10, 1154 inclusions. As compared to the encoder reported in [67], the proposed encoder is7 very of 11 robust against friction (eventually caused by unexpected contact between the encoder and the reader, as consequence, e.g., of vibrations), since the inclusions are simple apertures. Thus, these 3D-printed encoders may be of interest as quasi-absolute encoders in motion Figure 6 depicts the measured envelope functions inferred from the experimental control setup ofapplications. Nevertheless, Figure 6, obtained the reported by displacing prototype the encoder demonstrated at constant that velocity of v =by10opening mm/s. apertures in a 3D-printed material, it was possible to generate an encoder containing As it can be seen, the ID code was correctly read, and the clock signals were shifted, as an identifying ID code. Thus, with the results of this work, the possibility of generating consequence of the different positions of the resonators CSRRc and CSRRd in the reader. an identifyingto According tag embedded these results,inthe a certain fabricated clock chain (3D-printed) first crossed material the CSRR or consumer prod- c and then the CSRRd . uct during Thus, fabrication the direction was possible. of motion was determined. Figure 6. Measured Figure 6. Measured envelope functions when when the the encoder encoderofofFigure Figure11isisdisplaced displacedover overthe thereader reader of Figure of Figure 22 at at constant constant velocity velocity of of vv = = 10 mm/s. mm/s.The Thevertical verticaldistance distancebetween betweenthe theencoder encoderand andthe the readerisis0.2 reader 0.2mm. mm. This was the first electromagnetic encoder able to provide the ID code synchronously, 4. Conclusions and able to discriminate In this the direction paper, electromagnetic of motion, encoders implemented with synchronouswithout readingmetallic inclusions. and motion direc- As compared to the encoder reported in [67], the proposed encoder is very tion detection capability have been presented. The encoder is implemented by means robust againstof friction (eventually caused by unexpected contact between the encoder and the chains of apertures made in a host substrate, which can be any dielectric material, even reader, as consequence, flexible, with e.g., of vibrations), a dielectric constantsince the inclusions significantly arethan different simple the apertures. Thus, In one of vacuum. these this 3D-printed encoders may be of interest as quasi-absolute encoders in motion control work, 3D-printing techniques were employed due to their versatility and good perfor- applications. Nevertheless, the reported prototype demonstrated that by opening apertures mance already demonstrated with the implementation of other RFID devices. For that in a 3D-printed material, it was possible to generate an encoder containing an identifying purpose, a prototype 3D-printed electromagnetic encoder with 16 bits has been fabricated ID code. Thus, with the results of this work, the possibility of generating an identifying in dielectric PLA filament. On the other hand, the sensitive part of the reader detects the tag embedded in a certain fabricated (3D-printed) material or consumer product during presence of the apertures by virtue of the permittivity contrast with the host substrate fabrication was possible. material (PLA), since the sensitive part of the reader is, indeed, a permittivity sensor. In the 4. reported encoders, the relevant information relative to the ID code, encoder velocity Conclusions (and acceleration, if encoder motion is non-uniform), and motion direction is contained in In this paper, electromagnetic encoders with synchronous reading and motion direc- the envelope functions of the AM modulated signals generated at the output ports of the tion detection capability have been presented. The encoder is implemented by means of corresponding channels. The measured envelope functions are indicative of the function- chains of apertures made in a host substrate, which can be any dielectric material, even ality of the proposed reader/encoder system. Such systems, based on all-dielectric 3D- flexible, with a dielectric constant significantly different than the one of vacuum. In this work, 3D-printing techniques were employed due to their versatility and good perfor- mance already demonstrated with the implementation of other RFID devices. For that purpose, a prototype 3D-printed electromagnetic encoder with 16 bits has been fabricated in dielectric PLA filament. On the other hand, the sensitive part of the reader detects the presence of the apertures by virtue of the permittivity contrast with the host substrate material (PLA), since the sensitive part of the reader is, indeed, a permittivity sensor. In the reported encoders, the relevant information relative to the ID code, encoder velocity (and acceleration, if encoder motion is non-uniform), and motion direction is contained in the envelope functions of the AM modulated signals generated at the output ports of the corresponding channels. The measured envelope functions are indicative of the functionality of the proposed reader/encoder system. Such systems, based on all-dielectric Electronics 2021, 10, 1154 8 of 11 3D-printed encoders, can find applications in motion control (where robustness against mechanical friction is a key aspect), chipless-RFID tags (where the encoder can be embed- ded in the product during 3D-printing fabrication), and other potential applications in industrial scenarios. Author Contributions: Conceptualization, F.P., C.H., and F.M.; methodology, F.P. and C.H.; design, F.P. and C.H.; experimental validation, F.P. and C.H.; paper writing, F.P., F.M.; general supervision, F.M. 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