Modelling and Design of HF RFID Passive Transponders with Additional Energy Harvester

The huge progress in electronics technology and RFID technique gives the opportunity to implement additional features in transponders. It should be noted that either passive or semipassive transponders are supplied with energy that is derived from the electromagnetic field generated by the read/write device and its antenna.This power source is used to conduct radio-communication process and excess energy could be used to power the extra electronic circuits, but the problem is to determine the additional power load impact on the RFID system proper operation and size of interrogation zone.The ability to power the supplementary electronic blocks applied in the HF passive transponders is discussed in detail this paper. The simulation model and test samples with a harvester that recovers energy from the electromagnetic field of read/write device and its antenna have been developed in order to conduct investigations. The harvested energy has been utilized to supply a microprocessor acquisition block for LTCC pressure sensor developed in research previously described by authors.


Introduction
The (radio frequency identification) RFID technique is generally used in the processes of object identification.The permanent cost reduction of a single transponder and standardizations of operating conditions for all involved devices affect the broad implementation of these solutions in security and access control systems, industrial logistics (material supply or goods shipment), identification of measurement samples or valuable materials (in various areas of science, technology, or medicine), and forth [1][2][3][4][5].Further improvements are feasible in many cases thanks to great achievements in modern electronics and significant progress in new technology of hybrid microelectronic circuits [6][7][8].For example, it gives the opportunity to implement additional features in transponders.The extra functional blocks are usually powered by a built-in supply source-disposable battery.Unfortunately, since the batteries are used, the costs of applications with such transponders are very high and system maintenance is inconvenient with respect to totally passive solutions.However, it should be noted that either passive or semipassive transponders are supplied with energy that is derived from the electromagnetic field generated by the read/write device (RWD).This power source is used to conduct a radio-communication process and excess energy can be used to power the extra electronic circuits.But the problem is to determine the additional power load impact on the RFID system proper operation and size of interrogation zone (IZ).
The necessity to integrate a pressure sensor made in (low temperature cofired ceramic) LTCC technology [9] with a passive transponders consisted not only of a chip and antenna but also of a radio-frequency (RF) energy harvesting circuit (Figure 1) has been the key impulse to conduct the presented research.
The considerations also include interrogation zone (IZ) determination problems.The IZ is a space around the RWD where communication and energy conditions are met [6,10,11].Its shape and size is determined by the possibility of performing two tasks: providing transponders with the correct power supply and, as a result, establishing conditions for data radio communication with the RWD.It is the main parameter International Journal of Antennas and Propagation because it comprehensively covers the matter of energy and communication activity of all RFID system parts.Since the interrogation zone is determined for the whole application (not for a single device), a knowledge base about essential properties of RFID equipment is necessary.But unfortunately, producers very often do not specify some essential electrical and structural parameters for their products.Because of this, the calculation of basic parameters describing interrogation zone (e.g., the maximal distance/range between transponder and the RWD antenna centre) is impossible.At the present stage of knowledge, it is the main reason why the practical implementation of anticollision identification is restrained especially in automated systems operating in dynamic conditions.In this situation, inefficient and timeconsuming trial and error methods are commonly used during system configurations.It does not provide reliable information on the operation and efficiency of the automated identification processes.The problems of RFID application predictability become even more important in the context of placing the physical quantity sensors in battery-less passive transponders.
The problem is further complicated in anticollision systems.The main characteristic of the single identification is the possibility to recognize only one object labelled with a radio transponder that has to be alone in the interrogation zone.In the case of the anticollision system, algorithms of multiaccess to radio channel are used and the communication process is carried out simultaneously with several transponders [10].This makes it possible to automatically distinguish many objects appearing in the IZ at the same time.In both identification systems, it is assumed that labelled objects are present in the operating zone Ω ID (Figure 1), but there is no certainty that they will be recognized.The situation is even more complicated when the dynamic processes (with variable location and/or orientation of object in space) will be analyzed instead of the stationary one (with fixed location and orientation of objects in space) [12].
In RFID systems, the transponders are supplied with energy deriving from the electromagnetic field generated by the RWD.This supply source is always used to conduct a radio-communication process.But in transponders with sensors (sensors with RFID interface), it is necessary to provide energy for powering additional functional blocks.
The semipassive transponders (Figure 1) have a built-in extra source (e.g., lithium battery) which can be exchangeable or not [10].A part of additional energy can be utilized to enlarge the size of interrogation zone but most of it is used for powering blocks of additional autonomous functions, such as measurement of physical quantities (humidity [13][14][15], temperature [16][17][18][19], light intensity [18], pressure [20], acceleration [21], gas [22], etc.), and writing gathered data in a builtin memory.These extra functions are carried out without the participation of RWDs.Because of battery, these types of transponders are more expensive (with respect to totally passive solutions) and have limited durability and there is necessity to replace worn out batteries.According to practice rules of RFID system usage, there is necessity to mechanically protect the batteries against thefts or to utilize disposable solutions (such transponders have to be replaced when the battery is exhausted or spoiled).However, it should be emphasized that the RWD has to be still active for properly conducting radio communication process, because the extra battery system can never be used for activating the transmitting circuit.It means that the antenna of transponder does not emit the electromagnetic field as it is in the case of conventional short range devices (SRD) [23,24].
Disadvantages of the above mentioned power supply, development in the branch of new materials, and availability of low power integrated circuits as well as trends towards utilizing alternative energy that can be harvested from the operating environment of RFID systems are the reasons why many current investigations are focused on the integration of sensors with passive transponders [25][26][27][28][29].The research results are useful in many applications of contactless automatic identification (medicine [30] and healthcare [31,32], building industry [33], and supply chains [34]), although it is impossible to save measurement data in memory of such a hybrid construction without activation of RWD devices.
On the base of elaborated 3D IZ model, authors present (Section 2) the aspects connected with recovering excess energy which is supplied by an RWD device and with its utilization to power a pressure sensor with a passive RFID chip.Using the model and a passive chip with energy harvester, they propose the construction of sensor with RFID interface (Section 3) that is designed to work in the HF band of inductively coupled RFID systems (operating frequency  0 = 13.56MHz), according to the communication protocol ISO/IEC 15693 [35].Analyzing conducted tests and derived results (Section 4), readers can see that synthesis model of 3D interrogation zone can be used for predicting spatial placements of sensors in different kinds of RFID system applications [36].

Model of Passive Transponder with Energy Harvester
RFID systems working in the HF band utilize the typical operating frequency  0 = 13.56MHz.Because the wavelength  is about 22 m, the RWD and transponder antennas are made in the form of small loop in relation to .The inhomogeneous magnetic field generated into the RWD antenna vicinity is the medium for both transferring energy and wireless data exchange.The most common mean of data transmission is the load modulation with amplitude-shift keying.The load modulation with subcarrier is used because of the necessity for transferring energy to the passive transponders.These mechanisms are implemented in protocols normalized by ISO/IEC 15693, 14443, 18000-3, and others.The inductively coupled RFID HF systems operate in the zone for which an inhomogeneous magnetic field (characterised by the induction  or magnetic field strength ) and strong coupling (characterised by the mutual inductance ) between antennas of the communication set occur (Figure 2).
The efficiency estimation of energy transmission from RWD to passive transponder is complicated for this kind of medium, especially in the proposed solution with autonomous features (e.g., module for measuring physical quantities).Since the extra module disturbs the proper operation of transponder, the careful study of its impact on main parameters is compulsory.This problem is explained in details on the elaborated model of a passive transponder with the active build-in block for harvesting energy from the RFID system environment (Figure 3).
The presented model is valid for all antenna arrangements existing in proximity [37] or long range [38] RWD devices.It is suitable for the transponder located in a (, , ) point of the Cartesian coordinate system (Figure 2).It includes all elements of real solution: loop antenna, chip with extra energy harvester, and in addition microprocessor for controlling autonomous features.The microprocessor system tasks are to gather data from sensors (e.g., by using A/C converter) and write it to a chip internal memory by using a communication interface (e.g., I 2 C bus).It is powered by the harvester and it is active only when the transponder is in the interrogation zone and the energy conditions are satisfied.
The antenna loop is a parallel circuit in which   is the self-inductance and   represents the resistance of wire used for creating the winding and it also characterizes ohmic losses.The electric capacity   is the resultant of all capacitances between the coils and it results from a uniform distribution of wire electrical parameters along the entire length of winding.The equivalent of loop antenna has been also included in the general scheme in order to facilitate a subsequent experimental verification (  and   denote the serial resistance and the inductance of series antenna circuit).The source   represents voltage inducing in the antenna loop when the transponder is in the magnetic field of read/ write device.It is expressed by (1) where pulsation  = 2 0 and   means the current in the winding as The maximum value of voltage   on loop antenna terminals is obtained for the parallel resonance between the inductance   and the capacitance   of active chip.The capacitance   is expressed by (2) where   means the selfcapacity of chip (an energy storage cell for powering radio communication processes),   is the resultant of rectifier and voltage regulator capacitances, and   describes the electric capacity of energy harvester as The synthesis of rectifier and voltage regulator is made separately [39,40].Although the rectification takes place in a half-wave or full-wave rectifier [41], it is controlled by the voltage regulator [39].In the case of passive transponders working in the HF band, both of the circuits are commonly realized on fast switching diodes in MOS technology [42].It should be mentioned that the proposed model will be also valid for all blocks manufactured on a flexible substrate in a printed electronic technology which is possible thanks to the tremendous progress in the nanomaterial science [43,44].
The rectifier and voltage regulator are characterized by the capacitances included in the   quantity and by the resistances   and   .These elements of transponder equivalent affect electric circuit parameters which are seen at the loop antenna terminals.The   is a generalized parameter of voltage rectifier circuit obtained on the base of diode graphical model (static characteristic).Its value mainly depends on a circuit design and elements included in a specified construction of chip from a given manufacturer.Since the piecewise linear approximation of diode characteristic is generally used in electronic circuit analyses, the linear nature can be assumed for this value.The second quantity   represents the internal structure of voltage regulator.Although this block is used for powering the internal structure of transponders, it can be assumed that the stabilized voltage   is constant.This approximation is justified due to negligibly small current drawn and limited impact of this phenomenon on circuit parameters seen at the loop antenna terminals.
The variable resistance   represents the harvester internal structure.If minimum losses are assumed in this block (the power   at the input is almost equal to the power   supplied to the autonomous feature module), the resistance value can be determined from (3) in which  RMS means RMS voltage   and   ,   , respectively, dc current and voltage on the power output of energy harvesting block as Since the harvester derives energy from the electromagnetic field of read/write device and therefore affects the interrogation zone, the possibility of its functional property configuration is usually available by manufacturers.Depending on a chip design, it is possible to reduce or control the output values of voltage   and/or maximum current   [45,46].These properties describe appropriate operation boundaries of energy recovery block and ability to power the autonomous function modules together with connected sensors.
The modulator capacitance   and resistance   are also included in the model (Figure 3).The proper transponder operation does not depend on these parameters in the considered range.It is due to the fact that radio-communication processes are conducted according to adequate protocols (for the HF band: ISO/IEC 15693, 14443, 18000-3, and others) and the load modulation with subcarrier is used for transmitting data in the transponder-RWD direction.
Taking into account the underlying assumptions, it is possible to determine the voltage value induced at the loop antenna terminals under the load of modelled chip input circuits (4).Then, the voltage drops in the unloaded divider   and   are described by (5) as Because the   part is constant for   >   , the variable resistance   of voltage regulator is given by ( 6) The minimum value of   (  min ) voltage is the base for determining the interrogation zone.The   min value is the characteristic parameter of chip construction.It clearly impacts on (7) (where  0 = 4⋅10 −7 H/m) and on its base it is possible to determine the minimum value of magnetic field strength  min as The field strength  min is the elementary parameter that defines the IZ.It is differentiated on the base of value   min for the given direction of data transmission and the kind of operations (read/write) proceeded in the internal transponder's memory.The operation of transponderwith the active harvester is also described by (7).If the 1/  factor is omitted then the  min parameter is valid for the typical passive transponder.

Implementation of Passive Chip with Energy Harvester
The special laboratory stand has been made for experimental verification of the calculated value  min .The stand allows to measure the maximum distance between the RWD and transponder antennas for which the correct operation of RFID system is ensured.
The batteryless system has been made in order to exemplify the possibility of passive chip integration with the previously presented pressure sensor [9].The special circuit for recovering energy from the RFID system environment is the built-in chip (Figure 4).The presented solution is dedicated for transponders that work in inductively coupled RFID systems in the HF band ( 0 = 13.56MHz) and operate according to the communication protocol ISO/IEC 15693 [35].
The chip M24LR16E-R STMicroelectronics [45] is an integral part of transponder.It is equipped with a 16 kb EEP-ROM with a password protection.The memory organisation depends on the access mode: 2048 B in I 2 C mode and 512 blocks of 32 b in RF mode.The access is possible by a dual interface: wireless RFID and serial wire link I 2 C. The energy harvesting block can operate in four configurable ranges of current sink-the maximum value of   max (Figure 3) is equal to 6 mA, 3 mA, 1 mA, or 300 A.The internal tuning capacitance (  = 27.5 pF for 1 V pp ) is included at the loop antenna terminals.
The square antenna has been synthesised in order to carry out the experimental verification.The winding diagram of loop antenna (Figure 5(a)) has been developed in the HyperLynx 3D EM 15.21 package (Mentor Graphics).The test antenna has been realized practically on a PCB substrate by using a CNC plotter LPKF ProtoMat S100 (Figure 5(b)).
The project has been prepared for the typical twosided FR-4 laminate (thickness: 1.55 mm, permittivity: 4.85, dielectric losses: 0.025 for  = 10 MHz, thickness of copper: 17.5 m) and for the assumed parameters of applied chip (  = 27.5 pF,  0 = 13.56MHz).The loop parameters of model calculated in the HyperLynx 3D EM package have been confirmed experimentally by measuring the test antenna using a two-port network analyzer (Agilent PNA-X N5242A) and the differential probe (Figure 5(c)) which has been prepared especially for connecting equipment without any disturbances.The antenna impedance   has been calculated from (8) on the basis of measured -parameters [47] (for  0 = 50 Ω) as The convergence of measured and calculated data confirms the accuracy of realized project (Table 1).The parallel resonance with the   capacitance at  0 frequency is achievable for obtained parameters.It means that this test antenna construction allows to effectively transfer energy from the RWD to the chip and additionally to power the extra feature blocks.
The expanded sensor block is built on the C8051F988 microprocessor (Figure 6).This (integrated circuit) IC is the industry's lowest power microcontroller of 8051 family (MCU).It consumes as little as 10 nA in a sleep mode (with

Tests and Results
Designed elements of HF long range RFID systems with inductive coupling (working frequency:  0 = 13.56MHz) have been used to carry out final tests of the elaborated method useful for determining the minimum value of magnetic field strength  min (read range).All measurements of the experiment have been performed in the specialized RFID laboratory.The special test stand has been prepared for this reason (Figure 7).The parameters used in the equivalent calculations have corresponded to the parameters of devices available in the laboratory.The laboratory equipment has made it possible to conduct all kinds of experimental research in the range of inductively coupled RFID system activity, in both the energy and communication aspects.
The square RWD antenna (side length  = 0.3 m, number of loop turns   = 1) has been installed in the test stand.The antenna was supplied by the RWD ID ISC.LR200 FEIG (output power 1 W, current in the winding   = 0.45 A).The calculation and measurement results of magnetic field strength in the symmetry axis of RWD antenna are presented in Figure 8.
The calculation has been made for the antenna model that had been considered in [38] in details.The calculated results have been compared with the measurements obtained in the laboratory stand using spectrum analyzer R&S FSL18, oscilloscope Tektronix DPO71254B, magnetic near-field probe HZ-14, and also P7504, CT1.
On the base of calculated and measured result convergence, the correctness of magnetic field strength determination in the elaborated test stand has been confirmed.The possibility of energy harvesting on different kinds of its value level has been also estimated.It has been necessary in order to establish appropriate conditions for powering the autonomous functions of the designed transponder.The measurement results of chip harvester efficiency are presented in Table 2.The measurements have been made by a Tektronix DMM4020 digital multimeter.The power consumption variations have been obtained by using elaborated load simulator  (it changed the power levels according to predicted activities of microprocessor control system).
The main application parameters of test transponders have been determined on the base of above mentioned calculations and measurements (Table 3).
The calculations have been performed for transponder model described by ( 6) and (7).The mathcad environment has been used for preparing calculation program tools.The data in Table 3 represents three modes of designed transponder: (1) energy harvester OFF, (2) energy harvester ON and   = 1 mW, and (3) energy harvester ON and   = 4 mW.The power   has been measured by an oscilloscope Tektronix DPO71254B with probes P7504 CT1 (index a in Table 3).The real communication process has been carried out between the RWD device and the prepared test samples of analyzed transponder and the transmission correctness has been controlled by the spectrum analyzer Tektronix RSA 3408B.The parameter   has been determined as the basic quantity of the interrogation zone and it denotes the maximum operating distance (read range) of transponder located in the symmetry axis of RWD antenna loop.The index b in Table 3 means the measurement of unique identifier (UID) in the test stand for inventory command (0x01) ISO/IEC 15693 (FEIG ID ISOStart 2011 version 08.03.01).
Calculations of the  min have been made by two methods.The index c in Table 3 means that the calculations result from (7) for the RWD-transponder model (square RWD antenna model [38]; transponder antenna loop: HyperLynx 3D EM model; chip: product specification data).The index d in Table 3 means that the calculations result from RWD antenna model derived on the base of measured   .The minimum value of magnetic field strength  min (index e in Table 3) has been measured by means of the spectrum analyzer R&S FSL18 and magnetic near-field probe HZ-14.
The result convergence confirms the accuracy and correctness of developed simulation equivalent to real passive transponder with harvesting module.

Conclusion
The basic application parameters presented in Table 3 have been calculated for the passive transponder electric circuit equivalent and also the same quantities have been measured for the test electronic circuit.The verification of the developed model is confirmed on the base of the obtained result convergence.The developed methodology for testing passive and semipassive transponders with harvester that derives energy from RFID system environment is the key approach to determine the tree-dimensional interrogation zone (e.g., by using Monte Carlo method [36]) for both single and anticollision RFID systems.It provides reliable information on the operation and efficiency of automated identification processes and it allows RFID system designers to eliminate inefficient and time-consuming trial and error methods.

Figure 1 :
Figure 1: Block diagram of passive and semipassive RFID system.

Figure 2 :
Figure 2: Passive RFID system with inductive coupling and energy harvesting mechanism.

Figure 3 :
Figure 3: Model and electric equivalent of passive RFID transponder with energy harvester.

Figure 4 :
Figure 4: Block diagram of the elaborated transponder.

Figure 7 :
Figure 7: Test stand in the RFID laboratory in the Department of Electronic and Communications Systems.

Figure 8 :
Figure 8: Magnetic field strength in test stand.

Table 1 :
Calculated and measured parameters of the transponder antenna.

Table 2 :
Measurements of power efficiency for energy harvesting block.