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This paper presents a design for a self-powered radio frequency identification (RFID) tag with a thin film bulk acoustic resonate piezoelectric power supply (PPS), which can be used for portable remote temperature monitoring. We call this system a PPS-RFID for short. RFID systems have found many applications in the internet of things (IOT) in the past decade. But semi-active RFID tags require an onboard battery which limits their applications in many fields. For these reasons, our research focuses on power sources for RFID tags. This paper emphasizes the circuit design and simulation of the PPS. In our tests, 0.283 mW was generated by the PPS at 1 Hz vibration by a 650 N impact force. The results showed that the integrated PPS could supply sufficient power for the designed PPS-RFID tag. The PPS-RFID tag can be widely used for temperature monitoring during mobile transport of perishable items such as medicines or food.
self-powered RFID, piezoelectric, temperature monitoring, l energy harvesting, internet of things
The development of radio frequency identification (RFID) technology has led to its wide spread use in modern supply logistics, public administration and in the retail industry. RFID systems use two types of tags. (1) Passive RFID tags must obtain their operating power from a RF-wave transmitted from a RFID terminal. The readable range of passive tags is limited which also limits the practical applications; (2) Semi-active RFID tags have an integrated battery to provide power, the battery is sensitive to environmental conditions and its lifetime is limited.
There are many disadvantages of conventional RFID tags. (1) The battery cannot be changed. That is if the battery is completely discharged, the RFID tag expires, although some manufacturers have adopted low power consumption technologies to extend the tag lifetime. At present, the labeled lifetime of semi-active RFID tags are around 2 to 7 years. If the tag needs to be read and rewritten frequently, the lifetime will decrease greatly. (2) Furthermore, the conventional semi-active RFID will be too large for portable devices as the battery is in integrated into the tag. For example, the semi-active RFID system with a temperature monitoring function was studied by E. Abad et al. . (3) One major issue is that the user cannot accurately predict or estimate the expiration date on which the integrated battery will be drained. Once the battery is exhausted, the tag will not work and its recorded information is lost. In some important applications, this is a critical issue limiting the use of RFID tags .
The PPS-RFID proposed in this paper can solve these problems inherent in conventional semi-active RFID tags. The purpose of our research is to address the power supply issue in semi-active RFID tags. Piezoelectric energy harvesting has been extensively investigated, because it offers a high power density compared to other electromagnetic and electrostatic energy scavenging methods .
Many researchers have studied the concept of utilizing piezoelectric material for energy generation over the past few decades. In one commercial example of power harvesting in shoes, a piezoceramic unimorph and a Polyvinylidene Fluoride (PVDF) foil were implemented in a brand-named running shoe. The feasibility of generating electrical power "parasitically" while walking was explored in 1998 by Kymissis et al., at the MIT Media Laboratory. The Kymissis power-harvesting shoe produced an average power of 1.8mw and 1.1mw using either (Pbbased Lanthanumdoped Zirconate Titanates) PZT unimorph or PVDF stave-like configurations, respectively . Another investigation for power harvesting with a drum transducer was studied by Wang et al., in 2007 at the Hong Kong Polytechnic University. 11 Î¼W across an 18 KÎ© resistor was obtained with a single drum transducer under a cyclic force of 0.7 N at 590 Hz under a 0.15 N pre-stressed condition . A third investigation of a piezoelectric-based power harvester which converted the impact energy from various mechanical vibrations on RFID-Tags was performed by Takeuchi et al. in 2007 at Tamagawa University. They developed a safety system to monitor children commuting to school and a network system to monitor the utilization of a conference room using the RFID-tag . Recent progress in a power harvesting system with piezoelectric-based charge generation, was studied by Kalyanaraman et al. in 2010 as an energy harvesting device for mobile phones and laptops.. A piezoelectric transducer was used to harvest the mechanical vibrations produced while typing on the keyboard, a PZT with 1.5Mpa lateral stress operating at 15 Hz and volume is 0.2 cm3 was used in their system, the energy density was 6 mW/cm3, 1.2 W and a 9 V output was obtained.
Many piezoelectric energy harvesters have been studied previously. Generally, these PPS were too large (centimeter scale) for portable devices, especially for use with MEMS-based devices. A PPS with a single piezoelectric layer is studied in this paper. It is small enough to be integrated in semi-active RFID tags. The PPS utilizes energy from the application environment, such as vibrations during transport. This tag still uses a backscatter mode for communications with the RFID reader. Therefore, PPS-RFID can be used in a moving process to record information. Finally, compared with conventional RFID systems, PPS-RFID has a faster reaction speed and efficiency .
With the development of semi-active RFID tags, the PPS-RFID tag has been gaining more attention. Although there are many research reports on the PPS, there are few reports on RFID tags with PPS, especially on semi-active RFID tags with a temperature sensing function [8, 9]. For these reasons, this paper presents a design for a new semi-active RFID tag, where a piezoelectric energy harvester is used as a power supply. Besides, other functional chips can be integrated into a RFID tag to implement many extra functions, for example, temperature sensing or humidity sensing. In our designed PPS-RFID tag, a temperature-sensing chip was integrated to realize temperature monitoring.
2 Theory and design
2.A. Operating principles and objectives of PPS-RFID
The aims of our research are to design a new RFID tag with PPS, which can address the disadvantages of conventional RFID (Passive RFID and semi-active RFID with batteries) and to find many new applications in IOT, especially in temperature monitoring during transport.
The fundamental source of power is a direct piezoelectric effect, which generates energy from the weight of the attached items and motion due to environmental-induced vibrations. When a force is applied onto the surface of a piezoelectric material, an electrical charge will be generated on the surface. This is enough voltage difference to power a circuit. A PPS-based micro power supply can be realized by using the proper rectifier circuit and energy-storage circuit in a RFID tag. Compared with a traditional semi-active RFID tag, the PPS-RFID tag has some advantages, such as reusability, and a longer service life making it more environmentally friendly.
The operating principle of the PPS-RFID is shown in Figure 1. A PPS-RFID is pasted under the weight. It is self-powered and records the ambient temperature when the goods are handled or subject to vibrations during the transportation process.
Figure 1 Operating principle of the PPS-RFID.
There are at least two objectives of the PPS-RFID tag. First, the PPS-RFID must be able to monitor the temperature of its surroundings. Second, the PPS-RFID system can record data from other types of sensors integrated in the tags, such as pressure and humidity sensors. Thus, users can trace the transport history of the goods and cross correlate with physical condition data to determine whether the food or medicines have spoiled. The information recorded by the PPS-RFID tag will be of serious importance for medical quality control and food safety management systems (FSMS).
2.B. Circuit design
The semi-active RFID chip IDS-SL900A, prototyped by IDS Microchip AG Inc. in April 2010 and was expected to begin mass production in October 2010, is adopted in our tag circuit.
The IDS-SL900A is an EPC global class 3 tag chip optimized for single-cell and dual-cell, battery-assisted smart labels with sensor functionality. The chip is ideal for applications using thin and flexible batteries but can also be powered from an RF field. This chip includes a temperature sensor and real-time clock (RTC) to accommodate temperature sensor data logging .
The IDS-SL900A is suitable for our PPS-RFID tag, because the temperature sensors are integrated into the chip and two interfaces are supplied for expandability with other external sensors. The most important benefit of the IDS-SL900A is that its power consumption is very low. Its quiescent power consumption is
and the maximum power consumption is
so it can be easily driven by a piezoelectric power source.
The on-chip RTC of the IDS-SL900A is started through the START LOG command in which the start time is programmed in UTC format. The interval for sensing and logging can be programmed to sample data from 1second up to 8 hours. The accuracy of the timer is Â±3% .
The IDS-SL900A has excellent thermal properties due to its packaging, making it suitable for temperature sensing. The package size of the chip is 5Ã-5 mm and it has 16 pins. A block diagram of the IDS-SL900A is shown in Figure 2ï¼Œand the main performance parameters of the IDS-SL900A are shown in Table I .
Figure 2 Block diagram of the IDS-SL900A 
According to the datasheet for the IDS-SL900A, the measurement range for the temperature sensors in the IDS-SL900A is from -20 â„ƒ to 60 â„ƒ and the measurement accuracy is Â±0.5 â„ƒ. So the chip is very suitable for monitoring the temperature in perishable items such as medical drugs or food. The working current of the IDS-SL900A in various operating modes is shown in Table II .
Table 1 summary of the performance parameters of the IDS-SL900A chip
EPC Class 1 & Class 3
Table 2 IDS-SL900A working current in various operating modes
The LT3011 is used as voltage regulator in the circuit for the PPS-RFID tag. LT3011 is made by Linear Technology Corporation and is a high voltage, micro power, low dropout linear regulator. The low quiescent current (46 Î¼A operating and 1 Î¼A shutdown) is well controlled during dropout. .
The schematic of the PPS-RFID circuit is show in Figure 3. It is based on the reference circuit for a self-powered RF tag designed by Kymissis and Kendall . The circuit schematic is divided into two parts, part A and part B. There are two circuit components in part A. The first component is the PZT piezoelectric power source, the second component is the analog rectifier circuit. There are another two circuit components in part B, one is the voltage regulator and energy storage circuit, the other is the IDS-SL900A semi-active RFID chip.
(a) Part A
(B) Part B
Figure 3 Schematic of the PPS-RFID circuit..
The output voltage from the voltage regulator LT3011 is adjusted by the ratio between R5 and R6 in Figure 3. From the datasheet, the expression for the output voltage is shown as follows:
Where, VADJ=1.24 V, IADJ=30 nA, so the output voltage ranges from 1.24 V to 60 V by adjusting the ratio between R5 and R6. For the IDS-900A, the input voltage is 1.5 V, R5=100 KÎ©, R6=200 KÎ© can be calculated by using expression (1) .
2.C. Piezoelectric principles of PPS
The operating principle of the PPS in this paper is via a direct piezoelectric effect. A longitudinal field model of a PZT-4 piezoelectric vibrator is considered in this paper. When a force is applied on the surface, which is perpendicular to the direction of polarization, electrical charges will be generated on the surface. PZT-4 and platinum were used as the piezoelectric and the electrode materials, respectively.
A diagram of the longitudinal field model for a single-layer piezoelectric vibrator, which is polarized in the Z-axis direction, is shown in Figure 4. The Mason equivalent circuit for the single-layer piezoelectric vibrator is shown in Figure 5 .
In Figure 5,
Where, Zp is the acoustic impedance, kp is the wave number of the piezoelectric layer, dp is the thickness of the piezoelectric layer, C0 is the static capacitance of the piezoelectric vibrator, Îµ0 is the permittivity in a vacuum, Îµr is relative permittivity of the piezoelectric layer, S is the area of the resonance range. N is the electromechanical conversion rate of the piezoelectric vibrator, expressed as
Where is the piezoelectric modulus, is the elastic rigidity coefficient of a short circuit. Z4 is the input impedance of mechanical port 1,Z5 is the input impedance of mechanical port 2, F1 and v1 are the input stress and velocity of mechanical port 1 respectively, F2 and v2 the input stress and velocity of mechanical port 2 respectively. The input impedance of the mechanical ports are different for different boundary conditions, for example: when the outside electrodes of the piezoelectric vibrator is connected to air, so the boundary condition of piezoelectric vibrator is free, thus the input impedance of mechanical port is zero. When the electrodes of the piezoelectric vibrator are pasted onto another object, so the piezoelectric vibrator is fixed, the input impedance of the mechanical port is infinite.
Figure 4 Diagram of a single-layer piezoelectric vibrator.
Figure 5 The Mason equivalent circuit
From Figure 5, the single-layer piezoelectric vibrator is equivalent to a three-port network. This includes two symmetrical mechanical ports and an electrical port. The functions of the three ports are shown by the following expressions [14,15]. When force is applied on the mechanical port, the voltage at the electrical port can be calculated by using these expressions.
Where, V is the voltage at the electrical port, Ï‰ is the electrical signal angular frequency of the electrical port, I is the current of electrical port, h33 is the piezoelectric stiffness constant.
When the piezoelectric vibrator is used as a piezoelectric power supply, the electrical port in Figure 5 is the output port (Vo,Io) and the mechanical port is the input port. If a load impedance ZL is connected to the electrical port and an impact force (F1,v1) is applied on the mechanical port 1, the other mechanical port is fixed, then the piezoelectric vibrator will provide electricity to the load.
By ohm's law,
Substituting expression (9) in expression (6), (7) and (8), and noting that , , the output current I0 can be written as follows:
Therefore, the quantity of electricity is given by
2.D. Simulations of the PPS
The characteristics of piezoelectric vibrator can be seriously influenced by the boundary conditions. The electrical characteristics of the PPS in the ideal case can be calculated theoretically by using expression (12), but the boundary conditions are not considered. In order to account for the boundary conditions, a simulation using ANSYS11.0 software to calculate the characteristics of the PPS. A piezoelectric coefficient matrix, elastic stiffness matrix and dielectric coefficient matrix are used to define attributes of the piezoelectric material in ANSYS. The attribute matrix for PZT-4 is shown in Figrue 6, and its density is 7500 Kg/m3 .
Figure 6 PZT-4 attribute matrix
In order to simplify the simulation model, the modeling of the PPS includes the piezoelectric alone, and the voltages at the upper and lower surface are coupled respectively in the simulation. The thickness of the PPS electrode is neglected, since the upper and lower electrode is very thin in the actual system.
There are three steps of in the PPS simulation process. First, creating a model of the PPS; second, meshing the model of the PPS and applying the appropriate boundary conditions on the finite element model (FEM) of the PPS; third, solving the FEM.
The size and materials of the PPS simulated using the ANSYS software are shown in Table III.
Table 3 size and materials of the pps model
Coupled field method and transient simulation is done with the PPS. There are two parts in the finite element model of the PPS, one is the piezoelectric part and the other is the load resistance. The load resistance is used to calculate output power from the PPS. SOLID226 and CIRCUIT94 are used to simulate piezoelectric and load resistance, respectively .
The simulation boundary conditions on the lower surface of piezoelectric vibrator are fixed. A 650 Newton area force is applied on the upper surface of the piezoelectric material. And the voltage coupling contact on the lower surface is connected to the ground.
The transient time of the simulation is one second, and the load resistance is connected between the voltage coupling contact points of the upper and lower surfaces. The load resistance is set at 100 KÎ© based on simulations ran by Kendall C.J. . The PPS finite element model front view and isometric view are shown in Figure 7.
From Figure 7, the voltage across the upper and lower surface is coupled at a point, which is connected to the load resistance. And an area stress is applied to the upper surface.
(a) Front View
(b) Isometric View
Figure 7 Finite element model of PPS
3 Simulation results and discussion
The properties of the PPS can be obtained after solving the simulation and the results are shown in Figure 8. A 0.283 mW is generated on the load resistance of the PPS at 1 Hz and 650 Newton applied area force, and 5.3 V voltage was obtained with a 100 KÎ© load. Using the parameters for the RFID chip from Section 2.B, the maximum power consumption of the IDS-SL900A is calculated as
Hence, a one time charging process of the PPS would supply sufficient power for the PPS-RFID in the ideal case. However, most of the time, the power of the PPS-RFID is far below PMAX. When the PPS-RFID operates in a logging mode it will be at maximum power consumption, but the time interval for sensing and logging can be pre-programmed. So the simulation results show that the PPS can be used as a power source of the PPS-RFID for temperature sensing.
Figure 4 Simulation results for the PPS.
This paper presents a design for RFID tags with a piezoelectric power supply (PPS-RFID). The PPS circuit is composed of a piezoelectric power source, rectifier circuit, voltage regulator chip, energy-storage circuit and a semi-active RFID chip. The voltage regulator chip is a LT3011. The semi-active RFID chip is an IDS-SL900A.
The operating principles and objectives of the PPS-RFID tag are also presented. When the PPS-RFID tags are attached under some heavy item, then the PPS-RFID tag could be self-powered by the normal vibrations during transport. Because there are temperature sensors and a RTC integrated into the PPS-RFID tag, it could monitor the surrounding environment and recording the data for future review.
Furthermore, models the piezoelectric performance of the piezoelectric power source including the Mason equivalent circuit for a single-layer piezoelectric vibrator. Then the functions of the mechanical ports and electrical port are derived.
In order to consider many more boundary conditions when calculating the characteristics of the PPS, the PPS was simulated by using ANSYS11.0. A 0.283 mW output on the load resistance of the PPS was calculated at 1 Hz and a 650 Newton impact force, and a 5.3 V voltage was obtained on a 100 KÎ© load. The simulation results support the practical feasibility of the circuit design for use as a PPS-RFID tag.
In the future, we plan to build and use this PPS-RFID tag as an intelligent mote in an IOT, and expect to find many sensor network applications such as for the tracking of medical supplies or monitoring of perishable foods.
This work was supported by the MEMS subject construction fund of the Kunming University of Science and Technology (No. 14078024).
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