Chipless RFID ph Sensor for Food Spoilage Monitoring

Transcription

1 Chipless RFID ph Sensor for Food Spoilage Monitoring Formal Written Progress Report Prepared by: Group 05 Christian Espino John Baldwin Marvin Bataller Supervisor: Dr. G. Bridges Date of Submittal January 14, 2013

2 Table of Contents Glossary... ii 1. Introduction Summary of Progress Transceiver Resonator Circuit ph Sensing Circuit Future Work Budget Gantt Chart References i

3 Glossary Beamwidth: The intensity of an electromagnetic beam measured as an angle between the edges of the beam where its intensity is at half power (3dB point). Coplanar Waveguide (CPW): A layout where all components, including ground, lay on the same plane. Microstrip Line: A transmission line that is fabricated on a printed circuit board. ph Sensing Circuit: Device capable of measuring the ph levels of a solution. Radio Frequency Identification (RFID): Utilizing radio frequencies as a means of wireless data capturing. Spiral Resonator: A planar inductor etched onto a printed circuit board, tuned to resonate at a specific frequency. Transceiver: A device that receives and transmits communication signals. Ultra Wide Bandwidth (UWB): A device that utilizes a large range of more than 500MHz of the electromagnetic spectrum. Varactor: A diode whose characteristic capacitance changes as a function of bias voltage, commonly referred to as a variable capacitor. ii

4 1. Introduction The objective of the project is to design a wireless ph sensor used for food spoilage monitoring. Our ph sensor design utilizes Radio Frequency Identification (RFID) technology and comprises of the following main components: a transceiver, spiral resonators, and a ph sensing circuit. The design of the transceiver and the resonators are now complete. Our current main focus is to complete the design of the ph sensing circuit. According to our preliminary Gantt chart, the ph sensing circuit should have been finalized by early January. The team has been experiencing a set of drawbacks during this design stage, which has prevented us to move forward with the project. Our proposed work schedule indicates a prototype should have been built by January 11, and therefore the team is approximately one week behind schedule

5 2. Summary of Progress During the first semester of the course, the team had met with each other on a weekly basis every Tuesday and Thursday. Additional meetings were also scheduled during the weekend if the team felt it was required. Furthermore, bi-weekly meetings were also scheduled with our supervisor to discuss the progress and any concerns regarding the project. The first phase of the project was to research ph sensing techniques, chip inductor designs, and wireless-passive communication methods. By mid-september, each member had gained enough background to begin the design phase of the project. There were two main design approaches considered for a wireless passive integration. The first approach was to use coupled inductor coils as a means of wireless communication, while the second approach was to use RFID technology. With the current advancements of RFID and the flexibility it presented, this topology was selected. The design of our wireless RFID ph sensor comprises of the following components: a transceiver, a resonator circuit, and ph sensing circuit and can be shown in figure 1. Figure 1. Chipless RFID ph Sensor The following paragraphs provide the background and progress for each main component. 2.1 Transceiver The transceiver component comprises of a receiving and transmitting antenna, as well as a microstrip line. The design of the microstrip line began in early October and was completed within a week by Christian and Marvin. Once the microstrip design was complete, Marvin and John began the design of the antenna by mid-october. Ultra wide bandwidth (UWB), wide beamwidth, and linear - 2 -

6 polarization were the characteristics required for the antenna [1]. Two designs that met these requirements included the circular patch dipole antenna and the UWB monopole circular-disc antenna. We had initially started designing the antenna using the circular patch dipole antenna, but had encountered several complications. The dipole antenna was found to have large edge impedance which would require an impedance matching circuit. This matching circuit would involve using a quarter-wave transformer circuit, which would be impractical to fabricate with the equipment available to us. The matching circuit would have also increased the dimensions of the device where it can longer be incorporated into packaged goods for food spoilage monitoring. By late October we then started considering the UWB monopole circular-disc antenna, which either could have a microstrip fed or coplanar waveguide (CPW) fed configuration. Although the CPW fed configuration showed some advantages over the microstrip fed configuration, it would had further increase the complexity of the overall design. Therefore, the microstrip fed UWB monopole circular-disc antenna was chosen. When designing the UWB monopole circular-disc antenna, the equation used to calculate the radius was f L = 7.2 GHz /(L+r), where L = 2a, r = a/4, f L is the lower frequency in GHz of the bandwidth and a is the radius of the disk in cm [2]. Therefore, by varying the lower frequency we were able to adjust the dimensions of antenna within reasonable size. One main advantage that was found using the UWB monopole circular-disc antenna design was that the ground plane acted as an impedance matching circuit. The input impedance could therefore be adjusted by varying the width of the ground plane (W gnd ) and the length of the gap between the disk and the ground plane (L gap ) [3]. An impedance matching circuit that involved a quarter-wave transformer circuit was therefore no longer required, which was one of the issues we were encountering using the circular patch dipole antenna. We used High Frequency Structural Simulator (HFSS) software to determine W gnd and L gap that would provide optimal performance for the antenna. By late-november the antenna design was completed and built as scheduled. Christian and Marvin had then tested the antenna and the results were acceptable. 2.2 Resonator Circuit The resonator circuit design began in mid-october by both Christian and John. The first step was to partially replicate an existing RFID tag by using the same dimensions of three resonators. The main purpose of this step was to understand how spiral resonators behaved on RFID tags, and also to gain experience with various software programs such as Ansoft Designer and Ultiboard. Ansoft Designer is an electromagnetic simulator tool which was used primarily to simulate the behaviour of the resonators. Ultiboard, on the other hand, is a software tool used to create the schematic layouts of PCB designs and - 3 -

7 create exportable files for manufacturing. It took approximately two weeks to become familiar and experienced with these software programs. By early November we were able to replicate and build an existing RFID tag. When testing the replicated RFID tag, our interest was not to obtain identical results, but to observe three different resonances equally spaced from each other. Once this was accomplished, the next phase was to design a resonator circuit that would now meet our specifications. This phase began in mid-november. The group initially decided that one spiral resonator would be necessary for our ph sensor. This resonator would have a resonant frequency of 2.4GHz and would vary from this value due to the ph sensing circuit that would eventually be incorporated into it. The group then preferred that a second resonator should be implemented to act a reference, fixed at a resonant frequency of 2.4GHz. The reference resonator proved to be advantageous because it would help identify any drifting that may occur due to changes in the environment. Christian had researched and identified several parameters to adjust to obtain a resonant frequency of 2.4GHz. The parameters included the number of turns, the gap separation between the resonator and microstrip line, the length of the resonator, the width of the resonator, the width of the spiral arm, and the gap between two arms. Varying the spiral length and minimizing the number of turns was the parameter that showed significant change in the resonant frequency without compromising attenuation [1]. One design constraint that we had to keep in mind was the tool size of the milling machine which could not mill any features smaller than 0.3mm. Through simulation in Ansoft Designer, we were able to acquire a set of dimensions for a 2.4 GHz resonator. The resonator design was completed early January and is now ready to be sent to the technical shop for fabrication. 2.3 ph Sensing Circuit The ph sensing circuit design began in late September by all members. The ph sensing circuit will operate at a low voltage level with an expected change in voltage of approximately 60mV/pH [4]. Thus we needed a varactor whose characteristic capacitance curve s slope was greatest between 0-1V. Originally we chose the NXP Semiconductor BB NXP varactor, but there was a concern that its change in capacitance of 5pf between 0-1V was inadequate. We continued searching and in early October ordered a Skyworks SMV LF Hyperabrupt Junction Tuning Varactor which had a change of 40pf from 0-1V. For our electrodes, we decided to use silver chloride and iridium oxide as they provided a large ph sensing range of approximately 2-12pH [4]. Testing and simulation of the ph sensing circuit could not be completed until the resonator was fully designed and as such simulation did not begin until early January. Simulation was done through Ansoft Designer and HFS. Using the EM planar simulation mode we were able to simulate our - 4 -

8 microstrip line coupled to our resonator coils but the software did not allow the addition of lumped components. As we could not add lumped components we then decided to model our varactor in Ansoft Designer. We are currently in the process of brainstorming different ways to model the varactor, which has put us behind schedule

9 3. Future Work For the final semester of the course, the team has scheduled thee hour meetings every Tuesday, Thursday, and Saturday. This schedule is planned to continue until the end of the course. Informal meetings have also been arranged between two members every Monday, Wednesday, and Friday during the spares they have together. Due to some unforeseen drawbacks, we have updated our Gantt chart accordingly to have project completed before February. The current priority of the project is to finalize the design of our ph sensing circuit. This will involve the simulation model of the varactor and electrodes into our resonator circuit. This task is expected to be completed by mid-january. Once completed, we will be able to integrate all main components together and have a prototype built before the end of January. We expect to have our preliminary testing done by the first week of February. The remaining three weeks of February will be used to optimize the preliminary design and incorporate any changes if necessary

10 4. Budget We originally had a budget of $510.57, which was mainly due to the price of the Rogers Duroid 5880 board, the silver wire, and the iridium wire which cost $216.09, $60.00, and $ respectively. Marvin however contacted Rogers Corp. and was able to order 9x6 inch free samples of the Duroid 5880 board. For the silver and iridium wire, it will now be provided to us by Dr. G. Bridges. We originally planned on ordering five varactors from NXP but before placing the order we found a varactor manufactured by Skyworks that better suited our needs. The Skyworks varactor cost $0.77 a piece versus the $0.50 a piece for the NXP varactors. All parts have been ordered and have been received. Our current budget is a total of $80.02 as shown in table 1. Table I: Revised Budget ITEM UNIT UNIT COST TOTAL COST 1. Varactor part #: SMV LF ( 5 $0.77 $ Rogers RT/Duroid 5880 Glass Microfiber Reinforced PTFE Composite Material (.062 x9 x6 ) 2 1 $0.00 $ Iridium Oxide Wire % Metal Basis (5cm length x 0.5mm dia) 1 1 $0.00 $ Silver Chloride Wire 99.9% Metal Basis (1m length x 1.0mm dia) 1 1 $0.00 $ SMA Jack Flange Mount to PCB (Digikey Part #: ) 2 $6.80 $ % Milk (2L) 1 $2.57 $ ADSV 2.2 Software 1 1 $0.00 $ HFSS Software 1 1 $0.00 $ Network Analyzer - HP 8753E 1 1 $0.00 $ Impedance Analyzer - HP Agilent 4290A 1 1 $0.00 $ Commercial ph Sensor Exstik II 1 1 $0.00 $ Estimated Shipping and Brokerage Fees - - $ Provided by Dr. G. Bridges 2 Student Sample available at: GRAND TOTAL: $

11 5. Gantt Chart - 8 -

12 6. References [1] S. Preradovic and N.C. Karmakar. (2012). Multiresonator-Based Chipless RFID [Online]. Available: [Nov. 3, 2012]. [2] Dixena, P. K., Study and design of Directional Ultra Wideband antenna in planar technology [Online]. Available: [3] J. Liang, C. C. Chiau, X. Chen, and C. G. Parini. Study of a Printed Circualr Disc Monopole Antenna for UWB Systems, Antennas and Propagation, vol. 53 no. 11, pp , Nov [4] S. Bhadra, G. E. Bridges, D. J. Thomson, and M. S. Freund. Electrode Potential-Based Coupled Coil Sensor for Remote ph Monitoring, Sensors Journal, vol. 11 no. 11, pp , Nov