Chipless RFID pH Sensor for Food
Spoilage Monitoring
ECE 4600 – Group Design Project Proposal
Group #5
John Baldwin
Marvin Bataller
Christian Espino
Project Supervisor:
Dr. G. Bridges
Date of Submittal:
October 1, 2012
1. INTRODUCTION
Food quality is an important manufacturing requirement for consumer health and safety.
Current methods to monitor the quality of food during the transit and storage process are to use
temperature sensors, enzyme sensors, or gas sensors. Such sensors, however, have significant
drawbacks, which include low sensing accuracy in moist environments, high operating
temperatures, complicated fabrication processes, and several design constraints which prohibit a
wireless integration [1]. These factors reduce the cost-effectiveness and feasibility of their use in
commercial applications.
An alternative technique used to manage food quality is to monitor the change of pH
levels of the product. pH is essentially the measurement of the hydrogen ion concentration and is
usually expressed in the logarithmic scale [2]. A pH sensor commonly consists of two electrodes
which provide a linear potential response over a wide range of pH values [3]. During the food
spoilage process, the pH profile changes due to the growth and metabolic actions of microbes [1].
The pH level can therefore provide an accurate indication of the freshness of the product.
The objective of this project is to design and fabricate a pH sensing device which can be
embedded into packaged material. The device will have three main components, which will
include the pH detector, two resonator circuits, and the transceiver. Furthermore, the device will
also be magnetically coupled to an external source, where the resonant frequency of one of the
resonator circuits will be proportional to the pH of the material. The change of pH will vary the
potential across a set of electrodes integrated into the resonator circuit. The response of the
device will be measured by a network analyzer. Furthermore, existing Radio Frequency
Identification (RFID) concepts will be incorporated into the design for a wireless passive device
approach. This passive wireless design will be an efficient method of measuring food quality as
it can be embedded directly into packaged materials.
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2. SPECIFICATIONS
The pH sensing device will operate in a 4-7 pH range environment and have a ± 0.3 pH
accuracy. To determine the accuracy, the output of the sensor will be compared to a commercial
pH sensor. The device will also operate in a high moisture environment in an ambient
temperature between 2 – 30 °C. Since this device will be designed to be embedded into packaged
materials such as 2L carton of milk, the maximum length, width, and thickness will be 15.0cm,
10.0cm, and 0.250cm respectively. The device will be comprised of a pH detector, two resonator
circuits, and a transceiver as shown in figure 1.
The pH detector will use an iridium oxide electrode in tandem with a silver chloride
reference electrode. The hydrogen ion concentration will induce a change in voltage across the
electrodes of approximately 60mV/pH [3]. The induced voltage will be used to bias a variable
capacitor, also known as a varactor, which will be integrated into the resonator component.
The resonator component will consist of a reference and sensing circuit, and will each be
modelled as an RLC circuit shown in figure 2. The resonant frequency of an RLC circuit can be
calculated by
where
is the resonant frequency in Radians/Second, L is the
inductance in Henrys, and C is the capacitance in Farads. Both the reference and sensing circuits
will be designed to resonate at a frequency of approximately 2.4 GHz. The sensing resonator,
however, will have a varactor integrated into the circuit which will vary the resonant frequency
by approximately ± 0.1 GHz. To achieve the highest sensitivity in the change of reverse bias
voltage across the varactor, a characteristic capacitance curve slope that is greatest between 100
to 500 mV will be selected.
The transceiver will be comprised of two antennas which will be required to handle the
reception and transmission of a frequency sweep in the 2 to 3 GHz range. The antenna will be
designed to be cross-polarized to minimize the interference between the input and output signals.
Circular disk monopole antennas will be used in the design due to large bandwidth requirements
and its simplistic design [4]. The transmitted signal of the pH sensor will be designed to be
distinguishable at a distance of at least 5cm.
The specifications of the device are summarized in Table 1 on the following page.
.
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Table 1: Specifications
SPECIFICATIONS
pH Operating Range
4 – 7 pH
pH Sensing Accuracy
± 0.3 pH
Operating Environment
High Moisture
Operating Temperature
2 – 30 °C
Maximum Device Dimensions:
Length
15.0 cm
Width
10.0 cm
Thickness
0.250 cm
Reference Resonator
Fixed Frequency at 2.4 GHz
Sensing Resonator
Variable Frequency at 2.4 ± 0.100 GHz
Voltage Sensitivity Range
100 to 500 mV
Antennas
2 UWB Circular Monopole @2-3GHz Cross Polarized
Minimum Interrogation Distance
5.0 cm
Fluid Proximity Distance to Tag
0.2 cm
Rx
Tx
E-field
Network
Analyzer
Resonator
Circuit
RLC
Model
Rx
Tx
Electrodes
E-field
Figure 2: RLC Model of the
Resonator with Variable
Capacitor and Electrodes
Figure 1: Operating Principle of the
Chipless RFID System
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3. TASKS AND MILESTONES
Table 2 shows the milestones and the tasks to complete this project, as well as how the
work will be distributed among each group member. The timeline for each task is indicated in the
Gantt chart on the following page.
Table 2. Milestones, Tasks, and Division of Work
MILESTONES AND TASKS
ASSIGNED INDIVIDUAL(S)
1. Preliminary Work
Research Chipless RFID
Research pH Sensing
Ansoft Programming Review
Christian, John, Marvin
Christian, John, Marvin
Christian, John, Marvin
2. Materials
Gather Materials List and Pricing
Order Materials
Christian, John
Marvin
3. Replicate Existing Chipless RFID Systems
Simulate System in Ansoft
Build Chipless RFID System
Test Chipless RFID System
John, Marvin
Christian, John
Christian, John, Marvin
4. Simulate and Design Chipless pH Sensor
pH Sensing Circuit
Transceiver
Resonators
Build Chipless pH Sensor
Christian
Marvin
John
Christian, John, Marvin
5. Testing and Optimization
Monitor pH of Milk
Optimize Design
Christian, Marvin
John, Marvin
6. Administrative Work
Project Proposal
Informal Oral Progress Report
Formal Written Progress Report
Formal Oral Progress Report
Project Title
Final Report
Project Presentation
Christian, John, Marvin
Christian, John, Marvin
Christian, John, Marvin
Christian, John, Marvin
Christian, John, Marvin
Christian, John, Marvin
Christian, John, Marvin
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4. GANTT CHART
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5. BUDGET
The materials and cost required for this project is shown in Table 2. The total cost of the
project is estimated to be $510.57.
Table 2: Budget
ITEM
UNIT
UNIT
COST
TOTAL
COST
1.
Varactor – BB152-115 NXP
5
0.50
$2.50
2.
Rogers RT/Duroid 5880 Glass Microfiber Reinforced
1
$216.09
$216.09
1
$162.00
$162.00
1
$60.00
$60.00
PTFE Composite Material (.062”x12”x18”)
3.
Iridium Oxide Wire - 99.8% Metal Basis
(5cm length x 0.5mm dia.)
4.
Silver Oxide Wire – 99.9% Metal Basis
(1m length x 1.0mm dia.)
5.
SMA Jack Flange Mount to PCB
2
$22.41
$22.41
6.
2% Milk (2L)
1
$2.57
$2.57
7.
ADSV 2.2 Software 1
1
$0.00
$0.00
8.
HFSS Software 1
1
$0.00
$0.00
9.
Network Analyzer - HP 8753E 1
1
$0.00
$0.00
10. Impedance Analyzer - HP Agilent 4290A 1
1
$0.00
$0.00
11. Commercial pH Sensor – Exstik II 1
1
$0.00
$0.00
12. Estimated Shipping and Brokerage Fees
-
1
-
$45.00
Provided by the Electrical and Computer Engineering
Department
GRAND
TOTAL:
6
$510.57
6. REFERENCES
[1] W. Huang, S. Deb, Y. Seo, S. Rao, M. Chiao, J.C. Chiao. “A Passive Radio-Frequency pHSensing Tag for Wireless Food-Quality Monitoring”. IEEE Sensors Journal. Vol. 12, pp.
487-495. March 2012.
[2] R. Petrucci, W. Hardwood, F. Herring, J. Madura. General Chemistry. Upper Saddle River,
New Jersey: Pearson Prentice Hall, 2007, pp. 670.
[3] S. Bhadra. “Electrode Potential-Based Coupled Coil Sensor for Remote pH Monitoring”,
IEEE Sensors Journal. Vol. 11, pp. 2813-2819. November 2011.
[4] S. Preradovic and N. Karmakar. “Chipless Millimeter Wave Identification (MMID) Tag at
30 GHz,” in Microwave Conference (EuMC), 2011, pp. 123-126.
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