Projects:2015s1-28 Wireless Rotation Detector
Contents
Supervisors
Honours students
- 2015: Guowei Deng and Shuopeng Wang, see Wireless Rotation Detector
Project objects
Scope
- Design and construct a portable wireless device which can accurately measure the speed of rotating object.
Specific Aims
The aim of the project is to design and construct a portable wireless device which can be used for rotation speed measurement. The device is supposed to consist of antennas, absorbers, Arduino UNO Board with software, etc. The outcomes of the project will be as follows:
- A metamaterial absorber with resonant frequency of 2.4GHz
- A planar dual-antennas with capability of transmitting and receiving signals
- A software system for rotation speed calculation
The device should be able to measure the rotational speed of various objects such as sport equipment, car tyres, turbines, fans, etc. The system shall be operational under a set of circumstances and able to achieve reliable results.
Background & Significance
- Motivation
The rotation speed measurement devices have already ubiquitously used in industries. They are embedded on turbines, wheels or fans to extract the rotation speed of objects. However, most of commonly used angular speed measurement devices, such as laser tachometer, use visible light. One of the constraints in such devices is that the line of sight cannot be impaired [1]. Hence, the optical devices cannot be used if there is an obstacle between rotating objects and the test devices or under sophisticated circumstances such as fog or smoke.
In order to improve the performance of the measurement device, the 2.4GHz electromagnetic waves will be used instead of visible light due to its good ability of propagation. Accordingly, the devices can be used under more complicated situations, even if the line of sight has been impaired, once delivered.
- Technical Background
1. Polarization of Antennas
The polarization of antenna is defined as “the polarization of the wave transmitted (radiated) by the antenna” [2]. There are 3 different types of polarization including linear polarization, circular polarization and elliptical polarization [2]. All of them are defined as “time-varying direction and magnitude of the electric field vector” [2].
2. Polarization Loss Factor
Polarization loss is defined as the power loss due to polarization mismatch. For example, if a horizontally polarized antenna communicates with a vertically polarized antenna, the vertically polarized antenna will transmit vertically polarized waves, while the horizontally polarized antenna can only receive horizontally polarized wave. Thus, there is no power transfer between a horizontally polarized antenna and vertically antenna according to the reciprocity principle. In contrast, if 2 vertically polarized antennas communicate with each other, the power transfer will be maximum. Hence, if two linearly polarized antennas are rotated from each other by an angle θ, the polarization loss factor base on power loss for polarization mismatch is defined as
𝑃𝐿𝐹 = 𝑐𝑜𝑠2θ
3. Metamaterial Absorber
Metamaterials are man-made materials with the unique characteristics, such as negative permittivity and permeability, which cannot be found in natural materials [3]. The metamaterial absorber has been one of the most important applications due to its astonishing properties [4]. A single metallic metamaterial absorber typically consists of a LC resonant circuit and a ground plane, separated by a substrate in between [5]. For example, as shown in Figure 1, the capacitor-like structure can provide capacitance and is connected with two loops in parallel, which can be considered as two inductors in the resonant circuit [6]. The resonator can strongly respond to a properly oriented electric field due to the capacitor-like structure when the circuit is driven by EM field at resonant frequency.
The metamaterial absorber is expected to have the ability of dissipating the incident waves inside of the structure with limited energy reflected back from it [8]. As shown in Figure 2, the incident wave is transmitted into the substrate at working frequency. The surface current will be created along the ground plane when the wave reached the bottom layer. Similarly, remaining energy will be reflected back to the resonators which will also generate another surface current on the top layer. These two currents have same magnitude with opposite directions, resulting in a destructive interference [8]. Hence, the thickness of the substrate should be carefully chosen for the effective destructive interference.
Key Requirements
From the introduction of background and related work, several key requirements have been defined as following:
- Metamaterial absorber shall absorb at least 90% (-10db) power at 2.4GHz when it has
same polarization as the transmitting antenna.
- Dual antennas shall work at 2.4GHz and crosstalk between 2 antennas shall be less
than -40dB.
- Portable system shall be operational up to 50cm from the rotating object.
- Portable system shall accurately measure the angular speed of the rotating object in
both low and high speed.
- While the mutual coupling in dual antennas is reduced, the dimensions of dual antenna
should remain small, and specifically the antenna system should be planar.
- The bandwidth for the isolation should be able to cover the bandwidth of the antenna.
Technical Challenge
- Antenna Design
The technical challenge for antenna design is to figure out an appropriate method to reduce crosstalk between dual antennas with the frequency and substrate thickness specified. Some possible methods have already been mentioned in Section 3.2.2. However, all the methods will have side effects on the performance of the antenna including input impedance matching, efficiency, resonant frequency, etc. Hence, the method should be operational for coupling reduction with all side effects minimized. Furthermore, the method should not dramatically change the overall dimensions of the antenna, and specifically the antenna system is expected to be planar.
- Metamaterial Absorber
Resonant frequency and absorption ability are two key characteristics of the metamaterial absorber, which are influenced by the metallic structure and thickness of substrate. Hence, 2 variables need to be considered in absorber design in order to achieve the requirements. Moreover, the material of substrate is supposed to be FR4, a low-cost material with permittivity varying from 4.2 to 4.8. The variation of the permittivity has impact on the resonant frequency, which may lead to failure in absorber design. Hence, a relatively wide bandwidth has to be achieved in order to overcome the uncertainty.
Proposed Approaches
- Method exploited - ‘Divide-And-Conquer’.
- The entire project can be decomposed into several aspects.
- Hardware (design required)
- Metamaterial Absorber
- Dual-Antennas
- Software
- Calculation Algorithm for rotating speed
- GUI
- Other Components (from market or school)
- VCO
- Arduino UNO Board
- Power Detector
Hardware Design
Hardware design will begin with understanding the key requirements and specifications of the hardware. Most of the hardware design will be implemented in the commercial software package HFSS which is a professional software for simulating 3D full wave electromagnetic fields. The group member will fabricate the designed hardware after hardware has been designed in HFSS. The next task for hardware is to measure the prototype and compare the practical result with simulation results. If the prototype fails to meet the requirements, the group will refer back to design stage to redesign prototype until it meets requirements
Software Design
The software design will start after antenna and absorber design have been finished. The software design will be in C language based on the Fourier Transform. To test the validity of the software, the group members will combine software, hardware and other components together. If the software cannot accurately measure rotating speed of objects which is driven by a motor, the project group will return to previous stage to redesign the software.
Schedule and Milestone
- Milestone in Semester 1
- Milestone in Semester 2
Metamaterial Absorber Design
1.Electrical-LC Resonator
A typical electrical-LC resonant circuit was illustrated in Figure 1. In order to investigate the characteristics of the Electrical-LC resonator, we proposed a simple structure with dimensions of a=15mm, g=0.4mm, d=12.6mm, b=0.8mm, l=3.6mm and copper thickness of t=0.0035mm, as the dimension of the structure shown in Figure 1. The unit cell was hosted on the top of a 3.2mm thick FR-4 dielectric substrate with a loss tangent of 0.025 and a dielectric constant of εr=4.4. The current structure did NOT include the ground plane at the bottom because we only investigated the characteristics of the ELC resonator at this stage.
The simulation for ELC resonator was performed with a full-wave electromagnetic simulator, Ansoft HFSS. The unit cell was placed in a waveguide with Perfect Magnetic Conductor and Perfect Electric Conductor assigned in x-z plane, y-z plane respectively. The boundary condition was selected for the unit cell because of its symmetric structure with symmetric wave propagation. Figure 1 shows the reflection coefficient S11 and transmission coefficient S21 at the frequency ranging from 1 to 5GHz.
From Figure, we can observe that S11 has the resonant frequency at 3.16GHz with the magnitude of -16.22dB while the S21 resonates at 2.75 GHz with magnitude of -23.31dB. It is interesting to note that the S11 reaches the maximum value when S21 reaches the minimum 19 value and vice versa. This is because the relationship between S11 and S21 can be expressed as |𝑆21|2=1−|𝑆11|2 using scattering parameters. Moreover, we can observe that the bandwidth for S11 is 0.1152GHz and for S21 is 1.207GHz. In proposed design, a number of factors played a key role in determining the resonant frequency. The resonant frequency of a RCL circuit can be defined as
𝑓𝑟=12𝜋√𝐿𝐶
Thus, the most important factors to determine the resonant frequency are capacitance and inductance. The capacitance is affected by the dimensions of the capacitor-like structure including capacitor length l and gap g while the inductance is determined by the dimensions of the loops including the unit cell size d and width b. Hence, we used the “Optimetrics” in HFSS in order to figure out the effects of these 4 parameters.
1.1. Length l
By varying the length l of the capacitor from 3.2mm to 4.2mm, we sought to observe the effect of the range on the resonant frequency. It is beneficial in context of our project as it demonstrates the effects of the capacitance on the resonant frequency.
From Figure, we can observe that the resonant frequency of the ELC circuit decreases from 2.81GHz to 2.64GHz as a result of the increment of length l from 3.2mm to 4.2mm. The decreasing of the resonant frequency indicates that the capacitance increases as we increase the length l. This phenomenon can be explained by using the formula of parallel plate capacitor:
𝐶=𝜀𝐴/𝑔
The capacitance of parallel metallic plates of area A and separation g is given by the expression above where ε= permittivity of space between two plates. As we increase the length l of the capacitance, the area A increases and the capacitance increases as well. Accordingly, the resonant frequency will decrease.
1.2. Gap g The air gap g was varied from 0.2 to 1mm by step 0.2mm. Figure 8 illustrates the variations of the resonant frequency as the air gap increases from 0.2mm to 1mm.
It is clear from Figure 8 that the resonant frequency of the resonator remarkably increases as the gap increases. This phenomenon can be explained by the formula in previous section: the capacitance decreases as the air gap increases and the resonant frequency will increase as a result.
We can also observe the coupling strength decreases if the capacitance is increased. Weak coupling has the undesired effects of narrow bandwidth, as shown in Figure 8 where the bandwidth decreases as gap g decreases.
1.3. Size d The inductance of the RCL circuit is directly dependent on the size of the unit cell. By examining the resonant frequency from different sizes of unit cell it allows us to predict further resonator characteristics.
We can also observe the coupling strength decreases if the capacitance is increased. Weak coupling has the undesired effects of narrow bandwidth, as shown in Figure 8 where the bandwidth decreases as gap g decreases.
The above Figure 9 shows that the resonant frequency keeps increasing, which means the inductance of the ELC resonator keeps increasing as the size increases. Thus, we can directly adjust the resonant frequency by selecting the proper size of the unit cell.
1.4. Loop Width b
The variable presented in Figure 10 is called “cut-off”, which represents the width cut from the original loop. The “cut-off” increases from 0.1 to 0.7mm, which means the loop width decreases from 0.7 to 0.1mm. From Figure 10, it is obvious that the resonant frequency decreases as the width decrease, which indicates the narrower width the loop is, the higher inductance the loop has.
Antenna Design
Reference
[1] C.A Balanis, “Antenna Theory Analysis and Design”, 3rd ed., New York: Wiley, 2005.
[2] “Arduino FFT Library”, openmusiclabs, Available: http://wiki.openmusiclabs.com/wiki/ArduinoFFT.
[3] D. Schurig, J.J. Mock, and D. R. Smith, “Electric-filed-coupled resonators for negative permittivity metamaterials”, Appl. Phys. Lett. 88, art. no. 041109(2006).
[4] Y. Chen, “Wireless Rotation Detector for Sport Equipment”, 2014.
[5] T. Mark, “System Overview,” Wireless Rotation Detector for Sport Equipment, 2014, pp 6-8.
[6] H. M. Lee, “Absorption Bandwidth-Enhanced Metamaterial absorber Using In-planed ELC resonator and Cut-Wire,” available: http://ournal.sapub.org/eee [Accessed: 22 March 2015]
[7] A. Grasso, Notes of Project Management for Electrical Engineering & Sustainable Energy, 2014. [Online]. Available: https://myuni.adelaide.edu.au/bbcswebdav/pid-5997024-dt-content-rid-6342045_1/courses/3420_ELEC_ENG_COMBINED_0001/Project%20Management%20%283024%20%26%203029%29%20Lecture%20Notes_2014.pdf
