19-02-2013, 09:26 AM
MINOR PROJECT ON Design of an Antenna for a Wireless Sensor Network for Trains
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Abstract
An antenna for a wireless sensor network for trains . The network will monitor temperature and vibrations of the wheel bearings on the train wagons. Doing this will allow for an earlier detection of damaged wheels, which will ease planning of maintenance and reduce wear on the rails considerably. The requirement of the system is that it is to be installed without any cables attached to the sensor nodes. This calls for wireless communication, and that for that antennas are needed. A train is a difficult environment to transmit electromagnetic (EM) waves in. It is full of metal and EM-waves cannot pass through a conducting material. Having much metal in its vicinity also affects the function of the antenna. This needs to be taken into consideration when making the design. The constructed antenna is a small dual-layer patch antenna. Dual layer means that it is constructed out of two sheets known as substrates of isolating material with different characteristics. The lower one of these substrates is made in such a way that integration with a circuit board is possible. Such integration would reduce the production cost considerably. The antenna is designed for direct placement on a conducting surface. This surface could be part of the train. It uses the surrounding metal as a ground plane in order to reduce its size. The result is a small patch antenna with good radiation qualities in metallic surroundings.
Introduction
Project description
The goal of this project is to design, build and test an antenna for the 2.45 GHz ISM-band. This antenna is to be a part of a temperature sensor network placed on trains. The network will monitor temperature and in a later stage vibrations of the wheel bearings on the train wagons. This is done in order to detect faulty and broken wheels. The goal is to enable better planning of wagon maintenance by detecting which wheel will break before it actually does. Today detection is only possible after the wheel gets so badly damaged that it starts to cut into the rails. When this happens the wheel will damage the rail along the whole way from the point it starts to cut until it gets repaired or replaced. Being able to replace the wheels before this happens would thus reduce the need for rail maintenance. The goal that is to be reached at the end of this project - is to get a functioning prototype system with direct communication between the sensor nodes and the gateway. This is as far as antennas concern, the antenna is desired to be small. It should work in metallic surroundings, a condition that puts tough requirements on the antenna design. When currents are flowing on the surface of the antenna there will be a coupling between the antenna and the metal in its surroundings. This will affect the performance of the antenna. It has to be designed in such a fashion that it will work at the desired frequencies in a position very close to a metal wall. The design also has to make sure that the energy is actually radiated and not dissipated into the train.
Background
There are today a very large variety of antenna models. Many antennas made today are designed to work in empty space. This is the simplest environment, where only the antenna needs to be taken into consideration. Those antennas will not work for this application, since the antenna is mounted on a large piece of metal and has an electric circuit close to it. Both of these factors will affect the performance of the antenna. There are antennas designed for hostile environments. Some are designed to have a large bandwidth in order to make them work even after detuning .Detuning is the unwanted change in operating frequency that takes place due to the surroundings of the antenna. Others are made in such a way that they will be very robust to changes in their surroundings and can work in difficult
environments. The goal of those designs is to keep the fields within the structure of the antenna and in that way minimize the sensitivity of the antenna to its surroundings. There is however no general solution to this problem and there are no antennas available to buy that will work well in any hostile environment. This means that for each application one needs to find the solution that works the best in that particular environment.
Antennas
Antennas are devices especially designed for transmitting and receiving radio waves. Electromagnetic waves are formed by accelerating electric charges. The acceleration can be due to bending of the medium where the charges are travelling or by discontinuity in the medium be it a change of medium or a termination. If there is an oscillating current the charges in it are accelerating and so there will be radiation. This means that almost anything can act as an antenna albeit often a poor one. All alternating current will lead to radiation and even direct current in a bent conductor will radiate. What defines an antenna is that the radiation is the goal, instead of an unwanted side effect. Antennas are designed for an alternating, and in most cases sinusoidal, current. This current creates waves that are led through a transmission line to the antenna where they are radiated into space. What happens is that the electromagnetic waves that reach the antenna will not stop when the conductor ends but the waves will continue.
Patch Antennas
Patch or microstrip antennas are one kind of antennas. The classic patch antenna consists of
a ground plane and a thin layer of conducting material that are separated from each other
by a dielectric sheet called substrate. Current is led to the thin layer or patch and will induce
an electric field between the patch and ground. This field will directly beneath the patch be
straightly directed from the patch to ground or the other way around depending on the
phase of the current. But at the edges it will be more spread out since there is more room.
This effect is called fringing and it is this that makes patch antennas work.
The return loss
The first test was to measure how much energy is reflected back into the cable that connects to the antenna. This is called the S11-parameter. In order to do this test an Agilent E8364B PNA series network analyzer was used. It was calibrated using a Rosenberger kit to make sure that only the wave reflected at the antenna was measured. The antenna was connected, a signal was sent to it and the network analyzer recorded how much of the effect was reflected back into the cable as a function of frequency. This is one of the largest loss factors in wireless transmission, which means that if the results of this test were too bad the antenna would not work. It is also a quick and easy test and so it is always done
as a first test. This test gives an opportunity to see more characteristics of the antenna as well. It shows the return loss as a function of frequency, which shows over which frequencies the antenna could possibly work. It is quite common to find that the S11-parameter is good over a large spectrum but not for the spectrum that it was intended for. This problem is usually caused by either making the antenna slightly larger or smaller than it should or not taking proper notice of its surroundings. A third problem that can be identified
during this test is ground currents. Ground currents are caused by the reflections in the antenna and at the connection point. The way to detect them is to move one hand along the wire and look for disturbances in the pattern. If currents are flowing on the ground conductor of the coaxial cable part of the current will transfer into the hand and so the reflection coefficient measured in the network analyzer will change depending on where the hand is. If all currents are on the inner conductor the outer conductor will work as shielding and so there will be no dependence on conductors outside the cable.
Total and radiated efficiency
The efficiency of the antenna was measured in a reverberation chamber . It is a chamber made entirely of conducting material, in this case steel. Inside this chamber there are two reference antennas that are considered to be ideal in the frequency spectrum that the chamber supports. There are also two mechanical stirrers. The idea is that you have one antenna transmitting a signal that is reflected on all the surfaces. The two stirrers are rotating at different speeds making the environment different at all times. If another antenna measures its received signal and a time average is taken a measure of the radiated power in all angles is achieved. This is first done between the two reference antennas. Since both antennas are considered ideal the losses are only the losses in air and the losses to materials placed in the chamber. If there are losses caused by leakage they will also appear here. Then the antenna under test is used as transmitter. The losses during the reference test are the same which means that all additional losses occur in the tested device. The total efficiency of the antenna is the difference between these two measurements. For this test to work the environment inside the chamber has to be identical during the two tests. This means that both the transmitting antennas need to be inside the chamber and at the same place at all times. The results were analyzed using Matlab. The network analyzer also records the S11-parameter meaning that these losses can be subtracted and what is left is called the radiation efficiency. The final antenna was tested for 3 different frequencies: 2.40, 2.44,2.48 GHz. At 2.44 GHz the total efficiency was -1.52 dB which is the equivalent of 71%. This is a good result, but the 2.40 and 2.48 GHz results were not as satisfactory. At the end frequencies the efficiency was about 44%.
Application test in Train
This second application test was done around an actual train wagon. The wagon had a
rather high ground clearance, about as high as some lower freight wagons have. The ground
clearance is of interest since we want to transmit from one side of the wagon to the other
and the shortest path is under the wagon. For passenger cars this path is almost nonexistent
since the ground clearance is kept at a minimum, but for freight wagons there can be large
open spaces between the wheels. No signal can be sent through metal so the more space
the better it is. Since most freight wagons have similar or more open space than the wagon
used in the test, what works in this test should work on most other wagons.
The equipment used was two antennas made in this project and each one was connected to
a version 1 circuit. One of the circuits was programmed to transmit and the other one to
receive. The receiver was connected to a computer that recorded the data.The antennas
were placed against different surfaces of the train close to where an installation of the
system prototype was planned.
Discussion and Conclusions
The antenna
The goal of the project is to make an efficient, small, and robust antenna that would be
possible to produce at low cost. The antenna is efficient enough for the system to work, but
a higher efficiency would increase the robustness of the transmission from side to side on
the train. One should keep in mind that very few of the losses that occur when transmitting
under the train are due to the antenna. Most of the losses come from the hostile
environment. This means that the best way to improve the quality of the transmission is to
cut those losses. This can be done with a relay or with diversity and most importantly the
placement of the nodes needs to be considered. A relay has the draw back that it would be
yet another box that needs to be designed and installed. It would raise cost and is for now
not even discussed. Diversity is to use more than one antenna to enhance the received
signal. Two antennas with orthogonal polarization could for example be used in order to
read more of the reflected waves. Diversity is probably the most effective way to improve
the transmission quality and so it should be the first thing to do in the continuation of this
project. Then there is the placement of the nodes. The first tests were made with the
antennas close to the wheels since that is where the temperature and vibration
measurements will be made.