Projects:2014S1-11 Wireless Rotation Detector for Sport Equipment

This project focuses on developing a wireless non-optical solution for measuring the rotation of objects. The solution to this problem is approached through the use on antenna and the polarisation effect that antenna are subject to.

Project information

Motivation and Background

Rotation can be measured through optical means but this is subject to line of sight and may be blocked in some cases. Antennas do not suffer these same problems.
The most important principle used is that of the polarisation effect. This states that when two antennas are aligned parallel in the same plane then the power transmitted between them is maximised. When the two antennas are in perpendicular planes then the power is zero. Using this fact we can use the frequency of the changing power to measure the speed of rotation.

Previous work

Previous work has been done in this area through a honours project in 2012. This resulted in a device where a transmitting antenna and circuitry required to produce the signal are placed on the rotating object. This then sends a signal back to a receiving device that would then calculate the speed of rotation using the polarisation effect. This year we aimed to remove all the circuitry on the rotating object and just leave and antenna that will reflect a transmitted signal. This then gives us less weight and no power required on the rotating object. This approach is made possible by the use of an absorber and bi-directional coupler.

Aim

For this year project, we aim to create an improved device, which measure the rotation speed without the battery and signal generation on the target object and smaller physical size and weight of the integrated device. We also aim to build a more compact hand-held device with handle and easy access to user interface.

Outline of proposed work

Improve knowledge of antenna theory, antenna simulation software, signal generation, signal interference, calculation algorithm (fast Fourier transform), Arduino Board and Arduino development tool.
Develop the system overview of the project and identify the key components needed.
Compare the features of commercial devices with custom designs (e.g. size, price, weight, performance, working condition and low-profile configuration).
Test the commercial devices and custom-designed devices under laboratory conditions. Then integrated all the devices together and measure the performance of the project under real-world condition.

High-Level Plan

Phase 1 :
- Generating the ISM frequency by using the voltage control oscillator.
Phase 2 :
- Using the transceiver to send the signal.
- The absorber is attached to the target object.
- Receiving the reflected signal from absorber.
Phase 3 :
- Separating the received signal from the transmitted signal through using of bidirectional-coupler or crosstalk-reduced receiver.
Phase 4 :
- Using the power of the separated received signal to calculate the rotation speed of the target object.

Component Specification

Commercial Devices
- Voltage contolled Oscillator
- Bi-dirctional Coupler
- Power Detector
- Arduino Uno Board
Custom-designed Devices
- Transceiver
- Transmitter and Receiver
- Absorber

Component test results

Each of the components were tested individually before being incorporated together

VCO

The VCO was test with a power supply to see the tuning voltage needed before using the Arduino board to receive similar result. Using the Arduino board to supply a 3.2V tuning voltage the VCO produces at 2.4025 Ghz signal with 5.6 dBm power. This is the signal that is used in the device.

Bi-Directionl Coupler

The Bi-Directional coupler was tested with a network analyser. port 1 of the network analyser was connected to the the input and port 2 was connected to the coupled in reverse port of the coupler. The output of the coupler was connected to the transceiver and the input coupled forward port is terminated with a 50 ohm resister. Using this setup the s-parameters are found. For the use of the device the s11 and s21 parameters are the most important. The S11 shows the power reflected by the transceiver. The s21 parameter shows proportion of this power that we will receive at our desired port. From this we can tell that any signal that we receive stronger then 30dBm will be above the noise of the reflections of the transcevier and this will produce a signal at at the reversed coupling(Where the readings will be taken) of 50 dBm or above.

Power Detector

The power detector take an rf signal and gives an output voltage proportional to the input power. The power detector was tested at 2Ghz due to limitations of testing equipment. First it was tested with a variable power supply then using voltages taken from the Arduino board. The result were compared to the expected results give by the components data sheet. The results received were slightly higher than that expected but as the trend was the same as the expected results this is acceptable. As the calculations look at the frequency of the change in power as long as the output voltage changes with a steady rate this is fine.

Arduino Board

Along with the calculations the Arduino board is used to display results. The 2 most important things that it displays are the speed of the rotating object and the received voltage. The speed can be changed between Hz and RPM depending on the desired measurement. The voltage can also be change to a power reading. The display can also show recorded result. The Arduino Board can hold up to 5 speed recordings.

Transceiver

the resonate frequency of the transceiver is at 2.41GHz with -28.90dB reflection coefficient. Although the resonate frequency is not 2.4GHz sharp, the -10dB bandwidth covers the 2.4GHz. Hence the design is acceptable. The reflection coefficient very low, which ensures the quality of the received signal. A coaxial microstrip patch antenna is used for designing the transmitter. The coaxial microstrip patch antenna is a directional antenna with around 6-8dB gain. The general performance of it meets the design specification of this project.