To begin to understand certain processes involving motion it is essential to use a technology that delivers a non-invasive measure of the process, preferably in real-time for instant analysis. To date there are many real-time commercial technologies available to measure rotation and translation of a body or process in motion. Unfortunately, many of these technologies are problematic in portability and are often burdened by many wires which can affect the natural behavior and environment of the subject of study. Often, they require an external magnetic field for sensor excitation. This thesis proposal will detail the concept of a purely rotational sensor that would be wireless, real-time, operate from the Earth’s magnetic and gravitational fields, and be able to collecting data from several sensors simultaneously.
The utility of this type of sensing/measurement apparatus has broad application in many entertainment and research fields. A small wireless sensor (with multichannel capability) such as this would be valuable in Motion Picture special effects, replacing many of the current imaging technologies. Sports training/medicine could benefit greatly by knowing angles of motion of an athlete during training by helping optimize movement or assess risks. If coupled with imaging systems, this sensor could provide highly accurate 3D analysis for measurement of motion for consumer product research and development.
For this thesis work I propose to create a prototype wireless rotation sensing module. Due to limitations in availability of fabrication equipment, the prototype will be provided in a breadboard format. The general steps of this prototype creation will include: 1) Develop a familiarity with the sensing technologies as to properly implement them in the device. 2) Develop analog circuitry to provide adequate signal conditioning to the output of the sensors. 3) Program a microcontroller to perform the following: Multiplex sensor signals, analog-to-digital conversion, convert data to ASCII format, transmit data serially to the transceiver, accept user interrupt for reference/calibration of the device, provide user feedback, and program the firmware of the transceiver. 4) Properly implement transceivers for the wireless data communication. 5) Build a custom program in Visual Basic 6® to calculate the rotational vector and handle reference data. 6) Take necessary steps to ensure the data received from the device is accurate. 7) Provide a complete and concise account of the research activities involved in the creation of this device in a thesis document.
The rotational axis sensor will incorporate magnetic and gravitational sensors that will, from algorithms, calculate the angle from a reference of a subject or system during motion. The magnetic and gravitational sensing operates with no man-made fields present, purely from naturally occurring Earth’s magnetic and gravitational fields. This coupled to a small WiFi (wireless communication protocol used commonly with LAN systems, IEEE 802.11b) transceiver would allow for 1 to 82 of these rotational wireless sensors to operate on a single carrier frequency. This would be in effect active Radio Frequency Identification (RFID) application where the angular motion of multiple points of a subject or system in motion could be observed simultaneously.
The magnetic fields are sensed by Anisotropic Magnetoresistive (AMR) sensors. Each AMR is a Wheatstone bridge in which the differential voltage is the result of a response of magnetoresistive materials to the magnitude and direction of a magnetic field. For this application, three AMR sensors (combined package) are used to indicate the field strength and direction in all three axes (X, Y, and Z). The Permalloy material used in AMR sensors is most sensitive in the 1 μG to 10 G or mid range magnetic field level (parallel with the Earth’s field range). Translational motion is not possible when sensing with AMR sensors due to the low field strength gradient of the Earth’s Field.
Gravitational vectoring will be accomplished by using Micro Electro Mechanical Systems (MEMS) technology accelerometers. Two axis accelerometers can relate the degree of pitch and roll from a level position within ±90 degrees. By using two two-axis accelerometers, facing opposite directions, 360 degrees of rotation in the pitch and roll of the sensors can be determined.
By using the following published navigational algorithms, which combine the accelerometer (where θ is roll and φ is pitch) and AMR sensor outputs (X, Y, and Z), the yaw of the sensor can be determined.
A summed vector for the pitch, roll, and yaw can be calculated to tell the direction of the sensor from a predetermined reference point.
The wireless module that has been selected for this application is the Radiotronix Wi.232DTS. This wireless module combines a DTS/FSK (Digital Transmission System/Frequency Shift Key) data transceiver and a high performance protocol controller to create a UART-to-antenna (serial data protocol) module for direct wire replacement in embedded applications. The module is capable of data rates up to 152.43 kbs and can handle 32 channels in DTS mode and up to 84 in low power mode. This small (0.8” x 0.935” x 0.08”) module has extensive error checking protocols and a unique 48-bit address for identification.
Outputs from the sensors would be conditioned and collected by a small programmable microcontroller. The microcontroller would send the data in serial packets to the transceiver, and then the transceiver would send the data to a transceiver module serially connected to a computer. The computer would calculate the rotational data in a custom Visual Basic® program. With proper signal conditioning, rotational angles with a resolution of 1.0 degrees are achievable. Data rates of 57.6 kbs will be used for measurement (limits of the USART of the microcontroller).
It is with the utmost sincerity that I submit this thesis proposal for review and acceptance.
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