Research And Implementation Onultrasonic CDMA Based Indoor Location System

With the development of positioning technology, GPS based positioning system is able to achieve sub-meter positioning accuracy. But GPS is expensive and can’t work in indoor enviroment. At present, most of existing positioning methods are mainly designed for human, and based on existing hardware of user, they are little accauracy or expensive. And the problem of current high-precision ultrasonic based indoor position method is low capacity and low update rate. They can’t be used in fast moving robot control or multi-target team formation.In order to let all the targets get high-precision positioning information in indoor environment, this thesis realized accurate indoor positioning based on ultrasonic ranging. That is to install three or more ultrasonic transmitters in fixed positions in the room broadcasting ultrasonic signal toward ground periodically. As long as the robot is put into the effective working area of the ultrasonic signal, joined local wireless networks and time synchronization is completed, it is able to obtain its own location information continuously.This paper improved the algrizm and hardware circuit fully integrated with the features of ultrasonic sender and receiver, completed the circuit board and debugged the whole circuit. Finally we get the ultrasound realtime location system. In this paper, the precise time synchronization on wireless network and the way to avoid mutual interference of multi-ultrasonic transmitter is the focal point.At the accurate time synchronization part, this thesis using TI’s CC1101 wireless digital data transmission chip, working on 433 MHz ISM band to achieve wireless communication. By custom communication data structure and network topology to meet the mimimum communication requirements and minimizes communication latency. With measurement, time synchronization error is less than 100 microseconds. To prevent mutual interference between the ultrasonic transmitters, all ultrasonic sender broadcast CDMA encoded ultrasonic signals toward ground. When receivers despread the signal, it can not only distinguish different ultrasonic transmitter effectively, but also obtain accurate ultrasonic wave arriving time. And it is able to avoid multi-path factors that the environment brought, as well as the impact of interference to improve the performance of ranging system. At the same time, in order to improve the accuracy of positioning, Kalman filter is used to processe the obtained position data. As the test result, the positioning error is under 0.11 m.