The whole transfer-protocole may be omitted, if the receiver-circuit is changed at the D/A-converter stage. Replace the 4-bit converter by a well-designed 8-bit device. This increases dramatically the positionning speed. But for didactical purpose the 4-bit converter is a marvelous base to teach serial information transfer!
This project is consequently part of the robot positioning research we've already done.
The fundamental idea is to find the robot's position to three beacons. In a plane area you need at least three of them to fix the point exactly. Add the third dimension, and there must be four. (Actually GPS always needs 4 sending satellites to do its job). The robot's position is the intersection of three circles given by the center = beacon position and the radius = distance beacon-robot.
There are several ways to determine the distance to a beacon such as radar using the echo of a radio-signal or, in the case of a moving object, the Doppler-effect . We work on an indoor solution for a room no bigger than 5 x 5 m.
The trick is to send simultanously an infrared and an ultrasonic pulse, then to measure the delay between both's impacts.
As the light beam travels at the speed of light, the IR-receiver is activated nearly simultanously. Now the ultrasonic receiver must wait for the rather slow sound-pulse -compared to the speed of light- and count the time while waiting.
For this project you'll need a lot of stuff : volt-meter, oscilloscope, frequency-meter, PIC-programmer.
This is a major Lego Mindstorms project, where actually 4 (!) RCXs are involved. There are 3 beacons and a mobile robot equipped with a receiver. For the first time we will use the LEGO RCX incorporated IR-communication-system obeying to a certain protocole.
We follow some of the ideas of Lindsay Kleeman, Monash University, Australia, Optimal Estimation of Position and Heading for Mobile Robots using Ultrasonic Beacons and Dead-reckoning and Saeed Shiriy Ghidary et al, Kobe University, Japan, A new Home Robot Positioning System (HRPS) using IR switched multi ultrasonic sensors {Shiry, Tani, Takamori, Hattori}@octopus.cs.kobe-u.ac.jp .
Kleeman uses 6 only-ultrasonic beacons all linked to a central transmitter controller, which commands the firing of the beacons.The measured positions of the beacons and the approximate time between their firing are known to the ultrasonic receiver on board of the robot vehicle. Kleeman makes complicated calculations in order to incorporate dead-reckoning data. He insists on the fact that some values called echoed arrival times have to be rejected in the calculations. (In fact we observed this phenomenon, when the receiver was positioned out of the -6dB beam-angle of the transmitter, without literaly being out of range, so that it can't get the direct wave. )
The Japanese group is more specific about the room size (6mx4m). They obtain a positioning error less than 5 cm by using only one transmitter on the mobile base and 6 receivers located around in the room. They compare their procedure to landmarks positioning and find that it is much faster and may work in real time. The measuring time is less than 10ms. It is possible to find the heading of the robot without use of compass or gyroscope. The system is able to locate multi robots. The research group works with 23.1 kHz signals, considers the variations of the sound velocity with humidity and temperature in their calculations, even respects a certain circuit detection delay.
The fundamental difference between both approaches is that they answer to two complementary questions:
Simplifying these interesting concepts, we use only one receiver and three beacons which are not connected by wires to a central unit. Instead the robot takes the control of the beacons by its IR-communication system. The beacons do only send on the robot's demand. Receiving errors are compensated by dead-reckoning. The robot is therefore equipped with an altered direction-master gearing and one rotation sensor. For best reception, the beacons are placed higher than the robot. The receiving IR-module and ultrasonic transducer look upwards. In a further project, the beacons might be turnable, so that in error condition, they could search for a better angle. Even through the IR-communication-system, the robot could communicate its position back to the beacons, so that the beacons themselves were always able to fix their best sending direction. Of course, such a device would not permit multi robots. So, in this project both major questions are answered: the robot and the beacons-system know the positions of each other.
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