In designing a team of moving agents remotely-controlled by a computer, some factors need to be considered. Depending on the task required to be done by the system, the technical parameters are set considering the most profitable implementations. In the particular case of a robotic soccer environment for example, the moving agent's dimensions are established relative to the playing field size. In the current specification, an area of 2x2m was defined. These dimensions were used in choosing the dimensions of the robots ( approximately cm).
As the robots don't carry their own ``brains'' but are controlled from the computer, a very important part is the transmission link from the computer to the robots (Figure 2). As can be seen from Figure 2, the on-board system consists of motors, motor controllers, a decoder, and a receiver as well as batteries. The batteries are significant in that they comprise most of the weight of the on-board system. The off-board system consists of an IR transmitter, a coder, and a micro-controller.
Figure: The transmission link connects the (a) off-board components and the (b) on-board components of the system.
The transmission medium is IR radiation. This provides a robust and reliable connection that is free from interference. The emitter uses a carrier frequency of 40kHz ensuring very good noise rejection at the receiver. The information sent by the computer is first interpreted by a micro-controller (see Figure 2(a)). This interpretation is done by a C-program which converts the computer serial output to binary code. The commands for the robot motors are translated into 8-bit words while the robot addresses are transformed into 10-bit words. At the coder, the information bits are framed by synchronization blocks in order to allow for serial transmission. The framed control and address bit configuration is used for modulating the carrier IR frequency. The 40kHz-IR receiver on-board each robot filters and demodulates the signal and transmits it to the decoder (see Figure 3(b)). The decoder first examines the signal for the address bits and proceeds to analyze the command bits only in the case of a local address match.
A first attempt to get the system working was made using an RF AM transmitting-receiving system with the carrier frequency of 300 MHz. The transmitting and receiving modules were very small, light and compact, therefore suitable to be implemented in the small agents of the robotic system. Even though the modules worked well off the system, they failed to supply accurate data transmission when used in normal operating conditions. The main cause was the very noise caused by the brushed DC motors under the normal load. Its level was high enough to alter the data transmission in such a way that the decoders were not able to recognize any valid addresses. Thus the stopped robots always started to execute the first command but, after the motors were running, the noise altered the data and no robot was getting another command. Because the filtering methods being used did not work properly in this case (an amplitude modulated high frequency carrier and relatively noisy brushed DC motors) a simple IR solution was considered and implemented without the inconvenience caused by the RF noise.