In this thesis, a survey is made on the possibility to use the 24V power
lines as data communications medium in
heavy vehicles. The signal behavior of the electrical system of a truck is
studied theoretically and
experimentally. Existing systems for power line communications in AC
power nets and in vehicles are studied.
Some different spread spectrum modulation types used in power line
communications systems are studied.
The construction of a test system that uses binary frequency shift keying
(FSK) modulation is described. This
system uses a carrier frequency of about 1MHz and uses a signaling speed
of about 500bit/s. It is tested in a
laboratory and in a Scania truck.
It is found that capacitive loads on the power lines, for example
electronic control units with parallel
capacitances as part of their supply line filters, is a problem. These
capacitances attenuates the communications
signals severely.
The test system works and can communicate on the power lines. But
only if it is restricted to a part of the
electrical system with a minimum of capacitive loads. I.e. it must be
restricted to its own branch in the
electrical system tree structure. When the system is used to control
something it subjectively seems to work
properly, but when the bit error rate is measured it varies very much
between different testing occasions. The
system is also very sensitive to radiated interference from other
electrical equipment. At most the bit error rate
reaches several percent.
The conclusion is that the method that is tested is not good enough to
use in a commercial application. On the
other hand it shows that it is possible to have a communication on the
power lines with relatively simple
means. By using a different approach with a more advanced modulation
type it might well be possible to have a
power line communication that is good enough. This demands further
studies however. Recommendations of
how to proceed further is included in the report.
