Specifications
11
If yes turn left
If no, is right sensor clear?
If yes turn right
If no then reverse out or turn 180 degrees
Continue in a straight line (loop to START)
2.5 Dual Motor Drive Systems
Many systems use dual motors for directional control. A dual motor drive system refers
to having one motor for each drive wheel also known as ‘differential drive’. Both
motors driving forwards will make the unit move forwards; one motor forwards and one
motor in reverse will result in the unit turning etc. A well known example of this drive
system is in a skid steer (commonly known as a Bobcat). This method gives excellent
directional control and ‘on the spot’ turning allowing for tight corners and superior
manoeuvrability. The major problem with this method however is that one motor will
always turn slightly faster than the other resulting in the unit driving in a slight curve.
This may be due to efficiencies in the motors, gearbox friction, wheel to ground friction
and other uncontrollable factors. Some form of odometry for each wheel will be
necessary to ensure that the unit manoeuvres in a straight line.
This issue of non-straight driving is very common among robot builders and has been
overcome in several different ways. Many choose to change the drive system to a more
car like drive system with a single drive motor and steering mechanism. This is not
practical in many cases as it will sacrifice functionality. The problem has been
overcome before by pulse-width modulating the two drive motors and counting the
edges on the encoders located at each wheel. This seems to be a very successful method.
Knudsen J 2000 refers to a similar method of counting the revolutions of the drive
shafts and comparing the number or revs on each side. All of this information is based
on LEGO™ components however the methods are still relevant. It also explores the use
of a mechanical differential to ensure straight line driving is obtained. This is very
similar to a car differential however is used in a different manner. By attaching the two
drive motors to where wheels would normally be connected, the differential will not