User manual

RP6 ROBOT SYSTEM - 2. The RP6 in detail
Now we have found a way to reverse the motor's rotational direction. And how are we
going to accelerate or slow down the motor? A DC-Motor will rotate faster the higher
the voltage gets and we may control the motor speed by increasing or decreasing the
voltage. Let's have a closer look at the H-bridge again.
The figure shows what we can do. We
generate a square wave at a fixed fre-
quency and apply pulse width modulation,
which changes the
duty cycle. “Duty
cycle” means the ratio
between high and low
signal periods.
The motor will now get
a lower median DC-
voltage, corresponding
to the duty cycle.
The graph shows this
behaviour with a red
line (U
g
) and the red areas under the lines. For example if a bat-
tery voltage of 7 Volts is applied to the motor controller circuit and
the motor is being controlled at a PWM duty cycle of 50% the
equivalent median DC-voltage would be roughly 3.5 Volts! This is
not completely corresponding to the real circuit conditions, but it is
good to visualize it like this.
The RP6 uses a rather high gear reduction ratio (~ 1:72) which results in a quite
strong driving system, enabling the robot to carry heavier loads, for example com-
pared to the small ASURO robot. However with increasing weight, we must consider a
higher power supply load, resulting in an increased discharge rate…
Compared to remote controlled racing cars you might think the RP6 is a slow
vehicle - which is true - but we designed the robot to be slow! The robot is build to be
controlled by a microcontroller and if the programmer makes a mistake in the soft-
ware it would be rather unfavourable if the robot crashes into a wall at a speed of
10mph! So by using a moderate speed, the RP6 will not run into trouble that easy and
at a slower movement the sensors will have ample of time to react on obstacles. Addi-
tionally the robot is more powerful and the speed control is more accurate! Slower
speed enables the RP6 robot to drive very very slowly at a constant speed!
2.5. Expansion System
One of the most useful features of the RP6 is the expansion
system, allowing you to easily add additional things to the
Robot. The basic RP6 platform includes rather few sensors.
Still this number of sensors is well above the average of
comparable robots in this price-class, but the robot will only
become really attractive with several additional sensor mod-
ules. The ACS for example will only detect the existence of
obstacles in front of the robot. Using ultrasonic sensors or
improved additional IR-sensors you might be able to determ-
ine the distance and start sophisticated manoeuvres to avoid
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