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Motor Driver
H-Bridge
Driver
Simple
PWM Gen.
Handy
Method Measuring RPM
Measuring
RPM via Photo reflector
Introduction
to Robotic
DC,
Stepper,and Servo Motor
Microcontroller
Tutorial
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Interface
Tutorial
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3.2 Power
Four “AA” rechargeable alkaline batteries are used to provide
power and are mounted under the base. The weight of the batteries keeps
the center of gravity below the wheels’ horizontal axis and acts
as a pendulum in maintaining the position of the robot as it rolls along.
The batteries supply power for about 90 minutes of autonomy to the robot.
4 Wheel Motors
4.1 Modified Servos
To power the wheels, standard hobby servos were used. These servos were
modified for continuous rotation by taking apart and removing the mechanical
connection of the position feedback potentiometer. This is a common
practice with robot builders and several methods can be found on the
Internet. Sending pulse width modulation to the servo from the robot’s
microcontroller controls rotational speed. Forward, backward and full
stop are possible.
4.2 Power Saving Circuit
To prevent the servo’s idle current of 8 milliamps each from putting
a drain on batteries, a power saver circuit (fig. 3) was developed.
The servos can be completely powered down when not needed. A IRF520
mosfet, used as a switch, was inserted on the “high side”
of the positive power connection to the servo. A PVI5100 photovoltaic
isolator (PVI) IC from International Rectifier was used to drive the
mosfet. The PVI generates the turn-on voltage for the mosfet and eliminates
the voltage drop that would
occur when driving the mosfet in this fashion. Placement of the mosfet
on the high side keeps the input voltage relationship with the microcontroller
the same when off, further reducing idle current. A 10 megohm resistor
between gate and source, quickly turns off the mosfet when the PVI is
off.
Figure 3 Power Saving Circuit
Figure 4 Sensor Location
5 Sensors
5.1 Avoiding Obstacles
Obstacle avoidance is one of the primary activities of the robot when
moving. Avoidance is achieved by
emitting beams of IR light and detecting the reflected IR from objects
with a sensor (fig. 4). To prevent interference from other sources of
light, only light modulated at 38 kilohertz is emitted and received.
An IR LED on each side of the robot is pulsed to high power with currents
as high as 40ma. Pulses are a Panasonic PNA4602, made to respond only
to 38 kilohertz. The range of detection depends on the reflectance of
the obstacle and is in the range of 3 to 12 inches. The sensor, after
detecting IR, outputs a low state and remains in this state briefly.
This delay allows the microcontroller to pulse the IR LED, then check
for the response of the sensor without doing both at the same time.
5.2 Passive Infrared
A low power, passive infrared (PIR) sensor is used to detect people
or hot objects in the path of the robot. The PIR can only see objects
that move. Scanning the area by rotating horizontally allows the robot
to see people or other hot objects that are stationary. The relatively
wide field of view is restricted by a section of black heat shrink tubing
placed around the PIR. Restricting the field of view allows greater
accuracy in locating targets of interest.
6 Use of a Beacon
6.1 Charging plate
The use of a beacon enabling the robot to seek-and-find was an idea
to explore. Such a beacon could allow the robot to find its way “home”
so as not to wander too far away. If the wheels of the robot were constructed
of a conductive foam material and if the robot could make its way on
top of a sectional metal plate, then the robot could draw power and
charge it’s own batteries. The placement of this plate would need
to be marked with a beacon for the robot to locate.
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