where the AGV has to travel a long
distance before employing more
accurate positioning at the end points.
Steering Systems
The only type of steering that
most of us ever thought of was what
we use in our cars. Called Ackermann
steering, this type of system uses two
front wheels that turn at the same
time and at approximately the same
angle. The car’s body and the fixed
rear wheels then follow the curved
path quite accurately.
Ackermann steering has
advantages for robots and AGVs
because of its accuracy in following a
curved path. Its disadvantage is the
large turn radius required. AGVs that
tow carts or other objects use this
type of steering to prevent jack-knifing
of the carts.
A second type of steering is called
‘differential steering’ or ‘tank-type
steering’ as tanks and bulldozers use
this with two side treads running at
different speeds to turn. Differential
steering has gained favor with robot
experimenters as it is easy to drive
two side wheels at different speeds
and have a small robot turn at any
angle, or even spin on its axis. Of
course, with only two wheels one or
two free-swiveling casters are needed
at the front or back (or at both
ends) to keep the robot or AGV from
tipping over.
Figure 10 shows the DX- 40
AGV from Savant Automation with
differential steering. Smaller AGVs
that need to make tight turns or
even rotate on their axis use this type
of steering.
FIGURE 9.
FIGURE 10.
AGV Traffic Control
As you can imagine, it would not
be very productive if a warehouse full
of roving AGVs just ran about without
any concern for other AGVs or
humans in their paths. A flashing light
or even a constant beeping is not
enough to warn of an imminent
collision. The AGV must be able to
detect a human, another AGV, or an
obstacle and safely stop. Ultrasonic,
IR, and focused laser or optical beam
sensors can detect objects ahead and
a bumper sensor can be used as a last
fail-safe resort.
When it comes to AGV vs. AGV,
the best solution is to have each of
them operating in areas separate from
the others. This can be done in larger
systems with a master computer that
knows the location of each vehicle
and has bi-directional communications
with all the AGVs. The ‘zone control’
method uses an RF transceiver that
emits a signal in a specific area and
each AGV in that area receives and
re-transmits that signal back to the
master transceiver. The first AGV to
enter the area is given an ‘all clear’
signal to enter or cross, whereas
all others are given a ‘stop’ signal.
If the zone control goes down for
some reason, each AGV has its own
proximity sensors to prevent crashes.
The Evolution of
the OmniMate
I have long admired the HelpMate
that I mentioned at the beginning of
this article. It was first created by
Engelberger’s earlier company,
Transitions Research Corporation.
TRC later marketed the LabMate
robotic platform “trucks” which were
essentially a base from the HelpMate.
These differentially-driven bases have
been used in small AGV applications
and are equipped with a laser range
finder to detect obstacles and
accomplish basic navigation.
The University of Michigan’s
Advanced Technologies Lab and the
UM Mobile Robotics Laboratory have
FIGURE 12.
FIGURE 11.
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