Packmobile
with trailer AGV
Over the
years the technology has become more sophisticated and today automated vehicles
are mainly Laser navigated e.g. LGV (Laser Guided Vehicle). In an automated
process, LGVs are programmed to communicate with other robots to ensure product
is moved smoothly through the warehouse, whether it is being stored for future
use or sent directly to shipping areas. Today, the AGV plays an important role
in the design of new factories and warehouses, safely moving goods to their
rightful destination.
Wired
A slot is
cut in to the floor and a wire is placed approximately 1 inch below the
surface. This slot is cut along the path the AGV is to follow. This wire is
used to transmit a radio signal. A sensor is installed on the bottom of the AGV
close to the ground. The sensor detects the relative position of the radio
signal being transmitted from the wire. This information is used to regulate
the steering circuit, making the AGV follow the wire.
Guide tape
AGVs
(some known as automated guided carts or AGCs) use tape for the guide path. The
tapes can be one of two styles: magnetic or colored. The AGC is fitted with the
appropriate guide sensor to follow the path of the tape. One major advantage of
tape over wired guidance is that it can be easily removed and relocated if the
course needs to change. Colored tape is initially less expensive, but lacks the
advantage of being embedded in high traffic areas where the tape may become
damaged or dirty. A flexible magnetic bar can also be embedded in the floor
like wire but works under the same provision as magnetic tape and so remains
unpowered or passive. Another advantage of magnetic guide tape is the dual
polarity. small pieces of magnetic tape may be placed to change states of the
AGC based on polarity and sequence of the tags.
Laser target navigation
The
navigation is done by mounting reflective tape on walls, poles or fixed
machines. The AGV carries a laser transmitter and receiver on a
rotating turret. The laser is transmitted and received by the same sensor. The
angle and (sometimes) distance to any reflectors that in line of sight and in
range are automatically calculated. This information is compared to the map of
the reflector layout stored in the AGV's memory. This allows the navigation
system to triangulate the current position of the AGV. The current position is
compared to the path programmed in to the reflector layout map. The steering is
adjusted accordingly to keep the AGV on track. It can then navigate to a
desired target using the constantly updating position.
· Modulated Lasers The use
of modulated laser light gives greater range and accuracy over pulsed laser
systems. By emitting a continuous fan of modulated laser light a system can
obtain an uninterrupted reflection as soon as the scanner achieves line of
sight with a reflector. The reflection ceases at the trailing edge of the
reflector which ensures an accurate and consistent measurement from every
reflector on every scan. By using a modulated laser a system can achieve an
angular resolution of ~ 0.1 mrad (0.006°) at 8 scanner revolutions per second.
Pulsed
Lasers A typical pulsed laser
scanner emits pulsed laser
light at a
rate of 14,400 Hz which gives a maximum possible
resolution of ~ 3.5 mrad (0.2°) at 8 scanner revolutions per second. To achieve
a workable navigation, the readings must be interpolated based on the intensity
of the reflected laser light, to identify the centre of the reflector.
Inertial (Gyroscopic) navigation
Another
form of an AGV guidance is inertial navigation. With inertial guidance, a
computer control system directs and assigns tasks to the vehicles. Transponders
are embedded in the floor of the work place. The AGV uses these transponders to
verify that the vehicle is on course. A gyroscope is able to detect the
slightest change in the direction of the vehicle and
corrects
it in order to keep the AGV on its path. The margin of error for the inertial
method is ±1 inch. [1]
Inertial
can operate in nearly any environment including tight aisles or extreme
temperatures. [2]
Inertial navigation can include use of magnets embedded in the floor of the
facility that the vehicle can read and follow. [3]
Unit-load AGV using natural-features
navigation to carry steel to quality assurance lab
Natural features (Natural Targeting)
navigation
Navigation
without retrofitting of the workspace is called Natural Features or Natural
Targeting Navigation. One method uses one or more range-finding sensors, such as
a laser range-finder, as well as gyroscopes or inertial measurement units with
Monte-Carlo/Markov localization techniques to understand where it is as it
dynamically plans the shortest permitted path to its goal. The advantage of
such systems is that they are highly flexible for on-demand delivery to any
location. They can handle failure without bringing down the entire
manufacturing operation, since AGVs can plan paths around the failed device.
They also are quick to install, with less down-time for the factory. [4]
Vision guidance
Vision-Guided
AGVs can be installed with no modifications to the environment or
infrastructure. They operate by using cameras to record features along the
route, allowing the AGV to replay the route by using the recorded features to
navigate. Vision-Guided AGVs use Evidence Grid technology, an application of
probabilistic volumetric sensing, and was invented and initially developed by
Dr. Moravec at Carnegie Mellon University. The Evidence Grid technology uses
probabilities of occupancy for each point in space to compensate for the
uncertainty in the performance of sensors and in the environment. The primary
navigation sensors are specially designed stereo cameras. The vision-guided AGV
uses 360-degree images and build a 3D map, which allows the vision-guided AGVs
to follow a trained route without human assistance or the addition of special
features, landmarks or positioning systems
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