What is AGV - Wire Guidance Control?
Automated guided vehicle (AGV) is a automatic robot that follows sensor. The purpose of this system is to guide the vehicle to move in the predetermined path. This method is most often used in industrial applications to move around a manufacturing facility or warehouse.Wire guidance control is one of the method for AGV. This method uses a slot in the floor and wire is place below the floor surface. The slot is cut for AGV to follow. The wire have an electric signal. This wire is used for transmit a radio signal or magnetic field. In the vehicle, it have the sensor at somr parts in the bottom that close to floor. The sensor detect the radio signal or magnetic field that transmitted from wire. Then, it will use the signal to control how vehicle move.
In the experiment, we using the wire to generate magnetic field and sensors will detect and generate electricity. we will observe what does the relationship of voltage, offset distance and phase of sensors.
Experiment Apparatus
1. Dual trace oscilloscope
2. Function generator
3. Sensing coil
4. A spool of wire (20 meter or more)
Experiment setup
Complete setup in shown in Fig.1. Wire must be able to lay flat on the table for about 1-2 meter in order to simulate the travel of the AGV. Function generator set for sinewave at 50 Hz approximately. Output of the generator will connect in series with the wire through a current limiting resistor around 100 ohm. Some generator may limit the o/p by its own and can be connected directly to the wire. As the resistance of the wire may be small so the output voltage from the generator may drop substantially.
Figure 1. Experiment setup
Procedure
1. Place the sensing coils so that the wire is in the middle between 2 coil.
2. Connect both sensing coil to CH.1 and CH.2 of the oscilloscope. You may need to adjust the frequency for the perfect 180 degree out of phase between two channels.
3. Press the MATH function of the oscilloscope.
4. From the menu , choose A+B. You will see the purple trace that represent the sum of CH.1 and CH.2 (or A+B)
5. Measure the amplitude and phase of A+B as the function of offset distance from 1 to 20 mm. to right and to the left in 5 mm. per step.
6. Plot the result from 5 (Vertical axis as the voltage and horizontal axis as the offset distance)
Experiment Result
Analysis and discussion result
At the
middle point, Sensing coil produce the electricity around 0-30 mV. That
electricity is small value because both of 2 sensing coils produce an equal
amplitude but -180 phase different. So, the A+B signal will be small signal. From
the result, we see the changing of the A+B signal voltage in oscilloscope. When
we move the sensing coil far from middle point, the amplitude of A+B signal will
increase. All of the offset distance will make the amplitude increase that
because one of sensing coil that nearly with wire will produce the electricity
more than at the middle point. As the results, the amplitude of A+B increase. But
when the sensing coils move far at some point (in the experiment is around -15
and 15 mm.), the amplitude of A+B signal will decrease because the magnetic
field strength is lower than other point. The different between “-” and “+”
offset distance is phase of A+B signal. When “-” offset distance, the phase of
A+B signal will be -180 degree.
Conclusion
Wire guidance control uses to control the vehicle by
using wire and 2 sensing coils. Wire acts like the guiding part by using magnetic field. The wire will generate magnetic field and sensing
coils will change the electromagnetic field to be electrical signal. The
electric signal uses in control how the vehicle move.
Example of Wire Guidance control circuit diagram.
Other method for Automated guided vehicle (AGV).
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.
Inertial can operate in nearly any environment including tight
aisles or extreme temperatures. Inertial navigation can include use of
magnets embedded in the floor of the facility that the vehicle can read and
follow.
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.
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.
Geoguidance
A geoguided AGV recognizes its environment to establish its
location. Without any infrastructure, the forklift equipped with geoguidance
technology detects and identifies columns, racks and walls within the
warehouse. Using these fixed references, it can position itself, in real time
and determine its route. There are no limitations on distances to cover or
number of pick-up or drop-off locations. Routes are infinitely modifiable.
Reference in Other method for Automated guided vehicle (AGV) topic.
http://en.wikipedia.org/wiki/Automated_guided_vehicle
Notice : The data in the experiments maybe inaccurate or incorrect. Therefore, the results may be different on other trials or a distortion of the theory.
