Automated Guided Vehicle – AGV

An automated guided vehicle or automatic guided vehicle (AGV) is a mobile robot that follows markers or wires in the floor, or uses vision, magnets, or lasers for navigation. They are most often used in industrial applications to move materials around a manufacturing facility or warehouse.

Lower cost versions of AGVs are often called Automated Guided Carts (AGCs) and are usually guided by magnetic tape. AGCs are available in a variety of models and can be used to move products on an assembly line, transport goods throughout a plant or warehouse, and deliver loads.

There are several AGV navigation systems:
  • Wired – there is a wire placed in the floor, along the route that AGV needs to follow. The wire transmits a radio signal, and the sensor installed on the bottom of the AGV, close to the ground, detects the relative position of it. This information is used to regulate the steering circuit, making the AGV follow the wire.
  • Guide tape - The tapes can be magnetic or colored. The AGV 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.
  • Laser target – A reflective tape is mounted on the walls, poles or fixed machines. The laser, carried by AGV, is transmitted and received by the same sensor, and the angle and distance to any reflectors in line of sight and in range are automatically calculated.
    The navigation system triangulates current position of the AGV, by comparing received information to the map of the reflector layout stored in the AGV's memory. The current position is compared to the path programmed in to the reflector layout map and the steering is adjusted accordingly to keep the AGV on track. It can then navigate to a desired target using the constantly updating position.
  • Gyroscopic – There are transponders embedded in the floor of the work place, and the AGV uses these transponders to keep the vehicle 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.
  • Natural targeting – This system uses laser range-finder, as well as gyroscopes or inertial measurement units with Monte-Carlo/Markov localization techniques, in order to detect its position, and plans the shortest permitted path towards its target.
  • Vision guidance – This system uses cameras to record features along the route, allowing the AGV to replay the route by using the recorded features to navigate. 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.
  • Traffic control –  If there are more than one AGV in certain area, it is required to have traffic control so the AGVs will not run into one another.
  • Zone control uses a wireless transmitter to transmit a signal in a fixed area. Each AGV contains a sensing device to receive this signal and transmit back to the transmitter. If the area is clear the signal is set at “clear” allowing any AGV to enter and pass through the area. When an AGV is in the area the “stop” signal is sent and all AGV attempting to enter the area stop and wait for their turn.
  • Forward sensing control uses collision avoidance sensors to avoid collisions with other AGV in the area. These sensors include: sonic, which work like radar; optical, which uses an infrared sensor; and bumper, physical contact sensor. Most AGVs are equipped with a bumper sensor of some sort as a fail safe.

Why is this important?

The main benefits of using AGVs are reduced labor costs and a more reliable performance. Efficient, cost effective movement of materials is an important and common element in improving operations in many manufacturing plants and warehouses. Because automatic guided vehicles can delivery efficient, cost effective movement of materials, AGVs can be applied to various industries in standard or customized designs to best suit an industry’s requirements.

Automated Guided Vehicles can be used in a wide variety of applications to transport many different types of material including pallets, rolls, racks, carts, and containers. AGVs are best for applications with the following characteristics:

  • Repetitive movement of materials over a distance
  • Regular delivery of stable loads
  • Medium throughput/volume
  • When on-time delivery is critical and late deliveries are causing inefficiency
  • Operations with at least two shifts
  • Processes where tracking material is important

How it was before?

The first AGV was simply a tow truck that followed a wire in the floor instead of a rail. Later it was developed an automatic driverless control system for use in several industrial and commercial applications. Out of this technology came a new type of AGV, which follows invisible UV markers on the floor instead of being towed by a chain.

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.

How to build a magnetic track guided AGV?

Here will be given an example of how to build magnetic track guided AVG.
There will be used only two components made by ROBOTEQ - MGS1600C magnetic guide sensor, and MDC2260 dual channel motor controller.

The AGV will follow a track made of an adhesive magnetic tape affixed on the floor. The MGS1600C will measure how far from the center of the tape it is and provide the information to the motor controller which will then adjust the steering so that the vehicle remains at the center of the track.

Magnetic markers positioned on the left and right side of the track give the AGV location information that will be used to make stop and fork left/right decisions.
Magnetic tape is the easiest to lay and modify while providing excellent durability and reliability.

When designing the vehicle, there are four basic ways of providing drive and steering. Some types are easier to build, others have better steering characteristics.

In this case it will be used 4 wheel drive type. It is the simplest design and able to go in reverse, but not very precise for steering.
The sensor must be placed near the front edge of the chassis and for the best results at 30mm above the floor and it should be ensured that the height fluctuates within +/-10mm max as the AGV moves along the track.
The sensor data is output on a single wire in the form of a series of variable widths pulses, containing the Track Detect signal, Track Position and Left & Right Marker Detects signals. This pulse can be connected to any of the Roboteq motor controller’s pulse inputs.

Electrical Wiring
The wiring diagram below shows the magnetic guide sensor and motor controller in a typical 4 wheel drive chassis.

Sensor Configuration and Testing
The sensor is configured by default to output MultiPWM pulse and can therefore be used without further configuration if the track is made of Roboteq-supplied 25mm magnetic tape.

For configuration, monitoring and troubleshooting, connect the sensor to the PC via the USB connector located under the screw plug. Run the Magsensor PC utility to change the tape width if using a 50mm tape, or use the waveform display view to monitor the shape of the magnetic field.
 With no tape present, the PC utility must show a nearly flat line. For best results, always perform a zero calibration when operating the sensor in a new environment.

Moving a tape under the sensor will cause a "bell" curve to appear on the chart. The curve must be on the positive (up) direction. If the curve is going down, change the Tape Polarity setting in the configuration menu. Makers will also cause a bell curve, but the curve should be going down. The screenshot below shows the resulting curve when placing a left marker and a centered track.

Motor Controller Configuration
To receive and recognize data from the sensor, the controller must first be connected to a PC running the Roborun+ PC utility. In the configuration menu, the pulse input that is connected to the sensor must be enabled and configured as “Magsensor”.
Next, the controller must be configured to operate in mixed mode so that the a steering command will apply a different amount of power to the left and right motor for making turns.

Steering Control
The sensor outputs a value that is the tape’s distance from the center of the track. This information is then used to correct the steering. If the tape is centered, the value is 0 and no steering correction is needed. The further the track is from the center, in one or the other direction, the stronger the steering change.

Throttle Control
How the throttle power is controlled (when to start, stop, accelerate, slow down) is very application dependent. In this case, the AGV will be made to move when a tape is detected, take left or right forks and stop at precise locations. The AGV will then resume moving after a set time, or when a user button is pressed. The AGV will stop when the track is no longer present.

Fork and Merge Management
The sensor has an algorithm for detecting and managing up to 2-way forks and merges along the track. Internally, the controller always assumes that 2 tracks are present: a left track and a right track. When following a single track, the sensor considers that the 2 tracks are superimposed. When entering forks, the track widens, so does the distance between the left and right tracks. When approaching merges, the sensor will report a sudden spread of the left and right tracks, but will otherwise operate the same way as at forks.

Localization using Markers
Magnetic markers are pieces of magnetic tape of opposite polarity and that is located left and/or right of the center track. Markers provide a very simple and cost effective method to identify specific locations along the track.

Manual Steering Override
It is common to require that the AGV be driven manually, to place it in position, or to move it along an untracked path. Buttons, a joystick, a PLC or an RC Radio can be connected directly to the motor controller’s free inputs.

Test Track Description
The figure below shows a simple AGV track with several loading station and one stop station.

The flow chart below shows the structure of the MicroBasic program that will run inside the motor controller to move and steer the AGV along the track. The full source code is provided here.

Demo video can be seen here:
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