ISE 707 – Assignment I

8/10/2019 ISE 707 – Assignment I

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Collision Control of Automated Guided Vehicles – Warehouses

1.Introduction

Applications for Automated Guided Vehicles (AGV’s) have been on the rise since

the 20th century. AGV’s may be described as mobile robots used to move materials in a

manufacturing facility or a warehouse along fixed predefined paths, with the help of wires

in the floor, magnets, lasers or other sensors for navigation.

The major objective with using AGV’s was to improve efficiency and reduce costs in

situations that involve:

Repetitive movement of materials over a distance.

Regular delivery of stable loads.

Processes where tracking is important.

On-time delivery is critical and late deliveries cause inefficiency.

AGV’s can be deployed in any stage of a manufacturing process, raw material

handling stage where initial raw materials are received from the warehouse and delivered

directly to production lines to work in process stages where in-process inventory is

transported between machine stations where operations are performed to finish product

handling stage where goods are stored or sent to dock for delivery to final customer.

FIG 1.1 AGV in a warehouse. [wikipedia.org]



AGV applications are currently limited in a flexible manufacturing setting that

involves a large number of manufacturing operations as it may be required to transport in-

process inventory flexibly to all manufacturing cells on the line when a quality defect is

identified and cost of establishing control over navigation does not justify implementing an

AGV cause some paths may be used very sparingly.

One of the major challenges in implementing a system that involves multiple AGVs

is a collision avoidance system, which requires implementing a real time control system

that provides information about the location of all vehicles in the facility to account for

conflict free routing of vehicles.

2.1 Real Time Control for Collision Avoidance

Once an AGV is loaded, a path is generated for the AGV to its end point by a

centralized controller. Once assigned, manufacturing/warehousing systems that involve

multiple AGVs require real time traffic control so as to avoid collisions once a particular

path has been selected for the AGV. Some of the common methods employed to overcome

collisions are discussed here but a small description on how the path is selected by an AGV

is described prior to that.

2.2 Routing Control

The facility map for the use of AGVs is commonly divided into numerous zones to

help route the vehicle through the plant. The path to be followed by an AGV goes through

these different zones and is decided by a centralized system based on numerous algorithms

made available by the firm based on their needs. The most common algorithm adopted for

AGV systems is the Dijkstra’s algorithm or the shortest path algorithm which takes into

account the start point and end point the AGV has to travel to, to come up with a path for

the AGV. This algorithm is easy to adopt but does not take into account deadlocks that

may arise due to heavy traffic. Deadlock is a situation where multiple AGVs try to gain

access to a particular zone currently occupied by an AGV which results in none of the



AGVs being able to enter the site once the AGV vacates the site due to repetitive looping

of the transmitter signal.

Dynamic algorithms have now been developed that help avoid this error and account

for efficient functioning of the system. Once a path has been decided by the system,

localized control measures are used to avoid collisions among the AGVs.

2.3 Zone Control

Zone control is a cost efficient way to control AGVs. It involves the use of a wireless

transmitter that is used to transmit signals to AGVs from each zone in the plant. AGVs are

equipped with a sensor to receive and transmit signals back to the sensor. If there are no

AGVs in a particular zone, the signal is set to clear allowing an AGV to pass through the

zone. When an AGV is in a zone, the transmitter signal is set to stop and all AGVs entering

the area stop and wait for their turn to pass through. One of the issues with this form of

control is when an individual zone goes down, there is a risk that all AGVs directed to pass

through that zone are prone to collisions.

The transmitters on the shop floor, sensors and PLCs on the AGVs contribute the real

time control elements of the system. This system helps avoid multiple AGVs entering the

same storage isle or one blocking the other during travel in a large warehouse.

FIG 1.2 Zone control [moxa.com]



2.4 Forward Sensing Control

This system uses infrared sensors, optical sensors, physical contact sensors or sonic

sensors mounted directly on vehicles to avoid collisions with other AGVs in the area.

Disadvantages include difficulty in mounting the sensors and ability to detect collision on

all sides of the vehicle.

The sensor and PLCs mounted on the AGVs contribute the real time control features

of the system. This form of control is commonly used in a mfg. facility rather than in a

warehouse.

FIG 1.3 Range of Forward Sensing System [omran-ap.org]

2.5 Combination Control

This system uses a combination of the above two systems. Zone control is used

during normal operation and Forward sensing control is used as a fail-safe option.

3. Real Time Control – Features

All collision control methods detailed above work based on real time information and

are expected to provide results based on this data and provide results within the least

possible time so as to carry out functions effectively. Each form of control and its relation

to real time control is explained below:



Zone Control:

The transmitter signal is set to clear or stop based on if or not an AGV is currently in a

zone based on real time information and AGVs are expected to respond to this data

immediately.

Forward Sensing Control:

Forward sensing control helps avoid collisions if the sensor mounted on the vehicles

returns a positive reading which directs the AGV to come to a stop upon processing the

information.

Combination Control:

Combination control is a result of both zone and forward sensing control.

- The similarity between all the control mechanisms described here are that they work with

real time data to overcome collisions between systems once a path has been assigned.

4. Consequences of Failure

If the collision avoidance system in the facility fails, it leaves the AGVs prone to

collisions between each other and to equipment and personnel in the plant, which is a cause

of concern to both safety of personnel and financial needs of a firm.

Hence, it is vital that the system is able to function smoothly and make computational

decisions correctly and importantly in a timely manner once a path has been assigned.

FIG 1.4 Accidents [solico.nl]



5. Type of Real Time Control

These systems fall under the category of Hard Real Time Control as incorrect

processing of information or untimely processing of information may result in collisions

with equipment or personnel. These systems are expected to receive and process the

information they receive in a precise and timely manner so as to avoid complications

during operation.

6. Bibliography

1. ‘Dynamic Routing of Automated Guided Vehicles in Real Time’, Ewgenij

Gawrilow, Ekkehard Kohler Bjorn Stenzel, Rolf H. Mohring, October 10th,2007, Technische Universitat Berlin.

2. ‘Fundamentals of Modern Manufacturing: Materials, Process andSystems’, Mikell P Groover, Chapter 39.

3. ‘Automated Guided Vehicle’, Thomas Muller, IFS 2008, Chapter 3-4.

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ISE 707 – Assignment I