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The method of finding the location of moving Automatic Guided Vehicles (AGV) in pre-defined areas is by Distance Measurement Equipment (DME) which is based on ultrasound and triangulation technique. To reduce error,the ultrasound transmitter needs to be within the line of sight of the receiver. To get rid of this problem, powerful transmitters are used to ensure omni-directional transmission over large distances, which results in cone-shaped propagation..The new system shows that the vehicle can be more flexible, and angular restriction does not bind its movement.

Introduction

One pinnacle of development in the manufacturing system was the invention of Automated Guided Vehicles (AGV). It is a material handling equipment that works in cells without human intervention.The current method for AGV tracking is the Global Positioning System (GPS).But, because of to GPS receiversâ„¢ large size, limited accuracy, and satellite visibility requirements, this system is inappropriate to use within an enclosed area.Various methods of AGV navigation and their features are: 1) wire guided, 2) inertial guided, 3) laser guided, 4) grid, and 5)chemical path guided.Flexibility can be enhanced if the present track system is eliminated and the AGVs are made trackless. Therefore, there was a need to develop an AGV system that eliminated the pre-determined tracks.Inorder to Develop a trackless AGV, various modificationsare needed to the present system. One such requirement will be finding the location of AGV for navigation.

Methods to Find the Position for Navigational Aid
Infrared beacons mounted on the ceiling are used by The SENCAR AGV to triangulate the position. The matrix-based model is an improvement over conventional triangulation technique for coordinates. One of the different methods used to find coordinates is installing three or more transmitters at known locations and one receiver on board .It limits the accuracy as there is one receiver on board because it may not be possible to receive signals in any given direction. so there should be a line of sight between the transmitter and the receiver and all the transmitters have to face the receivers to catch the signal otherwise there can be error.
Another method uses three or more active transmitters mounted on the known location. The sensor rotates and measures the three angles. This system helps in measuring the coordinates and unknown vehicle rotation.
In total, it appears that none of the methods alone is always suitable, but an intelligent combination of two or more methods helps overcome the individual weaknesses. There are systems available in the market to overcome some of the problems. But the thing is that these systems are too large and expensive for operation . Therefore, a need for smaller and inexpensive system exists.

Distance Estimation of a Vehicle from the Base Station
The Distance Measuring System (DMS) is used to estimate the distance of the AGV from a known location.It consists of three parts”the base stations, mobile unit, and a time counter program.

Base Stations:
A base station consists of five parts: ultrasonic transmitters, amplifiers, counters, function generators, and a radio frequency receiver.
Mobile Unit:
It received the ultrasound signal from the base station and transmitted the radio signal in response. It consisted of an ultrasonic receiver, RF transmitter, amplifier, and micro controller.

Counter Program to Estimate Time
This C++ program was used to write the counter value in text format. It measured the number of counts between the latches and unlatches period. This was then multiplied by the average time period between any two adjacent counts.

Conclusion and Recommendations
By incorporating ultrasound receivers 90 degrees apart, the system requires no line of sight or specific angle to receive the signal from a transmitter. prediction of vehicle orientation was possible by adding a second ultrasound signal.

for more refer this pdf:
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