obstacle detection and avoidance for an autonomous farm tractor

7/29/2019 Obstacle Detection and Avoidance for an Autonomous Farm Tractor

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OBSTACLE DETECTION AND AVOIDANCE

FOR AN AUTONOMOUS FARM TRACTOR

by

Keith W. Gray

A thesis submitted in partial fulfillmentof the requirements for the degree

of

MASTER OF SCIENCE

in

Electrical Engineering

Approved:

_____________________ _____________________Kevin Moore Nick FlannMajor Professor Committee Member

_____________________ _____________________Kay Baker Noelle E. Cockett

Committee Member Interim Dean of Graduate Studies

UTAH STATE UNIVERSITYLogan, Utah

2000


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Copyright Keith W. Gray 2000

All Rights Reserved


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Abstract

Obstacle Detection and Avoidance

for an Autonomous Farm Tractor

by

Keith W. Gray, Master of Science

Utah State University, 2000

Major Professor: Dr. Kevin L. MooreDepartment: Electrical and Computer Engineering

The Center for Self-Organizing and Intelligent Systems (CSOIS) is developing

autonomous ground vehicles for use in farming applications. CSOIS has targeted

obstacle detection and avoidance as key challenges in real-world implementation of

autonomous farming vehicles. A range sensor, giving real-time updates of the

surrounding environment, performs obstacle detection. Obstacle avoidance is

accomplished through a combination of global and local avoidance subsystems that deal

with both known and unknown obstacles in the operating area of the vehicle. The global

avoidance subsystem is a mission-level path planner that preplans paths around all known

obstacles while the local avoidance subsystem maneuvers the tractor around unknown

obstacles. The local avoidance subsystem consists of an obstacle filter and an obstacle

avoidance algorithm. The obstacle filter reports unknown obstacles to the path planner

and enables the avoidance algorithm if the preplanned path is blocked. The avoidance


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algorithm learns of all the known and unknown obstacles from the obstacle filter so that

all obstacles can be safely avoided. The task of the avoidance algorithm is to maneuver

the vehicle around the obstacle with the goal of returning to the preplanned path as

quickly as possible. This thesis describes the unique challenges to autonomous farm

vehicle navigation and the solutions reached that enabled success.

(98 Pages)


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Acknowledgments

I would like to thank the many people that helped me on this project. First and

foremost I would like to thank the John Deere project team. Mel, Mitch, Sarah, and Don.

You four are the real reason the obstacle detection and avoidance worked on the tractor.

Your input and suggestions and patience helped me see past the obstacles in my thinking

and really shaped the final product. I would also like to thank my professors at Utah

State University, Dr. Moore and Dr. Flann, who gave valuable input and suggestions as

well as the opportunity to be part of this great team of engineers. I would like to thank

John Deere Inc. for their belief in our team and for the growth that I received because of

the project. I want to thank my wife, Malia, who supported me through this whole ordeal

and always encouraged me when I was burned out, even though I rarely saw her that

summer, I love her deeply. Finally I want to thank my God who has blessed me with the

desire to learn and grow and the ability to understand that I am nothing without him.

Keith W. Gray


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Contents

Page

Abstract ............................................................................................................................. iii

Acknowledgments.............................................................................................................. v

List of Tables...................................................................................................................viii

List of Figures ................................................................................................................... ix

1 Introduction ................................................................................................................... 1

1.1 Motivation ............................................................................................................... 11.2 Summary of Results ................................................................................................ 21.3 Thesis Outline ......................................................................................................... 4

2 Overall System............................................................................................................... 6

2.1 Base Station............................................................................................................. 82.2 Master Node ............................................................................................................ 82.3 Path Planner Node.................................................................................................112.4 Sensor Node ..........................................................................................................12

3 Obstacle Detection Technology ................................................................................... 16

3.1 Obstacle Detection Sensors ................................................................................... 173.1.1 CCD Camera .............................................................................................173.1.2 Ultrasonic Sensors..................................................................................... 203.1.3 Scanning Laser .......................................................................................... 213.1.4 3D Scanning Lasers...................................................................................233.1.5 Millimeter Wave Radar............................................................................. 23

3.2 Comparsion of Sensors.......................................................................................... 243.2.1 Weather ..................................................................................................... 24

3.2.2 Light ..........................................................................................................243.2.3 Detection Distance .................................................................................... 253.2.4 Response Time .......................................................................................... 253.2.5 Cost............................................................................................................263.2.6 Summary ...................................................................................................27

3.3 Detection Sensor Used on Tractor ........................................................................ 28


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4 Obstacle Avoidance ..................................................................................................... 31

4.1 Global and Local Obstacle Avoidance.................................................................. 31

4.2 Vision of Obstacle Avoidance............................................................................... 324.3 Obstacle Avoidance Used in Research.................................................................. 334.3.1 Wall-Following ......................................................................................... 344.3.2 Black Hole................................................................................................. 364.3.3 Path Computing......................................................................................... 384.3.4 Potential Fields.......................................................................................... 404.3.5 Histogram..................................................................................................41

5 Obstacle Detection and Avoidance ............................................................................ 52

5.1 Obstacle Detection and Avoidance Data Flow ..................................................... 52

5.2 Obstacle Detection ................................................................................................ 535.3 Obstacle Filter ....................................................................................................... 585.3.1 Obstacle Filter Flow of Control ................................................................ 605.3.2 Dynamic Obstacles Overview................................................................... 62

5.4 Avoidance.............................................................................................................. 64

6 Simulations and Vehicle Tests.................................................................................... 77

7 Conclusions and Recommendations .......................................................................... 81

References ........................................................................................................................ 84

Appendix A ...................................................................................................................... 87


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List of Tables

Table Page

5.1 LMS Continuous Data Report Packet. .................................................................54

5.2 PC to LMS Reset Packet. .....................................................................................56

5.3 PC to LMS Request for Continuous Data. ...........................................................56


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5.12 Stop state. ............................................................................................................. 75

5.13 Block diagram of steering control during avoidance. .......................................... 76

6.1 Map of VFH+ simulation. .................................................................................... 78

6.1 Map of VFH+ simulation (cont.). ........................................................................ 79


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Chapter 1

Introduction

This thesis presents the design and implementation of an obstacle detection and

avoidance system for use on an automated tractor. The research described in this thesis

was carried out as part of a larger project aimed at demonstrating obstacle detection and

avoidance for an autonomous tractor operating in a typical farming environment.

Specific contributions of the thesis include an assessment of the different obstacle

detection sensors and obstacle avoidance algorithms used in autonomous vehicle

research, and development of a new obstacle avoidance system for the tractor, including a

unique obstacle avoidance algorithm. Simulation and testing results show the

effectiveness of the approach.

1.1 Motivation

The idea of an autonomous tractor is not a new one. Every child who lived on a

farm had dreams of a farm vehicle that could take over the plowing and harvesting

chores, saving them from the boredom of driving long monotonous hours. With

advances in GPS technology and computerization of farm equipment this dream is now

closer to reality. Now the simple, yet boring, task of plowing and harvesting a field can

be turned over to an autonomous tractor that will never get tired and that will do the job it

is tasked to do. While an autonomous farm tractor is a realizable dream, there are still

several concerns that need to be addressed before they become common on the family

farm. Two of the biggest concerns are first, how the tractor will see its environment and,

second, how the tractor will react to its environment.


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The ability to sense the surrounding environment is an important issue for any

autonomous vehicle. As human beings we have the most powerful set of sensors and the

best possible computer available for interpreting our environment. For an autonomous

tractor, the question becomes: is it possible to find sensors that can gather sufficient

environmental data for safe vehicle navigation? Then, once the environmental data has

been captured, is there a fast and effective way to interpret the data and determine what it

means to the tractor? In order to take the farmer out of the tractor these two questions

must be answered in the affirmative.

Once the environment around the tractor has been determined then the vehicle

must react accordingly. A farmers reaction to an unexpected object in his way is nearly

instant and yet thought out. He knows that if he needs to swerve around a big rock that

has become unearthed that he cannot cross over, any hay that has been cut, or travel over

a fence line or ditch. As with the detecting of the environment, if the farmer is to be

taken out of the tractor the issue of tractor reaction must be fast and precise. It is

necessary to be able to make correct decisions about the appropriate action needed in

response to the data about the environment.

1.2 Summary of Results

The unique obstacle avoidance algorithm as developed by the author and presented

in this thesis was demonstrated before the project sponsors in a farm field west of Logan

Utah in August of 1999. Different scenarios were used to illustrate the need for obstacle

detection and avoidance as well as demonstrate the effectiveness of the avoidance

algorithm presented in this thesis. The different scenarios included an obstacle on the


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path of travel, obstacles to the right or left of the path of travel, known obstacles close to

the path of travel, and dynamic, or moving, obstacles.

In first scenario, an obstacle on the path of travel, the tractor was effective at

determining the environment around the obstacle to make the decision as weather to

avoid to the right or left of the obstacle. If the path of travel was close to the border of

the field then the avoidance algorithm would avoid towards the center of the field. The

next scenario, obstacles to the right or left of the path, the avoidance algorithm correctly

drove the tractor the shortest distance around the obstacle and back toward the path.

The next scenario, known obstacle close to the path of travel, demonstrated the

obstacle filter portion of the obstacle avoidance algorithm. When the obstacle detection

sensors detected a known obstacle, such as the fence line surrounding the field, the

obstacle filter did not inform the avoidance algorithm because the obstacle did not block

the path of travel. If, however, the tractor was avoiding an unknown obstacle and a

known obstacle was encountered in the process, that obstacle was also effectively

avoided.

The final scenario, dynamic obstacles, was demonstrated using a remote controlled

truck. The truck was fitted to be tall enough and wide enough to be seen by the detection

sensor. Three different scenarios with the dynamic obstacle showed the robustness of the

avoidance algorithm. The first scenario had the dynamic obstacle approach the tractor

from the right or left. As the tractor avoided the dynamic obstacle the obstacle continued

to move towards the tractor. When the obstacle was to close to the tractor the avoidance

algorithm stopped the tractor. The next scenario involving the dynamic obstacle had the


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obstacle move in front of the tractor, as the tractor started avoiding the obstacle the

obstacle quickly moved out of sight of the tractor and normal tractor driving resumed.

The final scenario with the dynamic obstacle had the obstacle approach the vehicle

causing the tractor to avoid. As the tractor attempted to maneuver around the obstacle the

obstacle moved with the tractor, pushing the tractor further from the desired path of

travel. When the tractor was more than one swath width away from the path of travel the

avoidance algorithm stopped the tractor.

1.3 Thesis Outline

The thesis is organized as follows. Chapter 2 discusses the tractor system,

illustrating how this work fits into the overall project. In Chapter 3 we discuss various

obstacle detection technologies used for autonomous vehicles. We also describe the

obstacle detection sensor used in this project. In Chapter 4 we examine obstacle

avoidance algorithms other researchers have used for autonomous vehicles. Section 4.1

discusses the difference between global and local or reactive avoidance. Section 4.2 lists

the obstacle avoidance requirements that were specified for the overall system developed

in the larger project. Section 4.3 then considers different obstacle avoidance algorithms

and compares them against the requirements in Section 4.2. In particular, subsection

4.3.5 is a detailed description on one obstacle avoidance algorithm that served as the

model for the avoidance algorithm developed by the author. Chapter 5 gives a detailed

explanation of the obstacle detection and avoidance system that was developed for the

autonomous tractor system. This chapter also describes how the system works together

to successfully detect and interpret the environment surrounding the vehicle and then


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react according to this environment. Section 5.3 and Section 5.4 detail the unique

approach to the obstacle avoidance algorithm. Chapter 6 presents the simulation and

actual testing of the detection and avoidance system. Chapter 7 concludes the thesis with

suggestions for future work.


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Chapter 2

Overall System

In this chapter we describe the overall autonomous tractor system for which the

obstacle detection and avoidance algorithm was developed. Each subsystem in the

system is discussed and we explain how the various subsystems interact with each other

to create a fully autonomous farm tractor. This architecture was developed by the

engineering team as part of the larger project [1].

As seen in Fig. 2.1 the autonomous tractor system is made up of four main

subsystems: the base station, the master node, the path planner node, and the sensor node.

The interaction of each of the subsystems will be illustrated by describing a typical task

UserGui

Wireless

Lan

EmergencyStop Button

Modem

Master

Node

Path Planner

Node

Sensor

Node

Wireless

Lan

Modem Sensors

Tractor

Control

Base Station

Coax

Coax

Coax

Tractor System Architecture

TCP/IP

RS-232

Fig. 2.1: Overall system architecture operation.


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for the autonomous tractor. The goal of the mission is to perform a simple sweep

pattern in a field. From the base station we use the User GUI to select the field of

operation. The sensor node conveys to the path planner the exact location of the tractor

and the current heading of the tractor. The path planner uses the position and attitude

data and the desired field of operation to plan a sweep path. The path planner takes into

account the turning radius of the vehicle as well as the size of the farming implement the

tractor may be using while planning the sweep pattern. Once the path planner has

planned a mission the mission is visible on the User GUI at the base station and we can

begin the mission. We then select on the GUI the start mission command. At this point

the path information generated by the path planner is forwarded to the master node and

the sensor node. The master node converts the path information into driving and steering

commands. These commands control the tractor along the path, thus performing the

desired task. The sensor node provides feedback to the master node controller on the

tractors position and attitude along the path. If the obstacle detection sensor senses an

obstacle that will impede the tractors travel then the obstacle avoidance algorithm

calculates driving and steering commands that will safely avoid the obstacle. These

commands are relayed to the master node, which carries out the commands. If for some

reason we feel that the tractor is behaving in an unsafe manner we have an emergency

stop button at the base station. This button allows us to shut off the tractor engine and

engage the emergency brake.


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The remainder of this chapter will discuss each of these different subsystems and

how they contribute to the vehicle operation as well as to obstacle detection and

avoidance.

2.1 Base Station

The base stations contribution to vehicle operation and obstacle detection and

avoidance is perhaps the most important of all the other subsystems. The reason for this

is a simple old clich that states, seeing is believing. If a farmer is to turn an

autonomous tractor loose in his field where the liabilities are high if something goes

wrong, then he will want accurate information about the tractor and he will want to stop it

immediately if something goes wrong. This is what the base station provides. The base

station is the communication link between the farmer and the tractor. The base station

consists of a graphical user interface (GUI) and a tractor kill button. The GUI allows the

farmer to task the tractor and then displays in real-time where the tractor is and what it is

doing. The base station also provides a graphical display of the instrument panel from

the tractor cockpit. The GUI also displays in real-time the environment around the

tractor by displaying the obstacle detection sensor information. If the farmer feels the

need to stop the tractor at any time during the course of the mission he can flip a switch

and instantly stop the tractors engine and lock the brakes.

2.2 Master Node

The master node performs all the functions needed to drive and steer the tractor.

During obstacle avoidance the avoidance algorithm calculates the proper heading for


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Master Controller

Motion Exception

HandlerPath Tracker

Drive ControllerStatus Broadcaster

Listeners

Script Parser &

Trajectory GeneratorWatchdogs

Position & Attitude

Processor

Path Store

from Tractor from N av/Sensor Node from Mission Plannerfrom Joystick

To other Nodes To Wheel Steering

VehicleState WheelDriveVectors

VehicleState

DriveVectors DriveVectors

VehicleState

HandledEnableEnable

PathSegments

PathSegments

PathScripts

CommFailure

KeepAlives

Position & Attitude

Odometry

DriveVectors

Joystick DriveVectors

GPS/FOG

GPS/FOG/ObstacleComputer PathScriptsJoystick DriveVectorsPathScripts

Fig. 2.2: Master node architecture.

collision-free travel, but it must communicate this heading to the master node whose task

is to steer the tractor to that heading. In the remainder of this section we describe how

the master node controls the tractor by discussing the architecture shown in Fig. 2.2 [1].

In order to operate the tractor the master node receives data from four different

sources. The tractor itself has an on-board computer that interfaces with the master node.

Through this link the master node can monitor engine RPM, throttle, and other

parameters necessary for driving. This same link also allows the master node to make

any changes to the tractors driving, such as increasing or decreasing the throttle and

steering angle. The joystick is the second source of input data into the master node. The

input from the joystick is interpreted as steering angle and throttle speed. All of the path

information computed by the path planner is the third data input into the master node.


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The path information is sent as start points and end points of the desired path along

with the radius of the path. If the radius is zero then the path is a straight line, if the

radius is greater than zero then the path is a circle. The final input to the master node is

from the sensor node. The sensor node communicates to the vehicles position and

heading. If the sensor node is avoiding obstacles then a motion exception is also

communicated to the master, indicating the speed and steering angle needed to avoid the

obstacle.

In addition to receiving data from the other nodes the, master node transmits data

to the planner and sensor nodes. In order to keep the other node up to date with the

current vehicle position and heading (heading and position from sensor are augmented in

the master node with vehicle odometery for more accurate readings) and other status

information, the master node has a status broadcaster. The status broadcaster updates

tractor parameters every 250 ms and can be accessed by the sensor and planner node

through a master listener function.

The reason for the inter-node communication is so the master node knows exactly

where the tractor needs to be and which direction it needs to be heading. It is then the

master nodes responsibility to drive the tractor to where it needs to be and steer it in the

direction it needs to be headed. This is the accomplished through the drive controller,

which resolves path commands from the path planner node and position and attitude

information from the sensor node. In the case of obstacle avoidance the path commands

from the planner node are disregarded as the sensor node provides the needed driving

information to the drive controller.


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2.3 Path Planner Node

The path planners contribution to obstacle detection and avoidance is to provide

mission information to the master and sensor node and to give a global understanding of

the tractors environment. In the remainder of this section we discuss how the path

planner creates missions and informs the other nodes of this mission. We also discuss

how the planner gives a global understanding of the tractors environment.

Path planning is done using a terrain map, a possibility graph, and a search. The

terrain map stores terrain information of the area where the vehicle will be driven,

including locations of roads, fields, obstacles, etc. The possibility graph, which

represents possible paths through the map, is made up of nodes and edges. Nodes can be

compared to street intersections and edges to streets. Traveling from node to node is

done by following an edge or a combination of edges that connects the two nodes.

Adding nodes and edges to the graph extends the graph and allows travel to previously

unattainable areas. Each edge is assigned a cost based on terrain information stored in

the map. An A* search algorithm, as described in [2], uses edge costs to find optimal

paths from one point to another in the graph.

In past projects, obstacle avoidance was done by rejecting edges that passed

through a known obstacle. This simple technique ensures that obstacles will be avoided;

however, it reduces the connectivity of the graph and may unnecessarily prevent access to

certain areas of the map. The path planning for the tractor uses edge-building techniques

that take obstacle shape into account and builds edges that circumvent obstacles.


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Once the mission has been constructed it is broken down into path segments.

Each path segment gives a start and endpoint of the path and a radius for the path. A

radius of zero indicates a straight path. Then, there are effectively two types of paths:

lines and curves. All the path segments for the entire mission are sent to the master and

sensor node and stored in order of execution.

Planning a mission is not the only function of the path planner. The other

function is to provide the sensor node with a global understanding of the environment in

which the tractor operates. This allows the sensor node to react during obstacle

avoidance to, not only the objects the detection sensor sees, but to objects the sensor

might not see but that are still present. An example of this might be if the tractor is

avoiding an obstacle that is on the desired path of travel with no other obstacles in view

of the detection sensor. From the obstacle detection and avoidance algorithms

perception, avoiding to the right or left of the obstacle makes no difference. However if

the map of the field indicated a pond was to the left of the obstacle then the obstacle

should be avoided by leaving the path to the right. Thus a global understanding of the

path planner assists the avoidance algorithm by indicating obstacles other than those seen

by the detection sensor.

2.4 Sensor Node

Having discussed how the path planner and master nodes contribute to obstacle

detection and avoidance we now focus on the node where obstacle detection and

avoidance are performed. In this section we take a closer look at how obstacle detection

and avoidance are performed by examining the architecture of the sensor node shown in


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correct for bias drift from the integration error.

Recall the two questions raised in the introduction: (1) is there a sensor that can

perceive the environment around the tractor well enough to provide data for collision-free

driving while accomplishing the predetermined mission as quickly as possible and (2)

how can we interpret this data and make correct decision about actions needed to avoid

obstacles. The question of reporting the environment will be addressed in chapter 3.

After the sensor reports the environment we then must interpret the data as quickly as

possible so that avoidance headings can be computed.

This leads us to the obstacle filter. The obstacle filter is a way of interpreting the

data from the detection sensor and combining that data with the map of known obstacles

from the path planner to give an up-to-date map of the environment surrounding the

vehicle. This up-to-date map is constructed by combining all known obstacles with any

new previously unknown obstacles that the detection sensor saw. The detection sensor

will see many obstacles, but to avoid dealing with the same obstacles twice, the obstacle

filter ignores any obstacle reported by the detection sensor that was previously known by

the path planner. An up-to-date map allows the tractor to make effective collision-free

heading calculations.

With the obstacle filter providing a map of the surrounding environment, the

obstacle controller can calculate proper heading corrections to avoid striking obstacles.

The algorithm for computing these heading corrections comprises chapter 5. While the

tractor is avoiding obstacles the obstacle controller also ensures that the master node is

steering the vehicle to the proper avoidance heading. It does this by monitoring the


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vehicle heading and issuing steering commands to the master node that will bring the

tractor to the calculated avoidance heading.

In this chapter we have discussed the tractor subsystems and how they all work

together to operate the vehicle and to aid in the detection and avoidance of obstacles. In

the next chapter we describe in more detail one of the subsystems, the obstacle detection

sensor.


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Chapter 3

Obstacle Detection Technology

Having discussed the tractor system as a whole, in this chapter we discuss the first

topic of this thesis, the obstacle detection sensor. First we consider why the detection

sensor is so vital to autonomous vehicles, whether they be tractors, airplanes, or robots.

We then examine what other researchers have used for detection sensors and what they

have to say about the various technologies that are available. Finally, we briefly describe

the obstacle detection sensor that was used on the tractor throughout the course of the

project.

For people to truly be taken out of the loop in the field of autonomous vehicles,

all of the functions that man performs must be mimicked. A microprocessor takes the

place of the brain; complex algorithms and programs take the place of decision-making;

actuators take the place of feet to accelerate or brake, and hands to steer. Unfortunately,

the most important feature of a person is the hardest to mimic. The ability of a person to

see and take in so much with his eyes is key. Without eyes the microprocessor crunches

the complex algorithms and programs blindly. The program expects a leisurely cruise in

the country, but the vehicle may actually be travelling on perilous terrain. This is the

biggest limitation for autonomous vehicles, with no single perfect solution. Before the

decision of which detection sensor to place on the tractor we examined many different

options. Section 3.1 discusses the different detection sensors that other researchers are

using in this field and then Section 3.2 discusses the detection sensor used for the tractor

project.


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3.1 Obstacle Detection Sensors

An examination of various research studies on autonomous vehicles/robots shows

that there are only five to six different types of effective obstacle detection sensors.

These sensors range in price from inexpensive to very expensive. Each of them has their

own unique advantages and disadvantages for different applications. The sensors are not

limited to obstacle detection. Some sensors are used for vehicle localization (the

detection of objects at known locations to determine the vehicles location in a map as

explained in [3] and [4]). Other sensors may be used to extract different features in

plants for plant characterization, allowing an autonomous robot to give the proper

fertilizer in the proper amounts to different plants as explained by Harper [5]. Again,

other sensors are used to create maps by mapping passable and impassable terrain as

explained by Howard [6] and by Matsumoto [7]. While all of these applications do not

address our problem of obstacle detection directly, they do give us an idea of how these

different sensors might work in our setup. If a sensor is used effectively to create

accurate maps for the vehicle environment then it is possible to detect obstacles in a

farming environment. Following will be a discussion of each of the different sensors

being used today. Their advantages and disadvantages will be highlighted and the

possibility of use on the tractor project will be examined.

3.1.1 CCD Camera

The first type of detection sensor we consider is the CCD camera. The camera is

considered a passive sensor since it is a sensor that requires ambient light to illuminate its

field of view. The camera also happens to be similar in a crude sense to the human eye.


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an obstacle. The background is often the same color as the plants Harper [5] wants to

detect, making cameras useless. The human brain has been trained to recognize so many

different patterns and color schemes that instantly we can decipher the data from our eyes

and determine backgrounds and obstacles. The computing power required for this is far

beyond our capabilities. If the computer brain can be trained to recognize background,

then a determination of where obstacles are on that background can be accomplished.

Howard [6] used visual data obtained from cameras to build maps. To do this he assumes

the floor is a constant color, thus making the distinction between passable and impassable

areas of the map. Navigation was accomplished by [7] placing patterns on the floors.

These patterns are easily deciphered from the camera and the robot then travels along the

pattern. Soto [13] used 5 color cameras for surveillance outdoors. He assumes that all

background is grass and is therefore green. This means anything not green in considered

an intruder. Another big problem with stereo vision is dust, rain, and snow. Foessel [15]

described the problems that cameras have in blowing snow. His research was conducted

in polar environments were there is an abundance of blowing snow. Corke [16]

described the problems associated with cameras and dust typical in mining environments.

To the sensor these conditions appear as obstacles when they are not. Again human eyes

adjust to these problems and filter them out. Indoor environments, where lighting and

coloring can be controlled, seem the best place to use stereo vision. The farming

environment offers too many challenges for stereo vision to be used effectively as an

obstacle detection sensor.


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3.1.2 Ultrasonic Sensors

A second type of detection sensor is the ultrasonic sensor, or sonar. Sonar is the

most widely used sensor for obstacle detection because it is cheap and simple to operate.

Their use ranges from the robot hobbyist to the serious researcher. In [3] and [11] sonar

is used in vehicle localization and navigation respectively. In [13], [17], [18], and [20]

sonar is used for obstacle detection. Borenstein [21-28] used a sonar ring around his robot

for obstacle detection, which allows him to develop an obstacle avoidance algorithm.

Harper [5] states that sonar works by blanketing a target with ultrasonic energy; the

resultant echo contains information about the geometric structure of the surface, in

particular the relative depth information. Unfortunately, a key drawback of sonar is one

sensor for one distance reading; that is, in order to obtain an adequate picture of the

environment around the vehicle many sensors must be used together. It is not uncommon

to have several rings of sonar sensors on omni-directional vehicles because the vehicle

can travel in any direction at any time, as the case with Borenstein [22], [23], [26]. The

number of sensors required for an adequate field of view is not the only drawback to

sonar. Borenstein [28] explains that Even though ultrasonic ranging devices play a

substantial role in many robotics applications, only a few researchers seem to pay

attention to (or care to mention) their limitations. He goes on to explain that for

ultrasonic sensors to be effective they must be as perpendicular to the target as possible to

receive correct range data. This is because the reflected sound energy will not be

deflected towards the sensors if the two are not perpendicular to each other. Borenstein


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[25] goes on to list three reasons for ultrasonic sensors being poor sensors when

accuracy is required. These reasons are:

1 Poor directionality that limits the accuracy in determination of the spatialposition of an edge to 10-50 cm, depending on the distance to the obstacle andthe angle between the obstacle surface and the acoustic beam.

2 Frequent misreadings that are caused by either ultrasonic noise from externalsources or stray reflections from neighboring sensors (crosstalk).Misreadings cannot always be filtered out and they cause the algorithm tosee nonexistent edges.

3 Specular reflections that occur when the angles between the wave front and

the normal to a smooth surface is too large. In this case the surface reflectsthe incoming ultra-sound waves away from the sensor, and the obstacle iseither not detected at all, or (since only part of the surface is detected) seenmuch smaller than it is in reality.

Perhaps the biggest limitation of sonar for the tractor application is the limited range of

sight. Given a one-ton tractor traveling at 5-8 mph, when the sonar detected an obstacle

it would already be too late to begin safe avoidance. However, despite all the limitations

of ultrasonic sensors, the technique can still have a good use. Sonar is a good safety net

sensor. If the other detection sensors miss an obstacle, or for some reason the vehicle

gets close enough for sonar to be triggered then a halt state can be executed and the

vehicle stops immediately.

3.1.3 Scanning Laser

A third type of detection sensor is a scanning laser. Scanning lasers use a laser

beam reflected off a rotating mirror. The beam reflects off the mirror and out to a target

and then is returned to the sensor for range calculations. Two main types of scanning

lasers are used. The first emits a continuous beam and from the return of that beam range


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data are calculated. A laser of this type, provided by Acuity Research Inc., was used

by Torrie in a different CSOIS project [20]. The Acuity Research laser is a class 1 laser

and is not recommended because it is not eye safe. The second type is a pulsed laser that

sends out many laser pulses and averages the range data on each pulse to determine the

range to an object. This type of laser is considered a class 3 and is better because it is eye

safe. Another advantage to the pulsed laser is that measurement errors can be filtered out

better than continuous beam lasers. Scanning lasers, as they are called, are starting to

make their presence known in autonomous vehicle research. Bailey [4] uses laser

scanners to determine the position of his robots. He knows the exact location of all

obstacles and by using the laser scanner can accurately calculate the robots position based

on distance to the obstacles and angle with respect to the obstacles. Apostolopoulos [10]

uses scanning lasers along with cameras to find meteorites in the Antarctic. In [11], [16],

and [20] scanning lasers are used to detect obstacles, which in turn aids in safe

navigation. Scanning lasers give better results for range data with far less computational

constraints compared to camera detection and the resolution is considerably better than

ultrasonic sensors. The angular resolution on a Sick Optic Inc Laser Measurement

System (LMS) laser range finder can be as small as 0.25 with a field window of 180.

Much of the research that places autonomous vehicles outdoors has moved away from

stereo vision to scanning lasers [16, 20] because no matter the amount of light, range data

are available. The scanning laser does have its drawbacks. Most notable is that the scans

are planar, which means that if an obstacle is above or below the scanning plane then

nothing is detected. The sensors also suffer from dust, rain, and blowing snow, causing


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false readings as was noted by Foessel [15], who operated his robots in polar

environments.

3.1.4 3D Scanning Lasers

3D scanning lasers make up a fourth type of detection sensor. The reason they are

classified differently than 2D scanning lasers is the jump in price and complexity.

Langer [14] describes a 3D-scanning laser his company has developed and how it might

be used. On the surface a 3D scanning laser looks attractive, but for real-time systems it

is unfeasible. Langer states the time required to scan 8000 pixels is 80 seconds. The

other draw back is the price of a unit. The author priced out another 3D scanning unit

with the same specifications as [14] and it was over $150,000.00, too much to put on any

farm vehicle even if the scanning speed was real-time.

3.1.5 Millimeter Wave Radar

The final type of detection sensor we consider is millimeter wave radar. This

sensor also suffers from a high price tag and not many researchers have the means to use

it. One place it has been used is at Carnegie Mellon University, on their Antarctic rover.

Foessel [15] explains why the harsh polar environment renders stereo vision and scanning

lasers useless: the blowing snow causes to many false objects. When millimeter wave

radar was tested in this environment blowing snow had almost no effect on millimeter-

wave range estimation. The narrow beam associated with radar also allows for good

angular resolution. When properly gimbaled the radar can collect a 3D image of the

environment in the path of the autonomous vehicle.


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3.2 Comparsion of Sensors

We have examined five different types of obstacle detection sensors and have

discussed some of their advantages and disadvantages separately. Now we will compare

each sensor side-by-side for our application. Given the farming environment we can set

five criteria for selecting the best obstacle detection sensor: operation in any weather,

operation in any light, detection out to at least 15 meters (50 feet), fast response time, and

cost that is significantly less than that of the tractor. Ideally the best sensor will meet our

five criteria, but if this is not possible the one that comes closest will be selected.

3.2.1 Weather

The farming environment brings out the whole spectrum where weather is

concerned. The sensor must be able to handle dust, rain, and snow, all of which can be

blowing and swirling at any given time. Because of this the camera, sonar, and scanning

laser can give false readings. The only suitable detection sensor is the millimeter wave

radar.

3.2.2 Light

On the farm the day starts in the darkness of the morning and then gradually the

light gets more intense as the day goes on until the noon day peak. At this point the light

recedes much as it started as the day winds down. If, however, the weather changes

throughout the day the amount of light may also change. This means that our detection

sensor must operate in any light. From the start this excludes the CCD cameras. They

function on ambient light and any changes in the lighting can affect the interpretation of


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their data. Sonar, scanning laser, and millimeter wave radar are relatively unaffected

by changing light conditions and would be good choices for dealing with changing light

conditions.

3.2.3 Detection Distance

For a tractor that travels at 8.8 ft/sec it only takes 3.4 seconds for the tractor to

travel 30 feet. The obstacle detection sensor must determine the presence of an obstacle

and the avoidance algorithm must calculate an avoidance heading in under 3.4 seconds,

preferably well under 3.4 seconds. This means that the detection sensor must be able to

detect obstacles soon enough that avoidance is performed safely. With this limitation we

determined that the obstacle detection sensor used on the tractor must have a maximum

detectable range of at least 15 meters (50 feet). This gives plenty of time for obstacle

identification and avoidance heading calculations. This requirement excludes most

ultrasonic sensors, which have a maximum range of about 5 meters. CCD cameras,

scanning laser, 3D scanning laser, and millimeter wave radar all have maximum

detectable ranges of at least 15 meters or more.

3.2.4 Response Time

In order for a detection sensor to be adequate the time required to see and obstacle

and cause the tractor respond is critical. In the case of the 3D-scanning laser the time

required to scan one image in front of the vehicle is 80 seconds, which is obviously

unacceptable for a real-time system. Sonar can determine fast enough if an obstacle is

present, but sonar can only see a short distance. That is, the tractor will already be too


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close to an obstacle before the sonar can sense an obstacle. A CCD cameras response

time depends on the image processing speeds and capabilities. With faster computers this

is not a problem and cameras response time is fast enough to detect obstacles at a safe

distance. 2D-scanning lasers have a very fast response time, on the order of 17 ms to 1 s

depending on the resolution and the speed that data are being output. This allows

scanning lasers to detect obstacles at a safe distance, allowing for proper obstacle

detection and avoidance. Millimeter wave radar also has a fast enough response time.

As with cameras, the higher speed computers allow for fast computation of the radar

signal. Radar can detect obstacles at safe distances.

3.2.5 Cost

For most research projects cost of sensors may not be a prime issue, but with the

autonomous tractor tested in this project there is a desire to keep cost down because of

possible future production considerations. If the cheapest detection sensor were the only

issue then hands down the sonar or CCD cameras would be the choice. The 2D scanning

laser costs significantly more than the sonar and the CCD camera, but it would not break

the bank to put it on a production type tractor. In fact, looking at our requirement of

costing significantly less than the cost of the tractor the scanning laser is acceptable. The

3D scanning laser, on the other hand, costs as much if not more than most tractors and

would not be feasible. The millimeter wave radar also has a high cost, while not as much

as the 3D-scanning laser, it is still significant enough to warrant the choice of a cheaper

technology.


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3.2.6 Summary

After examining the five criteria we must settle for the optimum sensor. Because

radar is the only sensor that works well in all weather conditions it would be a natural

choice. However due to the high cost and the relative newness of the technology we

opted not to use radar, hoping that within a few years the cost and technology would be

more practical. Based on the remaining four criteria the 2D scanning laser was the most

practical choice. The amount of ambient light has little effect on the sensor, the

maximum detection range is 30 meters, the response time is adequate, and the cost for a

unit is justifiable. The comparisons of each sensor with the five criteria are shown

graphically in Fig. 3.1.

In this section we have discussed five different sensors used for obstacle detection. We

investigated research that uses each of the five different sensors and we listed their

advantages and disadvantages. Finally we set forth four criteria that the

Operation

in any

weather

Operation

in any

light

Detection

of at least

15 meters

Fast

response

time

Cost less

than

tractor

CCD

Cameras

Ultrasonic

Scanning

Laser

3D Scanning

LaserMillimeter

Wave Radar

x x x

x x x

x x x x

x x

x x x x

Fig. 3.1: Comparision of obstacle detection sensors.


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detection sensor needed to meet in order to be used on the tractor. With each sensor

evaluated against these five criteria we decided that a 2D-laser scanner was the optimum

sensor at the present time for the autonomous tractor. The next section discusses in detail

the laser scanner that was chosen and used on the project.

3.3 Detection Sensor Used on Tractor

To detect obstacles on the tractor project, an LMS 220-laser range finder was

mounted on the front of the tractor (see Fig. 3.2). This sensor, from Sick Optic Inc.,

scans obstacles out to 30 meters in a 180 planar scan window (see Appendix A for

detailed specifications). The laser measures distance in meters with a range resolution of

0.01 m, from 0 to 180, in increments of 0.5. The angle resolution is configurable with

possible resolutions being measurements every 0.25, 0.5, or 1.0. With a resolution of

0.5 there are 361 distance measurements that the laser reports during continuous output

mode. Each measurement requires 2 bytes of information, with the low byte being

reported first and the high byte being reported last. This makes a total of 722 data bytes

that are reported by the laser to indicate the distance measured every 0.5 . In addition to

the 722 bytes of distance data there are also 10 additional bytes of data, 7 before the

distance data and 3 after the distance data, making the total number of bytes in one report

packet 732. In Section 5.1 we explain how this data is used for avoidance.

In Section 3.2.5 we selected a sensor based on four criteria. The only criterion

that the scanning laser did not meet was the problem of false readings due to inclement


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Sick Inc.

Laser RangeFinder

Obstacle Sensor

Fig. 3.2: Laser range finder mounted on tractor.

weather. In fact we saw that after two months of driving in a dirt field the amount of dust

created by the tractor tires while driving caused many false readings from the laser. To

remedy this problem, all range data closer than seven feet were ignored. This was not a

desirable solution, but without another detection sensor to back up the laser range finder

it was the only practical solution.

In this chapter we have discussed obstacle detection and how it relates to the

project. We first indicated why an obstacle detection sensor was so important to

autonomous vehicle research. Next we looked at the detection sensors being used by

researchers in the field of autonomous vehicles and described the basis for the selection

of a sensor for the autonomous tractor. Finally we briefly described the obstacle

detection sensor that was used on the tractor. Now we can focus our attention on how the


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vehicle will react to measurements of the surrounding environment that are obtained by

the detection sensor.


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Chapter 4

Obstacle Avoidance

In this chapter we present two different approaches to obstacle avoidance: global and

local avoidance. We then examine the desired attributes of the path planner/obstacle

avoidance algorithm used for the autonomous tractor project. We look at several local

obstacle avoidance techniques described in research literature, one of which is selected as

the model for the avoidance algorithm ultimately used on the tractor.

4.1 Global and Local Obstacle Avoidance

In autonomous vehicle research two levels of planning occur. One level of

planning, called global planning, examines the whole world an autonomous vehicle can

travel in and plans paths from one point to the next dependent upon this world. Obstacle

avoidance on a global level is accomplished by routing all paths away from potential

obstacles. If the vehicle encounters an unexpected obstacle during a mission then the

global planner reexamines the whole map with the added obstacle and adjusts either a

portion of the affected path or the rest of the path based on the newer map data.

Local or reactive planning plans a short path for the vehicle to traverse based only

on the environment surrounding the vehicle, typically using only the detection sensor

output. Local or reactive avoidance is aware of the desired path of travel, but due to path

obstructions, plots a path around the obstruction until the vehicle can safely reattach itself

to the desired path of travel.


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An example of global and local obstacle avoidance can be illustrated in my drive

to work. Each morning I drive to work my route is the same. If one day I drive to work

and a child jumps in front of my car I do not drive a different route to work, instead I

swerve around the child and continue on my way in the usual fashion. This is analogous

to reactive obstacle avoidance. However, one day while driving to work I come across a

roadblock. Now I choose a different road, or set of roads, that will get me to work. This

is analogous to global obstacle avoidance.

4.2 Vision of Obstacle Avoidance

At the start of the project the project team examined different approaches to

obstacle avoidance. We knew we wanted the knowledge of known obstacles in the

global map, but we wanted avoidance to be performed by a reactive algorithm. To

accomplish this we decided that the path planner would contain a list of positions for all

known obstacles in the map. These obstacles would determine the paths to be planned;

that is, no known obstacle would block a planned path. If an unknown obstacle were

encountered during the mission then a reactive algorithm would use the detection sensor

data as well as the list of known obstacles to decide the best route around the obstacle or

obstacles and back onto the desired path of travel.

Once the relationship between the path planner and the avoidance algorithm was

clarified we decided on five criteria that the reactive obstacle avoidance algorithm needed

in order to be married into our design architecture:

1 The algorithm must compute vehicle-heading changes based on a map of theobstacles in the tractors environment. This map would represent all obstacles


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seen by the detection sensor as well as known obstacles that may not beseen by the detection sensor.

2 The algorithm must consider the vehicle to have length and width, not simplybe a point source.

3 To avoid excessive computations the algorithm cannot examine the entire mapwhere the tractor may travel, but must examine only the portion of the map inthe vicinity of the tractor.

4 The algorithm must halt the tractor if an obstacle is too close to the vehicle orthe algorithm cannot compute a safe direction of travel around the obstacle.

5 The algorithm must handle dynamic, or moving obstacles.

This list served as a guide when evaluating the many different obstacle avoidance

algorithms that we found in various literatures. If an avoidance algorithm were lacking in

any of the five requirements then it would not be considered for use on the tractor. The

next section compares several different avoidance algorithms against the five

requirements and then describes in detail the algorithm chosen as the model for the

algorithm that was eventually used on the tractor.

4.3 Obstacle Avoidance Used in Research

In the previous section we outlined five criteria that our obstacle avoidance

algorithm needed to have to be compatible with our design architecture. We did not want

to completely reinvent the wheel so we did an extensive literature search on different

obstacle avoidance algorithms. This section will examine several of the predominate

algorithms being used today. Several of the references that will be discussed do not

outline a formal algorithm, but by reading the material a general understanding of the

algorithm is achieved. These papers usually refer to other works that explain in greater


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detail the algorithm being used. The remainder of the section is organized as follows.

Subsection 4.3.1 examines wall-following techniques of avoidance. Subsection 4.3.2

discusses black hole techniques. Subsection 4.3.3 presents the idea of path-computing

techniques. Subsection 4.3.4 considers potential field techniques. Finally Subsection

4.3.5 describes histogram techniques.

4.3.1 Wall-Following

Wall-following avoidance is a technique of following the contours of an obstacle until the

obstacle no longer blocks the desired path to the goal. Kamon [29] describes an

algorithm used to navigate a robot to a goal location in a real world environment. The

algorithm uses two basic modes of motion: motion towards the target and obstacle-

boundary following [29]. The robot proceeds to the goal until an obstacle is

encountered. Once the obstacle is reached it then moves along the obstacle boundary

until it determines that it is close enough to the goal to break away from the obstacle.

The robot then proceeds towards the goal until another obstacle is encountered. Fig. 4.1

gives an illustration of what this algorithm might do, and is similar to pictures found in

[29]. Kamon explains that in their experiments the algorithm worked quite well in most

situations, producing minimal path distances around obstacles compared with other wall-

following algorithms.

Even though wall-following is an effective means of obstacle avoidance, when

graded against our five criteria it is obviously not the technique we want to use. This

algorithm does not compute vehicle heading based on a map of the obstacle environment,

as the first criteria states; rather, heading is determined as the detection sensor senses the


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Start

Goal

Wall Following Technique

Fig. 4.1: Example of wall following technique as presented in [30].

contour of the obstacle. This effectively eliminates known obstacles that may not be

sensed by the detection sensor. This is a problem if the vehicle senses more than one

obstacle. The algorithm assumes the vehicle is a point source, which does not follow our

second criterion, although this could be changed in the algorithm such that non-point

source vehicles could be used. The third criterion does not factor into this evaluation

because the algorithm uses no map for obstacle avoidance. This is impossible to change

because the algorithm is truly a reactive algorithm, it only reacts to what the detection

sensor sees and nothing else. The fourth criterion, stating that the vehicle must halt if

obstacles are too close or if the obstacle is impassible, is not realized in this algorithm.

The very nature of the algorithm dictates that the vehicle positions itself very close to the

obstacle in order to follow the contours properly. The algorithm could be made to stop


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however, if the obstacle is impassible by forcing the vehicle to stop if it has traveled

too far off the desired path. The final criterion, handling dynamic obstacles, is also not

realizable. Kamon states the algorithm navigates a point robot in a planar unknown

environment populated by stationary obstacles with arbitrary shape.

4.3.2 Black Hole

The black hole technique of obstacle avoidance is a way of examining all of the

obstacles in front of the vehicle and heading towards the largest opening, or hole.

Bischoff [9] discusses how the black hole technique can be applied to humanoid service

robots. He explains that wandering around is a basic behavior that can be used by a

mobile robot to navigate, explore and map unknown working environments. When the

robot wanders it has no knowledge of its surroundings and must perform obstacle

avoidance. Bischoff explains that the onboard sensors segment the images around the

robot into obstacle-free and occupied areas and

After having segmented the image, the robot can pan its camerahead towards those obstacle-free regions that appear to be thelargest. If such a region is classified as large enough, the steeringangle of both wheels is set to half of the cameras head pan angleand the robot moves forward while continuously detecting,tracking and fixating this area.

In this wandering state the robot travels and avoids obstacles until a user stops the state.

If the robot becomes stuck in a corner or dead-end hallway then it backtracks until it can

wander freely again.

Chahl [8] describes a similar black hole avoidance algorithm. The algorithm is

different than Bischoffs but the general principle of finding the most open tunnel in front


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of the autonomous vehicle is the same. In the case of Chahl an autonomous airplane is

used. Here the vehicle can sense obstacles in three dimensions and has three constraints.

The constraints are:

1 continual progress towards a goal2 avoiding collisions3 maintaining a low altitude

With these constraints the goal is to find a free path in the panoramic range image

provided by the on-board sensors. A free path is described as at least a tunnel of given

minimum diameter. The diameter of the tunnel is determined by the safe minimum

distances between the vehicle and the nearest obstacle. These parameters would be

determined by the wingspan and controllability of the aircraft. This algorithm looks for

black holes, but unlike [9] the black hole that gets the airplane closest to the target is

taken.

Black hole obstacle avoidance is an effective means of using obstacle data in front

of the vehicle to determine the best course to take. When compared with our five criteria

for possible use on the tractor all but one of the criteria are acceptable. The black hole

approach does not meet the first criteria. The safest black hole to travel is calculated with

only the current detection sensor readings and therefore known obstacles not seen by the

detection sensor would go unnoticed. Even in [9] where the environment is mapped and

obstacle-free regions are established the robot must still examine each obstacle free zone

with its sensors to determine passability. In [9] there is a possibility of placing known

obstacles in to the map before obstacle-free regions are established. The second criterion

however is met because for any black hole to be passable it must be at least the


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dimension of the vehicle. The third criterion is also met because computations for

avoidance heading are made only on the obstacles within the immediate environment of

the vehicle. The fourth criterion of stopping the vehicle if obstacles are too close or if

there is no possible path of travel is also achievable in this type of avoidance. Finally, the

ability to handle dynamic obstacles is achievable because black holes are determined for

each sensor scan of the surrounding environments. If an obstacle moves then the next

sensor scan will indicate different black hole locations. An algorithm that is similar to

this approach is discussed in section 4.3.5 except black holes are found after obstacles are

placed into a map of the environment.

4.3.3 Path Computing

Path computing techniques limit the possible directions of travel by the robot to

certain number of headings. Usually three directions of travel are possible: right, left,

and straight-ahead. Every time the vehicle travels towards the goal these three directions

are determined to be free or blocked. Based upon the free or blocked nature of the paths

the vehicle navigates toward the goal. Fujimori [30] describes an adaptive obstacle

avoidance algorithm that assumes the following about the robot:

1 It moves only in the forward direction2 It turns to the right or left with a minimum rotation radius rmin3 Its velocity is constant except near the goal

Three distance sensors are used to determine if each of the three directions of travel is

free or blocked and the vehicle navigates safely to the goal.

Schwartz in [31] describes another variation on this technique when he describes

an avoidance algorithm used on a medium sized tracked vehicle called PRIMUS. To


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calculated the most recent obstacle map [31] or the most recent sensor reading [30] is

used.

4.3.4 Potential Fields

The potential field method of obstacle avoidance is used extensively in robotic

research. This method examines repulsion vectors around obstacles and determines the

desired heading and velocity of the autonomous robot needed to get around the obstacle.

Chuang [32] describes this method as collision avoidance in path planning is achieved

using repulsion between object and obstacle resulting from the Newtonian potential-

based model. Several examples of potential based obstacle avoidance follow.

Prassler [17] describes MAid, a robotic wheelchair that roams in a railway station. This

method first places all obstacles into a time stamp map. This map is similar to occupancy

grids (discussed in subsection 4.3.5) except that the time an obstacle resided in a

particular cell in the grid is recorded. With this information obstacles can be tracked and

an obstacle vector can be predicted. With this obstacle vector and the heading vector of

the robot a collision cone can be constructed. The collision cone represents the heading

and velocity vectors that would cause collision with the obstacle. The avoidance

maneuver consists of a one-step change in velocity to avoid a future collision within a

given time horizon. The new velocity must be achievable by the moving robot [17]. This

technique was performed very successfully in a moderately crowded railway station in

Germany.

Another robot that is similar to the one described above was designed by Carnegie

Mellon University and used as an automated tour guide in the Smithsonian Museum [18].


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As stated by Thrun, this algorithm generates collision-free motion that maximizes two

criteria: progress towards the goal, and the robots velocity. As a result the robot can ship

around smaller obstacles (e.g. a person) without decelerating.

Chuang [32] and Soto [13] describe two different algorithms for computing the

potential fields in the environment around autonomous vehicles. Each of these

algorithms have been simulated and tested on robots in real world environments with

good success.

The four papers described above all base their algorithms on the potential field

method. Even though all four algorithms are different the results are the same and this

allows us to compare the potential field method against our five criteria. The first

criterion is satisfied because the avoidance heading of the vehicle is calculated based on

obstacles placed in a map. The second criterion is also met because the potential fields

are computed based on the vehicle size. The computations for the avoidance heading are

made based on the portion of the map in the immediate environment of the vehicle, thus

satisfying the third criterion. The fourth criterion is met when the vehicle is stopped if

the potential fields indicate no collision-free path is possible. Finally the fifth criterion is

met because the algorithm can handle dynamic obstacles, as is the case for the tour guide

robot and the robot wandering in the railway station.

4.3.5 Histogram

Having discussed each of the different obstacle avoidance techniques we see that

there are desired aspects of each approach. The histogram approach to obstacle

avoidance contains many of the different aspects in one algorithm. Johann Borenstein


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[22-27] developed an obstacle avoidance algorithm based on histograms. Each of

these papers explains the progression of an algorithm that he finally calls the vector field

histogram plus (VFH+) algorithm in [22]. This VFH+ algorithm served as the model for

the algorithm that was developed for the tractor, as explained in Chapter 5. Here we give

a brief explanation of the algorithm found in [22]. For a more in depth understanding of

the algorithm the reader is encouraged to read [22-27].

The VFH+ algorithm consists of four stages, with each stage reducing the amount

of obstacle data to the point of calculating an avoidance-heading vector. The four stages

are the primary polar histogram, the binary polar histogram, the masked polar histogram,

and the selection of the steering direction.

Before any data reduction takes place a preliminary understanding of the algorithm is

needed. The world where the vehicle travels is resolved into a map of N x M cells. The

coordinates of this map are always fixed and the vehicle travels with respect to the map.

This map is called the certainty grid and values associated with each cell represent the

probability of obstacles residing at the cell locations in the grid. In Borensteins

algorithm [22-27] when the vehicle mission is first started each cell in the certainty grid

is initialized to zero, indicating the probability that obstacles reside in any particular cell

is impossible. In our implementation described in Chapter 5 all cells in the certainty grid

are initialized to zero except cells that contain known obstacles. Cells that contain known

obstacles are set to the highest possible certainty value, indicating that an obstacle resides

at that cell location (because of the prior knowledge of the path planner). As the vehicle

progresses in its mission the obstacle detection sensor detects obstacles at various grid


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locations. Each time an obstacle is sensed the value of the corresponding grid location

is updated by one. Thus the higher the certainty cell number the higher the probability

that an obstacle resides at that cell location. As per the third criterion, avoiding excess

calculations by using only a portion of the map in the immediate vicinity of the vehicle,

the VFH+ algorithm is compliant. Another map, called the active window, overlays that

portion of the certainty grid in the immediate vicinity of the vehicle. It is from all the

cells of the certainty grid overlaid by the active window that the avoidance heading in

calculated. Fig. 4.2 illustrates the certainty grid with an active window and several

known and unknown obstacles that have been placed into the grid. The avoidance

heading is calculated by reducing the obstacle data until the masked polar

50

50

50

50

50

50

50

50

50

50

50

50

Known Obstacle

10

25

50

0

0

35

0

0

25

0

0

10

Unknown Obstacle

50

50

50

0

0

0

0

0

0

0

0

0

Active Window Laser Scan Area

0

0

0

0

0

0

0

0

0

0

0

0

Unknown Obstacle

50

50

50

50

50

50

50

50

50

50

50

50

Fig. 4.2: Example of certainty grid.


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histogram is calculated. The masked polar histogram indicates the driving directions

that are opened and those that are blocked. The driving direction closest to the goal will

be selected and the vehicle will steer in that direction. This is illustrated in Fig. 4.3.

From the figure we see the vehicle is trying to track to the desired path of travel, but an

obstacle resides to its left. The masked polar histogram indicates that the vehicle cannot

choose a steering direction between 180 and 270, but since the path direction is less

than 180 then vehicle chooses the steering direction that will return it to the path.

With this preliminary understanding of the VFH+ algorithm we can discuss the four data

reduction stages.

Point Being Tracked

Avoidance Path

0

90

180

270

Fig. 4.3: Vehicle tracking a point on desired path.


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4.3.5.1 The Primary Polar Histogram

As stated by Borenstein, The first data reduction stage maps the active region of

certainty grid onto the primary polar histogram. The active window (Aij) is a circle of

diameter Ws with the vehicle always at its center. The content of each cell in the active

window contains a vector direction and magnitude from the cell Aij to the vehicle center

point (VCP). The vector is given by:

=

0

01tanxx

yyB

i

j

ij

where:

x0, y0: Present VCP coordinates.

xi, yj: Active cell, Aij, coordinates.

The magnitude is given by:

( )22ijijij dacm =

where:

cij: Certainty value of active cell Aij.

dij: Distance from active cell Aij to VCP.

a: A constant based on the diameter of the active circle.

The parameter a is chosen according to:

12

22

=

Wsa

For the vehicle to navigate safely around obstacles a certain distance from the obstacles

must be maintained. Enlarging each cell in the active window by the vehicle radius Rr

and some safe distance Ds (rr+s = Rr + Ds) creates a cushion between the vehicle and


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obstacles. Ds is the minimum allowable distance between the vehicle and the obstacle.

Each cell has an enlargement angle vij given by:

= +

ij

sr

ijd

rv

1sin

The primary polar histogram Hp

is created with an angular resolution so that n = 360/

is an integer. In Borensteins implementation, is specified to be 5, making n = 72 and

causing the discrete angle sector k = n. For each sector k, the polar histogram is

created by:

= Aji ijijp

k hmH , '

with:

h'ij = 1 if k [ Bij - vij, Bij + vij]

h'ij = 0 otherwise

It is important to note that the values Bij

, a d2

ij, and v

ijare properties of the active

window and never change; they can be calculated once and stored in a matrix of size Ws

x Ws for quick reference.

4.3.5.2 The Binary Polar Histogram

Once the primary polar histogram has been calculated for each sector k, the binary

polar histogram is easily constructed. The binary polar histogram, for each sector k, is

calculated by using two defined thresholds, low and high, the primary polar histogram,

and the following rules:


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1, =b

ikH

0, =b

ikH

b

ik

b

ik HH 1,, =

i f

i f

o t h e r w i s e

high

p

ikH >,

low

p

ikH >,

A sector k in the binary polar histogram is blocked if it has a value of 1 and is unblocked

if it has a value of 0. For example, if sectors k = 90 to k = 180 have values of 1, then

from 90 to 180 an obstacle is present.

4.3.5.3 The Masked Polar Histogram

The masked polar histogram takes into account the vehicle turning radius when

deciding if an obstacle blocks the vehicle path. The turning radius creates a trajectory

circle, and if an enlarged obstacle cell overlaps the trajectory circle, then all directions

from the obstacle to the backwards direction of motion are blocked. Borenstein, in [22,

Fig. 4] gives an example of blocked directions. Fig. 4.4 is a similar example of blocked

directions, but with a difference in the x and y directions. The figure shows the vehicle

near two enlarged active cells. Assuming that the active cells contain obstacles (labeled

obstacle 1 and obstacle 2), the vehicle must decide which direction to travel so not to

strike the obstacles. When obstacle 2 is enlarged, it overlaps the right trajectory circle

making travel to the right of obstacle 2 impossible. When obstacle 1 is enlarged, it does

not overlap the left trajectory circle; thus travel to the left of obstacle 1 is possible. The

possible directions of travel then become between the two obstacles or to the left of

obstacle 1.

The locations of the right and left trajectory centers presented here are different

than those presented in [22], by a sign change on the y, due to the different x and y


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rr+s

rr+s

free

rr+s

rryr

xr

dr

ylxl

rl

dl

free

blocked

blocked

blockedx

y

Obstacle 1

Obstacle 2

Fig. 4.4: Example of blocked directions.

orientations. The locations of the right and left trajectory centers, relative to the current

vehicle position and orientation are given by:

xr = rrsin yr = -rrcos

xl = -rlsin yl = r lcos

Thus the masked polar histogram is updated to reflect the right and/or left trajectory

circles being blocked. This is done by finding the distances from each active cell Aij to

the two trajectory centers as given by the formulas:

( ) ( )

( ) ( )222

222

jlill

jrirr

AyAxd

AyAxd

+=

+=

An obstacle blocks the vehicle direction to the right if:

( )srrr rrd ++

obstacle detection and avoidance for an autonomous farm tractor

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