Fuzzy Logic Based Autonomous Skid Steering Vehicle Navigation
L.Doitsidis,K.P.Valavanis,N.C.Tsourveloudis
Technical University of Crete
Department of Production Engineering and Management
Chania,Crete,Greece GR-73100
{Idoitsidis ,kimonv,nikost}@dpem.tuc.gr
Abstract-A two-layer fuzzy logic controller has been designed for 2-D autonomous Navigation of a skid steering vehicle in an obstacle filled environment. The first layer of the Fuzzy controller provides a model for multiple sonar sensor input fusion and it is composed of four individual controllers, each calculating a collision possibility in front, back, left and right directions of movement. The second layer consists of the main controller that performs real-time collision avoidance while calculating the updated course to be applicability and implementation is demonstrated through experimental results and case studies performed o a real mobile robot.
Keywords - Skid steering, mobile robots, fuzzy navigation.
Ⅰ .INTRODUCTION
The exist several proposed solutions to the problem of autonomous mobile robot navigation in 2-D uncertain environments that are based on fuzzy logic[1],[2],evolutionary algorithms [3],as well as methods combining fuzzy logic with genetic algorithms[4] and fuzzy logic with electrostatic potential fields[5].
The paper is the outgrowth of recently published results [9],[10],but it studies 2-D environments navigation and collision avoidance of a skid steering vehicle. Skid steering vehicles are compact, light, require few parts to assemble and exhibit agility from point turning to line driving using only the motions, components, and swept volume needed for straight line driving.
Skid steering vehicle motion differs from explicit steering vehicle motion in the way the skid steering vehicle turns. The wheels rotation is limited around one axis and the back of steering wheel results in navigation determined by the speed change in either side of the skid steering vehicle. Same speed in either side results in a straight-line motion. Explicit steering vehicles turn differently since the wheels are moving around two axes. The geometric configuration of a skid steering vehicle in the X-Y plane is shown in Fig1，while at is the heading angle, W is the robot width, θ the sense of rotation and S1, S2 are the speeds in the either side of the robot.
The derived and implemented planner a two-layer fuzzy logic based controller that provides purely” reactive behavior” of the vehicle moving in a 2-D obstacle filled environment, with inputs readings from a ring of 24 sonar sensors and angle errors, and outputs the updated rotational and translational velocities of the vehicle.
Ⅱ.DESIGN OF THE FUZZY LOGIC CONTROL SYSTEM
The order to the vehicle movement, a two-layer Madman-type controller has been designed and implemented. In the first layer, there are four fuzzy logic controllers repondible for obstacle detection and calculation of the collision possibleilities in the four main directions, front, back, left and right. The possibilities calculated in the first layer are the input to the second layer along with the angle error (the difference between the robot heading angle and the desired target angle), and the output is the updated vehicle’s translational and the rotational speed.
Fig. 1.Geometric configuration of the robot in the X-Y plane.
A .first layer of the fuzzy logic controller
The ATRV-mini is equipped with an array of 24 ultrasonic sensors that are vehicles as shown in Fig.2. The ultrasonic sensors that are used are manufactured by Polaroid.
After experiment with, and testing several methods concerning sonar sensor date grouping and management, it was first decided to follow the sensor grouping in pairs as proposed in [8](considering the ATRV –mini twelve sonar group Ais=1,…..,12, have been enumerated as shown in Fig.2) and then divide the sun of the provided pair sensor data by two to determine the distance from the (potential) obstacle. However, this method gave unsatisfactory results due to the ATRV –minis specific sensor unreliability. Even in cases with obstacles present in the vicinity of the vehicle, the sensors were detecting a “free path”. To overcome this problem, a modified, simpler, sensor grouping and data management method was tested that return much better and accurate results: The sensors were again grouped in pairs according to Fig.2, but the minimum of the (potential) obstacle. Each ATRV –mini sonar returns from obstacles at a maximum distance of 4metres (experimentally verified as opposed to different value provided by the sonar sensors manufacturer
Fig.2. Grouping of the Sensors.
The form of each first layer individual fuzzy controller, including the obstacle detection module, is shown in Fig.3.Observing Fig.3, data from group sensors A1, A2, ….,A5(5 inputs) and group sensors A7, A8 , …,A11(5 inputs) serve as inputs to the individual controllers responsible for the calculation of the front and back collision possibilities, respectively. Data from group sensors A5, A6, A7 (3 inputs) serve as input to calculate the left and right possibilities, respectively. The individual fuzzy controllers utilize the same membership functions to calculate the collision possibilities. The linguistic values of the variable distance_from_obstance are defined to be three, near, meium_distance, away with membership functions as shown in Fig.4 reflecting the maximum distance of 4 meters a sonar returns accurate information about potential obstacles.
Fig.3.Obstacle detection module.
Fig.4.Input Variable Distance_ From _ Obstacle.
The first layer output is a collision possibility in each direction taking values from 0 to 1.The linguistic variables describing each direction output variable collision possibility (with empirically
Derived for best performance) membership functions as shown In Fig.5.A part of the rules base for left collision is presented in TableⅠ.
An example of the rules used to extract front collision possibilities is: IF A1 is near AND A2 is near AND A3 is Near AND A4 is medium_distance AND A5is near THEN collision_possibility is high. Similar for the back collision possibility.For left (equivalently for right collision)possibilities the rule is of the form: If A5 is near And A6 is nearAnd A7 is near THEN collision_possibility is high.
Fig.5.Output Variable collision_possibility
TABLE Ⅰ
PART OF THE RULES BASE FOR LEFT COLLISION
Input Variables
High_Possibility
B. Second layer of the fuzzy logic controller
The second layer fuzzy controller recives as inputs the four collision possibilities in the four directions and the angle error, and outputs the translational velocity, which is responsible for moving the vehicle backward or forward and the rotational speed, which is responsible for the vehicle rotation as shown in Fig.6.
The angle error represents the difference between the robot-heading angle and the desired angle the robot should have in order to reach its target. The angle error takes values ranging from-1800to 1800. The linguistic variables that represent the angle error are: Backwards_1, Hard_Left, Left, right, Hard_right, Backwards_2 with (empirically derived from tests) membership functions as shown in Fig.7.
The translational velocity (m/sec) , which is one of the outputs of the second layer controller, is described with the following linguistic variables: back_full, back, back_solw, stop, front_slow, front, front full, with membership functions as in Fig.8.
Fig 6.Block diagram of the 2nd layer of the fuzzy logic controller
Fig 7.Input Variable Angle Error.
Fig 8.Output Variable Translational_Velocity.
The rotational_speed (rad/sc) is described with the following linguistic variables: Right_full, right, no_ratation, left, let_full with membership functions as in Fig.9.
An example of the rules that control the vehicle is demonstrated: If front_collision is Not_Possible AND Back_Collision is Not_possible And Left_Collision is Not_possible And Right_Collision is Not_possible And Angle Error is Ahead THEN Translational_velocity is Front_Full AND Rotational_Velocity is No_Ratation.
Fig. 9. Output Variable Rotational_velocity.
Ⅲ. RUSULTS
The fuzzy logic controller has been designed and implemented using C++ in an ATRV-mini manufactured by Real World Interface(RWI). In all experiments the robot is considered to have reached its target when stopping inside a circle with radius of 30 cm. This assumption has been dictated because all calculations have been made relative to the center of the robot. So if the robot stops inside that circle it is assumed that it has reached its target.
Several scenarios in an indoor 2-D obstacle filled environment have been tested to study the robot behavior and the controller’s applicability.
The arrow in Fig.10, Fig.15, Fig.20 is showing the initial direction of the vehicle.
In test case 1 we examine the behavior of the vehicle in an environment with three obstacles. The test case 1 is presented in Fig.10. Fig.11 shows the translational velocity, while the rotational velocity is given in Fig.12.Fig.13 presents the front collision possibility. In Fig.14, the solid line indicates the left collision possibility while the doted the right collision possibility. The behavior of the vehicle is defined from the surrounding obstacles.
In the beginning the left collision possibility is high due to the obstacle in the left. The robot moves forwards and it’s steering right in order to avoid the obstacle. Then it steers left and moves towards its target.
In the second test case presented in Fig.15, a more complicated environment with three obstacles has been tested. Fig.16 shows the translational velocity, while the rotational velocity is given in Fig.17. Fig.18 presents the front collision possibility while in Fig.19 the solid line indicates the left collision possibility while the doted the right collision possibility. In Fig.15 we can see that the path in front of the robot is blocked. The robot uses only the rotational velocity in order to steer and avoid the obstacle. Then it moves in a curve towards its target.
The third test case considers an environment with many small obstacles. The path the vehicle is following is presented in Fig.20. Due to the obstacles that are around the vehicle, the vehicle is forced to make a small right turn an then it escapes from the closed area. Fig.21 shows the translational velocity, while the rotational velocity is given in Fig.22.Fig.23 presents the front collision possibility while in Fig.24 the solid line indicates the left collision possibility while the doted the right collision possibility.
The behavior of the vehicle in each case can verified by observing the relative figures concerning the collision possibility in each direction.
Fig. 10.Test Case 1. Environment with three obstacles and remote target point.
Fig.11. Translational Velocity in Test Case Fig. 14. Left and right Collision Possibilities in Test Case 1.
Fig.12. Rotational Velocity in Test Case 1. Fig. 15. Test Case 2. Environment with three obstacles.
Fig.13. Front Collision Possibility in Test Case 1. Fig.16. Translational Velocity in Test Case 2.
Fig.17.Rotational Velocity in Test Case 2. Fig.20. Test Case . Environment with many small obstacles.
Fig.18. Front Collision Possibility in Test Case 2. Fig.16. Translational Velocity in Test Case 3.
Fig.19. Left and right Collision Possibilities in Test Case 2 Fig.22.Rotational Velocity in Test Case 3.
Fig.23. Front Collision Possibility in Test Case 3.
Fig.24. Left and right Collision Possibilities in Test Case 3
Ⅳ.DISCUSSION AND CONCLUSIONS
We have presented a navigation system for a skid steering vehicle with the use of a two-layer fuzzy logic controller. The first layer of the fuzzy logic controller is composed of four fuzzy logic controllers. The rule base of the controllers responsible for front and back collision contains 60 rules and the rule base of the controllers responsible for front and back collision contains 57 rules. The rule base of the second layer fuzzy logic controller responsible for real-time navigation and collision avoidance contains 238 rules.
The fuzzy logic controller has performed satisfactorily. The results show that the vehicle has the ability to move in complicated environments. The controller, which is proposed in this paper, is based in the controller proposed in [9] but it is implemented in a skid steering vehicle.
Future directions of the research include the testing of dynamic environments, and the use of other sources of information.. The goal is to create an autonomous vehicle that will use for navigation and the collision avoidance combined information from visual inputs, sonars and outdoors GPS data that will guide the vehicle in remote target points.
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