To be able to carry out experiments in the TruckLab which give reliable and repeatable test results and to perform autonomous manoeuvres with the desired accuracy and precision, the hardware of the TruckLab needs to be improved. Therefore this project will focus on improving the scaled tractors which are used in the TruckLab. The improvement which is gained needs to be demon-strated by means of experiments done with the new hardware and will be compared with the current hardware.
The overhead camera system and the Wi-Fi based communication system are not physically attached to the scaled tractor semitrailer and the problems related to these two systems, will be addressed in a master thesis [29] by Dan Cristian Chirascu.
In previous projects involving the TruckLab, Simulink control systems have been implemented to fulfill autonomous manoeuvres with the scaled tractor semitrailers. The problems which need to be solved and the choices which need to be made regarding these systems have been partially addressed already. Although more research in this area is needed, the developed Simulink control systems cannot be properly tested with the current hardware of the TruckLab. The problems which have been identified related to the overhead camera system, the Wi-Fi based communica-tion system and Simulink are listed in Appendix A.
In the next section the TruckLab is divided into subsystems. For each of these subsystems the associated problems and limitations are discussed. This list of problems and limitations has been compiled on the basis of observations described in [1], [2], [3], [4] and [5]. Based on this information the research objective for this graduation project will be formulated.
1.2.1 Problem definition
This section describes the problems which should be solved. Some other problems will not be addressed by this project, they are described in Appendix A.
CHAPTER 1. INTRODUCTION
Tractor system in general
• The tractor contains too many unnecessary parts, for example a fake spare wheel and a sound and light module. All these additional components make it difficult to fit and reach the components on the 1/13.3 scaled tractor which are necessary to enable the research objectives of the TruckLab.
• Certain components are fragile because they are made of plastic.
Steering system
The layout of the existing tractor is shown in Figure1.1. The figure indicates the location of the most important components of the tractor and is useful to understand where problems occur.
Figure 1.1: Layout of the old tractor
• The steering servo which actuates the steering system with a frequency of 50 Hz has a resolution of around 1.1 degrees (47 steps and 52 degrees of range). This low resolution results from the small PWM range which is used to control the steering servo.
• The maximum steering angle is limited to 26 degrees due to components of the steering system contacting each other, this results in a larger turning radius than desired. The maximum steering angle of a real tractor is approximately 49 degrees according to [2].
• The steering servo is not equipped with position feedback, therefore the steering system can only be controlled in open loop.
• A free play of around 3 degrees is present in the steering system. This results in the steering angle output not being reliable nor repeatable. This free play occurs in the connection between the steering servo and the chassis, the connection between the wheel hub and the wheel axle and the connection between the wheel axle and the steering knuckle.
CHAPTER 1. INTRODUCTION
• The relation between the PWM input and the steering angle output differs between the two scaled trucks.
• The steering system suffers from a dead time of around 0.37 s, which is the time to observe a response to a step in the PWM steering input value. The analogue servo in one of the tractors has an actuation time delay of 0.2 seconds according to [6], the digital servo which is used in the other tractor has an actuation time delay of 0.09 seconds according to [7].
• The interaction between the TruckLab’s surface and the tires of the tractor result in tire slip.
Therefore even if the steering system can be accurately actuated, the desired path might not be followed due to tire slip.
• The steering system is not equipped with Ackermann steering, this is due to the steering rod connecting the front of the steering knuckles, this results in reverse Ackermann steering.
The wheel on the outside of the curve turns more than the wheel on the inside of the curve.
This results in additional resistance while cornering.
• The steering system has too many links, resulting in additional free play.
• The steered front wheels can move up and down unpredictably due to the leaf-spring sus-pension which connects the front axle with the chassis. This makes the direction of travel of the tractor more unpredictable.
Propulsion system
• The drive motor which propels the tractor has a low resolution, which results from the small PWM range (47 steps) which is available to control the drive motor. This low resolution results in oscillations in longitudinal velocity. Furthermore there is a motor delay of 0.6s.
• The longitudinal velocity of the tractor is controlled in open loop, because the tractor is not equipped with sensors to measure velocity. Therefore closed loop control of the longitudinal velocity of the tractor is not possible.
• The propulsion system suffers from a longitudinal delay of 0.52 s, which is the time to observe a response to a step in the PWM velocity input value.
• The tractor is not equipped with a brake system, therefore the tractor cannot be decelerated quickly enough. The only option is to decrease the velocity of the drive motor, this how-ever results in stopping distances which are too long when decelerating from high forward velocities.
• A decrease in longitudinal velocity has been observed when the steering angle of the tractor increases. Therefore the prescribed longitudinal velocity cannot be maintained during a manoeuvre which involves cornering.
• The rear axle, which includes the differential, contains plastic parts which results in wear over time.
• The driven rear wheels can move up and down unpredictably due to the leaf-spring suspension which connects the rear axle with the chassis. This makes the velocity of the tractor more unpredictable.
• The drive time is limited by the battery capacity to approximately 30 minutes according to [5].
CHAPTER 1. INTRODUCTION
Olimex E407 micro-controller
• For development of the software on the Olimex control board there is a dependency on HANcoder to enable the use of Simulink models on the Olimex.
• The Olimex control board lacks voltage and current diagnostics, filtering and some protec-tions are needed to safely connect the steering servo and drive motor. The Olimex can otherwise be damaged by current peaks generated by the motor or the servo.
• The location of the Olimex control board in the semitrailer of one of the scaled tractors is a disadvantage, because some wires need to be routed from tractor to trailer.
1.2.2 Research objective
Based on the problem definition a research objective will now be formulated.
The research objective for this graduation project is to improve the steering system and the propulsion system and to address the problems formulated for the scaled tractor in general. The problems related to the Olimex E407 micro-controller and the possible replacement or extension of this board with additional hardware can be addressed in case it is necessary to improve the other subsystems.
The improvement which is gained in terms of accuracy and precision by these individual subsystems will be demonstrated by means of experiments. If possible within the time span of the project, experiments which test the entire closed loop system including the position feedback from the overhead camera system via the Wi-Fi based communication system are proposed.