In this section the integration of Turtlebot components, RC components and in house developed components is discussed. The integrated components form the new tractor. The location and orientation of some components is explained. Before the components have been ordered several 2D and 3D CAD drawings in NX Siemens have been made to ensure the components would fit.
Examples of these drawings are shown in Appendix D.
4.4.1 Steering system
The new steering system is shown in Figure4.3. The main design question for the steering system has been how the Dynamixel should be connected to the front wheels. The orientation and position of the Dynamixel servo with respect to the front axle influences the relation between servo position and steering angle. This is further influenced by the length of the steering link and the connection points of the steering link. The front axle has multiple connection points to attach the steering link.
The other end of a steering link is usually attached to the servo by means of a lever, known as the servo horn. The position of the connection point of the steering link on the servo horn influences the ratio between servo rotation and steer angle. This ratio influences the resolution with which the steer angle can be set. The resolution increases if the servo rotates over a larger angle. Of further importance is that the steering mechanism should be able to move without coming into contact with other components of the tractor. Furthermore the number of steering joints of a steering mechanism influences the amount of free play in the system. The manufacture-ability and the ease of mounting the steering system to the tractor chassis should also be considered.
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Figure 4.3: The new steering system
Several options for the connection between the Dynamixel servo motor and front wheels that have been considered are:
1. Mount the steer actuator in such a way that its axis of rotation is not parallel to the in-clined axis of the kingpin. Then connect the steer actuator with the steering knuckle or with another connection point via one or multiple links.
2. Mount the steer actuator in such a way that its axis of rotation is parallel to the inclined axis of the kingpin. Then connect the steer actuator with the steering knuckle or with another connection point via one or multiple links.
3. Mount the steer actuator in such a way that its axis of rotation is parallel to and coincides with the inclined axis of the kingpin. Then connect the steer actuator with the steering knuckle via one or multiple links.
Options which fall into category 1, result in one or multiple links which need to translate in three directions, up-down, left-right, forwards-backwards. Therefore it is difficult to analyze be-havior of such a mechanism and the influence of design parameters as for instance length of links.
This can be done with a multibody model of the mechanism. The relation between the position of the actuator and the position of the steering knuckle will be nonlinear. Mounting of the servo is probably easiest choosing an option out of category 1. Steering accuracy is probably degraded by the use of several joints necessary to make the required connection. Joints will result in free play.
Options which fall into category 2, result in one or multiple links which need to move in a plane.
This plane is perpendicular to the inclined axis of the kingpin. A mechanism which moves inside this plane is probably somewhat easier to analyze compared to options out of category 1. The relation between the rotation of the actuator and the rotation of the steering knuckle will still be nonlinear. Mounting of the servo is a bit more difficult, because it needs to be inclined.
Options which fall into category 3, result in a link which only needs to rotate around the co-inciding axis of the kingpin and the steering actuator. This results in a linear relation between the rotation of the actuator and the rotation of the steering knuckle. As the relation is linear, the servo angle is equal to the steering knuckle angle, analysis of this mechanism is simple. Steering accuracy is highest as the servo, link and steering knuckle can rotate as a single body. Mounting of
CHAPTER 4. SYSTEM DESCRIPTION OF THE TRACTOR
the servo is more difficult as it also needs to be inclined. A limitation is that the servo only rotates 75 degrees, which is equal to the rotation of the steering knuckle. When choosing an option out of category 1 or 2 the angle over which the servo rotates can probably be higher, which results in a higher resolution. However when choosing an option which falls into category 3, still around 853 steps of 4096 steps can be used. This results in a resolution of 0.08 degrees of steering knuckle rotation per step of the servo motor.
Despite the benefits of the connection options which fall into category 2 or category 3, a steering system has been created that falls into category 1. The mounting position and orientation of the servo is the main reason. It has been concluded that options in category 2 or category 3 take up to much valuable space or affect ease of manufacturing too much. The steering system that has been designed is compact, relatively simple to manufacture and is mounted easily to the chassis. Free play can still mostly be eliminated by using high quality ball joints. The more difficult relation between servo position and steer angle can still be obtained by a combination of measurements and modelling of the system. The steering system now uses a single steering link, connected to a servo horn. The orientation of the steering link can be adapted by adding or removing nuts which are screwed on the thread of the ball joints. The servo horn has multiple mounting points for adaptability. The servo is directly mounted underneath the top chassis plate, slotted holes also give adaptability. The position of the servo above the front axle results in a compact system.
4.4.2 Propulsion system
The new propulsion system is shown in Figure4.4.
Figure 4.4: The new propulsion system, right wheel removed
CHAPTER 4. SYSTEM DESCRIPTION OF THE TRACTOR
Two Dynamixels propel the rear wheels individually. A test done with Turtlebot3 in its original configuration has shown that the rear wheels can directly be coupled to the Dynamixels. Additional gears are not needed in between to increase torque. The test has shown that an additional 6 kg on top of Turtlebot3 can be propelled with sufficient velocity, a mass of 6 kg is similar to the weight of the trailer which the tractor needs to be able to move. Therefore the Dynamixels have been rigidly connected to the rear wheels by means of custom made coupling discs. These discs exactly fit into the rims and thereby the wheels are held in place. This rigid connection results in less free play, which should make the behaviour more predictable and repeatable.
It has been considered to also rigidly attach the Dynamixels to the bottom chassis plate. However in combination with the rigid front axle this would result in a statically indeterminate system.
This means that in case of an uneven surface, not all four tyres can maintain in contact with the road surface. Therefore it has been decided to rigidly connect the Dynamixels together by means of two pivot plates. These pivot plates, can rotate about a pivot rod which is connected to the chassis. This results in a statically determined system. The tyres do not loose traction in case of a road undulation, this should decrease the amount of slip of the tyres. The pivot construction is made of aluminum and therefore durable. The complete construction, consisting of few components, fits in between the rear wheels and therefore there is more space left for other components. This is ideal as space is limited at this scale. The orientation of the Dynamixels has been chosen such that there is enough space to pivot without hitting the trailer. The chosen orientation also provides the most ground clearance.
4.4.3 Computing system with sensors
The components of the computing system together with the sensors already formed an integrated system in Turtlebot3. Therefore the main question has not been how to interconnect these com-ponents mutually, but rather where to locate these comcom-ponents inside the tractor.
It has been decided to use some of the waffle plates of Turtlebot3. These plates are designed to easily mount electronic components. Therefore they are used as the connection between the electronic components and the chassis of the tractor. The alternative of mounting the electronics directly to the chassis would require a lot of additional holes in the chassis in order to obtain the same level of adaptability.
The battery, openCR board and bluetooth module have been located at the center of the chassis.
The orientation and position of these components have been optimized for maximum reachability and optimal cable routing. The USB port of the openCR is easily reachable for software uploads.
Also easily reachable are the on/off switch, power adapter connector and battery connector. The most suitable location and orientation of the sensors have also been considered. The lidar is po-sitioned on top of the waffle plate stack above the front wheels. The lidar can scan through the openings in the cabin and thereby detect obstacles at that particular height. This height has been chosen such that other trucks and trailers are detectable for the lidar.
The Raspberry Pi camera is attached to the waffle plates at the bottom of the stack. The cam-era can be used for obstacle detection at heights not visible for the lidar. The Raspberry Pi is mounted to the same set of waffle plates. The location of the Raspberry Pi has mostly been chosen to optimize the cable routing. The camera cable and cabling for the lidar are completely located inside the cabin, this limits cable clutter.
4.4.4 Chassis
The chassis forms the connection between the steering system, propulsion system and computing system. The fifth wheel and cabin are also mounted to the chassis. The chassis consists of a bottom plate and a top plate which are connected by vertical spacers. The reason behind this concept is the maximization of space to fit components. A typical tractor chassis normally consists of two parallel beams which run from the front to the back of the tractor. However such beams
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take up valuable space which could be used for components. Therefore the decision has been made to use plates instead of beams. By connecting the plates with spacers, the chassis obtains it’s stiffness. Furthermore the surface area where components can be attached is higher than for a typical beam chassis.
Conceptual CAD drawings of the chassis have been made before the manufacturing process. These drawings have shown that the chassis is capable to fulfill the task of connecting the subsystems.
Furthermore these drawings have helped in the manufacturing process as a guide for the dimensions of the chassis. The first prototype chassis however does not fully adhere to the scaled dimensions necessary to resemble an actual tractor at scale 1:13.3. Therefore some adaptations need to be made in future versions of the chassis. The wheelbase should be made 13 mm shorter by moving the rear axle forward. The chassis may be shortened by 5 mm at the rear and 12 mm at the front. The cabin should then be placed exactly flush with the front end of the chassis plates. The fifth wheel should move 12 mm rearward and 18 mm downwards. Each set of rear wheels should move 1.5 mm inward. New CAD drawings have been made. The desired dimensions of the scaled tractor are listed in Table4.1.
Name of dimension Reference dimension (m) scaled dimension (m) current dimension (m)
WB 3.60 0.271 0.284
AE 0.99 0.074 0.066
AC 0.88 0.066 0.040
CE 3.71 0.279 0.310
CH 2.56 or 2.94 ((super) space cab) 0.193 or 0.221 0.203
KA 0.47 0.035 0.060
Fifth wheel height 1.190 (equal to kingpin height) 0.090 0.108
Table 4.1: Desired dimensions of scaled tractor including reference and current dimensions
The desired dimensions of the scaled trailer are listed in Table 4.2. The explanation of the names of the tractor and trailer dimensions and the sources of the reference dimensions can be found in Appendix B. Future versions of the chassis will be manufactured such that the scaled tractor adheres to these desired dimensions listed in Table 4.1. Adhering to these dimensions further ensures that the scaled tractor is suitable for the scaled trailer once that has also been adapted to adhere to the dimensions found in Table4.2.
CHAPTER 4. SYSTEM DESCRIPTION OF THE TRACTOR
Name of dimension Reference dimension (m) scaled dimension (m) current dimension (m)
Total length 13.675 1.028 0.980
Front overhang of trailer 1.675 0.126 0.080
King pin-rear of trailer 12.000 0.902 0.900
King pin height 1.190 0.090 0.096
Total height 4.000 0.301 0.292
wheelbase 7.620 0.573 0.619
Rear overhang 2.499 0.188 0.140
Total width 2.550 0.192 0.196
Axle spacing 1.310 0.099 0.100
Table 4.2: Desired dimensions of scaled trailer including reference and current dimensions
4.5 Summary
In this chapter a system description of the new tractor has been given. The scaled tractor semi-trailers are part of a closed loop control system which is used in the TruckLab. An overhead camera system is used to localize the tractor semitrailers. MATLAB/Simulink and ROS are used to control the tractor. The component integration in the new tractor has been discussed on the basis of design choices related to the connection of components. The main design question for the steering system has been how the Dynamixel servo motor should be connected to the front wheels.
Two Dynamixel servo motors are integrated in the pivoting rear axle and propel the rear wheels individually. The components of the computing system and the sensors have been positioned tak-ing into account functionality, reachability and optimal cable routtak-ing. The reasontak-ing behind the concept of the new chassis is the maximization of space to fit components. Some adaptations to the dimensions need to be made in future versions of the prototype chassis.
Chapter 5
Open loop control of the tractor
5.1 Introduction
In this chapter the control of the tractor is discussed. The main aspects are control of the steering system and control of the propulsion system. The propulsion system consists of two motors that allow for individual control of the left and right rear wheel velocities. The steering system has been calibrated by measuring the steer angles of the front wheels and comparing these against the values predicted for an Ackermann steering geometry. Ackermann steering geometry is desired as it results in low amount of side slip of the tires. The Ackermann principle and Ackermann geometry is explained in [25] and [26]. A polynomial relation has been determined between the steer servo position and the steer angle of the front wheels. For control of the tractor several software packages interact, this interaction is described. Some adaptations needed to be made to the available software.