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Space robots have attracted increasing attention for performing, autonomously or telerobotically, on-orbit servicing missions such as repairing, refueling, and upgrading spacecraft; reusing space assets; and on-orbit assembly. The extension of robot application to space can release astronauts from risky, time-consuming, and expensive extravehicular activities [1]. However, in the microgravity environment, the floating base of a space robot will be disturbed by the robot's arm motion when it approaches or manipulates a target. The motion of the spacecraft base resulting from this disturbance will, conversely, affect the motion of end effectors (known as coupling dynamics), making control of space robots more complicated than that of fixed-base robots. In addition, such a disturbance of spacecraft attitude may result in a communication interruption between the spacecraft and the ground station or a failure of energy accumulation caused by disorientation of solar panels [2].

This chapter presents the background and a brief introduction to the need to develop mathematical models that will eventually be used in designing the controllers. First, it introduces the kinematic model of a wheeled vehicle which is front-wheel-steered and rear-wheel driven. Subsequently, this kinematic model of the wheeled vehicle is extended to incorporate a trailer attached to the vehicle at an off axle hitch point. For example, such a trailer may represent an agricultural implement attached to an agricultural vehicle. The model is developed to reflect real-life scenarios by incorporating wheel slip, both in the longitudinal and the lateral directions. The trailers are steerable and driven and therefore represent the most general scenario. The model can be easily converted to represent a passive trailer by setting the steering and propulsion of the active trailer to zero. The chapter then goes on to derive the kinematic model of a tracked vehicle which is also subsequently extended to incorporate an active trailer coupled to the tracked vehicle at an off axle hitch point. Finally, the kinematic model of a four wheel driven and four wheel steered vehicle is presented. The rest of the chapter consists of the derivation of the dynamic models of all the scenarios mentioned above. A unique part of this chapter is the presentation of partial dynamic models which describe the leading part of an articulated vehicle system using its kinematic model and the trailing part of the articulated vehicle system using its dynamic model.

In the future, large satellites may be automatically assembled on-orbit in order to reduce weight and increase volume efficiency in launchers. During this process, it may be important to maintain satellite attitude. This paper presents a dynamic model, including reaction wheels and a twisting-algorithm-based sliding-mode controller, to maintain attitude during a simple self-assembly process. Simulation results of this controller are compared with results from other controllers found in the literature.

This paper presents a strategy for autonomously resolving intersection contention between platoons of autonomous vehicles. Each member of a platoon is represented by a dynamic model and a path tracking controller as well as a vehicle gap controller. These models and controllers are incorporated in simulation environments. Multiple vehicles in convoys simultaneously arriving at an intersection are simulated to ensure an optimized intersection negotiation taking into account a raft of widely varying requirements that exist in a public transport environments. A set of concepts such as initiative, infrastructure adaptation, and priority propagation are introduced and used to promote globally and locally optimal crossing patterns, avoiding vehicle stoppage in most circumstances. Results from a multiple convoy dynamic model simulation validate the strategy presented.

It is essential to have the ability to control the attitude of a spacecraft while an onboard robotic manipulator is operating. The paper presents a comprehensive dynamic model for a reaction wheel actuated spacecraft and a robust controller that can maintain the attitude of the spacecraft as required while the manipulator follows the desired trajectory. Unlike previous work, this paper reformulates the dynamic equation of a Free-Flying Space Robot (FFSR) with actuating Reaction Wheels (RWs) by taking into account the contribution of RWs to the angular momentum of the entire system. Given strong nonlinearities and multiple inputs of the system, diagonalization is first used to transform the strongly coupled problem into multiple single-input problems by introducing virtual torques. When involving system uncertainties, the defined virtual torque is accurately associated with the actual torque using nominal value of the uncertain inertia matrix in order to guarantee stability of original system. Smoothed Sliding Mode (SMC) controllers are designed for each single-input system provided that the bounds of uncertainties can be estimated. A spacecraft mounted with a 3-DOF manipulator is used in simulation to demonstrate the robustness of the control law when applied to space manipulators.

To ensure efficient transportation of large space structures in the future, it has been suggested that they be transported in an unassembled and compact form and then assembled inorbit using space robots. Forces generated during open contact operations have to be taken into careful consideration so that structural components can be successfully acquired and assembled by the robot. Unlike ground-based robots, the free-flying space robot base will deviate from its desired orientation when the spacecraft comes in contact with structural elements. This effect on spacecraft attitude may adversely affect communication, solar energy collection and in extreme cases the stability of the spacecraft. While thrusters can be used to provide regulation torques, in a scenario where the spacecraft is surrounded by structural components that are to be assembled, filing clusters is undesirable. Further, the force impulses generated by thrusters have poor resolution and hence are unsuitable for providing fine regulation. In this paper, the authors utilize reaction wheels (RWs) to provide accurate and smooth control torques without the need to have on-board fuel. By adopting the commonly used canonical-form dynamic model of a space robot as found in the literature, force tracking may be achieved, but a desired orientation of the spacecraft cannot be guaranteed. To solve this problem, the authors build dynamic equations of the space manipulator and the spacecraft base separately to form basis for separate contact-force control and attitude control of the platform. By taking a further step from previous literature which mainly try to maintain contact between the end-effector and the target, hybrid force/motion control is extended to a space robot in this paper in order to track a desired force trajectory and simultaneously maintain a desired relative position between the end-effector and the target to avoid inducing tumbling torques. A recently proposed adaptive variable structure control method is applied to design a robust attitude controller taking into account system uncertainties and external disturbances. A two-link planar space robot model is simulated to track a desired uni-axial contact force and regulate the spacecraft attitude during contact with a free-floating target. The simulation results have demonstrated better robustness and performance of the proposed controller, such as shorter settling time in the presence of disturbance and smaller force tracking error, in comparison to a conventional feedback linearisation attitude controller.

This paper proposes a controller to stabilize the attitude of Vectored Thrust Aerial Vehicle (VTAV). VTAV has high sensitivity to realize quick maneuver, which also means the system is easily effected by uncertainties or disturbances. In order to overcome these undesired effects, the controller is required to be robust. One of the major uncertainty factors of VTAV is uncertain aerodynamic forces, which leads a disturbance to the input to the dynamics. In order to compensate this, high gain output feedback approach with a filter inserted in the loop is proposed. This approach is able to ease Parallel Feedforward Compensator (PFC) selection that is necessary for the controller, considering both stability and disturbance attenuation simultaneously. Numerical simulations comparing with conventional methods showed validity of the proposed method.

Using a compact agricultural tractor, an automated mobile platform is being developed to support precise agricultural tasks. The task being considered is fruit picking, however the platform developed should be generic. The need for precise control of the tractor, loader mechanism and the task implement attached to the tractor, is paramount. Development of the automated mobile platform can be broken down into various components, namely, the steering, loader, and traction control systems, and safety system. Development for each system includes modifications to hardware, and design of appropriate software to drive the systems automatically. Much work is needed to provide the expert control for the types of precision tasks envisaged. Preliminary work shows simple control of the steering and loader systems.

This paper presents an application where a Fuzzy-Logic Controller (FLC) is used at a supervisory level to implement mutual coordination of the steering of the two front wheels of a motorcar. The two front wheels are steered by two independent discrete time state feedback controllers with a view to optimize the steering slip angles. The functions of the two controllers are tied together by way of a FLC. Because of the presence of unmodelled dynamics and disturbances acting on the two sides, it is difficult to achieve the desired performance using conventional control systems. This is the primary reason that FLC is emploged to solve the problem. The results show that the implemented system achieved desired coupling between the two independent systems and thereby reduces the difference between the two steered angles.

This paper presents an approach for formulating dynamic cooperative maneuvers for platoons of autonomous vehicles. Each member of a platoon is represented by a dynamic model and has a set of tactical and operational controllers in the form of hierarchical event-chains and low-level longitudinal/latitudinal controllers. These models and controllers are incorporated in simulation environments and demonstrate the ability for platoons to cooperatively maneuver around each other and other hazards. Multiple vehicles in convoys drive autonomously down a two-lane roadway and are simulated to ensure a safe and cooperative operation taking into account a raft of widely varying requirements that exist in a public transport environments. A set of concepts such as event conditions, sub-chaining and limited progression are introduced and used to promote responsive maneuver patterns. Results from a multiple convoy simulation validate the strategy presented.

In this paper, a sliding mode controller with disturbance observer (DOB-SMC) is developed for a tractor as a farm vehicle operating in the presence of longitudinal and lateral slips at front and rear wheels. Farm vehicles intended for autonomous operations usually traverse at low speeds along straight lines. As very low accelerations are involved kinematic models seem sufficiently accurate for developing controllers. However undulating, sloping and very uncertain terrain, besides rough environment, introduces significant disturbances which makes the control design challenging. The proposed DOB-SMC is able to guarantee precision as well as robustness for a typical tractor. In the modelling, lateral and longitudinal sliding are incorporated in the otherwise ideal kinematic model and an error model is developed for the kinematic model. Then a DOB-SMC is developed to cope with disturbances. We prove that the proposed controller is stable in the presence of significant slip. Simulation results prove acceptable performance of the proposed controller in comparison to two other controllers in the literature.

This paper presents the autonomous platoon formations that could be used to form a platoon of buses on dedicated roadways. A non-linear dynamic model and a path-following and car-following controller are presented and their performance evaluated. Using the dynamic model a platoon of three vehicles is simulated along a multi-segment reference path, demonstrating the adequate lateral and longitudinal performance required for simulations of scale, meaning the operation of multiple vehicles in a convoy or the coordinated operation of multiple convoys. The models and controller presented here will form the basis for future research involving simulations of autonomous vehicles when maneuvering through a series of complex intersections and roadways.

Precise path tracking control of autonomous vehicles is an area which is beginning to benefit from an increased awareness of its importance and its challenge. One of the challenges comes about due to the need for increased accuracy and robustness. Kinematic models are primarily used for path tracking control due to their generic and more simplistic nature when compared to dynamic models. However, control based solely on the kinematic model rarely takes into consideration any of the dynamics that are present, which can degrade the performance. This paper looks at the path tracking control of vehicles, specifically agricultural tractors, incorporating compensation for inevitable steering dynamics. The path tracking control is achieved via a state transformation and full state feedback, equivalent to the backstepping algorithm. The backstepping approach with steering dynamics compensation is compared to more traditional kinematic model only approaches which assume such dynamics are insignificant when compared to the bandwidth of the path tracking controller itself. For comparison, such traditional approaches are implemented with both PD and 2-sliding mode controllers employed for low level steering control. Simulation results are carried out and show that incorporating the steering dynamics into the design can yield improvements in performance.

This paper presents the derivation of the mathematical model for a three-body articulated agricultural vehicle such as a tractor that drags behind two agricultural implements connected in series. It is then used in a simulation to study the effects of slippage. The model is developed with the aim of designing robust controllers that ensure high-precision path-tracking control of such articulated systems. In the simulations, the model was subjected to real conditions experienced in agricultural applications such as disturbances and uncertainties due to ground undulation, gravitational forces due to sloping ground, and lateral wheel slippage. The implement attached to the tractor is assumed to be steerable to enhance the pathtracking capability. This work aims to provide an insight in to the articulated tractor behaviour under the influence of real life farming condition.

Precise guidance of agricultural vehicles is an area which is beginning to benefit from an increased awareness of its importance and its challenge. It is important as farming is becoming increasingly and justifiably automated, and because precision and autonomous farming is more necessary now, in order for the industry and individual farms to be competitive. It is a challenge because agricultural vehicles operate in quite uncertain and unpredictable environments. It is important that vehicles are guided with precision in a lateral as well as longitudinal direction, while being subjected to often significant disturbance forces, or slip, due to uncertain and sloping terrain. Little work has been carried out thus far in the guidance of such vehicles where vehicle slip is significant. In this paper, the authors provide an alternative approach to the robust trajectory tracking of an agricultural tractor under the influence of lateral and longitudinal velocity slip. This approach takes into account the dynamics of the steering system, and caters for slip velocities which are bounded but time varying. This is achieved via the use a robust combination of sliding mode control and integrator backstepping. Robust stability is achieved, ensuring the trajectory error is bounded to an adjustable region around the origin. Simulation results show that the alternative approach proposed yields suitable robust trajectory tracking.

This work presents the derivation of a comprehensive mathematical model for an off-road vehicle such as an agricultural tractor that drags behind it a heavy implement. The models are being developed with the aim of designing robust controllers that will enable the high precision control of the implement's trajectory. The developed model is subjected to real conditions, such as ground undulation and uncertainty, sloping terrain, tyre slippage, and constrained steering of the tractor. The implement is assumed to possess independently steered wheels for aiding in implement alignment. A complete model is presented and simulated under varying conditions. Primarily this work demonstrates and validates the trailed vehicle system behavior when the trailing implement is subjected to large drag forces due to ground engagement and the significantly large lateral disturbances that occur in real life broad acre farming conditions.

We have defined a new paradigm of robot navigation, by using sensors in the environment instead of onboard robotic platform. In this scenario, determining the output of the most relevant and validated sensor is of crucial importance when heterogeneous sensors are available for measuring a given process. We are using an IEEE 1451 TEDS (Transducer Electronic Data Sheets) compliant sensor model proposed earlier for heterogeneous sensor networks. The presented method computes at each time step the usefulness of sensor information from discovered sensors, based upon Kullback-Leibler divergence between the sources of information. The robotic application consequently can obtain the relevance of the sensor output and the robot can update its estimate of the environment. We present results of a real time implementation of heterogeneous sensor networks using distributed multi-sensing 3D Real-time Robotics Software Player/Gazebo on a simulated pioneer robot. The results show that the proposed model can be utilized in the real-time scenario and can help reduce the computational cost of a system which occurs due to lack of discriminatory information.

A number of methods have been developed for finding collision free paths for a manipulator moving from one configuration to another. In each of these cases the desired goal configuration must be chosen beforehand. However, in most applications there may be more than one configuration that satisfies the desired position and orientation requirements of the end effector. One of these configurations must be chosen for the path planner; generally we want to select the configuration that results in the lowest cost movement from the manipulator's current position. Until the motion plan is calculated, we cannot know which goal configuration to choose. This paper demonstrates a method for selecting the lowest cost path without needing to calculate a motion plan from the start position to all of the possible goal configurations. Generating motion plans is computationally expensive and it is desirable to limit these computations where ever possible. This is especially true in the case of mobile manipulators where the surrounding terrain for the motion planning calculation changes at each step, meaning that computational savings of reusing calculations from previous runs cannot be realised. The method demonstrated in this paper is shown to be much more suited to these types of applications than more traditional motion planning methods.

This work demonstrates how to carry out contact operations in a very stiff contact situation. The experimental set up used is a CNC machine that carries out metals spinning. The controlled forces are the metal forming forces. The designed controller operates in a dynamic environment. The complete control system operates in a manner similar to reaction compensation. The force controller used can be viewed as an integrating on-off controller which has tightly controlled release of control efforts to the controllers. It brings in artificial damping that ensures the stability of the control system. The designed force controller operates within limited tool path disturbances. The controller requires the feed forward of a nominal tool path. The accuracy of the nominal tool path brings the force deviations to within a manageable level. A secondary contribution of this work is the design of a generic external control loop for commercial CNC machines that enables the sensor integration for visual servoing or active force control

This paper presents the hardware and software architectures of a compact agricultural tractor, that is currently being developed as a generic mobile platform for agricultural tasks. The specific task that is being addressed is fruit picking. Such systems require precision outdoor maneuvering and coordination with the implement attached to the vehicle. The system described also includes a loader attached to the front of the tractor which carries the fruit picking robot. First the hardware associated with the safety subsystem, steering, traction and loader control systems are described. Then the real-time software that coordinates the control of all subsystems while maintaining a functional radio link to a remote station is described.

Comparison of advantages and disadvantages of control methods used in nonlinear systems is presented. To take advantage of both adaptive and robust control methods, a novel method which switches between the two is proposed. Robust control is used during transients and situations where parameters are uncertain. During steady state operations, adaptive control is used. Switching between the two methods is carried out based on the standard deviation of the estimated parameter vector. The method has been implemented on a 2 DOF articulated robot. Experimental results are presented to prove the robustness and the accuracy of the new control method., 43rd IEEE Conference on Decision and Control; Atlantis, Paradise Island, Bahamas

This paper presents a general approach for the identification of contact and grasping uncertainties, and the monitoring of contact situation changes in force controlled assembly operations. The identification problem is solved using virtual contact manipulators and Kalman filter techniques. Monitoring is solved by carrying out a statistical test on the sum of normalized and squared innovations of the Kalman filter, within a moving window, identification and monitoring are verified by experimental results. The paper explains how the error covariance matrix of the Kalman filter is interpreted to analyse the observability of the Kalman filter's states. Preliminary simulation results are presented for an ad hoc active sensing strategy to achieve complete state observability.

There are numerous examples of partially autonomous systems that are controlled at some level by a human operator or user. Generally control at the lowest levels is autonomous while the human user is primarily responsible for decision making at the highest levels. Examples of such systems include passenger automobiles, HVAC systems in buildings, CNC machines in job shops, and security systems. An important class of systems are mobile agents with embedded computers that are directly controlled by a human pilot or navigator in the loop. This chapter addresses (i) the design of interfaces between the human user and the computercontrolled system and (ii) the design of control algorithms that enable autonomous motion of the vehicle when desired. The performance of such human in the loop systems is very sensitive to the persons ability to interact with the embedded computer and sensors (Rahman et al., 1994).