| 1. |
- Haghshenas, Hamed, 1990-
(författare)
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Time-Optimal Cooperative Path Tracking for Multi-Robot Systems
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Robotic systems are nowadays the key technology in a wide variety of applications. The increasing demand for performance of robotic systems is often met by employing a team of cooperating robots for a specific task.When the task carried out by the robots involves manipulation of an object, the multi-robot system is said to perform a cooperative manipulation task.Cooperative manipulation is an important capability for extending the domain of robotic applications.This thesis studies the time-optimal path tracking problem for a cooperative manipulation scenario where an object is rigidly grasped by multiple manipulators. The goal is to move the object along a predefined geometric path in minimum time while satisfying the imposed constraints on the motion. First, it is shown that the time-optimal path tracking problem for cooperative manipulators can be cast as a convex optimization problem. A fundamental property of convex optimization problems is that any locally optimal solution is also a globally optimal one. Furthermore, by recognizing and formulating a problem as a convex optimization problem, it can be solved very reliably and efficiently using interior-point or other methods for convex optimization.These results are presented in two separate studies. In the first one which is a preliminary study, the manipulation setup is a particular setup comprised of two planar manipulators and a bar. Furthermore, the load distribution among the manipulators is considered to be equal. The second study extends the results in the preliminary study to a general scenario with $N$ generic manipulators and an object with a desired orientation during the motion. Here, the load distribution among the manipulators is determined via a generic pseudo-inverse of the grasp matrix that can be chosen by the user.The freedom in the choice of the pseudo-inverse allows to consider different load distributions which can be exploited to account for the potential differences in the capabilities of the manipulators.The second part of this thesis is devoted to finding load distributions that are free of internal forces. A drawback of using multiple manipulators in a cooperative manipulation task is that internal forces can be introduced.Internal forces are forces exerted by the end-effectors at the grasping points that do not contribute to the motion of the manipulated object. While a certain amount of such forces can be useful in some cases, in general they must be avoided to prevent object damage and unnecessary effort of the manipulators.This thesis proposes a new approach to obtain internal force-free load distributions.The proposed approach results in a new pseudo-inverse of the grasp matrix parameterized by coefficients that have the meaning of the inertial parameters of some parts of the object. The freedom in the choice of the parameters of the pseudo-inverse allows to assign different loads to the manipulators. This can be exploited to account for the differences in the power capabilities of the manipulators.The results are further explored for scenarios where the object is three-dimensional and convex and has uniform mass density. Finally, the proposed pseudo-inverse is combined with the results in the first part of the thesis to solve the problem of time-optimal cooperative path tracking subject to zero internal forces during the motion.
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| 2. |
- Robertsson, Magna, 1973-
(författare)
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Personligt mod : Om krigsdekorationer som mjuk normstyrning under insatsen i Afghanistan åren 2008-2012
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the 2000s, Sweden has pursued an active foreign and security policy. This has meant participation in several international military operations and has left many Swedish soldiers and officers with combat experience. Thus, the Swedish parliament decided in 2010 on a political reform of the veterans area, with more extensive societal support, war decorations to express the government recognition of personal sacrifices made in the service and a new medal for courage in combat. Considered as a reward system, it functions as an immaterial and emotionally established incentive, in contrast to the financial and bonus reward systems that are otherwise common.From a governance perspective, this setting is fundamentally interesting. The complexity of military operations and the demands for efficiency in armed combat are in contrast with the democratic state's need to guarantee the rule of law, even from a distance and under difficult conditions. Traditional government is not sufficient because the situation is characterized by high contextual uncertainty and therefore requires more situational adaptive control. Organizing in a professional model implies autonomy for military officials, and this means that there is a form of gap in terms of civilian control.In the dissertation, a concept and an analytical model are formulated to understand this phenomenon, entitled soft norm governance, that also form one of the main results. The model combines four levels of analysis to describe the dynamics of the steering mix: organizational metagovernance, rule control, policy work and professional ethics. One conclusion is that professional ethics has a two-way function in the chain of implementation steering, both as a decisive factor for concrete decision-making, but also as an objective for the government’s soft norm governance. Thus, this control gap does not mean absence of control, but that other value-based norm systems govern our actions. In this way, soft norm governance also reaches beyond the scope of the law.The theoretical framework is metagovernance, the idea that the modern state is steering at a distance and with subtle methods, such as by organizational measures. It opens for the importance of soft law, social norms and ethics in governmental steering. The case study of the veterans policy and medal of courage contributes empirically to the specification of these theories. Furthermore, new institutionalism adds an explanatory value with a rationality of action for the officials, a so-called logic of appropriateness based on the professional role and on adaptation to the situation at hand and to applicable rules. Theoretically, the thesis contributes by supplementing with a logic of values, which takes into account the profession's ethical and moral rationality of action, which is particularly important in situations such as armed conflict.The methodological approach combines a structural statistical perspective with a qualitative and understanding-oriented perspective and can, with the support of the analytical model, illuminate both pattern and function. The material base is a total selection from the medal preparation of eight contingents in Afghanistan during the years 2008-2012, i.e. FS16-FS23. It consists of the archive material from the nominations as well as in-depth interviews with responsible commanders at the international units and at the national headquarters, including the Commander-in-Chief.
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| 3. |
- Hedberg, Erik, 1986-
(författare)
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Control, Models and Industrial Manipulators
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The two topics at the heart of this thesis are how to improve control of industrial manipulators and how to reason about the role of models in automatic control.On industrial manipulators, two case studies are presented. The first investigates estimation with inertial sensors, and the second compares control by feedback linearization to control based on gain-scheduling.The contributions on the second topic illustrate the close connection between control and estimation in different ways. A conceptual model of control is introduced, which can be used to emphasize the role of models as well as the human aspect of control engineering. Some observations are made regarding block-diagram reformulations that illustrate the relation between models, control and inversion. Finally, a suggestion for how the internal model principle, internal model control, disturbance observers and Youla-Kucera parametrization can be introduced in a unified way is presented.
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| 4. |
- Wang, Yang, 1991-
(författare)
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A Position Control-based Approach to Stiff Objects Haptic Rendering
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With electronic components and computational power becoming more portable and available to general consumers, applications like virtual reality (VR) and mixed reality (MR) are on the rise and renovating the industry of training simulators. Though it is not fully accepted in surgical robots, haptic force feedback can be safely introduced into various surgical training simulators to convey the necessary haptic cues needed to develop hand-eye coordination and motor skills. However, with the current generation of haptic device hardware, it is still challenging to render high stiffness virtual objects while stability allows. Passivity-based approaches are commonly used to ensure stability but they may lead to conservative behavior. Therefore, new approaches are needed to explore other ways of rendering high stiffness without losing stability. This thesis proposes a position control-based haptic rendering approach that designs a position controller to do force computation for the rendering of high-stiffness objects. Instead of computing a penalty force signal that mimics the desired interaction and softly constrains the objects to have little penetration, this proposed approach aims to achieve no penetration with the help of a position controller and sensors. The approach can be easily adapted and applied on any haptic device since it relies on the building of an implicit simulation model which can be linearized at one or several operating points to describe the full dynamics of the system. Different user scenarios, disturbance signals, and evaluation metrics are also proposed to assess and compare the performance with other common approaches. The results show that the proposed approach can stably render a virtual wall with stiffness higher than the common way of modeling the wall as a spring-damper system. By using a piece-wise linear model generated based on multiple operating points and a gain scheduling controller with linear interpolation, the performance of the system is further improved.
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| 5. |
- De Backer, Jeroen, 1987-
(författare)
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Robotic Friction Stir Welding for Flexible Production
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Friction Stir Welding (FSW) is a modern welding process that joins materials by frictional heat, generated by a rotating tool. Unlike other welding processes, the material never melts, which is beneficial for the weld properties. FSW is already widely adopted in several industries but the applications are limited to simple geometries like straight lines or circular welds, mostly in aluminium. The welding operation is performed by rigid FSW machines, which deliver excellent welds but puts limitations on the system in terms of flexibility and joint geometries. Therefore, several research groups are working on the implementation of the FSW process on industrial robots. A robot allows welding of three-dimensional geometries and increases the flexibility of the whole system. The high process forces required for FSW, in combination with the limited stiffness of the robot brings some extra complexity to the system. The limitations of the robot system are addressed in this licentiate thesis.One part of the thesis studies the effect of robot deflections on the weld quality. A sensor-based solution is presented that measures the path deviation and compensates this deviation by modifying the robot trajectory. The tool deviation is reduced to an acceptable tolerance and root defects in the weld are hereby eliminated. The sensor-based method provided better process understanding, leading to a new strategy that uses existing force-feedback for path compensations of the tool. This method avoids extra sensors and makes the system less complex. Another part of this work focuses on the extra complexity to maintain a stable welding process on more advanced geometries. A model is presented that allows control of the heat input in the process by control of the downforce. Finally, the robot’s limitations in terms of maximal hardness of the materials to be welded are investigated. Parameter tuning and implementation of preheating are proposed to allow robotic FSW of superalloys.
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| 6. |
- Myklebust, Andreas, 1985-
(författare)
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Modeling and Estimation for Dry Clutch Control
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing demands on comfort, performance, and fuel efficiency in vehicles lead to more complex transmission solutions. One such solution is the Automated Manual Transmission (AMT). It works just like an ordinary manual transmission but the clutch and gear selection are computer controlled. In this way high efficiency can be accomplished with increased comfort and performance. To be able to control and fully utilize an AMT it is of great importance to have knowledge about how torque is transmitted in the clutch. The transmitted torque in a slipping dry clutch is therefore studied in experiments with a heavy duty truck (HDT). It is shown that material expansion with temperature can explain torque variations up to 700 Nm for the same clutch actuator position. A dynamic clutch temperature model that can describe the torque variations is developed. The dynamic model is validated in experiments, and shown to reduce the error in transmitted torque from 7 % to 3 % of the maximum engine torque compared to a static model.The clutch model is extended with lock-up/break-a-part dynamics and an extra state describing wear. The former is done using a state machine and the latter using a slow random walk for a parameter corresponding to the clutch disc thickness. An observability analysis shows that the augmented model is fully or partially observable depending on the mode of operation. In particular, by measuring the actuator position the temperature states are observable, both during slipping of the clutch and when it is fully closed. An Extended Kalman Filter (EKF) was developed and evaluated on measurement data. The estimated states converged from poor initial values, enabling prediction of the translation of the torque transmissibility curve. The computational complexity of the EKF is low and it is thus suitable for real-time applications.The clutch model is also integrated into a driveline model capable of capturing vehicle shuffle (longitudinal speed oscillations). Parameters are estimated to fit an HDT and the complete model shows good agreement with data. It is used to show that the effect of thermal expansion, even for moderate temperatures, is significant in launch control applications.An alternative use of the driveline model is also investigated here. It is found that the amplitude discretization in production road-slope sensors can excite vehicle shuffle dynamics in the model, which is not present in the real vehicle. To overcome this problem road-slope information is analyzed and it is shown that a third-order butterworth low-pass filter can attenuate the vehicle shuffle, while the shape of the road profile is maintained. All experiments in the thesis are performed using production HDTs only, i.e. production sensors only. Since all modeling, parameter estimation, observer design and validation are performed with production sensors it is straight forward to implement the results in a production HDT following the presented methodology.
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| 7. |
- Nauclér, Peter, 1978-
(författare)
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Estimation and Control of Resonant Systems with Stochastic Disturbances
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The presence of vibration is an important problem in many engineering applications. Various passive techniques have traditionally been used in order to reduce waves and vibrations, and their harmful effects. Passive techniques are, however, difficult to apply in the low frequency region. In addition, the use of passive techniques often involve adding mass to the system, which is undesirable in many applications. As an alternative, active techniques can be used to manipulate system dynamics and to control the propagation of waves and vibrations. This thesis deals with modeling, estimation and active control of systems that have resonant dynamics. The systems are exposed to stochastic disturbances. Some of them excite the system and generate vibrational responses and other corrupt measured signals. Feedback control of a beam with attached piezoelectrical elements is studied. A detailed modeling approach is described and system identification techniques are employed for model order reduction. Disturbance attenuation of a non-measured variable shows to be difficult. This issue is further analyzed and the problems are shown to depend on fundamental design limitations. Feedforward control of traveling waves is also considered. A device with properties analogous to those of an electrical diode is introduced. An `ideal´ feedforward controller based on the mechanical properties of the system is derived. It has, however, poor noise rejection properties and it therefore needs to be modified. A number of feedforward controllers that treat the measurement noise in a statistically sound way are derived. Separation of overlapping traveling waves is another topic under investigation. This operation also is sensitive to measurement noise. The problem is thoroughly analyzed and Kalman filtering techniques are employed to derive wave estimators with high statistical performance. Finally, a nonlinear regression problem with close connections to unbalance estimation of rotating machinery is treated. Different estimation techniques are derived and analyzed with respect to their statistical accuracy. The estimators are evaluated using the example of separator balancing.
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| 8. |
- Robertsson, Karin, 1986-
(författare)
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Mellan civilsamhälle och folkrörelse : Ett aktörsperspektiv på rekrytering och organisering av medborgerligt engagemang
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Civic engagement, commonly seen as concerning people's actions and activities,is generally studied at the individual level. In this doctoral thesis the term is used to examine and analyse how recruitment and organisation of civic engagement has developed since the mid-1990s at the organisational level in a Swedish context, as well as the conditions that have influenced and shaped this development. Three forms of civic engagement are examined: associational membership, volunteering and giving. In this thesis, interviews and text analyses are used. The analysis of archived webpages and annual reports demonstrates a more diversified range of forms of civic engagement available at the organisational level. It is expressed as an increased demand for different types of volunteering and giving, in addition to the more traditional form of civic engagement that associational membership constitutes in a Swedish context. The diversified range of civic engagement is accompanied by various recruitment practices such as marketing and fundraising, characterised by different institutional logics that guide the actual recruitment work within organisations. Interviews were conducted with elected representatives and employees at a senior level within non-profit human service organisations. The interviewees reasoning about the changing conditions for recruiting and organising civic engagement is understood as actions through which they reproduce or change logics. The study concludes that in practice, the recruitment and organisation of civic engagement can be seen as an expression of institutional complexity. It is argued that the collectively shared view of how civic engagement can and should be organised in a Swedish context is characterized by a complex composition of logics that prescribe different patterns of action for participants and organisations.
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