Friction is an unavoidable phenomenon that exists in all mechanical systems incorporating parts with relative motion. It is well-known that friction is a serious impediment for precise servo control, hence the interest to devise a procedure to compensate for it – a subject that has been studied by many researchers for many years. The vast majority of friction compensation schemes reported in the literature rely on the availability of velocity measurements, an information that is hard to obtain. A second limitation of the existing procedures is that they rely on mathematical models of friction that contain several unknown parameters, some of them entering nonlinearly in the dynamic equations. In this paper we propose a globally convergent tracking controller for a mechanical system perturbed by static and Coulomb friction, which is a reliable mathematical model of the friction phenomenon, that does not rely one measurement of velocity. The key component is an immersion and invariance-based adaptive speed observer, used for the friction compensation. To the best of our knowledge, this is the first globally convergent solution to this challenging problem. We also present simulation results of the application of our observer for systems affected by friction, which is described by the more advanced LuGre model.
In this article, we apply the recently developed generalized parameter estimation-based observer design technique for state-affine systems to the practically important case of linear time-varying descriptor systems with uncertain parameters. We proceed from the general description of the system given by the standard canonical form and try to develop a comprehensive theory for the design of adaptive observers for these systems. A consequence of our intention to keep the analysis as general as possible is that the final results are a little bit cryptic. Certainly, proceeding from this general framework, we can address particular cases and achieve some practically workable designs. We have identified several critical aspects of the problem. For instance, the highly relevant role played by the strangeness index underscores the differences between the time-variant and the time-invariant cases, which are quite significant. We give simulation results of benchmark examples that illustrate the performance of the proposed adaptive observer.
This paper investigates the role of beliefs and stigma in shaping students’ use of professional mental health services at a large private university in Mexico, where supply-side barriers are minimal and services are readily accessible. In a survey experiment with 680 students, we find that nearly 50% of students in distress do not receive professional mental health support despite a high level of awareness and perceived effectiveness, constituting a substantial treatment gap. We document stigmatized beliefs and misconceptions correlated with the treatment gap. As three-quarters of students incorrectly believe that those in distress perform worse academically and that the majority of students going to therapy are in severe distress, we implement an information intervention to correct these beliefs. We find that it increases students’ sharing of on-campus mental health resources with peers and encourages them to recommend these resources when advising a friend in distress. Interestingly, we find that it lowers respondents’ willingness to pay for private therapy at the end of the intervention. Yet, this effect does not translate into a long-run reduction in self-reported therapy use 6 months after the experiment, with prior therapy users showing increased off-campus take-up.
The length of a double category is a numerical invariant that quantifies the amount of ”work” required to rebuild the double category from its purely globular data. The minimal possible length is 1. It is conjectured that every framed bicategory realizes this minimal value. In this paper we confirm the conjecture for a subclass of framed bicategories: those whose unit squares are all cartesian/op-cartesian. We refer to such structures as fully-faithful/absolutely dense framed bicategories. The conjecture follows from a result relating the internal structure of fully faithful/absolutely dense framed bicategories to crossed products of decorated bicategories by π _2 -indexings.
In this paper, we survey recent econometric contributions to measure the relationship between economic activity and climate change. Due to the critical relevance of these effects for the well-being of future generations, there is an explosion of publications devoted to measuring this relationship and its main channels. The relation between economic activity and climate change is complex with the possibility of causality running in both directions. Starting from economic activity, the channels that relate economic activity and climate change are energy consumption and the consequent pollution. Hence, we first describe the main econometric contributions about the interactions between economic activity and energy consumption, moving then to describing the contributions on the interactions between economic activity and pollution. Finally, we look at the main results on the relationship between climate change and economic activity. An important consequence of climate change is the increasing occurrence of extreme weather phenomena. Therefore, we also survey contributions on the economic effects of catastrophic climate phenomena.
