For several decades, it was widely believed that a noninteracting disordered electronic system could only undergo an Anderson metal-insulator transition due to Anderson localization. However, numerous recent theoretical works have predicted the existence of a disorder-driven non-Anderson phase transition that differs from Anderson localization. The frustration lies in the fact that this non-Anderson disorder-driven transition has not yet been experimentally demonstrated in any system. Here, using angle-resolved photoemission spectroscopy, we present a case study of observing the non-Anderson disorder-driven transition by visualizing the electronic structure of the Weyl semimetal NdAlSi on surfaces with varying amounts of disorder. Our observations reveal that strong disorder can effectively suppress all surface states in the Weyl semimetal NdAlSi, including the topological surface Fermi arcs. This disappearance of surface Fermi arcs is associated with the vanishing of the topological invariant, indicating a quantum phase transition from a Weyl semimetal to a diffusive metal. These observations provide direct experimental evidence of the non-Anderson disorder-driven transition occurring in real quantum systems, a finding long anticipated by theoretical physicists.

Non-Hermitian physics, studying systems described by non-Hermitian Hamiltonians, reveals unique phenomena not present in Hermitian systems. Unlike Hermitian systems, non-Hermitian systems have complex eigenvalues, making their effects less directly observable. Recently, significant efforts have been devoted to incorporating the non-Hermitian effects into condensed matter physics. However, progress is hindered by the absence of a viable experimental approach. Here, the discovery of the surface-selectively spontaneous reconstructed Weyl semimetal NdAlSi provides a feasible experimental platform for studying non-Hermitian physics. Utilizing angle-resolved photoemission spectroscopy (ARPES) measurements, surface-projected density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements, it is demonstrated that surface reconstruction in NdAlSi alters surface Fermi arc (SFA) connectivity and generates new isolated non-topological SFAs (NTSFAs) by introducing non-Hermitian terms. The surface-selective spontaneous reconstructed Weyl semimetal NdAlSi can be viewed as a Hermitian bulk – non-Hermitian boundary system. The isolated non-topological SFAs on the reconstructed surface act as a loss mechanism and open boundary condition (OBC) for the topological electrons and bulk states, serving as non-Hermitian boundary states. This discovery provides a good experimental platform for exploring new physical phenomena and potential applications based on boundary non-Hermitian effects, extending beyond purely mathematical concepts. Furthermore, it provides important enlightenment for constructing topological photonic crystals with surface reconstruction and studying their topological properties.

Altermagnets constitute a novel, third fundamental class of collinear magnetic ordered materials, alongside with ferro- and antiferromagnets. They share with conventional antiferromagnets the feature of a vanishing net magnetization. At the same time they show a spin-splitting of electronic bands, just as in ferromagnets, caused by the atomic exchange interaction. On the other hand, topology has recently revolutionized our understanding of condensed matter physics, introducing new phases of matter classified by intrinsic topological order. Here we connect the worlds of altermagnetism and topology, showing that the electronic structure of the altermagnet CrSb is topological. Using high-resolution angle-resolved photoemission spectroscopy, we observe the large momentum-dependent spin-splitting in CrSb that induces altermagnetic Weyl nodes. We observe the related topological Fermi-arcs, which in electronic structure calculations are spin polarized. This indicates that in altermagnets the large energy scale intrinsic to their spin-splitting creates its own realm of robust electronic topology.

Haptic rendering often deals with interactions between stiff objects. A traditional way of force computing models the interaction using a spring-damper system, which suffers from stability issues when the desired stiffness is high. Instead of computing a force, this paper continues to explore shifting the focus to rendering an interaction with no penetration, which can be accomplished by using a position controller in the joint space using the encoders as feedback directly. In order to make this approach easily adaptable to any device, an alternative way to model the dynamics of the device is also presented, which is to linearize a detailed simulation model. As a family of linearized models is used to approximate the full dynamic model of the system, it is important to have a smooth transition between multiple sets of controller gains generated based on these models. Gain scheduling is introduced to improve the performance in certain areas and a comparison among three controllers is conducted in a simulation setup.

In traditional force rendering approaches, it is quite popular to model a virtual stiff wall as a spring-damper system to compute the interaction force, which can easily lead to unstable behavior. In this paper, we present an approach to ensure no penetration into the wall by position control. The approach approximates the nonlinear model of a 6-DOF parallel-structure haptic device by a piece-wise linear model to improve the performance compared with a controller designed from a one-point linearized model in haptic rendering. A simulation-based performance comparison study shows that the new controller can render higher stiffness than the previous solution.

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.

Under senare år har vi haft en trend att elektronik blir allt mer kraftfull och portabel och samtidigt mer tillgänglig för större grupper av konsumenter. Detta har även medfört att utveckling och användning av applikationer inom ”Virtual Reality” och ”Mixed Reality” har ökat och även medfört förändringar för industriella tillämpningar av simulatorer där syftet är att träna upp olika slags färdigheter hos användaren.

Även om det inte är fullt accepterat med kirurgiska robotar, haptisk kraftåterkoppling kan introduceras på ett säkert sätt för simulatorer inom kirurgi och innefatta de haptiska sinnen som behövs för att utveckla koordination mellan syn och rörelse och även motoriska färdigheter.

Det är dock fortfarande en utmaning att på ett stabilt sätt återge en kontakt med ett objekt med en hård yta med de haptiska enheter som finns kommersiellt  tillgängliga idag.

Passivitetsbaserade ansatser är vanligt förkommande för att lösa problemet med stabilitet, men ger ett alltför konservativt beteende. Vi behöver därför nya ansatser för att utvärdera alternativa sätt att återge hög kontaktstyvhet utan att äventyra stabiliteten.

Den här avhandlingen föreslår en ansats för haptisk kraftåtergivning som är baserad på att använda ett reglersystem för lägeskontroll där reglersystemet beräknar den kraft som ska återkopplas till användaren när kontakt inträffar med ett styvt objekt. 

Istället för att beräkna den interaktionskraft som försöker efterlikna den verkliga kontakten och förhindra att man tränger in i materialet strävar den här ansatsen efter att förhindra penetrering genom att använda ett reglersystem för lägeskontroll för att beräkna kraften.

För att skapa en generell ansats som kan anpassas och appliceras på andra mekanismer än den vi använt som testcase har vi tillämpat ett angreppssätt där vi först skapar en implicit simuleringsmodell som därefter linjäriseras för en eller flera positioner inom arbetsrymden. Därigenom kan det dynamiska beteendet för systemet beskrivas i form av en eller flera linjäriserade modeller. 

För att utvärdera ansatsen har olika scenarios studerats där olika störsignaler har använts. Ett antal utvärderingskriteria har definierats och nyttjas som bas för utvärdering av prestanda och jämförelse med andra ansatser.

Resultaten visar att den utvecklade ansatsen kan med ett stabilt beteende, återge klart högre krafter och därmed en mer realistisk känsla än den traditionella metoden att beräkna kraften baserat på en fjäder-dämparmodell, när interaktion sker med ett styvt objekt i en virtuell värld.

Conventional force rendering methods in haptic applications often suffer stability issues when simulating interactions with stiff objects such as a virtual wall. This paper argues that the emphasis in such scenarios is to minimize the penetration into the virtual wall instead of modeling the wall as a spring-damper system. Therefore, we propose an approach using a position controller to achieve better haptic rendering of the virtual wall. The proposed approach exploits model-based development tools to obtain the linear control system model without the need for an analytical model of the dynamics of the haptic device. A simulation-based performance comparison of two different controllers has been made for a 6-DOF parallel structure haptic device.

Background: The complex anatomical structure, limited field of vision, and easily damaged nerves, blood vessels, andother anatomical structures are the main challenges of a cranio-maxillofacial (CMF) plastic surgical robot. Bearing thesecharacteristics and challenges in mind, this paper presents the design of a master-slave surgical robot system with a forcefeedback function to improve the accuracy and safety of CMF surgery.

Methods: A master-slave CMF surgical robot system based on force feedback is built with the master tactile robot andcompact slave robot developed in the laboratory. Model-based master robot gravity compensation and force feedbackmechanism is used for the surgical robot. Control strategies based on position increment control and ratio control areadopted. Aiming at the typical mandibular osteotomy in CMF surgery, a scheme suitable for robot-assisted mandibularosteotomy is proposed. The accuracy and force feedback function of the robot system under direct control and masterslavemotion modes are verified by experiments.

Results: The drilling experiment of the mandible model in direct control mode shows that the average entrance pointerror is 1.37+/-0.30 mm, the average exit point error is 1.30+/-0.25 mm, and the average posture error is 2.27+/-0.69 degrees. The trajectory tracking and in vitro experiment in the master-slave motion mode show that the average position followingerror is 0.68 mm, and the maximum force following error is 0.586N, achieving a good tracking and force feedbackfunction.

Conclusion: The experimental results show that the designed master-slave CMF robot can assist the surgeon in completingaccurate mandibular osteotomy surgery. Through force feedback mechanism, it can improve the interactionbetween the surgeon and the robot, and complete tactile trajectory movements.

Haptic rendering has been developing for decades with different rendering approaches and many factors that affect the stability when rendering rigid-body interactions have been investigated. To get an overall understanding of the challenges in haptic rendering, we approach this topic by conducting a systematic review. This review examines different haptic rendering approaches and how to deal with instability factors in rendering. A total of 25 papers are reviewed to answer the following questions: (1) what are the most common haptic rendering approaches for rigid-body interaction? and (2) what are the most important factors for instability of haptic rendering and how to address them? Through the process of investigating these questions, we get the insight that transparency can be further explored and technical terms to describe haptic rendering can be more standardized to push the topic forward.

Conventional force rendering methods in haptic applications often suffer stability issues when simulating stiff objects such as a virtual wall. This paper argues that the emphasis in such scenarios is to minimize the penetration into the virtual wall instead of emulating the force to the operator. Therefore, position controllers are developed to achieve better haptic rendering of the virtual wall. The new rendering method is implemented on a complex 6-DOF parallel structure haptic device. The position controllers are developed by both LQR and 6-joint independent PID methods. The control method exploits model-based development tools to obtain the linear control system model without deriving the mathematical model of the complex haptic device. The performance of the two controllers is compared on a simulated prototype of the haptic device.