Wave energy is a huge power source that can make a significant contribution to the production of renewable energy in the future. It can generate as much as 10% of the world´s present energy consumption [1]. However, Wave Energy Converters (WECs) is a relatively new field, which requires new technology development, and testing before efficient WECs can give any significant contribution to the energy production. The main difficulties lie in the harsh marine environment and the fact that wave power is delivered with high forces and low speeds. The high forces necessitates strong and durable mechanical structures and the slow speed makes efficient energy conversion challenging. Further, there is a very large variation in wave height over time, and during storms, the largest waves can be 10-25 times higher than the average wave height at the site. The maximum forces and motions can thereby be tremendous if not handled properly, and makes the development of efficient Wave Energy Converters (WEC) challenging. The dimensioning forces strongly depend on the wave power concept, the WEC implementation and the actual Power Take-Off (PTO) system. Of the many different types of WECs being researched and developed, this paper focuses on the development of a winch based point absorber wave energy converters (WBPA-WECs). This paper presents the development of a solution for a flexible chain where an elastomeric bearing is used as a means to achieve the relative motion between the links in a chain. With this solution no sliding is present and the motion is achieved as a deformation in the elastomeric bearing. A design process is formulated for determining a suitable configuration of layers of steel shims and elastomers. A test rig for bearing tests has been developed and results from these experimental tests in combination with material tests and FE analysis are presented in the paper. These are presented in the context of illustrating the proposed design process for designing these type of laminated elastomer bearings. The different steps in this design process have been illustrated for the defined test bearing and we can conclude that our findings indicate that the proposed process can be an important support method for further development of this approach with elastomer bearings.

According to the European Council, 75% of the greenhouse emissions are caused by energy production and use. In order to decarbonize the energy sector, more research is focused towards the renewable energy sources. Among the many available resources, wave energy is one of the key resources to consider. As part of the research, KTH is developing a winch based point absorber wave energy converter. The winch consists of a chain link with elastomer bearings wound around a drum. The winding and unwinding motion of chain converts wave motion into electricity. A typical winch based wave energy converter undergoes around 80 million cycles in its lifetime. This calls for a durable system design requiring minimal service and maintenance. With an elastomer bearing, the winding and unwinding of the chain over the drum is realized as deformation of the elastomer thereby eliminating sliding. While the use of elastomer enables an efficient design, it also makes it more challenging due to its highly non-linear and viscoelastic behavior. A constitutive model is necessary to determine material characteristics of an elastomer for different loading conditions. In this work, a systematic design process is outlined and an attempt is made to determine a suitable hyperelastic material model for the elastomer. The study is focused on two materials - silicone and polyurethane. The test samples are compressed and followed by shear in deformation. For material model determination, the test data is curve fit and later verified using finite element method. The material is assumed incompressible.

Wave energy has an immense potential to provide clean electricity with almost zero emission, among the many renewable energy resources. While it has a huge potential to cater 10% of the current electricity demand, there are many uncertainties involved in the development of a wave energy converter (WEC). This article discusses those challenges and benefits of a product development process to address them. A winch based point absorber (WBPA) concept being developed at KTH Royal Institute of Technology has been used as reference for all discussions. A systematic design approach provides an effective way to rationalize the project from start to end. The value analysis approach is quite useful in order to eliminate rudimentary functions thereby reducing the cost. The stage gate methods makes the systematic design approach interactive and provides an understanding if a concept must be eliminated or continued with at an early stage in the project. Elements of value analysis approach and stage gate methods have been implemented along side complementing the systematic design approach to outline a design process for the WBPA-WEC. An effort has been made to address the challenges faced in the development of a WBPA-WEC.

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.

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.

Ocean wave power is a promising renewable energy source. It has, however, been difficult to find a cost-effective solution to convert wave energy into electricity. The harsh marine environment and the fact that wave power is delivered with large forces at low speed make design of durable mechanical structures and efficient energy conversion challenging. The dimensioning forces strongly depend on the wave power concept, the wave energy converter (WEC) implementation, and the actual power take-off (PTO) system. A WEC with a winch as a power take-off system, i.e., a winch-based point absorber (WBPA), could potentially enable a low levelized cost of energy (LCOE) if a low-cost, durable and efficient winch that can deal with peak loads can be developed. A key challenge for realizing such a winch is to find a force transmitting solution that can deal with these peak loads and that can handle up to 80 million cycles during its life. In this article, we propose a design solution for a force transmitting chain with elastomer bearings connecting the links of the chain. With this solution no sliding is present, and the angular motion is realized as elastic shear deformations in the elastomer bearings when the chain is wound onto the winch drum. The elastomer bearings were designed for low shear stiffness and high compression stiffness, and the links were designed primarily to minimize the number of joints in the chain. Thereby, the maximum allowed relative angle between the links when rolled up over the drum should be as large as possible within practical limits. Finite element-based topological optimization was performed with the aim to increase the link strength to weight ratio. A test rig for a first proof of concept testing has been developed, and preliminary test results indicate that this chain concept with elastomer bearings can be a potential solution for a durable chain and should be analyzed and tested further for fatigue and sea operations.

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.