A main distinguishing feature of a wireless network compared with a wired network is its broadcast nature, in which the signal transmitted by a node may reach several other nodes, and a node may receive signals from several other nodes simultaneously. Rather than a blessing, this feature is treated more as an interference-inducing nuisance in most wireless networks today (e.g., IEEE 802.11). The goal of this paper is to show how the concept of network coding can be applied at the physical layer to turn the broadcast property into a capacity-boosting advantage in wireless ad hoc networks. Specifically, we propose a physical-layer network coding (PNC) scheme to coordinate transmissions among nodes. In contrast to "straightforward" network coding which performs coding arithmetic on digital bit streams after they have been received, PNC makes use of the additive nature of simultaneously arriving electromagnetic (EM) waves for equivalent coding operation. PNC can yield higher capacity than straight-forward network coding when applied to wireless networks. We believe this is a first paper that ventures into EM-wave-based network coding at the physical layer and demonstrates its potential for boosting network capacity. PNC opens up a whole new research area because of its implications and new design requirements for the physical, MAC, and network layers of ad hoc wireless stations. The resolution of the many outstanding but interesting issues in PNC may lead to a revolutionary new paradigm for wireless ad hoc networking.

Hot topic: physical-layer network coding

For measuring machines and machine tools, geometrical accuracy is a key performance criterion. While numerical compensation is well established for CMMs, it is increasingly used on machine tools in addition to mechanical accuracy. This paper is an update on the CIRP keynote paper by Sartori and Zhang from 1995 [Sartori S, Zhang GX (1995) Geometric error measurement and compensation of machines, Annals of the CIRP 44(2):599–609]. Since then, numerical error compensation has gained immense importance for precision machining. This paper reviews the fundamentals of numerical error compensation and the available methods for measuring the geometrical errors of a machine. It discusses the uncertainties involved in different mapping methods and their application characteristics. Furthermore, the challenges for the use of numerical compensation for manufacturing machines are specified. Based on technology and market development, this work aims at giving a perspective for the role of numerical compensation in the future.

Geometric error measurement and compensation of machines : an update

The Stewart platform manipulator: a review

Although the concepts of Jacobian matrix, manipulability, and condition number have existed since the very early beginning of robotics their real significance is not always well understood. In this paper we revisit these concepts for parallel robots as accuracy indices in view of optimal design. We first show that the usual Jacobian matrix derived from the input-output velocities equations may not be sufficient to analyze the positioning errors of the platform. We then examine the concept of manipulability and show that its classical interpretation is erroneous. We then consider various common local dexterity indices, most of which are based on the condition number of the Jacobian matrix. It is emphasized that even for a given robot in a particular pose there are a variety of condition numbers and that their values are not coherent between themselves but also with what we may expect from an accuracy index. Global conditioning indices are then examined. Apart from the problem of being based on the local accuracy indices that are questionable, there is a computational problem in their calculation that is neglected most of the time. Finally, we examine what other indices may be used for optimal design and show that their calculation is most challenging.

/pdf/jacobian-manipulability-condition-number-and-accuracy-of-5ecm213mgd.pdf

Jacobian, Manipulability, Condition Number, and Accuracy of Parallel Robots

Parallel robots are nowadays leaving academic laboratories and are finding their way in an increasingly larger number of application fields such as telescopes, fine positioning devices, fast packaging, machine-tool, medical application. A key issue for such use is optimal design as performances of parallel robots are very sensitive to their dimensioning. Optimal design methodologies have to rely on kinetostatic performance indices and accuracy is clearly a key-issue for many applications. It has also be a key-issue for serial robots and consequently this problem has been extensively studied and various accuracy indices have been defined. The results have been in general directly transposed to parallel robots. We will now review how well these indices are appropriate for parallel robots.

/pdf/jacobian-manipulability-condition-number-and-accuracy-of-3enayjm37o.pdf

Jacobian, manipulability, condition number and accuracy of parallel robots

An effective algorithm for the identification of the kinematic parameters of a Stewart platform is presented and verified through simulations The algorithm can be applied to both the reduced and the full models, which differ by the number of parameters to be identified Compensation procedures for models are also presented The algorithm was tested using simulated measurements that included realistic measurement noise, and the results showed that the platform pose error was reduced by at least one order of magnitude The identification algorithm is computationally expensive and has to be performed on a powerful computer for practical implementation >

On the accuracy of a Stewart platform. II. Kinematic calibration and compensation

This paper focuses on the accuracy enhancement of Stewart platforms through kinematic calibration. The calibration problem is formulated in terms of a measurement residual, which is the discrepancy between the measured leg length and the computed leg length. With this formulation, one is able to identify kinematic error parameters of the Stewart platform without the necessity of solving the forward kinematic problem, thus avoiding the numerical problems associated with the solution of the forward kinematic problem. The error parameters are basically the installation errors of the platform ball and U-joints as well as the leg length offsets. By this formulation, a concise differential error model with a well-structured identification Jacobian, which relates the pose measurement residual to the errors in the parameters of the platform, is derived. A measurement procedure that utilizes a single theodolite was devised to determine the poses of the platform. Experimental studies reveal that the proposed calibration method is effective in enhancing the accuracy performance of Stewart platforms.

Kinematic calibration of a Stewart platform using pose measurements obtained by a single theodolite

Manufacturing tolerances, installation errors and link offsets contaminate kinematic parameters of a nominal platform kinematic model. Since the platform controller determines the length of the leg actuators according to the nominal model, the resulting pose of the platform is inaccurate. To enhance the positioning accuracy of the platform, there is a need to estimate the actual values of these parameters and to incorporate these estimates into the inverse kinematic procedure performed by the machine controller. The objective of this paper is 2-fold. First, a new kinematic model is proposed to represent the errors due to imperfection of the U-joints and the ball-joints, which are key components of the Stewart platform. This model may be used to assist the design of the Stewart platform as the effects of mechanical tolerances of different components on the system accuracy can be fully explored. Secondly, an error-model-based method is devised for Stewart platform calibration, by which the accuracy of the Stewart platform can be greatly improved.

Kinematic modeling and calibration of a Stewart platform

There are many different types of robots that are used in different fields to make our lives easier. Currently, a popular assistive robot used in health care includes the “support robot” that helps persons? mobility and the “rehabilitation robot” that facilitates the performance of range-of-motion exercises. A robot performs tasks with mechanical precision on its own, or with guidance. Many fundamental questions exist that challenges the development of assistive health care robots, such as (1) What is a caring robot? (2) Should caring robots be humanoids or not? (3) What necessary functions are required for these caring robots so that it can provide quality nursing care? (4) What level of intelligence should robots have so that it can function effectively as a responsive ‘assistant’ to patients? (5) What levels of decision-making should the robot have to be a dependable partner solving patients' care problems? Based on nursing caring theories, the authors of this paper propose a set of necessary functions that a caring robot should have, so that these questions can be answered. A caring robot has functional features like that of a human nurse. Developing humanoid robots will enhance the necessary functions of a caring robot particularly in task for example, taking vital signs and delivery of meals and medications. The most important ability of the caring robot is to carry-out basic nursing care functions as expected by the patients. This paper aims to apply nursing caring theory as a framework for the patient human-robot interaction.

Functions of a caring robot in nursing

Manufacturing tolerances, installation errors, link offsets, and controller errors effect the positioning repeatability of serial and parallel manipulators. A variety of measurement methods have been proposed and used to measure the repeatability of serial manipulators. However, these methods might not be suitable for parallel manipulators due to limited instrumentation accessibility and the large structure involved. This article describes two measurement procedures by which the position of a target point in three-dimensional space can be determined using a single theodolite that is used to collect two sets of measurements taken from two arbitrary locations. The required hardware needed for the implementation of this method is two standard measuring tapes installed vertically at two arbitrary locations, or one horizontal bar. If the target points are located on the end effector of a manipulator, its repeatability can be measured. The procedure that uses two vertical tapes was used to measure the repeatability of a Stewart Platform and experimental results are provided. © 2995 John Wiley & Sons, Inc.

Measurement of pose repeatabilty of stewart platform

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