László L. Kovács

Interference-Tolerant Spatio-Temporal Dynamic Spectrum Allocation

The radio spectrum is a scarce and thus expensive resource. The exclusive licences of todays practice easily solve the problem of interference, but is clearly inadequate for providing optimal spectrum efficiency for dynamically varying demands. We propose a new model for dynamic spectrum allocation (DSA) to increase spectral efficiency, that handles interference issues in a flexible way. The level of interference between different radio technologies and between geographic regions is captured by the introduced radio technology coupling and geometric coupling parameters. The tolerance for interference from the radio network providers point of view is also taken into account. The rules for a feasible spectrum allocation are given, and the optimal allocation that maximizes the achievable gain is calculated using heuristics. Simulation results show the effectiveness of the proposed DSA model for different scenarios.

Spatio-Temporal Dynamic Spectrum Allocation with Interference Handling

As for today, radio spectrum resource is rigidly partitioned for dedicated purposes. The exclusive license of fixed size spectrum blocks separated by guard bands easily solves the interference problems; however, the rigid allocation of spectrum is clearly inadequate for providing optimal spectrum efficiency for spatially and temporarily varying loads. Dynamic spectrum allocation (DSA) is a new and promising alternative where the assigned spectrum blocks may vary in time and space, too. In this paper we describe a spatio-temporal DSA model that splits the complex problem into temporal and spatial dynamic spectrum allocation. In our architecture the spectrum is allocated by regional spectrum brokers (RSB) that also coordinate spectrum access between regions. The problem of interference between different regions and providers is handled by a flexible description using the proposed geographical and radio technology coupling parameters. We also show how the optimal allocation can be found by giving the ILP solution to the problem. To evaluate the efficiency of the proposed DSA method, different gains are defined from the regulators point of view. The performance evaluation is carried out using computer simulations, and the results are compared with the cases where either there is no interference allowed at all, or interference does not occur between regions.

Spatio-temporal spectrum management model for dynamic spectrum access networks

Currently radio network resource is rigidly partitioned for dedicated purposes. Most of the spectrum is already allocated, but a large part of it is underutilized and the utilization varies greatly in time and space. The exclusive license of fixed size spectrum blocks separated by fixed guard bands easily solves the interference problems; however, the fixed allocation of spectrum is clearly inadequate for providing optimal spectrum efficiency for spatially and temporarily varying loads.n Dynamic spectrum allocation is a new and promising alternative to fixed allocation schemes. In Dynamic Spectrum Access (DSA) networks the assigned spectrum block may vary in time and space, too.n In this paper we describe a new spatio-temporal spectrum management model for DSA networks. We also describe an architecture that splits the complex problem into Temporal and Spatial Dynamic Spectrum Allocation (TDSA and SDSA). In our model Regional Spectrum Brokers (RSB) coordinate the temporal dynamic spectrum allocation for a given region within which we assume that the spatial distribution of the spectrum demand is homogeneous. There is a centralized entity also, called Spectrum Broker Coordinator (SBC), which stores the spectrum demands of the regions, and the spectrum management at the borders of the regions is realized based on this information.

Motion Control of an Under-Actuated Service Robot Using Natural Coordinates

The recent work presents the motion control of a pendulum like under—actuated service robot AC ROBOT ER. This robot is designed to be applied in indoor environments, where it can perform pick and place tasks autonomously and/or with close cooperation with humans. It can also serve as a platform that carries other service robots with lower mobility. The cable suspended robot has a complex structure and its dynamics is difficult to model using conventional robotic approaches. Instead, in this paper natural (Cartesian) coordinates are used to describe the configuration of the robot, while its dynamics is modeled as a set of differential algebraic equations. The method of computed torque control with a PD controller is applied to the investigated under—actuated system. The inverse dynamics solution is obtained via direct discretization of the DAE system. Results for a real parameter case study are presented by numerical simulations.

Dynamics of Digital Force Control Applied in Rehabilitation Robotics

Controlled structures are often required to keep desired contact forces between some of its elements. A classical example is the controlled interaction of a robot with its environment when the control of the contact force between the robotic actuator and the workpiece is prescribed. Experiments often call the attention to the destabilising digital effects, like sampling, in these systems. In this paper the stability of a newly developed force based teaching-in method is analysed. The method is applied in rehabilitation robotics. The stability limits are presented in the parameter space of the sampling time, control gains and mechanical parameters of the robot. The least force error and the fastest settling force signal are calculated. The influence of the elasticity of the force sensor is analysed as well as the possible bifurcations. Real parameter case study confirms the analytical predictions.

The role of mechanical properties on the behaviour and performance of multi-dof haptic devices

The dynamics behaviour of haptic interfacing can significantly depend on the mechanical properties of the haptic device. A systematic framework is outlined in this paper for the development of representative models to characterize the mechanical effects. These models reflect the influence of overall system properties in a parametric form. It is shown that simplified models can be representative in different regions of the virtual environment parameters and operating conditions. Closed-form stability conditions are obtained, which extend the linear stability conditions commonly used in the literature. Using these, quantitative performance measures can be established for the performance of haptic interfaces, which reflect the effect of mechanical design parameters.

Stability case study of the ACROBOTER underactuated service robot

The dynamics of classical robotic systems are usually described by ordinary differential equations via selecting a minimum set of independent generalized coordinates. However, different parameterizations and the use of a nonminimum set of (dependent) generalized coordinates can be advantageous in such cases when the modeled device contains closed kinematic loops and/or it has a complex structure. On one hand, the use of dependent coordinates, like natural coordinates, leads to a different mathematical representation where the equations of motion are given in the form of differential algebraic equations. On the other hand, the control design of underactuated robots usually relies on partial feedback linearization based techniques which are exclusively developed for systems modeled by independent coordinates. In this paper, we propose a different control algorithm formulated by using dependent coordinates. The applied computed torque controller is realized via introducing actuator constraints that complement the kinematic constraints which are used to describe the dynamics of the investigated service robotic system in relatively simple and compact form. The proposed controller is applied to the computed torque control of the planar model of the ACROBOTER service robot. The stability analysis of the digitally controlled underactuated service robot is provided as a real parameter case study for selecting the optimal control gains.

Servo-constraint based computed torque control of underactuated mechanical systems

This paper aims to generalize the computed torque control method for underactuated systems which are modeled by a non-minimum set of generalized coordinates subjected to geometric constraints. The control task of the underactuated robot is defined in the form of servo constraint equations that have the same number as the number of independent control inputs. A PD controller is synthesized based on projecting the equations of motion into the nullspace of the distribution matrix of the actuator forces/torques. The results are demonstrated by numerical simulation and experiments conducted on a two degrees-of-freedom device.Copyright

Experiments on the Stability of Digital Force Control of Robots

Digital force control has a great importance in many robotic applications. For example, in automated manufacturing processes the contact force between the robot and the workpiece has to be controlled in order to achieve the desired product quality. Deburring and polishing of castings can be mentioned as typical examples for this situation [1, 2]. In addition, force control has an important role in contour following tasks, where the robot recognizes the contour of a given object by applying a prescribed contact force normal to the object’s surface [3]. Similarly, for on-line trajectory generation of robots (teaching-in) by a human operator, force control is a possible and obvious solution. In this case, the operator grasps the handle of the teaching-in device and leads the robot through the desired trajectory, which requires the compensation of the contact force to be zero between the handle and the robot’s flange. This control technique can successfully be applied for the teaching in of robots used in medical rehabilitation, where the physiotherapist having no robot programming skills and he/she has to teach-in complex motion trajectories to the robot depending on the patient’s actual condition [4]. Nowadays, service robotic applications come to the front of force control related research [5, 6]. These applications are ranging from simple cleaning robots to humanoids, which operate in the vicinity of humans. Hence, they have to be safe and dependable, which requirements may be provided via force feedback based control.

Act-and-Wait Control Concept for a Force Control Process with Delayed Feedback

The act-and-wait control concept is applied to a force control problem with feedback delay. The point of the concept is that the feedback loop is switched off and on periodically during the control process so that the duration of the switch off period is larger than the feedback delay. The concept is compared to the traditional, continuous control concept, when the feedback loop is continuously active. Stability charts are constructed that plots the critical proportional gains, where the process looses stability, as function of the feedback delay. It is shown that the proportional gains can significantly be increased without loosing stability, if the act-and-wait concept is used. Consequently, the force error can significantly be decreased this way. The theoretical results are confirmed by experiments.