Esa Kostamo

Performance and Properties of Ultra-Thin Silicon Nitride X-ray Windows

The spectral transmittance of a new generation of SiN based X-ray windows is characterized. The windows are strengthened by low aspect-ratio support grid. As expected for this unprecedented thin window material, the transmittance in the soft X-ray spectral region outperforms the present technologies. A detailed study of the various performance properties of the fabricated SiN X-ray windows is presented. Besides their high transmittance, the windows also have high uniformity, high mechanical strength and good leak tightness. The windows can withstand temperatures from cryogenic range to approximately 250°C. SiN foils are the first real nanotechnology-based choice for the practical realization of X-ray windows and bring the performance to a level that only nanotechnology can offer.

Ultra-Thin Silicon Nitride X-Ray Windows

We have demonstrated the fabrication of ultra-thin Si fine grid supported silicon nitride X-ray windows. These X-ray windows exhibit unequaled transmission of soft X-rays, high strength and excellent thermal stability. Measured soft X-ray transmission performance is significantly enhanced compared to typical polymer or beryllium based X-ray window structures. A double sided grid structure is used to demonstrate the scaling of the technology to larger areas.

Magnetorheologically Damped Compliant Foot for Legged Robotic Application

The aim of this work is to enhance the controllability and the balance of a legged robot by improving the traction between the foot tip and the ground, since the stability of the robot can be influenced only during the phase when the foot is touching the ground. Within the framework of the hydraulically actuated quadruped robot, called HyQ, this paper presents an innovative solution for bouncing reduction between a robotic leg and the ground by means of a semi-active compliant foot. The compliant foot is custom-designed for quadruped walking robots and it consists of a linear spring and a magnetorheological damper. By utilizing magnetorheological technology in the damper element, the damping coefficient of the compliant foot can be altered in a wide range without any additional moving parts. The content of this paper is twofold. In the first part the design, the prototype and a model of the semi-active compliant foot are presented, and the performances of the magnetorheological damper are experimentally studied in quasi-static and dynamic cases. Based on the quasi-static measurements the damping force can be controlled in a range from 15 N to 310 N. From the frequency response measurements it can be analyzed that the generated damping force has a bandwidth higher than 100 Hz. The second part of this paper presents an online stiffness identification algorithm and a mathematical model of the HyQ leg. Using this model the relevant physical parameters are identified. A critical damping control law is proposed and implemented in order to demonstrate the effectiveness of the device that makes use of smart materials. Further on, drop-down experiments have been carried out to assess the performance of the proposed control law in terms of bounce reduction and settling time. In the test setup the HyQ leg was attached to a vertically sliding test setup and in the leg the compliant foot was mounted to the lower limb segment. With the total mass of 7 kg the robotic leg was dropped from the heights of 0.1 m, 0.2 m and 0.3 m. In the results it will be demonstrated that by real time control of the damping force 98% bounce reduction with settling time of 170 ms can be achieved.

Effect of the plate surface characteristics and gap height on yield stresses of a magnetorheological fluid

Effects of the plate material, surface roughness and measuring gap height on static and dynamic yield stresses of a magnetorheological (MR) fluid were investigated with a commercial plate?plate magnetorheometer. Magnetic and non-magnetic plates with smooth (Ra???0.3??m) and rough (Ra???10??m) surface finishes were used. It was shown by Hall probe measurements and finite element simulations that the use of magnetic plates or higher gap heights increases the level of magnetic flux density and changes the shape of the radial flux density profile. The yield stress increase caused by these factors was determined and subtracted from the measured values in order to examine only the effect of the wall characteristics or the gap height. Roughening of the surfaces offered a significant increase in the yield stresses for non-magnetic plates. With magnetic plates the yield stresses were higher to start with, but roughening did not increase them further. A significant part of the difference in measured stresses between rough non-magnetic and magnetic plates was caused by changes in magnetic flux density rather than by better contact of the particles to the plate surfaces. In a similar manner, an increase in gap height from 0.25 to 1.00?mm can lead to over 20% increase in measured stresses due to changes in the flux density profile. When these changes were compensated the dynamic yield stresses generally remained independent of the gap height, even in the cases where it was obvious that the wall slip was present. This suggests that with MR fluids the wall slip cannot be reliably detected by comparison of flow curves measured at different gap heights.

Magnetorheological valve in servo applications

In this article, the servo property of a high-performance magnetorheological valve will be evaluated by closing the pressure feedback loop. The magnetorheological valve developed in this study has two separately controllable fluid flow channels and is especially designed for high-frequency applications. A state space model of the magnetorheological valve from the control signal to the pressure output will be identified, and the identified model is used for tuning a proportional–integral–derivative controller and for simulation of the closed-loop system. Finally, the controller will be implemented to a control computer, and the pressure output will be controlled in a real-time control loop. By analyzing the dynamic and static performance of the magnetorheological servo valve, it can be stated that the magnetorheological valve has a good potential for high-frequency pressure and force control applications.

Development and Performance Evaluation of a Magnetorheological Valve Designed for Control Applications

Magnetorheological fluids are often proposed for applications requiring fast response and good controllability but the dynamic characteristics of the MR devices are seldom analyzed in detail. The aim of this study is to present a magnetorheological valve optimized for fast dynamical response. The fundamental design criteria for fast MR valves are discussed and an experimental valve designed for high frequency actuation is analyzed. It is shown the performance figures generally reported for MR technology can be significantly improved. The results show a step pressure difference of 7 MPa can be controlled with a response time of 0.7 ms. The maximum rate of change for controllable pressure was measured to achieve 20.2 MPa in one millisecond.Copyright