Jari Kostamo

RandomPOD—A new method and device for advanced wear simulation of orthopaedic biomaterials

A 16-station wear simulator of the pin-on-disc type, called RandomPOD, was designed, built, and validated. The primary area of application of the RandomPOD is wear studies of orthopaedic biomaterials. The type of relative motion between the bearing surfaces, generally illustrated as shapes of slide tracks, has been found to have a strong effect on the type and amount of wear produced. The computer-controlled RandomPOD can be programmed to produce virtually any slide track shape and load profile. In the present study, the focus is on the biomechanically realistic random variation in the track shape and load. In the reference test, the established combination of circular translation and static load was used. In addition, the combinations of random motion/static load, and circular translation/random load were included. The pins were conventional ultra-high molecular weight polyethylene (UHMWPE), the discs were polished CoCr, and the lubricant was diluted calf serum. The UHMWPE wear factor resulting from random motion was significantly higher than that resulting from circular translation. This was probably caused by the fact that in the random motion the direction of sliding changed more than in circular translation with the same sliding distance. The type of load, random vs. static, was unimportant with respect to the wear factor produced. The principal advantage of using the present random track is that possible unrealistic wear phenomena related to the use of fixed track shapes can be avoided.

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.

Pilot operated miniature valve with fast response and high flow capacity

This paper introduces a pilot operated miniature digital hydraulic valve with a high flow capacity, in comparison to the size of the valve, and a fast response. The valve is designed to be used as a part of a digital valve system, which consists of a large number of similar valves. This paper presents the structure of the valve as well as the response time, flow capacity and leakage measurements of the prototype. The presented valve has a volume of approximately 4 cm3 and a flow capacity of 9 l/min with a 3.5 MPa pressure difference. Its response time is approximately 1 ms and the maximum operating pressure exceeds 30 MPa.

Robot Competition as a Teaching and Learning Platform

Abstract Field Robot Event is an annual student competition for small field robots. A Field Robot project has been seen as a motivating challenge for project-based learning as it results to a working prototype. In Finland the team has consisted of students from different major subjects (mechatronics, automation and agricultural engineering) and interdisciplinary educational goals have been set: 1) to let students apply theoretical knowledge in practice; 2) to teach team working skills; and 3) to get acquainted with robot design. This requires well-planned student guiding and teaching. Dividing the team into separate groups makes schedule keeping hard, decreases working motivation and hinders learning from each other. Dividing education into consequent work tasks enables schedule follow-up and learning evaluation. Too much freedom leads to less learning and a decreased work motivation. Detailed robot specifications have enabled the students to focus on the essential work. Robot building project has been found to be a good platform for learning technology and practising theoretical skills studied in other courses.

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