The 31st Aerospace Mechanisms Symposium

The proceedings of the 31st Aerospace Mechanisms Symposium are reported. Topics covered include: robotics, deployment mechanisms, bearings, actuators, scanners, boom and antenna release, and test equipment. A major focus is the reporting of problems and solutions associated with the development and flight certification of new mechanisms

Mechanical and optical studies for an extremely large telescope mid-infrared instrument

Extremely Large Telescopes are considered worldwide as one of the highest priorities in ground-based astronomy, since they have the potential to vastly advance astrophysical knowledge. ESO is building its own Extremely Large optical and infrared Telescope, the ELT. This new telescope will have a 39m main mirror and will be the largest optical telescope in the world, able to work at the diffraction limit. METIS, one of the first light instruments of the ELT, has powerful imaging and spectrographic capabilities on the thermal wavelengths. It will allow the investigation of key properties of a wide range of celestial objects. METIS is an extremely complex instrument, weighing almost 11t, and requiring high positioning and steering precisions. Here I present the ELT’s METIS’ Warm Support Structure. It consists of a seven leg elevation platform, an hexapod capable of providing METIS with sub-millimetre and arcsecond positioning and steering resolutions, and an access platform where personnel can perform in-situ maintenance activities. The structure weighs less than 5 t and is capable of surviving earthquake conditions with accelerations up to 5g. The current design is supported by FEM simulations in ANSYS®, and was approved for Phase C. I also study the impact of the Talbot effect on the optics of METIS. This near-field effect reimages high frequencies of the phase into the amplitude, having the potential to harm the High contrast imaging (HCI) modes of the instrument. I analyse the phase errors resulting from the surface form errors of optical elements and conclude that they have an impact of less than 3% on the amplitude considering the current specifications. Finally, I develop a way of replicating the behaviour of a vortex coronagraph with raytracing software. I use this to assess the straylight caused by this kind of coronagraphs

Motion cueing in driving simulators for research applications

This research investigated the perception of self-motion in driving simulation, focussing on the dynamic cues produced by a motion platform. The study was undertaken in three stages, evaluating various motion cueing techniques based on both subjective ratings of realism and objective measures of driver performance. Using a Just Noticeable Difference methodology, Stage 1 determined the maximum perceptible motion scaling for platform movement in both translation and tilt. Motion cues scaled by 90% or more could not be perceptibly differentiated from unscaled motion. This result was used in Stage 2‟s examination of the most appropriate point in space at which the platform translations and rotations should be centred (Motion Reference Point, MRP). Participants undertook two tracking tasks requiring both longitudinal (braking) and lateral (steering) vehicle control. Whilst drivers appeared unable to perceive a change in MRP from head level to a point 1.1m lower, the higher position (closer to the vestibular organs) did result in marginally smoother braking, corresponding to the given requirements of the longitudinal driving task. Stage 3 explored the perceptual trade-off between the specific force error and tilt rate error generated by the platform. Three independent experimental factors were manipulated: motion scale-factor, platform tilt rate and additional platform displacement afforded by a XY-table. For the longitudinal task, slow tilt that remained sub-threshold was perceived as the most realistic, especially when supplemented by the extra surge of the XY-table. However, braking task performance was superior when a more rapid tilt was experienced. For the lateral task, perceived realism was enhanced when motion cues were scaled by 50%, particularly with added XY-sway. This preference was also supported by improvements in task accuracy. Participants ratings were unmoved by changing tilt rate, although rapid tilt did result in more precise lane control. Several interactions were also observed, most notably between platform tilt rate and XY-table availability. When the XY-table was operational, driving task performance varied little between sub-threshold and more rapid tilt. However, while the XY-table was inactive, both driving tasks were better achieved in conditions of high tilt rate. An interpretation of these results suggests that without the benefit of significant extra translational capability, priority should be given to the minimisation of specific force error through motion cues presented at a perceptibly high tilt rate. However, XY-table availability affords the simulator engineer the luxury of attaining a slower tilt that provides both accurate driving task performance and accomplishes maximum perceived realism

A demonstration and comparative analysis of haptic performance using a Gough-Stewart platform as a wearable haptic feedback device

In many hazardous work environments, contact tasks ranging from manufacturing to disassembly to emergency response are performed by industrial manipulators. Due to the hazardous and complex nature of these environments, teleoperation is often employed. When such is the case, the operator is left to interpret a large amount of data during task completion due to the complexity of modern robotic systems and the possible complexity of the tasks. This information is usually processed visually but can lead to sensory overload. To mitigate this, the information processing can also be distributed through other modes of sensory such as auditory or haptic. The University of Texas at Austin's TeMoto hands-free interface reduces the burden on the operator of commanding remote systems by enabling the use of gestural and verbal commands to complete a range of tasks, but the removal of a mechanical interactive device from the operator interface complicates the inclusion of haptic feedback. In this work, a standalone Gough-Stewart platform previously configured as a wearable haptic feedback device for the Nuclear and Applied Robotics Group at the University of Texas at Austin provides real-time haptic feedback to the unconstrained hand(s) of the operator. In doing so, this haptic interface can be employed with the intent of enhancing situational awareness and minimizing operator stress by imparting forces and torques to the user based on those imparted on the end-effector of the industrial manipulator. While multiple technical issues and human factor issues must be addressed, this effort focuses on integrating the system and evaluating its performance for various industrial manipulator designs and sensor modalities. After testing various digital signal processing techniques, functionality was demonstrated among one series-elastic and two rigid industrial manipulators, each with different force/torque data acquisition characteristics and a comparative analysis in haptic performance was performed. Furthermore, it was demonstrated with the TeMoto hands-free teleoperation system. Overall, the demonstrations and experiments performed in this work prove the system to be a viable, hardware agnostic means of haptic feedback and a strong basis for future effortsMechanical Engineerin

Finite element code-based modeling of a multi-feature isolation system and passive alleviation of possible inner pounding

The existing seismic isolation systems are based on well-known and accepted physical principles, but they are still having some functional drawbacks. As an attempt of improvement, the Roll-N-Cage (RNC) isolator has been recently proposed. It is designed to achieve a balance in controlling isolator displacement demands and structural accelerations. It provides in a single unit all the necessary functions of vertical rigid support, horizontal flexibility with enhanced stability, resistance to low service loads and minor vibration, and hysteretic energy dissipation characteristics. It is characterized by two unique features that are a self-braking (buffer) and a self-recentering mechanism. This paper presents an advanced representation of the main and unique features of the RNC isolator using an available finite element code called SAP2000. The validity of the obtained SAP2000 model is then checked using experimental, numerical and analytical results. Then, the paper investigates the merits and demerits of activating the built-in buffer mechanism on both structural pounding mitigation and isolation efficiency. The paper addresses the problem of passive alleviation of possible inner pounding within the RNC isolator, which may arise due to the activation of its self-braking mechanism under sever excitations such as near-fault earthquakes. The results show that the obtained finite element code-based model can closely match and accurately predict the overall behavior of the RNC isolator with effectively small errors. Moreover, the inherent buffer mechanism of the RNC isolator could mitigate or even eliminate direct structure-tostructure pounding under severe excitation considering limited septation gaps between adjacent structures. In addition, the increase of inherent hysteretic damping of the RNC isolator can efficiently limit its peak displacement together with the severity of the possibly developed inner pounding and, therefore, alleviate or even eliminate the possibly arising negative effects of the buffer mechanism on the overall RNC-isolated structural responses

Extension of the Application Potential of Wheeled Mobile Driving Simulators to Uneven Grounds

Driving simulators are an important element of vehicle development, since the design of driver assistance systems in particular requires the investigation of the driver-vehicle interaction. In the future, an even greater application potential is to be expected with regard to automated driving, since, for example, handover strategies can be investigated in a secure environment. However, today's driving simulator concepts have reached a limit with regard to the achievable quality of motion simulation. Especially urban driving scenarios require a range of motion that is not economically viable with the sled systems applied in current high-end systems. One way out of this limitation is provided by wheeled mobile driving simulators, which generate the demanded accelerations through tire forces. This enables an application on different driving surfaces, which allows flexible adaptation of the movement area to the requirements of the scenario. However, due to the contact between tire and driving surface, unevenness induces vibrations into the system which disturb the immersion of the subject. The known previous research on wheeled mobile driving simulators gathered in literature neglected this aspect and postulated a sufficient driving surface quality. However, it is unclear what sufficient means in this context. In addition, the flexibility advantage of the concept may be significantly limited by the requirement of a high quality surface. Thus, this work aims at quantifying the required driving surface quality and the development and evaluation of approaches for the reduction of disturbances induced by unevenness. First, an analysis of the current development state of the driving simulator at FZD, which includes a purely tire-sprung system with solid rubber tires, is conducted. This analysis shows that driving surface qualities with a maximum height deviation of 0.01 mm over a length of 4 m (so-called depth gauge) are required to use a driving simulator of this configuration without deteriorating the immersion of the subject. This quality is not achievable with asphalt surfaces, which offer the highest application potential for WMDS. The minimum achievable depth gauge amounts to 2 mm. Thereupon, an active compensation of the driving surface-induced vibrations with the Hexapod, which is already available in simulators, is investigated. The active approach increases the tolerable depth gauge by a factor of 4 compared to the passive tire-sprung system. Nevertheless, it is still only 3 % of the target value. Especially the high dead time of the hexapod as well as the low damping and the parameter fluctuations of the tire limit the potential of the concept. Therefore, the potential of implementing an additional suspension in combination with the active approach is investigated. In order to achieve a low natural frequency, which is advantageous in terms of vibration isolation, a kinematics is developed that reduces the suspension movements of the omnidirectional motion platform by support forces. In addition, the motion control of the driving simulator is adapted in order to adjust the wheel force distribution to the demands of the suspension. These measures reduce the disturbances caused by suspension movements to values below the perception threshold up to a horizontal acceleration of 4.5 m/s². The simulation of an urban driving scenario with a multibody model shows that this covers the majority of the occurring accelerations and that within more than 99% of the simulation time the disturbance motions remain below the perception threshold. With pneumatic tires, the acceleration range with ideal support can be increased to 5.4 m/s². With regard to the required driving surface quality, this allows an increase of the acceptable depth gauge to 0.8 mm, which corresponds to an improvement of almost two orders of magnitude compared to the initial situation. Nevertheless, the value is slightly below the minimum of 2 mm achievable with asphalt surfaces. However, the determined value is only required to remain below the perception threshold with the disturbance vibrations. As vibration in vehicles is not uncommon, the negative effects on the immersion could possibly be lower, allowing a slight exceeding of the threshold. Future subject studies must examine this aspect in more detail