Mojtaba Azadi

Towards a bio-inspired leg design for high-speed running

High-speed terrestrial locomotion inevitably involves high acceleration and extensive loadings on the legs. This imposes a challenging trade-off between weight and strength in leg design. This paper introduces a new design paradigm for a robotic leg inspired by musculoskeletal structures. The central hypothesis is that employing a tendon-bone co-location architecture not only provides compliance in the leg, but can also reduce bone stresses caused by bending on structures. This hypothesis is applied to a leg design, and verified by simulations and the experiments on a prototype. In addition, we also present an optimization scheme to maximize the strength to weight ratio. Using the tendon-bone co-location architecture, the stress on the bone during a stride is reduced by up to 59%. A new foam-core prototyping technique enables creating structural characteristics similar to mammalian bones in the robotic leg. This method allows us to use lighter polymeric structures that are cheaper and quicker to fabricate than conventional fabrication methods, and can eventually greatly shorten the design iteration cycle time.

High-bandwidth AFM-based rheology is a sensitive indicator of early cartilage aggrecan degradation relevant to mouse models of osteoarthritis

Murine models of osteoarthritis (OA) and post-traumatic OA have been widely used to study the development and progression of these diseases using genetically engineered mouse strains along with surgical or biochemical interventions. However, due to the small size and thickness of murine cartilage, the relationship between mechanical properties, molecular structure and cartilage composition has not been well studied. We adapted a recently developed AFM-based nano-rheology system to probe the dynamic nanomechanical properties of murine cartilage over a wide frequency range of 1 Hz to 10 kHz, and studied the role of glycosaminoglycan (GAG) on the dynamic modulus and poroelastic properties of murine femoral cartilage. We showed that poroelastic properties, highlighting fluid-solid interactions, are more sensitive indicators of loss of mechanical function compared to equilibrium properties in which fluid flow is negligible. These fluid-flow-dependent properties include the hydraulic permeability (an indicator of the resistance of matrix to fluid flow) and the high frequency modulus, obtained at high rates of loading relevant to jumping and impact injury in vivo. Utilizing a fibril-reinforced finite element model, we estimated the poroelastic properties of mouse cartilage over a wide range of loading rates for the first time, and show that the hydraulic permeability increased by a factor ~16 from knormal=7.80×10(-16)±1.3×10(-16) m(4)/N s to kGAG-depleted=1.26×10(-14)±6.73×10(-15) m(4)/N s after GAG depletion. The high-frequency modulus, which is related to fluid pressurization and the fibrillar network, decreased significantly after GAG depletion. In contrast, the equilibrium modulus, which is fluid-flow independent, did not show a statistically significant alteration following GAG depletion.

Evaluating Vibrotactile Dimensions for the Design of Tactons

Vibrotactile stimuli are defined in terms of their amplitude, frequency, waveform and temporal profile all of which have been varied to create tactons. A number of approaches have been adopted to design tactons including multidimensional scaling, iterative empirical methods and using perceptual processing models. The objective of the present set of experiments was to create sets of tactons based on the properties of the dimensions of vibrotactile stimuli. An absolute identification paradigm was used in which each of nine tactons was presented eight times using a tactor mounted on either the index finger or forearm. It was found that tactons created by varying the frequency, amplitude and temporal profile of the vibrotactile stimuli were correctly identified on 73-83 percent of the trials, with a mean information transfer of 2.41 bits. The latter metric indicates that for these sets of nine tactons between five and six could be reliably identified. The vibrotactile stimuli delivered in the experiments were identified as consistently on the forearm as the hand and the IT values were similar at the two locations. This suggests that sites other than the hand can be used effectively in tactile communication systems and that it is channel capacity that ultimately determines performance on this type of task.

Identification of vibrotactile patterns: building blocks for tactons

Vibrotactile stimuli vary along a number of dimensions including frequency, amplitude, waveform and temporal profile all of which can be varied to create tactons. The objective of the present experiment was to measure tactile pattern recognition using eight vibrotactile stimuli that varied with respect to frequency, amplitude and pulse duration. An absolute identification paradigm was used in which each stimulus was presented eight times to the index finger or forearm and participants had to identify the visual image associated with the tacton. The results from the experiment indicated that in the absence of spatial cues, tactons were relatively difficult for participants to identify, with an overall mean recognition rate of 57% correct and an IT of 1.72 bits. However, there were significant differences in the identification rates among the tactons, with mean scores ranging from 30% to 83 % correct. Tactons created using higher frequencies and amplitudes were easier to identify than those with lower frequencies and amplitudes. Surprisingly, there was no difference between the finger and the forearm in tacton identification. The dimension that appeared to be most difficult to encode was amplitude, as reflected in the patterns of misidentification in the confusion matrices. These findings indicate that the specific dimensions and stimulus ranges selected to create tactons can profoundly affect the ability to identify tactile patterns and that differences in spatial and temporal acuity across the skin are not predictive of these abilities.

Vibrotactile actuators: Effect of load and body site on performance

The objective of the present set of experiments was to characterize the dynamics of two types of actuator used in vibrotactile displays: a linear resonant actuator and a C2 tactor. Of particular interest was to determine how much the inputs delivered by these actuators changed under varying load conditions and when they were mounted at different locations on the body. When on a rigid surface the force gain frequency response functions of the two tactors were found to be very similar, although the C2 tactor generated considerably higher forces. When the dynamics of the C2 tactor were measured on the skin, there was a substantial decrease in the displacement of the skin with load. When the two tactors were compared under the same loading conditions, the displacement produced by the LRA on the skin was an order of magnitude less than that of the C2 tactor. The resonant frequency of the LRA did vary with the site on the body to which it was attached, which appeared to be related to the stiffness and damping of the underlying skin. These studies indicate the importance of characterizing the properties of actuators used in tactile displays and of controlling how tactors are attached to the skin.

A Variable Spring Using a Tensegrity Prism

Prestressable pin jointed structures are statically indeterminate but kinematically they can be either overdeterminate or indeterminate. Tensegrities are special cases of the latter class. Although, all prestressable structures have variable stiffness that can be controlled by prestress, it is only a subgroup of them such as tensegrities that make effective variable stiffness springs. An N-gon tensegrity prism is investigated as a basis for making a variable stiffness spring called Tensegrity Prism spring. The appropriate deflection direction is found and force-displacement and stiffness equations of a translational tensegrity prism spring with n bars are found. In addition, the main characteristics of a variable tensegrity prism spring and the methods that improve them are discussed.Copyright

Introducing a new semi-active engine mount using force controlled variable stiffness

This work introduces a new concept in designing semi-active engine mounts. Engine mounts are under continuous development to provide better and more cost-effective engine vibration control. Passive engine mounts do not provide satisfactory solution. Available semi-active and active mounts provide better solutions but they are more complex and expensive. The variable stiffness engine mount (VSEM) is a semi-active engine mount with a simple ON–OFF control strategy. However, unlike available semi-active engine mounts that work based on damping change, the VSEM works based on the static stiffness change by using a new fast response force controlled variable spring. The VSEM is an improved version of the vibration mount introduced by the authors in their previous work. The results showed significant performance improvements over a passive rubber mount. The VSEM also provides better vibration control than a hydromount at idle speed. Low hysteresis and the ability to be modelled by a linear model in low-frequency are the advantages of the VSEM over the vibration isolator introduced earlier and available hydromounts. These specifications facilitate the use of VSEM in the automotive industry, however, further evaluation and developments are needed for this purpose.

Wide bandwidth nanomechanical assessment of murine cartilage reveals protection of aggrecan knock-in mice from joint-overuse.

Aggrecan loss in human and animal cartilage precedes clinical symptoms of osteoarthritis, suggesting that aggrecan loss is an initiating step in cartilage pathology. Characterizing early stages of cartilage degeneration caused by aging and overuse is important in the search for therapeutics. In this study, atomic force microscopy (AFM)-based force-displacement micromechanics, AFM-based wide bandwidth nanomechanics (nanodynamic), and histologic assessments were used to study changes in distal femur cartilage of wildtype mice and mice in which the aggrecan interglobular domain was mutated to make the cartilage aggrecanase-resistant. Half the animals were subjected to voluntary running-wheel exercise of varying durations. Wildtype mice at three selected age groups were compared. While histological assessment was not sensitive enough to capture any statistically significant changes in these relatively young populations of mice, micromechanical assessment captured changes in the quasi-equilibrium structural-elastic behavior of the cartilage matrix. Additionally, nanodynamic assessment captured changes in the fluid-solid poroelastic behavior and the high frequency stiffness of the tissue, which proved to be the most sensitive assessment of changes in cartilage associated with aging and joint-overuse. In wildtype mice, aging caused softening of the cartilage tissue at the microscale and at the nanoscale. Softening with increased animal age was found at high loading rates (frequencies), suggesting an increase in hydraulic permeability, with implications for loss of function pertinent to running and impact-injury. Running caused substantial changes in fluid-solid interactions in aggrecanase-resistant mice, suggestive of tissue degradation. However, higher nanodynamic stiffness magnitude and lower hydraulic permeability was observed in running aggrecanase-resistant mice compared to running wildtype controls at the same age, thereby suggesting protection from joint-overuse.

A NEW VARIABLE STIFFNESS SPRING USING A PRESTRESSED MECHANISM

A novel design of a semi-active variable stiffness element is proposed, with possible applications in vibration isolation. Semi-active vibration isolators usually use variable dampers. However, it is known from the fundamental vibration theory that a variable spring can be far more effective in shifting the frequencies of the system and providing isolation. Geometry change is a common technique for building variable springs, but has disadvantages due to the complexity of the required mechanism, and slow response due to the inertia of moving parts. In the variable spring introduced here (VS), the stiffness is changed by force control in the links which corresponds to infinitesimal movements of the links, and does not need a change of geometry to provide a change of stiffness. This facilitates a fast response. The proposed VS is a simple prestressed cable mechanism with an infinitesimal mechanism. Theoretically the level of the prestress in the cables can be used to control the stiffness from zero to a maximum value that is only limited by the strength of the links. In this work, the statics, kinematics and stability of the VS are studied, the stiffness is formulated, and possible configurations of the VS are found.Copyright

An Application of Parallel Singularity in Variable Stiffness Elements

Variable stiffness elements (VSE) or variable springs have wide applications in vibration control systems and robotics. In this work, the concept of a new VSE is introduced and the criteria for the design are explained. The new VSE is based on a special case of cable-driven mechanisms at a singular configuration and is called antagonistic variable stiffness element (AVSE). Singularity of the mechanism provides interesting characteristics for the AVSE such as zero elastic (passive) stiffness and fully controllable stiffness along the infinitesimal flex. Based on the given criteria, the possible configurations of the AVSE are found.Copyright