Passive Variable Compliance for Dynamic Legged Robots

Recent developments in legged robotics have found that constant stiffness passive compliant legs are an effective mechanism for enabling dynamic locomotion. In spite of its success, one of the limitations of this approach is reduced adaptability. The final leg mechanism usually performs optimally for a small range of conditions such as the desired speed, payload, and terrain. For many situations in which a small locomotion system experiences a change in any of these conditions, it is desirable to have a tunable stiffness leg for effective gait control. To date, the mechanical complexities of designing usefully robust tunable passive compliance into legs has precluded their implementation on practical running robots. In this thesis we present an overview of tunable stiffness legs, and introduce a simple leg model that captures the spatial compliance of our tunable leg. We present experimental evidence supporting the advantages of tunable stiffness legs, and implement what we believe is the first autonomous dynamic legged robot capable of automatic leg stiffness adjustment. Finally we discuss design objectives, material considerations, and manufacturing methods that lead to robust passive compliant legs

Identifying ground-robot impedance to improve terrain adaptability in running robots

To date, running robots are still outperformed by animals, but their dynamic behaviour can be described by the same model. This coincidence means that biomechanical studies can reveal much about the adaptability and energy efficiency of walking mechanisms. In particular, animals adjust their leg stiffness to negotiate terrains with different stiffnesses to keep the total leg-ground stiffness constant. In this work, we aim to provide one method to identify ground-robot impedance so that control can be applied to emulate the aforementioned animal behaviour. Experimental results of the method are presented, showing well-differentiated estimations on four different types of terrain. Additionally, an analysis of the convergence time is presented and compared with the contact time of humans while running, indicating that the method is suitable for use at high speeds

Climbing and Walking Robots

Nowadays robotics is one of the most dynamic fields of scientific researches. The shift of robotics researches from manufacturing to services applications is clear. During the last decades interest in studying climbing and walking robots has been increased. This increasing interest has been in many areas that most important ones of them are: mechanics, electronics, medical engineering, cybernetics, controls, and computers. Today’s climbing and walking robots are a combination of manipulative, perceptive, communicative, and cognitive abilities and they are capable of performing many tasks in industrial and non- industrial environments. Surveillance, planetary exploration, emergence rescue operations, reconnaissance, petrochemical applications, construction, entertainment, personal services, intervention in severe environments, transportation, medical and etc are some applications from a very diverse application fields of climbing and walking robots. By great progress in this area of robotics it is anticipated that next generation climbing and walking robots will enhance lives and will change the way the human works, thinks and makes decisions. This book presents the state of the art achievments, recent developments, applications and future challenges of climbing and walking robots. These are presented in 24 chapters by authors throughtot the world The book serves as a reference especially for the researchers who are interested in mobile robots. It also is useful for industrial engineers and graduate students in advanced study

Understanding preferred leg stiffness and layered control strategies for locomotion

Despite advancement in the field of robotics, current legged robots still cannot achieve the kind of locomotion stability animals and humans have. In order to develop legged robots with greater stability, we need to better understand general locomotion dynamics and control principles. Here we demonstrate that a mathematical modeling approach could greatly enable the discovery and understanding of general locomotion principles. ^ It is found that animal leg stiffness when scaled by its weight and leg length falls in a narrow region between 7 and 27. Rarely in biology does such a universal preference exist. It is not known completely why this preference exists. Here, through simulation of the simple actuated-SLIP model, we show that the biological relative leg stiffness corresponds to the theoretical minimum of mechanical cost of transport. This strongly implies that animals choose leg stiffness in this region to reduce energetic cost. In addition, it is found that the stability of center-of-mass motion is also optimal when biological relative leg stiffness values are selected for actuated-SLIP. Therefore, motion stability could be another reason why animals choose this particular relative leg stiffness range. ^ We then extended actuated-SLIP by including realistic trunk pitching dynamics. At first, to form the Trunk Spring-Loaded Inverted Pendulum (Trunk-SLIP) model, the point mass of actuated-SLIP is replaced by a rigid body trunk while the leg remains massless and springy. It is found that exproprioceptive feedback during the flight phase is essential to the overall motion stability including trunk pitching. Either proprioceptive or exproprioceptive feedback during stance could generate stable running motion provided that exproprioceptive feedback is used during flight. When both kinds of feedback are used during stance, the overall stability is improved. However, stability with respect to speed perturbations remains limited. ^ Built upon Trunk-SLIP, we develop a model called extended Trunk-SLIP with trunk and leg masses. We then develop a hierarchical control strategy where different layers of control are added and tuned. When each layer is added, the overall motion stability is improved. This layer by layer strategy is simple in nature and allows quick controller design and tuning as only a limited number of control parameters needs to be added and tuned at each step. In the end, we propose a future control layer where the commanded speed is controlled to achieve a higher level target such as might be needed during smooth walking to running transitions. ^ In summary, we show here that the simple actuated-SLIP model is able to predict animal center-of-mass translation stability and overall mechanical cost of transport. More advanced models are then developed based upon actuated-SLIP. With a simple layer by layer control strategy, robust running motion can be discovered. Overall, this knowledge could help better understand locomotion dynamics in general. In addition, the developed control strategy could, in principle be applied to future hip based legged robot design

Magnetorheological Variable Stiffness Robot Legs for Improved Locomotion Performance

In an increasingly automated world, interest in the field of robotics is surging, with an exciting branch of this area being legged robotics. These biologically inspired robots have leg-like limbs which enable locomotion, suited to challenging terrains which wheels struggle to conquer. While it has been quite some time since the idea of a legged machine was first made a reality, this technology has been modernised with compliant legs to improve locomotion performance. Recently, developments in biological science have uncovered that humans and animals alike control their leg stiffness, adapting to different locomotion conditions. Furthermore, as these studies highlighted potential to improve upon the existing compliant-legged robots, modern robot designs have seen implementation of variable stiffness into their legs. As this is quite a new concept, few works have been published which document such designs, and hence much potential exists for research in this area. As a promising technology which can achieve variable stiffness, magnetorheological (MR) smart materials may be ideal for use in robot legs. In particular, recent advances have enabled the use of MR fluid (MRF) to facilitate variable stiffness in a robust manner, in contrast to MR elastomer (MRE). Developed in this thesis is what was at the time the first rotary MR damper variable stiffness mechanism. This is proposed by the author for use within a robot leg to enable rapid stiffness control during locomotion. Based its mechanics and actuation, the leg is termed the magnetorheological variable stiffness actuator leg mark-I (MRVSAL-I). The leg, with a C-shaped morphology suited to torque actuation is first characterised through linear compression testing, demonstrating a wide range of stiffness variation. This variation is in response to an increase in electric current supplied to the internal electromagnetic coils of the MR damper. A limited degrees-of-freedom (DOF) bipedal locomotion platform is designed and manufactured to study the locomotion performance resulting from the variable stiffness leg. It is established that optimal stiffness tuning of the leg could achieve reduced mechanical cost of transport (MCOT), thereby improving locomotion performance. Despite the advancements to locomotion demonstrated, some design issues with the leg required further optimisation and a new leg morphology

Biomechanics and Energetics of Bipedal Locomotion on Uneven Terrain.

Humans navigate uneven terrain in their everyday lives. From trails, grass, and uneven sidewalks, we constantly adapt to various surfaces in our environment. Past research has shown that walking on natural terrain, compared to walking on smooth flat surfaces, results in increased energy expenditure during locomotion. However, the biomechanical adaptations responsible for this energetic increase are unclear, since locomotion research is often conducted either on short walkways or in an outdoor setting, thus limiting data collections. To further our understanding of human locomotion on uneven terrain, I focused on quantifying the biomechanical and energetic changes due to increased terrain variability during walking and running. First, this thesis presents modifications to a regular exercise treadmill to allow for attachment of a separate uneven surface. Using this treadmill, I collected kinetic, kinematic, electromyographic, and energy expenditure data during continuous human walking and running. I showed that humans walking at 1.0m/s on an uneven surface, with a 2.5cm height variability, increased energy expenditure by 0.73W/kg (approx. 28%) compared to walking on smooth terrain. Greater energy expenditure was primarily caused by increased positive work at the hip and knee, with minor contributions from increased muscle activity and step parameter adaptations. I then showed that running at 2.3m/s on the same surface resulted in an energetic increase of 0.48W/kg (approx. 5%) compared to running on even terrain. In contrast to walking, humans compensated for uneven terrain during running by reducing positive work produced by the ankle and adapting a more crouched leg posture. The similar absolute increases in energetic cost between walking and running implied that much of this increase is likely due to surface height variability and changes in mechanical work. Finally, this work presents analytical and simulated analyses for the rimless wheel and simplest walker models. These analyses explored the relationship between gait dynamics, energy input strategies, surface unevenness and the energetic cost of walking. Together, these studies advance our understanding of the relationship between mechanics and energetics of human walking on uneven surfaces and could potentially lead to more robust and energetically efficient legged robots, prostheses and more effective clinical rehabilitation interventions.PhDKinesiology and Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111616/1/voloshis_1.pd

An Overview on Principles for Energy Efficient Robot Locomotion

Despite enhancements in the development of robotic systems, the energy economy of today's robots lags far behind that of biological systems. This is in particular critical for untethered legged robot locomotion. To elucidate the current stage of energy efficiency in legged robotic systems, this paper provides an overview on recent advancements in development of such platforms. The covered different perspectives include actuation, leg structure, control and locomotion principles. We review various robotic actuators exploiting compliance in series and in parallel with the drive-train to permit energy recycling during locomotion. We discuss the importance of limb segmentation under efficiency aspects and with respect to design, dynamics analysis and control of legged robots. This paper also reviews a number of control approaches allowing for energy efficient locomotion of robots by exploiting the natural dynamics of the system, and by utilizing optimal control approaches targeting locomotion expenditure. To this end, a set of locomotion principles elaborating on models for energetics, dynamics, and of the systems is studied