Optimal Design Methods for Increasing Power Performance of Multiactuator Robotic Limbs

abstract: In order for assistive mobile robots to operate in the same environment as humans, they must be able to navigate the same obstacles as humans do. Many elements are required to do this: a powerful controller which can understand the obstacle, and power-dense actuators which will be able to achieve the necessary limb accelerations and output energies. Rapid growth in information technology has made complex controllers, and the devices which run them considerably light and cheap. The energy density of batteries, motors, and engines has not grown nearly as fast. This is problematic because biological systems are more agile, and more efficient than robotic systems. This dissertation introduces design methods which may be used optimize a multiactuator robotic limb's natural dynamics in an effort to reduce energy waste. These energy savings decrease the robot's cost of transport, and the weight of the required fuel storage system. To achieve this, an optimal design method, which allows the specialization of robot geometry, is introduced. In addition to optimal geometry design, a gearing optimization is presented which selects a gear ratio which minimizes the electrical power at the motor while considering the constraints of the motor. Furthermore, an efficient algorithm for the optimization of parallel stiffness elements in the robot is introduced. In addition to the optimal design tools introduced, the KiTy SP robotic limb structure is also presented. Which is a novel hybrid parallel-serial actuation method. This novel leg structure has many desirable attributes such as: three dimensional end-effector positioning, low mobile mass, compact form-factor, and a large workspace. We also show that the KiTy SP structure outperforms the classical, biologically-inspired serial limb structure.Dissertation/ThesisDoctoral Dissertation Mechanical Engineering 201

Rapid acceleration of legged robots: a pneumatic approach

For robotics to be useful to the public in a multifaceted manner, they need to be both legged and agile. The legged constraint arises as many environments and systems in our world are tailored to ablebodied adults. Therefore, a practically useful robot would need to have the same morphology for maximum efficacy. For robots to be useful in these environments, they need to perform at least as well as humans, therefore presenting the agility constraint. These requirements have been out of reach of the field until recently. The aim of this thesis was to design a planar monopod robot for rapid acceleration manoeuvres, that could later be expanded to a planar quadruped robot. This was achieved through a hybrid electric and pneumatic actuation system. To this end, modelling schemes for the pneumatic cylinder were investigated and verified with physical experiments. This was done to develop accurate models of the pneumatic system that were later used in simulation to aid in the design of the platform. The design of the platform was aided through the use of Simulink to conduct iterative testing and multivariate evaluations using Monte Carlo simulation methods. Once the topology of the leg was set, the detail design was conducted in Solidworks and validated with its built in simulation functions. In addition to the mechanical design of the platform, a specialist boom was designed. The design needed to compensate for the forces the robot exerts on the boom as well as the material constraints on the boom. This resulted in the development of a cable-stayed, four bar mechanism boom system. An embedded operating system was created to control the robot and take in and fuse sensor inputs. This was run using multiple sensors, sub-controllers and microcontrollers. Sensor fusion for the system was done using a Kalman Filter to improve readings and estimate unmeasured states of the robot. This Kalman Filter took LiDAR and accelerometer readings as inputs to the system to produce a subcentimetre accurate position measure for the system. Finally, the completed platform was validated using fixed-body forward hopping tests. These tests showed a significant degree of similarity to the simulated results and therefore validated the design process

Development of a new hydraulic ankle for HYDROID humanoid robot

For humanoid robots, design of the ankle mechanism is still open research problem since high torque is required while compact structures have to be maintained. This paper investigates an enhanced design of 3 degree-of-freedom hydraulic hybrid ankle mechanism. The design is based on (US9327785) Alfayad et al. (2011). Using a hybrid kinematic structure with hydraulic actuation, allows us to reach a slender humanoid ankle shape while enabling the high torque performances required for stable walking. Performances analysis of the first version ankle mechanism designed for HYDROïD humanoid robot showed some limits mainly induced by seal friction and pistons misalignment. In this paper, the influence of the friction parameters is explored. A virtual model is developed to evaluate the performances of a new flexion/extension and adduction/abduction pistons arrangement. Then, a control algorithm is simulated and implemented, as an example, to the flexion/extension motion of the new ankle mechanism. Finally, an experimental validation for the performances of the new proposed hydraulic ankle is conducted using the built hardware prototype, the results show significant improvemen

Développement d'une unité de valves motorisées et algorithme de transition pour actionnement hydrostatique bimodal d'une jambe robotique

Les robots mobiles, tels que les exosquelettes et les robots marcheurs, utilisent des actionneurs qui doivent satisfaire à une large plage de requis de force et de vitesse. Par exemple, pour le cycle de marche d’une jambe robotique, la phase d’appui nécessite une force élevée tandis que la phase de balancement requiert une grande vitesse. Pour satisfaire ces requis opposés, le dimensionnement d’un système d’actionnement traditionnel à rapport de réduction unique conduit généralement à un moteur électrique lourd, surdimensionné et à une faible efficacité énergétique. Ainsi, l’alternative explorée est une architecture hydrostatique à deux vitesses où des valves motorisées sont utilisées pour reconfigurer dynamiquement le système entre deux modes de fonctionnement : fort ou rapide. La complexité réside dans le choix d’une technologie de valve légère ainsi que dans le développement d’un algorithme de contrôle permettant de réaliser les transitions de manière rapide et fluide. Un prototype d’une unité de valves motorisées est conçu et intégré dans l’architecture hydrostatique complète de l’actionneur et un banc d’essai d’une jambe robotique est fabriqué. Trois stratégies de contrôle des moteurs sont comparées lors du changement de mode : une vitesse constante, une diminution de vitesse et une réduction du courant. La méthode choisie, le contrôle en courant, est ensuite utilisée pour la démonstration des phases d’appui et de balancement de la jambe robotique. Par cette méthode, il est possible d’effectuer des transitions rapides, de maintenir une force suffisante et de minimiser les oscillations qui surviennent lors du contact avec le sol. Ces travaux offrent donc un premier point de comparaison au niveau du choix de valves, de la masse, de la vitesse d’actionnement et de la stratégie de contrôle

Design, modelling and control of a rotorcraft landing gear for uneven ground conditions

The ability to perform vertical take-off and landing, hovering and lateral flight provides rotorcrafts crucial advantages over other aircrafts and land vehicles for operations in remote areas. However, a major limitation of rotorcrafts is the requirement of a flat surface to land, increasing the difficulty and risk of landing operations on rough terrain or unstable surfaces. This limitation is mainly due to the use of conventional landing gear like skids or wheels. The growing use of Unmanned Aerial Vehicles (UAVs) also increases the necessity for more landing autonomy of these systems. This thesis presents the investigation into the development of an adaptive robotic landing gear for a small UAV that enhances the landing capabilities of current rotorcrafts. This landing gear consists in a legged system that is able to sense and adapt the position of its legs to the terrain conditions. This research covers the development of effective tools for the design and testing of the control system using software and hardware platforms. Mathematical models using multibody system dynamics are developed and implemented in software simulations. A hardware robot is designed and built to validate the simulation results. The system proposed in this thesis consists in a landing gear with four robotic legs that uses an Inertial Measurement Unit (IMU) to sense the body attitude, Force Sensing Resistors (FSR) to measure feet pressure and a distance sensor to detect ground approach. The actuators used are position-controlled servo motors that also provide angular position feedback. The control strategy provides position commands to coordinate the motion of all joints based on attitude and foot pressure information. It offers the advantage of being position-controlled, so it is easier to be implemented in hardware systems using low-cost components, and at the same time, the feet forcecontrol and leg design add compliance to the system. Through software simulations and laboratory experiments the system successfully landed on a 20° slope surface, substantially increasing the current slope landing limit

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

Biomimetic leg design and passive dynamics of Dolomedes aquaticus

Spiders provide working models for agile, efficient miniature passive-dynamic robots. Joints are extended by haemoplymph (hydraulic) pressure and flexed by muscle-tendon systems. Muscle contraction in the prosoma leads to an increase in hydraulic pressure and subsequently leg extension. Analysis of body kinematics the New Zealand fishing spider, Dolomedes aquaticus indicates that elastic plates around the joints absorb energy from the ground reaction force when the force vector points backwards (i.e. would decelerate the spider’s body in the direction of locomotion) and release it to provide forward thrust as the leg swings backwards. In addition to improving energy efficiency, this mechanism improves stability by passively absorbing energy from unpredictable foot-ground impacts during locomotion on uneven terrain. These principles guided an iterative design methodology using a combination of 3D modelling software and 3D printing techniques. I compared and contrasted compliant joints made of a variety of plastic materials. The final 3D-printed spider leg prototype has a stiff ABS exoskeleton joined by a compliant polypropylene backbone. The entire structure envelopes a soft silicone pneumatic bladder. FEA analysis was used to determine the ideal shape and behavior of the pneumatic bladder to actuate the exoskeleton. The spider leg can be flexed and contracted depending on the input pressure. To laterally actuate this pneumatic spider leg I designed and developed a fabrication system that uses vacuum injection molding to produce an integrated mesh sleeve/elastomer pneumatic actuator. I designed an apparatus to measure pressure and contraction of silicone and latex pneumatic muscles when inflated. I analyzed the non-linear pressure-contraction relationships of silicone versus latex pneumatic muscles, and also derived force-contraction relationships. From efficiency studies, both media muscles proved to be inefficient and the measuring apparatus needs to be more robust to prevent leaking air. The fabrication process still offers the possibility of a quick and efficient method of creating pneumatic muscles. A spider-like robot that implements these pneumatic muscles and pneumatic leg design could be used to explore the efficiency and stability of passive dynamic legged locomotion in spider-like robots