Impact at each leg transition is one of the main causes of energy dissipation in most of the current bipedal walking robots. Minimizing impact can reduce the energy loss. Instead of controlling the joint angle profiles to reduce the impact which requires significant amount of energy, installing elastic mechanisms on the robots structure is proposed in this research, enabling the robot to reduce the impact, and to store part of the energy in the elastic form which returns the energy to the robot. Practically, this motivates the development of the bipedal walking robots with adjustable stiffness elasticity which itself creates new challenging problems. This thesis addresses some of the challenges through five consecutive stages. Firstly, an adjustable compliant series elastic actuator (named ACSEA in this thesis) is developed. The velocity control mode of the electric motor is used to accurately control the output force of the ACSEA. Secondly, three different conceptual designs of the adjustable stiffness artificial tendons (ASAT) are proposed each of which is added at the ankle joint of a bipedal walking robot model. Simulation results of the collision phase (part of the gait between the heel-strike and the foot-touch-down in bipedal walking) demonstrate significant improvements in the energetics of the bipedal walking robot by proper stiffness adjustment of ASAT. In the third stage, in order to study the effects of ASATs on reducing the energy loss during the stance phase, a simplified model of bipedal walking is introduced consisting of a foot, a leg and an ASAT which is installed parallel to the ankle joint. A linear spring, with adjustable stiffness, is included in the model to simulate the generated force by the trailing leg during the double support phase. The concept of impulsive constraints is used to establish the mathematical model of impacts in the collision phase which includes the heel-strike and the foot-touch-down. For the fourth stage, an energy-feedback-based controller is designed to automatically adjust the stiffness of the ASAT which reduces the energy loss during the foot-touch-down. In the final stage, a speed tracking (ST) controller is developed to regulate the velocity of the biped at the midstance. The ST controller is an event-based time-independent controller, based on geometric progression with exponential decay in the kinetic energy error, which adjusts the stiffness of the trailing-leg spring to control the injected energy to the biped in tracking a desired speed at the midstance. Another controller is also integrated with the ST controller to tune the stiffness of the ASAT when reduction in the speed is desired. Then, the local stability of the system (biped and the combination of the above three controllers) is analyzed by calculating the eigenvalues of the linear approximation of the return map. Simulation results show that the combination of the three controllers is successful in tracking a desired speed of the bipedal walking even in the presence of the uncertainties in the leg’s initial angles. The outcomes of this research show the significant effects of adjustable stiffness artificial tendons on reducing the energy loss during bipedal walking. It also demonstrates the advantages of adding elastic elements in the bipedal walking model which benefits the efficiency and simplicity in regulating the speed. This research paves the way toward developing the dynamic walking robots with adjustable stiffness ability which minimize the shortcomings of the two major types of bipedal walking robots, i.e., passive dynamic walking robots (which are energy efficient but need extensive parameters tuning for gait stability) and actively controlled walking robots (which are significantly energy inefficient). / May 2008

Impact at each leg transition is one of the main causes of energy dissipation in most of the current bipedal walking robots. Minimizing impact can reduce the energy loss. Instead of controlling the joint angle profiles to reduce the impact which requires significant amount of energy, installing elastic mechanisms on the robots structure is proposed in this research, enabling the robot to reduce the impact, and to store part of the energy in the elastic form which returns the energy to the robot. Practically, this motivates the development of the bipedal walking robots with adjustable stiffness elasticity which itself creates new challenging problems. This thesis addresses some of the challenges through five consecutive stages. Firstly, an adjustable compliant series elastic actuator (named ACSEA in this thesis) is developed. The velocity control mode of the electric motor is used to accurately control the output force of the ACSEA. Secondly, three different conceptual designs of the adjustable stiffness artificial tendons (ASAT) are proposed each of which is added at the ankle joint of a bipedal walking robot model. Simulation results of the collision phase (part of the gait between the heel-strike and the foot-touch-down in bipedal walking) demonstrate significant improvements in the energetics of the bipedal walking robot by proper stiffness adjustment of ASAT. In the third stage, in order to study the effects of ASATs on reducing the energy loss during the stance phase, a simplified model of bipedal walking is introduced consisting of a foot, a leg and an ASAT which is installed parallel to the ankle joint. A linear spring, with adjustable stiffness, is included in the model to simulate the generated force by the trailing leg during the double support phase. The concept of impulsive constraints is used to establish the mathematical model of impacts in the collision phase which includes the heel-strike and the foot-touch-down. For the fourth stage, an energy-feedback-based controller is designed to automatically adjust the stiffness of the ASAT which reduces the energy loss during the foot-touch-down. In the final stage, a speed tracking (ST) controller is developed to regulate the velocity of the biped at the midstance. The ST controller is an event-based time-independent controller, based on geometric progression with exponential decay in the kinetic energy error, which adjusts the stiffness of the trailing-leg spring to control the injected energy to the biped in tracking a desired speed at the midstance. Another controller is also integrated with the ST controller to tune the stiffness of the ASAT when reduction in the speed is desired. Then, the local stability of the system (biped and the combination of the above three controllers) is analyzed by calculating the eigenvalues of the linear approximation of the return map. Simulation results show that the combination of the three controllers is successful in tracking a desired speed of the bipedal walking even in the presence of the uncertainties in the leg’s initial angles. The outcomes of this research show the significant effects of adjustable stiffness artificial tendons on reducing the energy loss during bipedal walking. It also demonstrates the advantages of adding elastic elements in the bipedal walking model which benefits the efficiency and simplicity in regulating the speed. This research paves the way toward developing the dynamic walking robots with adjustable stiffness ability which minimize the shortcomings of the two major types of bipedal walking robots, i.e., passive dynamic walking robots (which are energy efficient but need extensive parameters tuning for gait stability) and actively controlled walking robots (which are significantly energy inefficient).

Biological systems are able to perform complex movements with high energy-efficiency and, in general, can adapt to environmental changes more elegantly than traditionally engineered mechanical systems. The Equilibrium Point Hypothesis describes animal motor control as trajectories of equilibrium joint angle and joint stiffness. Traditional approaches to robot design are unable to implement this control scheme because they lack joint actuation methods that can control mechanical stiffness, and, in general, they are unable to take advantage of energy introduced into the system by the environment. In this paper, we describe the development and implementation of an FPGA-controlled, servo-actuated robotic joint that incorporates series-elastic actuation with specially developed nonlinear springs. We show that the joint's equilibrium angle and stiffness are independently controllable and that their independence is not lost in the presence of external joint torques. This approach to joint control emulates the behavior of antagonistic muscles, and thus produces a mechanical system that demonstrates biological similarity both in its observable output and in its method of control.

No description available.

Researchers have offered a variety of solutions for overcoming the old and challenging problem of undesired vibrations. The optimum vibration-control solution that can be a passive, semi-active or active solution, is chosen based on the desired level of vibration-control, the budget and the nature of the vibration source. Mechanical vibration-control systems, which work based on variable stiffness control, are categorized as semi-active solutions. They are advantageous for applications with multiple excitation frequencies, such as seismic applications. The available mechanical variable stiffness systems that are used for vibration-control, however, are slow and usually big, and their slowness and size have limited their application. A new semi-active variable stiffness solution is introduced and developed in this thesis to address these challenges by providing a faster vibration-control system with a feasible size. The new solution proposed in this thesis is a semi-active variable stiffness mount/isolator called the antagonistic Variable Stiffness Mount (VSM), which uses a variable stiffness spring called the Antagonistic Variable stiffness Spring (AVS). The AVS is a kinematically singular prestressable mechanism. Its stiffness can be changed by controlling the prestress of the mechanisms links. The AVS provides additional stiffness for a VSM when such stiffness is needed and remains inactive when it is not needed. The damping of the VSM is constant and an additional constant stiffness in the VSM supports the deadweight. Two cable-mechanisms - kinematically singular cable-driven mechanisms and Prism Tensegrities - are developed as AVSs in this thesis. Their optimal configurations are identified and a general formulation for their prestress stiffness is provided by using the notion of infinitesimal mechanism. The feasibility and practicality of the AVS and VSM are demonstrated through a case study of a typical engine mount by simulation of the mathematical models and by extensive experimental analysis. A VSM with an adjustable design, a piezo-actuation mechanism and a simple on-off controller is fabricated and tested for performance evaluation. The performance is measured based on four criteria: (1) how much the VSM controls the displacement near the resonance, (2) how well the VSM isolates the vibration at high frequencies, (3) how well the VSM controls the motion caused by shock, and (4) how fast the VSM reacts to control the vibration. For this evaluation, first the stiffness of the VSM was characterized through static and dynamic tests. Then performance of the VSM was evaluated and compared with an equivalent passive mount in two main areas of transmissibility and shock absorption. The response time of the VSM is also measured in a realistic scenario.

No description available.

Target of this thesis was to design servo-mechanism suitable for control system of small and speedy aircraft, such as business jets and light multi-purpose attack aircraft. Main task was to use suggested characteristics for computer analysis and determine impact of their changes on servo-mechanism properties. Further on, experimental way of determining these parameters.