20 accepted papers
We present an adjustable-equilibrium parallel elastic actuator (AE-PEA) that combines a large direct-drive motor (DDM) and a 3D printed torsional spring with a small motor that can continuously adjust the equilibrium position of the actuator. We demonstrate that the AE-PEA achieves high torque contr
The bicycle transmission enables effortless gear shifting to amplify the force produced by human limbs, but it cannot amplify limb power. Conventional variable stiffness springs, placed in series or parallel with human limbs, can amplify both force and power by increasing stiffness. However, amplify
This study introduces an innovative actuator that resembles a motor with a non-uniform permanent magnetic field. We have developed a prototype of the actuator by combining a standard motor, characterized by a uniform magnetic field, with a custom rotary magnetic spring exhibiting a non-uniform magne…
The Spring-Loaded Inverted Pendulum (SLIP) is one of the simplest models of robot locomotion. SLIP is commonly used to predict the center of mass motion and derive simple control laws for stable locomotion. However, the SLIP model is not integrable, which means that no closed-form relation can be de…
Springs are efficient in storing and returning elastic potential energy but are unable to hold the energy they store in the absence of an external load. Lockable springs use clutches to hold elastic potential energy in the absence of an external load, but have not yet been widely adopted in applicat…
This article presents a data-driven algorithm to compute optimal control inputs for input-constrained nonlinear optimal control problems with switched dynamics. We consider multi-stage optimal control problems where the control inputs and the switching instants are both unknown. Our key contribution
Springs are commonly used in wearable robotic devices to provide assistive joint torque without the need for motors and batteries. However, different tasks (such as walking or running) and different users (such as athletes with strong legs or the elderly with weak legs) necessitate different assisti…
Springs can provide force at zero net energy cost by recycling negative mechanical work to benefit motor-driven robots or spring-augmented humans. However, humans have limited force and range of motion, and motors have a limited ability to produce force. These limits constrain how much energy a conv…
Sitting down and standing up from a chair and, similarly, moving heavy objects up and down between factory lines are examples of cyclic tasks that require large forces but little to no net mechanical energy. Motor-driven artificial limbs and industrial robots can help humans do these tasks, but moto
Energetically-passive robot exoskeletons, mimicking the function of the bicycle, could enable humans to reach previously unprecedented mobility. However, energetically-passive robot exoskeletons require a sophisticated mechanism to enable the human to supply energy, similar to what is enabled by the…
Theory suggests a linear relation between stiffness and the energy stored by a linear helical spring at constant deformation. This relation implies that increasing the stiffness of a helical spring upon deformation requires more energy at larger deformations. State-of-the-art variable stiffness spri…
In this paper, we introduce a new type of compliant actuator named the Parallel Variable Stiffness Actuator (PVSA) which consists of a variable stiffness spring placed in parallel with a direct-drive motor. Parallel variable stiffness actuators provide (i) high-fidelity force control and (ii) contro…
Theory suggests an inverse relation between the stiffness and the energy storage capacity for linear helical springs: reducing the active length of the spring by 50% increases its stiffness by 100%, but reduces its energy storage capacity by 50%. State-of-the-art variable stiffness actuators used to…
We present a differential-algebraic formulation with switching constraints to model the nonsmooth dynamics of robotic systems subject to changing constraints and multiple impacts. The formulation combines a single structurally simple governing equation, a set of switching kinematic constraints, and…
Prioritized multi-objective optimization has been widely used within the operational space inverse dynamics control framework. In this paper, we present a constrained prioritized multi-objective optimization-base control formulation that extends to impedance control, including the `simple' impedance…
Constrained optimal feedback controllers can be used to mitigate external perturbations and modeling errors in robot control. However, these controllers require extensive calculation that is one of their main limitations in real-time implementation. Here we present two locally optimal feedback contr
Stiffness modulators are devices that promote a novel means of actuation; they provide stiffness modulation without deliberately doing mechanical work. These type of compliant actuators may be used for human augmentation to complement co-contracted antagonistic muscles and as such reduce muscle acti…
This paper introduces an analytical approach for the design of variable stiffness mechanisms. The basis of this approach is a general model - representing the potential energy function and the physical constraints - covering the design space of variable stiffness mechanisms. Using this model, we pre…
Compliant actuators have found their place in areas of prosthetics, rehabilitation and robot locomotion because they enable safe human-robot and stable robot-environment interaction, both non-trivial to achieve using conventional rigid actuation. These actuators are capable of varying their equilibr…
Energy efficient compliant actuation is the missing ingredient and key enabler of next-generation autonomous systems, domestic robots, prosthetic devices, orthotic devices, and wearable exoskeletons, to name a few. For all these devices, one would wish to develop actuators enabling wide range impeda