• Energy Research

Abstract We developed a new actuator, the plus Series-Parallel Elastic Actuator (+SPEA), which is a redundant actuator using brakes. This actuator can achieve additional objectives, other than performing a given task, such as minimum electrical energy consumption. In this paper, we developed a novel control strategy which consists of solving an optimal control problem. Solving this problem allows to use the redundancy of the actuator, as the brakes, in order to achieve lower electrical consumption. The results are then compared to the consumption of an equivalent non-redundant stiff actuator. The results highlight the capability of the actuator to reduce energy consumption in comparison with the latter actuator.

Turbomachine rotors, supported by little damped rolling element bearings, are generally sensitive to unbalance excitation. Accordingly, most machines incorporate squeeze film damper technology to dissipate mechanical energy caused by rotor vibrations and to ensure stable operation. When developing a novel geared turbomachine able to cover a large power range, a uniform mechanical drivetrain needs to perform well over the large operational loading range. Especially, the rotor support, containing a squeeze film damper and cylindrical roller bearing in series, is of vital importance in this respect. Thus, the direct objective of this research project was to map the performance of the envisioned rotor support by estimating the damping ratio based on the simulated and measured vibration response during run-up. An academic test rig was developed to provide an in-depth analysis on the key components in a more controlled setting. Both the numerical simulation and measurement results exposed severe vibration problems for an insufficiently radial loaded bearing due to a pronounced anisotropic bearing stiffness. As a result, a split first whirl mode arose with its backward component heavily triggered by the synchronous unbalance excitation. Hence, the proposed SFD does not function properly in the lower radial loading range. Increasing the static load on the bearing or providing a modified rotor support for the lower power variants will help mitigating the vibration issues.

Abstract Several works have shown that series and parallel elasticity can reduce peak power and energy consumption in prosthetic ankles. Setting the right stiffness of the elastic elements is essential to unlock this potential. In this work, we perform a thorough optimization of series and parallel elastic elements for a prosthetic ankle driven by a geared DC motor. Through simulation, we study the effect of drivetrain limitations and compare different mechanical and electrical optimization objectives. The results highlight the importance of selecting a motor and gearbox in an early stage of the design process. Drivetrain inertia causes peaks in electrical power in the swing phase, which would go unnoticed in an optimization based solely on mechanical power. Furthermore, limitations of the drivetrain and controller reduce the range of applicable springs. This has a direct influence on the optimized spring stiffness values, which, as a result, are different from other works. Overall, the results suggest that, by integrating motor selection into the early stages of the design process, designs can be made lighter, more compact and more efficient.

This paper presents an algorithm to adjust the configuration of a variable-stiffness actuator to reduce its electrical energy requirements during the execution of repetitive tasks. The algorithm is based on the gradient descent optimization method with a modification of the adaptation step size, a forgetting term, and a projection rule to cope with the variation of the actual objective function and signal noise. The performance of the algorithm is validated with experimental results that confirm its capability to reduce the energy requirements of the actuator's driving mechanism. Simulation results illustrate the use of the algorithm in a two-degree-of-freedom system.

The use of active prostheses for lower limb replacement brings new challenges like power optimization, energy efficiency and autonomy. The use of series and parallel elasticity is often explored to reduce the necessary motor power but this does not necessarily have a positive influence on the energy consumption of the prosthesis. This paper presents the experiments performed with the variable compliance actuator used in an active ankle prosthesis and the electromechanical model of this actuator. The results show that the measurements can be matched using the model, and this model can thus be used to optimize the energy efficiency of the actuator. Simulations show that the electrical efficiency can be increased by 10% compared to parameters selected by an optimization method that only takes mechanical properties into account.

The majority of the commercial transtibial prostheses are purely passive devices. They store energy in an elastic element during the beginning of a step and release it at the end. A 75 kg human, however, produces on average 26 J of energy during one stride at the ankle joint when walking at normal cadence and stores/releases 9 J of energy, contributing to energy efficient locomotion. According to Winter, a subject produces on average of 250W peak power at a maximum joint torque of 125 Nm. As a result, powering a prosthesis with traditional servomotors leads to excessive motors and gearboxes at the outer extremities of the legs. Therefore, research prototypes use series elastic actuation (SEA) concepts to reduce the power requirements of the motor. In the paper, it will be shown that SEAs are able to reduce the power of the electric motor, but not the torque. To further decrease the motor size, a novel human-centered actuator concept is developed, which is inspired by the variable recruitment of muscle fibers of a human muscle. We call this concept series-parallel elastic actuation (SPEA), and the actuator consists of multiple parallel springs, each connected to an intermittent mechanism with internal locking and a single motor. As a result, the motor torque requirements can be lowered and the efficiency drastically increased. In the paper, the novel actuation concept is explained, and a comparative study between a stiff motor, an SEA and an SPEA, which all aim at mimicking human ankle behavior, is performed.

Future robots will need to perform complex and versatile tasks comparable to those of humans. Due to the unavailability of suitable actuators, however, novel intelligent and agile robots are often restricted in their performances and development. The limited output torque range and low energy efficiency of current robotic actuators are the main bottlenecks. We have developed a SPEA with intermittent mechanism that addresses these problems. The SPEA is a novel compliant actuator concept that enables variable recruitment of parallel elastic elements and adaptive load cancellation. This paper describes how a SPEA lowers the motor torque and increases the energy efficiency. Experiments on the first proof of concept set-up endorse the practicability of the SPEA concept and the modeled trend of a lowered motor torque and increased energy efficiency. We expect that features of the biologically inspired SPEA with intermittent mechanism will prove exceedingly useful for robotics applications in the future.