Bi-directional series-parallel elastic actuator and overlap of the actuation layers

Abstract

Several robotics applications require high torque-to-weight ratio and energy efficient actuators. Progress in that direction was made by introducing compliant elements into the actuation. A large variety of actuators were developed such as series elastic actuators (SEAs), variable stiffness actuators and parallel elastic actuators (PEAs). SEAs can reduce the peak power while PEAs can reduce the torque requirement on the motor. Nonetheless, these actuators still cannot meet performances close to humans. To combine both advantages, the series parallel elastic actuator (SPEA) was developed. The principle is inspired from biological muscles. Muscles are composed of motor units, placed in parallel, which are variably recruited as the required effort increases. This biological principle is exploited in the SPEA, where springs (layers), placed in parallel, can be recruited one by one. This recruitment is performed by an intermittent mechanism. This paper presents the development of a SPEA using the MACCEPA principle with a self-closing mechanism. This actuator can deliver a bi-directional output torque, variable stiffness and reduced friction. The load on the motor can also be reduced, leading to a lower power consumption. The variable recruitment of the parallel springs can also be tuned in order to further decrease the consumption of the actuator for a given task. First, an explanation of the concept and a brief description of the prior work done will be given. Next, the design and the model of one of the layers will be presented. The working principle of the full actuator will then be given. At the end of this paper, experiments showing the electric consumption of the actuator will display the advantage of the SPEA over an equivalent stiff actuator.

Analysis, Design, and Preliminary Evaluation of a Parallel Elastic Actuator for Power-Efficient Walking Assistance

“…The simulation was carried out by solving (12)(13)(14)(15)(16)(17)(18)(19)(20) Fig. 11 overlays the performance of the PEA in the light of two indexes, average (rms) and maximum positive electrical P of the motor in a gait cycle when springs with different stiffnesses were attached and no spring was attached.…”

“…The parameters in (12)(13)(14)(15)(16) are summarized in Table 6. Given the largest mass (20.5415 g) and the outer radius (3.406 cm) of the selected springs, the equivalent moment of inertia of those springs should be much smaller than 238 g•cm 2 , which accounts for 4.5 % of the moment of inertia of the motor (5301 g•cm 2 ).…”

“…Based on (12)(13)(14)(15)(16)(17), the necessary current, I , of the BLDC motor in the PEA during a gait cycle can be obtained.…”

“…A clutch is needed to engage and disengage the PE during the gait for the knee joint because PEAs require large active torques to counter the elastic recoil for rapid stopping in stance or leg placement in swing [20]. In addition, the parallel extension spring is also utilized in the exoskeleton hip actuator [31], HAL for lumbar support [34], the hopping robot ETH Cargo for carrying a load [35] and other cases [14], [15], [36]. The second type used widely is the torsion spring as it can provide the torque directly.…”

Design, modelling and characterisation of a magnetic variable stiffness mechanism

“…While SEAs and VIAs are successful in changing the speed profile required of drive motors, the load borne by the motor has not changed since the compliant element is placed in series with the motor [164]. For robotic joints where high torques and low speeds are typical this means that motor selection remains a choice between that of using a large bulky motor that will largely operate well outside its high efficiency operating window of high speed and low torque, or coupling smaller motors with large reduction ratios where frictional losses greatly affects the efficiency of the joint [164].…”