Tom Verstraten

Energy Consumption of Geared DC Motors in Dynamic Applications: Comparing Modeling Approaches

In recent years, many works have appeared, which present novel mechanical designs, control strategies, or trajectory planning algorithms for improved energy efficiency. The actuator model is an essential part of these works, since the optimization of energy consumption strongly depends of the accuracy of this model. Nevertheless, various authors follow very different approaches, often neglecting speedand load-dependent losses and inertias of components such as the motor and the gearbox. Furthermore, there is no consensus on how negative power affects power consumption. Some authors calculate energy consumption by integrating the electrical power entirely, by integrating its absolute value, or by integrating only positive power. This letter assesses how well commonly used models succeed in predicting the energy consumption of an 80 W geared DC motor performing a dynamic task, by comparing the results they produce to experimental baseline measurements.

Series and Parallel Elastic Actuation: Influence of Operating Positions on Design and Control

It is well-established that properly tuned elastic elements can make robotic actuators more energy-efficient, especially in cyclic tasks. Considering a drive train topology, two important subcategories of elastic actuators are series elastic actuation (SEA) and parallel elastic actuation (PEA). There is still no definite answer to the fundamental question which topology consumes less energy in a given task. This paper approaches the problem by studying oscillatory motions of a single degree-of-freedom link in a gravitational field. The imposed motion is a sinusoid with a nonzero offset requiring a static torque that needs to be compensated by the actuation system. Simulations and experiments show that the SEA consumes less energy up to certain offset angles. At high offsets, the PEA becomes the more energy-efficient alternative, provided that its no-load angle is properly tuned. Inverse dynamics simulations show how a threshold offset angle can be determined for a given task.

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

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.

Toward Self-Healing Actuators: A Preliminary Concept

Natural organisms have a unique property not yet available in robotics, i.e., a self-healing (SH) ability. This powerful biological healing function has inspired chemists to impart similar properties to synthetic materials to create “SH materials.” Recent developments in SH polymers led us to investigate the potential of using these materials in robotics. This paper presents an innovative approach of using SH polymers, based on the reversible Diels-Alder (DA) reaction, in a compliant actuator. Using DA polymers, a sacrificial SH mechanical fuse (SH-MF) is designed, developed, and validated by placing it in a cable-driven robotic system. The fuse is designed as weakest element and will sacrificially fail if a damaging overload occurs, protecting the compliant element and other components of the system. The experimental results showed that this SH-MF could be healed at a relatively low temperature, recovering the initial mechanical properties. This first working prototype indicates the feasibility to use SH materials in robotics. “SH robotics” will lead to more sustainable and lighter systems, and eventually to more efficient designs.

Cylindrical cam mechanism for unlimited subsequent spring recruitment in Series-Parallel Elastic Actuators

Series-Parallel Elastic Actuators (SPEA) enable variable recruitment of parallel springs and variable load cancellation. In previous work, we validated a MACCEPA-based SPEA prototype with a self-closing intermittent mechanism, to reduce motor load and improve energy efficiency. However, the mechanism only allowed for 4 parallel springs and a limited equilibrium angle range, which limits the variable load cancellation and operation range. Therefore, we developed a novel cylindrical cam mechanism for unlimited subsequent spring recruitment. This paper describes and validates the working principle of the cylindrical cam mechanism. Furthermore, the latest MACCEPA-based SPEA is presented with a maximum output torque of 40Nm and variable stiffness. Additive and traditional manufacturing techniques go hand in hand to overcome the actuators complexity. The experiments endorse the working principle, demonstrate the variable stiffness, and prove the motor torque can be reduced to 5Nm while an output torque of 40Nm can be achieved.

+SPEA introduction: Drastic actuator energy requirement reduction by symbiosis of parallel motors, springs and locking mechanisms

Modern actuation schematics become increasingly ingenious by deploying springs and locking mechanisms in series and/or parallel. Many of these solutions are, however, tailored for a specific application and a general schematic that allows for drastic energy reduction remains a challenge. We have developed a series-parallel elastic actuator (SPEA) based on a symbiosis of multiple motors, springs and locking mechanisms in parallel, which we call +SPEA. This paper introduces the novel +SPEA concept. We present a first prototype, a +SPEA model and a control strategy that optimizes the energy consumption, and experiments to verify the working principle and recruitment strategy. The experiments show a good fit with the model and currently the actuator reduces the required energy in blocked output experiments by more than a factor 4.

On the Importance of a Motor Model for the Optimization of SEA-driven Prosthetic Ankles

Several examples in literature demonstrate the potential impact of motor inertia on the electrical energy consumption of actuators. Nevertheless, optimizations of actuated prosthetics are often based on the mechanical energy consumption, disregarding the potential effects of motor inertia. In this short abstract, we simulate the electrical energy consumption of a powered prosthetic ankle actuated by a Series Elastic Actuator. Its compliant element is optimized for mechanical energy consumption, a typical strategy in state-of-the-art prosthetics. Our results confirm the importance of motor inertia. Due to the resulting changes in the operating points of the motor, the average motor efficiency is lowered by 17 %.

A Series Elastic Dual-Motor Actuator Concept for Wearable Robotics

Series Elastic Actuators (SEAs) are used extensively in the field of wearable robotics because of their energy efficiency. Redundant drivetrains enable a further reduction in electrical energy consumption, as they use the actuator’s motors in a more energy efficient way. In this work, we present a Series Elastic Dual-Motor Actuator (SEDMA), a kinematically redundant actuator with series elasticity. We simulate its use in an ankle prosthesis and compare its energy efficiency to that of a traditional SEA. Results indicate an energy reduction of 16% compared to the SEA.

Introducing Compound Planetary Gears (C-PGTs): A Compact Way to Achieve High Gear Ratios for Wearable Robots

In the field of wearable robots, high power density and highly efficient actuators are required to handle the high-power motion without becoming heavy and bulky and hence hamper their mobility. Typically, electrical motors are used in combination with high gear ratio gearheads or lever arms in order to achieve the required torques. These gearboxes consist mainly out of several stages of simple Planetary Gear Trains (PGTs). However, this approach leads to big and heavy gearboxes when high torque is needed. An alternative, more compact, design to obtain the required torque increase can be achieved using Compound Planetary Gears (C-PGTs). It is shown that the latter mechanism can obtain gear ratios up to 1:600 while withstanding an output torque of 100 Nm.

Failure Mode and Effect Analysis (FMEA)-Driven Design of a Planetary Gearbox for Active Wearable Robotics

Conducting an FMEA for the design of a planetary gear transmission for exoskeletons enables decision making based on the interdependence between design parameters and the device requirements, as well as an early identification of several functional risks. Therefore, the use of FMEAs in the design of wearable robotic devices could contribute to higher design robustness, and ultimately result in a broader acceptance of future active wearable robotic devices.