Ejercicios Prácticos

In fact, the use of compliant actuators is increasing in finger systems [121], exoskeletons [168], prostheses [61], humanoid robots [60] and musculoskeletal upper torsos [95]. Raibert demon- strated in the 80s that springy legs can substantially contribute to more energy-efficient locomo- tion and simplified control [139]. The concept of storing and releasing energy during contact with the ground, observed in animals and humans, has been realized on many robots so far. The idea of using springs, particularly serial springs, was promoted very early also by Alexander. He suggested mainly three uses [10]:

1. the pogo stick principle to bounce along on springs helps to save energy and to reduce unwanted heat production,

2. return springs to halt the legs at the end of each forward/backward swing and start them swinging the other way help to save further energy,

3. compliant foot pads that can reduce forces at impact of feet with the ground and support better road holding by preventing vibrations.

Today the reasons for the use of compliant actuators are manifold and vary depending on the application from safe human-robot interaction to dynamic smooth and energy-efficient legged

locomotion. Aiming at increased performance and safety of robots, various types of compliant actuators can be differentiated. In the following we give an overview of the various research directions that have emerged since Raibert’s hopping machines.

Pneumatic Artifical Muscles

Pneumatic artificial muscles (PAMs) are muscle-like actuators aiming at mimicking the properties of human muscles: high power-to-weight ratio, relatively efficient operation, scalable force, elastic energy storage, and power output [92].

The McKibben muscle is the probably most well-known pneumatic artificial muscle [21]. Due to the high compressibility of air, such PAM behaves very compliant like a spring. It can only exert pulling forces, i. e., usually two PAMs in a antagonist-agonist configuration are neces- sary to drive a joint in both directions. Therefore, it falls into the category of antagonistically controlled stiffness actuator according to [56]. The advantages of a PAM typically include back- drivability and high force production at low speeds and low mass. The disadvantages are the nonlinear characteristics—like muscles have, too—, hysteresis which makes control very difficult, high threshold of pressure to generate force, and the need for pressurized air which makes it energetically inefficient and loud.

Another PAM recently developed is the pleated pneumatic artificial muscle, which according to the authors reduces the hysteresis and overcomes the threshold of pressure [56]. It has been installed in the biped Lucy to realize slow walking in the sagittal plane [169]. The robot has in total six DoFs, three in each leg for the pitch motions in each joint, and is prevented from falling sidewards by a guiding mechanism.

Series Elastic Actuators

A further famous compliant actuator is the series elastic actuator (SEA), developed by Pratt and Williamson [126], who laid the foundations for a paradigm shift in design and philosophy for the next generation of actuators. In the original SEA an electrical geared motor is connected by a rotational spring with fixed stiffness to the joint. The physical compliance of the actuator is limited by the spring constant and cannot be changed during operation. Force respectively impedance control is enabled by measuring the spring elongation and returning it in a feedback loop. Since the actuator controls the equilibrium position of the spring, it belongs to the group of equilibrium-controlled stiffness actuator.

This actuation principle enables lower reflected inertia by the decoupling of the joint from the motor, shock tolerance, energy storage and release, and less damage to the environment. The SEA is characterized in general by low impedance, high force-fidelity, low friction and good bandwidth. As a result, SEAs are well suited for use in legged robots. The 2-D biped Spring Flamingo is one of the first robots in which these actuators had been deployed for improved walking performances [20].

Tendon- and cable-based versions of SEAs followed and were implemented not only in manip- ulators, but also in bipedal robots. The Delft bipedal robots Flame and TUlip use several Bowden cable-driven SEAs for the actuation of the hip, knee, and ankle pitch joints which are required to perform highly fast dynamic motions [60]. However, the maximum walking speeds reported amount to only 0.45m_{/s. Furthermore, due to the high reduction actuation, these robots are,}

in general, not tailor-made for fast gaits that require high energy input during ground contact, such as running or hopping. Hutter et al. argued that high spring compliance in combination with low damping enables storing and releasing a substantial amount of energy during ground

contact phases [70]. The authors have built an articulated, two-segmented leg, ScarlETH, to analyze its performance and suitability for deployment in a quadrupedal running robot. The legs are actuated by SEAs based on a chain/cable pulley design.

Also for other applications, such as wearable assistive devices or exoskeletons, novel actuators based on the series elastic actuation principle provide clear advantages. The Robotic tendon, for instance, reduces substantially the energy consumption of a wearable assistive device [61]. An impedance-controlled gait rehabilitation robot was shown to yield good performance results, provided a well chosen spring stiffness [168].

While the original intention of the SEA’s pioneers was solely a more accurate and stable force/torque control, the additional advantages that came along the SEA inspired many new developments of compliant actuators, in order to come close to the functionalities of our natural archetypes. Adjustable physical compliance has emerged as most desirable property of an actuator, in order to control the natural frequency of the system for safer human robot-interaction and improved legged locomotion behavior.

Variable Impedance Actuators

The AMASC (Actuator with Mechanically Adjustable Series Compliance) is an antagonistically controlled stiffness actuator where two motors independently control the no-load position and physical compliance of the joint [67]. It was specifically designed for running robots to apply variable stiffness as an additional control parameter in order to adjust the overall leg property. The MACCEPA (Mechanically Adjustable Compliance and Equilibrium Position Actuator) [57] is a further actuator with adaptable compliance targeted at enabling the biped Veronica to walk at different speeds. Results of this testing and any gained benefits by using these variable compliant actuators, however, are not known to the author.

Recently we experience an even huger wealth of research on variable impedance actuators. Within the EU project VIACTORS, a number of novel actuators with variable stiffness and damp- ing have been developed [6]. The constructed actuators often comprise several custom-made motors, Harmonic drive gears, nonlinear spring mechanisms and tendons where additional motors are employed to adjust the lever arm or spring stiffness. Such actuation units present modular, integrable solutions that can be well adaptable to the desired task. By this modular solution, dynamic coupling effects that may be undesirable from a controller point of view are eliminated.

While such actuation units represent impressive sophisticated engineering approaches, it still remains to be shown that they provide additional advantages in fast dynamic legged robotics. To date, research studies have mostly focused on manipulation tasks as application for these actuators. One main disadvantage of actuators with adaptable compliance is the increased model complexity which poses a challenge for the development of the motion controller. Further, due to the additional motor for tuning the joint behavior it is very difficult to keep the reflected mass and inertia low.

In fact, the ideas behind the AMASC eventually were integrated in the development of the electric cable differential (ECD) leg eliminating the mechanically adjustable stiffness intention- ally because of the additional mechanical complexity and the reduced energy storage capacity of the springs [68]. The bipedal robot MABEL was assembled of two such ECD legs to overcome the problems the predecessor BiMASC had with the AMASC actuator. So far, it has demonstrated successfully walking over small obstacles.

These incremental advances show how difficult it really is to realize a bipedal walking and running machine moving in a more “human-like” fashion than existing robots. So, the question is whether the desired leg properties can be also achieved by less complex actuators. Accord- ing to the ideas expressed in [85], it may be sufficient to enhance conventional actuators by addition of design features: for instance add a powered latch, a mechanical stop to avoid knee hyperextension or a clutch to engage or disengange the motor as desired.

Active Compliance

An active compliant actuator is a completely stiff actuator that mimics the behavior of a spring by software control. The controller calculates the deviation from the desired torque/force based on measurement of the current torque/force and commands it to the actuator. Obvious disad- vantages are that such actuator cannot store any energy and that shocks cannot be absorbed. The advantage is online adaptable compliance. However the bandwidth of actively controlled compliance depends on the bandwidth of the sensors, actuators, and controller frequency. De- spite these constraints, several research groups have successfully utilized active compliance for high-speed locomotion [149, 1].