Pieter Beyl

Exploiting Natural Dynamics to Reduce Energy Consumption by Controlling the Compliance of Soft Actuators

Exploiting natural dynamics for bipedal locomotion, or passive walking, is gaining interest because of its energy efficiency. However, the natural trajectories of a passive walker are fixed during the design, thus limiting its mobility. A possible solution to this problem is creating a “semi-passive walker” equipped with actuators with adaptable compliance, which allows the natural dynamics to be changed according to the situation. This paper proposes a compliance controller, a strategy for continuously changing the compliance in such a way as to adapt the natural motion of the system to a desired trajectory. This opens up the possibility of following a range of different trajectories with a relatively low energy consumption. The idea is to fit the controllable actuator compliance to the “natural” compliance of the desired trajectory, and combine that with trajectory tracking control. This strategy was implemented and tested on a 1-DOF pendulum setup actuated by an antagonistic pair of pleated pneumatic artificial muscles. Both simulations and measurements show that the proposed strategy for choosing actuator compliance can significantly reduce the amount of control activity and energy consumption without harming tracking precision.

Prosthetic feet: State-of-the-art review and the importance of mimicking human ankle–foot biomechanics

Numerous prosthetic feet are currently on the market for individuals with a transtibial amputation, each device aimed at raising the 3C-level (control, comfort and cosmetics) with slightly different characteristics. In general, prosthetic feet can be classified into three categories. These are, following the time line: conventional feet (CF), energy-storing-and-returning (ESR) feet and the recent so-called ‘bionic’ feet. Researchers have shown enhanced performance properties of ESR feet compared with early CF. However, even with the advanced technology, none of the ESR feet is capable of significantly reducing energy cost of walking or enhancing prosthetic gait (Nielsen et al. J Prosthet Orthotics 1989;1:24–31; Waters et al. J Bone Joint Surg Am 1976;58:42–46; Torburn et al. J Rehabil Res Dev 1990;27:369–384). From the 1990s, gradually more attention has been paid to the incorporation of active elements in prosthetic feet as the passive devices are not capable of providing the individual with sufficient ankle power during gait. Most part of the ‘bionic’ devices are still on the research level nowadays but one can expect that they will become available on the market soon. In this article, the evolution of prosthetic feet over the last two decades is reflected. The importance of mimicking human ankle–foot biomechanics with prosthetic feet is briefly discussed. Prior work in both objective and subjective evaluation of prosthetic gait is reported.

Development of a compliance controller to reduce energy consumption for bipedal robots

In this paper a strategy is proposed to combine active trajectory tracking for bipedal robots with exploiting the natural dynamics by simultaneously controlling the torque and stiffness of a compliant actuator. The goal of this research is to preserve the versatility of actively controlled humanoids, while reducing their energy consumption. The biped Lucy, powered by pleated pneumatic artificial muscles, has been built and controlled and is able to walk up to a speed of 0.15 m/s. The pressures inside the muscles are controlled by a joint trajectory tracking controller to track the desired joint trajectories calculated by a trajectory generator. However, the actuators are set to a fixed stiffness value. In this paper a compliance controller is presented to reduce the energy consumption by controlling the stiffness. A mathematical formulation has been developed to find an optimal stiffness setting depending on the desired trajectory and physical properties of the system and the proposed strategy has been validated on a pendulum structure powered by artificial muscles. This strategy has not been implemented on the real robot because the walking speed of the robot is currently too slow to benefit already from compliance control.

Safe and Compliant Guidance by a Powered Knee Exoskeleton for Robot-Assisted Rehabilitation of Gait

In the research field of robot-assisted gait rehabilitation there is increased focus on the improvement of physical human–robot interaction by means of high-performance actuator technologies and dedicated control strategies. In this context we propose a combination of lightweight, intrinsically compliant, high-torque actuators (pleated pneumatic artificial muscles) with safe and adaptable guidance along a target trajectory by means of proxy-based sliding mode control. We developed a powered knee exoskeleton (KNEXO) to evaluate these concepts. In addition to the trajectory-based controller a torque controller was implemented with a view to minimizing the interaction during unassisted walking. First, various treadmill walking experiments were performed with unimpaired subjects wearing KNEXO to evaluate the performance of the proposed controllers. Test results confirm the ability of KNEXO to display low actuator torques in unassisted mode and to provide safe, adaptable guidance in assisted mode. Subsequently, a multiple sclerosis patient participated in a series of pilot experiments. Provided there was some patient-specific controller tuning KNEXO was found to effectively support and compliantly guide the subjects knee.

Design and control of a lower limb exoskeleton for robot-assisted gait training

Robot-assisted rehabilitation of gait still faces many challenges, one of which is improving physical human-robot interaction. The use of pleated pneumatic artificial muscles to power a step rehabilitation robot has the potential to meet this challenge. This paper reports on the development of a gait rehabilitation exoskeleton with a knee joint powered by pleated pneumatic artificial muscles. It is intended as a platform for the evaluation of design and control concepts in view of improved physical human-robot interaction. The design was focused on the optimal dimensioning of the actuator configuration. Safety being the most important prerequisite, a proxy-based sliding mode controller PSMC was implemented as it combines accurate tracking during normal operation with a smooth, slow and safe recovery from large position errors. Treadmill walking experiments of a healthy subject wearing the powered exoskeleton show the potential of PSMC as a safe robot-in-charge control strategy for robot-assisted gait training.

Third–Generation Pleated Pneumatic Artificial Muscles for Robotic Applications: Development and Comparison with McKibben Muscle

Abstract This paper introduces the third generation of Pleated Pneumatic Artificial Muscles (PPAM), which has been developed to simplify the production over the first and second prototype. This type of artificial muscle was developed to overcome dry friction and material deformation, which is present in the widely used McKibben muscle. The essence of the PPAM is its pleated membrane structure which enables the muscle to work at low pressures and at large contractions. In order to validate the new PPAM generation, it has been compared with the mathematical model and the previous generation. The new production process and the use of new materials introduce improvements such as 55% reduction in the actuator’s weight, a higher reliability, a 75% reduction in the production time and PPAMs can now be produced in all sizes from 4 to 50 cm. This opens the possibility to commercialize this type of muscles so others can implement it. Furthermore, a comparison with experiments between PPAM and Festo McKibben muscles is discussed. Small PPAMs present similar force ranges and larger contractions than commercially available McKibben-like muscles. The use of series arrangements of PPAMs allows for large strokes and relatively small diameters at the same time and, since PPAM 3.0 is much more lightweight than the commong McKibben models made by Festo, it presents better force-to-mass and energy to mass ratios than Festo models.

Estimating robot end-effector force from noisy actuator torque measurements

This paper discusses two ways to estimate the interaction force at the end-effector of a robot. The first approach that is presented combines filtered dynamic equations with a recursive least squares estimation algorithm to provide a smoothened force signal, which is useful in the (common) case of noisy torque measurements.

The Safety of a Robot Actuated by Pneumatic Muscles—A Case Study

In situations where robots share their workspace with humans, and where physical human-robot interaction is possible or even necessary, safety is of paramount importance. This paper presents a study of the safety of a lightweight robot actuated by pneumatic muscles. Due to its low weight, it has excellent hardware safety characteristics. In spite of this, it is shown that the system can be unsafe when under PID control. It is also shown that safety can be greatly increased by using Proxy-Based Sliding Mode Control (PSMC). The role of passive compliance in safety is also investigated. It is argued that passive compliance can have positive as well as negative effects on robot safety, depending on the situation.

A proof-of-concept exoskeleton for robot-assisted rehabilitation of gait

Robotic gait rehabilitation faces many challenges regarding ankle assistance, body weight support and physical human-robot interaction. This paper reports on the development of a gait rehabilitation exoskeleton prototype intended as a platform for the evaluation of design and control concepts in view of improved physical human-robot interaction. The performance of proxy-based sliding mode control as a “robot-in-charge” control strategy is evaluat both in simulation and in experiments on a test setup. Compared to PID control, test results indicate good tracking performance and in particular safe system behavior.

Safe and compliant guidance in robot-assisted gait rehabilitation using Proxy-based Sliding Mode Control

Research in robot-assisted gait rehabilitation has seen significant improvements in human-robot interaction, thanks to high performance actuator technologies and dedicated control strategies. In this context we propose a combination of lightweight, intrinsically compliant, high power actuators (Pleated Pneumatic Artificial Muscles, PPAMs) with safe and adaptable guidance along a trajectory by means of Proxy-based Sliding Mode Control (PSMC). Treadmill walking experiments performed by a healthy subject wearing a powered knee exoskeleton indicate two main challenges: synchronizing the compliant device and the subject, and tuning the control parameters in view of safe guidance. The exoskeleton is able to compliantly guide the test persons knee along various target trajectories, while ensuring a smooth response to large perturbations.