Jeffrey Ackerman

Energy Efficiency of Legged Robot Locomotion With Elastically Suspended Loads

Elasticity is an essential property of legged locomotion. Elastically suspending a load can increase the efficiency of locomotion and load carrying in biological systems and for human applications. Similarly, elastically suspended loads have the potential to increase the energy efficiency of legged robot locomotion. External loads and the inherent mass of a legged robot, such as batteries, electronics, motors, and fuel, can be elastically suspended from the robot with compliant springs, passively reducing the energetic cost of locomotion. An experimental elastic load suspension mechanism was developed and utilized on a hexapod robot to test the energetic cost of legged robot locomotion over a range of suspension stiffness values. While running at the same speed, the robot with an elastically suspended load consumed up to 24% less power than with a rigidly attached load. Thus, elastically suspended loads could increase the operation time, load-carrying capacity, or top speed of legged robots, enhancing their utility in many roles.

Energetics of bio-inspired legged robot locomotion with elastically-suspended loads

Elasticity is an essential property of locomotion. In biology, elastically-suspended loads increase the efficiency of locomotion and load carrying. Similarly, elastically-suspended loads have the potential to increase the energy efficiency of legged robot locomotion. External loads and the inherent mass of a legged robot, such as batteries, electronics, and fuel, can be elastically-suspended from the robot with compliant springs, passively reducing the energetic cost of locomotion. An experimental prototype hexapod robot with a novel elastic load suspension mechanism based on the Christie suspension system is developed and utilized to test the energy efficiency of legged robot locomotion with elastically-suspended loads versus rigidly-attached loads. Elastically-suspended loads are shown to reduce the energetic cost of locomotion compared to rigidly-attached loads. Thus, the speed, operation time, or load carrying capacity could be increased for robots that utilize elastically-suspended loads.

A model of human walking energetics with an elastically-suspended load

Elastically-suspended loads have been shown to reduce the peak forces acting on the body while walking with a load when the suspension stiffness and damping are minimized. However, it is not well understood how elastically-suspended loads can affect the energetic cost of walking. Prior work shows that elastically suspending a load can yield either an increase or decrease in the energetic cost of human walking, depending primarily on the suspension stiffness, load, and walking speed. It would be useful to have a simple explanation that reconciles apparent differences in existing data. The objective of this paper is to help explain different energetic outcomes found with experimental load suspension backpacks and to systematically investigate the effect of load suspension parameters on the energetic cost of human walking. A simple two-degree-of-freedom model is used to approximate the energetic cost of human walking with a suspended load. The energetic predictions of the model are consistent with existing experimental data and show how the suspension parameters, load mass, and walking speed can affect the energetic cost of walking. In general, the energetic cost of walking with a load is decreased compared to that of a stiffly-attached load when the natural frequency of a load suspension is tuned significantly below the resonant walking frequency. The model also shows that a compliant load suspension is more effective in reducing the energetic cost of walking with low suspension damping, high load mass, and fast walking speed. This simple model could improve our understanding of how elastic load-carrying devices affect the energetic cost of walking with a load.

Coupled-Oscillator Model of Locomotion Stability With Elastically-Suspended Loads

Elasticity is a fundamental property of dynamic locomotion and is generally thought to affect the efficiency and stability of motion. In particular, it is becoming increasingly apparent that elastically-suspended loads are common in biology and useful for carrying loads. For example, the Suspended Load Backpack reduces the peak forces and energy cost during locomotion. In this paper, we present a simple model of locomotion to examine the effect of elastically-suspended loads on the peak forces, energy cost, and stability during locomotion. The results from the model show that elastically-suspended loads reduce the peak forces, energy cost, and stability of locomotion compared to rigidly-attached loads, thus indicating that a tradeoff exists between the decreased stability of locomotion and the reduction of peak forces and energy cost. We discuss this tradeoff and the implications of reduced stability on locomotion over rough terrain and the maneuverability of a system.Copyright

Dynamics of carrying a load with a handle suspension

Carrying loads with a compliant pole or backpack suspension can reduce the peak forces of the load acting on the body when the suspension natural frequency is tuned below the stepping frequency. Here we investigate a novel application for a load suspension that could be used to carry a load by hand, which is a common yet difficult method of load carriage and results in inherently asymmetric dynamics during load carriage. We hypothesize that the asymmetric dynamics of carrying a load in one hand will result in multiple locomotion frequency modes which can affect the forces of carrying a load with a handle suspension. We tested an adjustable-stiffness hand-held load suspension with four different natural frequency values while walking and running compared to a rigid handle. As expected, the peak forces acting on the body decrease compared to a rigid handle as the effective suspension stiffness decreases below the stepping frequency. However, the asymmetric dynamics of carrying a load with one hand introduce another frequency mode at half the stepping frequency which increases the peak forces acting on the body when the natural frequency of the handle is tuned near this frequency. We conclude that hand-held load suspensions should be designed to have a natural frequency below the half-stepping frequency of walking to minimize the peak forces and the musculoskeletal stress on the human body while carrying loads with one hand.

Design of Compliant Bamboo Poles for Carrying Loads

Carriage of heavy loads is common in developing countries and can impart large repetitive forces on the body that could lead to musculoskeletal fatigue and injury. Compliant bamboo poles have been used to carry heavy loads in Asia for generations and could be a low-cost, sustainable, and culturally acceptable way to minimize the forces acting on the body during load carriage. Experimental evidence of running with a 15 kg load suspended from a pair of compliant poly(vinyl chloride), or PVC, poles shows that the poles act as a vibration-isolating suspension, which can reduce the peak forces on the body during locomotion. However, it is currently not well-understood how to design and optimize poles for load carrying such that the peak forces on the body are minimized during carrying. Further, current users of bamboo poles do not have a reliable way to measure forces on the body and so cannot empirically optimize their poles for force reduction. Our objective is to determine the geometric and material design parameters that optimize bamboo poles for load carriage and to develop recommendations that could make it easier for load carriers to fabricate well-suited poles. Our approach is to synthesize a predictive model of walking and running from the field of biomechanics, which can predict the peak forces on the body as a function of pole stiffness, with a bending beam model of the bamboo pole that relates pole geometry and material to the effective pole stiffness. We first check our model’s ability to predict the experimental results from a well-established study with PVC poles. We then extend the predictive design study to include a wider range of stiffness values and pole geometries that may be more effective and realistic for practical load carrying situations. Based on stiffness, deflection, strength, and pole mass design constraints, we specify an appropriate range of dimensions for selecting bamboo poles to carry a 15 kg load. The design methodology presented could simplify the selection and design of bamboo carrying poles in order to reduce the likelihood of musculoskeletal injury. [DOI: 10.1115/1.4028757]

Effects of independently altering body weight and mass on the energetic cost of a human running model

The mechanisms underlying the metabolic cost of running, and legged locomotion in general, remain to be well understood. Prior experimental studies show that the metabolic cost of human running correlates well with the vertical force generated to support body weight, the mechanical work done, and changes in the effective leg stiffness. Further, previous work shows that the metabolic cost of running decreases with decreasing body weight, increases with increasing body weight and mass, and does not significantly change with changing body mass alone. In the present study, we seek to uncover the basic mechanism underlying this existing experimental data. We find that an actuated spring-mass mechanism representing the effective mechanics of human running provides a mechanistic explanation for the previously reported changes in the metabolic cost of human running if the dimensionless relative leg stiffness (effective stiffness normalized by body weight and leg length) is regulated to be constant. The model presented in this paper provides a mechanical explanation for the changes in metabolic cost due to changing body weight and mass which have been previously measured experimentally and highlights the importance of active leg stiffness regulation during human running.

Design of Stabilizing Arm Mechanisms for Carrying and Positioning Loads

Stabilizing arm mechanisms are used to support and position a load with minimal force from the user. Further, stabilizing arm mechanisms enable operators to stabilize the motion of the load while walking or running over variable terrain. Although existing stabilizing arm mechanisms have reached fairly broad adoption over a range of applications, it remains unknown exactly how the spring properties and geometric parameters of the mechanism enable its overall performance. We developed a simplified model to analyze the vertical dynamics of stabilizing arms to determine how the spring properties and mechanism geometry affect the natural frequency of the load mass, the range of load masses that can be supported, and the equilibrium position of the load mass. We found that decreasing the unstretched spring free length is the most effective way to minimize the natural frequency; the spring lever arm can be used to adjust for a desired load mass range, and the linkage length can be used to adjust the range of motion of the stabilizing arm. The spring stiffness should be selected based on the other parameters. This work provides a systematic design study of how the parameters of a stabilizing arm mechanism affect its behavior and fundamental design principles that could be used to improve existing mechanisms, and enable the design of new mechanisms.

Energetic and Dynamic Analysis of Multifrequency Legged Robot Locomotion With an Elastically Suspended Load

Elastically suspended loads can reduce the energetic cost and peak forces of legged robot locomotion. However, legged locomotion frequently exhibits multiple frequency modes due to variable leg contact times, body pitch and roll, and transient locomotion dynamics. We used a simple hexapod robot to investigate the effect of multiple frequency components on the energetic cost, dynamics, and peak forces of legged robot locomotion using a high-speed motion tracking system and the fast Fourier transform (FFT). The trajectories of the robot body and the suspended load revealed that the robot was excited by both a body pitching frequency and the primary locomotion frequency. Both frequency modes affected the dynamics of the legged robot as the natural frequency of the elastic load suspension was varied. When the natural frequency of the load suspension was reduced below the primary locomotion and body pitching frequencies, the robot consumed less average power with an elastically suspended load versus a rigidly attached load. To generalize the experimental results more broadly, a modified double-mass coupled-oscillator model with experimental parameters was shown to qualitatively predict the energetic cost and dynamics of legged robot locomotion with an elastically suspended load. The experimental results and the theoretical model could help researchers better understand locomotion with elastically suspended loads and design load suspension systems that are optimized to reduce the energetic cost and peak forces of legged locomotion.

Energy Efficiency of Legged Robot Locomotion With Elastically-Suspended Loads Over a Range of Suspension Stiffnesses

Elastically suspending a load from humans and animals can increase the energy efficiency of legged locomotion and load carrying. Similarly, elastically-suspended loads have the potential to increase the energy efficiency of legged robot locomotion. External loads and the inherent mass of a legged robot, such as batteries, electronics, and fuel, can be elastically-suspended from the robot chassis with a passive compliant suspension system, reducing the energetic cost of locomotion. In prior work, we developed a simple model to examine the effect of elastically-suspended loads on the energy cost of locomotion from first principles. In this paper, we present experimental results showing the energy cost of locomotion for a simple hexapod robot over a range of suspension stiffness values. Elastically-suspended loads were shown to reduce the energy cost of locomotion by up to 20% versus a rigidly-attached load. We compare the experimental results to the theoretical results predicted by the simple model.Copyright