Mechanical Impedance Control in the Human Arm While Manually Transporting an Open-Top Fluid Filled Dish

Abstract

The present study deals with stabilizing aspects of a hand-held dish filled with liquid while walking steadily. This is an attempt to decipher the neuro-muscular strategies employed and the mechanical responses of the arm during certain tasks of manual materials handling. The experimental configuration included a cup and the test-subject’s hand as an ‘end-effector’ of a serial three-link system representing the upper limb. These links are connected together by the wrist, elbow and shoulder joints. The tested subjects walked at constant speed on a treadmill while aiming to minimize liquid spillage from the cup. The motion of the limb and shoulder girdle served as inputs to a model to reveal the impedance adjustments during the simultaneous control of grasping and walking under ordinary conditions, and when one of the joints is affected. A regressive function used to express stiffness, included first-order dependence on angle and on angular velocity. The function used for damping included first-order dependence on angular velocity. Redundancies in the numerical solution were eliminated using multicollinearity diagnostic algorithms. The results revealed that the wrist joint was found to have constant stiffness and damping and no regulation of these coefficients was necessary during gait. Both in the elbow and shoulder joints stiffness included a constant coefficient as well as an angular velocity-dependent coefficient. Although all tested subjects demonstrated ability to prevent spillage of liquid, there was a considerable variability among the results obtained, indicating that the compensatory mechanisms employed by each subject to regulate the mechanical impedance were subjective. These results can help in the optimization of manual materials handeling tasks in industrial settings as well as future design of prosthetic arms, robotic appliances and man machine interfacing devices.

Mechanical Impedance and Its Relations to Motor Control, Limb Dynamics, and Motion Biomechanics

“…This sheds light on the design of spring-based artificial and robotic arms [14]. The wrist joint was found to have constant stiffness and damping and thus no regulation of these coefficients was necessary during gait.…”

“…Alternatively, velocity-based inverse-dynamics equations using Kane’s [14, 99] method can be used. For single-stance standing, open-chain inverse-dynamics was used [4, 100, 101].…”

“…Accordingly, in defining impedance characteristics, stiffness and damping coefficients have been mostly assumed to be variable [61, 167] with resulting dependencies upon deformation and rate of deformation [14]. …”

“…This method has been implemented for the modeling of various physical activities, including reaching motion for the upper limb [10–14] or, for the lower limbs, walking, hopping, and running. In this latter case, the foot- or heel-strike period during the landing phase has been modeled by means of one-dimensional lumped models (with linear displacements) or rotational lumping (with angular displacements) [15–23].…”

“…Indeterminacy of the locomotor system can also be addressed by implementing the lumping method, whereby the material elements of the human body, e.g., muscles, tendons, ligaments, bones, and joints, are lumped together in functional units so that the overall musculoskeletal system is represented as a one or more degrees-of-freedom (DOFs) damped elastic mechanism, interconnecting the masses of the body segments [ 9 ]. This method has been implemented for the modeling of various physical activities, including reaching motion for the upper limb [ 10 – 14 ] or, for the lower limbs, walking, hopping, and running. In this latter case, the foot- or heel-strike period during the landing phase has been modeled by means of one-dimensional lumped models (with linear displacements) or rotational lumping (with angular displacements) [ 15 – 23 ].…”