Hannes Höppner

Variable impedance actuators: A review

Variable Impedance Actuators (VIA) have received increasing attention in recent years as many novel applications involving interactions with an unknown and dynamic environment including humans require actuators with dynamics that are not well-achieved by classical stiff actuators. This paper presents an overview of the different VIAs developed and proposes a classification based on the principles through which the variable stiffness and damping are achieved. The main classes are active impedance by control, inherent compliance and damping actuators, inertial actuators, and combinations of them, which are then further divided into subclasses. This classification allows for designers of new devices to orientate and take inspiration and users of VIAs to be guided in the design and implementation process for their targeted application.

The DLR hand arm system

An anthropomorphic hand arm system using variable stiffness actuation has been developed at DLR. It is aimed to reach its human archetype regarding size, weight and performance. The main focus of our development is put on robustness, dynamic performance and dexterity. Therefore, a paradigm change from impedance controlled, but mechanically stiff joints to robots using intrinsic variable compliance joints is carried out.

Variable Stiffness Actuators: Review on Design and Components

Variable stiffness actuators (VSAs) are complex mechatronic devices that are developed to build passively compliant, robust, and dexterous robots. Numerous different hardware designs have been developed in the past two decades to address various demands on their functionality. This review paper gives a guide to the design process from the analysis of the desired tasks identifying the relevant attributes and their influence on the selection of different components such as motors, sensors, and springs. The influence on the performance of different principles to generate the passive compliance and the variation of the stiffness are investigated. Furthermore, the design contradictions during the engineering process are explained in order to find the best suiting solution for the given purpose. With this in mind, the topics of output power, potential energy capacity, stiffness range, efficiency, and accuracy are discussed. Finally, the dependencies of control, models, sensor setup, and sensor quality are addressed.

Analysis and Synthesis of the Bidirectional Antagonistic Variable Stiffness Mechanism

Variable stiffness actuation promises many benefits regarding mechanism robustness, energy efficiency, and dynamic performance. Here, we analyze the bidirectional antagonistic variable stiffness (BAVS) joint. A comprehensive overview of several aspects is given with a focus on the stiffness and torque characteristics of the joint. First, the functionality and properties of the abstract joint model are considered. Then, implementation details influencing the stiffness properties are discussed based on cam disc variable stiffness mechanisms. In general, an analytic approach is chosen to enable a generalization of the results. Experiments conducted on a BAVS joint of the variable stiffness actuated robot DLR Hand Arm System verify the theoretical findings.

Wrist and forearm rotation of the DLR Hand Arm System: Mechanical design, shape analysis and experimental validation

The DLR Hand Arm System is based upon the variable stiffness concept which has been recently developed to improve impact robustness and energy efficiency of modern robots. This paper continues the work on the bidirectional antagonistic variable stiffness (BAVS) joint concept which is an extension of antagonistic joints. Three mechanical setups utilizing different spring and cam disc combinations to implement a desired torque-stiffness characteristic are analyzed. Two BAVS joint solutions as used for the wrist and forearm rotation of the DLR Hand Arm System are presented. Furthermore in the experimental section torque-deflection calibration and drive redundancy are validated.

The Grasp Perturbator: Calibrating human grasp stiffness during a graded force task

In this paper we present a novel and simple handheld device for measuring in vivo human grasp impedance. The measurement method is based on a static identification method and intrinsic impedance is identified inbetween 25 ms. Using this device it is possbile to develop continuous grasp impedance measurement methods as it is an active research topic in physiology as well as in robotics, especially since nowadays (bio-inspired) robotics can be impedance-controlled. Potential applications of human impedance estimation range from impedance-controlled telesurgery to limb prosthetics and rehabilitation robotics. We validate the device through a physiological experiment in which the device is used to show a linear relationship between finger stiffness and grip force.

Task Dependency of Grip Stiffness—A Study of Human Grip Force and Grip Stiffness Dependency during Two Different Tasks with Same Grip Forces

It is widely known that the pinch-grip forces of the human hand are linearly related to the weight of the grasped object. Less is known about the relationship between grip force and grip stiffness. We set out to determine variations to these dependencies in different tasks with and without visual feedback. In two different settings, subjects were asked to (a) grasp and hold a stiffness-measuring manipulandum with a predefined grip force, differing from experiment to experiment, or (b) grasp and hold this manipulandum of which we varied the weight between trials in a more natural task. Both situations led to grip forces in comparable ranges. As the measured grip stiffness is the result of muscle and tendon properties, and since muscle/tendon stiffness increases more-or-less linearly as a function of muscle force, we found, as might be predicted, a linear relationship between grip force and grip stiffness. However, the measured stiffness ranges and the increase of stiffness with grip force varied significantly between the two tasks. Furthermore, we found a strong correlation between regression slope and mean stiffness for the force task which we ascribe to a force stiffness curve going through the origin. Based on a biomechanical model, we attributed the difference between both tasks to changes in wrist configuration, rather than to changes in cocontraction. In a new set of experiments where we prevent the wrist from moving by fixing it and resting it on a pedestal, we found subjects exhibiting similar stiffness/force characteristics in both tasks.

Key Insights into Hand Biomechanics: Human Grip Stiffness Can Be Decoupled from Force by Cocontraction and Predicted from Electromyography

We investigate the relation between grip force and grip stiffness for the human hand with and without voluntary cocontraction. Apart from gaining biomechanical insight, this issue is particularly relevant for variable-stiffness robotic systems, which can independently control the two parameters, but for which no clear methods exist to design or efficiently exploit them. Subjects were asked in one task to produce different levels of force, and stiffness was measured. As expected, this task reveals a linear coupling between force and stiffness. In a second task, subjects were then asked to additionally decouple stiffness from force at these force levels by using cocontraction. We measured the electromyogram from relevant groups of muscles and analyzed the possibility to predict stiffness and force. Optical tracking was used for avoiding wrist movements. We found that subjects were able to decouple grip stiffness from force when using cocontraction on average by about 20% of the maximum measured stiffness over all force levels, while this ability increased with the applied force. This result contradicts the force–stiffness behavior of most variable-stiffness actuators. Moreover, we found the thumb to be on average twice as stiff as the index finger and discovered that intrinsic hand muscles predominate our prediction of stiffness, but not of force. EMG activity and grip force allowed to explain 72 ± 12% of the measured variance in stiffness by simple linear regression, while only 33 ± 18% variance in force. Conclusively the high signal-to-noise ratio and the high correlation to stiffness of these muscles allow for a robust and reliable regression of stiffness, which can be used to continuously teleoperate compliance of modern robotic hands.

Blindfolded robotic teleoperation using spatial force feedback to the toe

This paper examines the capability to incorporate spatial force feedback to the human toe when teleoperating a robotic arm in a force task. Due to the growing complexity of teleoperated systems new means of feedback get increasingly important. To investigate the viability of spatial toe-feedback, experiments with 12 subjects were conducted. The participants had to teleoperate a DLR Light-Weight Robot (LWR) via optical tracking of one finger in order to push a toy train. The orientation of the rail was unknown to the subject and had to be explored using the haptic feedback — a three-dimensional spatial force to the toe, reflecting the contact forces at the robotic end-effector — in absence of visual feedback. The rail was mounted in one of four possible orientations (differences of 45°). The main task of the experiment was to identify the present orientation. In our study subjects could successfully identify the orientation of the rail in more than two thirds of all trials (68%). In almost half of the trials (44%) the subjects were able to move the train along the rails long enough to reach the bumpers at the end and identify them as such. Assuming no feedback would be provided at all, the first metric has a chance level of 25%, and reaching the bumper can be considered impossible. Thus, we can conclude that humans can incorporate spatial force feedback to the toe into their sensorimotor loop.

A new biarticular joint mechanism to extend stiffness ranges

We introduce a six-actuator robotic joint mechanism with biarticular coupling inspired by the human limb which neither requires pneumatic artificial muscles nor tendon coupling. The actuator can independently change monoarticular and biarticular stiffness as well as both joint positions. We model and analyse the actuator with respect to stiffness variability in comparison with an actuator without biarticular coupling. We demonstrate that the biarticular coupling considerably extends the range of stiffness with an 70-fold improvement in versatility, in particular with respect to the end-point Cartesian stiffness shape and orientation. We suggest using Cartesian stiffness isotropy as an optimisation criterion for future under-actuated versions.