STIFF EU project on enhancing biomorphic agility through variable stiffness - DLR hands - logo by Ian Saunders - artificial arm and hand by TU Delft

STIFF is a research project on enhancing biomorphic agility of robot arms and hands through variable stiffness & elasticity. It is funded by the 7th framework programme of the European Union (grant agreement No: 231576).

Check our 2011 Summer School on Impedance

Institutional Partners
German Aerospace Center (DLR), Germany:
Project coordinator. Responsible for integrating a variable-impedance robotic system in the project. Development of a novel EMG system for human impedance measurements. Integration of human and robotic impedance control approaches.

Technische Universiteit Delft, Netherlands:
Responsible for modelling the human neuromuscular system from muscle to joint level. Developent of time varying system identification and parameter estimation techniques to quantify the model parameters from recorded data using haptic manipulators.

IDSIA, Switzerland:
Responsible for learning high-level task-specific controllers based on reinforcement signals for the flexible variable-impedance robot arm developed by DLR, and for inverse reinforcement learning to extract cost functions in collaboration with UEDIN.

University of Edinburgh, United Kingdom:
Responsible for the development of 'Optimal Feedback Control' based closed loop control paradigms, specifically tailored to redundant and variable impedance actuators. Developing methods to extract cost functions and comparing control policies to evaluate improvement in performance when modulating impedance optimally.

Université Paris Descartes - CNRS, France:
Responsible for studies of impedance control in humans, using a variety of techniques including direct physiologicial measurements (EMG, H-reflex), mathematical modeling and robotic simulation. The main emphasis is 1) to suggest biologically-inspired strategies to be applied to robotics control and 2) to use analogies with robotic devices to better understand human behaviour in terms of impedance.

artificial DLR hand grabs a glass; humanoid robot javelin thrower cartoon by Juergen Schmidhuber




Human Stiffness Behaviour

In the earliest studies of human arm impedance it was shown that the fundamental spring-like properties of the neuro-musculo-skeletal system generalise to multiple dimensions such that displacing the hand with a robot generates restoring forces toward the initial position. A fundamental question therefore is that of whether humans modulate limb stiffness in order to optimise performance depending on task constraints. Work toward understanding this question has been carried out on three fronts – methods, behaviour and neural mechanisms – during the second year of the STIFF project:

Methods: TUD has implemented methods to extract time-varying impedance values based on a new 'linear parameter varying' (LPV) algorithm originally developed for cyclic behaviour of wind turbines, while DLR has evaluated the use of their light weight robot arms as a means to perturb a human arm and measure stiffness in three dimensions. DLR has also developed a hand-held device to measure the stiffness of the human grip during manipulation tasks and are developing methods to map EMG to stiffness in an effort to measure human stiffness behaviour without having to perturb the arm.

Behavior: UPD and TUD have tested experimentally whether the principle of 'minimum intervention' governs the regulation of limb impedance for targeted movements of the hand. Whereas reducing the sensitivity of the limb to noise by having the hand move along a rigid constraint induced impedance changes consistent with this principle, increasing the tolerance for noise by relaxing the accuracy demands at the target did not. These human data are being modelled by UEDIN and IDSIA in the context of optimal feedback control to try and understand the performance criteria or 'cost-functions' that the CNS uses to optimize impedance in these situations.

Mechanisms: UPD is using physiological techniques to study the neural basis for human stiffness behaviour. They have looked at the sensitivity of the motoneuron to proprioceptive feedback from the muscle using the H-reflex technique, to clarify the relationship between stretch reflex gain modulation and the level of muscle force or net applied force on the environment. They have also prepared experiments to test for a direct influence of cortical signals on the reversal of the stretch reflex during catching, by means of TMS, and to test whether muscles act as natural wave-variable processors to counteract significant transmission delays in the neural feedback circuits.

artificial DLR arm and hand; artificial hand squeezes STIFF
artificial DLR hand holding a wine bottle