Grasp analysis for underacuated hands with synergies

This  post summarizes the research at SIRSLab on grasping with underactuated compliant robotic hands in the recent years. These studies have been realized  SIRSLab also in collaboration with other Universities and research centres, e.g. University of Pisa and DLR.

The complexity of robotic hands is needed to adapt themselves to the many kinds of tasks, but the large number of motors needed to fully actuate the DoFs comes at the cost of size, complexity and weight of devices. A possible approach to solve this problem consists of reducing the number of actuators thus resulting more efficient, simpler and reliable than their fully actuated alternatives. Reducing control inputs seems to inspire also biological systems and in particular motor control of human hands, which share with robotic hands the large number of DoFs.

Reducing the number of control inputs, from fully actuated joints to few synergies, might reduce the dimension of the force and motion controllability subspaces thus compromising the dexterity of the grasp. In [PrMaBi11] and later in [GaBiPrMa11] we introduced the compliance in the quasi-static model at the contact points, at the joints and at the synergy actuation synergies. In particular, the introduction of compliance at the synergy level was referred to as soft synergies.

The solution of the quasi-static grasp problem when only a few actuators are present in the hand is possible if compliance is introduced in the model. In

We investigated the main structural properties of grasping with underactuated hands and in particular to what extent a hand with many DoFs can exploit postural synergies to control force and motion of the grasped object.

An underactuated robotic hand with compliance at the contact, joint and synergy level.


The analysis of grasp in terms of possible motions of the manipulated object and reachable internal forces has been investigated for robotic hands with underactuation in [PrMaBi11] using explicit manipulation of input and output variables and in [PrMaGaBi11] an using implicit analysis based on the study of the kernel of a system of equations. Both approaches explicit and implicit lead to the same results but the explicit one is more easy to read in terms of control actions.

In [GaBiPrMa11] the authors investigated the role of synergies in the optimal choice of contact forces in grasping.

In [MaPr13] we described some preliminary evaluations on grasping properties with underactuated hands and in particular we evaluated grasp stiffness in hands with passive joints.


In [MaPr11, PrMaAgWi12] the authors studied an interesting problem: when the robotic hand has not enough degrees of freedom and present compliance, it is possible that if you change the internal force the grasped object moves, due to the different deformation of the equivalent contact springs. These papers implicitly consider the underactuated hands as those hands with fewer DoFs with respect to those needed to control internal forces without moving the object. The results of this study have been applied to a real robotic hand, the experimental tests are described in [PrMaAgWi13].

Main publications on this topic:

[PrMaBi11] D. Prattichizzo, M. Malvezzi, A. Bicchi. On motion and force controllability of grasping hands with postural synergies. In Robotics: Science and Systems VI, pp. 49-56, The MIT Press, Zaragoza, Spain, June 2011. [pdf]

[PrMaGaBi11] D. Prattichizzo, M. Malvezzi, M. Gabiccini, A. Bicchi. On the Manipulability Ellipsoids of Underactuated Robotic Hands with Compliance. Robotics and Autonomous Systems, Elsevier, 2012. [pdf]

[GaBiPrMa11] M. Gabiccini, A. Bicchi, D. Prattichizzo, M. Malvezzi. On the role of hand synergies in the optimal choice of grasping forces. Autonomous Robots, Springer, 31:235-252, 2011. [pdf]

[MaPr11] M. Malvezzi, D. Prattichizzo. Internal force control with no object motion in compliant robotic grasps. In Intelligent Robots and Systems (IROS), 2011 IEEE/RSJ International Conference on, Pages 1008-1014, September 2011. [pdf]

[PrMaAgWi12] D. Prattichizzo, M. Malvezzi, M. Aggravi, T. Wimboeck. Object motion-decoupled internal force control for a compliant multifingered hand. In Proc. IEEE Int. Conf. on Robotics and Automation, 2012. [pdf]

[MaPr13] M. Malvezzi, D. Prattichizzo, Evaluation of Grasp Stiffness in Underactuated Compliant Hands,  Accepted at IEEE Int. Conf. on Robotics and Automation, 2013. [pdf-draft]

[PrMaAgWi13] D. Prattichizzo, M. Malvezzi, M. Aggravi, T. Wimböck Compliant robotic hands with a low number of actuators: controlling grasping forces without affecting the object motion, submitted to the International Journal of Robotics Research, 2013.

Robotic hand, object displacements and contact forces obtained in the numerical experiments presented in [PrMaAgWi12], first row: controlling only the contact forces, without compensating the object motion; second row: with the proposed object motion-decoupled control. The left figures show the initial

hand and object configuration, the right ones the final.

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