A tensegrity structure is an assembly of compressive elements (bars) and tensile elements (cables, springs) held together in equilibrium [1],[2]. Tensegrity is known in architecture and art for more than a century [3] and is suitable for modeling living organisms [4]. Tensegrity mechanisms have been more recently studied for their promising properties in robotics such as low inertia, natural compliance and deployability [5]. A tensegrity mechanism is obtained when one or several elements are actuated. This work falls within the context of the AVINECK project involving biologists and roboticists with the main objective to model and design bird necks. Accordingly, a class of planar tensegrity manipulators made of a series assembly of several Snelson's X-shape mechanisms [6] i.e. crossed four-bar mechanisms with springs along their lateral sides, has been chosen as a suitable candidate for a preliminary planar model of a bird neck.

The prototype is built with two Snelson's X-shape mechanisms stacked in series. The bars are assembled in different layers to avoid self collisions. Each mechanism is composed of four bars and two springs. The manipulator is driven with cables parallel to the springs and passing through the axes thanks to small holes. Each cable is attached to a drum. The manipulator is actuated with two cables, thus, several actuation strategies can be used for this mechanism. In particular, one single mechanism can be actuated with two independent cables in an antagonist way, and the cables can be attached in different ways to the two modules. With the designed prototype, switching between actuation strategies can be done easily and several actuation schemes will be tested.

Structural parts (bars, support, drums) are 3D-printed in ABS. Each revolute joint between bars and axes is built with two bearings that ensure a long centering, and all parts are axially stopped with shaft collars. The sizing of the prototype has been chosen arbitrary. We decided to have a cross bar length of 100 mm and a top bar length of 50 mm. Those dimensions are adapted to several sets of springs available, i.e the considered springs are always in tension and not extended too much for all reachable positions of the manipulator. Once the length and spring stiffness is defined, the static model is computed in order to obtain the maximum input force for the cables. This force needs to be sufficient enough to actuate the mechanism in a large workspace and to resist external loads. The force applied by the cables is directly linked to the drum radius and motor torque. The drum radius also affects the translational speed of the cable. A compromise is made to have enough force and sufficient speed of the motor.

Two servo controllers interact with a microprocessor that contains the control loop. Each motor is equipped with an encoder to know the real position of the mechanism. The good behavior of the mechanism is ensured with a control loop.

[1] Fuller, Tensile-integrity structures, United States Patent 3063521,1962

[2] Motro, Tensegrity systems : the state of the art, Int. J. of Space Structures, 7 (2), pp 75–83, 1992

[3] Skelton et al., Tensegrity Systems. Springer, 2009

[4] Levin, The tensegrity-truss as a model for spinal mechanics : biotensegrity, J. of Mechanics in Medicine and Biology, Vol. 2(3), 2002

[5] P. Wenger and D. Chablat, Kinetostatic Analysis and Solution Classification of a Planar Tensegrity Mechanism, Robotica, https ://doi.org/10.1017/S026357471800070, Published online : 20 August 2018.

[6] Snelson, Continuous Tension, Discontinuous Compression Structures, US Patent No. 3,169,611.