This paper introduces a geometrico-static analysis of an intrinsically safe parallel manipulator called R-Min. This robot was designed to reduce the risk of injury during  
a collision with a human operator, thanks to an underactuated architecture which enables large internal displacements in case of a collision.   
Indeed, the R-Min architecture is based on a modification of the well-known planar five-bar mechanism, where additional passive joints are introduced on the distal links in order to create a planar seven-bar mechanism with two degrees of underactuation. These two additional degrees of freedom are passively driven through the use of a supplementary passive leg, in which a tension spring is mounted between the base and the end-effector.

In this paper, the conditions satisfying the equilibrium and the stability of the mechanism are introduced, based on a geometrico-static analysis.   
The direct and inverse problems are then solved using a numerical approach.   
Solutions to both problems are presented and classified.   
One subset of solutions to the inverse problem is isolated and projected in the Cartesian space in order to obtain the payload-invariant workspace of the R-Min robot.

Introduction :

Collaborative robots are intended to help humans with arduous working situations. Operating in the vicinity of human beings requires robots to evolve in highly dynamic environment and deal with unpredictable behaviour of operators while ensuring their safety in case of unavoidable human-robot collision.

Avoiding human injuries can be achieved through the use of proprioceptive force sensing technologies allowing the detection of collision and the appropriate reaction of the robot defined by the controller. However, although being able to detect a collision, conventional robots must operate at low-speed, due to their high mass and inertia, in order to fulfill the energy thresholds given in the standard ISO/TS 15066. Other strategies are based on exteroceptive sensors allowing to anticipate dangerous situation and adapt the robot's behaviour with respect to the operator's proximity. In practice, this approach relies on algorithms that are hard to certify.

The drawbacks of the control approach highlight the need for intrinsically safe robots through proper mechanical design. This can be achieved by designing robots with lighter architecture. However, these robots remain stiff and must operate at low-speed to be able to react fast enough to a collision. A complementary approach consists in using compliant joints to reduce the peak force during a collision, but this kind of joints lead to complex, heavy and costly mechanisms.

In a previous paper, authors introduced the R-Min robot whose architecture is based on the well-known planar five-bar mechanism, where additional passive joints are introduced to the distal links. The obtained mechanism is a planar seven-bar mechanism with two degrees of underactuation, authorizing the robot to passively self-reconfigure in case of collision. A supplementary passive leg, in which a tension spring is mounted, is added between the base and the end-effector in order to constrain the additional degrees of freedom.

Simulations were conducted in order to analyze the impact force and the HIC (Head Injury Criteria) during a collision with the head of an operator. The results of these simulations showed that R-Min is intrinsically much safer than a regular rigid five-bar mechanism.

The purpose of the present paper is (i) to investigate the computation of the forward and inverse geometrico-static problems of the R-Min robot and (ii) analyze its workspace properties. As a result, the paper is divided as follows. Section II introduces the robot architecture and recalls its behaviour under the application of several types of loadings. Section III explains the resolution of the the forward and inverse geometrico-static problems. Indeed, because the robot is under-actuated, its equilibrium configurations are necessarily defined by both the geometric and the statics equations. In Section IV, we analyze the joint and Cartesian robot spaces, and identify solutions to the direct and inverse geometrico-static problems. Finally, in Section V, conclusions are drawn.


Geometrico-Static Analysis of a New Collaborative Parallel Robot for Safe Physical Interaction

Division / Sponsors: 
Design Engineering Division and Computers & Information In Engineering Division


Paper Proceedings: https://asmedigitalcollection.asme.org/IDETC-CIE/IDETC-CIE2020/volume/83907

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