WO2004053470A2

WO2004053470A2 - Balancing spring for articulated mechanism, in particular for robot arm

Info

Publication number: WO2004053470A2
Application number: PCT/FR2003/050149
Authority: WO
Inventors: Yann Perrot
Original assignee: Commissariat A L'energie Atomique; Compagnie Generale Des Matieres Nucleaires
Priority date: 2002-12-03
Filing date: 2003-12-02
Publication date: 2004-06-24
Also published as: WO2004053470A3; FR2847958A1; FR2847958B1

Abstract

The invention concerns a balancing spring (8) between two articulated segments (2, 3), in particular of a robot arm, comprising a core made of composite material whereof the fibers have elongation direction identical to the spring tensile direction, in the alignment of the fastening points to the segments. Such a spring is compact and has high spring rate in said application.

Description

BALANCING SPRING, FOR ARTICULATED MECHANISM, ESPECIALLY FOR ROBOT ARMS

DESCRIPTION

The subjects of this invention are an articulated mechanism, suitable in particular for a robot arm, the essential feature of which is a balancing spring, and the spring itself. The usual robot arms are composed of a number of segments articulated together in chain and which move in a work volume according to the orders. The end of the arm exerts a sometimes important mechanical moment if it is related to the resistance capacities of the segments and of the force feedback mechanisms arranged at the joints to maintain the arm in the commanded position. We have therefore designed balancing springs arranged between successive segments to reduce the share of the moment that the force feedback mechanisms must resume. The usual springs sin in that they are generally too bulky, and at the same time often too flexible, since in this application they are subjected only to slight variations in elongation but sometimes to significant forces.

A spring of a new kind is proposed here, which satisfies these requirements of high stiffness and can be installed in robot arms of small section thanks to its reduced size. One aspect of the invention is therefore an articulated mechanism comprising a first hollow segment, a second segment, an articulation of the segments and a balancing spring disposed between a first attachment point secured to the first segment and a second attachment point secured of the second segment, characterized in that the spring is disposed in the hollow of the first segment and comprises a composite material containing fibers elongating between the two attachment points and tensioned, and parts for attachment to the attachment points; and a second aspect ^' is a spring meeting the preceding definition.

Other aspects of the invention will appear from the detailed description of the following figures: FIG. 1 is a general view of the mechanism of the invention,

- Figures 2, 3, 4, and 5 illustrate respective embodiments of the spring.

The first figure represents a robot arm 1 comprising at least three segments 2, 3, 4, etc. successive and linked together in chain by joints 5, 6, etc. Force feedback motors and angular position encoders are placed at the joints but, foreign to the invention, have not been shown. The first segment 2 is installed on a base 7 in a fixed or pivoting manner, here again according to known methods. If we particularly consider the first two segments 2 and 3 and their intermediate articulation 5 (but the invention can be placed between any pair of segments), we note that a spring 8 is placed between a first fixing point 9, integral with the first segment 2, and a second point of attachment 10, integral with the second segment 3 and close to the joint 5. The spring 8 is included in the tubular structure of the first segment 2, as is recommended both for the sake of aesthetics and to reduce the risks of 'accident. It is stretched between the fixing points 9 and 10, the latter of which is situated on the side opposite the extension of the arm 1 relative to the articulation 5, so that it helps to maintain the second segment 3 (and the following). The spring 8 characterizing the invention can take several different forms having several common characteristics. As shown in FIG. 2, it essentially consists of a core 11 arranged between two attachment parts 12 and 13 at the ends, fixed to the fixing points 9 and 10.

These attachment parts 12 and 13 may consist of rings which are slid over the fixing points 9 and 10 if they are materialized by cylindrical pins. Other forms of construction are obviously conceivable.

The core 11 of the spring 8 is made of composite material and comprises fibers extending from one attachment part 12 to the other 13 so as to take up the tensile forces exerted on the spring 8 by the second section 3. The fibers 13 are embedded in a hardened matrix as usual. The spring 8 thus constructed has good stiffness thanks to the favorable direction of the fibers combined with their rather low flexibility. The attachment parts 12 and 13, essential to the local resistance of the spring 8, are advantageously metallic, in this case steel, but which can also be made of aluminum for example. The union of the core 11 and the attachment parts 12 and 13 can be achieved in several ways. In the embodiment of FIG. 2, the fibers 14 are pre-impregnated fibers which form a strip 15 which is wound around the annular attachment parts 12 and 13 by forming yokes 16 and 17 there and closing the strip 15 on herself. The core 11 also comprises a block 18 disposed between the two parallel expanses of the strip 15 and glued to it to stiffen the core 11 well, without this block 18 participating much in the recovery of the tensile force.

In a typical embodiment, where the spring is responsible for balancing a robot arm such as that of FIG. 1 by exerting a moment of approximately 300Nm, it will have a stiffness of 5000 N / m, a Young modulus of 25000 MPa , a diameter of 15 mm and a weight of 0.15 kg. An ordinary metal spring, with helical coils, would typically have a stiffness of 20 N / mm, a diameter of 50 mm and a weight of 5 kg under the same conditions and to accomplish the same service. The smaller diameter allows the new spring to be accommodated in the envelope of the segment without modifying it, and the high stiffness gives a high reaction of the spring for only a small extension. Another spring 20 is shown in FIG. 3 and also comprises a central core 21 between two attachment parts 22 and 23 each comprising a metal ring 24 similar to that of the preceding attachment parts, but here extended by a pair of tongues 25 at least, between which is retained the end of a strip of pre-i pregnated fabric 26 composed as above of fibers elongating between the attachment parts 22 and 23 to properly take up the tensile forces exerted between - them, and of an all-encompassing matrix. If several of the pairs of tabs 25 exist, it is possible to accommodate as many parallel strips 26 to increase the stiffness and the resistance capacity of the spring. In several embodiments of the invention, it is possible not to immediately exploit the great stiffness of the fibers. The spring 28 in FIG. 4 comprises two attachment parts 29 and 30 similar to those of the previous embodiment and an intermediate core 31 here also formed of a strip of prepreg fabric but provided with a multitude of undulations 32 : the elongation of the spring 28 begins with the crushing of the corrugations 32 with a moderate stiffness, before the fibers of the core 31 work fully in traction with a much greater stiffness.

The spring 34 of FIG. 5 comprises, between two attachment parts 35 and 36, a core 37 composed of two strips 38 and 39 of prepreg fabric joining the ends and both retained between the pairs of tabs of the parts d '' clip 35 and 36, but which move apart in the center and retain a core 40 between them: the elongation of the spring 34 is here accompanied by a crushing of the cores 40, the resistance of which thus conditions the initial stiffness of the spring 34. When the core 40 is crushed, the stiffness of spring 34 here again depends on that of the fibers of bands 38 and 39.

The fibers may be glass, aramid or carbon in particular, but there is no reason to restrict in principle the list of materials usually used for composites for this invention.

Claims

1) Articulated mechanism comprising a first hollow segment (2), a second segment (3), an articulation (5) of the segments and a balancing spring (8) disposed between a first fixing point (9) integral with the first segment (2) and a second fixing point (10) integral with the second segment (3), characterized in that the spring is disposed in the hollow of the first segment and comprises a composite material containing fibers (14) extending between the two attachment points and tensioned, and parts for attachment to the attachment points. 2) Spring comprising, between two attachment parts (12, 13), at least one core (11) of a composite material containing fibers (14) elongating between the attachment parts.

3) Spring according to claim 2, characterized in that the composite material is a strip (11) wound in an endless loop around the attachment parts (12, 13).

4) Spring according to claim 2, characterized in that the composite material is at least one strip (26) having pinched ends between pairs of tongues (25) associated with the attachment parts (22, 23).

5) Spring according to claim 2, characterized in that the core has undulations (32). 6) Spring according to claim 2, characterized in that the core is composed of two arcuate bands (38, 39), an elastic core (40) being retained between the bands.

7) Mechanism according to claim 1, wherein the spring conforms to any one of claims 2 to 6.