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  SISOM 2008 and Session of the Commission of Acoustics, Bucharest 29-30 May 3D – CAD MODEL OF A GRIPPING SYSTEM WITH THREE FINGERS – STRUCTURAL ANALYSIS Marian BOLBOE*, Petre ALEXANDRU** * Drd. ing. Univ. "Transilvania" Bra ş ov, e-mail: marian.bolboe@schaeffler.com ** Prof. dr. ing. Univ. "Transilvania" Bra ş ov, e-mail: alex.p@unitbv.ro The writing shows a new mechanical structure for gripping mechanism with three fingers. The gripping mechanism was designed and simulated CAD with Solid Works software. For this structure there was also made a structural analysis. 1. THE MECHANICAL STRUCTURE This writing presents a new mechanical structure for gripping systems wich are capable to assure the five phases of gripping: central (against the object), the catch, the maintainance, the deposition, and the detachment from the object. [2] The gripping system is composed of three fingers, each with three phalanxes. The relative positions of the fingers are equidistant, spatially disposed like a hood. The mechanism is capable to assure catching of the object not just with the third phalanxe; it can allso to catch the objects with at least two phalanxes. That means, the gripping mechanism can catch all kind of objects (all kind of geometries) and between objects and mechanism there are at least six points of contact. The structure is based on the following elements: pneumatic cylinder (1); bars (5, 6, 7, 9, 10, 11); phalanxes (4, 8, 12) and a passive locking mechanism (4-13-14). The slide 14 and joint „R” can be treated like passive joints or locking joints (pneumatik brake). The structure has an helix-spring for couppling phalanxe 4 and bar 3 (fig. 3). 0 1 235641314101112 8 79 R A BCEDGOPHKLMH'JN G'F'IFVIIIIIIIIVV Fig. 1 Fig. 2    3D – Cad model of a gripping system with three fingers – structural analysis 145 The entrapment mechanism of the finger is presented in a schematized shape in figure three, having 6 contours, with the graph from figure 2. 20632 i  MfkS  = − = − ⋅ = ∑ , i  f  ∑ - the sum of the joints mobility. S  is the cinematic degree of the mono-contour;  k= number of mono-contours (1) Fig. 3 2. ASPECTS REGARDING THE STRUCTURE’S MECHANISM OF OPERATION OF PHALANXES The functionality of the mechanism, which is presented in figure 2, is based on two aspects: 1) Joint „R” is open ( is not locked); pneumatik cylinder „A” acting phalanxe 4 with bars 1 and 2 (bar 3 is treated solidar with phalanxe 4 because of the helix-spring; in this case the mobility of joint „D”=0 ). In this situation, the parts 5 and 6 are fixed with part 4 ≡ 3 (the parts 5 and 6 are linked from phalanxe 4). Therefore all three phalanxes (parts 4, 8 and 12) can be treated like a rigid structure which is rotateing in  joint „G”. For the passive mechanism 4-13-14 joint „R” is free (figure five- a and b). Based on figure number three, we can see three kinematic parts: 1, 2, 3 ≡ 4 (13 and 14 are passive parts and the rest 5...12 are treated like rigid with 4). In consequence, the mobility summ of the mechanism is: Fig. 4- graph 0   1   256   3=4 13   14   10   11   12   8 7   9   R   A BC   EG   O   PHKL   M   H'   J   N   G'   F'   I   F   VII   IIIIIIVV ML   K   H'   IJ   N   HG   G'   F’D   O   P   C   B A   R   E   F14   1   2   3   4   6   13   5   7   9   10   11   12   8   spring    Marian BOLBOE, Petre ALEXANDRU  146   4131 i  MfkS  = − = − ⋅ = ∑ , i  f  ∑ - the sum of the joints mobility. i  f  ∑ =4 (A; B; C; G) (2) Fig. 5 -a Fig. 5- b 2) Joint „R” (pneumatik brake) is closed, so parts 14, 13, 4 are fixed. Engine „A” is acting part 3 (through parts 1 and 2). The spring is compressed and joint „D” is unlocked, therefore (through parts 5-6 and 7-8) phalanxe 8 is acted and through the parts 9-10 and 11-12, phalanxe 12 is acted. In this case, the graph of the structure is presented in figure 6. The passive mechanism 4-13-14 is in fact a safe-mechanism for locking of part 4. When the force of gripping (in phalanxe 4) is up to set value, joint „R” is closed, phalanxe 4 is locked on the object, but the phalanxes 8 and 12 continue to grip the object. (figure seven- a and b). The passive mechanism 4-13-14, will be fix; joint „D is acted, so we will find 16 joints (A, B, C, D, E, F, F’, G’ H, H’, I, J, K, L, M N,); k  =5. 16531 i  MfkS  = − = − ⋅ = ∑ , i  f  ∑ - the sum of the joints mobility. i  f  ∑ =16; S  is the cinematic degree of the mono-contour;  k= number of mono-contours (3) Fig. 7- a Fig. 7- b – axonometric view 1 23564=0101112 8 79 ABCEDHKLMH'JN G'F'IFIIIIIIIVV   Fig. 6  3D – Cad model of a gripping system with three fingers – structural analysis 147 Even if the mechanism has just one degree of mobility, it presents many posibilities of catching. The gripping system is designed to catch first with its first phalanxe (part number 4). This gives the possibility to have at least two points of contact between the object and the finger. In figure number eight we present the result of the simulation for example one (the finger is considered to be rigid). Fig. 8 a Fig. 8 b The simulation of the gripping process, for example of function number two (when the first phalanxe –part 4- is locked), is presented in figure number nine. Fig. 9 a Fig 9 b 3. CONCLUSIONS The precursory phases of execution and testing of the mechanical structure are held by performant software, capable to modelate virtual structures, to verify them, to make kinematic and dinamic complex analyses and to optimize them. Based on the simulation made by the authors, we reached the conclusion that, for catching cylindrical, spherical objects, or other complicated shapes, this gripping system presents a simple structure and flexibility and safeness in usage. The gripping mechanism is composed of three modules – one for every finger – each of it having one engine element (fig. 1). Another advantage is based on an equidistant disposal of the fingers, like a cupola shape. [1] Even if it has just three fingers (one degree of mobility per finger), the mechanism gives the possibility to touch the objects (even of complicated geometry) in at least six points. REFERENCES   1. BOLBOE, M., ALEXANDRU, P., 3D – CAD model of the antropomorphic gripping system with 4 fingers – structural and kinematic analysis , SISOM, Bucuresti, 2007.   2. STARETU, I., Sisteme de prehensiune , Ed. LuxLibris, Brasov, 1996.  
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