The growth of the functional safety and the control of the clamping force of the grip devices
THE GROWTH OF THE FUNCTIONAL SAFETY AND THE CONTROL OF THE CLAMPING FORCE OF THE GRIP DEVICES
Eng. Marinela Cherciu1, Eng. Mugur Spirescu2, PhD. Eng. Iulian Vasile3
1,2,3National Instituteof Researchand Development for Mecatronics and Measurement
Technique – Bucharest, Romania
Abstract: Thepaperpresents someconstructive-functionalaspects of the development of a grip device, which canbedesignedfora wide range ofthe clamping forces,depending on the application and which allowsprogramming of the clamping force depending on the loadto bemanipulated, while ensuringhighoperationalsafety. Thegrip device canbe usedformanipulators androbotswithindustrial, laboratory, orotherspecialapplicationsallowing adaptation theconstructivesolutionaccording to the shape, sizeand weight ofobjectsto bemanipulated.
Keywords: robots, mechanisms, gripping.
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The Romanian Review Precision Mechanics, Optics & Mechatronics, 2012, No. 42
The growth of the functional safety and the control of the clamping force of the grip devices
1. Introduction
Inthe specific literature there area lot ofconstructivesolutions for grippingmechanismssuch asthosearticulatedlevers, witharticulatedlevers andbackstages, withleversandguides, slidingfingersetc., operatedhydraulic, pneumatic orelectric, directorviacammechanisms, screw – nut mechanisms,pinion-rack mechanisms,etc., the grippingfingerscando, if appropriate, movementsof rotation, translational, or plane-parallel. Strong development of the sensorsand transducers field, suchastheirdiversification andminiaturizationand increasingnumber ofcompaniesoperatingin this arealed tolower pricesfortheirproductionandtheir usewidelyforrigorouscontrolofprocesses. Inthis way, given the currentopportunitiesin the sensorsand transducers field, and considering thatINCDMTMhasexperiencein the robots andmanipulators field, in the Institutehasdevelopeda new constructive solutionfor agrip device. Thissolution,byintroduction of anelastic elementand aforce transducer in the drivechain, allows the programming andcontrol of the clamping force andmaintains constantpreload , evenifthe drivesignal is lost.
Figure 1 shows a longitudinal section (A - A) of the grip device. It is composed of: an electric actuator 1, which is fixed by means of a holder of motor 2 and of screws 3 and 8, on the grip device body 7; on the drive shaft of the electric actuator is attached, through a cylindrical pin 4, a drive screw 5, which transmits the motion of a slide 6, threaded for this purpose at one end and it can slide into the body 7 of the device, forming in this way a screw – nut mechanism ; in thetransmission of the motion, the rotation of the slide 6 is no possible by form, in cross section, the outer shape of the slide and the shape of the bore practiced in the body 7, in which glides, are square; at the other end, the slide 6 has a cylindrical bore, in which, using a cover 13, secured by some screws, are mounted prestressed a drive rod 9, provided at one end with a collar, a helical compression spring 10, a bushing with collar 11 and a force transducer 12, annular shaped; the rod drive 9 is guided and can slide in the cover 13; by mounting helical compression spring 10, in prestressed condition, is ensured the contact of the rod 9 with the slide 6, on the bottom of its bore, the elastic prestressing force being also transmitted through the bushing with collar 11, which can slide on the rod 9, to the force transducer 12, as preloaded force, with which it is calibrated; at the other end, the drive rod 9 has a leap of the diameter and it is threaded, having fixed, with two nuts 15, a plate 14, on which are articulated, by means of the pins 16, two link rods 17, that transmit motion to a gripping mechanism; the gripping mechanism consists of two fingers 24, articulated by means of the pins 23, two right levers 21 and two levers with elbow 22, which in turn are articulated by means of pins 20 and the spacer washers 19, on the body 7, so as to form two articulated parallelograms; the two articulated parallelograms receive the movement from the link rods 17, which are articulated of the levers with elbow 22, through the pins 18, the mechanism providing a plane parallel movement of the fingers 24.
To a displacement to the rightof the slide6, respectively at its pushing, themovementwillbetransmitted throughthe driverod9,theplate14 andthe twolink rods17 to the twolevers with elbow22,whichdrivearticulatedparallelograms, and thefingers24 willperforman opening movement.
To a displacementto the leftof the slide6, respectively whenitis pulled,throughthe same chainof transmission of the movementdescribed above, the fingers 24 willperform amovementnearby, the prestressingforceofhelicalcompressionspring10maintaining the contact between the rod9 and theslide6.When theobjectto bemanipulatediscaughtbetweenfingers24,thegrippingmechanismis blocked, the rod 9is blocked too, the movingfurtherto the leftofslide6leading to arelative movementbetween theslide6 and the rod 9, to the compression of the helicalcompressionspring10and thus toincreaseitselastic deformation force, force whichistransmittedto theforce transducer12throughthe bushing with collar11. The amount of thecompressiveforceofhelicalspring10 iscontinuouslymeasuredbyforce transducer12 andcomparedwiththe amountoriginally planned, and whenthe twovaluesareequal is isueda signal to cut-off the voltage oftheelectricactuator 1, respectivelyblockingmovement.
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The Romanian Review Precision Mechanics, Optics & Mechatronics, 2012, No. 42
The growth of the functional safety and the control of the clamping force of the grip devices
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The Romanian Review Precision Mechanics, Optics & Mechatronics, 2012, No. 42
The growth of the functional safety and the control of the clamping force of the grip devices
The system thusblocked ensures the transmission of the elasticdeformationforce of thehelicalcompressionspring10 to the fingers 24, thusmaintainingconstant the clampingforce and rigorouslycontrolled.
Afterhandling of the object,in order ofhis release, the actuator1 willtransmit throughthe drivingscrew5,a moveto the rightof theslide6,having initiallybeen arelativemovementbetweenthe slide9 and the rod6, without moving the rod9 and the grippingmechanismuntiltherod9isnotin contactwith theslide6,whilethespring10extends, then, the continuing displacement of theslide6istransmittedthroughthe rod9 to thegrippingmechanism, and the mecanism will open, thus releasingthemanipulated object.
Thegrip device can be designedforhandlinga widerange ofobjects, the length of the fingers and their formin the gripping areacan bedesignedfor objects with various dimensions andconfigurations. Inthis case, dimensional features of the elements of the grippingmechanismand the length of the fingers were chosen so thatat catchingof themanipulatedobject, the shortarms of the levers with elbow24 tobeextended, situation in which the twolink rods17 areparallel.
Figure2 shows thekinematicscheme of the grippingmechanismwith the following significance:
-length of the link rod: AB= a;
-lengthof the shortarm of the leverwith elbow:BO=r;
- length ofthe long armof the leverwith elbow:OC=R;
In order to determine whichis the opening"d" of thefingersaccording to the movementdriverod9, inFigure 3are shown schematically the link rod17 and the shortarm of the leverwith elbow22, intheinitialpositionABandBOand after theraceAA’= x, respectively A’B’ and B’O.
We make the following notations:
- AA’ = x;
- BOB’ = α;
- BA’B’ = β;
At first, theangleαis determinedaccording to the movement x:
α = A’OB’ - A’OB [1]
From the triangle A’OB’ it results:
A’OB’ = arccos [2]
From the triangle A’OB it results:
A’OB = arctg [3]
It results:
α = arccos- arctg [4]
Taking into account the Figure2 we can determine the opening “d” of the finger:
d = R[ sin (30° + α ) – 1/2] [5]
The opening of the grippingmechanismis
2d = 2R[ sin (30° + α ) – 1/2] [6]
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The Romanian Review Precision Mechanics, Optics & Mechatronics, 2012, No. 42
The growth of the functional safety and the control of the clamping force of the grip devices
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The Romanian Review Precision Mechanics, Optics & Mechatronics, 2012, No. 42
The growth of the functional safety and the control of the clamping force of the grip devices
Figure 4presents aaxonometric view of the grip devicethrough whichwe can forma clearer picture ofthe entire ensemble and itscomponents.
Regarding theadvantagesofusingthis solutionin the constructionof agrip device, we mention that the designed solutionisa relativelysimple construction, compactandrobust, which canbecarried out inmultipleversions, it provides acontrolledclamping force, the clampingforceiskept constanteven if the electricactuator poweris interrupted, clamping forcecanbescheduled according to theweight of the object to handle andnot the least it is suitable forminiaturization.
References
[1] Demian Traian ş.a. – Mecanisme de Mecanică Fină, Editura Didactică şi Pedagogică, Bucureşti – 1982.
[2] Demian Traian ş.a. – Bazele Proiectării Aparatelor de Mecanică Fină, Editura Tehnică, Bucureşti – 1986.
[3] Artobolevski I. I. – Mechanism in Modern Engineering Design, MIR Publishers Moscow 1979.
[4] Dorin Alexandru ş.a. – Roboţi Industriali şi Manipulatoare, Editura Tehnică, Bucureşti - 1985 .
[5] FESTO, AFAG, YASKAWA, ABB, TECHNOSOFT a.s.o. catalogues.
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The Romanian Review Precision Mechanics, Optics & Mechatronics, 2012, No. 42