Modeling of magnetic field of magnetic and electromagnetic chucks.

Modeling of magnetic field of magnetic and electromagnetic chucks. DAVID HELŠTÝN VŠB – Technical University of Ostrava, Faculty of Electrical Engineering and Informatics, Department of Electrical Machines and Apparatus, Ostrava, Czech Republic

Anotace Magnetické pole permanentních, elektropermanentních a elektromagnetických upínadel je předmětem tohoto článku. Ve spolupráci s výrobcem upínadel byly modelováním zjišťovány upínací síly na velikosti, drsnosti a materiálu upínaného předmětu, jajož I na pólové rozteči upínadel.. Pro užité materialy byly zjištěny BH charakteristiky, které sloužily jako jedny ze vstupních dat. Výsledky budou použity k návrhu nového upínače.

Abstract In our case the magnetic field in the magnetic and electromagnetic chuck and its working area is subject of this paper. In the cooperation with producer of magnetic clamping systems we researched effects of size and sort of materials of clamping object and fabric of chuck (pole pitch) for magnetic field. The changes of chucking forces in dependence on size (thickness) and sort of materials of the clamping object there was also researched. For all used materials was measured BH-loops, which was used as one of input data for modeling by final elements method (FEM) in program environment ANSYS. Results was used to design new electromagnetic chuck.

12. ANSYS Users’ Meeting, 30.září – 1.října 2004 Hrubá Skála

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Introduction Magnetic clamping systems producer (producer did not wish present his name and all results of our cooperation) entered VŠB-TU Ostrava, department of Electrical Machines and Apparatus, to try to find the solution of a new type of magnetic chuck and the optimization of older magnetic chucks in view of the use of new materials and new types of permanent magnets. To find out the course of magnetic induction, the size of a clamping force in dependence on the size and the thickness of a clamping workpiece and in dependence on the surface roughness by the help of modelling of these chucks was also our business. This paper will not deal with concrete chucks but will deal generally with clamping force dependence on size of clamping workpiece and dependence on surface roughness. The program ANSYS and Finite Element Method (FEM) was used for model creation and solution.

Model creation Finding out the characteristic values of materials and the course of the magnetic field and the modelling of this field are the principal problems of electrical engineering. First step for modeling is magnetic characteristics determination of all used materials. BH-loops and virgin magnetization curves of ferromagnetic materials and permanent magnets used for the chuck construction were measured. Curves were set as input data. There were created three magnetic chuck types models: Permanent magnetic chuck Electromagnetic chuck Electro-permanent magnetic chuck Permanent magnetic chuck in this case consists (not only) of a pole board, of a movable U form block with permanent magnets, an immobile W form block with permanent magnets and of a ferromagnetic baseplate. The U form block is nesting to the W form block. The pole board consists of steel and brass lamellas. (Fig.1) The magnetic force is independent on the outside power supply. Switch on and switch off are performed by a hand control or a pneumatic bar.


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Fig.1 Permanent magnet model with lamellas and workpiece.

Electro-permanent magnetic chucks have the same parts as permanent magnetic chuck and (Fig.2) in addition they include coils for permanent magnets magnetization and demagnetization. To switch on it is necessary to magnetize magnets, to switch off to demagnetize these magnets by voltage impulses of 80 200 ms duration. The coil current is about 260 - 300 A. These chucks haven’t movable parts and they are dependent on outside power – pulse source.

Fig.2 Electro-permanent magnet: 1.baseplate; 2.working outlet; 3.chuck head; 4.chuck side; 5.magnet; 6.pole; 7.epoxy resin; 8.magnetization and demagnetization coil

Electromagnetic chucks are wholly dependent on the outside power supply (generally 110 V DC). Their construction is without permanent magnets. (Fig.3)


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Fig.3 A quarter of model of electromagnetic chuck with coils

Magnets are replaced by electromagnetic coils. The coils may be located in the same direction as lamellas or perpendicularly to lamellas. (Fig.4)

Fig.4 The electromagnetic chuck: 1.north pole lamellas; 2.south pole lamellas; 3.coils; 4.base plate; 5.brass seal plate.

How to model the air gap between the pole board and the workpiece in all chuck types was the fundamental problem. Regarding the size of chucks (for example 310 x 600 mm) the air gap is exiguous. None the less to omit the air gap is not possible. For simplification we replaced roughness by air gap with proportions 12. ANSYS Users’ Meeting, 30.září – 1.října 2004 Hrubá Skála

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from 0,017mm to 0,4 mm in our case. It is necessary to split air gap to minimum three layers of mesh elements to obtain magnetic force in air gap in ANSYS. If we create air gap (thickness 0.017mm) only one mesh element was in this gap. We split this gap to three parts with accuracy 10-19m, but solution was impossible. So we must create three air gaps with total thickness 0,017 mm. The element with eight angular points was used for meshing. Magnetic scalar analysis was used for solution.

Results It holds generally for all chuck types that magnetic lines of force salient above a pole board to one half pole pitch height (Fig.5)

Material is saturated if a

workpiece thickness is lower and clamping force therefore decreases. Clamping force decreases also if surface roughness increases (air gap). (Fig.6)

Fig.5 Force dependence on workpiece thickness

Fig.6 Force dependence on air gap size


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As well the workpiece size and the workpiece location on the pole board are for the clamping force very important. Results for extreme workpiece size are given in Tab.1. The workpiece width was as size of pole pitch, the length as the length of the lamella.

Tab.1. Force dependence on workpiece position toward poles

Relative force [%] workpiece across poles

100

workpiece above south pole

7,17

workpiece above north pole

7,05

workpiece above ½ north and ½ 56,5 south pole

Conclusion Results will be used to development new chuck and optimizing chucks till this time produced. The weight of some chucks may be reduced by this optimizing about 10 to 15% and the use of new permanent magnet types reduces height of chucks and increases the clamping force.

References: [1]- ANSYS - Electromagnetic Fields Analysis Guide [2]- Draxler, K. Magnetic components and measuring, CVUT- Prague 1991 [3]- Dedek, L. Theory of electromagnetic field, VUT, Brno 1990 [4]- Levol, M. Electromagnet chuck solving, VŠB-TU Ostrava 1994 [5]-Jurek,M. Magnetic field of electro-permanent chuck solving, VŠB-TU Ostrava 1994 [6]-Helštýn,D. Permanent magnet chuck model, ANSYS user’s meeting, Znojmo


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