Unitek UNIBOND 2, Руководство пользователя

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7 . 0 0 . T H E O R Y OF OPERATIOF
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7 . 0 1 . I n r e s i s t a n c e s p o t welders. i t i s o f prime importance t o d e l i v e r t h e same
amount o f energy t o each weld s i t e . assuming t h a t t h e weld s i t e s a r e i d e n t i c a l . F o r
t h i s reason, many schemes have been devised t o d e l i v e r a p r e c i s e l y c o n t r o l l e d b u r s t o f
voltage o r c u r r e n t . i n order t o provide uniform welds. T h i s would accomplish t h e o b -
j e c t i v e . i f t h e weld s i t e s were i d e n t i c a l . I n production s i t u a t i o n s . however, p a r t s
vary i n thickness. s u r f a c e f i n i s h , c l e a n l i n e s s and p u r i t y. T o f u r t h e r complicate m a t -
t e r s , e l e c t r o d e a r e a and pressure may v a r y w i t h t i m e . A l l o f these f a c t o r s i n f l u e n c e
the weld schedule. r e q u i r i n g an adjustment i n v o l t a g e a n d / o r c u r r e n t . b u t i n most i n -
stances, t h e instantaneous power r e q u i r e d w i l l remain t h e same.
Many devices have been designed which a l l o w t h e instantaneous power t o v a r y. a s a r e -
s u l t o f t h e aforementioned i n f l u e n c e s . a n d compensate f o r t h e e r r o r i n energy b y a d -
justing t h e d u r a t i o n o f t h e weld p u l s e p r o p o r t i o n a t e l y. T h i s w i l l work s a t i s f a c t o r i l y
over a narrow range, b u t h a l f t h e power f o r t w i c e t h e t i m e may n o t produce a f i n a l
temperature high enough t o y i e l d a s t r o n g weld, o r may produce a m e t a l l u r g i c a l l y d i f -
ferent w e l d .
The Unibond I I uses v o l t a g e and c u r r e n t feedback t o produce an output impedance which
i s approximately t h e same a s t h e i n t e r f a c e r e s i s t a n c e o f t h e weld s i t e . T h e power d e -
livered t o t h e weld versus t h e degree o f impedance mismatch i s shown i n Figure 2 4 .
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PERCENT
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W ER.
ito
80
10
60
SO
40
30
20
I 0
0
RL
R 0
Figure 2 4 - Percent o f Power d e l i v e r e d t o t h e Welding Load a s a f u n c t i o n
of t h e R a t i o o f Load Resistance t o Unibond Output Resistance.
This p r i n c i p l e can be r e a d i l y demonstrated by l o o k i n g a t t h e simple voltage d i v i d e r
shown i n Figure 2 5 . T h i s a r t i f i c i a l o u t p u t impedance can be produced i n an a m p l i f i e r
by reducing t h e v o l t a g e feedback b y a f a c t o r o f two. t o double i t s v o l t a g e g a i n , a n d
making up t h e d i f f e r e n c e w i t h c u r r e n t feedback.
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