Time
Second
(a)PRA
1 2
-10
T ime
Second
(b)Fig. 3.37 a) The Speed I nput of the D i e l e s s Wire Drawing P r o c e s s b) The PRA o u t p u t of t h e D i e l e s s Wire Drawing P r o c e s s
Z.O.H.
S e t p o i n t The PRA
o u t p u t
F i g . 3.38 Block Diagram of t h e Closed-Loop Speed C o n t r o l System
F i l t e r i n g I n i t i a l i z a t i o n Send c o n t r o l o u t C a l c u l a t e t h e PRA p r o p o r t i o n a l c o n t r o l 20 ms time d e l a y s u b r o u t i n e Sampling d a t a Enable tim e r 1 and tim e r 2 Read d a t a from th e t im e r c o u n t e r s and LSB b u f f e r r e g i s t e r s
Fig. 3.39 The F l o w c h a r t of t h e C l o s e d - l o o p C o n t r o l System
3 .8 - R e s u l t s and D i s c u s s i o n T e s t P r o c ed u r e
The f o l l o w i n g p ro ced u re was f o l l o w e d t o c a r r y o u t a t e s t . The power s u p p l i e s f o r h e a t e r bands and o t h e r i n s t r u m e n t a t i o n were f i r s t s w i t c h e d on. The u s e r program was loaded i n t o t h e m icro co m p u ter from a t a p e and t h e program was e x e c u t e d from i t s s t a r t i n g a d d r e s s by an e x e c u t i o n command. F o l lo w in g t h e prompt on t h e s c r e e n , t h e o p e r a t o r i n p u t t h e s p e c i f i c a t i o n v i a th e keyboard, and th en th e program e x e c u t i o n was
r e s t a r t e d . The t e m p e r a t u r e o f the polymer m e l t in t h e polymer hopper and chamber were c o n t r o l l e d by t h e m icrocom puter a t th e p r e s e t v a l u e s . A f t e r
80 min u t e s f o r t e m p e r a t u r e t o r e a c h a uniform d i s t r i b u t i o n s t a t e , w i r e from t h e c o i l was passed th ro u g h t h e g u id es and over t h e p u l l e y b e f o r e b e in g i n s e r t e d and p u l l e d through th e PRA t r a n s d u c e r and t h e DRU, t h e n wound o n t o t h e b u l l block. The a i r p r e s s u r e was put on, t h e h y d r a u l i c pump was s t a r t e d and th e p r e s s u r e va lv e was t u r n e d down t o b u i l d h y d r a u l i c p r e s s u r e up t o 100 p si. Again th e c o n t r o l program e x e c u t i o n was r e s t a r t e d and t h e w i r e draw ing p r o c e s s s t a r t e d .
R e s u l t s
The h y d r a u l i c motor c o n t r o l l e d by t h e m ic r o co m p u ter o p e r a t e d
sm oothly a t i t s speed s e t p o i n t and th e p e rfo rm a n c e o f t h e speed c o n t r o l sy stem was show n by t h e s p e e d s t e p r e s p o n s e i n F i g . 3*30 (p. 1 0 3 ). The 0.M p e r c e n t q u a n ti z a t i o n e r r o r could l e a d t o a h u n t i n g problem a t slow speed i n which t h e system o u tp u t v a r i e d betw een two d i s c r e t e p o s i t i o n s ab o u t t h e p a r t i c u l a r s e t p o i n t . This h u n t i n g problem was n o t of much p r a c t i c a l i m p o r ta n c e s i n c e th e motor u s u a l l y o p e r a t e d a t h ig h sp e ed f o r maximum PRA . Even i n t h e t a p e r e d w ir e draw in g p r o c e s s i n w hich t h e draw ing speed v a r i e d in a r a t h e r wide r an g e and th e h u n t i n g problem
o c c u r r e d , t h e e r r o r cau sed by t h e h u n t i n g was s t i l l w i t h i n t h e a ll o w e d r a n g e of ±2 p e r c e n t . Fig. 3-40 shows t h e speed / t i m e r e s p o n s e o f th e h y d r a u l i c m otor d u r i n g a t a p e r e d w i r e d raw in g p r o c e s s , in which t h e speed r a n g e i s s e t from 0.02 ms"'' t o 1.14 ms” ^ and t h e t o t a l d r aw in g tim e i s a b o u t 5 s e c o n d s .
S t a r t draw in g
p r o c e s s F i n i s h draw ing p r o c e s s I n c r e a s e draw ing
speed t o 0.02 m/s
Fig. 3.40 Speed / Time Response During a Tapered Wire Drawing P r o c e s s
The t e m p e r a t u r e o f th e polymer both i n t h e polymer hopper and m e l t chamber was c o n t r o l l e d t h e r m o s t a t i c a l l y a t i t s p r e s e t l e v e l t o w i t h i n ±3°C v a r i a t i o n s and t h e e x p e r i m e n t a l r e s u l t s showed t h a t w i t h t h i s a c c u r a c y , t h e r e q u i r e m e n t of p r oducing p r o d u c t s of t h e c o r r e c t q u a l i t y was m e t .
During t h e d raw in g p r o c e s s w i t h a c o n s t a n t sp e ed , t h e PRA i n d i c a t o r worked w e l l over a wide measurement r a n g e from l e s s t h an 1 p e r c e n t t o t h e maximum p e r c e n t a g e r e d u c t i o n in a r e a . In p a r t i c u l a r , a t h i g h d r a w i n g speed, t h e f l u c t u a t i o n of t h e r e a d i n g o f t h e i n d i c a t o r was v e r y s m a l l . P l a t e 5 shows two p u l s e t r a i n s from e n c o d er s 1 and 2 r e s p e c t i v e l y a t a drawing sp eed of about 1 m s"1 , and a r e d u c t i o n in a r e a of a b o u t 10
p e r c e n t . The d i f f e r e n c e between t h e p e r i o d s o f t h e two p u l s e t r a i n s i s c l e a r l y e v i d e n t . However, when a r a p i d change o c c u r r e d i n d r a w i n g s p e e d , t h e r e a d i n g o f t h e i n d i c a t o r f l u c t u a t e d s e r i o u s l y and o n l y a f t e r a
r e l a t i v e l y lo n g p e r i o d , d id i t become s t a b l e a g a in . The m ost s e r i o u s c a s e o f t h e m easurem ent f l u c t u a t i o n i s i l l u s t r a t e d i n F ig . 3.^1* Here a f t e r a r a pi d change i n t h e d raw ing speed as shown i n Fig. 3.25
(p. 96), t h e r e a di n g s of the i n d i c a t o r f l u c t u a t e d over a wide r a n g e . In F ig. 3*^1, a l l t h e r e a di n g s above 11.6 p e r c e n t a r e f a u l t y and t h e a c t u a l m easured v a l u e s of th e PRA d u r in g t h i s p r o c e s s never showed l a r g e r
v a l u e s t h a n 11.6 p e r c e n t . The main problem c a u s i n g t h i s f a u l t y , r e a d i n g stemmed from th e m ec h an ical d r i v i n g p a r t s . The encoder was d r i v e n by w i r e t h r o u g h a " f r i c t i o n ” c o n t a c t . During th e p r o c e s s o f a r a p i d change in draw ing sp e e d , b e c a u s e of th e i n e r t i a o f t h e encoder and t h e d r i v i n g d i s c , s l i p b etw een t h e w ir e and th e d i s c m ight o c c u r. D uring t h e p r o c e s s o f a slo w change in draw ing speed, t h e i n d i c a t o r m a i n t a i n e d t h e c o r r e c t r e a d i n g .
The f l u c t u a t i o n i n r e a d i n g s d u r in g t h e p r o c e s s of a r a p i d change in draw ing speed gave r i s e t o d i f f i c u l t i e s i n u s i n g t h i s t r a n s d u c e r t o implement a c l o s e d lo o p c o n t r o l system . Some c a u t i o n had t o be e x e r c i s e d i n o r d e r t o r e d u c e t h e o s c i l l a t i o n d u r i n g t h e t r a n s i e n t p a r t o f t h e p r o c e s s . A d i g i t a l f i l t e r i n g method was employed i n sa m p le d d a t a
p r o c e s s i n g . Even so, t h e fi n a l c l o s e d lo o p c o n t r o l s y s te m a p p e a r e d n o t t o work a s w e l l as e x p e c t e d , and i t s p e rfo rm a n c e d id n o t meet t h e r e q u i r e m e n t o f d r aw in g t h e t a p e r e d w i r e .
A q u a l i t a t i v e t e s t was conducted t o examine t h e p r o d u c t s u s i n g d i e l e s s w i r e d r a w in g method c o n t r o l l e d by m ic r o co m p u te r . The r e s u l t s of