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h a s t o b e c u t and a l o g i c h i g h s i g n a l a p p l i e d t o p i n 38 of
J1.
jumper i n , p i n 38 i s h e l d low ( l o g i c z e r o ) , s o i n o r d e r t o e n a b l e t h e remote
s e t p o i n t t h e jumper h a s t o b e removed.
With t h e
To remove t h e r e m o t e - s e t - p o i n t - e n a b l e ,
remove t h e i n s t r u m e n t c o v e r
and c u t a jumper between
U10, U11,
and C 2 1 ( s e e F i g u r e 6 . 1 and 6 . 4 ) .
Once t h e s e t p o i n t h a s b e e n i n p u t , a n e r r o r s i g n a l i s d e t e r m i n e d ,
w i t h t h e e r r o r s i g n a l d e f i n e d as:
E
=
TAD
-
TSP
where :
TAD
i s c a l c u l a t e d t e m p e r a t u r e
TSP
i s s e t p o i n t t e m p e r a t u r e
The g a i n f u n c t i o n f o r t h e e r r o r s i g n a l i s g i v e n i n F i g u r e
4 . 4 .
The
f u n c t i o n i s based on t h e t e m p e r a t u r e and s e t p o i n t .
For a t e m p e r a t u r e which
i s more t h a n 2.55 K below t h e s e t p o i n t , t h e c o n t r o l power i s f u l l on ( o r on
f o r 100% of t h e c y c l e ) .
as t h e e r r o r r e d u c e s .
The p e r c e n t a g e of on t i m e i s g i v e n by:
From 2.55 K t o 0 K , t h e power i s reduced l i n e a r l y
100
E
%
Power On
=
2.55
For example, i f t h e e r r o r i s 1 . 5 2
°K,
t h e c o n t r o l power i s on 1 . 5 2 / 2 . 5 5 o r
59% of t h e p e r i o d .
However, i f o n l y t h e p r o p o r t i o n a l t e r m from F i g u r e 4.4 w a s c o n s i d e r e d
i n t h e d e t e r m i n a t i o n of t h e c o n t r o l s i g n a l , i t i s obvious t h e c o n t r o l l e r
would c o n t r o l below t h e s e t p o i n t .
T h e r e f o r e , t h e l i n e a r t e r m i s i n t e g r a t e d
( a n i n t e g r a l term a d d e d ) .
s i g n a l and i n t e g r a l t e r m added.
I n a n under t e m p e r a t u r e c o n d i t i o n , power i s
added w h i l e i n a n o v e r t e m p e r a t u r e c o n d i t i o n , power i s t a k e n away.
F i g u r e
4.5
shows t h e r e l a t i o n s h i p between e r r o r
The e r r o r l i m i t of 2.55 i s chosen f o r two r e a s o n s .
The f i r s t i s t h e
l i m i t a t i o n s of t h e hardware s i n c e one s o f t w a r e i n s t r u c t i o n ( o r one b y t e )
h a s
8
b i t s , t h e maximum d e c i m a l number t h a t can b e manipulated
i s
255 ( o r FF
b a s e 1 6 ) .
The second i s t h e p e r i o d of t h e F8 i n t e r n a l t i m e r .
The t i m e r t h a t
i s s e t f o r c o n t r o l g e n e r a t e s a n i n t e r r u p t e v e r y 4 m s e c .
The c o n t r o l s i g n a l
i s g e n e r a t e d by s e t t i n g a n e x t e r n a l f l i p - f l o p and c o u n t i n g t h e number of
i n t e r r u p t s u n t i l t h e pre-determined c o n t r o l count i s r e a c h e d .
A t t h i s p o i n t ,
t h e t i m e r s h u t s i t s e l f o f f and resets t h e f l i p - f l o p .
Here, 255 c o u n t s t h a t
are
4
m sec a p a r t e q u a l j u s t o v e r 1 second ( o r j u s t l o n g e r t h a n t h e p e r i o d
f o r t h e program t o r u n o n c e ) .
I n t h i s way, t h e r e i s c o n t i n u o u s h e a t on f o r
more t h a n 2.55 K below s e t p o i n t c o n d i t i o n .
A l i g h t h a s been hard-wired
t o i n d i c a t e when t h e power i s b e i n g
a p p l i e d t o t h e h e a t e r t e r m i n a l s .
T h i s l i g h t i s a decimal p o i n t i n t h e
lower r i g h t c o r n e r of t h e least s i g n i f i c a n t d i g i t .
4.3.6
D i g i t a l D i s p l a y Board
The d i g i t a l d i s p l a y d a t a P i n 1 and c l o c k P i n 2 and ground P i n 4
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