3.5.4 Structure of electrical control systems
Elect rical co n t ro l syst em s are bro ken d o wn in t o fo u r m ain areas: – t h e co n ven t io n al p art ,
– t h e p ro gram m able lo gic co n t ro ller PLC, – in p u t an d o u t p u t syst em s an d
– t h e co m m u n icat io n syst em . Th e co n ven t io n al p art o f t h e elect rical syst em co n sist s o f t h e p o wer
su p p ly, d rive co n t ro llers, t h e in d ivid u al elect rical an d elect ro n ic co n - t ro l d evices an d t h e lo w-vo lt age swit ch gear fo r p o wer co n t ro l m o n it o r-
in g an d at t ach in g t h e co m p o n en t s in t h e co n t ro l cabin et s an d t h e m ach in e. Th e safet y co n t ro l rep resen t s a p art icu larly im p o rt an t co m -
p o n en t o f t h e co n ven t io n al p art fo r all fu n ct io n s relevan t t o safet y su ch as t h e em ergen cy st o p , m o n it o rin g o f safet y gat es o r safet y d o o rs an d
t h e en gagem en t o f t h e clu t ch brake in m ech an ical p resses. In ad d ition , th e electrical con trol system m u st reliably p reven t th e
p ress from startin g u p or ru n n in g th rou gh in d ep en d en tly, followin g th e occu rren ce of an y in d ivid u al fau lt. In case of failu re of an y com p on en t,
it m u st en su re th at n o fu rth er p ress closin g m ovem en t can be in itiated . Th e safet y fu n ct io n o f t h e co n ven t io n al p art o f a h yd rau lic p ress
m o n it o rs t h e valves resp o n sible fo r reliable st an d st ill o f a m o vem en t wh ich co u ld lead t o in ju ry. Th ese in clu d e t h e valves lo cat ed in t h e lo w-
er cylin d er area wh ich co n t ro l d ie clo su re. Hyd rau lic red u n d an cy is ach ieved by m ean s o f t wo m o n it o red valves. Failu re o f o n e o f t h ese
d o es n o t lead t o t h e execu t io n o f a h azard o u s m o vem en t . If o n e valve fails, t h e p ress’s valve m o n it o rin g circu it au t o m at ically brin gs t h e p ress
t o a st an d st ill. Th e PLC co n t ro ls an d m o n it o rs all m ach in e fu n ct io n s n o t co vered by
t h e co n ven t io n al co n t ro l circu it s an d au t o m at ic d ie ch an ge, p erfo rm s fau lt d iagn o st ics an d p ro vid es t h e in t erface t o o t h er m ach in e sect io n s.
Th e sen so rs an d act u at o rs o f t h e m ach in e are co n n ect ed t o t h e cen - t ral PLC by m ean s o f t h e in p u t o u t p u t syst em . Th ere are t wo d ifferen t
t ech n o lo gies bein g u sed : first t h e cen t ral in p u t o u t p u t m o d u les wh ich are in t egrat ed in t h e PLC racks o r in ad d it io n al u n it s o f t h e co n t ro l cab-
in et ; seco n d , o n an in creasin g basis, lo cal in p u t o u t p u t m o d u les wit h in t h e p ress co n n ect ed via fast co m m u n icat io n syst em s t o t h e cen t ral PLC
u n it Fig. 3.5.3
.
97
Press control systems
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
98
Fundamentals of press design
Fig. 3.5.3
PLC w ith central and distributed inputoutput systems
distributed IO
. . .
. . .
. . .
. . .
sw itch cabinet
PLC
machine
sensors and actuators distributed
IO module distributed
IO module distributed
IO module distributed
IO module
serial bus
central IO
. . .
. . .
. . .
. . .
sw itch cabinet PLC
terminal box terminal box terminal box terminal box machine
parallel individual signals
parallel individual signals
sensors and actuators IO
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th e co m m u n icat io n syst em co n n ect s t h e PLC, an d p ro vid es d at a exch an ge wit h t h e visu al d isp lays wit h in t h e m ach in e an d also co m -
m u n icat io n wit h ext ern al syst em s.
3.5.5 Functional structure of the control system