3 Fundamentals of press design

Press control systems
3.5.4 Structure of electrical control systems
Electrical control systems are broken down into four main areas:
– the conventional part,
– the programmable logic controller (PLC),
– input and output systems and
– the communication system.
The conventional part of the electrical system consists of the power
supply, drive controllers, the individual electrical and electronic control
devices and the low-voltage switchgear for power control monitoring
and attaching the components in the control cabinets and the
machine. The safety control represents a particularly important component
of the conventional part for all functions relevant to safety such
as the emergency stop, monitoring of safety gates or safety doors and
the engagement of the clutch/brake in mechanical presses.
In addition, the electrical control system must reliably prevent the
press from starting up or running through independently, following the
occurrence of any individual fault. In case of failure of any component,
it must ensure that no further press closing movement can be initiated.
The safety function of the conventional part of a hydraulic press
monitors the valves responsible for reliable standstill of a movement
which could lead to injury. These include the valves located in the lower
cylinder area which control die closure. Hydraulic redundancy is
achieved by means of two monitored valves. Failure of one of these
does not lead to the execution of a hazardous movement. If one valve
fails, the press’s valve monitoring circuit automatically brings the press
to a standstill.
The PLC controls and monitors all machine functions not covered by
the conventional control circuits and automatic die change, performs
fault diagnostics and provides the interface to other machine sections.
The sensors and actuators of the machine are connected to the central
PLC by means of the input/output system. There are two different
technologies being used: first the central input/output modules which
are integrated in the PLC racks or in additional units of the control cabinet;
second, on an increasing basis, local input/output modules within
the press connected via fast communication systems to the central PLC
unit (Fig. 3.5.3).
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
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