STEM’s
safety system is used to control the correct closing of guards,
gates or protections on dangerous machine tools. The safety
system is composed by magnetically coded sensors and magnets
associated to a control unit. The magnetic sensor detects
the presence or the absence of the coded magnet in the active
region, and converts it in a correspondent signal. The coded
magnet is a device that, with the alteration of the magnetic
field from itself created, makes possible a commutation process.
The control unit converts finally the signal from the sensor
in an output signal. The advantage in using a contact-less
safety sensor is on the absence of mechanical wear and on
the total protection against liquids and dust. The sensor
could be mounted behind a plastic or diamagnetic wall, without
drilling and with no changes in the activation distances.
Utilization of this system makes moreover impossible the evasion
of the sensor by a non-coded magnet.
SAFETY CATEGORIES
The Machine directive 89/392/EC – implemented in Italy
with the DPR 459/96- include the list of the safety essential
requirements that have to be implemented on the machines in
order to put on the CE mark and therefore being sold or manufactured
in the European Union Countries.
The contact-less safety sensors have to be conform to the
European safety standards.
The safety of a machine and the risk of injury for the operator
is determinated with the EN 1050 standard. Further information
are in the EN 292 (Safety of machinery- Basic concepts, general
principles for design), in the EN 1088 (Safety of machinery.
Interlocking devices associated with guards) and in the EN
954-1 (Safety of machinery – Safety-related parts of
control systems). The last standard is a guideline for the
design of the safety systems and applies to all the safety-related
parts of control systems regardless the type of energy used.
The EN 954-1 standard classify the control systems of the
machines and of the protecting equipment in five different
categories (B, 1, 2, 3, and 4), with increasing level of reliability
and safety. For example, a control system realized in category
3 gives more safety guarantee than a category 1 system.
The requirements of the five categories are described in the
following table extracted from the EN 954-1 standard.
TABLE 1
The guideline that follow is to determinate the correct category
(B,1, 2, 3, 4) of the control system upon the risk assessment.
The correct safety category could be chosen following the
below table
TABLE 2
As shown in the picture, the steps for the risk estimation
is based upon three different parameters:
| S |
The
usual consequences of accidents and normal healing
processes should be taken into account in determining
S1 and S2, e.g. bruising and/or lacerations without
complications would be classified as S1 whereas an
amputation or death would be classified as S2.
|
| F |
The period of exposure
to the hazard should be evaluated on the basis of
an average value which can be seen in relation to
the total period of time in which the equipment is
used.
For example, if it’s necessary to reach regularly
between the tools of the machine during cycling operation
in order to feed and move work pieces, then F2 should
be selected. If access is only required from time
to time, then F1 can be selected.
|
| P |
When an hazard
arises it is important to know if it can be recognised
and whether it can be avoided before it leads to an
accident. When a hazardous situation occurs P1 should
only be selected if there is a realistic chance of
avoiding an accident or of significantly reducing
its effect. P2 should be selected if there is almost
no chance of avoiding the hazard.
|
