Efficient Plant February 2018 - 26
feature | human/machine safety
the vicinity of
potentially whip up as bending occurs.
A second approach involves pull-backs
and restraints. These types of devices are
restrictive and have limitations-and operators hate them. Both shackle the operator to
a machine and restrict mobility.
The two-hand, down/foot-through device
is yet another approach. While this method
will work in some cases, it raises ergonomic
issues and is very slow.
Then there are light curtains and laser
active-optic protective devices (AOPDs).
They represent more advanced press-brake
safeguarding options. The diagram in Fig. 1
(p. 25) shows how these technologies work.
SHEDDING LIGHT ON SAFETY
Light curtains started out as simple product
detection devices, then developed into
machine-guarding solutions. Early versions
used incandescent lamps strung together
with a corresponding line of light detectors.
Presses were one of the first machine-safety
applications where safety light curtains were
used. A light curtain is a photoelectric
presence-sensing device. It protects against
access into hazardous points and areas.
These solutions range from very compact
to larger, more robust and resistant models
that can withstand demanding ambient conditions. Note that a stop-time measurement
(STM) device is needed to calculate the
safety distance on a regular basis, just as it is
needed with two-hand controls.
Safety light curtains safeguard personnel
in the vicinity of point-of-operation hazards. This is done with an LED transmitter
and receiver. Any interruption of the plane
of light by an object equal to/or larger than
the "minimum object sensitivity" initiates
an output signal. That could be a hand or a
finger or a misplaced tool, and it causes the
machine to stop or it doesn't allow a cycle
until the blockage is removed. To not initiate this output signal, the operator must be
outside the protected area through the entire stroke of the press-brake ram. The safety
distance between the light curtain and the
machine depends on the application, the
type of light curtain, and the machine's
OSHA has established the following set of
regulations for light curtains:
1. The machine must be able to stop the
movement of the ram anywhere in the
2. The stopping time of the ram must be
3. The stopping time of the ram must be
monitored for deviation in stopping
time on each stroke.
4. The minimum distance the light curtains can be located to the pinch point
must be known.
5. The light curtains must be control
6. The machine stop circuit, with which
the light curtains are interfaced, must
be control reliable.
7. The light curtains must be self
checking for proper operation on