22 Rockwell Automation Publication 450L-UM001D-EN-P - December 2019
Chapter 3 Operation
Sequential pulses of infrared light are sent between the two sticks. The beam
status analysis and the processing are done by the receiver. When one or more
beams are interrupted, the two output signal switching devices (OSSDs) turn off.
A safety control, safety relay, or safety contactor evaluates the signal to stop the
dangerous state of the machine.
The first or last beam near the end of the sticks optically synchronizes the timing
of the emission and reception of infrared light pulses. These beams are referred to
as the synchronization beams. Because the GuardShield systems are optically
synchronized, no electrical connection is required between the transmitter and
receiver sticks.
The unique design of the GuardShield platform allows every 450L transceiver to
transmit and receives infrared light. In contrast to the classical architecture of
common pure transmitter and receiver light curtain systems, GuardShield
transceiver sticks exchange protective field information via infrared light. So
infrared light is not only sent from a transceiver working as a transmitter to a
receiver. As indicated in Figure 8
, a transceiver stick configured as a receiver
transmits some infrared light to the transmitter. The dashed lines in Figure 8
indicate the synchronization beams.
Figure 8 - Schematic Display of Beam Center Lines
The slightly slanted beams and the maximum operating angle of the IR light
require that a transmitter and a receiver stick are installed beyond a minimum
operating distance. A 450L system is safe against manipulation because a
transmitter stick cannot be installed directly in front of a receiver stick.
A GuardShield transceiver, which acts as a receiver, has two OSSD safety outputs
and depending on the plug-in that is used, one additional non-safety auxiliary
output. When the GuardShield transmitter and receiver are properly powered,
aligned, not obstructed, and the start/restart mode has been correctly initiated,
both the OSSDs and the auxiliary output are ON (output voltage = +24V DC).
The two safety OSSDs are cross-fault monitored and short-circuit protected.
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