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Safety Light Curtains
Safety Principles
1-39
Safety Light Curtains
YES = Key operated guard locking with personnel key
may be required.
Can a crash stop cause machine or control system damage?
YES = Conditional guard locking may be required.
Will there be guard wear causing misalignment at the interlock
device?
YES = A noncontact device may be required.
Does the device need to be totally sealed e.g. for washdowns or
high humidity environment?
YES = A noncontact device may be required.
Is frequent access required?
YES = Control interlocking may be most suitable.
Is it likely that there will be attempts to cheat the interlock
device?
YES = A device with increased security may be required.
Safety Light Curtains
Safety light curtains are most simply described as photoelectric
presence sensors specifically designed to protect plant personnel
from injuries related to hazardous machine motion. Also known as
AOPDs (Active Opto-electronic Protective Devices), light curtains
offer optimal safety, yet they allow for greater productivity and are
the more ergonomically sound solution when compared to
mechanical guards. They are ideally suited for applications where
personnel need frequent and easy access to a point of operation
hazard.
Operation
Safety light curtains consist of an emitter and receiver pair that
creates a multi-beam barrier of infrared light in front of, or around, a
hazardous area. When any of the beams are blocked by intrusion in
the sensing field, the light curtain control circuit sends a signal to the
machines E-Stop. The emitter and receiver can be interfaced to a
control unit that provides the necessary logic, outputs, system
diagnostics and additional functions (muting, blanking, PSDI) to suit
the application. When installed alone, the light curtain pair will
operate as a control reliable switch.
To eliminate susceptibility to false tripping attributed to ambient
light and interference (crosstalk) from other opto-electronic devices,
the LEDs in the emitter are pulsed at a specific rate (frequency
modulated), with each LED pulsing sequentially so that an emitter
can only affect the specific receiver associated with it. While this
method lends itself to increased safety, light curtains must offer
further protection in terms of Control Reliability.
Control Reliability
Control Reliability is a concept that appears in the ANSI B11 series
of standards and OSHA 1910.217. The definition, appearing in the
most recent B11 standards is,
The capability of the machine control system, the
safeguarding, other control components and related
interfacing to achieve a safe state in the event of a failure within
their safety related functions.
Achieving a system that meets this requirement can be accomplished
by a design strategy, method or feature that separates the safety-
related functions of a system into components, modules, devices or
systems that can be monitored or checked by other components,
modules, devices or systems.
Applying the following three simple concepts help ensure the
performance of the safety function
Redundancyuse of identical methods
Diversityuse of dissimilar methods
Monitoringuse of verification methods
The selection and integration of components, modules, devices and
systems that have been specifically designed and intended for use in
safety-related functions also contributes to the achievement of
control. A disciplined design process is vital for achieving a
complete and accurate design, and should be used ensure that
control reliability is achieved.
In terms of opto-electronic safeguarding, this means the light curtain
system must be able to send a stop signal to the machine even in the
event of a component failure(s). Light curtains have two cross
monitored outputs that are designed to change state when the safety
light curtain sensing field is broken. If one of the outputs fails, the
other output responds and sends a stop signal to the controlled
machine and as part of the cross monitored system detects that the
other output did not change state or respond. The system would
then go to a lock out condition, which prevents the machine from
being operated until the safety light curtain is repaired. Resetting the
safety light curtains or cycling power will not clear the lock out
condition.
Types and Functions
Allen-Bradley Guardmaster safety light curtains fall into three
product families: Point of Operation Control (POC), Area Access Control
(AAC), and Perimeter Access Control (PAC).
Point of Operation Control (POC)
POC light curtains use a pair of optic heads to create a curtain of
infrared light that allows personnel frequent and easy access to point
of operation hazards. These light curtains are used when the
operator is required to interface with machinery at a relatively short
distance from the hazardous area. Allen-Bradley Guardmaster Point
of Operation Control light curtains are available in 14mm and 30mm
resolutions for finger and hand protection, respectively. 1-40
Safety Principles
Safety Light Curtains
Figure 71: POC
Area Access Control (AAC)
AAC is a long range system (up to 275 feet) that uses single-beam
emitter and receiver units to create a protective barrier around
hazardous machines including robotic cells, transfer stations,
palletizers and loading machines. Breakage of the beam is used to
stop the hazardous machine motion. Two-beam or multi-sided
access control is attained through the use of corner mirrors.
Figure 72: AAC
Perimeter Access Control (PAC)
PAC light curtains use an emitter and receiver pair to create a single-
or multi-beam barrier (1, 2, 3 or 4 beams) for full body detection
around hazardous machinery. When the guarded perimeter is
breached the safety light curtain sends a stop signal to the guarded
machine, stopping hazardous machine motion and protecting
personnel.
Figure 73: PAC
BlankingFixed and Floating
Blanking allows portions of a light curtains sensing field to be
disabled to accommodate objects typically associated with the
process to be ignored by the light curtain.
When this object is stationarye.g. mounting hardware, a machine
fixture, tooling or conveyorso is the blanked portion of the beam.
Known as fixed blanking, this function requires that the object be in
the specified area at all times. If any of the beams programmed as
blanked are not blocked by the fixture or workpiece, a stop signal
is sent to the machine.
Figure 74: Fixed blanking
Floating blanking allows an object such as feed stock to penetrate
the sensing field at any point without stopping the machine. This is
Work
Piece
Roller
Work
Path
Fixed
Blank
Beam Safety Principles
1-41
Safety Light Curtains
accomplished by disabling up to two light beams anywhere within
the sensing field. Instead of creating a fixed window, the blanked
beams move up and down, or float, as needed. Depending on the
number of beams floated, any one or two beams can be blocked
anywhere in the sensing field without the system sending a stop
signal to the protected machinery.
Figure 75: Floating blanking
When using blanking, fixed or floating, the Safety Distance (the
minimum distance the light curtain can be from the hazard such that
an operator cannot reach the hazard before the machine stops) is
affected. Since blanking increases the minimum object size that can
be detected, the minimum safety distance must also increase based
on the formula for calculating the minimum safety distance (see
Calculating Safety Distance, page 1-43).
Muting
Sometimes the process requires that the machine must stop when
personnel enters the area, yet remain running when automatically-
fed material enters. In such a case, a muting function is necessary.
Work
Piece
Press Brake
Die
Floating
Blank
Beam
Figure 76: Muting
Muting requires the combination of a light curtain, two or four
muting sensors and a control unit to process the signals and
determine if and when to activate the muting function. Muting
sensors are mounted in front of and behind the light curtain and
only a specific combination of sensor outputs will initiate the muting
function. For example, when the two sensors in front of the light
curtain change state within a predetermined timeframe, the light
curtain goes into muting. The muting sensors must be mounted far
enough apart so that personnel cannot feasibly walk into the area
and trip both muting sensors simultaneously and for a long enough
period to initiate the muting of the light curtain.
ANSI B11.19 states the safety perfo