Presence Sensing Devices - Introduction
Presence Sensing Devices, also known as "Light Curtains" have been around for over 25 years.
There are over 15-20 different Presence Sensing Device manufacturers/marketers.
There are two basic choices for Presence Sensing Devices on the market, the 2-box system, and
the 3-box system.
- 3 - box systems
- Transmitter, Receiver, and Control box
- Presence Sensing Devices are now commonly being used to replace traditional
- Pull-backs, Type A & B gates, Fixed barrier guards
- They have many significant advantages over the traditional guarding methods:
- Freedom of movement for operator
- Flexibility of use, increased mounting options
- Reduction of operator fatigue
- Presence Sensing Devices increase productivity through:
- Quick set-up
- Less Maintenance
- Increased visibility of work area
Presence Sensing Devices - Basic Operation
The Transmitter unit sends
modulated infrared light from LED array to a corresponding array of photo-transistors
in the receiver unit.
When an opaque object interrupts
one or more of the beams, the light curtain sends a stop signal to the
Typically, the infrared
light is both pulsed and sequenced at a specific frequency. This is so
the photo-transistors in the receiver unit may be designed to accept only
the signal from the specific corresponding transmitter. This helps eliminate
the acceptance of ambient light or light from other external sources.
No electrical connection
is required between transmitter and receiver. Both are independently powered.
In a 2 - Box system: -Receiver
must be wired to main press control enclosure for interface of “stop signal”.
In a 3 - Box system -Both
receiver and transmitter must be cabled to the third box (i.e. presence
sensing device control box), which in turn is wired back to the main press
control enclosure for interface of “stop signal”.
With 3 - Box systems, the
“logic” elements are in the third (i.e. control) box. This typically allows
for a smaller profile for the transmitter and receiver, which can increase
the mounting options for the system, but slightly increases amount of interconnection
Presence Sensing Devices - Mounting
Lights must be mounted the
proper distance from the closest pinch-point as determined by the distance
formula. The OSHA formula for distance is as follows:
The Ansi formula takes into
account some additional factors that affect overall stopping time of the
machine, and is listed below:
Presence Sensing Devices - Control Reliability
“The device shall be designed
and constructed so that any single component failure, including output
devices, shall not prevent the normal STOP command from being sent to the
press, but shall prevent operation of the press stroke until the failure
has been corrected. In the event of a power failure to the device, it shall
initiate a STOP command to the press control system.”
To meet this requirement
manufacturers typically use self-checking circuitry to monitor the Presence
Sensing Device for internal faults. The requirement for control reliability
applies to the composite control system. If the press clutch/brake control
system does not meet control reliability requirements, addition of a presence
sensing device will not bring the composite control into compliance.
If internal fault is detected:
a stop signal is sent to machine control
the presence sensing device goes to locked condition
it remains so until failed component is replaced
a controlled reset must be used to restore to operative condition
Presence Sensing Devices - Other Terms
Blocking out one or more
beams to allow material or tooling to extend through sensing field without
sending stop signal
Allows any one beam to be
interrupted without stopping the press. The interrupted beam moves as material
passes through the sensing field
particularly useful in bending
operations with press brakes.
- The protective function is bypassed
(muted) during the non-hazarduous portion of the stroke (i.e. the upstroke).
- Used primarily for hand-fed
operations or where parts ejection would interrupt sensing field and stop
Presence Sensing Devices - Conclusions
The light curtain leads
to the initiation of stopping action of the machine. It does this by sending
a stop signal to the machine's control system. The machine’s control system
and brake must be functioning properly in order to stop the machine.
The requirement for control
reliability applies to the composite control system. If the press clutch/brake
control system does not meet control reliability requirements, addition
of a presence sensing device will not bring the composite control into
Do not use light curtains
on full-revolution presses or other machines which cannot be stopped in