Cut Sheet

Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com V8-T12-7
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Limit Switches
Let’s now take an in-depth
look at the limit switch. It is
a commonly used device. If
you look around your kitchen,
you can find a number of limit
switches. For example, limit
switches stop your micro-
wave oven from operating
unless the door is closed, and
they ensure the light in your
refrigerator is only on when
the door is opened.
Remember, a limit switch is
a mechanical device that
requires the physical
contact of an object with
the switch’s actuator to
make the contacts change
state. The term limit switch
is derived from this operation
of the switch. As the object
(or target) makes contact
with the operator of the
switch, it eventually moves
the actuator to the “limit”
where the contacts change
state.
Limit Switch with
Adjustable Roller Arm
This mechanical action either
opens (in a Normally Closed
[NC] circuit) or closes (in a
Normally Open [NO] circuit)
the electrical contacts. The
contacts then start or stop
the flow of current in the
electrical circuit.
The switching function can
be used to control loads from
simple relays to high-current
solenoids, from logic devices
to PLCs.
Strengths and Weaknesses
As with all devices, the limit
switch has its strengths and
weaknesses:
Limit Switch Attributes
Applications
Limit switches are used in a
variety of applications.
Consider these:
Limit switches can be used to
turn off a washing machine if
the load becomes
unbalanced. In automobiles,
they turn on lights when the
door is opened.
In industry, limit switches are
used to limit the travel of
machine parts, sequence
operations or to detect
moving items on a conveyor
system.
Attributes
Strengths
Can be used in almost any industrial
environment because of its rugged design
Can switch high inductance loads up to
10 amps
Very precise in terms of accuracy and
repeatability
Low cost
Weaknesses
Moving mechanical parts wear out
Must touch target to sense
Relatively slow (five times/sec.) compared
to electronic devices
Limit Switch Components
A limit switch consists of
three main components.
Limit Switch Components
The actuator is the part of
the limit switch that physically
comes in contact with the
target. In some limit switches,
the actuator is attached to an
operating head. The
operating head translates a
rotary, linear or perpendicular
“triggering” motion into the
motion type needed to open
or close the electrical
contacts of the switch.
The switch body is the
component that contains the
electrical contact mechanism.
The terminal screw or screw/
clamp assembly necessary
for wiring is referred to as the
receptacle.
In the Workplace
At the Marathon T-Shirt Company, boxes of apparel approach
the end of the packaging line, ready to be stacked onto pallets.
A palletizer with suction-cup grippers picks up a box and swings
it around to a waiting pallet.
A Limit Switch in Action
How does the unit know it has reached its sixth layer of boxes?
When the pivot arm reaches the top of its vertical travel rod, the
arm hits a limit switch. The switch signals the system to send
the full pallet down line and sets up an empty pallet to restart
the process.
Actuator/Operating Head
Receptacle/
Terminals
Receptacle/
Terminals
Switch
Body