Cut Sheet

V8-T12-8 Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Limit Switch Movement
Let’s take a closer look at what actually happens as a limit
switch is activated. Imagine a target object moving toward a
limit switch actuator.
Limit Switch Movement
1. The actuator is at its
initial position. The limit
switch contacts are in
their normal
“untriggered” position.
2. Contact is made with the
target object and the
actuator moves its pre-
travel distance. Contacts
are still in their normal
“untriggered” position.
3. The actuator reaches its
operating point where
the contacts change
from their normal
“untriggered” position to
their “triggered”
position.
4. The differential is the
difference between
the operating and release
point. Differential is
engineered into the
switch to guard against
the effects of vibration
and rapid ON/OFF
oscillations of the
switch right at the
operating point.
Notes
In the case of a lever actuator, there is some over-travel allowing the
lever to move beyond the operating point.
On plunger actuators, the overtravel distance is a safety margin for the
sensor to avoid breakage.
The actuator begins the return to its initial position. The contacts return to
their normal “untriggered” position as the actuator reaches its release
point and resets the contacts.
Differential
Reset Point
Initial Position
Operating
Point
Pre-Travel
Over-Travel
Limit Switch Movement Definitions
Here are a few other terms
that are used in describing
the movement of the limit
switch actuator:
The operating force is the
force required to move the
actuating element.
The minimum return force
is the minimum force
required to return the
actuator to its initial position.
The total travel is the
maximum allowable distance
the actuating element can
travel.
The ability of a switch to
repeat its characteristics
precisely from one operation
to the next is called the
switch’s repeat accuracy.
In the Workplace
Inside this sawmill, a high-speed saw quickly reduces logs to
construction beams.
Limit Switches Working Where People Cannot
In the process, chips and dust hang in the air. Breathing is
impossible in the area without a mask. Even with goggles, it
would be impossible to inspect the cutting.
The production department devised a system of limit switches
to do the inspecting automatically. A remote operator can
configure a set of limit switches to allow the log to be cut to the
desired dimensions.