What is an industrial limit switch?
The standard limit switch is a mechanical device that uses physical contact to detect the presence of an object (target). (opposite to Proximity sensors) When the target comes in contact with the actuator, the actuator is rotated from its normal position to the operating position. This mechanical operation activates contacts within the switch body.
limit switch of Siemens in operation
Some terminologies related to limit switches
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free position
The free position is the position of the actuator when no external force is applied.
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Pretravel
Pretravel is the distance or angle traveled in moving the actuator from the free position to the operating position.
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operating position
The operating position is where contacts in the limit switch change from their normal state (NO or NC) to their operated state.
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Overtravel
Overtravel is the distance the actuator can travel safely beyond the operating point.
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Differential travel
Differential travel is the distance traveled between the operating position and the release position.
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Release position
The release position is where the contacts change from their operated state to their normal state.
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Release travel
Release travel is the distance traveled from the release position to the free position.
Siemens limit switch terminologies explained
Different Operations
Momentary Operation
limit switch momentary operation
One type of actuator operation is momentary. When the target comes in contact with the actuator, it rotates the actuator from the free position, through the pretravel area, to the operating position. At this point the electrical contacts in the switch body change state. A spring returns the actuator lever and electrical contacts to their free position when the actuator is no longer in contact with the target.
Maintained Operation
limit switch maintained operation
In many applications it is desirable to have the actuator lever and electrical contacts remain in their operated state after the actuator is no longer in contact with the target. This is referred to as maintained operation. With maintained operation the actuator lever and contacts return to their free position when a force is applied to the actuator in the opposite direction. A forkstyle actuator is typically used for this application.
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