Monday, January 22, 2018

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Discrete Process Measurement - Proximity Switches

A proximity switch is one detecting the proximity (closeness) of some object. By definition, these switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity of objects.

Recall that the “normal” status of a switch is the condition of minimum stimulus. A proximity switch will be in its “normal” status when it is distant from any actuating object. Being non-contact in nature, proximity switches are often used instead of direct-contact limit switches for the same purpose of detecting the position of a machine part, with the advantage of never wearing out over time due to repeated physical contact. However, the greater complexity (and cost) of a proximity switch over a mechanical limit switch relegates their use to applications where lack of physical contact yields tangible benefits.

Most proximity switches are active in design. That is, they incorporate a powered electronic circuit to sense the proximity of an object. Inductive proximity switches sense the presence of metallic objects through the use of a high-frequency magnetic field. Capacitive proximity switches sense the presence of non-metallic objects through the use of a high-frequency electric field. Optical switches detect the interruption of a light beam by an object.

The schematic diagram symbol for a proximity switch with mechanical contacts is the same as for a mechanical limit switch, except the switch symbol is enclosed by a diamond shape, indicating a powered (active) device:

proximity switch symbols - Normally open and normally closed.

Many proximity switches, though, do not provide “dry contact” outputs. Instead, their output elements are transistors configured either to source current or sink current. The terms “sourcing” and “sinking” are best understood by visualizing electric current in the direction of conventional flow rather than electron flow.

The following schematic diagrams contrast the two modes of switch operation, using red arrows to show the direction of current (conventional flow notation). In both examples, the load being driven by each proximity switch is a light-emitting diode (LED):



modes of switch operation showing sinking and sourcing outputs

 

These photographs show two different styles of electronic proximity switch:

 

two kinds of proximity switches, same operation, different casings...

 

The next photograph shows a proximity switch detecting the passing of teeth on a chain sprocket, generating a slow square-wave electrical signal as the sprocket rotates. Such a switch may be used as a rotational speed sensor (sprocket speed proportional to signal frequency) or as a broken chain sensor (when sensing the rotation of the driven sprocket instead of the drive sprocket):

 

hotograph shows a proximity switch detecting the passing of teeth on a chain sprocket, generating a slow square-wave electrical signal as the sprocket rotates.
 
 
 

Click here to continue reading to the next page, Pressure Switches


Click here to go to the previous page, Limit Switches

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