Limit Switches

     
 

   The switch, which is one of the most basic of all sensors, comes in two types? normally open and normally closed. Prior to advances in sensor technology, mechanical switches were used extensively in control applications. Due to improved reliability and performance, mechanical switches are still used for this purpose, but they are primarily used where switch actuation and wear are minimal. The standard limit switch is a mechanical device that uses physical contact to detect the target. A typical limit switch consists of a switch body and an operating head.

Industrial Limit Switches showing different styles of actuator

   
     
     
     
     
     
     
           
 

    The switch body contains electrical contacts to energize or de-energize a circuit. The operating head incorporates a lever arm or plunger. This is also called an actuator. The actuator rotates when the target applies force. This movement changes the state of contacts within the switch body. Several types of actuators are available...
   The roller type actuator is most suited to applications where a sliding contact causing the rotary part to rotate would otherwise cause contact wear to take place over a period of time.

   
     
     
     
     
   

    The fork-style actuator must be physically reset after each operation and is suitable forLimit switch showing spring rod actuator critical stop applications in movement control. i.e. where a limit of movement has been exceeded and a manual reset is required following an emergency stop.
    Flexible loop and spring rod actuators can be actuated from all directions, making them suitable for applications where the direction of approach is constantly changing.
    Plunger-type actuators are ideal where short, controlled machine movements are present, or where space or mounting does not permit a lever-type actuator. The plunger can be activated in the direction of plunger stroke, or at a right angle to its axis.

   
       
                   
    All switches use the following common definitions of contact type....    
    Single Pole, Single Throw (SPST)
A switch that makes or breaks the connection of a single conductor in a single branch circuit. This switch typically has two terminals. It is commonly referred to as a "Single-Pole" Switch.

Single Pole, Double Throw (SPDT)
A switch that makes or breaks the connection of a single conductor with either of two other single conductors. This switch typically has 3 terminals, and is commonly used in pairs and called a "Three-Way" switch.

Double Pole, Single Throw (DPST)
A switch that makes or breaks the connection of two circuit conductors in a single branch circuit. This switch typically has four terminals.

Double Pole, Double Throw (DPDT)
A switch that makes or breaks the connection of two conductors to two separate circuits. This switch typically has six terminals and is available in both momentary and maintained contact versions.
   
       
       
       
       
       
                   
         
                   
 

.      .

 
 

Copyright pc-control.co.uk 2008