Automation Instrumentation

What is a limit switch and how does it work?

What is the purpose of a limit switch? Is limit switch a sensor?

A limit switch is a low power switch which, is a snap-action device and it would open or close its contact according to the position of the mechanical part. This switch would operate when there is a motion of the machine parts or it would also operate during the presence of an object. So we can define the limit switch as a sensor due to its ability to determine the presence of an object. This switch can be used to control a machine as a safety interlock, or it can also count the object which passes through a specified point.

Limit switch representation

What are the major components of a limit switch?

Limit switch

Actuator

This is the limit switch section that would make contact with the object.

Head

This section of the limit switch would convert the actuator movement to the contact movement.  So if the actuator is moved properly then the switch contact would operate according to it.

Contact block

This part of the limit switch would cover the electrical contact element of the switch. Mostly it has two or four contact pairs.

Terminal block

This part has the screw terminations, and this is the place where the switch and the rest of the control circuits make the electrical connections.

Switch body

This section of the limit switch would cover the contact and terminal block

Base

This section of the limit switch would house the terminal block in a plug-in switch.

How does a limit switch operate?

This switch is an electromechanical device and its actuator is connected to a set of contacts. In case if any object comes near to the actuator or makes contact with it then this switch would make or break the electrical connection. The limit switch is named according to its operation. Mostly the limit switch would be composed of a spring-loaded push button and it would be connected to an electrical circuit. So when the object comes in contact with the push button (actuator), then this push button will be pushed inside and thus it would activate the electrical circuit. So after that, the electrical circuit would provide the signal to control the motion of the machine element.

What are the types of limit switches?

The actuator would be in rest position if no object or any force acts on it, the actuator would change its position in case if an object comes in contact with it and this position can be called an operating position.

Classification of the limit switch according to the actuator type

  • Side Rotary
  • Side or top push
  • Wobble stick

Side rotary actuator

Side Rotary Actuator Representation

This type of actuator can be considered as a shaft which will be sticking out from the side of a limit switch and it would operate the switch head during the rotation. This actuator can move in clock wise and anti-clock wise and it is capable for the unidirectional and bidirectional operation of the contacts. A lever arm would be attached to the shaft, so when the objects that passes would make contact with the lever and thus the switch will be activated.

Side or top push actuation

In this type of limit switch, it would have a short rod (button) on the side or top of a limit switch, and this would operate the limit switch when it is pressed. This rod has a spring return mechanism and it would come back to its original position, when the actuation force is removed. In certain type of rods, it doesn’t have any spring return and in this case it must be pushed in the opposite direction to reset the contacts.

Side or top push actuation limit switch representation

Wobble stick (Whisker actuation)

In this type of limit switch, its actuator is a long narrow rod, which can be seen on the top of the limit switch. This rod would operate the switch contact when it is deflected from the vertical position. These types of actuators are capable to operate in any direction and they would return to their original position if the force is removed.

Wobble stick type limit switch representation

Classification of the limit switch contacts

Maintained and momentary contacts

The limit switch contact would change its state if a force is applied to the actuator. A spring return limit switch contact would come to its original position during the removal of the operating force.

The limit switch that has maintained contact would stay in its actuated position till an opposite force is applied to it.

Two-circuit and four-circuit contacts

Mostly a limit switch commonly has two or four contact pairs. In order to open or close a control circuit, each contact pair would be utilized. Thus, the switches are called two-circuits or four-circuit devices.

Normally open or normally closed contacts

This type of contact shows the state of the contact when the switch is in an un-actuated position. The normally open contact would stay open if there is no actuation and the normally closed contact would stay close if there is no actuation.

Snap action contacts

In this type, the actuator movement would make the contact change its state, when the trip point or limit is reached. So when the actuator movement is reversed to a certain reset point it would make the contact snap back to its original state. We can use snap-acting contacts for continuous performance in applications which has low-speed actuators. The amount of travel of the contact is not dependent on the actuator’s amount of travel.

Slow make and break contacts

slow make and break contact movement

In this type of contact, the travel distance and the speed of the contact is depended on the speed and the travel distance of the actuator. Each contact pair has its own trip point. This can be used when we don’t need to change all of the contacts to change their state at the same time.

Direct open action contact

In this type of contact, the direct opening action would directly couple the force of the actuator to the contacts. So this force is really huge and due to this, it can open a welded contact. This type of contact can be integrated into the snap action and also the slow make and break limit switch.

How should we choose a limit switch?

  • We should consider the electrical rating of the limit switch
  • Limit switches are available in different operating modes and we can select this device according to it
  • Contact configuration
  • It should be selected according to the operating and releasing force
  • It should be selected according to the overtravel
  • It should be selected by considering the mechanical and electrical durability
  • It should be selected according to the torque force which is required for the operation
  • We should also consider the total travel and the movement differential

What are the advantages of a limit switch?

  • The absence of an object can also be determined by this switch
  • Ruggedness
  • The position of the object can be determined
  • Easy installation
  • Reliability
  • It can be useful for positioning
  • It can also determine the end of travel of an object
  • It can be used in a harsh environment
  • Durable
  • High repeatability

Electrical advantages of the limit switch

  • It can handle electrical noise interference
  • High power loads can be switched
  • It provides immunity to radio frequency interference
  • No leakage current
  • Voltage drop is minimum

What are the disadvantages of a limit switch?

  • Contact life could be short
  • Moving mechanical parts would wear out
  • We can only use this device for contact sensing

What are the applications of a limit switch?

  • Solenoid valve
  • Emergency pull chords
  • Conveyor system
  • Transfer machines
  • Milling and boring machine
  • Radial drills
  • This switch would be used with a control relay
  • It would be used with a motor contactor control circuit
  • It can also be used to provide inputs to PLC
  • Lathe machines
  • Automatic doors
  • Hoists and cranes
  • Elevators
  • Escalators

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