The Automatic operation of machine requires the use of switches that can be activated by the motion of the machine, limit switch is used to convert this mechanical motion into an Electrical signal to switch circuits.
A limit switch is an Electromechanical device that consists of a mechanical actuator linked to a series of electrical contacts. When an object comes in physical contact with the actuator, the actuator tends to move and this movement is used to either open or close an electrical circuit.
Limit switches are one of the easiest reliable way to convert mechanical movements into electrical signals.
Components of a Limit Switch
A limit switch consists of an actuator with operating head, the switch body mechanism and a series of electrical terminals that are used to connect the switch to the electrical circuit that is controlling.
The Actuator is a part of the switch which physically comes in contact with the target. In some limit switches the actuator is attached to an operating head which translates a rotary, linear or perpendicular motion to open or close the electrical contact of the switch.
The switch body contains the switch contact mechanism whose state is controlled by the actuator. The electrical terminals are connected to the switch contacts and enables wires to be connected to the switch through terminal screws.
Limit Switch Contacts
Limit switch contacts may be
1. Normally open which is represented as a NO,
2. Normally closed which is represented as NC,
3. Any combination of NO and NC contacts.
1. Normally Open Contact:
In normally open condition circuit connected with the limit switch is not linked with the power supply as a switch is normally open.
Let’s consider that a limit switch is used to switch ON a red light when an object comes in contact with the actuator.
Now when the actuator of the limit switch comes in contact with an object, the switch is actuated and the switch contacts gets closed and as a result the circuit gets completed and the red-light glows.
When the object moves away from the limit switch the actuator and the switch contact returns back to original position and as a result the circuit becomes open and the red light is switched OFF.
2. Normally Closed Contact:
Normally closed switch works in the opposite manner in this switch contacts are normally closed when the limit switch is not actuated.
When it is actuated, the contacts will get opened.
When the above circuit is connected in normally closed manner then the red-light glows even if the limit switch is not actuated. When it gets actuated, the light is switched OFF.
3. Combination contact of a NO and NC:
In this type different combinations of a NO and NC are used. Consider that we want a green light to switch ON during normal working condition and a red light to switch ON when the limit switch comes in contact with an object.
Then circuit of green light is connected in NC contact of the limit switch and the circuit of red light is connected in NO contact terminal of the limit switch.
Now during normal operation green light will be ON as it is connected in the normally closed contact
and when the limit switch comes in contact with an object the switch gets actuated and the red light switches ON.
As the object moves away the switch comes to the normal position and the green light is switched ON.
Working of a Limit Switch
Let’s take an example of a stamping or labelling system that stamps a logo on a product as it moves on a conveyor. The conveyor is used to lead the product to the stamping base and limits switch is used to detect the position of product on the conveyor.
When the product comes in physical contact with actuator of the limit switch the actuators movement results in the electrical contacts within the switch to close the electrical connection.
Limit switch uses this mechanical movement of the actuator to change the electrical switch state to activate stamping assembly. The stamping assembly presses the stamp on to the product.
As the product moves after stamping the actuator and switch state moves back to normal position.
Different Types of Actuators in Limit Switch.
Depending on the application limit switches may use different types of Actuators.
Choosing the proper actuator depends on:
Shape, Speed, Direction and Total travel of the machine part being used to trip the limit Switch.
Now we will discuss some of them.
In this type of limit switch livers and roller livers rotate on an axis. Roller livers with a roller to lessen the friction and the force required to activate the liver. Liver spring return to neutral when the forces removed roller livers are the most popular option for operators.
Adjustable Roller lever:
An adjustable roller lever is similar to a standard roller lever. The length of the lever is adjustable from a few millimeters to up to 5 cm.The rollers are also available in various diameters.
Plunger heads comes in various varieties and sizes. Plungers are actuated by a perpendicular force applied directly to the end of the plunger.
When a plunger switch is needed but the force will not be applied directly at a 90 degree angle a roller plunger can be used. The roller converts some of the non-perpendicular force into perpendicular force that can actuate the plunger.
Whisker Limit Switch:
This type of limit switch can detect motion in any direction, that is any movement of the whisker can cause the switch to activate. This type of switch is also much more sensitive than other type of limit switches.
Now let’s discuss another the type of limit switch which is commonly found in control circuits that is the Micro limit switch or Micro switch
Micro limit switches are much smaller than standard limit switch allowing them to be installed in narrow or small spaces that would normally be inaccessible to other switches.
Micro switch needs very less amount of force to operate. It operates at a very rapid speed. A micro switch is also known as a miniature snap actions switch.
Micro limit switches are available in different case styles and with different operating levers.
Parts of a Micro Switch:
Micro limit switches generally consist of a common terminal C, normally closed terminal NC & a normally open terminal NO. Micro switches usually have an actuating plunger that only has to travel a small distance in order to trigger the contact sequence.
The actuating plunger must be pressed a predetermined amount before it activates. A small degree of movement is able to change the contact positions due to a spring loading mechanism that causes movable contacts to snap between NO and NC terminals.
Working of a Micro Switch:
Here the input is given to the common terminal. When the liver is pressed the switch is a normally closed mode that means the liver terminal C or the movable contact is connected to the NC terminal and there is no contact between C and NO terminals.
So initially Green LED glows which is connected at the NC terminal of the switch.
When slight force is imposed on the liver the C and NO terminals get connected and the Red LED glows as there is now contact established between C and NO terminals.
As there is no contact between C terminal and NC terminal the green light will be OFF.