Limit Switches Working Principles – Types of Limit Switches
What is Limit Switches :
Mechanical limit switches are contact sensing devices widely used for detecting the presence or position of objects in industrial applications. The term limit switch is derived from the operation of the device it-self. As an object (or target) makes contact with the operator of the switch, it eventually moves the actuator to the “limit” where the electrical contacts change state.
A limit switch is an electromechanical device that consists of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.
Mechanical Limit Switches :
Mechanical limit switches typically consist of a mounted actuator arm that operates a set of a electrical contacts when the arm is displaced.
Types Of Mechanical limit Switches :
Two examples are shown in illustrates the operation of a Lever-type limit switch and illustrates a plunger, or push-type, limit switch.
Working Of Limit Switches :
In the case of the lever type, the actuator arm is a rod connected to a lever shaft, which is free to rotate when the rod is displaced. When the forces displacing the rod are removed, the lever shaft is returned to its normal position by a return spring. The lever shaft has a roller mounted on its bottom, which rotates a rocker as it changes position from right side to left side. This mechanical action operates one or more sets of contacts, which are mounted on the other side of the limit switch, as shown in the back view. The rocker shaft is connected through the housing to the contact lever assembly, the head of which moves a set of electrical contacts. The electrical contact may either be closed or open initially. The action of the actuator and lever arms takes it from its normal, or deactivated, state to the other state. Hence, a normally open limit switch will be closed when activated and a normally closed limit switch will be open when activated.
Limit Switch Diagram :
Figure b. illustrates the more direct action of a push-type limit switch. It shows a set of contacts operated from the contact lever assembly. Contact set a-b is normally closed; set c-d is normally open. When the lever is depressed, each contact goes to its opposite state. When installed, the user wires the appropriate contact pair back to the controller, which distinguishes the state of the system by sensing the presence of voltage or current supplied through the contact.
Limit switches come in several varieties and designs; Figure simply shows two concepts. They are designed for heavy-duty applications in which there is physical
contact between the actuator and the process being sensed. For example, limit switches are often used on machine tools to limit the travel of a machine axis. They are sometimes used in materials handling applications, e.g. to indicate the passage of a part along a conveyor. They are typically designed to handle relatively high voltages, both AC and DC. This means they cannot be directly wired to the input port of a computer without having their signal converted to TTL level.
Features and Benefits of Limit Switches
- Can be used in almost any industrial environment
- Very precise in terms of accuracy and repeatability
- Consume little electrical energy
- Can switch loads with high inductance
- Can control multiple loads
Limitations of Limit Switches
- Generally restricted to equipment operating at relatively low speeds.
- Must make direct contact with target.
- Moving mechanical parts will wear out.
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