A limit switch is a mechanical device that is commonly used in industrial automation systems to detect the presence or absence of an object and to trigger a response. It works by using a movable actuator that comes into contact with a stationary target such as a lever or roller to open or close an electrical circuit. The basic working principle of a limit switch is based on the mechanical movement of the actuator and the electrical circuit it controls.
This article will provide a technical explanation of the limit switch working principle.
How Does a Limit Switch Work?
The limit switch operates only when a predetermined limit is reached and it is usually actuated by contact with an object such as a cam.
One of the most common detection sensors found in automated systems is the limit switch. A mechanical limit switch is comprised of a switch and the operator (actuator). The most common switches have a contact block with one N.O. and one N.C. set of contacts. There are a variety of operators including lever-type, push roller-type, wobble stick and fork-lever.
A limit switch detects the presence of an object when the object contacts the lever of the switch.
The limit switch is a very common type of mechanically operated motor control device. Many industrial production lines use this type of switch to limit the travel of various devices on the line.
This device takes the place of human operators. It is often used in the control circuits of machine processes to govern the starting, stopping or reversal of motors.
The limit switch is constructed of two main parts: the body and the operator head (also called the actuator). The body houses the contacts that are opened or closed in response to the movement of the actuator. Contacts may consist of the normally open, normally closed, momentary (spring-returned) or maintained-contact types. The terms “normally open” and “normally closed” refer to the state of the contacts when the switch is in its normally deactivated state.
The standard symbols used to represent limit switch contacts are as follows:
- The N.O. held closed symbol indicates that the contact is wired as an N.O. contact but when the circuit is in its normal off state, some part of the machine holds the contact closed.
- The N.C. held open symbol indicates that the contact is wired as an N.C. contact and some part of the machine in its normal off state holds the contact open.
- A contact block with one N.O. and one N.C. set of contacts is the most common configuration. When you have two or more sets of contacts in a limit switch that are electrically isolated, you must wire the loads that these contacts are controlling on the same side of the line.
The figure below shows the control circuit for starting and stopping a motor in the forward and reverse directions with two limit switches providing overtravel protection.
The operation of the circuit can be summarized as follows:
- Pressing the momentary forward push button completes the circuit for the F coil, closing the normally open maintaining contact and sealing the circuit for the forward starter coil.
- At the same instant, the normally closed interlock contact F opens to prevent the reverse direction of the motor.
- To reverse the motor direction the operator must first press the stop button to de-energize the F coil and then press the reverse push button.
- If the overtravel position should be reached in either the forward or reverse direction, the respective N.C. limit switch will open to prevent any further travel in that direction.
- The forward direction is also interlocked with a normally closed R contact.