Limit Switch Explained - What it is, How it works & How it is used in Manufacturing
What is the Function of Limit Switch?
A limit switch is an electro-mechanical device used to send an electrical signal based on a physical interaction. Limit Switches are used to detect the presence of objects, thus allowing the system to take desired action.
Limit Switches can be found in a number of everyday devices:
- They are used in microwaves to check that the door is closed before operation is allowed.Â
- They are used in elevators to notify the system that the cabin has reached the desired floor.Â
- Limit Switches are used in washing machine lids as a permissive to start the wash cycle.
Limit Switches in Manufacturing
Countless limit switches are found in manufacturing. They are used as control devices and safety devices for machinery and personnel. In all cases, the limit switches will send a digital signal to the control system. Based on the hardware and software tied to said switches, the system is able to take appropriate action.
Why is the limit switch important?
Limit switches are an inexpensive way to create a link between the physical and electrical domains. They have been developed a number of decades ago and the mass adoption of their use significantly lowered their cost for the end user. They thus play an important role in manufacturing due to their simplicity and low cost.
Use Cases of Limit Switches
- Product Detection & Count - As a product pushes against a limit switch, a signal is sent to the control system. Through simple PLC ladder logic, the user can count the number of times the product goes by the limit switch and display the counter for the operator.
- Personnel Safety - A limit switch can be used to detect the opening of a safety guard that stops and de-energizes the machine. If the guard is opened during operation, the machine stops. If the guard is opened while the machine is stopped, the limit switch prevents the machine from starting. In both cases, the limit switch is used to safeguard the operator from potential harm.
- Machine Safety - A limit switch can be used to protect machinery from unintentional damage. This includes components that are part of changeovers (end of arm tools), components that may wear-out over time (motor clutch) and components that may damage others if they fail (gears, shafts, etc.).
Types of Limit Switches
Limit switches can be classified into four main types based on the type of actuator:
- Whisker: Whisker limit switches have a long, thin actuator that can be bent or deflected by even a small amount of force. They are often used in applications where space is limited, such as in robotic arms and grippers.
- Roller: Roller limit switches have a roller-shaped actuator that is rotated by the object being detected. They are often used in applications where the object being detected is moving quickly, such as on conveyor belts and packaging machines.
- Lever: Lever limit switches have a lever-shaped actuator that is moved by the object being detected. They are often used in applications where the object being detected is large or heavy, such as on machine tools and industrial cranes.
- Plunger: Plunger limit switches have a plunger-shaped actuator that is pushed in or pulled out by the object being detected. They are often used in applications where the object being detected is small or delicate, such as in electronic assemblies and medical devices.
In addition to the type of actuator, limit switches can also be classified by their electrical configuration. The two most common electrical configurations are:
- Normally open (NO): NO limit switches are open when the actuator is not activated and close when the actuator is activated.
- Normally closed (NC): NC limit switches are closed when the actuator is not activated and open when the actuator is activated.
Some limit switches also have a double throw (DPDT) electrical configuration, which means that they have two sets of electrical contacts, one NO and one NC. When the actuator is activated, the NO contacts close and the NC contacts open.
Limit switches are available in a wide variety of sizes, shapes, and materials to meet the specific needs of different applications. They are also available in a variety of electrical ratings, so that they can be used to control a wide range of loads.
Here are some examples of how different types of limit switches are used in real-world applications:
- Whisker limit switches: Whisker limit switches are often used in robotic arms and grippers to detect the presence of objects. They are also used in some types of safety interlocks.
- Roller limit switches: Roller limit switches are often used on conveyor belts and packaging machines to detect the presence of products. They are also used in some types of machine tools to detect the position of the cutting tool.
- Lever limit switches: Lever limit switches are often used on machine tools and industrial cranes to detect the position of the moving parts. They are also used in some types of safety interlocks.
- Plunger limit switches: Plunger limit switches are often used in electronic assemblies and medical devices to detect the position of small or delicate components. They are also used in some types of safety interlocks.
Limit switches are a versatile and reliable way to control and monitor machines and processes. They are used in a wide variety of industries, including manufacturing, construction, agriculture, and transportation.
Operation of a Limit Switch
As discussed above, a limit switch is used to translate a change in mechanical state into an electrical signal. How does the device actually accomplish that?
A limit switch is a simple device. The lever is used to close a contact within the switch. When the contact is closed, it allows the electrical signal to go through. The mechanical operation is similar to that of a light switch. However, a spring is added to return the limit switch into its original position.
The critical parameter of a limit switch is at which point it closes the contact within. This parameter will vary between switch types as well as brands. It is important to verify the datasheet to confirm this position. It will influence the system design that will utilize the limit switch.
Limit Switch - Electrical Signal Configurations
As discussed previously, once the actuator is displaced a certain distance, the switch is made. At this point, it needs to transmit a signal to an electrical device which may be a Programmable Logic Controller (PLC), relay, control circuit, or an additional limit switch. A limit switch is similar to a relay in the sense that it can be purchased in one of the following configurations:
- Normally Open | The switch will allow the current to pass through once it is actuated. In resting state, no current will pass through the switch.
- Normal Closed | The switch will stop the current once it is actuated. In resting state, current will flow freely.
- Combination of Two | The switch will provide a set of contacts that can be used in Normally Open (NO) and Normally Closed (NC) circuits.
There is an argument to be made for either configuration. However, when it comes to limit switches being used for safety purposes, it’s always advised to have current circulating in “resting state”. This is important as during a problem in the circuit, the safety should trigger. Should it be set to no-power in the “resting state”, the circuit may fail to prevent injury or damage.
Limit Switch - Real Applications & Datasheet
In this section, we’re looking at the Allen Bradley 801 General Purpose Limit Switch configurations. Below, you’ll find the “Contact Arrangement” provided by the manufacturer. Let’s review each section in detail
Section 1
A - Normally Open (NO) limit switch symbol.
B - Normally Closed (NC) limit switch symbol.
AC - Alternating Current Ratings for light duty limit switches at different voltage levels. It is important to remember that although your application may require a switch to operate within a 24VDC circuit, it’s possible to use these across multiple industries in which voltage level standards are different.
DC - Direct Current Ratings for light duty limit switches.
Section 2
C - Normally Open (NO) and Normally Closed (NC) set of contacts on a single limit switch.
H - Dual Normally Open (NO) and Normally Closed (NC) set of contacts on a single limit switch.
N - Dual Normally Open (NO) set of contacts on a single limit switch.
O - Dual Normally Closed (NC) set of contacts on a single limit switch.
Section 3 - Dual Position Limit Switch
In the first example of a limit switch we had reviewed, there was a single position at which the switch would trigger the signal. In this case, there are two points at which the signals will be different. These signals can be picked up in discrete logic, but most commonly, this would be done using a PLC. A PLC programmer can utilize such a limit switch to detect a difference between a partial and a full limit switch closure.
Position of Lever = OFF
- Contact 1 = Closed
- Contact 2 = Open
Position of Lever = Partially Closed
- Contact 1 = Open
- Contact 2 = Open
Position of Lever = Fully Closed
- Contact 1 = Open
- Contact 2 = Closed
Section 3 - Normally Open (NO) Differential Location Limit Switch
Until now, we’ve talked about limit switches that would actuate the same one way or the other. This is the case for most general purpose limit switches. However, it is possible to detect if the switch was pushed in one way or the other through the interlock present in the differential location limit switch. The operation will be as follows.
Position of Lever = OFF
- Contact 1 = Open
- Contact 2 = Open
Position of Lever = Closed to the Left
- Contact 1 = Open
- Contact 2 = Closed
Position of Lever = Closed to the Right
- Contact 1 = Closed
- Contact 2 = Open
Section 4 - Normally Closed (NC) Differential Location Limit Switch
The Normally Closed (NC) variation of the previous switch will operate as follows. The only difference is the state of the contacts in the “Off” position.
Position of Lever = OFF
- Contact 1 = Closed
- Contact 2 = Closed
Position of Lever = Closed to the Left
- Contact 1 = Open
- Contact 2 = Closed
Position of Lever = Closed to the Right
- Contact 1 = Closed
- Contact 2 = Open
Section 5 - Dual Normally Closed (NC) & Normally Open (NO) Differential Location Limit Switch
This variation of a limit switch features four contacts. Two of these contacts can be used as Normally Open (NO) connections and two other as Normally Closed (NC).
Section 6
Current ratings for regular duty limit switches. Note that these ratings are higher than the ones found in Section 1.
What is a Limit Switch & How Does a Limit Switch Work
A limit switch is an electromechanical device that consists of an actuator mechanically linked to an electrical switch. When the actuator is moved by an object, it causes the electrical switch to open or close, depending on the type of limit switch.
There are two main types of limit switches: normally open (NO) and normally closed (NC). An NO limit switch is open when the actuator is not activated, and closes when it is activated. An NC limit switch is closed when the actuator is not activated, and opens when it is activated.
Limit switches are used in a wide variety of applications, such as:
- To control the movement of machinery, such as conveyor belts and robot arms
- To detect the presence or absence of objects
- To signal the start or end of a process
- To trigger safety alarms
Here is a simplified explanation of how a limit switch works:
- The actuator is attached to a moving object or part of a machine.
- When the object or machine part moves, it activates the actuator.
- The actuator causes the electrical switch to open or close, depending on the type of limit switch.
- The open or closed electrical switch sends a signal to a controller, such as a PLC or relay.
- The controller then performs the desired action, such as stopping the machine or triggering an alarm.
Here is an example of how a limit switch is used in a real-world application:
A conveyor belt is used to transport products from one area of a factory to another. A limit switch is installed at the end of the conveyor belt to detect the presence of products. When a product reaches the end of the conveyor belt, it activates the limit switch. The limit switch then sends a signal to a controller, which stops the conveyor belt. This prevents the product from falling off the conveyor belt.
Limit Switches - Other Types & Applications
A limit switch used in manufacturing is rated for hundreds of thousands of cycles. However, there are various flavors of this device that aren’t as robust for other everyday applications.
Limit Switches in Elevator Systems
Elevator systems commonly require limit switches for a variety of purposes by building codes. They are used for position detection, safety and operational state of the elevator. The types used in this application are rated for an even greater number of cycles as they constantly experience actuator movement through the life of the system.
Limit Switches in Household Systems
Limit Switches are used in everyday appliances: fridges, tea kettles, mixers, washers, dryers and more. The limit switches in these appliances are the same in principle, but different in reliability. In other words, they operate the same, but may fail much faster then their manufacturing counterparts.
How do I know if my limit switch is bad?
Using a basic ohmmeter or Digital Multimeter (DMM), it’s possible to determine whether or not a limit switch is bad. Disconnect the limit switch from the system and apply the ohmmeter leads to each terminal. If the limit switch is normally open (NO), the resistance should be very high. If it’s normally closed (NC), the resistance should be close to zero. Toggle the limit switch into the active position and measure the resistance. It should be the opposite in this setting. If the transition does not occur, the limit switch is bad.
Conclusion on Limit Switches
Limit Switches are used in a wide array of applications across the production floor as well as our daily lives. A limit switch is one of the most basic mechanical / electronics components on a production floor. They are used for object detection and machine / personnel safety. The limit switch will send a signal to a control circuit once it is in a certain position. Although the function is straight-forward, a wide range of flavors of such switches exist in order to provide flexibility to the end user. In a real-world example, we’ve looked at the datasheet that specifies multiple functions for general purpose allen bradley limit switches.
