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.

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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. 
• They are used in elevators to notify the system that the cabin has reached the desired floor. 
• 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

1. 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.

2. 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.
3. 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:

1. The actuator is attached to a moving object or part of a machine.
2. When the object or machine part moves, it activates the actuator.
3. The actuator causes the electrical switch to open or close, depending on the type of limit switch.
4. The open or closed electrical switch sends a signal to a controller, such as a PLC or relay.
5. 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.

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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

What is Limit Switch & Types of Limit Switch?

Introduction to Limit Switches

In the realm of industrial automation, efficiency, accuracy, and safety are vital. To achieve these, various components are used, and limit switches are one of the most significant. These small but essential devices help control machinery by detecting physical presence, position, or motion of an object. Limit switches are frequently used to monitor and limit the movement of machines, ensuring safe and precise operations. In this article, we will explore what limit switches are and their different types in great detail.

A limit switch is an electromechanical device used to detect the presence or position of an object. When the object comes in contact with the actuator of the limit switch, it changes the state of the switch contacts&#;either completing or breaking an electrical circuit. This makes them incredibly useful for safety, control, and feedback within automated systems. Understanding the key features and variations of limit switches can significantly enhance their effective implementation in different industrial environments.

How Do Limit Switches Work?

Limit switches work through mechanical action&#;when an object makes contact with the actuator, the internal contacts within the switch either open or close. This change in the contact position results in an electrical signal, which is then transmitted to the control system to either start, stop, or alter a machine's operation.

There are different components in a limit switch, such as:

• Actuator: The part that interacts with the moving object.

• Contacts: Internal elements that switch between open or closed states when the actuator is triggered.

• Housing: The external casing that protects the internal mechanism from the environment.

These switches are typically used in situations where there is a need to detect movement or establish the position of a part. For instance, on a conveyor belt, limit switches are placed at the end of the conveyor to detect products, providing control feedback to stop or redirect them. They are also used in elevators to determine the stopping position and in machine covers to ensure they are correctly positioned before a machine starts.

Types of Limit Switches

Limit switches come in various types, each designed to serve specific applications and environments. Understanding the different types will help in selecting the correct limit switch for your needs. Here, we discuss the main types of limit switches:

1. Mechanical Limit Switches

Mechanical limit switches are the most common type of limit switch. They use direct physical contact between the object and the actuator to trigger the switch. The actuator may be a lever, plunger, or roller.

• Lever Actuator: These actuators use a lever, sometimes with a roller at the end, to detect linear motion. Lever actuators are widely used in conveyor systems to detect product movement.

• Plunger Actuator: These actuators are designed to detect direct pressure and are used when an object must press the plunger to actuate the switch. They are often seen in industrial presses and safety interlocks.

• Roller Actuator: Roller actuators are commonly used in applications with sliding surfaces, ensuring smooth operation and reducing wear and tear.

Mechanical limit switches are preferred in applications where robustness is a priority. They can handle mechanical force effectively and are typically used in heavy-duty industrial environments.

Non-contact limit switches operate without physical contact between the switch and the object. These switches often use inductive, capacitive, or optical sensing to detect the position of an object.

• Inductive Limit Switches: These detect metal objects by generating an electromagnetic field. When a metallic object enters this field, it is detected. Inductive switches are perfect for environments where mechanical contact would cause excessive wear or damage.

• Capacitive Limit Switches: These switches detect both metallic and non-metallic objects. They work by sensing changes in the dielectric constant. They are often used for detecting liquids, powders, and other materials.

• Optical Limit Switches: These switches use light beams to detect an object's position. They are ideal for applications where precision is critical, such as in robotic arms or assembly lines.

Non-contact limit switches are commonly chosen in environments where maintaining contact between the object and switch could lead to wear or contamination.

3. Snap-Action Limit Switches

Snap-action limit switches are characterized by a fast switching action, in which the internal contacts "snap" into position when the actuator reaches a specific point. This type of switch reduces arcing and ensures a fast, reliable response, making it ideal for high-speed applications.

4. Rotary Limit Switches

Rotary limit switches are used to detect rotational movements. They are often found in cranes, hoisting systems, and other applications where monitoring the rotation angle is important. By limiting the rotation, these switches prevent overtravel and ensure safety.

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Applications of Limit Switches

Limit switches are used across various industries, playing a vital role in ensuring operational safety and efficiency. Here are some common applications of limit switches:

• Conveyor Systems: Limit switches are placed at different points along the conveyor to detect the movement of products, regulate speed, and ensure a smooth workflow.

• Elevators and Lifts: Limit switches are used to determine the exact stopping position of an elevator, ensuring safety for users.

• Industrial Presses: In presses, limit switches control the depth of each stroke, ensuring parts are formed or punched consistently.

• Safety Interlocks: Limit switches act as interlocks in machinery, disengaging operations if any part of the system is out of position or a safety guard is open.

• Robotics: Limit switches are also used in robotic arms to ensure they operate within their designated range, providing positional accuracy.

Their diverse applications make limit switches indispensable for automation. They ensure that machinery is operating within its designated limits, helping to avoid accidents and equipment damage.

Advantages of Using Limit Switches

The use of limit switches in automation provides several advantages that make them an essential component in industrial settings:

1. Operational Safety: By limiting movement and preventing overtravel, limit switches ensure that machinery operates safely.

2. Precision Control: Limit switches provide accurate control over the position of mechanical parts, improving the precision of automated operations.

3. Reliability: Limit switches are known for their rugged construction, which allows them to perform reliably even in harsh environments.

4. Cost-Effective: Compared to other types of sensors, limit switches are relatively simple in design and cost-effective, making them a preferred choice for many automation solutions.

These advantages make limit switches a practical solution for enhancing both the safety and efficiency of automated systems.

How to Select the Right Limit Switch

When selecting a limit switch, it is essential to consider various factors that align with the specific requirements of your application:

1. Application Requirements: Understand what kind of movement needs to be detected&#;linear or rotary&#;and whether direct contact is acceptable.

2. Environmental Factors: Evaluate the environment in which the switch will operate. Is it dusty, wet, or exposed to chemicals? The switch's housing and IP rating should match these conditions.

3. Actuator Type: Select the actuator type based on the interaction between the object and the limit switch. Lever actuators, for instance, are ideal for detecting sliding motions.

4. Switching Frequency: Consider how often the limit switch will be actuated. High-speed applications require switches with a high switching frequency rating to ensure reliable performance.

By taking these factors into account, you can ensure that the chosen limit switch meets the specific requirements and challenges of the application.

FAQs

1. What is a limit switch?

A limit switch is an electromechanical device used to detect the presence or position of an object, often used for controlling machinery in industrial automation.

2. What are the different types of limit switches?

Common types include mechanical, non-contact, snap-action, and rotary limit switches, each serving different applications based on their operating principle.

3. How do you choose the right limit switch?

Factors to consider include the type of movement, environmental conditions, actuator type, and switching frequency.

Conclusion: The Role of Limit Switches in Automation

Limit switches are an integral part of industrial automation, providing the necessary control and feedback to maintain safety, efficiency, and precision. From understanding their working principle to recognizing their different types, limit switches prove to be versatile and reliable components in various applications. Their simplicity and robustness make them a preferred choice for industries that prioritize operational safety and accuracy.

Whether it&#;s detecting the position of a conveyor belt or acting as an interlock in a high-speed press, limit switches ensure that machinery functions smoothly within defined limits. Choosing the right limit switch and implementing it effectively can enhance the productivity and safety of an automation system.

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