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.

Link to TMAZTZ

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.

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.

Q&A on Limit Switches

What happens when a limit switch goes bad?

When a limit switch fails, it can either remain stuck open (not allowing the circuit to close) or stuck closed (always completing the circuit). This can cause issues such as:

• Machines failing to stop at a designated point.
• Safety interlocks not functioning properly.
• HVAC systems shutting down or overheating.
• Inconsistent or erratic operation of automated systems.

If you suspect a faulty limit switch, test it with a multimeter and replace it if needed.

Can you run a furnace without a limit switch?

No, running a furnace without a high-limit switch is dangerous. The limit switch is a safety device that prevents overheating by shutting down the burner if temperatures exceed safe levels. Removing or bypassing it could result in:

• Overheating and damage to the furnace.
• Risk of fire hazards.
• Carbon monoxide buildup.

If your furnace limit switch is malfunctioning, it should be replaced rather than bypassed.

Is a limit switch a relay?

No, a limit switch is not a relay, though both control electrical circuits. The key differences:

• A limit switch is mechanically actuated, meaning physical movement (such as a machine part reaching a position) triggers the switch.
• A relay is electrically actuated, meaning it switches a circuit based on an electrical signal.

However, in some applications, limit switches work alongside relays to control motors, actuators, and other devices.

How do I reset my limit switch?

To reset a manual furnace limit switch:

1. Turn off power to the furnace.
2. Locate the limit switch (typically near the blower motor).
3. Look for a reset button—press it if available.
4. Restore power and observe if the furnace operates normally.

For mechanical limit switches used in automation:

• If the switch is stuck, manually depress and release the actuator.
• If the issue persists, the switch may need cleaning or replacement.

What is the difference between a limit switch and a solenoid valve?

The primary difference lies in function and operation:

• A limit switch is a sensing device that detects physical movement and opens or closes an electrical circuit.
• A solenoid valve is an electromechanical device that controls the flow of fluid or air when an electrical signal is applied.

They are often used together in industrial automation, where a limit switch detects position and a solenoid valve controls a process.

Why use a solenoid instead of a switch?

A solenoid is used instead of a mechanical switch when:

• Remote operation is needed (e.g., turning a valve on/off from a distance).
• High-speed actuation is required (solenoids operate faster than mechanical switches).
• Repeated actuation is involved (solenoids have fewer moving parts and can handle frequent switching).
• Automation is required (solenoids can be controlled via electrical signals rather than manual interaction).

How much does it cost to replace a solenoid switch?

The cost of replacing a solenoid switch depends on the type and application:

• Automotive solenoid switches (e.g., starter solenoids) range from $20 to $200, depending on brand and model.
• Industrial solenoid switches (used in automation and heavy machinery) can cost between $50 and $500, depending on voltage, material, and durability.
• HVAC solenoid valves typically cost $30 to $150, with professional installation adding extra labor costs.

If replacing a solenoid, always verify compatibility with your system to avoid additional expenses.

Conclusion on Limit Switches

What is a Limit Switch? A Complete Guide

Essential accessories like sensors and switches are critical in automating industrial equipment and gadgets today. Of these, limit switches are unavoidable. But what is a limit switch, and how does it function? This detailed guide will help you understand what Limit switches are, what types exist, how they work, and where they are applied. It also gives information on how to install the limit switches properly.

What is a Limit Switch?

A limit switch is an electromechanical control device utilized as a turn-on/off control to detect the position or absence of an object through physical touch. These switches are mainly used in operating tools or as part of operational security features. They are initiated when an object in motion touches the actuator, leading to a change in the internal contacts of the switch to open or close circuits.

Limit Switch Types

It is essential to have a basic knowledge of limit switches to determine which type is suitable for a given use. To date, there are four types of limit switches. In general, limit switches are categorized depending on the type of actuator and the operational conditions. Here are the most common types:

• Rotary Limit Switches: These are of the plunge type with a rotating lever arm. They are instrumental in applications where angle is to be detected.

• Plunger Limit Switches: They engage in a linear movement when an object pushes the plunger.

• Lever Limit Switches: They include a 'contact trigger' lever arm, which rotates when touched.

• Proximity Limit Switches: If possible, these switches avoid using direct contact means like mechanical contact involving magnetic or inductive sensing.

• Safety Limit Switches: These are used in safety-critical applications so that some operations can stop the necessary machinery when activated.

Limit Switch Working Principle

The limit switch functioning concept is quite simple but very efficient and helpful in lots of mechanical applications. Whenever an object comes into contact with the actuator of the switch, the actuator is mechanically displaced. This movement either makes or breaks the electrical contacts contained in the switch, thus changing the circuit's state.

Depending on the application, this change in state can:

• Trigger an alarm

• Start or stop a motor

• Stop a machine to avoid an incident

Limit Switches in Industrial Application

Limit switches are essential components in industrial settings, known for their reliability and versatility. Here are some of their key applications:

1. Position Monitoring:

Limit switches are integrated into circuits to monitor the position of specific components within a production line. For example:

• Conveyor systems: To track the movement and location of items.

• Robotic equipment: To identify the exact position of mechanical arms or other moving parts.

2. Safety Systems:

These switches play a critical role in machine safety by halting operations when necessary. For instance:

• Stopping machinery if a door is opened.

• Shutting down equipment when safety guards are removed or displaced.

3. Automated Systems:

In automated manufacturing, limit switches control the sequence of operations by detecting the presence or absence of an object. This ensures precise and efficient workflow.

4. Elevators:

Limit switches help ensure safe and accurate elevator operation. They assist in:

• Identifying floor positions.

• Preventing the elevator from stopping at incorrect levels or bypassing the nearest floors.

By combining functionality and precision, industrial limit switches contribute significantly to operational safety, efficiency, and automation.

How to Install a Limit Switch?

For maximum utilization and safety of a limit switch, it is essential to install it properly. Here is a step-by-step guide on how to install a limit switch:

• Determine the Mounting Location: Select an area that will offer contact between the actuator and the moving object.

• Mount the Switch: Fix the limit switch with screws or brackets.

• Connect the Wiring: It is recommended that the switch be connected to the control circuit.

• Adjust the Actuator: It is essential to regulate the actuator response in the desired operating range.

• Test the Installation: Manipulate the machinery to enable a confirmation that the switch performs appropriately.

Function of a Limit Switch

The Role and Applications of Limit Switches

Limit switches are critical components in many industrial systems. Their primary role is to detect the presence, absence, or position of an object and send a signal to the power control system. This functionality is leveraged in various applications, including:

• Motor Control: Stopping a motor when an object on a conveyor belt reaches its endpoint.

• Door Monitoring: Detecting whether a door is fully open, partially open, or completely closed.

• Reducing Oscillations: Minimizing oscillation in moving mechanical parts within a system.

Safety Limit Switches: A Key to Industrial Safety

Safety limit switches are specialized versions of limit switches designed to enhance workplace safety. These switches are often used in environments where heavy machinery or loads pose risks to workers. Standard features of safety limit switches include:

• Durability: Heat-resistant components that can endure harsh conditions and rugged terrains.

• Enhanced Safety: Features to prevent interference, ensuring consistent performance.

• Quick Response: A very short response time to halt machinery immediately during critical or emergency situations.

Why Limit Switches Are Essential?

Limit switches play a pivotal role in industrial automation and safety systems. Understanding their types, operations, and applications ensures they are used efficiently across various sectors. Whether it's a basic plunger switch for position sensing or a robust industrial-grade switch for safety purposes, these devices are indispensable for safe and effective operations.

Also Read: An Essential Guide to Understanding Pressure Switches

Conclusion

For high-quality limit switches, Schneider Electric eShop is a reliable choice. They offer solutions tailored to meet diverse industrial needs, combining superior performance and safety features. Explore their catalog today to enhance your business operations with reliable and efficient limit switches.

Frequently Asked Questions

Q1. Can limit switches operate in extreme environments?

Ans: Many limit switches function under harsh conditions. They can function in high temperatures, humidity, or exposure to chemicals. However, it's essential to review the product specifications to ensure suitability for your specific environment.

Q2. How long does a limit switch last?

Ans: The lifespan depends on the type of switch and its operating environment. High-quality limit switches can endure millions of operation cycles, with more reliable models offering enhanced durability.

Q3. Can limit switches be used for non-contact applications?

Ans: Yes, proximity limit switches allow for touch-free operation, unlike traditional models that require physical contact.

Q4. What factors should you consider when selecting a limit switch?