What is Supply Top-roller Plunger Actuator Limit Switch and Why Do We Use Them?

There's probably not a day that goes by where you don't use or encounter a limit switch at home, or at your workplace.

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Types of Limit Switches

There are 4 general types of limit switches:

1. Whisker

2. Roller

3. Lever

4. Plunger

Depending on the application, a limit switch may be a combination of two types, such as a roller-lever.

What are Limit Switches?

A limit switch is an electromechanical device operated by physical force applied to it by an object.

Limit switches are used to detect the presence or absence of an object.

Originally, they were used to define the limit of travel of an object, hence the name Limit Switch.

Limit Switch Applications

When you open the fridge door, a light comes on inside. How does that happen? Yes, you guessed it! A limit switch is used to detect if the fridge door is open or closed.

Another application you may encounter at home is with overhead garage doors. A limit switch stops the movement of the door when it reaches its fully opened position.

How Do Limit Switches Work?

Now that we've looked at a couple of limit switch applications, let’s closely examine the device itself.

Limit switches are electromechanical devices consisting of an actuator mechanically linked to an electrical switch.

When an object contacts the actuator, the switch operates, causing an electrical connection to make or break.

Configurations of Limit Switches

Limit switches are available in several configurations: Normally Open, Normally Closed, or one of each.

Symbols of Limit Switches

Depending on the electrical schematic's origin, you may see limit switches drawn differently.

The International Electrotechnical Commission (IEC) and the National Electrical Manufacturers Association (NEMA) have slightly different symbols.

Microswitch

Let’s look inside a microswitch, a type of limit switch.

A microswitch has two limit switches operating together and sharing a common terminal. One switch is normally open, and the other is normally closed.

Technically, the switch configuration is Single Pole Double Throw (SPDT). The dashed line indicates both switches are mechanically connected and operate simultaneously.

Microswitch Simple Circuit

Let's connect the microswitch to a lamp circuit. In the inactive state, the red lamp is on as the device is not being operated by an object pushing on the trigger.

When the trigger is pushed, the device activates, and the green lamp comes on.

Limit Switches in Action

You've seen the limit switch in action and might be thinking about where you've encountered them.

For example, you might see limit switches operated by a container on an assembly line, a rotating machine part, or other moving mechanical objects.

Limit switches can be used to count passing objects or determine the position of a hydraulic cylinder.

Proximity Sensor vs. Limit Switch

Limit switches are being replaced by proximity sensors in many industrial applications.

Unlike limit switches, proximity sensors have no mechanical moving parts.

A proximity sensor performs the switching action with electronic switches.

Limit switches still surpass proximity sensors in ruggedness and reliable operation in harsh environments.

Generally, limit switches handle much higher current values than proximity sensors.

You might want to review one of our other articles:

What is a Sensor? Different Types of Sensors, Applications

Summary

Let's review:

There are 4 general types of limit switches: whisker, roller, lever, and plunger.

Limit switches are electromechanical devices operated by physical force applied by an object.

A limit switch consists of an actuator mechanically linked to an electrical switch.

Limit switches come in various configurations: Normally Open, Normally Closed, or a combination of both.

Depending on the schematic's origin, limit switches may be drawn differently.

Limit switches are being replaced by proximity sensors in many applications.

If you have any questions about using limit switches, add them in the comments below, and we will get back to you within 24 hours.

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The RealPars Team

Limit Switch Explained - What it is, How it Works & How it is used in Manufacturing

Beginner

What is the Function of Limit Switch?

A limit switch is an electro-mechanical device that sends an electrical signal based on physical interaction. They detect objects' presence, allowing the system to take desired action.

Limit switches can be found in numerous everyday devices:

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• They are used in microwaves to check that the door is closed before operation is allowed.
• They notify the system that the elevator cabin has reached the desired floor.
• In washing machine lids, they act as a permissive to start the wash cycle.

Limit Switches in Manufacturing

Countless limit switches are found in manufacturing. They are used as control and safety devices for machinery and personnel. These switches send a digital signal to the control system, which takes appropriate action based on hardware and software configurations.

Why is the Limit Switch Important?

Limit switches are an inexpensive way to create a link between the physical and electrical domains. They were developed decades ago, and their widespread use has significantly lowered costs for users. This makes them vital in manufacturing due to their simplicity and low cost.

Use Cases of Limit Switches

1. Product Detection & Count - When a product pushes against a limit switch, a signal is sent to the control system. Using simple PLC ladder logic, the user can count the number of times the product passes the limit switch and display the counter for the operator.

Limit Switch - Box Detection on a Conveyor Example

2. Personnel Safety - A limit switch can detect the opening of a safety guard to stop and de-energize 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. This safeguards the operator from potential harm.
3. Machine Safety - A limit switch can protect machinery from unintentional damage. This includes components in changeovers, worn-out components (like motor clutches), and components that could damage others if they fail (such as gears and shafts).

Types of Limit Switches

Limit switches can be classified into four main types based on the actuator type:

• Whisker: Whisker limit switches have long, thin actuators that can be bent with minimal force. They are used where space is limited, such as in robotic arms and grippers.

Limit Switch | Whisker Limit Switch - Example of an Omron Whisker Limit Switch

• Roller: Roller limit switches have roller-shaped actuators rotated by the detected object. They are used where the object moves quickly, such as on conveyor belts and packaging machines.

Limit Switch | Roller Limit Switch - Example of a Renew Roller Limit Switch

• Lever: Lever limit switches have lever-shaped actuators moved by the detected object. They are used where the object is large or heavy, such as on machine tools and industrial cranes.

Limit Switch | Lever Limit Switch - Example of a Generic Lever Limit Switch

• Plunger: Plunger limit switches have plunger-shaped actuators pushed or pulled by the detected object. They are used where the object is small or delicate, such as in electronic assemblies and medical devices.

Limit Switch | Plunger Limit Switch - Example of a COMEPI Plunger Limit Switch

Limit switches can also be classified by their electrical configuration. The two most common are:

• Normally open (NO): NO limit switches are open when the actuator is not activated and close when it is activated.
• Normally closed (NC): NC limit switches are closed when the actuator is not activated and open when it is activated.

Some limit switches have a double throw (DPDT) configuration, meaning they have two sets of contacts, one NO and one NC. When activated, the NO contacts close, and the NC contacts open.

Limit switches are available in various sizes, shapes, and materials to meet specific application needs. They also come in multiple electrical ratings to control a broad range of loads.

Here are some examples of how different types of limit switches are used in real-world applications:

• Whisker limit switches: Used in robotic arms and grippers to detect objects' presence, and in some safety interlocks.
• Roller limit switches: Used on conveyor belts and packaging machines to detect products and in machine tools to detect cutting tool positions.
• Lever limit switches: Used on machine tools and industrial cranes to detect moving parts' positions and in some safety interlocks.
• Plunger limit switches: Used in electronic assemblies and medical devices to detect small or delicate components' positions and in some safety interlocks.

Limit switches are versatile and reliable for controlling and monitoring machines and processes. They are used in various industries, including manufacturing, construction, agriculture, and transportation.

Operation of a Limit Switch

A limit switch translates a change in mechanical state into an electrical signal. How does it accomplish that?

A limit switch is simple. The lever closes a contact within the switch. When the contact closes, it allows an electrical signal to pass through. This mechanical operation is similar to a light switch but adds a spring to return the limit switch to its original position.

The critical parameter of a limit switch is the point at which it closes the contact. This parameter varies between switch types and brands. It's essential to verify the datasheet to confirm this position, as it influences the system design utilizing the limit switch.

Limit Switch - Electrical Signal Configurations

Once the actuator is displaced a certain distance, the switch is made. It then transmits a signal to an electrical device like a Programmable Logic Controller (PLC), relay, control circuit, or another limit switch. A limit switch is similar to a relay and can be purchased in the following configurations:

• Normally Open (NO) | The switch allows current to pass through once actuated. In the resting state, no current passes through.
• Normally Closed (NC) | The switch stops current once actuated. In the resting state, current flows freely.
• Combination of Both | The switch provides a set of contacts for Normally Open (NO) and Normally Closed (NC) circuits.

There is an argument for either configuration. However, for safety purposes, it's always advised to have current circulating in the resting state. This ensures that during a problem, the safety triggers. If 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 review the Allen Bradley 801 General Purpose Limit Switch configurations. Below is the Contact Arrangement provided by the manufacturer. Let's review each section:

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. Although your application may require a switch to operate within a 24VDC circuit, it's possible to use these across multiple industries with different voltage level standards.

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, the switch triggered a signal at a single position. In this case, there are two points where the signals differ. These signals can be picked up in discrete logic, usually with a PLC. A PLC programmer can detect a difference between partial and 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

Most general-purpose limit switches actuate the same way. However, a differential location limit switch detects if pushed one way or the other through the interlock. The operation is 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 operates similarly but with a different 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 features four contacts: two NO and two NC connections.

Section 6

Current ratings for regular duty limit switches. These ratings are higher than those in Section 1.

What is a Limit Switch & How Does a Limit Switch Work

A limit switch is an electromechanical device consisting of an actuator mechanically linked to an electrical switch. When the actuator is moved by an object, it opens or closes the electrical switch, depending on the type of limit switch.

The two main types of limit switches are 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 various applications, such as:

• Controlling the movement of machinery, such as conveyor belts and robot arms
• Detecting the presence or absence of objects
• Signaling the start or end of a process
• Triggering 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 transports products from one area of a factory to another. A limit switch is installed