Limit Switch Explained - What it is, How it works & ...

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

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Beginner

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. 
• 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.

Limit Switch - Box Detection on a Conveyor Example

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.

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

• 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.

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

• 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.

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

• 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.

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

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 Switch | How Does a Limit Switch Work - Example of a Mechanical Limit Switch

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.

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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.

There are many choices for detecting presence and position in machine automation applications, but limit switches are often the best option when high precision is required.

By Tim Wheeler, Technical Applications Engineer, AutomationDirect

Many types of sensors are available for detecting presence and position in industrial automation applications, from course to fine precision. Photo eyes and inductive switches are commonly used, but when a high degree of accuracy is needed, precision limit switches are often a better option because they provide repeatable accuracy down to the sub-micron level.

Industry workhorse
Industrial limit switches are the battleships of the sensor world and have been used in manual, semi-automatic and automated machinery applications for more than 100 years to detect presence and position of parts and mechanisms. These sensors come in several configurations including NEMA, heavy-duty IEC, double-insulated IEC, compact, miniature and precision, Figure 1.

The major difference between NEMA and IEC limit switch is the robustness, a common distinguisher in these comparisons. The NEMA is designed for demanding applications such as in heavy machinery, foundries and mining, and tends to be more expensive. In applications such as material handling and gantry equipment, a similar but more cost-effective heavy-duty IEC limit switch is usually suitable. Both the NEMA and heavy-duty IEC housings are typically made of metal, die-cast zinc alloy or aluminum depending on the device.

Double-insulated IEC limit switches are typically made of plastic, and the compact IEC switches are made of plastic or metal. Many of the precision touch limit switches are made of stainless steel. Precision limit switches have a barrel design, similar to a very small (M5 x 0.5) proximity switch. These stainless steel, plunger actuation switches have a high degree of repeatable accuracy.

It is worth mentioning that safety switches are used to protect personnel and equipment from hazards by monitoring the position of movable guarding. They are often used on hinged guard doors where an actuator key engages the switch body when the guard is closed. Cable-operated safety switches are also used to activate an emergency stop when the cable is pulled.

Key selection considerations
Limit switches are suitable for use in a wide range of applications and harsh environments on the factory floor due to their ease of installation, reliable operation and rugged design, Figure 2. Limit switches are typically used in physical contact applications that cause wear and tear on the switch actuator and electrical contacts, so exceeding two operations per second should be avoided. When selecting a limit switch, consider the application and actuation method first, as often they are the clearer determining factors.

Other sensing options include inductive proximity switches and photo eyes, both of which are a touch-free method to sense position and presence. An inductive proximity (prox) switch detects ferrous and non-ferrous metals with no mechanical contact needed. A prox switch typically has a solid-state output, instead of mechanical contacts as with a limit switch. Its non-contact sensing method and solid-state output make it a good choice for high cycle-rate applications, and when debris may interfere with limit switch actuation.

However, there are many quality and long-lasting limit switches available. With a properly specified limit switch, it&#;s not unusual for it to have a mechanical life of 30 million actuations, and an electrical life of 5 million operations. The limiting factor is often electrical contact life, but replacement contact blocks are available for quick replacement at a low cost with some limit switches.

NEMA and heavy-duty IEC limit switches are both suitable for used in harsh environments. Foundries, shipping and dockside operations are common applications. They are also found in machines in industries such as automotive, food and beverage, and pulp and paper, and power. These switches also work well on large conveyors and welding equipment.

Limit switches are also used in more medium-duty applications such as earth moving equipment, agricultural machinery, and farm and tractor implements. Other medium-duty applications include CNC machine tools, overhead hoists, large cranes, and textile and printing machinery, Figure 3.

Limit switches are often used in consumer grade machines and equipment such as escalators and elevators, industrial automatic doors, aircraft access platforms, point-of-sale dispensing kiosks, scissor lifts and slot machines. With careful design, limit switches work well in most of these applications, and are easily interfaced to programmable logic controllers.

Limit Switch Selection Criteria
The leading limit switch selection criteria are listed in Table 1 and explained below.

Table 1, Limit switch selection criteria

• Actuation method
• Repeatability
• Travel to operate and reset
• Force to operate
• Contact configuration
• Environment requirements

Limit switches are actuated several ways including side rotary, top and side push, and wobble stick. These actuators are often mounted onto 90° adjustable heads. Travel to operate contacts and the amount of force needed varies, and should be determined with a high degree of certainty before selecting a switch.

Levers are adjustable to any angle on the operating shaft and need around 5° rotation, from a total of 90° travel or more, in either direction to operate contacts. Typical actuator types include various length stainless steel levers with nylon or metal rollers. Spring stainless steel rods (whiskers), and loops of nylon or metal cable, are available as well.

Push-operated (plunger type) limit switches are top or side actuation in a pushbutton or roller style to operate contacts, and require 2 mm or less travel. Wobble head limit switches are typically top operated using different rod-type operators such as a nylon rod, stainless steel rod or spring steel&#;and will operate with about 10° of actuation, perpendicular to the rod, from any direction.

The repeatability of an assembled limit switch is determined by the type of operating head used. The switches with the best repeatability are the direct-operated pushbutton or plunger type, typically 0.003-in. or less repeatable accuracy. The addition of rollers and operating levers adds tolerance stickup due to concentricity. The tolerance can easily double when rollers are used.

Limit switch operation starts at an initial, normal position at rest. When operated they move through a pretravel range to an operating point, where the electrical contacts close, and then into the overtravel range. The travel to operate and the travel to reset are different. There is hysteresis in limit switch actuation. It may take 1 to 2 mm, or 5° to 10° of pretravel, to operate a limit switch&#;s contacts, but only half of that to reset upon return, but precision limit switches often have zero pretravel.

The force to operate a limit switch varies widely depending on the device. Many heavy-duty switches require 4 lb or more to operate the contacts. Small, precision limit switches may only take 0.1 lb to operate.

Many limit switches have single-pole, single-throw electrical contact configuration, either normally open or closed. Some have more contact configurations available. Snap-action contacts are available where the contact motion is independent of the speed of the actuator. The contacts will still close quickly even with very slow-moving actuators. Alternatively, slow-make/slow-break contacts are available where contact motion is dependent on actuator speed.

A limit switch will operate reliably in extreme environments; at high or low temperatures; and in moist, wet or contaminated areas. Degree of protection varies from IP40 to IP67/NEMA/UL. Common enclosure Types are 1, 3, 3S, 4, 4X, 6 and 6P.

Design considerations
Limit switch design considerations listed in Table 2 and can affect the operation and reliability, but careful design can mitigate potential problems.

Table 2, Limit switch design considerations

• Mechanical life
• Sensor impact
• Switching frequency
• Maximum actuation speed

Mechanical life of a limit switch can be extended by adhering to these guidelines. Limiting the severity of the impact of the target material on the limit switch operator is important, as is limiting the overtravel of the switch during operation as much as possible. There is no need to rotate or push the switch actuator more than necessary for reliable actuation, and levers are adjustable to any angle on the operating shaft to minimize impact and overtravel. Pressing a limit switch plunger to the stroke end may cause malfunction due to the impact, but this can be mitigated by installing a hard stop.

Some limit switches allow right-angle operation to the plunger with an appropriate lead in cam angle, often 30°. However, many limit switches require contact with the object at a right angle or within a few degrees. Action is limited between the tip end and the edge of the internal bearing, but side loads can cause damage to the switch over time. The end of the housing may deform if impacted, causing failure in the return.

Shock and vibration specifications should be carefully considered during design. When severe, it could operate the switch and cause a fault. Mounting a limit switch out in the open where it can be accidently impacted, placing a side load on the axis of rotation, or causing overtravel of the plunger should be avoided.

As discussed previously, switching frequency must be less than 2 cycles per second as mechanical and electrical activations are limited. The actuation speed of a limit switch can be too fast or too slow as well. If activated too fast, the switch could bounce or wear quickly. Slow activation, less than 50 mm/minute, can cause repeatability errors, especially in precision limit switches.

Precision Limit Switches
Traditional limit switches, proximity sensors and photo eyes are somewhat limited in terms of accuracy and repeatability, typically in the 25- to 100-µm range depending on the device. However, there are some cost-effective and ultra-precise mechanical limit switches with repeatability in the 0.5- to 10-µm range, Figure 4. Considering a human hair is typically 50- to 60-µm in diameter, that&#;s precise sensing.

These precision limit switches are often used to replace fiber sensors used for positioning parts. When using a photo eye, uneven surfaces, water droplets and even fingerprints on the part can make precision position sensing less accurate. However, since the precision limit switch contacts the part, it minimizes the effects of an uneven surface, and can push through a water drop or fingerprint.

In a sheet thickness application, such as detecting a double-fed label, designers may consider limit switches unsuitable for detection of an extra label due to low repeatability. However, newer high precision touch and tool setter switches are capable of 0.5 µm repeatability, and can reliably detect 50 µm or thinner sheets. Take-up material can be checked to ensure the label was removed.

These precision limit switches can be used in harsh environments including CNC machine tools with coolant spray and metal shavings, and in motion control and robotic automation. In these applications, the switches are typically used to find a home position, an edge of the tooling, or an edge on a part. Applications include finding the soldering tool tip to adjust the z-axis on a robot, homing x-y tables, and detecting the edge(s) of a grinding wheel in a CNC grinding application

On automated machines and robots, there are many sensors such as vision systems, lasers and photo eyes. Finding the position of the tooling or part is often critical to operation. Precision limit switches are an excellent option to find a repeatable home or start position. Finding a position within 0.5 micron, 1/ of a millimeter, with a mechanical switch adds the word precision in front of machining and assembly.

Limit switches are tried and true in machine automation applications, and you have probably used them before. Don&#;t run a switch too hard or fast, and use the correct activation method. And don&#;t forget about the precision limit switch. With repeatability starting well under one thousandth of an inch (10 µm), and down to sub-micron repeatable accuracy, finding precise position has never been easier.

Automation Direct
Automationdirect.com

All figures courtesy of AutomationDirect

 

 

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