When to Use Safety Limit Switch？

When to Use Safety Limit Switch?

Discover the essential roles of Safety Limit Switches in various industrial applications, ensuring operator and machinery safety while enhancing operational efficiency.

Limit switches play a crucial role in industries, acting as an electro-mechanical device that detects physical interactions and sends an electrical signal in response. These switches are used to determine the presence of objects, allowing systems to take appropriate actions.

The Function of Limit Switches

Limit switches are pervasive in everyday devices:

• In microwaves, to confirm the door is closed before operation.
• In elevators, to indicate the cabin has reached the desired floor.
• In washing machines, as a permissive to start the wash cycle.

Limit Switches in Manufacturing

In the manufacturing realm, limit switches serve as critical control and safety devices for machinery and personnel. Upon activation, they send a digital signal to the control system, allowing it to execute the necessary action.

The Importance of Limit Switches

Limit switches provide an economical solution for bridging the physical and electrical domains. Their long-standing use in industries has led to a reduction in cost, making them crucial due to their simplicity and affordability.

Use Cases of Limit Switches

Product Detection & Count

When a product activates a limit switch, a signal is sent to the control system. By employing PLC ladder logic, one can count the number of times a product passes the switch, displaying this count for the operator.

Personnel Safety

Limit switches can detect the opening of a safety guard which leads to de-energizing and stopping the machine. This prevents the machine from operating if safety protocols are compromised, protecting operators from harm.

Machine Safety

Limit switches protect machinery from accidental damage by monitoring components susceptible to wear and tear, such as motor clutches, and ensuring operational integrity.

Types of Limit Switches

Limit switches can be categorized based on the actuator type:

Whisker:

These switches have a thin actuator capable of bending with minimal force, suitable for spaces like robotic arms and grippers.

Roller:

Featuring roller-shaped actuators, these switches are ideal for fast-moving objects, such as conveyor belts and packaging machines.

Lever:

Lever actuators make these switches apt for detecting large or heavy objects in machine tools and industrial cranes.

Plunger:

With a plunger-shaped actuator, these switches detect small or delicate items in electronic assemblies and medical devices.

Operation of a Limit Switch

A limit switch converts mechanical state changes into electrical signals. When the actuator is displaced, it closes a contact within the device, allowing electrical current to pass. The return to the original state is often managed by an internal spring.

Electrical Signal Configurations

Limit switches typically come in:

• Normally Open (NO): Allows current upon activation.
• Normally Closed (NC): Stops current upon activation.
• Combination: Provides both NO and NC contacts.

For safety, it's advisable to use configurations that let current flow in the resting state, ensuring the circuit triggers safety mechanisms during a fault.

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Applications & Case Study

Allen Bradley's 801 General Purpose Limit Switch configurations offer insight into practical applications.

Section 1

• A - Normally Open (NO) symbol
• B - Normally Closed (NC) symbol
• AC - Light duty ratings for AC at various voltage levels
• DC - Light duty ratings for DC

Section 2

• C - Single limit switch with both NO and NC contacts
• H - Dual NO and NC contacts on the same switch
• N - Dual NO contacts
• O - Dual NC contacts

Section 3

Dual Position Limit Switches:

Utilized in PLC for detecting partial or full closures.

• OFF position: Contact 1 = Closed, Contact 2 = Open
• Partially Closed: Contact 1 = Open, Contact 2 = Open
• Fully Closed: Contact 1 = Open, Contact 2 = Closed

The differential location limit switch can detect if it was actuated to the left or right.

For example:

• Position OFF: Both contacts open
• Closed (Left): Contact 1 open, Contact 2 closed
• Closed (Right): Contact 1 closed, Contact 2 open

Limit Switches in Real-World Applications

Limit switches efficiently manage and control machinery across several industries:

• Manufacturing, ensuring operational safety
• Construction for heavy machinery management
• Agriculture, managing equipment safely
• Household appliances for safe operational states

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Checking Your Limit Switch

To verify the integrity of a limit switch, use a Digital Multimeter (DMM). Disconnect the switch and measure resistance across terminals. In NO switches, resistance should be high when not activated. In NC switches, it should be close to zero. Activate and observe for the opposite reading; if not, the switch may be defective.

Conclusion

Safety Limit Switches are indispensable in various domains due to their crucial role in ensuring both machinery and personnel safety. Understanding their types, configurations, and applications can significantly enhance operational reliability and efficiency. Whether you are looking to integrate them in manufacturing or household devices, these switches provide an effective, low-cost solution to safety and operational challenges.