Limit switches for automatic sensing appllications


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Limit switches

A limit switch is electromechanical device consisting of an actuator operated by the motion of part of a machine or an object. The operation of the device makes or breaks an electrical connection.

When an object comes in contact they can be used as part of a control system to control machinery, as safety interlocks, or to count objects passing a point

Limit switches are rugged, easy to installation, and reliable. They can determine the presence or absence, passing, positioning, and end of travel of an object. As the name suggests They were first used to define the limit of travel of an object Due to these reasons they are used in a variety of applications and environments. The operator types of a standard industrial limit switches are lever, roller, plunger, and whisker type.

Working of a limit switch

As the object makes contact with the actuator of the switch, it moves the actuator to its maximum position where the contacts change state. Other varieties of sensors/switches are proximity sensors, light sensors, electric switches etc.

It can be mounted into remote locations so that it is actuated by an object other than a human operator

  • Limit switches can be directly operated mechanically by the motion of the operating lever.

  • A reed switch may be used to indicate proximity of a magnet mounted on some moving part.

  • Proximity switches operate by the disturbance of an electromagnetic field, by capacitance, or by sensing a magnetic field.

Outputs of limit switch

  • A control relay,

  • A motor contactor control circuit,

  • As an input to a programmable logic controller.

  • Direct control of the final controlling device

basic functions of limit switches

  • Detecting presence/absence
  • Counting
  • Detecting range of movement
  • Detecting positioning & travel limit
  • Breaking a live circuit when unsafe conditions arise
  • Detecting speed

Selection of a limit switch

Due to its versatility, a limit switch can be used in various applications. Selection of the limit switch is design specific.

Mechanical limit switches can be found in any industrial or commercial application where detection or safety is needed.

Advantages and Disadvantages

Limit switches are a practical solution for sensing in most situations. There are, however, a few disadvantages to using limit switches.

Advantages

*Switching high currents is no problem (up to 10A)

  • High precision, accuracy, and repeatability

  • Economic sensing solution

  • Can withstand most environments

Disadvantages

  • Must make physical contact with an object to actuate

  • Mechanical component can wear out

Applications of limit switch

Industries & Markets

• keeps the microwave from starting without the door being shut.

• turns the light off when the refrigerator door is closed?

• car’s dome lights switching on when on opening the door?

• stops the washing machine with a unbalanced load

Industrial Application

  • Packaging . Boxes of paper approach the end of the packaging line, ready to be stacked onto pallets . A palletizer with suction-cup grippers picks up a box and swings around to a waiting pallet. Limit switch is used to detect the limit of stacking. When the pivot arm reaches the top of its vertical limit, the arm hits a limit switch signaling the system to send the full pallet down line and setting up an empty pallet to restart the process.
  • Remote location sensing Working where people cannot. Inside a sawmill, a high-speed saw quickly reduces logs into construction beams. In the process, chips and dust float in the air. Breathing is impossible in the area without a mask. Even with goggles, it would be impossible to inspect the cutting. The production department devised a system of limit switches to do the inspecting automatically. A remote operator can configure the switches to allow the log to be cut to the desired dimensions.
  • Food and beverage . At a frozen food processor, an automatic pallet stacking system is used. This system uses a wobble stick limit switch to detect when the pallets have been loaded to their desired level. The switch then signals the conveyor to send the load through an automatic vertical rise door into the freezer for quick freezing.
  • Fire safety. In a manufacturing plant, rooms need to be closed off quickly in case of contamination or fire. To help facilitate this process, high-speed doors have been developed. These doors may move as quickly as six feet per second. At such speeds, the door would destroy itself quickly, if not for the use of limit switches. The limit switches are used to slow the door just before it is fully opened or fully closed.

Some of many industries with limit switch needs:

• Material handling – packaging, moving, warehousing, distribution

• Food & beverage packaging, distribution

• Manufacturing – automotive/heavy equipment, machining, marine/aviation, glass & plastics

• Metals – mining, refining, processing, forming

• Commercial applications

• Control cabinets

Variations of a limit switch

Mechanical limit switch operators are available in many shapes and sizes based on their functionality and application. Switches can be divided into two types

  • momentary

Momentary, or “spring return” switches return to their normal state as soon as the actuator is released from the object it is sensing.

  • maintained.

Maintained switches will remain in the actuated position even after the actuator has been released.

Types of limit switches - operator heads and their applications

A-Shape & E-shape:

Roller Levers actuate radially rotating on an axis. Roller levers are tipped with a roller to lessen the friction and the force required to activate the lever. Levers spring-return to neutral when the force is removed. This is one of the most popular styles of switch operators.

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Adjustable roller lever is similar to a standard roller lever, but the length of the lever is adjustable from a few mm up to 5 cm. The rollers are also available in various diameters.

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B-Shape:

Rounded plunger heads come in many varieties and sizes. Plungers are actuated by a perpendicular force applied directly to the end of the plunger.

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C-Shape:

Roller plunger When a plunger switch is needed where the force will not be applied directly at a 90º angle, a roller plunger can be used. The roller translates some of the non perpendicular force into perpendicular force that can actuate the plunger.

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D-Shape:

Rod levers are also actuated radially like levers; however these are thin rods that are much longer. Adjustable rods can be as long as 10 cm or more.

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F-Shape:

Lateral rounded plunger head varies from the standard B-shape in that the plunger is perpendicular to the switch.

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G-Shape:

Lateral roller plunger head varies from the standard C-shape in that the plunger is perpendicular to the switch.

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Additional Operators:

Flexible rod is similar to a rod lever in their length and appearance, rods (whiskers, spring rods, etc.). However, unlike levers that actuate only in one plane, flex rods activate in 360º.

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Cable pull are switches that are tipped with a lanyard to which can be attached a cable. Pulling or tightening the cable draws out the spring loaded lever, which activates the switch.

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Rotative Axis switches can be installed onto an axis (i.e., a door hinge) so that when the hinge or axis turns, the switch actuates.

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Latch/Reset switches come with a assortment of operating head styles. These switches do not spring return to neutral; instead, they latch when activated and must be physically, manually released before being deactivated.

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Key Latch switch is coupled with a “key”. Once removed from the keyhole, the switch is actuated. Returning the key to the hole deactivates the contacts. Commonly used in door jams to shut off power when opened.

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Mounting considerations

Considering which type of switch to use is important when applying a limit switch. But just as important is determining where and how to mount the switch.

Cam design

In many situations, such as a conveyor system, a cam is used to operate the actuator. The cam should be of a shape that does not allow the actuator to receive a severe impact or that releases the actuator suddenly, allowing it to snap back freely.

Mounting location

Limit switches should never be mounted in locations that could allow false operations by normal movements of operator or machine components. They should be mounted rigidly, be maintenance accessible, and have the side of the body with the cover screws facing outward. If liquid intrusion is a possibility, the switch should be mounted face down to allow gravity to prevent seepage through the seals on the operating head. All conduit connections should be tightly sealed. In applications where machining chips or other debris accumulates, the limit switch should be mounted in a location or at such an angle that minimized build-up on the operating head.