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What is the difference between A Limit Switch and A Proximity Switch?

What is the difference between A Limit Switch and A Proximity Switch?

There are many application areas for travel switches, many of which are found inside electrical appliances. What role can such a simple switch play? It mainly plays the role of chain protection. We hope the explanation in this article will make it easier for you to understand the various switches.

1.1 Principle of operation

Travel switches are also known as limit switches. When the animal is close to the static object, the connecting rod of the switch drives the contact of the switch to cause closure, and then the contact breaks or disconnects to produce closure. Thus, changing the switch’s contact opening and closing state controls the circuit and mechanism’s action.

1.2 Applications

1.2.1 Refrigerator

In our daily lives, we are most likely to encounter the example of the Refrigerator. I do not know if you noticed, but when you open the Refrigerator, the Refrigerator inside the light will light up, and you close the door and go out; this is because there is a switch on the door frame, the door was pressed tightly when the lights of the circuit are disconnected, the door opens on the relaxation, and then automatically close the circuit so that the lights are lit. This switch is the travel switch; therefore, the travel switch is a displacement by the object that determines the circuit switch.

1.2.2 Washing machines

The dehydration (drying) process in the washing machine is speedy. If someone, due to negligence, opens the door or cover of the washing machine and then puts their hands in, it is easy to cause harm to people. To avoid the occurrence of such accidents, the washing machine door or cover installed an electrical contact; once someone opens the door or cover of the washing machine, the motor will automatically cut off the power, and even rely on mechanical means of linkage, so that the door or cover is opened immediately “brake,” forcing the rotating parts to stop to avoid injury to the person.

1.2.3 Recorder

In tape recorders and VCRs, we often use fast forwarding or rewinding, where the tape spins rapidly but stops automatically when it reaches the end of the tape.

1.3 Role of travel switches in the industry

The real place for travel switches is in industry, where they are used with other devices to form more complex automated equipment.

We use them to control the movement of the workpiece or the travel of the automatic tool feed to avoid collision accidents. Sometimes, the travel switch automatically changes the direction of the controlled object between two specified positions to get a constant reciprocating motion.

For example, the automatic material transportation cart reaches the endpoint. It touches the travel switch, which turns on the overturning mechanism to dump the material from the cart and return to the starting point. After arriving at the starting point, it touches the travel switch at the starting point again, connects the circuit of the loading mechanism, and starts to load the car automatically. Always go on like this: it becomes a set of automatic production lines without human management, working day and night, saving human manual labor.

2.1 Principles of operation

Proximity switches are also known as non-contact travel switches. It can not only complete the travel control and limit protection but also be a non-contact detection device used to detect the parts’ size, speed measurement, etc. It can also be used for frequency counters, frequency pulse generators, liquid level control, and processing programs such as automatic connection.

2.2 Characteristics

Reliable operation, long service life, low power consumption, high repositioning accuracy, high operating frequency, and adaptability to harsh working environments.

2.3 Species

Among the various types of switches, there is a component that can “sense” the proximity of an object – the displacement sensor. It uses displacement sensors on the sensitive characteristics of the approaching object to control the switch, which is the proximity switch.

When an object moves toward the proximity switch and approaches a certain distance, the displacement sensor can “sense,” and then the switch will act; usually, this distance is called the “detection distance.”

2.3.1 Eddy current proximity switches

This type of switch is sometimes called an inductive proximity switch. It uses conductive objects near the electromagnetic field proximity switch so that the object generates internal eddy currents. This eddy current reacts to the proximity switch so that the switch’s internal circuit parameters change, thus recognizing the conductive object to move closer and then controlling the switch on or off. This proximity switch can detect the object must be conductive.

2.3.2 Capacitive proximity switches

Measurement of this type of switch usually constitutes one pole plate of the capacitor. In contrast, the other pole plate is the switch’s enclosure, which is generally grounded or connected to the equipment’s chassis during measurement. When an object moves close to the switch, regardless of whether it is a conductor, due to its proximity, always make the capacitance of the dielectric constant changes, so that the capacitance changes, creating and measuring the head connected to the circuit state also change, which can control the switch on or off. This proximity switch detection object, not limited to conductors, can be insulated liquid or powder.

2.3.3 Hall Proximity Switches

A Hall element is a magnetically sensitive element. Switches made with Hall elements are called Hall switches. When a magnetic object moves close to the Hall switch, the Hall element on the detection surface of the switch produces the Hall effect and causes the internal circuit state of the switch to change, thus recognizing the presence of a magnetic object in the vicinity and controlling the switch on or off. The detection object of this proximity switch must be magnetic.

2.3.4 Optical Proximity Switches

The switch made by utilizing the photoelectric effect is called a photoelectric switch. The light-emitting and photoelectric devices go in a certain direction in the same detection head. When a reflective surface (the object to be detected) closes, the photoelectric device receives the reflected light, which will be in the signal output, which can be “sensed” there is an object close.

2.3.5 Pyroelectric Proximity Switches

Switches made of components that sense temperature changes are called pyroelectric proximity switches. This type of switch has a pyroelectric device mounted on the detection surface of the switch. When an object with a temperature different from that of the environment is approached, the output of the pyroelectric device changes to detect the approach of the object.

2.3.6 Other types of proximity switches

When the distance of an observer or system from a wave source changes, the frequency of the approaching wave is shifted, a phenomenon known as the Doppler effect. This phenomenon is called the Doppler effect. Sonar and radar are made by utilizing the principle of this effect. The Doppler effect can be used to make ultrasonic proximity switches, microwave proximity switches, etc. When an object approaches, the reflected signal the proximity switch receives will generate a Doppler shift, which can be recognized as an approaching object.

2.4 Main uses

In daily life, hotels, restaurants, garages, automatic doors, and automatic heaters have applications. Security and anti-theft measures such as data archives, accounting, finance, museums, vaults, and other important places are usually equipped with various proximity switches composed of anti-theft devices. In the measurement technology, such as length and position measurement, the control technology, such as displacement, speed, acceleration measurement, and control, also uses a large number of proximity switches.

2.5 Notes

Because these two kinds of proximity switches on the environment of the lower requirements of the conditions. When the measured object is conductive or can be fixed on a piece of metal object, the

Generally, we use eddy current proximity switches because of their high response frequency, good resistance to environmental interference, a wide range of applications, and lower prices.

If the object to be measured is non-metal (or metal), liquid level height, powder height, plastic, tobacco, etc. Should be used capacitive proximity switch. The response frequency of this switch is low but has good stability.

The influence of environmental factors should be considered when installing the switch. Suppose the object is a magnetically conductive material or the magnet is buried in the object to be measured to distinguish it from the object moving together. In that case, Hall proximity switches should be used, which are the least expensive.

No dust pollution occasions can be used as a photoelectric proximity switch in relatively good environmental conditions. The photoelectric proximity switch works on the object to be measured with almost no shadow. Therefore, it is widely used in demanding fax machines and tobacco machines.

In the anti-theft system, automatic doors usually use pyroelectric, ultrasonic, and microwave proximity switches. Sometimes, to improve the reliability of identification, the above types of proximity switches are often combined.

No matter which kind of proximity switch is selected, attention should be paid to the operating voltage, load current, response frequency, detection distance, and other indicator requirements.

① Use a travel switch for single-direction position detection.

② Travel switches are used for protection functions.

③ Proximity switches are used when it is necessary to detect the passage of an object through a certain point.

④ Proximity switches for bi-directional position detection

The travel switch has a trigger pendulum, so the position and angle of the detection object must be determined (communicate with the mechanical design).

⑥ Whichever switch bracket should be adjustable (some switches can be fine-tuned themselves)

A travel switch is generally one normally open and one normally closed contact; generally, only one pair is used, so two wires can be used.

⑧ Proximity switches typically have a 2-wire, 3-wire, or 4-wire system (2-wire proximity is best not to make because its leakage current is larger), so the line is more than the travel switch.

Proximity switches should always be aware of PNP and NPN issues and should be consistent with the equipment to which they are connected.

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