Help! Regarding the limit switch?

The capacitive proximity switch is also a type of position sensor with a digital output. Its sensing element is usually one of the plates of a capacitor, while the other plate is the object itself. When the object moves toward the proximity switch, the dielectric constants of the object and the proximity switch change, which in turn alters the state of the circuit connected to the sensing element; this allows for the control of the switch’s on and off states. The objects that can be detected by this proximity switch are not limited to metal conductors; they can also be insulating liquids or powdery substances. When detecting objects with a low dielectric constant ε, it is possible to increase the sensing sensitivity by turning the potentiometer (located at the back of the switch) clockwise. Generally, the potentiometer is adjusted so that the capacitive proximity switch operates at a value of 0.7–0.8 Sn. Author: Dongli Yong, posted on January 21, 2007, at 13:02:00, in: “Electrical Components Forum”

An inductive proximity switch is a type of position sensor with a digital output; it consists of an LC high-frequency oscillator and an amplification circuit. When a metal object approaches this oscillating induction head that generates an electromagnetic field, eddy currents are induced within the object. This eddy current acts back on the proximity switch, reducing its oscillation capacity and altering the parameters of its internal circuit; this allows detection of whether a metal object is approaching, thereby controlling whether the switch is turned on or off. The objects that this proximity switch can detect must be metal objects. Author: Unknown Source: Shiwan Ziliao Jishu Wang

When a metal or semiconductor sheet carrying an electric current is placed vertically in a magnetic field, a potential difference is generated at its two ends; this phenomenon is known as the Hall effect. The potential difference at both ends is called the Hall voltage U. Its expression is U=K·I·B/d, where K is the Hall coefficient, I is the current passing through the thin sheet, B is the magnetic flux density of the external magnetic field (Lorentz force), and d is the thickness of the thin sheet. The input of a Hall switch is characterized by the magnetic induction strength B; when this value reaches a certain level, the flip-flop inside the Hall switch flips, and accordingly the output level of the Hall switch also changes. The output terminal generally uses a transistor for output. Hall switches feature no electric shock risk, low power consumption, a long service life, and a high response frequency. They are integrated using epoxy resin internally, which enables them to operate reliably in various harsh environments. Hall switches can be used as proximity switches, pressure switches, odometers, etc., as a new type of electrical component. The function of a Hall switch is similar to that of a reed magnetically controlled switch, but it has a longer lifespan, faster response time, and no wear and tear. When installing it, it is important to pay attention to the polarity of the magnets; if the polarity is reversed, the switch will not work. Extracted from 110wplc.cn; author unknown. Link: http://110wplc.cn/Article/Catalog4/201.html. This post was last edited by Guo Ke Ye on 2008-1-14 at 12:50

I’m not sure what type of limit switch the original poster is using; those with mechanical impact rods are contact-type, while proximity switches are non-contact type.

The most commonly used limit switches are of the microswitch type and the proximity type.

The most commonly used type nowadays is the Hall-effect proximity switch, which features reliable operation, high sensitivity, and a low failure rate.

A limit switch is a type of field instrument used in automatic control systems for indicating the valve position and providing signal feedback. It outputs signals in the form of switches (contacts) to indicate whether the valve is open or closed; these signals are received by programmable controllers or computers. Once confirmed, they enable the next sequence of operations to be carried out. Limit switches can also serve as an important means for valve interlock protection and remote alarm indication within automatic control systems. They can work together with explosion-proof valve control boxes to provide complete signal control and feedback functions. There are both contact-type and non-contact-type limit switches. The contact method is relatively straightforward: on the moving parts of mechanical equipment, a travel switch is installed, while at the fixed point that moves in opposition to them, a stop block at the extreme position is installed, or the installation positions may be reversed. When the mechanical contacts of the travel switch come into contact with the stop block, the control circuit is interrupted (or altered), sending back a signal indicating on or off. There are many forms of non-contact detection; common ones include reed switches, photoelectric types, inductive types, and so on. Generally, mechanical types are more reliable for cut-off valves; when the quality of proximity-type valves is poor, their sensitivity is insufficient, and they often give false alarms in harsh environmental conditions.