Characteristics of High Frequency Oscillatory Contactless Switch

Proximity switch AR-NLG-LBM15 is normally closedContactless output, long operating life, and stable detection even in harsh environments with water or oil splashes. It belongs to a position sensor with switch output, consisting of an LC high-frequency oscillator and an amplification processing circuit. When a metal object approaches this oscillation induction head that can generate an electromagnetic field, eddy currents are generated inside the object. This eddy current reacts on the proximity switch, causing the oscillation ability of the proximity switch to decay, and the parameters of the internal circuit to change, thereby identifying whether a metal object is approaching and controlling the on or off of the switch. The object that can be detected by this proximity switch must be a metal object.

The high-frequency oscillation type proximity switch consists of a detection element, a detection unit, an amplification unit, an integer unit, and an output unit. The detection element installed inside this sensor is composed of a detection coil and a high-frequency oscillator. After being powered on, the detection coil generates an alternating electromagnetic field. When a metal object approaches the electromagnetic field, the magnetic flux density on the metal surface changes, generating induced currents - eddy currents. The magnetic flux generated by eddy currents is always in the opposite direction of the magnetic flux of the detection coil. Due to the effect of eddy currents, the energy consumption of the detection coil increases, the quality coefficient decreases, the amplitude decreases, and even the oscillator stops oscillating. On the contrary, when the metal object moves away from this active area, the oscillator begins to oscillate again. After detecting the state change of the amplitude sensor, the detection circuit converts it into a switch signal.

Principle of high-frequency oscillation proximity switch:

When using proximity switches, it is important to pay attention to the operation method, as everything related to electricity can be quite dangerous. A proximity switch is a type of position sensor with a switch output, which can be divided into inductive and capacitive types based on its working principle. Inductive proximity switches are composed of LC high-frequency oscillators and amplification processing circuits, which use metal objects to generate eddy currents inside the object when approaching the oscillating sensing head that can generate an electromagnetic field. High temperature resistant proximity switches can be made according to customer needs, with a temperature of 150C.

The AC power supply is connected to the rectifier module, and after filtering and three-phase full wave rectifier, it is converted into DC. Then, it is connected to the high-frequency inverter circuit to convert DC into high-frequency AC. After passing through the high-frequency transformer, rectifier bridge, and filter, it outputs a stable DC.

The high-frequency switch circuit mainly consists of rectification and filtering circuit, full bridge conversion circuit, PVm space-time control circuit, voltage stabilization and limiting circuit, current stabilization and limiting circuit, protection circuit, and auxiliary power supply circuit.

After passing through the power switch, the voltage of the three-phase power grid (or single-phase) is rectified and filtered to obtain a smooth DC voltage of 520Vdc (300Vdc for single-phase), which is supplied to the inverter circuit..

The inverter circuit is mainly composed of high-power IGBT modules (or field-effect MOSFET modules) to form a full bridge conversion circuit. When P brews out the control signal and drives the power module through isolated drivers, the two sets of diagonal tubes alternately conduct, generating high-frequency pulse voltage at the primary of the high-frequency transformer. The secondary voltage is transformed by the high-frequency transformer and rectified to provide energy to the load.

The output terminals are respectively connected with voltage stabilization, current limiting, and feedback circuits such as current stabilization and voltage limiting. When placed in a stable voltage state, the voltage stabilization and current limiting circuits come into effect. When the output voltage rises or falls, the sampled voltage is compared with the reference voltage through the internal voltage comparator of the stabilizing circuit. The error signal voltage is added to Pw to control the circuit, causing the PV output pulse width to change accordingly, thereby stabilizing the output voltage. If the load current is too high, the current limiting circuit works to limit the output current within the current limiting set value.

Similarly, in a steady current state, the steady current circuit functions to stabilize the output current within the set value, while when overvoltage occurs, the voltage limiting circuit clamps the output voltage at the voltage limiting value. When there is an abnormal situation (such as input overvoltage or undervoltage, overcurrent or overheating, etc.) that generates a protection signal and applies it to the protection control circuit, the protection circuit outputs a voltage to the PVM circuit, causing the PM circuit to stop outputting, thus achieving the protection purpose.

The difference between normally open and normally closed proximity switches:

Normally open: When no object is detected approaching, the proximity switch is in the "off" state and the LED is turned off. The so-called off refers to the transistor inside the proximity switch not conducting.

Normally closed: When no object is detected approaching, the proximity switch is in the "fully closed" state, and the LED lights up. The so-called "fully closed" refers to the conduction of the transistor inside the proximity switch.

Analysis of NPN near switch state:

NPN proximity switch uses NPN transistor to control signal output internally, and the load needs to be connected between the brown line (VCC) and black line (output) of NPN proximity switch

Normally open: When no detection object approaches, the internal transistor is turned off, the output line is clamped by an internal pull-up resistor, the output line is at a high level, and the load does not work; When the detection object approaches, the internal transistor conducts, and due to the connection between the output line and the collector of the transistor, it is at a low level, causing the load to operate.

Normally closed type: The signal state of normally closed type is opposite to that of normally open type. When there is no detection object approaching, the internal transistor conducts and the load works. Due to the connection between the output line and the collector of the transistor, the output line is at a low level; When the detection object approaches, the internal transistor is turned off, and the output line is clamped by the internal pull-up resistor to a high level, and the load does not work.

LED, as an indicator light for proximity switches, will light up when the transistor is closed. For normally open proximity switches, the LED will light up when metal approaches; For normally closed proximity switches, the LED will turn off when metal approaches.