Principle and application of photoelectric switch

Principle and application of photoelectric switch Deng Zhongyi

Abstract: The principle, terminology, types and matters needing attention of photoelectric switch are introduced. The application of photoelectric switch is illustrated with examples. Keywords: sensor; Photoelectric switch; Principle; App application

China Library Classification Number: TP2 12. 14 Document Identification Number: A.

I. Introduction

The photoelectric switch is a member of the sensor family, which converts the light intensity change between the transmitter and the receiver into the current change to achieve the purpose of detection. Because the output circuit and input circuit of photoelectric switch are electrically isolated, it can be used in many occasions.

Second, the introduction of photoelectric switch

1, working principle

Photoelectric switch (photoelectric sensor) is the abbreviation of photoelectric proximity switch, which uses the blocking or reflection of the light beam by the detected object to gate the circuit through the synchronous loop, thus detecting the presence or absence of the object. Objects are not limited to metals, and all objects that can reflect light can be detected. The photoelectric switch converts the input current into an optical signal on the transmitter and emits it, and then the receiver detects the target object according to the intensity of the received light. The working principle is shown in figure 1. Most photoelectric switches use infrared light waves with wavelengths close to visible light. Fig. 2 is an external view of some photoelectric switches of SICK Company in Germany.

2. Classification and terminology of photoelectric switch

(1), classification

① Diffuse reflection photoelectric switch: it is a sensor integrating transmitter and receiver. When an object passes by, the object reflects enough light from the photoelectric switch transmitter to the receiver, so the photoelectric switch generates a switching signal. When the surface of the detected object is bright or its reflectivity is extremely high, diffuse reflection photoelectric switch is the first choice for detection.

(2) Mirror reflective photoelectric switch: It also integrates transmitter and receiver. The light emitted by the photoelectric switch transmitter is reflected back to the receiver through the mirror. When the detected object passes by and completely blocks the light, the photoelectric switch generates a detection switch signal.

(3) Opposite photoelectric switch: it includes a transmitter and a receiver which are separated from each other in structure and have opposite optical axes. The light from the transmitter goes directly to the receiver. When the detected object passes between the transmitter and the receiver and blocks the light, the photoelectric switch generates a switching signal. When the detected object is opaque, the opposite photoelectric switch is the most reliable detection device.

(4) Slotted photoelectric switch: The standard U-shaped structure is usually adopted, and its transmitter and receiver are located on both sides of the U-shaped slot and form the optical axis. When the detected object passes through the U-shaped groove and blocks the optical axis, the photoelectric switch generates a switching signal. Groove photoelectric switch is suitable for detecting high-speed moving objects, which can distinguish transparent and translucent objects and is safe and reliable to use.

⑤ Optical fiber photoelectric switch: Using plastic or glass optical fiber sensor to guide light, the measured object can be detected at a long distance. Generally, optical fiber sensors are divided into two types: retroreflection type and diffuse reflection type.

Their working lights are shown in Figure 3.

(2) Interpretation of terms

The schematic diagram of common terms is shown in Figure 4.

① Detection distance: refers to the detection from the measured reference position (sensing surface of photoelectric switch) when the detector moves in a certain way.

The spatial distance of the surface. Rated operating distance refers to the nominal value close to the operating distance of the switch.

② Return distance: the absolute value of action distance and reset distance.

③ Response frequency: the number of times the photoelectric switch is allowed to operate within the specified time interval of1s.

④ Output status: normally open and normally closed. When no object is detected, the load connected to the normally open photoelectric switch does not work because the output transistor inside the photoelectric switch is turned off. When an object is detected, the transistor is turned on and the load is energized to work.

⑤ Detection mode: According to the different ways in which the light emitted by the transmitter is reflected back to the receiver when the photoelectric switch detects an object, it can be divided into diffuse reflection, specular reflection and counter reflection.

⑥ Output forms: NPN-2, NPN-3, NPN-4, PNP-2, PNP-3, PNP-4, AC-2, AC-5 (with relay), DC NPN/PNP/ normally open/normally closed function.

⑦ Pointing angle: See the schematic diagram of the pointing angle of photoelectric switch, that is, as shown in the next three figures of Figure 4.

⑧ Surface reflectivity: The light emitted by the diffuse reflective photoelectric switch can only be reflected back to the receiver of the diffuse reflective switch after passing through the surface of the detected object, so the detection distance and the surface reflectivity of the detected object will determine the intensity of the light received by the receiver. The intensity of light reflected by rough surface must be less than that of smooth surface, and the surface of the detected object must be perpendicular to the light emitted by photoelectric switch. See table 1 for the reflectivity of common materials.

Table 1 reflectivity of common materials

Environmental characteristics: the application environment of photoelectric switch will also affect its long-term working reliability. When the photoelectric switch works at the maximum detection distance, the optical lens will be stuck by dirt in the environment and even corroded by some strong acidic substances, which will reduce its use parameters and reliability. A simpler solution is to determine the optimal working distance according to the derated use of the maximum detection distance (Sn) of the photoelectric switch.

(3) Precautions for use

① The infrared sensor is a diffuse reflection product, and the standard detector used is a flat white drawing paper.

② The infrared photoelectric switch can work stably under the condition of high ambient illumination, but in principle, the optical axis of the sensor should be prevented from facing strong light sources such as sunlight.

③ The minimum detectable width of the opposite photoelectric switch is 80% of the lens width of this photoelectric switch.

④ When using inductive loads (such as lamps, motors, etc.). ), its transient surge current is large, which may deteriorate or damage the photoelectric switch of AC secondary line. In this case, please transfer the load through the AC relay.

⑤ The lens of infrared photoelectric switch can be wiped with mirror paper, and chemicals such as diluted solvent are prohibited to avoid permanent damage to plastic mirror.

⑥ According to the actual requirements of users, under some harsh conditions, such as dusty occasions, the sensitivity of the produced photoelectric switch is increased by 50% to meet the requirement of prolonging the maintenance cycle of photoelectric switch in long-term use.

⑦ All products are manufactured by SMD process, and can only be delivered after strict testing, and will not be damaged in general use. In order to avoid accidents, please check whether the wiring is correct and the voltage is rated before connecting the power supply.

The schematic diagram of the above precautions is shown in Figure 5.

Third, the application examples

Various applications of photoelectric switches can be seen from fig. 6. Fig. 6(a) shows a photoelectric switch for material positioning and shearing control; Fig. 6(b) shows that the upper and lower limit values of the liquid level are controlled by photoelectric switches. When the liquid level is higher or lower than the upper and lower limits, the photoelectric switch control circuit can open or close the threshold door to keep the liquid level between the upper and lower limits. Fig. 6(c) shows how to detect the number of passing objects or whether objects exist by using the light shielding effect of objects; Fig. 6(d) shows whether products are arranged at the same height by using linear propagation of light; Fig. 6(e) shows a photoelectric switch for detecting the number of products in an assembly line; Fig. 6(f) shows the level of the liquid level detected by the photoelectric switch.

Four. Concluding remarks

In addition to the above examples, photoelectric switch has many applications, such as stroke control, diameter limitation, speed detection, airflow control and so on. We believe that photoelectric switch will be more and more advanced and its application will be more and more extensive.