Introduction
A photoelectric sensor is a sensor that uses a photoelectric component as a conversion element. In electronics, the photoelectric sensor converts the light intensity into electrical signals. It is generally composed of three parts: transmitter, receiver, and a detection circuit.
The transmitter emits a light beam to the target. The emitted light beam is generally a semiconductor light source, such as a light-emitting diode (LED), a laser diode, and an infrared emitting diode. The beam is emitted continuously, or change the pulse width to accomplish it. The receiver is composed of a photodiode, a phototransistor, and a photocell commonly. In front of the receiver, optical components such as lens and aperture are installed. Behind it is the detection circuit, which can filter out the effective signal and make actions.
1 Photoelectric Sensor Basic
1.1 Photoelectric Sensor Characteristics
Photoelectric sensors can be used to detect non-electrical physical quantities that directly cause changes in light, such as light intensity, illuminance, radiation temperature measurement, gas composition analysis, etc., and can also be used to detect other non-electrical quantities that can be converted into light quantity changes, such as part diameter, surface roughness, displacement, vibration, speed, acceleration, as well as the recognition of the shape and working state of the object. It has the characteristics of non-contact, fast response, reliable performance, etc., So it is widely used in industrial automation devices and robots.
1.2 Photoelectric Effect
The principle of photoelectric sensors is usually divided into three categories:
(1) The phenomenon that can cause electrons to overflow the surface of an object under the action of light is called the external photoelectric effect, such as a photoelectric tube, photomultiplier tube, etc.
(2) Under the action of light, the change of resistance is called the internal photoelectric effect, such as a photoresistor, phototransistor, etc.
(3) An object that generates a certain directional electromotive force under the action of light is called the photovoltaic effect, such as photovoltaic cells.
1.3 Optical Signals into Electrical Signals
Photoelectric sensors that use photoelectric components based on the photoelectric effect as sensing parts have various applications. According to the nature output of the device, it has two principles:
Semiconductor optoelectronic components are widely used in the civil industry and have an important role in the military. 1) A continuously changing photocurrent can be used to measure the intensity of light and physical quantities such as temperature, light transmission ability, displacement, and surface state of the object. For example, illuminance meter for measuring light intensity, photoelectric pyrometer, photoelectric colorimeter, turbidity meter, photoelectric alarm, automatic detection device for quality testing products. ndFor example, lead sulfide photoresistors can be made into infrared night vision devices, infrared cameras, and infrared navigation systems.
Various photoelectric automatic devices are made with the characteristics of "with" or "without" electrical signal output when photoelectric elements are irradiated by light or no light. 2) Convert the measured stuff into a continuously changing photocurrent. The photoelectric element is used as a switch-type photoelectric conversion element. For example, the photoelectric input device of the electronic computer, the switch type temperature adjusting device and the digital photoelectric speed measuring instrument, etc.
Based on the above mentioned, the sensing types of the photoelectric sensors include through-beam arrangement, reflective arrangement, laser-reflective, and diffuse type.
2 Photoelectric Sensor Applications
2.1 What are Photoelectric Sensors Used?
1) Smoke and Dust Concentration Monitor
Preventing industrial smoke and dust pollution is one of the important tasks of environmental protection. To eliminate it, we must first know the amount of smoke and dust emissions. Therefore, the smoke and dust source must be monitored, automatically displayed, and alarmed for exceeding the standard. The turbidity of the smoke and dust in the flue is detected by the size of the change during the transmission of light in the flue. If it increases, the light source's light is absorbed and refracted by the soot particles, and the light reaching the photodetector decreases. Therefore, the strength of the output signal of the photodetector can reflect the change of flue turbidity.
2) Barcode Scanner
When the scanning pen moves on the bar code, if it encounters a black line, the light of the light-emitting diode will be absorbed by the black line, and the phototransistor will not receive the reflected light present a high impedance and be in a cut-off state. When encountering the white interval, the light emitted by the LED is reflected the base of the photosensitive triode, which generates a photocurrent to conduct. After the entire bar code is scanned, the phototransistor deforms the bar code into electrical pulse signals, which are amplified and then reshaped to form a pulse train, and finally processed by a computer to complete the identification of the bar code information.
3) Product Counter
When the product is running on the conveyor belt, it continuously shields the light path from the light source to the photoelectric sensor so that the photoelectric pulse circuit generates an electrical pulse signal. Every time the light is shaded by-products, the photoelectric sensor circuit generates a pulse signal. Therefore, the number of output pulses represents the number of products. The pulse is counted by the counting circuit and displayed by the display circuit.
4) Photoelectric Smoke Detector
When there is no smoke, the light emitted by the LED travels straight, and the phototransistor does not receive a signal. When there is no output but having smoke, the light emitted by the light-emitting diode is refracted by the smoke particles so that the triode receives the light, a signal output sends an alarm.
5) Measuring Speed
Coated with black and white on the rotating shaft of the motor. When rotating, the reflected light and non-reflected light alternately appear. The photoelectric sensor correspondingly receives the reflected light signal and outputs the intermittent electrical signal, amplified by the amplifier and the shaping circuit. The square wave signal is shaped and output, and finally, the motor speed is output by the electronic digital display.
6) Photocells in Photoelectric Detection and Automatic Control
When photocells are used as photodetectors, their basic principles are the same as photodiodes, but their basic structures and manufacturing processes are not the same. Since the photovoltaic cell does not require an external voltage when working, it has high photoelectric conversion efficiency, wide spectral range, good frequency characteristics, low noise, etc. They are widely used in the photoelectric readout, photoelectric coupling, grating ranging, laser collimation, film sound recovery, UV monitors, flameout protection devices for gas turbines, etc.
3 Photoelectric Sensor Development
3.1 Advantages
` Long Detection Distance
If the detection distance of 10m or more is set in the through-beam type, it can detect remotely that other detection methods (magnetic, ultrasonic, etc.).
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