How Does a Photoelectric Sensor Work?

IntroductionPhotoelectric sensor is a sensor that uses a photoelectric component as a conversion element. In electronics, the photoelectric sensor converts the light intensity into the electrical signals. It is generally composed of three parts: transmitter, receiver and detection circuit.The transmitter emits a light beam to the target, and 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 Basic1.1 Photoelectric Sensor CharacteristicsPhotoelectric 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 EffectThe 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 external photoelectric effect, such as photoelectric tube, photomultiplier tube, etc.(2) Under the action of light, the change of resistance is called the internal photoelectric effect, such as photoresistor, phototransistor, etc.(3) An object 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 SignalsPhotoelectric sensors that use photoelectric components based on photoelectric effect as sensing parts have a wide variety of applications. According to the nature output of the device, it has two principles: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 and turbidity meter, photoelectric alarm, automatic detection device for quality testing products. Semiconductor optoelectronic components are not only widely used in the civil industry, but also have an important role in the military. For example, lead sulfide photoresistors can be made into infrared night vision devices, infrared cameras and infrared navigation systems.2) Convert the measured stuff into a continuously changing photocurrent. 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. 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 photoelectric sensor include through-beam arrangement, reflective arrangement, laser-reflective, and diffuse type.

2 Photoelectric Sensor Applications2.1 What are Photoelectric Sensors Used?1) Smoke and Dust Concentration MonitorPreventing 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 source of the smoke and dust 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 emitted by the light source 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 ScannerWhen 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 to 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 CounterWhen 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 DetectorWhen there is no smoke, the light emitted by the LED travels in a straight line, 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 SpeedCoated 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 intermittently receives the reflected light signal, and outputs the intermittent electrical signal, which is 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 ControlWhen photocells used as photodetectors, their basic principles are the same as photodiodes, but their basic structures and manufacturing processes are not exactly the same. Since the photovoltaic cell does not require an external voltage when working, it has the advantages of high photoelectric conversion efficiency, wide spectral range, good frequency characteristics, low noise, etc. They are widely used in photoelectric readout, photoelectric coupling, grating ranging, laser collimation, film sound recovery, UV monitors and flameout protection devices for gas turbines, etc.

3 Photoelectric Sensor Development3.1 Advantages` Long Detection DistanceIf 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.).a Less Restrictions on Detecting ObjectsAs the detection principle is based on the shading and reflection caused by the detection object, unlike proximity sensors, which limit the detection object to metal. Photoelectric sensor can detect almost all objects such as glass, plastic, wood, and liquid.b Short Response TimeThe light itself is high speed, and the circuit of the sensor is composed of electronic parts, so there is no mechanical working time, and the response time is very short.c High ResolutionHigh-resolution can be achieved by using advanced design technology to concentrate the projected light beam on a small spot, or by forming a special light-receiving optical system. It can also detect small objects and high-precision position.d Non-contact DetectionThe detection can be achieved without mechanically touching the detection object, so it will not cause damage to the detection object and the sensor. Therefore, the photoelectric sensor can be used for a long time.e Color DiscriminationThe reflectance and absorptivity of the light formed by the detection object differ according to the combination of the wavelength of the light projected and the color of the detection object. Using this characteristic, the color of the detected object can be detected.f Easy to AdjustIn the type of projected visible light, because the projected light beam is visible to the eyes, the position of the detection object can be moved easily.

3.2 Smart Photoelectric SensorsDiscover the best Photoelectric Sensors in the future. Smart photoelectric sensors provide essential input for every process chain. They support dynamic, real-time-optimized, and self-organized industry processes. The functions of the smart photoelectric sensor are: self-compensation, self-calibration, self-diagnosis, numerical processing, two-way communication, information storage, and digital output. With the development of science and technology, the functions of smart photoelectric sensors will gradually increase. It will use artificial neural networks, artificial intelligence, and information processing technologies (such as sensor information fusion technology, fuzzy theory, etc.) to enable sensors to have more advanced intelligence, analysis skill, and judgment. So its self-adaptation, and self-learning functions can complete complex tasks such as image recognition, feature detection, and multi-dimensional detection.

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