Photoelectric sensor is a sensor which uses photoelectric devices as conversion elements. It can be used to detect non-electrical physical quantities which directly cause the change of light quantity, such as light intensity, illumination, radiation temperature measurement, gas component analysis, etc. It can also be used to detect other non-electric quantities which can be converted into light quantity change, such as the diameter of parts, surface roughness, strain, displacement, vibration, speed, acceleration, and the shape of objects. Identify the working state, etc. Photoelectric sensors are widely used in industrial automation devices and robots because of their non-contact, fast response and reliable performance. The emergence of new optoelectronic devices, especially the birth of CCD image sensor, has opened a new page for the further application of optoelectronic sensors.
The Photelectric Sensor Classification
The photoelectric sensor generally consists of two parts: processing path and processing element. Its basic principle is based on the photoelectric effect, which converts the measured changes into changes of optical signals, and then further converts non-electric signals into electrical signals by means of photoelectric components. Photoelectric effect refers to illuminating an object with light. It can be seen as a series of photons bombarding the object with a certain energy. At this time, the photon energy is transmitted to the electron, and all the energy of a photon is absorbed by an electron at one time. The state of the electron will occur when it gets the energy transferred by the photon. Change, so that objects exposed to light produce corresponding electrical effects. Generally, photoelectric effects are classified into three categories:
(1) The phenomenon that electrons escape from the surface of an object under the action of light is called external photoelectric effect, such as phototube, photomultiplier tube, etc.
(2) The phenomenon that the resistivity of an object changes under the action of light is called internal photoelectric effect, such as photoresistor, phototransistor, etc.
(3) Under the action of light, the phenomenon that an object produces an electromotive force in a certain direction is called photovoltaic effect, such as photovoltaic cells.
Photoelectric detection method has the advantages of high accuracy, fast response, non-contact, and many measurable parameters. The sensor has simple structure and flexible forms. Therefore, photoelectric sensors are widely used in detection and control.
Photoelectric sensor is the key component to realize photoelectric conversion in various photoelectric detection systems. It is a device that converts optical signals (visible and ultraviolet laser) into electrical signals.
Photoelectric sensors are generally composed of light source, optical path and photoelectric components. The measured changes are converted into changes of optical signals, and then the optical signals are further converted into electrical signals by means of photoelectric components.
Principle of Photoelectric Effect
Photoelectric element is the most important part of photoelectric sensor. Its core principle is different types of photoelectric effect. According to wave-particle duality, light is composed of photons moving at the speed of light. When an object is irradiated by light, its internal electrons absorb the energy of photons and change their state, and their electrical properties will also change. This phenomenon is called photoelectric effect.
According to the different changes of electrical properties, the photoelectric effect can be divided into three types:
1) External photoelectric effect
The phenomenon that electrons escape from the surface of an object under the action of light is called external photoelectric effect. Photoelectric components based on external photoelectric effect include photoelectric tube, photomultiplier tube, etc.
2) photoconductive effect
Electrons in semiconductors can not jump out of semiconductors after absorbing photons, so that the conductivity of objects changes, or the phenomenon of photoelectromotive force is called internal photoelectric effect. According to its working principle, internal photoelectric effect can be divided into photoconductive effect and photovoltaic effect. Photoelectric components based on photoconductive effect include photoresistors, phototransistors, etc.
3) Photovoltaic effect
Under the action of light, the phenomenon that an object produces an electromotive force in a certain direction is called photovoltaic effect. Photovoltaic devices based on photovoltaic effect include photovoltaic cells, photosensitive diodes, transistors, etc.
Operating Principle of Photoelectric Components
Based on different photoelectric effects, let’s see how they work respectively:
External Photoelectric Effect Device
The optoelectronic devices made from the external photoelectric effect of the electron emitted by the substance under the irradiation of light are usually vacuum or inflatable optoelectronic devices, such as phototubes and photomultipliers.
For example, when the incident light is irradiated on the cathode, a single photon transfers all its energy to a free electron in the cathode material, thus increasing the energy of the free electron. When the energy of the electron is greater than the work of the cathode material, it can overcome the bondage of the metal surface and escape, forming an electron emission, which is called photoelectron. Photoelectrons are generated only when the frequency of incident light is higher than the limit frequency.
After photoelectrons are generated, they are absorbed by the anode in the vacuum tube, which generates an electric current. If the illumination intensity is increased at this time, more photons will be irradiated to the cathode material, which will produce more photoelectrons, and the photocurrent will increase accordingly. When the R value of resistance is determined, the photoelectric current in the circuit is a function of the illumination intensity of the incident light, so that the photoelectric conversion can be realized. The illumination intensity can be calculated by measuring the number of current read by the circuit.