An Overview of Photoelectric Sensors

Catalog

Ⅰ Introduction

A photoelectric sensor is a device that converts an optical signal into an electrical signal, and it is based on the photoelectric effect. The photoelectric effect refers to the phenomenon that electrons are emitted from the surface of a metal or its compound under the irradiation of light of a certain frequency. This is the phenomenon in which the electrons of a substance absorb the energy of photons and a corresponding electrical effect occurs. According to different photoelectric effect phenomena, the photoelectric effect is divided into three categories: external photoelectric effect, internal photoelectric effect, and photovoltaic effect. Photoelectric devices include photocells, photomultiplier tubes, photoresistors, photodiodes, phototransistors, photocells, etc.

The photoelectric detection method has the advantages of high precision, fast response, non-contact, etc. The structure of the sensor is simple and the form is flexible and diverse. Therefore, the photoelectric sensor is widely used in detection and control. Photoelectric sensors can be used to detect non-electrical physical quantities that directly cause changes in light quantity, such as light intensity, illuminance, radiation temperature measurement, and gas composition analysis. It can also be used to detect other non-electric quantities that can be converted into changes in the amount of light, such as part diameter, surface roughness, strain, displacement, vibration, speed, acceleration, and the identification of the shape and working state of objects. The photoelectric sensor has the characteristics of non-contact, fast response, and reliable performance, so it is widely used in industrial automation devices and robots. New photoelectric devices are constantly emerging, especially the birth of CCD image sensors, which has opened a new page for the further application of photoelectric sensors.

Ⅱ How do photoelectric sensors work?

The photoelectric sensor is generally composed of two parts: a processing path and a processing element. The photoelectric sensor working principle is based on the photoelectric effect, which converts the measured changes into optical signal changes, and then further converts the non-electrical signals into electrical signals with the help of photoelectric elements. The photoelectric effect refers to illuminating an object with light, which can be seen as a series of photons with certain energy bombarding the object. At this time, the photon energy is transferred to the electron, and the entire energy of a photon is absorbed by one electron at a time. After the electron gets the energy transferred by the photon, its state will change, so that the object irradiated by the light will produce a corresponding electrical effect.

Schematic of the photoelectric effect

Generally, the photoelectric effect is 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 an external photoelectric effect. (photocell, photomultiplier tube, etc.); 

(2) The phenomenon that the resistivity of the object is changed under the light is called an internal photoelectric effect. (photoresistor, phototransistor, etc.); 

(3) Under the light, the phenomenon that an object generates a certain direction of electromotive force is called a photovoltaic effect. (photovoltaic cell).

The photodiode is the most common light sensor. The appearance of the photodiode is the same as the general diode. When there is no light, it is the same as the ordinary diode. The reverse current is very small, called the dark current of the photodiode. When it is under the light, the carrier is excited, producing electron-empty holes, called photoelectric carriers. Under the effect of an external electric field, photoelectric carriers participate in conduction, forming a reverse current much larger than the dark current. This reverse current is called photocurrent. The magnitude of the photocurrent is proportional to the light intensity, so the electrical signal that changes with the light intensity can be obtained on the load resistance.

In addition to the function of the photodiode to convert the optical signal into an electrical signal, the phototransistor also has the function of amplifying the electrical signal. The appearance of the phototransistor is not much different from that of the general triode. Generally, the photosensitive triode only leads out two poles-the emitters and the collector, and the base is not led out. The shell also has a window for light to enter. To increase the illumination, the base area is very large. Because the emission area is small, the incident light is mainly absorbed by the base area. The collector junction is reverse biased during operation, and the transmitter junction is biased forward. The current flowing through the tube when there is no light is the dark current Iceo=(1+β)Icbo, which is smaller than the penetration current of the general triode. When there is light, a large number of electron-hole pairs are excited, making the current Ib generated by the base electrode increase. The current flowing through the tube at this moment is called the photocurrent. The emitter current Ie=(1+β)Ib. It can be seen that the phototransistor has a higher sensitivity than the photodiode.

Photoelectric sensors are controlled by converting changes in light intensity into changes in electrical signals. In general, the photoelectric sensor is composed of three parts, which are divided into a transmitter, a receiver, and a detection circuit.

Working principle of the photoelectric sensor

The transmitter is aimed at the target to emit a light beam. The emitted light beam generally comes from a semiconductor light source, a light-emitting diode (LED), a laser diode, and an infrared emitting diode. The beam is emitted uninterrupted, or the pulse width is changed. The receiver consists of a photodiode, phototransistor, and photocell. In front of the receiver, optical components such as lenses and apertures are installed. Behind it is the detection circuit, which can filter out the valid signal and apply the signal.

The transmitter has a calibrated lens to focus the light towards the receiver, and the receiver outlet cable connects the device to a vacuum tube amplifier. There is a small incandescent lamp as a light source in the metal cylinder. These small and strong incandescent lamp sensors are the prototype of today's photoelectric sensors. The receiver consists of a photodiode, phototransistor, and photocell. In addition, the structural elements of the photoelectric sensor include an emitting plate and optical fibers. The corner reflector is a solid-structured launching device. It consists of a small triangular pyramid reflective material, which can accurately return the light beam from the reflector. It can change the emission angle in the range of 0 to 25 with the optical axis so that the light beam is almost from one emission line. After reflection, it still returns from this reflection line.

Ⅲ Photoelectric sensor Types

1 Slot photoelectric sensor

It is a slot-shaped photoelectric device that mounts an optical transmitter and a receiver face to face on both sides of a slot. The light emitter can emit infrared light or visible light, and without blocking, the light receiver can receive light. However, when the detected object passes through the slot, the light is blocked and the photoelectric switch operates. It outputs a switch control signal to cut off or switch on the load current to complete a control action. The detection distance of the slot switch is generally only a few centimeters because of the limitation of the overall structure.

Slot photoelectric sensor

2 Collimator photoelectric sensor

If the light emitter and the light receiver are separated, the detection distance can be increased. A photoelectric switch composed of a light emitter and a light receiver is called a split-beam photoelectric switch. Its detection distance can reach several meters or even tens of meters. When in use, the light emitter and the light receiver are respectively installed on both sides of the detection object path. When the detection object passes, the light path is blocked, and the light receiver acts to output a switch control signal.

Collimator photoelectric sensor

3 Reflective photoelectric switch

The light-emitting device and the light-receiving device are installed in the same device, and a reflective plate is installed in front of it. The reflective plate-type photoelectric switch is used to complete the photoelectric control function by the reflection principle. Under normal circumstances, the light emitted by the light emitter is reflected back by the reflector and received by the light receiver. Once the light path is blocked by the detection object and the light receiver does not receive the light, the photoelectric switch acts and outputs a switch control signal.

Reflective photoelectric switch

4 Diffusion reflective photoelectric switch

Its detection head is also equipped with a light emitter and a light receiver, but there is no reflector in front. Under normal circumstances, the light emitted by the light emitter cannot be found by the light receiver. When the detection object passes through, it blocks the light and reflects the light part back. The receiver receives the signal and outputs a switching signal.

Ⅳ Applications

There are many types of photoelectric sensors that use photoelectric elements as sensitive elements, and they are widely used. According to the output characteristics of the photoelectric sensor, it can be divided into two categories:

(1) photoelectric measuring instruments made by converting the measured into a continuously changing photocurrent, which can be used to measure the intensity of light and the temperature, light transmission capacity, displacement of objects and surface state, and other physical quantities. There are many sensitive components that use photoelectric components. For example illuminance meter, photoelectric pyrometer, photoelectric colorimeter, and turbidity meter for measuring light intensity, photoelectric alarm for preventing fire, automatic detection device for checking the diameter, length, ellipse, and surface roughness of processed parts. Semiconductor optoelectronic components are not only widely used in the civil industry, but also have an important position in the military. For example, lead sulfide photoresistors can be used to make infrared night vision devices, infrared cameras, and infrared navigation systems, etc.

(2) Convert the measured light into a continuously changing photocurrent. Various photoelectric automatic devices made by using the characteristics of "with" or "without" electrical signal output of photoelectric elements when exposed to light or without light. The photoelectric element is used as a switching photoelectric conversion element. For example, the photoelectric input device of the electronic computer, the switch-type temperature adjustment device, and the digital photoelectric speedometer for speed measurement.

Application of photoelectric sensor

1 Smoke and Turbidity Monitor

Preventing industrial smoke pollution is one of the important tasks of environmental protection. In order to eliminate industrial smoke and dust pollution, we must first know the amount of smoke and dust emissions. The source of smoke and dust must be monitored, automatically displayed, and exceeded the alarm. The smoke turbidity in the flue is detected by the change in the transmission of light through the flue. If the flue turbidity 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 in the flue turbidity.

2 Barcode scanning pen

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, showing high impedance and being in the cut-off state. When white space is encountered, the light emitted by the light-emitting diode is reflected in the base of the phototransistor, and the phototransistor generates a photocurrent and turns on. After the entire bar code is scanned, the phototransistor deforms the bar code into electric pulse signals. These pulse signals are amplified and shaped to form pulse trains, which are then processed by the computer to complete the recognition of the bar code information.

3 Product counter

When the product is moving on the conveyor belt, it continuously blocks the light path from the light source to the photoelectric sensor, so that the photoelectric pulse circuit generates electrical pulse signals. Each time the product is shaded, the photoelectric sensor circuit generates a pulse signal. Therefore, the number of pulses output represents the number of products. The pulse is counted by the counting circuit and displayed by the display circuit.

4 Photoelectric smoke alarm

When there is no smoke, the light from the light-emitting diodes travels in a straight line, and the phototransistor does not receive the signal. When there is smoke, the light emitted by the light-emitting diode is refracted by the smoke particles, so that the triode receives the light, and there is a signal output to send an alarm.

5 Photocell

When a photocell is used as a photodetector, its basic principle is the same as that of a photodiode, but its basic structure and manufacturing process are not exactly the same. The photovoltaic cell does not require an external voltage during operation and the photoelectric conversion efficiency is high, the spectral range is wide, the frequency characteristics are good, and the noise is low. It has been widely used in the photoelectric readout, photoelectric coupling, grating distance measurement, laser collimation, and film replay, UV light monitor and gas turbine flameout protection device, etc.

6 laser weapon

Because the photoelectric sensor is particularly sensitive to infrared radiation, or visible light, or both, it is more likely to be the target of laser attack. In addition, the electronic system and the sensor itself are extremely susceptible to interference from thermal noise and electromagnetic noise generated by the laser. There are mainly the following ways for laser weapons on the battlefield to attack the photoelectric sensor: Use a laser beam of appropriate energy to "blind" the sensor so that it cannot detect or continue to track the detected target. Or, if the sensor is directing the weapon to fly to the target, blinding will cause it to lose its target. In summary, as sensors play an increasingly important role on the battlefield and are vulnerable to laser attacks, they have become the target of choice for low-energy laser weapons.

7 Automatic meter reading system

With the development of microelectronic technology, sensor technology, computer technology, and modern communication technology, photoelectric sensors can be used to develop automatic meter reading systems. The aluminum disk of the electric energy meter is rotated by the torque generated by the eddy current and the magnetic field. The photoelectric sensor can convert the rotation number of the aluminum disc into the pulse number. For example, if the rotating bright aluminum plate is partially blackened, and then it is equipped with a reflective photoelectric transmitter-receiver pair tube. When the aluminum plate rotates, a pulse will be generated at the blackened area, and the rotation speed of the aluminum plate can be converted to the corresponding pulse number and sent to the T0 port of the CPU through the photoelectric coupling isolation circuit for counting processing. The use of photoelectric coupling isolators can effectively prevent interference signals from entering the microcomputer. Combined with other transmission methods, an automatic meter reading system can be formed.

Article Recommended:

All You Need to Know about Ultrasonic Sensors

Image Sensor: How do CCD and CMOS Sensors work?