A photoelectric sensor is a sensor that uses photoelectric elements as its detection element. It first converts the change in the measured quantity into a change in a light signal, and then, with the help of photoelectric elements, further converts the light signal into an electrical signal. A photoelectric sensor generally consists of three parts: a light source, an optical path, and photoelectric elements.
Working Principle: A photoelectric sensor generally consists of two parts: a processing path and a processing element. Its basic principle is based on the photoelectric effect, converting the change in the measured quantity into a change in a light signal. Then, with the help of photoelectric elements, the non-electrical signal is further converted into an electrical signal. The photoelectric effect refers to the phenomenon where light irradiates an object, which can be seen as a series of photons with a certain energy bombarding the object. At this time, the photon energy is transferred to an electron, and the entire energy of one photon is absorbed by one electron at a time. After receiving the energy transferred by the photon, the electron's state changes, thus causing a corresponding electrical effect in the irradiated object.
The photoelectric effect is usually classified into three categories:
(1) The phenomenon that electrons can escape from the surface of an object under the action of light is called the external photoelectric effect, such as phototubes and photomultiplier tubes;
(2) The phenomenon that can change the resistivity of an object under the action of light is called the internal photoelectric effect, such as photoresistors and phototransistors;
(3) The phenomenon that an object generates an electromotive force in a certain direction under the action of light is called the photovoltaic effect, such as a photovoltaic cell.
Photoelectric sensors are sensors that use photoelectric devices as conversion elements. They can be used to detect non-electrical physical quantities that directly cause changes in light intensity, such as light intensity, illuminance, radiation thermometry, and gas composition analysis; they can also be used to detect other non-electrical quantities that can be converted into changes in light intensity, such as part diameter, surface roughness, strain, displacement, vibration, velocity, acceleration, as well as the shape and working status of objects. Photoelectric sensors are characterized by non-contact operation, fast response, and reliable performance, and are therefore widely used in industrial automation devices and robots.
I. Smoke and Dust Turbidity Monitor
Preventing industrial dust pollution is a crucial task in environmental protection. To eliminate industrial dust pollution, it's essential to first understand the amount of dust emitted; therefore, monitoring, automatic display, and alarm systems for exceeding emission limits are necessary. The turbidity of dust in a flue is detected by measuring the change in light intensity as it travels through the flue. If the flue turbidity increases, the absorption and refraction of light emitted from the light source by dust particles increases, reducing the amount of light reaching the photodetector. Consequently, the strength of the photodetector's output signal reflects the change in flue turbidity.
II. Barcode Scanning Pen
When the scanning pen moves across the barcode, if it encounters a black line, the light emitted by the LED is absorbed by the black line, and the phototransistor receives no reflected light, exhibiting high impedance and remaining in a cutoff state. When it encounters a white space, the light emitted by the LED is reflected to the base of the phototransistor, generating a photocurrent and turning it on. After the entire barcode has been scanned, the phototransistor transforms the barcode into a series of electrical pulse signals. These signals are amplified and shaped to form a pulse train, which is then processed by a computer to complete the recognition of the barcode information.
III. Product Counter
As the product moves along the conveyor belt, it continuously blocks the light path from the light source to the photoelectric sensor, causing the photoelectric pulse circuit to generate electrical pulse signals. Each time the product blocks the light, the photoelectric sensor circuit generates a pulse signal. Therefore, the number of output pulses represents the number of products. These pulses are counted by the counting circuit and displayed by the display circuit.
IV. Photoelectric Smoke Detector
When there is no smoke, the light emitted by the LED travels in a straight line, and the phototransistor does not receive a signal, resulting in no output. When there is smoke, the light emitted by the LED is refracted by smoke particles, allowing the phototransistor to receive the light, resulting in a signal output and triggering an alarm.
V. Measuring Rotational Speed
The rotating shaft of the electric motor is painted with black and white colors. When it rotates, reflected light and non-reflected light appear alternately. The photoelectric sensor receives the reflected light signal intermittently and outputs an intermittent electrical signal. The signal is then amplified and shaped by an amplifier and shaping circuit to output a square wave signal. Finally, the motor speed is output by an electronic digital display.
VI. Applications of photovoltaic cells in photoelectric detection and automatic control
When photovoltaic cells are used as photodetectors, their basic principle is the same as that of photodiodes, but their basic structure and manufacturing process are not exactly the same. Because photovoltaic cells do not require an external voltage to operate; they have high photoelectric conversion efficiency, a wide spectral range, good frequency characteristics, and low noise, they have been widely used in photoelectric readout, photoelectric coupling, grating ranging, laser collimation, movie sound reproduction, ultraviolet monitors, and flameout protection devices for gas turbines.
Classification 1: Classified by the nature of the output quantity of photoelectric sensors
According to the output properties of photoelectric sensors, they can be divided into two categories: (1) Photoelectric measuring instruments that convert the measured quantity into a continuously changing photocurrent can be used to measure the intensity of light and physical quantities such as the temperature, light transmittance, displacement and surface condition of an object. For example: illuminance meters for measuring light intensity, photoelectric pyrometers, photoelectric colorimeters and turbidimeters, photoelectric alarms for fire prevention, and automatic detection devices and instruments for checking the diameter, length, ellipticity and surface roughness of processed parts, all of which use photoelectric elements as their sensitive elements. Semiconductor photoelectric elements are not only widely used in civilian industrial fields, 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) Converting the measured quantity into a continuously changing photocurrent. Various photoelectric automatic devices are made by utilizing the characteristic of photoelectric elements to output electrical signals "with" or "without" when exposed to light or no light. Photoelectric elements are used as switching photoelectric conversion elements. For example, photoelectric input devices for electronic computers, switching temperature control devices and digital photoelectric speed measuring instruments for measuring speed, etc.
2. Standard Classification
(1) Slot-type photoelectric sensor
A slotted photoelectric switch consists of a light emitter and a receiver mounted face-to-face on opposite sides of a slot. The emitter emits infrared or visible light, which the receiver can receive under unobstructed conditions. However, when an object passes through the slot, the light is blocked, triggering the photoelectric switch to output a control signal that cuts off or connects the load current, thus completing a control action. The detection distance of the slotted switch is typically only a few centimeters due to the limitations of its overall structure.
(2) Through-beam photoelectric sensors: By separating the emitter and receiver, the detection distance can be increased. A single emitter and receiver constitute a through-beam split photoelectric switch, or simply a through-beam photoelectric switch. The detection distance of a through-beam photoelectric switch can reach several meters or even tens of meters. When using a through-beam photoelectric switch, the emitter and receiver are installed on opposite sides of the path of the object being detected. When the object passes through and blocks the light path, the receiver activates and outputs a switch control signal.
(3) Reflector type photoelectric switch
A photoelectric switch that integrates the emitter and receiver into a single device, with a reflector mounted in front, utilizes the principle of reflection to achieve photoelectric control. Normally, the light emitted by the emitter is reflected back by the reflector and received by the receiver. However, if the object being detected blocks the light path, and the receiver cannot receive the light, the photoelectric switch activates, outputting a switch control signal.
(4) Diffuse-reflective photoelectric switch
The detection head of a diffuse reflective photoelectric switch also contains a light emitter and a light receiver, but there is no reflector in front of the diffuse reflective photoelectric switch. Under normal circumstances, the light emitted by the light emitter cannot be detected by the light receiver. During detection, when the object being detected passes by, it blocks the light and partially reflects the light back. The light receiver then receives the light signal and outputs a switching signal.
Troubleshooting the cause of no signal output
The first thing to consider is the wiring or configuration. For through-beam photoelectric sensors, a combination of a light-emitting part and a light-receiving part must be used, and both ends need to be powered; while retroreflective sensors must be used in combination with a sensor probe and a retroreflector. At the same time, the user must provide a stable power supply to the sensor. If it is a DC power supply, the positive and negative terminals must be confirmed. If the positive and negative terminals are connected incorrectly, there will be no output signal.
The above analysis considers the photoelectric sensor itself. We also need to consider the position of the object to be detected. If the object is not within the detection area, the detection is futile. The object must be within the sensor's detection range, that is, within the photoelectric sensor's sensing range. Secondly, we need to consider whether the sensor's optical axis is aligned. For through-beam sensors, the light-emitting and receiving axes must be aligned; similarly, for retroreflective sensors, the probe and reflector axes must be aligned. We also need to consider whether the object meets the standard for a standard or minimum detection object. The object cannot be smaller than the minimum detection object standard to avoid problems with through-beam and reflective sensors detecting transparent objects. For example, reflective sensors have requirements regarding the color of the object; the darker the color, the shorter the detection distance.
If the above situations can be clearly ruled out, the next step is to check for environmental interference factors. For example, the light intensity must not exceed the rated range; if there is dust in the environment, the surface of the photoelectric sensor probe needs to be cleaned regularly; multiple sensors installed too closely may interfere with each other; another significant factor is electrical interference, and if there are high-power devices nearby, appropriate anti-interference measures must be in place. If you have checked all of the above and these factors can be clearly ruled out, but there is still no signal output, it is recommended to return the device to the manufacturer for testing and diagnosis.
