In industrial automation or other industrial control fields, photoelectric sensors are one of the important components. Currently, there are many types of sensors on the market, and their applications vary in different fields and production environments. For example, they can be used for detection, measurement, analysis and processing. According to type, they can also be divided into photoelectric sensors, proximity sensors, fiber optic sensors, displacement sensors, etc.
In the field of general industrial automation, these types of sensors are generally powered by DC voltage, and the power supply voltage is relatively wide, ranging from DC6 to DC36V.
I. Photoelectric Sensors: Photoelectric sensors are devices that convert light signals into electrical signals. Their working principle is based on the photoelectric effect. The photoelectric effect refers to the phenomenon where, when light shines on certain materials, the electrons in the material absorb the energy of the photons, resulting in a corresponding electrical effect. Based on different phenomena, the photoelectric effect is divided into three categories: external photoelectric effect, internal photoelectric effect, and photovoltaic effect. Photoelectric devices include phototubes, photomultiplier tubes, photoresistors, photodiodes, phototransistors, and photovoltaic cells.
Their classifications can be roughly as follows:
1. Diffuse reflection type: general type or energy type (-8), focused type (-8-H), type with background suppression function (-8-H), type with background analysis function (-8-HW)
2. Reflector type: General type (-6), type with polarization filter function (-54, -55), type with transparent body detection function (-54-G), type with foreground suppression function (-54-V)
3. Through-beam type
4. Channel type
5. Fiber optic sensors: plastic fiber optic type, glass fiber optic type
6. Color mark sensor, color sensor, fluorescence sensor
7. Optical Communication
8. Laser ranging: triangular reflection principle type, phase difference principle type, time difference principle type
9. Grating
10. Explosion-proof/flameproof type
Characteristics of photoelectric sensors:
1. Long detection distance
Photoelectric sensors are used to detect presence or absence, with a detection distance of at least tens of centimeters.
2. Fewer restrictions on the objects being detected
The detection function can detect the presence or absence of most substances with a distinct color, except for those with special colors that require a dedicated photoelectric sensor.
3. Short response time
Light itself is high-speed, and the circuitry of the sensor is made up of electronic components, so it does not include mechanical working time and has a very short response time.
4. High resolution
High resolution can be achieved by focusing the projected light beam onto a small spot through advanced design techniques or by constructing a special light-receiving optical system. It can also be used for the detection of tiny objects and high-precision position detection.
5. Enables non-contact detection
Detection can be performed without mechanical contact with the object being detected, thus preventing damage to the object and the sensor. Therefore, the sensor can be used for a long time.
6. Can perform color discrimination
The color of an object can be detected by measuring the differences in reflectance and absorptivity of light emitted from it, which vary depending on the wavelength of the emitted light and the color of the object.
7. Easy to adjust
The sensor's detection sensitivity and focusing performance can be adjusted. In the type that projects visible light, the projected beam is visible to the eye, making it easy to adjust the position of the detected object.
II. Proximity Sensors : Proximity sensors are a general term for sensors that replace contact-based detection methods such as limit switches, aiming to detect objects without physical contact. They can detect the movement and presence of an object and convert it into an electrical signal. A proximity sensor is a device capable of sensing the approach of an object. It utilizes the sensitivity of displacement sensors to approaching objects to identify their proximity and outputs a corresponding switching signal; therefore, proximity sensors are often called proximity switches. They detect magnetic losses caused by eddy currents generated on the surface of a conductor by the influence of an external magnetic field. Proximity switches are mostly used to detect metals, with short detection distances, often only a few millimeters. They are generally divided into capacitive and inductive types.
Characteristics of proximity sensors:
1. Because it can be performed in a non-contact manner, it will not wear out or damage the object being tested.
2. Due to the use of contactless output, the lifespan is extended (except for magnetic type). The use of semiconductor output has no impact on the lifespan of the contacts.
3. Unlike optical detection methods, this method is suitable for use in environments with water and oil, and is virtually unaffected by stains, oil, or water on the object being tested. Furthermore, it includes Teflon-coated models and products with good chemical resistance.
4. Compared with contact switches, it can achieve high-speed response.
5. It can handle a wide range of temperatures.
6. Unaffected by the color of the object being tested, it detects changes in the physical properties of the object being tested, so it is almost unaffected by surface color, etc.
7. Unlike contact sensors, they are affected by ambient temperature, surrounding objects, and the influence of similar sensors, including inductive and capacitive sensors. Sensors can also interfere with each other. Therefore, mutual interference must be considered when setting up sensors. Furthermore, in inductive sensors, the influence of surrounding metal must be considered, while in capacitive sensors, the influence of surrounding objects must be considered.
Regarding the wiring of the two types of sensors mentioned above
Photoelectric and proximity sensors can generally be divided into NPN and PNP types, and the wiring is uniformly marked with blue wire color in the industry.
Brown is connected to the positive power supply, blue to the negative power supply, and black to the signal output. They can be connected individually or in series. See the diagram below:
Third, fiber optic sensors generally require the use of fiber optic amplifiers, which makes them slightly more expensive. However, in terms of their nature, they are fundamentally different from the photoelectric and proximity sensors mentioned earlier. They can perform qualitative measurements, meaning they can perform quantitative detection, such as measuring distances in millimeters.
A fiber optic sensor is a sensor that converts the state of a measured object into a measurable optical signal. The working principle of a fiber optic sensor is as follows: a light beam incident from a light source is sent through an optical fiber to a modulator. Within the modulator, the light interacts with the external parameters to be measured, causing changes in the optical properties of the light, such as intensity, wavelength, frequency, phase, and polarization state, resulting in a modulated optical signal. This modulated signal is then sent through an optical fiber to a photoelectric device and, after passing through a demodulator, the measured parameters are obtained. Throughout the process, the light beam is guided through the optical fiber, passes through the modulator, and then exits. The optical fiber's primary function is to transmit the light beam, and secondarily, it acts as an optical modulator.
Features of fiber optic sensors:
1. High sensitivity;
2. Its geometry offers versatility, allowing for the fabrication of fiber optic sensors of any shape;
3. It can manufacture devices that sense various physical information (sound, magnetism, temperature, rotation, etc.);
4. It can be used in high-voltage, electrical noise, high-temperature, corrosive, or other harsh environments;
5. Moreover, it has inherent compatibility with fiber optic telemetry technology.
IV. Displacement sensors, also known as linear sensors, are linear devices that utilize metal induction. Their function is to convert various measured physical quantities into electrical quantities. In production processes, displacement measurement is generally divided into two types: measuring physical dimensions and measuring mechanical displacement. Based on the form of transformation of the measured variable, displacement sensors can be divided into analog and digital types. Analog sensors can be further divided into property-based and structural types. Commonly used displacement sensors are mostly analog structural types, including potentiometer-type displacement sensors, inductive displacement sensors, synchro sensors, capacitive displacement sensors, eddy current displacement sensors, and Hall effect displacement sensors. A significant advantage of digital displacement sensors is their ease of direct signal transmission to computer systems. These sensors are developing rapidly and are increasingly widely used.
Classification:
Conductive plastic displacement sensor
Metal-glass uranium displacement sensor
Metal film displacement sensor
Magnetic displacement sensor
Photoelectric displacement sensor
Magnetostrictive displacement sensor
Digital laser displacement sensor
Performance metrics:
Nominal resistance: The resistance value marked on the potentiometer.
Repeatability: The smaller this parameter is, the better.
Resolution: The smallest displacement value that the displacement sensor can report. The smaller this parameter is, the better. The resolution of conductive plastic displacement sensors is infinitesimal.
Tolerance: The percentage of the difference between the nominal resistance and the actual resistance to the nominal resistance is called the resistance deviation, which indicates the accuracy of the potentiometer. Tolerance is generally acceptable as long as it is within ±20%, because displacement sensors typically use voltage dividers, and the specific resistance value has no impact on the sensor's data acquisition.
Linearity accuracy: linearity error. The smaller this parameter is, the better.
Lifespan: Conductive plastic displacement sensors have a lifespan of over 2 million cycles.
