Sensors are a basic type of electronic information equipment in the manufacturing industry, and they are special components of new electronic devices that are being developed.
The sensor industry is recognized both domestically and internationally as a high-tech industry with great development prospects, owing to its high technical content, good economic benefits, strong penetration ability, and broad market prospects.
Driven by the booming electronic information industry, sensor industry has formed a certain industrial foundation and has made significant progress in technological innovation, independent research and development, achievement transformation, and competitiveness, making important contributions to promoting national economic development.
With the advent of the information age, sensors have become the main means and method for people to obtain information in the natural and production fields.
In modern industrial production, especially in automated production processes, various sensors are used to monitor and control various parameters in the production process, ensuring that equipment operates in a normal or optimal state and produces high-quality products. In fundamental research, sensors have an outstanding status.
Nowadays, sensors have already penetrated into extremely wide areas such as industrial production, space development, ocean detection, environmental protection, resource investigation, medical diagnosis, bio-engineering, and even cultural relics protection.
It can be seen that the important role of sensor technology in developing the economy and promoting social progress is very obvious. Statistical figures show that the annual revenue of the global intelligent sensor market will increase at a rate of 10% per year.
Currently, the number of sensor devices installed with processors worldwide is 65 million, and this number will reach 2.8 trillion by 2019.
Key Points of Sensor Selection
Sensor knowledge is also a relatively large electrical discipline that requires abundant experience to master proficiently. We will explain more about it in the future, but today we will mainly talk about the selection.
1. Determine the Type According to the Measured Object and Output Conditions
To carry out a specific measurement task, it is necessary first to consider what type of principle of sensor to use. This requires careful analysis of various factors to make a determination.
For example, taking a flowmeter as an example, there is an electromagnetic flowmeter, a vortex flowmeter, and an ultrasonic flowmeter, which depend on the specific target when selecting a flowmeter.
In addition, it is also necessary to refer to what kind of output mode needs to be used, such as a 2-wire or a 4-wire current signal, a 0-20mA, a 4-20mA, a 0-10V voltage signal, or some protocol communication.
2. Selection Based on Sensitivity
Generally, within the linear range of the sensor, it is desirable to have a higher sensitivity of the sensor. This is because only when the sensitivity is high enough, the value of the output signal corresponding to the measured change is relatively large.
Moreover, this sensitivity is advantageous for signal processing. However, it is important to note that when the sensitivity of the sensor is high, external interference signals unrelated to the measured object can be amplified by the amplification system and affect the measurement accuracy.
Therefore, the sensor itself should have a higher signal-to-noise ratio, and the interference signals introduced from the outside should be reduced as much as possible.
The sensitivity of the sensor is directional. When the measured object is a single vector and requires high directionality, another sensor with lower sensitivity in other directions should be selected. If the measured object is a multidimensional vector, the cross-sensitivity of the sensor should be as small as possible.
3. Determine Frequency Response Characteristics
The frequency response characteristics of sensors determine the frequency range of the measured object, which must be kept undistorted within the allowable frequency range.
In practice, there is always a certain delay in the response of the sensor, and it is preferable for the delay time to be as short as possible. The higher the frequency response of the sensor, the wider the signal frequency range that can be measured.
In dynamic measurement, the response characteristics should be based on the characteristics of the signal (steady-state, transient, random, etc.) to avoid significant errors.
4. According to Sensor Stability
After using the sensor for a certain period, the ability of the sensor to maintain its performance is called stability. The factors that affect the long-term stability of the sensor are not only related to the sensor structure but also mainly related to the sensor’s usage environment.
Therefore, to ensure that the sensor has good stability, the sensor must have a strong environmental adaptability.
Before selecting a sensor, the usage environment should be investigated, and appropriate sensors should be selected based on the specific usage environment or appropriate measures taken to reduce the impact of the environment.
5. The Range and Accuracy of Sensors Are the Most Difficult Pair to Coordinate
Accuracy is an important performance indicator of sensors, and it is an important link related to the measurement accuracy of the entire measurement system. However, the accuracy of the sensor is limited by its range.
Generally, the larger the range, the lower the accuracy, but high-precision sensors are very likely to have insufficient range. This causes high-precision large-range sensors to be very expensive.
Therefore, adjustments need to be made when selecting sensors according to these considerations.
When selecting a sampling sensor, it is necessary to ensure that the device can meet the basic operating conditions of the application (can refer to the data sheet provided by the manufacturer).
The 6 most important operating conditions include:
- Temperature range;
- Protection level;
- Voltage range;
- Discrete or analog output;
- Parameter changes, i.e. “whether changeable parameters are beneficial”.
When considering the use of sensors with IO-Link, there are also 6 other things to consider:
- Response speed;
- Sensing range;
- Repetition accuracy;
- Electrical connection;
- Installation type;
- Visual display: whether there is a need for a visible display on the sensor for the application.
In modern industrial production, especially in automated production processes, various sensors are used to monitor and control various parameters in the production process, so that equipment works in a normal or optimal state and products achieve the best quality.
Therefore, it can be said that without many excellent sensors, modern production will lose its foundation. Next, we will provide a detailed introduction of several of the most common types of sensors in manufacturing, along with some application techniques and insights.
The Most Common Types of Sensors
1. Proximity Sensors
Proximity sensors detect the presence of objects in the vicinity without physical contact. They are discrete output devices.
Typically, magnetic proximity sensors detect whether an actuator has reached a specific position by sensing a magnet located in the actuator.
It is generally not a good idea to purchase an actuator from one company and a magnetic proximity sensor from another. Although the sensor manufacturer may say that the sensor is compatible with X, Y, and Z actuators, the reality is that changes in magnets or mounting position can cause sensing issues.
For example, the sensor may either energize or fail to energize when the magnet is not in the correct position. If the actuator manufacturer offers proximity sensors that are matched to the actuator, these should be the preferred sensors.
Transistor-based proximity sensors have no moving parts and have a long service life. Spring-loaded proximity sensors use mechanical contacts, have a shorter service life, but are less costly than transistor types. Spring-loaded sensors are best suited for applications requiring AC power and those that operate in high-temperature environments.
2. Position Sensors
Position sensors have analog output and display the position of the actuator based on the position indicator of the magnet mounted on it. From a control standpoint, position sensors offer a great deal of flexibility. Control engineers can establish a series of setpoint values matched to component changes.
Since these position sensors are based on magnets (such as proximity sensors), it is best to purchase sensors and actuators from the same manufacturer (if possible). With IO-Link functionality, data from position sensors can be obtained, which can also simplify control and allow for parameterization.
3. Inductive Sensors
Inductive proximity sensors use Faraday’s Law of Induction to measure the presence or analog output position of an object. The most critical factor when selecting an inductive sensor is determining the metal type the sensor will detect, thus determining the sensing range.
Compared to black metals, the sensing range of colored metals is reduced by over 50%. The manufacturer’s product manual should provide information on the sample selection needed.
4. Pressure and Vacuum Sensors
Ensure that pressure or vacuum sensors can measure the pressure range in both imperial (pounds per square inch) and metric (bars) measurements. Specify the most appropriate shape and size for the allocated space.
When installing the equipment, consider whether the sensor should be configured with indicator lights or a display screen to facilitate operator use. If fast changes to set values are necessary, pressure and vacuum sensors with IO-Link configuration can be considered.
5. Flow Sensors
Like pressure and vacuum sensors, flow sensors can be selected based on flow range, size, and variable set values. Display options can be specified when ordering sensors.
Flow sensors with relatively low flow rates can be selected for a particular area of the equipment or for the entire equipment.
6. Optical Sensors
The most common types of optical sensors are photoelectric scattering, reflection, and through-beam. Laser sensors and fiber optic sensing devices also fall under the category of optical sensors.
Photoelectric sensors are mostly presence sensors that detect objects by reflecting or interrupting a beam of light. Due to their low cost, versatility, and high reliability, these sensors are among the most widely used in the manufacturing industry.
Diffuse photoelectric sensors do not require reflectors. They are cost-effective sensors used to detect the presence of nearby objects.
Through-beam photoelectric sensors can provide the longest sensing range. These sensors have separate transmitting and receiving units installed at two points. Garage door safety sensors are beam sensors. Interruption of the beam indicates the presence of a target.
Slot-type photoelectric sensors are an interesting through-beam variant; they combine a transmitter and a receiver in a compact unit. Slot-type sensors are used to detect the presence and absence of small parts.
Reflective photoelectric sensors have a sensor and a reflector and are used for mid-range presence sensing. In terms of precision and cost, these sensors fall between diffuse and through-beam sensors.
Fiber optic sensing devices are used for presence and distance sensing. The parameters on these multifunctional sensors can be adjusted to detect various colors, backgrounds, and distance ranges.
Laser sensors can be used for long-range presence sensing and are the most accurate for short-range measurement applications.
Vision sensors can be used for barcode reading, counting, shape verification, and more. Vision sensors are an economical and efficient visual application that can be used in situations where camera systems are expensive and complex.
Vision sensors are used for barcode reading, tracking individual components, and executing process steps tailored to the component. Sensors can verify the functionality of the number of parts present on the component. Vision sensors can determine if a specified curve or other shape has been achieved.
Since these sensors deal with light, testing the sensors in conditions as close as possible to the operating environment is crucial, taking into account ambient light and background reflectivity.
In most applications, it is recommended to place vision sensors inside a housing to isolate them from external light sources. Seeking the help of the vision sensor manufacturer during sensor testing is a good idea. Also, remember to ensure the selection of the appropriate fieldbus.
Signal converters convert the analog output signal from a sensor to a binary signal on the converter, or convert it to IO-Link process data.
7. Other sensors
(1) Magnetic switch:
It is a specialized name for sensors used in cylinders, mainly used to detect the position of the cylinder pistons. Usually, it is provided by the cylinder supplier according to the customer’s usage. As the name suggests, the magnetic switch detects the target object through electromagnetic induction, so its detection accuracy is relatively low.
(2) Proximity switch:
The proximity switch is also designed and manufactured based on the principle of electromagnetic induction, so it can only measure metal target objects, and there is a slight difference in the sensing distance for different metals.
Currently, the commonly used sensing distances for proximity switches are as follows: 1mm, 2mm, 4mm, 8mm, 12mm, etc. There are usually two types of proximity switches: embedded and non-embedded.
The so-called embedded type refers to the fact that the sensing head of the proximity switch does not detect the metal target in its circumferential direction, only detects the metal target in front of it, and the sensor sensing head can be installed without exposing the metal mounting brackets.
The so-called non-embedded type means that the sensing head of the proximity switch will detect both the metal target in front of it and the metal target in its circumferential direction at the same time, and the sensor sensing head must expose the metal mounting bracket for a certain distance, and there must be no metal target within a certain range in the circumferential direction to avoid incorrect judgments.
The detection accuracy of proximity switches is higher than that of magnetic switches. Proximity switches are usually used in situations where the position accuracy requirements for judging the presence or absence of products and the positioning of fixtures are relatively low.
(3) Photoelectric switch:
The photoelectric detection method has the advantages of high accuracy, fast response, and non-contact, and can measure multiple parameters. The structure of the sensor is simple and flexible, so photoelectric sensors are widely used in detection and control.
There are roughly three types of photoelectric switches we usually mention: one is a reflective photoelectric sensor, another is a through-beam photoelectric sensor, and the other is a photoelectric sensor that uses a reflection plate to reflect light.
The latter two are detected by shading due to the target object, while the former is achieved by reflecting light through the target object.
Therefore, the latter two usually have longer sensing distances and higher accuracy. Due to the relatively high detection accuracy of photoelectric sensors, they are usually used in detecting the precise position of products or workpieces, as well as feedback devices for stepper and servo systems.
(4) Fiber optic sensor:
Fiber optic sensor is also a type of detection element that uses photoelectric signal conversion. Compared with photoelectric switches, it can usually detect smaller target objects, have a longer sensing distance, and higher accuracy.
Therefore, fiber optic sensors are usually used in more precise detection applications and positioning feedback devices for stepper and servo systems.
The grid is also a sensor that uses photoelectric signals. The detection area of the grid is large, so it is also called an area sensor. The main application area of the grid is interlocking and safety functions between equipment, especially for protecting people.
Thermocouples are mainly used to detect the ambient temperature around them.
(7) Laser detector:
The main function of the laser detector is to accurately measure the external dimensions of the target object.
(8) Industrial camera:
The industrial camera is also commonly known as CCD (Charge-coupled Device) in engineering, mainly used to detect the external shape and position of the target object. With the improvement of current CCD technology, high-resolution industrial cameras can now be applied to precise measurement fields.
According to working principles, encoders can be divided into incremental and absolute types. Incremental encoders convert displacement into periodic electrical signals, and then convert this electrical signal into counting pulses, where the number of pulses represents the size of the displacement.
The absolute encoder corresponds to a specific digital code for each position, so its indication is only related to the starting and ending positions of the measurement, and is not related to the measurement process in the middle.
Encoders are usually used in closed-loop or semi-closed-loop control systems with stepper motors or servo motors.
The microswitch is a contact-type sensor, mainly used for the connection between equipment or the detection of the status of safety and protective doors of equipment.