Based on classical electromagnetism, physical quantities such as displacement, position, liquid level, size, flow rate, velocity, and vibration, which are difficult to detect and process quantitatively, have been converted into electrical quantities that are easy to detect quantitatively and facilitate information transmission and processing. These are the displacement sensors that are widely used in production and daily life.
Displacement sensor
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. Displacement is a quantity related to the movement of an object's position during motion, and the methods for measuring displacement cover a wide range. Small displacements are typically detected using strain gauge, inductive, differential transformer, eddy current, and Hall effect sensors, while larger displacements are commonly measured using inductive synchros, optical gratings, capacitive gratings, and magnetic gratings. Among these, optical grating sensors are increasingly widely used in machine tool processing and testing instrumentation industries due to their advantages such as easy digitization, high accuracy (currently, the highest resolution can reach the nanometer level), strong anti-interference ability, absence of human reading errors, convenient installation, and reliable use.
Classification and Principle of Displacement Sensors
Classified by working principle:
potentiometer displacement sensor
It converts mechanical displacement into a resistance or voltage output that has a linear or arbitrary functional relationship with it through a potentiometer element. Ordinary linear potentiometers and circular potentiometers can be used as linear and angular displacement sensors, respectively. However, potentiometers designed for displacement measurement require a definite relationship between displacement change and resistance change. The movable brush of a potentiometer-type displacement sensor is connected to the object being measured.
The displacement of an object causes a change in resistance at the moving end of a potentiometer. The amount of resistance change reflects the magnitude of the displacement, while the increase or decrease in resistance indicates the direction of the displacement. A power supply voltage is typically applied to the potentiometer to convert the resistance change into a voltage output. Wire-wound potentiometers, because their brushes move with resistance changing in steps per turn, also exhibit a stepped output characteristic. If this type of displacement sensor is used as a displacement feedback element in a servo system, excessively large step voltages can cause system oscillations. Therefore, the resistance per turn should be minimized during potentiometer manufacturing. Another major disadvantage of potentiometer-type sensors is their susceptibility to wear. Their advantages are: simple structure, large output signal, ease of use, and low cost.
Magnetostrictive displacement sensor
Magnetostrictive displacement sensors use internal non-contact measurement and control technology to accurately detect the absolute position of a moving magnetic ring to measure the actual displacement value of the product being tested.
It utilizes the principle of magnetostriction, generating a strain pulse signal through the intersection of two different magnetic fields to accurately measure position. The measuring element is a waveguide, and the sensing element inside the waveguide is made of a special magnetostrictive material. The measurement process involves generating a current pulse in the sensor's electronic chamber. This current pulse propagates within the waveguide, creating a circular magnetic field outside the waveguide. When this magnetic field intersects with the magnetic field generated by a movable magnetic ring fitted on the waveguide to indicate position change, a strain mechanical wave pulse signal is generated within the waveguide due to magnetostriction. This strain mechanical wave pulse signal propagates at a fixed speed of sound and is quickly detected by the electronic chamber.
The propagation time of this strained mechanical wave pulse signal within the waveguide is directly proportional to the distance between the movable magnetic ring and the electronic chamber. By measuring the time, this distance can be determined with high precision. Since the output signal is a true absolute value, rather than a proportional or amplified signal, there is no signal drift or value variation, and there is no need for periodic recalibration.
Magnetostrictive displacement sensors are high-precision, long-stroke absolute position measurement sensors manufactured based on the magnetostrictive principle. They employ an internal non-contact measurement method; since the moving magnetic ring used for measurement and the sensor itself have no direct contact, they are not subject to friction or wear. Therefore, they have a long service life, strong environmental adaptability, high reliability, good safety, and are easy to automate in systems. They can operate normally even in harsh industrial environments (such as those susceptible to oil spills, dust, or other contaminants). The sensor uses high-tech materials and advanced electronic processing technology, enabling its application in high-temperature, high-pressure, and high-vibration environments. The sensor output signal is an absolute displacement value; even if the power is interrupted and reconnected, the data will not be lost, and there is no need to reset to zero. Because the sensing element is non-contact, even repeated measurements will not cause any wear to the sensor, greatly improving the reliability and service life of the detection. The stroke can reach 3 meters or longer, with a nominal accuracy of 0.05% F·S. Sensors with a stroke of more than 1 meter can achieve an accuracy of 0.02% F·S and a repeatability of 0.002% F·S, thus finding wide application.
Classified by mode of movement
Linear displacement sensor
The function of a linear displacement sensor is to convert linear mechanical displacement into an electrical signal. To achieve this, a variable resistance rail is typically fixed at a fixed point on the sensor, and the resistance value is measured by the displacement of a slider on the rail. The sensor rail is connected to a steady-state DC voltage, allowing a small current of microamperes to flow. The voltage between the slider and its starting point is proportional to the length the slider moves. Using the sensor as a voltage divider minimizes the requirement for high accuracy in the total resistance of the rail, because resistance changes caused by temperature variations do not affect the measurement results.
Angular displacement sensor
Angle displacement sensors are used for obstacle detection: using angle sensors to control your wheels can indirectly detect obstacles. The principle is very simple: if the motor angle sensor is running but the gear is not turning, it means your machine is blocked by an obstacle. This technology is very simple to use and very effective; the only requirement is that the moving wheel cannot slip on the floor (or slip too many times), otherwise you will not be able to detect obstacles. This problem can be avoided by connecting an idler gear to the motor. This wheel is not driven by the motor but is driven by the movement of the device: if the idler gear stops while the drive wheel is rotating, it means you have encountered an obstacle.
Classified by the material being tested
Hall effect displacement sensor
Its measurement principle is to keep the excitation current of the Hall element (see semiconductor magnetic sensing element) constant and move it in a uniformly gradient magnetic field. The displacement is proportional to the output Hall potential. The larger the magnetic field gradient, the higher the sensitivity; the more uniform the gradient change, the closer the relationship between the Hall potential and the displacement is to linear. Figure 2 shows three magnetic systems that generate gradient magnetic fields: system a has a narrow linear range, and the Hall potential ≠ 0 when the displacement Z=0; system b has good linearity when Z<2 mm, and the Hall potential = 0 when Z=0; system c has high sensitivity and a measurement range of less than 1 mm. In the figure, N and S represent the positive and negative magnetic poles, respectively. Hall displacement sensors have low inertia, high frequency response, reliable operation, and long lifespan, so they are often used in applications where various non-electrical quantities are converted into displacement before measurement.
Photoelectric displacement sensor
It measures the displacement or geometric dimensions of an object based on the amount of light it blocks. A key feature is that it is a non-contact measurement method and can perform continuous measurements. Photoelectric displacement sensors are commonly used for continuous measurement of wire diameter or as edge position sensors in strip edge position control systems.
Selection of displacement sensors
The selection of displacement sensors must meet the following requirements:
1. Technical specifications regarding sensitivity
For an instrument, higher sensitivity is generally better, because the more sensitive it is, the easier it is to detect changes in acceleration in the surrounding environment. A large change in acceleration naturally leads to a larger change in the output voltage, making measurement easier and more convenient, and the measured data will be more accurate.
2. Zero point temperature
Zero-point balance change caused by changes in ambient temperature. It is generally expressed as a percentage of the rated output when the temperature changes by 10°C, that is, the drift of the sensor's input caused by temperature changes when it is not under pressure.
3. Technical specifications regarding bandwidth
Bandwidth refers to the effective frequency band that a sensor can measure. For example, a sensor with a bandwidth of hundreds of Hz can measure vibration, and a sensor with a bandwidth of fifty Hz can effectively measure tilt angle.
4. Technical specifications of the output method
There are two output methods: digital and analog. Digital sensors input digital signals to the instrument, such as quantity and weight; analog sensors input analog signals to the instrument, such as voltage and current.
5. Technical specifications regarding measuring range
The range of measurement required to measure the motion of different things is different, and it must be measured according to the actual situation.
6. Ultimate overload
The maximum load a sensor can withstand without rendering it inoperable. This means that if the load exceeds this value, the sensor will suffer permanent damage.
7. Sensor Gain
It refers to the amplification factor of the sensor's original signal output.
Common Faults and Troubleshooting Methods
The working principle of a linear displacement sensor is the same as that of a sliding rheostat. It is used as a voltage divider and uses a relative output voltage to represent the actual position of the measured position.
1. If the linear displacement sensor's ruler has been used for a long time, and the seal has aged, containing many impurities, and the mixture of water and oil severely affects the contact resistance of the brushes, causing the displayed numbers to fluctuate continuously, then the linear displacement sensor's ruler is likely damaged and needs to be replaced.
2. If the power supply capacity is too small, many problems can arise: the movement of the molten plastic will cause fluctuations in the display of the mold closing electronic ruler, or the movement of the mold closing will cause fluctuations in the display of the injection electronic ruler, resulting in large measurement errors. These problems are more likely to occur if the solenoid valve's drive power supply and the electronic ruler's power supply are connected simultaneously. In severe cases, voltage fluctuations can even be measured using a multimeter in voltage mode. If the problem is not caused by high-frequency interference, electrostatic interference, or insufficient neutralization, then it is likely due to insufficient power supply capacity.
3. Both frequency modulation interference and electrostatic interference can cause the display numbers on the linear displacement sensor's electronic ruler to fluctuate. The signal cable of the electronic ruler must be kept in a separate cable tray from the equipment's high-voltage lines. The electronic ruler must be equipped with a grounded bracket, and the ruler's casing must have good contact with the ground. Shielded cables must be used for the signal cable, and one end of the cable should be grounded to the shield. With high-frequency interference, a multimeter voltage measurement will usually show a normal reading, but the displayed numbers will fluctuate continuously; the same phenomenon occurs with electrostatic interference.
To verify if electrostatic interference is the cause, you can use a power cord to short-circuit the cover screws of the electronic ruler with some metal parts on the machine. The electrostatic interference will immediately disappear once short-circuited. However, eliminating high-frequency interference is much more difficult using the above method. Variable frequency drives and robotic arms frequently experience high-frequency interference. Therefore, you can try stopping the high-frequency drive or robotic arm to verify if it is indeed high-frequency interference.
4. If the linear displacement sensor's electronic ruler displays data that jumps regularly at a certain point during operation, or if no data is displayed, it is necessary to check whether the insulation of the connecting wire is damaged and whether it is making regular contact with the machine's casing, resulting in a short circuit to ground.
5. The power supply voltage must be stable. Industrial voltages need to meet a stability requirement of ±0.1%. For example, if the reference voltage is 10V, a fluctuation of ±0.01V is permissible. Otherwise, it will cause fluctuations in the display. However, if the amplitude of the display fluctuation does not exceed the amplitude of the voltage fluctuation, then the electronic ruler is normal.
6. The alignment of the linear displacement sensor needs to be excellent, but a parallelism error of ±0.5mm and an angle error of ±12° are permissible. However, if both the parallelism and angle errors are too large, the displayed numbers will fluctuate. In such cases, the parallelism and angle must be adjusted.
7. During the connection process, extra care must be taken. The three wires of the electronic ruler must not be connected incorrectly, and the power wire and output wire cannot be interchanged. If the above wires are connected incorrectly, a large linear error will occur, which will be difficult to control, the control accuracy will become very poor, and the display will easily exhibit fluctuations, etc.
Displacement sensors come in a wide variety of types and their applications are constantly expanding. At the same time, more and more innovative technologies are being incorporated into sensors, such as OEM-based LVDT technology, ultrasonic technology, magnetostrictive technology, fiber optic technology, and time-grating technology. Displacement sensor technology has made groundbreaking progress. Due to technological advancements, the performance of various sensors has been significantly improved, and costs have been drastically reduced, thereby greatly expanding their application scope and forming a rapidly growing industry.
