Eddy current displacement sensor is widely used in machining, automobile manufacturing, precision instruments, welding machinery, scientific research and teaching fields for its wide range of environmental adaptability, ultra-high detection frequency and precision. The principle of eddy current measurement is a non-contact measurement principle. This type of sensor is particularly suitable for measuring rapid displacement changes without the need for external forces on the object being measured. However, non-contact measurement is of great significance for applications where the measured surface is not allowed to be contacted, or the sensor needs to have an ultra long life. Strictly speaking, the principle of eddy current measurement should be an inductive measurement principle. The eddy current effect originates from the energy of the oscillating circuit. Eddy currents need to be formed in conductive materials. An alternating current is supplied to the inner coil of the sensor probe to form a magnetic field around the sensor coil. If a conductor is placed in this magnetic field, according to Faraday's law of electromagnetic induction, eddy current will be excited in the conductor. According to Lenz's law, the magnetic field direction of eddy current is opposite to that of coil, which will change the impedance value of coil in probe. The change of the impedance value is directly related to the distance between the coil and the measured object. After the sensor probe is connected to the controller, the controller can obtain the variation of voltage value from the sensor probe, and calculate the corresponding distance value based on this. The principle of eddy current measurement can be applied to all conductive materials. Because the eddy current can penetrate the insulator, even the metal material covered with insulator can also be used as the object to be measured. The unique coil winding design not only achieves the extremely compact shape of the sensor, but also meets the requirements of high temperature measurement environment.
So how to choose an appropriate eddy current displacement sensor? We suggest that you pay attention to three points:
1) Pay attention to the material to be measured. Generally, eddy current displacement sensor can only be used to measure metal objects. For non-metallic objects, do not consider the use of eddy current sensors from the beginning. In metal objects, it can be divided into ferromagnetic and non ferromagnetic materials. At present, the mainstream international eddy current displacement sensor brands, such as German iridium and American Kaman, require customers to inform the measured material in advance when choosing eddy current displacement sensor. If it belongs to special materials, such as some kind of stainless steel, its magnetic properties may be between ferromagnetic and non ferromagnetic materials, it is necessary to carry out one-to-one factory calibration for the sensor in advance. For the eddy current sensor system without one-to-one calibration with the tested material at the factory, if you want to achieve high accuracy, you can also require the manufacturer to do the calibration on site, but usually can not reach the accuracy level of factory calibration.
2) Parameter selection. Many manufacturers provide multiple levels of eddy current displacement sensors for customers to choose from. The parameters commonly used to select eddy current sensors include the accuracy of the sensor, which is also called by other names, such as linearity, absolute error, etc. It refers to the deviation degree of the measured value of the sensor from the theoretical real value. This parameter can not be measured directly. The second is the resolution, which refers to the minimum displacement change required for the sensor to make the indication change. Generally, the resolution parameter value should be less than the accuracy. The third is the measurement speed. Taking the German iridium eddyncdt3300 as an example, the measurement speed can reach 100 kHz. The measurement speed directly determines whether the measurement can keep up with the change speed of the measured object and whether it can completely reflect the whole process of displacement change. Vibration measurement is often used in high speed measurement. Of course, in addition to this, the sensor can withstand a lot of parameters, including vibration and temperature. Why choose the right indicators? Because the higher the technical parameters must mean that the manufacturing process is more complex and difficult, and it is also bound to be expensive. Therefore, when you formulate the measurement requirements, you must not imagine in vain and propose an ultra-high measurement requirement. I have seen that some sensors use units that often require measurement accuracy of several microns or even nanometers, and the measurement speed is still very high. Is it really necessary to put forward such high requirements? The answer is often unnecessary, or requires high margin. However, we should bear in mind that there is no index promotion without cost. Behind every high index, there is real gold and silver to pay.
3) Brand choice. Among the displacement sensors with many measuring principles, the eddy current displacement sensor market is the most chaotic and the brand is the most uneven. There are even merchants who claim to be the original manufacturer of a foreign brand, but they claim that they are produced by the original factory. When problems arise, as the preliminary technical scheme has been finalized, they can only adopt it with a firm will, which will often lead to greater problems in the follow-up. To identify whether a brand is an original product, in addition to the common original factory certification (this can also be counterfeited), the most direct way is to look at its Chinese website. Foreign brands usually use their own brand name as their website address. If you see that the website of a certain brand is a mixture of Chinese and English, it is unlikely to be a regular manufacturer. In addition, the Chinese website of regular imported brands can be found in the website of its foreign parent company, and those that cannot be found must not be subsidiaries. The choice of brand is particularly critical, because theoretically speaking, the structure of eddy current displacement sensor is not complicated, and small domestic factories can also make it, but the accuracy and repeatability are quite different from those of large brands. In addition, service life, temperature drift, etc., will greatly affect the customer's use effect. I have seen some manufacturers switch from imported brands to domestic brands for a price difference of several hundred yuan. As a result, one sensor will not be broken in the past three years. It has broken four times in a year, which is more expensive, but also greatly affects the use process. If the production line stops once, it can not be compensated by the cost saving of several hundred yuan.
The above are some methods to choose eddy current displacement sensor. We hope that we can choose the right products among many brands, which can save money and meet the measurement requirements.