With the development of modern technology, the improvement of production efficiency, and the advent of non-contact measurement technology, customers are pursuing higher and higher quality.
Measurement is a very important part of quality control, which requires that the measurement technique should also be efficient, accurate and non-invasive.
Traditional vernier calipers, micrometers and other contact gauges gauges are inefficient and unstable, and because the gauges are in direct contact with the surface of the workpiece, it is inevitable that they will cause damage to the workpiece or the gauge.
This has led to the emergence of non-contact measurement technology.
After a long period of development, more and more types of non-contact measurement techniques have appeared.These techniques are mainly based on optoelectronic, electromagnetic, and ultrasonic technologies.
A wide variety of optical and non-optical non-contact measurement techniques have been developed.Examples include nuclear magnetic resonance, X-ray scanning, eddy current measurement, and structured light.
Other techniques include laser triangulation, laser ranging, interferometric measurement, and stereoscopic vision.Ultrasonic ranging is another non-contact measurement technique that has emerged.
In this paper, the two categories of non-contact measurement technology optical method and non-optical method to do a brief description, but also gives examples of the corresponding part of the product, so that engineers and technicians can have a rough understanding of non-contact measurement technology.
Current situation
1. Non-optical method
1.1 Acoustic measurement method
Acoustic measurement method is mainly used for distance measurement, in which ultrasonic distance measurement technology is more widely used. Ultrasonic waves are mechanical waves with a frequency higher than 20Hz.
In order to use ultrasonic waves as a means of detection, ultrasonic waves must be generated and received. A high frequency acoustic transducer is required for the emission and reception of ultrasonic waves.
Ultrasonic waves are very directional, have a small energy loss when propagating in solid media, and travel long distances, so they are commonly used to measure distances.
The principle of ultrasonic distance measurement is in the case of known ultrasonic propagation speed in a medium, when the ultrasonic pulse through the medium to reach the measured surface, it will be reflected back, through the measuring instrument to measure the time interval between the emission of ultrasonic waves and the received echo, you can calculate the distance from the instrument to the measured surface.
Ultrasonic distance measuring instrument and ultrasonic thickness gauge are two typical examples of the application of ultrasonic technology. Ultrasonic ranging technology is affected by the environment of temperature, humidity, propagation medium, the measurement accuracy is often not high, and there is still to develop high-precision and highly adaptable ultrasonic ranging technology.
1.2 Magnetic measurement method
Magnetic measurement method is to test the distribution of magnetic field in a specific space where the object is located, to complete the measurement of the external or internal parameters of the object.
The MRI technique is a representative technique of the magnetic measurement method, which is based on the following principles.
Using the principle of nuclear magnetic resonance, an additional gradient magnetic field is applied to the main magnetic field. Specific electromagnetic waves are irradiated into the magnetic field of the object under test.
This causes specific atomic nuclei within the object to undergo nuclear magnetic resonance. As a result, radio frequency signals are released by these atomic nuclei.
These signals are processed by a computer to determine the species and location of the atomic nuclei. This allows the construction of a three-dimensional image of the object’s interior.
Nuclear magnetic resonance imaging, which emerged rapidly in the late 1970s, has become the predominant means of studying the structure of polymer chains, and is able to maintain the integrity of the sample compared to other traditional detection methods.
However, the accuracy of the MRI technique is still not as good as that of high-precision mechanical measurement techniques, and the measurement speed is slow, and there are also material and volume requirements for the object to be measured.
1.3 X-ray scanning method
X-rays were one of the three major discoveries in physics in the late 19th and early 20th centuries, marking the emergence of modern physics. Industrial CT, or industrial computed tomography imaging, is mainly used for non-destructive testing of industrial components.
It is based on ray scanning technology, the main principle is: with an X-ray beam at one end along a certain way to irradiate the object under test, a highly sensitive detector at the other end to receive the X-rays through the object under test, the resulting signal to the computer for processing, reconstructed by the object under test three-dimensional image or tomographic image.
The industrial CT system inspects large, high-density objects. It does not require sophisticated fixed equipment or other pre-processing measures.
The method is not limited by the complexity of the surface of the object.
It can non-destructively measure both the internal and external surfaces of the object. However, the system has a high cost for acquiring data over a long period of time.
X-rays used in the process can be harmful to the human body. Additionally, the resolution of industrial CT depends on the shape of the measured workpiece.
The resolution is different for various types of workpieces.
High-sensitivity detectors and linear accelerators for increasing ray power are the focus of industrial CT development.
1.4 Eddy Current Measurements
According to Faraday’s principle of electromagnetic induction, a lump metal conductor placed in a changing magnetic field or in the magnetic field for cutting the magnetic line of force movement, its body will produce a vortex-like induced current, this current is called eddy current, the above phenomenon is known as eddy current effect.
The eddy current sensor is a sensor based on the eddy current effect, with high reliability, high sensitivity, fast response speed and other characteristics.
The principle of the sensor coil involves generating a magnetic field by an alternating current.
This magnetic field causes a metal conductor to produce an induced current.
The magnetic field generated by the induced current weakens the magnetic field of the coil.
This weakening affects the inductance of the coil.Changes in the distance between the metal conductor and the coil cause changes in the induced current.
These changes in the induced current result in corresponding changes in the inductance of the sensor coil. By measuring the changes in the inductance, the distance between the coil and the conductor can be determined.
Eddy current sensors are small in size, high in reliability for continuous operation, and capable of non-contact measurement of displacement, speed, stress, thickness, surface temperature, material damage, etc., especially in the state analysis of high-speed sports machinery is widely used.
The representative ones are eddy current speed sensor and eddy current thickness sensor.
The disadvantage of eddy current measurement technology is that the object to be measured must be a metal conductor of a certain thickness with a smooth surface, and no other metal end surfaces are allowed around the sensor coil.
2. Optical methods
2.1 Structured light
Structured light method, as a new active, non-contact 3D vision measurement technology, has unparalleled advantages in reverse engineering, quality inspection, digital modelling and other fields.
Projected structured light method is a typical application of structured light measurement technology.The principle (see Figure 1) is as follows: the projector grating projects onto the surface of the object under test.
The grating stripes undergo deformation after being modulated by the object’s surface shape.
The degree of deformation depends on the object’s surface height and the relative position of the projector and the camera.
The receiving camera captures the deformation image. The image is handed over to the computer for further processing.
The computer uses the system’s structural parameters for the processing. This allows the measurement of the object’s structural parameters. Finally, the deformation of the object is obtained.
Structured light visual inspection has the advantages of large range, non-contact, fast speed, good system flexibility, moderate accuracy and so on. However, due to the constraints of its principle, it is not favourable to the measurement of objects with complex surface structure.
Fig. 1 Principle diagram of projected structured light 3D measurement system
2.2 Laser triangulation
Laser triangulation is an important form of non-contact optical measurement, which is widely used and the technology is relatively mature.
The principle is: a beam of laser light emitted by the light source is irradiated on the plane of the object to be measured, and finally imaged on the detector through reflection.
When the position of the object surface changes, its image on the detector also occurs corresponding displacement.
Through the relationship equation between the image shift and the actual displacement, the real object displacement can be obtained by the detection and calculation of the image shift.
This method is simple in structure, fast in measurement speed, high in accuracy, flexible in use, and suitable for measuring objects of large size and complex shape.
However, it is impossible to measure the surface of the object that the laser can not irradiate, and at the same time, the measurement accuracy of the laser triangulation method is relatively affected by the environment and the characteristics of the measured object surface, and it is necessary to vigorously study the high-precision triangulation method of measurement products.
2.3 Laser ranging methods
Laser has good collimation and very small divergence angle, so that the instrument can carry out point-to-point measurement, adapted to very narrow and complex measurement environment.
The laser distance measurement method uses the characteristics of the laser. The laser signal is emitted from the transmitter and irradiates the surface of the object.
After reflection, the laser travels back along the same path to the receiving device. By detecting the laser signals, either the time elapsed or phase changes can be measured.
This allows calculation of the distance between the laser rangefinder and the object being measured.
Laser distance measurement is mainly divided into two categories: pulse distance measurement and phase distance measurement.
For pulse ranging method, its system structure is simple, the detection distance is far, but the traditional ranging system uses direct counting to measure the round-trip time of the light pulse, low precision.
Phase ranging system structure is relatively complex, but its accuracy is higher, with the rapid development of optoelectronic technology, phase laser ranging technology has been continuously optimised and improved, has been able to meet the ultra-short distance and ultra-high precision measurement needs.
With the laser rangefinder towards miniaturisation, intelligent direction, due to the unique advantages of laser distance measurement technology, will have more and more broad application prospects in various types of distance measurement field.
2.4 Interferometry
Commonly used laser interferometer is a laser as the light source of the Michelson interferometer, that is, a beam of light emitted by the light source by the beam splitter mirror is divided into the measurement light and reference light, respectively, directed to the reference plane and the target plane, the two beams of light after the reflection of the overlap in the beam splitter mirror and interferes with each other.
When the target plane moves, the light and dark stripes of the interference pattern will change the corresponding number of times and the number of changes will be recorded by the photoelectric counter, from which the distance moved by the target plane can be calculated.
According to the different optical paths, there are two types of split optical paths and common optical paths.
Laser interferometry is characterised by very high measurement accuracy and fast measurement speed, but the measurement range is limited by the wavelength of the light wave, which is not suitable for the detection of large-scale objects, nor is it suitable for the measurement of complex surfaces with large changes in concavity and convexity, and it can only measure the change of small displacements.
2.5 Image analysis method
Image analysis method is also called stereo vision, and its research focuses on the geometric size of the object and the position and attitude of the object in space.
Stereo vision measurement is based on the principle of parallax, which is the difference in the position of a point between the corresponding points in two images. By calculating the distance from the parallax of the point, the spatial three-dimensional coordinates of the point can be obtained.
Generally from one or more camera systems from different orientations and angles to determine the distance information from multiple two-dimensional images of the object, the formation of three-dimensional images of the object surface topography, monocular, polyocular vision.
Stereo vision measurement belongs to the passive three-dimensional measurement method, often used for the identification of three-dimensional targets and the position of the object, morphological analysis, using this method of system structure is simple, in the field of machine vision is more widely used.
The basic geometric model of stereo vision is shown in Figure 2.
Figure 2 The basic geometric model of stereo vision
Binocular stereo vision is to shoot the same scene by two cameras in different positions by moving or rotating, and obtain the three-dimensional coordinate value of the point by calculating the parallax of the spatial point in the two images.
Its measurement principle is shown in Fig. 3. A complete stereo vision system usually includes six major parts: image acquisition, camera calibration, feature extraction, image matching, 3D information recovery, and post-processing.
Stereo vision method is widely used in aviation measurements, robot vision system, binocular, multi-camera and multi-frame image sequences and other stereo vision problems have become the focus of international academic research and hot spots.
Fig. 3 Principle of three-dimensional measurement by binocular stereo vision
Shortcomings
After decades of development, non-contact measurement technology has been greatly improved, but in addition to the outstanding advantage of not contacting with the measurement object, there are also a variety of shortcomings.
(1) The precision is not considered to be high compared to the contact measuring instrument articulated arm, coordinate machine, etc.
Non-contact measurement technology due to the gauge sensor element is not in contact with the measured object, the middle of the interval medium and distance greatly affects the measurement accuracy, the majority of cases the measurement accuracy is lower than the contact measurement technology.
(2) Higher price
Non-contact measurement technology using a large number of high-precision optical and electronic components, the price is too high relative to the contact measurement products, not conducive to the popularity of non-contact measurement technology.
(3) Part of the non-contact measurement methods on the workpiece and the environment have special requirements
Non-contact measurement methods due to its special nature, the workpiece and the environment also have some special requirements. Such as nuclear magnetic resonance instrument can not measure magnetic metal objects, ultrasonic measurement technology for the ambient temperature is more sensitive, and optical measurement technology based on the gauge can not adapt to the harsh working environment.
Development trend
Through the analysis of various non-contact measurement technologies and products at home and abroad, the development trend of non-contact measurement technology mainly includes the following points.
1. Highly integrated
The principle of non-contact measurement technology is mature, but most of the measurement products at this stage have a single function and cannot realise multi-purpose. The future of non-contact measurement instruments will achieve a high degree of integration.
Stereo vision camera, for example, integrated non-contact temperature measurement, displacement measurement, vibration measurement, etc. will greatly improve the applicability of the product, which is conducive to reducing the cost of the purchase of measuring instruments, and accelerate the promotion and application of non-contact measurement technology.
2. High precision and accuracy
At this stage of non-contact measurement technology products are still less accurate than the contact measurement technology, but also need to vigorously develop high-precision optical components, electronic components and excellent analysis and processing software to improve the measurement accuracy of the instrument, in due consideration of the cost of the case, and ultimately be able to achieve a comprehensive beyond the contact measurement technology products.
3. Reduced requirements for the working environment
Non-contact measurement technology relies mostly on optical components, poor environment for the optical components of the work of precision and accuracy has a greater impact, in the poor working environment, optical measurement instruments can not even work properly, which is the urgent need to solve the technical problems, but also non-contact measurement technology, a direction of development.
4. Highly Intelligent
With the rapid development of industrial technology, measuring instruments are required to carry out highly intelligent analysis of the measured object, which requires non-contact measurement technology to develop in the direction of intelligence, such as non-contact measuring instruments can intelligently analyse the measured object and thus automatically select the most optimal measurement method.
Conclusion
Non-contact measurement technology has an increasingly wide range of applications in industrial production. Due to the long measuring time required for contact measurement, it is difficult to measure objects with large or small dimensions, complex surface contours or long measuring cycles.
At the same time, with the development of optical technology and electronic circuit technology, the accuracy of non-contact measurement is getting higher and higher, and in some cases the accuracy even exceeds that of contact measurement, so non-contact measurement will become the main measurement method in the field of industrial production in the future.
