1 Introduction
The probe is one of the key components of the precision measuring instrument. As the sensor provides the geometric information of the tested workpiece, its development level directly affects the precision, performance, efficiency and flexibility of the precision measuring instrument. The coordinate measuring machine is a typical precision measuring instrument. Its development history also shows that only after the precision measuring head provides a new touch measuring principle and a new measuring accuracy for the coordinate measuring machine can the coordinate measuring machine undergo a fundamental change. In other words, precision probes are the main factors that limit the accuracy and speed of precision measuring instruments. The ability of precision measuring instruments to meet modern measurement requirements also depends on the constant innovation and development of precision probe systems.
2 Evolution of Precision Probes
The development of precision probes has a long history. It can be traced back to the appearance of the inductance micrometer in the 1920s. However, the rapid development has benefited from the emergence of the three coordinate measuring machines in the late 1950s. So far, precision probes are generally classified into contact probes and non-contact probes. The contact probes are further classified into mechanical probes, trigger probes and scanning probes; non-contact probes. Laser probes and optical video probes.
Mechanical probes, also called contact-type hard probes, are an earlier type of probe used by precision gauges. Through the direct contact between the measuring probe and the workpiece under test, the positioning and aiming are performed to complete the measurement, which is mainly used for manual measurement. This type of probe has a simple structure and convenient operation, but its accuracy is not high, and it is difficult to meet the requirements of the current numerical control precision measuring instruments. Except for individual occasions, such probes are rarely used at present.
The precision probes currently widely available in the market are triggered probes. The first triggering probe was developed in 1972 by the British company Renishaw. The measuring principle of the triggering probe is that the precision measuring instrument sends out the sampling pulse signal when the measuring end of the probe touches the measured workpiece, and the coordinate value of the measuring center of the measuring end is latched by the positioning system of the instrument to determine the measurement. The coordinates of the point of contact with the part under test. The probes have the advantages of simple structure, convenient use, low production cost and high triggering accuracy, and are the most widely used probes in three-dimensional probes. However, these probes also have errors such as anisotropy (triangular effect) and pre-travel, which limits the further improvement of their measurement accuracy. The highest accuracy can only reach a few tenths of a micron. When using a trigger type probe to measure on a precision measuring instrument, it is usually a two-point fixed line, a three-point fixed plane, a three-point or four-point fixed circle. The essence of this method is to use several coordinates to determine the ideal geometric element. The size of the device shows obvious defects in the measurement of form and position error; the appearance of the scanning probe compensates for the shortcomings of the trigger probe.
The scanning probe is also called the quantized probe. The output of the probe is proportional to the offset of the probe. It is a high-accuracy, strong-function, and adaptable probe. It also has the position detection and curved surface of the spatial coordinate point. The scanning measurement function. The measuring principle of this type of probe is that after the measuring probe touches the measured workpiece, it continuously measures the contact displacement, and the conversion device of the probe outputs a signal proportional to the small deviation of the measuring rod. This signal and the precision measuring instrument The corresponding coordinate values ​​are superimposed to obtain the precise coordinates of the point on the workpiece under test. If the deformation of the measuring rod is not taken into account, the scanning probe is isotropic, so its accuracy is much higher than the triggering probe. The disadvantages of this type of probe are its complex structure and high manufacturing cost. At present, only a few companies in the world can produce it.
Whether it is a touch probe or a scanning probe, contact probes are used to contact the measured workpiece to collect contour points, and then data processing is performed to obtain the position or shape information of the workpiece. Since the contact probe has a certain size, it is not possible to measure the workpieces with small internal dimensions such as holes and grooves; in addition, the pressure generated when the tip of the probe touches the workpiece to be measured may cause the deformation and the stroke of the workpiece to be measured. Injury, it is also difficult to measure some sheets, contours and soft materials. Non-contact probes avoid the limitations of contact probes due to the optical method.
Non-contact probes generally use optical methods for measurement. Because the probe does not need to touch the measured workpiece, there is no measurement force, and it will not scratch the measured workpiece. At the same time, it can measure the surface topography of soft media. However, there are many external influence factors on this type of probe, such as the topography characteristics, radiation characteristics, and surface reflection of the measured object will affect the measurement results. So far, the measuring accuracy of the non-contact probe has not been very high, and it cannot replace the position of the contact probe in the precision measuring instrument.
3 Status Quo of Various Precision Probes
3.1 touch probe
When the measuring end of the triggering probe touches the workpiece under test, the triggering signal is detected. The core is to judge the contact or not, similar to the nature of the electronic switch, it is also known as switch probe. The methods to realize this function include the on-off of the electromechanical switch, the piezoelectric effect of the piezoelectric crystal, and the deformation of the strain gauge.
Early triggering probes used a mechanical positioning mechanism under the action of a spring force. When the measuring end comes into contact with the workpiece under test, the resulting contact force is transmitted from the spindle to the triggering mechanism inside the probe body when the force is increased enough. When the pre-stress of the internal spring is overcome, the mechanical contact of the trigger mechanism is disengaged and a trigger signal is issued. The probes triggered by this method have the following disadvantages: When the probe touches the workpiece from different directions, the contact force required to overcome the internal spring is not the same, which results in the touch probe when the probe touches the workpiece from different directions. The pre-stroke is also different, reducing the repeatability accuracy. This is the biggest source of error that causes measurement errors, and this design approach cannot avoid the changes in the pre-stroke at all. In addition, when the tip of the probe is in contact with the workpiece under test, the rod will undergo bending deformation due to the force, and the measurement error will also be introduced, and the error increases as the length of the rod increases. Renishaw's early touch probe TP2 can only support a 10mm long probe with a pre-travel variation of 3.28μm and a one-way repeatability of 0.35μm.
The use of solid-state sensing technology in the triggering probe significantly improves the accuracy of the probe, and in particular, reduces the measurement error due to pre-stroke changes when using longer probes. Renishaw's new-generation touch probe TP200 uses a higher-sensitivity strain gage technology to trigger, greatly reducing the probe's anisotropy and reducing pre-stroke changes. The structural design uses inductive contact deformation strain gauges and mechanical reset mechanism to isolate, this design can eliminate a large part of the measurement error caused by vibration; Moreover, the design of the probe is very compact, only 13mm in diameter, can easily stretch Into the measured workpiece to measure, increase the measuring range of the probe. The TP200 probe not only has a great improvement in accuracy, measuring range and working life, but also reduces the pre-stroke change when using a longer measuring rod. In the case of a measuring rod up to 100 mm, the one-way repeatability accuracy is 0.5 μm, and the pre-stroke variation is less than 1 μm; in addition, the probe has a very low measuring force, and the XY force is only 0.02 N. Z-direction measurement The force is 0.07N.
In order to adapt to different measurement requirements, two or more triggering technologies can be integrated into one triggering probe, namely double triggering or multiple triggering technology. German Zeiss ST3 probe adopts the combination of piezoelectric sensor and electromechanical switch. When using the piezoelectric sensor to trigger, the measurement force can be reduced to 0.01N. With electromechanical triggering, the sensitivity can be avoided. The piezoelectric sensor causes false triggering. In addition, Renishaw's TP800 probes incorporate electromechanical switches, piezo-electric sensors, and strain gage technology to work in one of three trigger modes. When using the piezoelectric sensor triggering method, the probe pre-stroke variation is minimal; when using the electromechanical switch triggering method, the pre-stroke variation is the largest and the accuracy is the lowest; when the strain gauge triggering method is adopted, the pre-stroke variation is moderate. The measuring accuracy of the probe is also very high. When carrying a 50 mm length rod, the unidirectional repeatability of each direction is only 0.25 μm, and the pre-stroke variation is less than 0.5 μm. Probes with multiple triggering methods generally support longer and longer spindles than traditional touch probes. For example, Renishaw's TP800 probe can support up to 350 mm probe bars; different options can be selected based on actual measurement needs. Triggering methods, measuring speeds and measuring rods extend the application range of the probe.
3.2 Scanning probes
Scanning probe is also called linear probe. Different from the trigger probe, when the measuring probe touches the measured workpiece, it not only sends out the aiming signal, but also gives the micro displacement of the measuring end. Micrometric function. The key technology of this type of probe is whether it can provide a three-dimensional micro guide system with no friction, no return error, high sensitivity and good linearity of motion.
The Zeiss company's early scanning probes used three-layer leaf spring guides. Each guide rail was equipped with an inductive sensor to sense the micro-displacement. When the measuring probe touches the workpiece, the probe sends a zero signal. And the deviation signal is processed by the electric box and received by the computer and stores the measurement data. The probe adopts static detection technology. Before the probe touches the workpiece, the probe gets a preset measurement direction. After touching the workpiece in this direction, the speed of the hair is measured, then the micro-motion is detected, and the probe is issued when the probe passes zero. Zero signal, then the probe will be transferred to the next rapid feed movement. The characteristic of this working mode is that if the surface of the measured workpiece is known in advance, the movement direction of the probe can be preset. At this time, only a few control techniques are needed, and it is relatively easy and simple to implement. However, the probe also has its disadvantages. When the probe moves in one direction (such as X direction), the other two directions (Y and Z direction) are guided by the locking part, and the probe touches the probe at this time. The movement direction of the measured workpiece is not consistent with the normal direction of the measured point, which will introduce cosine error. In addition, when measuring the commutation (if the X direction is changed to the Y direction), the locking parts are locked and released. The axis will produce a mechanical zero error, and the inductive sensors of each axis will also generate an electronic zero error when they are replaced.
The TRAX scanning probe system manufactured by Germany's Leitz company has the same principle of flexible guide rails as Zeiss's scanning probe, but its measurement principle is completely different. The TRAX probe adopts dynamic detection technology. When the measuring probe touches the measured workpiece, the guide rails in the three directions of the probe do not need to be fixed. The measuring end reads the contact force along the normal direction and can also measure the direction of the force measurement. The motion direction of the measuring head is always consistent with the normal direction of the measured point, which not only avoids the existence of cosine error, but also eliminates the mechanical zero error in the Zeiss probe, making the measurement accuracy very high. In addition, since the measuring force generated when the measuring end comes into contact with the workpiece reaches the preset force of the user, the measuring head returns in the direction of the outer normal direction of the measuring point, so that there is almost no friction between the measuring end and the surface of the measured workpiece. High-speed scanning measurement. The working principle of the TRAX probe determines that it is suitable for scanning measurements of both known and unknown surfaces, greatly expanding its scope of use.
A three-dimensional contact probe was developed jointly by Mecartex Switzerland and METAS (Swiss Federal Metrology and Inspection Agency). The probe adopts a new type of mechanical mechanism that limits its own rotational motion and divides its translational motion into three directions of x, y, z, making the probe completely three degrees of freedom; Each direction of movement can be measured by an inductive sensor. Since all axes and probes in the mechanism have the same angle, gravity has the same effect on each axis, making the probes have the same force in all three directions. Connecting a probe with the probe body with a permanent magnet makes it easy to replace and clean the probe; it protects the probe body from damage when an accidental impact occurs.
The latest developments in precision probes are the Renishaw Revo probes introduced by Renishaw this year. It can be said that the probe is a revolutionary progress of the precision probe in terms of measurement principle. The probe system uses Renscan5 technology to achieve high-precision, ultra-high-speed five-axis scanning measurements on coordinate measuring machines, measuring speeds up to 500mm/s. The feature of this technology is that most of the inspection movements are carried by the head. The measuring machine can move along a vector direction at a constant speed, minimizing the dynamic error caused by the structure and weight of the CMM. Basically eliminates the measurement errors usually present in three-axis scanning systems.
Both shafts of the Revo probe have spherical air bearing technology and are driven by a high-resolution encoder brushless motor that provides fast, ultra-high-accuracy positioning. In order to reduce the influence of the probe mechanism on the dynamic inertia under high-speed motion, the Revo probe uses an optical method to measure the precise position of the tip of the probe. The realization method is: the probe body is equipped with a laser light source and a position sensor ( PSD), the beam emitted by the laser source passes through a hollow probe and hits the mirror at the end of the probe. When the probe contacts the workpiece, it is bent and deformed. Displacement of the mirror causes the displacement of the mirror to directly lead to the optical path. Changes can be made and the PSD can be used to measure the changing light path to determine the exact position of the probe tip.
The technical indexes for assessing the performance of the touch-trigger probe include the size of the measurement force, the amount of pre-stroke change, and the unidirectional repeatability. Compared with the touch probe, the scanning probe has a more complex structure and is more widely used. More technical parameters to evaluate its performance. Table 1 compares the performance of the typical products of the three probe manufacturers. Through this table, you can understand the technical status of the scanning probes.
Table 1 Comparison of several scanning probes
United States EMD - Germany Zeiss - Renishaw, United Kingdom
Probe Model: EMD Scanning-VAST-Renishaw SP600
Resolution: 0.05μm-0.2μm-0.1μm
Repeatability: 0.1μm-1.5μm-<5.0μm
Measuring force: 2～200g-24～480g-120g/mm
Scanning points per second: 300-100-60 ~ 100
Scanning points per geometry: 32700-8000-/
With or without zero force measurement: Yes - Yes - None
Multiple probe combination application (contact/non-contact): Yes -/- Some systems
Unknown Surface Scan: Energy-Energy-No
Scanning capability: dynamic-active-passive
Companion software: Menu or Text - Menu - Menu
Roughness measurement: yes-no-no
Dynamic Test: Yes-No-No
Vibration Analysis: Energy-No-No
Hardness test: yes-no-no
Linear Accuracy: 0.75μm+L/600mm-2.2μm+L/350mm-5.0μm
Spatial precision: 1.75μm+L/600mm-2.5μm+L/300mm-7.5μm
CMM real-time online analysis: Yes - No - No
3.3 Non-contact probe
Non-contact probes can also be divided into one-dimensional, two-dimensional and three-dimensional probes. Laser probes are usually one-dimensional probes. One-dimensional probes generally use contact probes to measure point-by-point measurements on the measured workpieces. Therefore, the measurement speed is slow, which is not conducive to the development of precision measuring instruments to high-efficiency; three-dimensional probes due to existing technologies and Theoretical limitations, its complex structure, accuracy is not high; the main direction of development of non-contact probe is mainly two-dimensional probe. Optical vision probes are usually two-dimensional probes.
In recent years, the development of non-contact probes is a research focus of precision measuring instrument manufacturers worldwide. There are many ways to implement non-contact measurement. The number of non-contact probes currently used in practice and production has been quite a few. Wolf & Beck's OTM series optical probes from Germany, Mitutoyo QVP image probes from Mitutoyo Corporation, SCANWORKS 3D laser scanning probes from Perceptron, USA, LC series optical scanning probes from METRIS, Belgium, NEXTEC, Israel The WIZPROBE laser scanning probe and Viscan optical scanning probe from Germany's Zeiss Company have been widely used for non-contact measurement of precision measuring instruments.
The WIZPROBE laser scanning probe of NEXTEC of Israel adopts the unique triangulation measurement principle, and the measurement accuracy is less affected by the material type, surface processing form, laser beam angle and environmental conditions. Suitable for scanning small, precision, soft, thin, and other special parts, can also be used to scan the cover. The probe has a sampling frequency of 50 dots/s, a laser spot diameter of 30 μm, a measurement range of 50 mm±5 mm, a measurement resolution of 1 μm, a single-point accuracy of 6 μm, and a single-point repeatability of up to 0.3 μm.
Perceptron's SCANWORKS laser scanning probe in the United States uses triangulation for three-dimensional measurement. When the laser projection plane intersects the surface of the object, a CCD camera obtains a projection image containing the intersection line. Through an accurate calibration procedure, the intersection line is converted to a three-dimensional space, thereby obtaining a three-dimensional value of the surface of the measured object. The probe system has a measurement accuracy of 50 μm and a repeatability of 20 μm.
3 Development Trends of Precision Probes
From an overall perspective, in recent years, no matter which type of precision measuring head, the trend toward higher precision, smaller size, better interchangeability, more comprehensive functions, and digital development.
Due to its low cost, simple structure, and ability to meet common measurement requirements, the touch-trigger probe is still the most widely used probe in the market for a certain period of time. Its development direction is small size, high integration, high precision, and anisotropy. small. At present, Renishaw company occupies 90% of the world market for these types of probes, and this situation will not change in the near future.
The current scanning probes, due to their complex structure, large size, and high price, have affected their popularization and application. The development direction of the scanning probe is to develop a new type of high-precision scanning probe with simple structure and low cost while not affecting its accuracy and scanning speed. In addition, probes with large ranges that can reach into small holes for measuring micro parts are also developing. In particular, it is an important research direction to have micro-probe with nanometer resolution. The digitized precision probe using a grating sensor is a general development trend of the scanning probe, and the final development result will be an intelligent precision probe.
Although the scanning measurement method is more efficient than the point measurement method, it is still limited by the touch detection. Non-contact probes do not need contact. Samples and measurements during movement avoid frequent accelerations, decelerations, and collisions of precision measuring instruments. This greatly improves the measurement efficiency. Since the non-contact probe has zero measuring force, it can be measured. A variety of soft, easy to scratch the workpiece; In addition, you can form a small spot, some contact with the end of the probe is not easy to measure, or to measure some of the details, and have a large range of measurement; Therefore, precision measurement The application trend of the head is that the non-contact probe will be more and more widely used.
The probe is one of the key components of the precision measuring instrument. As the sensor provides the geometric information of the tested workpiece, its development level directly affects the precision, performance, efficiency and flexibility of the precision measuring instrument. The coordinate measuring machine is a typical precision measuring instrument. Its development history also shows that only after the precision measuring head provides a new touch measuring principle and a new measuring accuracy for the coordinate measuring machine can the coordinate measuring machine undergo a fundamental change. In other words, precision probes are the main factors that limit the accuracy and speed of precision measuring instruments. The ability of precision measuring instruments to meet modern measurement requirements also depends on the constant innovation and development of precision probe systems.
2 Evolution of Precision Probes
The development of precision probes has a long history. It can be traced back to the appearance of the inductance micrometer in the 1920s. However, the rapid development has benefited from the emergence of the three coordinate measuring machines in the late 1950s. So far, precision probes are generally classified into contact probes and non-contact probes. The contact probes are further classified into mechanical probes, trigger probes and scanning probes; non-contact probes. Laser probes and optical video probes.
Mechanical probes, also called contact-type hard probes, are an earlier type of probe used by precision gauges. Through the direct contact between the measuring probe and the workpiece under test, the positioning and aiming are performed to complete the measurement, which is mainly used for manual measurement. This type of probe has a simple structure and convenient operation, but its accuracy is not high, and it is difficult to meet the requirements of the current numerical control precision measuring instruments. Except for individual occasions, such probes are rarely used at present.
The precision probes currently widely available in the market are triggered probes. The first triggering probe was developed in 1972 by the British company Renishaw. The measuring principle of the triggering probe is that the precision measuring instrument sends out the sampling pulse signal when the measuring end of the probe touches the measured workpiece, and the coordinate value of the measuring center of the measuring end is latched by the positioning system of the instrument to determine the measurement. The coordinates of the point of contact with the part under test. The probes have the advantages of simple structure, convenient use, low production cost and high triggering accuracy, and are the most widely used probes in three-dimensional probes. However, these probes also have errors such as anisotropy (triangular effect) and pre-travel, which limits the further improvement of their measurement accuracy. The highest accuracy can only reach a few tenths of a micron. When using a trigger type probe to measure on a precision measuring instrument, it is usually a two-point fixed line, a three-point fixed plane, a three-point or four-point fixed circle. The essence of this method is to use several coordinates to determine the ideal geometric element. The size of the device shows obvious defects in the measurement of form and position error; the appearance of the scanning probe compensates for the shortcomings of the trigger probe.
The scanning probe is also called the quantized probe. The output of the probe is proportional to the offset of the probe. It is a high-accuracy, strong-function, and adaptable probe. It also has the position detection and curved surface of the spatial coordinate point. The scanning measurement function. The measuring principle of this type of probe is that after the measuring probe touches the measured workpiece, it continuously measures the contact displacement, and the conversion device of the probe outputs a signal proportional to the small deviation of the measuring rod. This signal and the precision measuring instrument The corresponding coordinate values ​​are superimposed to obtain the precise coordinates of the point on the workpiece under test. If the deformation of the measuring rod is not taken into account, the scanning probe is isotropic, so its accuracy is much higher than the triggering probe. The disadvantages of this type of probe are its complex structure and high manufacturing cost. At present, only a few companies in the world can produce it.
Whether it is a touch probe or a scanning probe, contact probes are used to contact the measured workpiece to collect contour points, and then data processing is performed to obtain the position or shape information of the workpiece. Since the contact probe has a certain size, it is not possible to measure the workpieces with small internal dimensions such as holes and grooves; in addition, the pressure generated when the tip of the probe touches the workpiece to be measured may cause the deformation and the stroke of the workpiece to be measured. Injury, it is also difficult to measure some sheets, contours and soft materials. Non-contact probes avoid the limitations of contact probes due to the optical method.
Non-contact probes generally use optical methods for measurement. Because the probe does not need to touch the measured workpiece, there is no measurement force, and it will not scratch the measured workpiece. At the same time, it can measure the surface topography of soft media. However, there are many external influence factors on this type of probe, such as the topography characteristics, radiation characteristics, and surface reflection of the measured object will affect the measurement results. So far, the measuring accuracy of the non-contact probe has not been very high, and it cannot replace the position of the contact probe in the precision measuring instrument.
3 Status Quo of Various Precision Probes
3.1 touch probe
When the measuring end of the triggering probe touches the workpiece under test, the triggering signal is detected. The core is to judge the contact or not, similar to the nature of the electronic switch, it is also known as switch probe. The methods to realize this function include the on-off of the electromechanical switch, the piezoelectric effect of the piezoelectric crystal, and the deformation of the strain gauge.
Early triggering probes used a mechanical positioning mechanism under the action of a spring force. When the measuring end comes into contact with the workpiece under test, the resulting contact force is transmitted from the spindle to the triggering mechanism inside the probe body when the force is increased enough. When the pre-stress of the internal spring is overcome, the mechanical contact of the trigger mechanism is disengaged and a trigger signal is issued. The probes triggered by this method have the following disadvantages: When the probe touches the workpiece from different directions, the contact force required to overcome the internal spring is not the same, which results in the touch probe when the probe touches the workpiece from different directions. The pre-stroke is also different, reducing the repeatability accuracy. This is the biggest source of error that causes measurement errors, and this design approach cannot avoid the changes in the pre-stroke at all. In addition, when the tip of the probe is in contact with the workpiece under test, the rod will undergo bending deformation due to the force, and the measurement error will also be introduced, and the error increases as the length of the rod increases. Renishaw's early touch probe TP2 can only support a 10mm long probe with a pre-travel variation of 3.28μm and a one-way repeatability of 0.35μm.
The use of solid-state sensing technology in the triggering probe significantly improves the accuracy of the probe, and in particular, reduces the measurement error due to pre-stroke changes when using longer probes. Renishaw's new-generation touch probe TP200 uses a higher-sensitivity strain gage technology to trigger, greatly reducing the probe's anisotropy and reducing pre-stroke changes. The structural design uses inductive contact deformation strain gauges and mechanical reset mechanism to isolate, this design can eliminate a large part of the measurement error caused by vibration; Moreover, the design of the probe is very compact, only 13mm in diameter, can easily stretch Into the measured workpiece to measure, increase the measuring range of the probe. The TP200 probe not only has a great improvement in accuracy, measuring range and working life, but also reduces the pre-stroke change when using a longer measuring rod. In the case of a measuring rod up to 100 mm, the one-way repeatability accuracy is 0.5 μm, and the pre-stroke variation is less than 1 μm; in addition, the probe has a very low measuring force, and the XY force is only 0.02 N. Z-direction measurement The force is 0.07N.
In order to adapt to different measurement requirements, two or more triggering technologies can be integrated into one triggering probe, namely double triggering or multiple triggering technology. German Zeiss ST3 probe adopts the combination of piezoelectric sensor and electromechanical switch. When using the piezoelectric sensor to trigger, the measurement force can be reduced to 0.01N. With electromechanical triggering, the sensitivity can be avoided. The piezoelectric sensor causes false triggering. In addition, Renishaw's TP800 probes incorporate electromechanical switches, piezo-electric sensors, and strain gage technology to work in one of three trigger modes. When using the piezoelectric sensor triggering method, the probe pre-stroke variation is minimal; when using the electromechanical switch triggering method, the pre-stroke variation is the largest and the accuracy is the lowest; when the strain gauge triggering method is adopted, the pre-stroke variation is moderate. The measuring accuracy of the probe is also very high. When carrying a 50 mm length rod, the unidirectional repeatability of each direction is only 0.25 μm, and the pre-stroke variation is less than 0.5 μm. Probes with multiple triggering methods generally support longer and longer spindles than traditional touch probes. For example, Renishaw's TP800 probe can support up to 350 mm probe bars; different options can be selected based on actual measurement needs. Triggering methods, measuring speeds and measuring rods extend the application range of the probe.
3.2 Scanning probes
Scanning probe is also called linear probe. Different from the trigger probe, when the measuring probe touches the measured workpiece, it not only sends out the aiming signal, but also gives the micro displacement of the measuring end. Micrometric function. The key technology of this type of probe is whether it can provide a three-dimensional micro guide system with no friction, no return error, high sensitivity and good linearity of motion.
The Zeiss company's early scanning probes used three-layer leaf spring guides. Each guide rail was equipped with an inductive sensor to sense the micro-displacement. When the measuring probe touches the workpiece, the probe sends a zero signal. And the deviation signal is processed by the electric box and received by the computer and stores the measurement data. The probe adopts static detection technology. Before the probe touches the workpiece, the probe gets a preset measurement direction. After touching the workpiece in this direction, the speed of the hair is measured, then the micro-motion is detected, and the probe is issued when the probe passes zero. Zero signal, then the probe will be transferred to the next rapid feed movement. The characteristic of this working mode is that if the surface of the measured workpiece is known in advance, the movement direction of the probe can be preset. At this time, only a few control techniques are needed, and it is relatively easy and simple to implement. However, the probe also has its disadvantages. When the probe moves in one direction (such as X direction), the other two directions (Y and Z direction) are guided by the locking part, and the probe touches the probe at this time. The movement direction of the measured workpiece is not consistent with the normal direction of the measured point, which will introduce cosine error. In addition, when measuring the commutation (if the X direction is changed to the Y direction), the locking parts are locked and released. The axis will produce a mechanical zero error, and the inductive sensors of each axis will also generate an electronic zero error when they are replaced.
The TRAX scanning probe system manufactured by Germany's Leitz company has the same principle of flexible guide rails as Zeiss's scanning probe, but its measurement principle is completely different. The TRAX probe adopts dynamic detection technology. When the measuring probe touches the measured workpiece, the guide rails in the three directions of the probe do not need to be fixed. The measuring end reads the contact force along the normal direction and can also measure the direction of the force measurement. The motion direction of the measuring head is always consistent with the normal direction of the measured point, which not only avoids the existence of cosine error, but also eliminates the mechanical zero error in the Zeiss probe, making the measurement accuracy very high. In addition, since the measuring force generated when the measuring end comes into contact with the workpiece reaches the preset force of the user, the measuring head returns in the direction of the outer normal direction of the measuring point, so that there is almost no friction between the measuring end and the surface of the measured workpiece. High-speed scanning measurement. The working principle of the TRAX probe determines that it is suitable for scanning measurements of both known and unknown surfaces, greatly expanding its scope of use.
A three-dimensional contact probe was developed jointly by Mecartex Switzerland and METAS (Swiss Federal Metrology and Inspection Agency). The probe adopts a new type of mechanical mechanism that limits its own rotational motion and divides its translational motion into three directions of x, y, z, making the probe completely three degrees of freedom; Each direction of movement can be measured by an inductive sensor. Since all axes and probes in the mechanism have the same angle, gravity has the same effect on each axis, making the probes have the same force in all three directions. Connecting a probe with the probe body with a permanent magnet makes it easy to replace and clean the probe; it protects the probe body from damage when an accidental impact occurs.
The latest developments in precision probes are the Renishaw Revo probes introduced by Renishaw this year. It can be said that the probe is a revolutionary progress of the precision probe in terms of measurement principle. The probe system uses Renscan5 technology to achieve high-precision, ultra-high-speed five-axis scanning measurements on coordinate measuring machines, measuring speeds up to 500mm/s. The feature of this technology is that most of the inspection movements are carried by the head. The measuring machine can move along a vector direction at a constant speed, minimizing the dynamic error caused by the structure and weight of the CMM. Basically eliminates the measurement errors usually present in three-axis scanning systems.
Both shafts of the Revo probe have spherical air bearing technology and are driven by a high-resolution encoder brushless motor that provides fast, ultra-high-accuracy positioning. In order to reduce the influence of the probe mechanism on the dynamic inertia under high-speed motion, the Revo probe uses an optical method to measure the precise position of the tip of the probe. The realization method is: the probe body is equipped with a laser light source and a position sensor ( PSD), the beam emitted by the laser source passes through a hollow probe and hits the mirror at the end of the probe. When the probe contacts the workpiece, it is bent and deformed. Displacement of the mirror causes the displacement of the mirror to directly lead to the optical path. Changes can be made and the PSD can be used to measure the changing light path to determine the exact position of the probe tip.
The technical indexes for assessing the performance of the touch-trigger probe include the size of the measurement force, the amount of pre-stroke change, and the unidirectional repeatability. Compared with the touch probe, the scanning probe has a more complex structure and is more widely used. More technical parameters to evaluate its performance. Table 1 compares the performance of the typical products of the three probe manufacturers. Through this table, you can understand the technical status of the scanning probes.
Table 1 Comparison of several scanning probes
United States EMD - Germany Zeiss - Renishaw, United Kingdom
Probe Model: EMD Scanning-VAST-Renishaw SP600
Resolution: 0.05μm-0.2μm-0.1μm
Repeatability: 0.1μm-1.5μm-<5.0μm
Measuring force: 2～200g-24～480g-120g/mm
Scanning points per second: 300-100-60 ~ 100
Scanning points per geometry: 32700-8000-/
With or without zero force measurement: Yes - Yes - None
Multiple probe combination application (contact/non-contact): Yes -/- Some systems
Unknown Surface Scan: Energy-Energy-No
Scanning capability: dynamic-active-passive
Companion software: Menu or Text - Menu - Menu
Roughness measurement: yes-no-no
Dynamic Test: Yes-No-No
Vibration Analysis: Energy-No-No
Hardness test: yes-no-no
Linear Accuracy: 0.75μm+L/600mm-2.2μm+L/350mm-5.0μm
Spatial precision: 1.75μm+L/600mm-2.5μm+L/300mm-7.5μm
CMM real-time online analysis: Yes - No - No
3.3 Non-contact probe
Non-contact probes can also be divided into one-dimensional, two-dimensional and three-dimensional probes. Laser probes are usually one-dimensional probes. One-dimensional probes generally use contact probes to measure point-by-point measurements on the measured workpieces. Therefore, the measurement speed is slow, which is not conducive to the development of precision measuring instruments to high-efficiency; three-dimensional probes due to existing technologies and Theoretical limitations, its complex structure, accuracy is not high; the main direction of development of non-contact probe is mainly two-dimensional probe. Optical vision probes are usually two-dimensional probes.
In recent years, the development of non-contact probes is a research focus of precision measuring instrument manufacturers worldwide. There are many ways to implement non-contact measurement. The number of non-contact probes currently used in practice and production has been quite a few. Wolf & Beck's OTM series optical probes from Germany, Mitutoyo QVP image probes from Mitutoyo Corporation, SCANWORKS 3D laser scanning probes from Perceptron, USA, LC series optical scanning probes from METRIS, Belgium, NEXTEC, Israel The WIZPROBE laser scanning probe and Viscan optical scanning probe from Germany's Zeiss Company have been widely used for non-contact measurement of precision measuring instruments.
The WIZPROBE laser scanning probe of NEXTEC of Israel adopts the unique triangulation measurement principle, and the measurement accuracy is less affected by the material type, surface processing form, laser beam angle and environmental conditions. Suitable for scanning small, precision, soft, thin, and other special parts, can also be used to scan the cover. The probe has a sampling frequency of 50 dots/s, a laser spot diameter of 30 μm, a measurement range of 50 mm±5 mm, a measurement resolution of 1 μm, a single-point accuracy of 6 μm, and a single-point repeatability of up to 0.3 μm.
Perceptron's SCANWORKS laser scanning probe in the United States uses triangulation for three-dimensional measurement. When the laser projection plane intersects the surface of the object, a CCD camera obtains a projection image containing the intersection line. Through an accurate calibration procedure, the intersection line is converted to a three-dimensional space, thereby obtaining a three-dimensional value of the surface of the measured object. The probe system has a measurement accuracy of 50 μm and a repeatability of 20 μm.
3 Development Trends of Precision Probes
From an overall perspective, in recent years, no matter which type of precision measuring head, the trend toward higher precision, smaller size, better interchangeability, more comprehensive functions, and digital development.
Due to its low cost, simple structure, and ability to meet common measurement requirements, the touch-trigger probe is still the most widely used probe in the market for a certain period of time. Its development direction is small size, high integration, high precision, and anisotropy. small. At present, Renishaw company occupies 90% of the world market for these types of probes, and this situation will not change in the near future.
The current scanning probes, due to their complex structure, large size, and high price, have affected their popularization and application. The development direction of the scanning probe is to develop a new type of high-precision scanning probe with simple structure and low cost while not affecting its accuracy and scanning speed. In addition, probes with large ranges that can reach into small holes for measuring micro parts are also developing. In particular, it is an important research direction to have micro-probe with nanometer resolution. The digitized precision probe using a grating sensor is a general development trend of the scanning probe, and the final development result will be an intelligent precision probe.
Although the scanning measurement method is more efficient than the point measurement method, it is still limited by the touch detection. Non-contact probes do not need contact. Samples and measurements during movement avoid frequent accelerations, decelerations, and collisions of precision measuring instruments. This greatly improves the measurement efficiency. Since the non-contact probe has zero measuring force, it can be measured. A variety of soft, easy to scratch the workpiece; In addition, you can form a small spot, some contact with the end of the probe is not easy to measure, or to measure some of the details, and have a large range of measurement; Therefore, precision measurement The application trend of the head is that the non-contact probe will be more and more widely used.
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