Preliminary knowledge of three-coordinate measuring instrument

Coordinate measuring machine (CMM) is a new type of high-efficiency precision measuring instrument developed in 1960s. Its appearance, on the one hand, is due to the efficient processing of automatic machine tools and CNC machine tools and the demand for fast and reliable measuring equipment for more and more complex parts; On the other hand, the development of electronic technology, computer technology, numerical control technology and precision machining technology provides a technical basis for the emergence of CMM. 1960, FERRANTI Company of Britain successfully developed the world's first CMM. By the end of 1960s, more than 30 companies in nearly 10 countries were producing CMM, but CMM in this period was still in its infancy. Since 1980s, many companies, such as Zeiss, Lai Ci, DEA, LK, Sanfeng, SIP, Ferranti and Moore, have continuously introduced new products, which has accelerated the development of CMM. Modern CMM can not only complete all kinds of complicated measurements under the control of computer, but also control machining by exchanging information with CNC machine tools, and also realize reverse engineering according to measured data. At present, CMM has been widely used in machinery manufacturing, automobile industry, electronics industry, aerospace industry and national defense industry, and has become an indispensable general measuring equipment for modern industrial inspection and quality control.

Second, the composition and working principle of CMM

(A) the composition of CMM

Coordinate measuring machine is a typical mechatronics equipment, which consists of mechanical system and electronic system.

(1) Mechanical system: Generally, it consists of three orthogonal linear motion axes. In the structure shown in Figure 9- 1, the X-direction guide rail system is installed on the workbench, the cross beam of the movable bridge is the Y-direction guide rail system, and the Z-direction guide rail system is installed in the central carriage. Grating rulers are installed on all three axes to measure the displacement of each axis. Manual wheels and motor drives and CNC motors are usually close to each axis. A probe for contacting the surface of the detected part is installed at the end of the Z axis.

(2) Electronic system: generally composed of grating counting system, probe signal interface and computer, it is used to obtain the data of the measured coordinate points and process the data.

(2) The working principle of CMM.

Coordinate measuring machine is a general digital measuring equipment based on coordinate measurement. Firstly, it transforms the measurement of each measured geometric element into the measurement of the coordinate position of some point sets on these geometric elements. After measuring the coordinate positions of these points, the size and shape and position errors are calculated according to the spatial coordinate values of these points through mathematical operation. As shown in Figure 9-2, to measure the diameter of a cylindrical hole on the workpiece, you can touch three points (points 1, 2,3) on the inner hole wall in the section I perpendicular to the hole axis, and then you can calculate the diameter and central coordinate oi of the hole according to the coordinate values of these three points; If there are many contact points in the cross section (point 1, 2, …, n, n is the measuring point), the roundness error of the cross section circle can be calculated by the least square method or the minimum condition method; If several cross-sectional circles perpendicular to the hole axis are measured (i, II, …, m, m are the number of cross-sectional circles to be measured), the cylindricity error of the hole and the center coordinates of each cross-sectional circle can be calculated according to the coordinate values of the measured points, and then the position of the hole axis can be calculated according to the coordinate values of each center; If three more points are contacted on the hole end face A, the position error of the hole axis relative to the end face can be calculated. It can be seen that this working principle of CMM makes it universal and flexible. In principle, it can measure any parameter of any geometric element of any workpiece.

Third, the classification of CMM

(A) according to the technical level of CMM classification

1. Digital display and print type

This kind of CMM is mainly used for geometric dimension measurement, and can display and print the coordinate data of the measured point. However, it needs manual operation to obtain the required geometric size, shape and position errors, and its technical level is low, which has been basically eliminated at present.

2. Computer data processing type

This kind of CMM has a slightly higher technical level and is widely used at present. The measurement is still carried out manually or by moving, but the computer can be used to process the measured data, such as automatic correction calculation of workpiece installation inclination, coordinate transformation, hole center distance calculation, deviation value calculation and so on.

3. Computer numerical control type

The technical level of this CMM is very high, and it can automatically measure according to the compiled program like a CNC machine tool.

(2) Classification according to the measuring range of CMM.

1. Small coordinate measuring machine

The measuring range of this coordinate measuring machine is less than 500mm in its longest coordinate axis direction (generally X axis direction), and it is mainly used for measuring small precision molds, cutters and cutters.

2. Medium-sized coordinate measuring machine

The measuring range of this CMM is 500 ~ 2000 mm in the direction of the longest coordinate axis, and it is the most widely used model, mainly used for measuring boxes and die parts.

3. Large coordinate measuring machine

The measuring range of this CMM in the direction of its longest coordinate axis is more than 2000mm, which is mainly used to measure large parts such as automobile and engine casing, aero-engine blades and so on.

(C) according to the accuracy of CMM classification

1. precision coordinate measuring machine

Its uniaxial maximum measurement uncertainty is less than 1× 10-6L (L is the maximum range in mm), and its spatial maximum measurement uncertainty is less than (2 ~ 3 )× 10-6L, so it is usually placed in a constant temperature measuring room for precise measurement.

2. Medium and low precision coordinate measuring machine

The maximum measurement uncertainty of low-precision coordinate measuring machine is about 1× 10-4L, the maximum measurement uncertainty of space is about (2 ~ 3 )× 10-4L, and the maximum measurement uncertainty of medium-precision coordinate measuring machine is about/kloc-0 /×14l. This kind of CMM is usually placed in the production workshop and used for production process inspection.

(D) According to the structure of CMM classification

According to the structural form, CMM can be divided into mobile bridge type, fixed bridge type, gantry type, cantilever type and column type. , as shown in the next section.

Section 2 Mechanical Structure of Coordinate Measuring Machine

I. Structural form

The coordinate measuring machine is composed of three orthogonal linear motion axes, and the mutual configuration position of these three axes (that is, the overall structure form) has great influence on the accuracy of the measuring machine and its applicability to the measured workpiece.

Second, the workbench

In the early days, the worktable of CMM was generally made of cast iron or cast steel, but in recent years, manufacturers began to widely use granite to make worktable, because granite has small deformation, good stability, wear resistance, no rust and low price.

Low cost and easy processing. Some measuring machines are equipped with lifting table to expand the measuring range of Z axis, while others are equipped with rotating table to expand the measuring function.

Third, the guide rail

Guide rail is the guiding device of measuring machine, which directly affects the accuracy of measuring machine, so it requires high linear accuracy. The guide rails used in CMM include sliding guide rail, rolling guide rail and air-floating guide rail, but sliding guide rail and air-floating guide rail are commonly used, and rolling guide rail is rarely used because the rolling guide rail has poor wear resistance and lower stiffness than sliding guide rail. In the early CMM, many models used sliding guide rails. The sliding guide rail has high precision and strong bearing capacity, but it has large friction resistance and is easy to wear. It is easy to crawl at low speed and difficult to run at high speed, and it tends to be gradually replaced by air bearing guide rail. At present, most coordinate measuring machines adopt aerostatic guideway (also known as air bearing guideway and air cushion guideway), which has the advantages of simple manufacture, high precision, low friction and stable operation.

With the development of air flotation technology, CMM has made great breakthroughs in processing cycle and accuracy. At present, many manufacturers are looking for high-strength lightweight materials as guide rail materials, and some manufacturers have chosen ceramics or carbon fiber with high film content as the materials for moving parts on mobile bridges and beams. In addition, in order to speed up the heat conduction and reduce the thermal deformation, Zeiss Company uses coated anti-aging alloy to make the guide rail, which makes the aging deformation extremely small and makes the temperature of each part more uniform, thus improving the measurement accuracy of the whole machine and relaxing the requirements for the ambient temperature.

Section 3 Measuring System of Coordinate Measuring Machine

The measuring system of CMM consists of scale system and probe system, which are the key components of CMM and determine the measuring accuracy of CMM.

First, the hierarchy

The scale system is used to measure the coordinate values of each axis. At present, there are many kinds of calibration systems used in CMM, which are basically the same as those used in various machine tools and instruments. According to its nature, it can be divided into mechanical scale system (such as precision screw plus differential roller, precision rack and pinion, rolling ruler), optical scale system (such as optical reading scale, optical encoder, grating, laser interferometer) and electronic scale system (such as inductosyn and electronic scale system). According to the statistical analysis of the scale system used in the production of CMM at home and abroad, it is known that grating is the most used, followed by inductosyn and optical encoder. Some high-precision CMM calibration systems use laser interferometer.

Second, the probe system

(1) probe

CMM uses probe to pick up signals, so the performance of probe directly affects the accuracy and efficiency of measurement. Without advanced probe, the function of CMM can't be fully exerted. According to the structural principle, the probe used in CMM can be divided into mechanical type, optical type and electrical type. According to the measurement method, it can be divided into contact type and non-contact type.

1. mechanical contact probe

Mechanical contact probes are rigid probes, which can be divided into conical probes, cylindrical probes, spherical probes, semicircular probes, point probes and V-shaped block probes according to the shape of their contact parts (as shown in Figure 9-5). This probe is simple in shape and easy to manufacture, but the measuring force depends on the experience and skill of the operator, so the measuring accuracy is poor and the efficiency is low. At present, except for a few manual measuring machines, most measuring machines no longer use this probe.

2. Electrical contact probe

At present, most coordinate measuring machines adopt electric contact probes, which can be divided into dynamic probes and static probes according to their working principles.

(1) dynamic detector

The measuring rod is installed on the iron core, and the iron core is placed on three pairs of contacts through three steel balls distributed along the circumference of 1200. When the measuring rod does not bear the measuring force, the steel ball on the iron core keeps in contact with the three pairs of contacts. When the spherical end of the measuring rod is in contact with the workpiece, no matter which direction the contact force is applied in X, Y and Z, at least one steel ball will be out of contact with the contact, which will cause the circuit to break and generate a step signal.

Visible, the probe is in contact with the workpiece surface movement process, instantaneous measurement sampling, so it is called dynamic probe, also known as trigger probe. The dynamic probe is simple in structure and low in cost, and can be used for high-speed measurement, but its accuracy is slightly lower. Moreover, the dynamic probe can not stay on the surface of the workpiece in contact state, but can only measure the surface of the workpiece point by point discretely, and can not carry out continuous scanning measurement. At present, most manufacturers choose the trigger probe produced by RENISHAW Company in Britain.

(2) Static probe

The static probe not only has the function of triggering the probe to sample, but also is equivalent to a subminiature coordinate measuring machine. There is a three-dimensional geometric sensor in the probe. When the probe is in contact with the workpiece surface, there are corresponding displacement outputs in X, Y and Z directions, thus driving the servo system to automatically adjust, stopping the probe at the specified displacement, and collecting three-dimensional coordinate data when the probe is near static, so it is called a static probe. When the static probe moves along the surface of the workpiece, it can always keep contact and carry out scanning measurement, so it is also called scanning probe. Its main characteristics are high precision and continuous scanning, but it is difficult to manufacture, slow sampling speed and expensive, so it is suitable for high-precision measuring machines. At present, the electrostatic probes produced by Lai Ci, Zeiss and Kerry all use inductive displacement sensors, and the electrostatic probes at this time are also called three-way inductive probes. Figure 9-7 shows the structure of the three-dimensional induction probe with double leaf springs produced by Zeiss Company.

The probe is in the form of a three-layer spring guide rail with three layers in three directions, and each layer is suspended by two leaf springs. The adapter 17 can move in the x direction through a parallelogram mechanism consisting of two x-direction leaf springs 16. The parallelogram mechanism is fixed below the parallelogram mechanism composed of the Y-direction leaf spring 1, and the adapter can move in the Y-direction by means of the leaf spring 1. The Y-direction parallelogram mechanism is fixed below the parallelogram mechanism composed of the Z-direction leaf spring 3, and the adapter can move in the Z direction by its leaf spring. In order to enhance the stiffness and stability of the leaf spring, a metal splint is installed in the middle of the leaf spring. In order to ensure the sensitive and accurate measurement, the leaf spring should not be too thick, generally 0.1mm. Because the Z guide rail is installed horizontally, it is balanced by three groups of springs 2, 14 and 15. There is a thread adjusting mechanism above the adjustable spring 14. By adjusting the balance force, the micromotor 10 rotates the balance force adjusting screw 1 1 to make the balance force adjusting nut sleeve 13 rise and fall, and automatically adjust the balance force. In order to reduce the displacement of Z-direction spring plate caused by shear force, spring 2 and 15 are set to balance the dead weight of Y-direction and X-direction components of the probe respectively.

Each layer of guide rail has three parts: ① locking mechanism: as shown in Figure 9-7b, its positioning block 24 has a groove, which is precisely matched with the locking steel ball 23 on the locking rod 22 to determine the "zero position" of the guide rail. When opening is required, the motor 20 can be reversed by an angle, and the directional guide rail is in a free state. When locking is needed, rotate the motor forward by an angle. ② Displacement sensor: used to measure displacement. As shown in fig. 9-7c, the magnetic core 27 is fixed on one side of the double-layer guide rail, and the coil 26 and the coil bracket 25 are fixed on the other side of the double-layer guide rail. ③ Damping mechanism: used to reduce the influence of external vibration during high-resolution measurement. As shown in Figure 9-7d, damping sheet 29 and 30 are respectively fixed on the upper damping bracket 28 and the lower damping bracket 3 1 which move relatively, so that a capillary gap is formed between the two damping sheet, and viscous silicone oil is put in the middle, so that the two layers of guide rails generate damping force when moving, thus avoiding the swing caused by an excessively sensitive leaf spring mechanism.

(3) Optical probe

In most cases, there is no mechanical contact between the optical probe and the measured object. This non-contact measurement has some outstanding advantages, mainly as follows: 1) Because there is no measuring force, it is suitable for measuring all kinds of soft and thin workpieces; 2) due to non-contact measurement, the surface of the workpiece can be quickly scanned and measured; 3) The measuring range of most optical probes is relatively large, which is difficult for general contact probes to achieve; 4) It can detect the parts on the workpiece that are difficult to be detected by general mechanical probes. In recent years, the optical probe has developed rapidly. At present, there are many kinds of optical probes used in coordinate measuring machines, such as triangulation probe, laser focusing probe, optical fiber probe, three-dimensional probe and contact grating probe. The following briefly introduces the working principle of triangular probe. (2) Probe accessories

In order to expand the function of the probe, improve the measurement efficiency and detect different parts of various parts, it is often necessary to configure various accessories for the probe, such as probe, connector, probe rotating accessories and so on.

1. Measuring terminal

For the contact probe, the measuring end is the part that is in direct contact with the surface of the workpiece to be measured. Different shapes of surfaces require different measuring heads. Figure 9-9 shows some common measuring head shapes.

Step 2 investigate

A probe is a replaceable measuring rod. In some cases, in order to facilitate the measurement, it is necessary to choose different probes. The probe has a great influence on the measurement ability and accuracy. It should be noted that: 1) The probe should be as short as possible on the premise of meeting the measurement requirements; 2) The diameter of the probe must be smaller than the diameter of the measuring end. Under the condition of non-interference, try to choose a large diameter probe; 3) When long probes are needed, cemented carbide probes can be selected to improve the rigidity. If you need a particularly long probe, you can choose a lightweight ceramic probe.

3. Connector

In order to connect the probe to the probe, and to connect the probe to the rotating body or the spindle of the measuring machine, various connectors are needed. Commonly used are star probe connectors, connecting shafts and star probe seats.

Step 4 rotate the attachment

It is used for the detection of some workpiece surfaces, such as some inclined surfaces and integral impeller blade surfaces. It is impossible to complete the required measurement only with a probe perpendicular to the workbench. At this time, it is necessary to make the probe or the whole probe rotate at a certain angle with the help of certain rotating accessories to expand the function of the probe.

The common rotating accessory is the probe rotating body, as shown in Figure 9-11a.. It can rotate around horizontal axis A and vertical axis B, and there is a precise indexing mechanism in its rotating mechanism, and its indexing principle is similar to that of multi-tooth indexing disk. There are 48 cylinders evenly distributed along the circumference in the static disk, while there are 48 corresponding steel balls in the dynamic disk, which are graduated in steps of 7.5o Its rotation range around the longitudinal axis is 360o, ***48 positions, and its rotation range around the horizontal axis is 0o ~ 105o, * *15 positions. When the rotation angle around the horizontal axis is 0o (that is, the probe is vertically downward), the position of the measuring end remains unchanged when rotating around the vertical axis, so that the measuring end can have 48× 14+ 1 = 673 positions in space. It can change the attitude of the probe to expand the ability to approach the workpiece from all directions. At present, the probe rotors used in measuring machines are all kinds of probe rotors produced by RENISHAW Company.

The fourth quarter coordinate measuring machine control system

A, the function of the control system

Control system is one of the key components of coordinate measuring machine. Its main functions are: reading the space coordinate values, controlling the real-time response of the measurement aiming system and processing the probe signals, controlling the mechanical system to realize the necessary movement of measurement, and monitoring the state of the coordinate measuring machine in real time to ensure the safety and reliability of the whole system.

Second, the structure of the control system

According to the degree of automation, coordinate measuring machines are divided into manual, electric and numerical control. In the early days, the coordinate measuring machines were mainly manual and electric, and the measurement was completed by the operator directly manually or through the joystick, and then the data was processed in the computer. With the development of computer technology and numerical control technology, numerical control system has become more and more popular. It controls the automatic feeding and data sampling of CMM through programs, and completes data processing in the computer.

1. Manual and mobile control system

This control system has simple structure, convenient operation and low price, and is widely used in workshops. The calibration system of these two kinds of coordinate measuring machines is usually grating, and the probe generally adopts trigger probe. The working process is as follows: whenever the trigger probe touches the workpiece, the probe sends a trigger signal, and sends an interrupt signal to the CPU through the probe control interface. The CPU executes the corresponding interrupt service program, reads the value of the counting interface unit in real time, calculates the corresponding space length, forms sampling coordinate values X, Y and Z, and sends them to the sampling data buffer for subsequent data processing.

2. Numerical control system

The measurement feed of numerical control system is controlled by computer. The motion of each axis of the measuring machine can be controlled by the program, and the running state of the measuring machine can be monitored in real time to realize automatic measurement. In addition, it can also be measured manually through the joystick. CNC control system can be divided into centralized control and decentralized control.

(1) centralized control

The main CPU realizes centralized control, monitors and samples coordinate values, and completes the tasks of receiving, interpreting and executing host commands, real-time returning and displaying status information and data, keyboard input of control commands and safety monitoring. Its motion control is completed by an independent module, which is a relatively independent computer system, completing single-axis servo control, three-axis linkage and motion state monitoring. From the functional point of view, the motion control CPU should not only complete the operation of digital regulator, but also carry out interpolation operation, which has a large amount of operation, and its real-time performance and measuring feed speed depend on the speed of CPU.

(2) Distributed control

Distributed control refers to the use of multiple CPUs in the system, and each CPU completes specific control. At the same time, these CPUs work in coordination to complete the measurement task together, so the speed is fast and the real-time performance of the control system is improved. In addition, distributed control is characterized by multi-CPU parallel processing. Because it is single, it is easy to maintain and expand. If you want to add a turntable, you only need to extend a single-axis control unit in the system, define its address on the bus and add the corresponding software.

Third, metering and feeding control

Coordinate measuring machines other than manual control the rotating speed of servo motor through joystick or CNC program, so as to control the probe and measuring table to move relative to each other according to the set trajectory and realize the measurement of the workpiece. The measuring feed of CMM is basically the same as that of CNC machine tools, but it requires higher motion accuracy, motion stability and response speed. The motion control of CMM includes single-axis servo control and multi-axis linkage control. Single-axis servo control is relatively simple, and the motion control of each axis is completed by its own single-axis servo controller. However, when the probe is required to move relative to the workpiece according to a predetermined trajectory in three-dimensional space, the CPU needs to control three axes according to a certain algorithm to realize the spatial movement of the probe. This control is completed by the above single-axis servo controller and interpolator. In the control system of CMM, the interpolator is controlled by CPU program. According to the set trajectory, CPU constantly provides coordinate axis position instructions to the three-axis servo control system, and the single-axis servo control system keeps tracking, so that the probe moves from the starting point to the end point step by step.

Fourth, the communication of the control system.

The communication of control system includes internal communication and external communication. Internal communication refers to the mutual transmission of commands, parameters, status and data between the host and the control system, which is realized by the communication bus connecting the host and the control system. When CMM is a part of FMS system or CIMS system, external communication refers to the communication between control system and other equipment. At present, serial RS-232 standard and parallel IEEE-488 standard are mainly used for the communication of coordinate measuring machines.

Section 5 Software System of Coordinate Measuring Machine

Modern coordinate measuring machines are equipped with computers, which collect data through operation and output the required measurement results. The function of its software system directly affects the function of the measuring machine. Therefore, all CMM manufacturers attach great importance to the research and development of software systems, and the proportion of manpower and financial resources invested in this area is increasing. Let's briefly introduce the software used in CMM.

A, programming software

In order to realize automatic measurement of CMM, it is necessary to compile the corresponding measurement program in advance. These measurement procedures are compiled as follows.

(a) icon and window programming mode

Graphic and window programming is the simplest way. Select the measured element through the graphic menu, establish the coordinate system, select the operation process and input parameters through the "window" prompt, and compile the measurement program. This method is only suitable for relatively simple programming of single geometric element measurement.

(B) self-study programming mode

This programming method is that on the CNC measuring machine, the operator guides the measuring process and types the corresponding instructions until the measurement is completed, and the computer automatically records the manual operation process and related information of the operator and automatically generates the corresponding measuring program. If you want to measure the same parts repeatedly, you can automatically complete all the measurement processes recorded before by calling the measurement program. This method is suitable for batch inspection and is also a relatively simple programming method.

(3) off-line programming

In this way, the measuring program is pre-programmed on other general-purpose computers by using the special measuring machine language provided by the CMM manufacturer, which has nothing to do with the opening of the CMM. After compiling the program, go to the measuring machine for trial operation, and modify it when errors are found. Its advantage is that it can solve very complicated measurement work, but its disadvantage is that it is easy to make mistakes.

(4) Automatic programming

In computer integrated manufacturing system, measurement program is usually automatically generated by CAD/CAM system. On the one hand, CMM reads the design drawing data file generated by CAD system and automatically constructs virtual workpiece; On the other hand, CMM accepts the actual workpiece processed by CAM, and automatically generates the measurement path according to the virtual workpiece to realize unmanned automatic measurement. The measurement program in this process is completely automatically generated by the system.

Second, the measurement software package

The measurement software package can contain a variety of data processing programs to meet various engineering needs. The measurement software package of CMM is generally divided into general measurement software package and special measurement software package. The general measurement software package is mainly a software package for measuring the basic geometrical features of points, lines, surfaces, circles, columns, cones and spheres, and their shape and position errors and relationships. Usually, every CMM is equipped with this software package. Special measurement software package refers to various measurement software packages developed by CMM manufacturers in order to improve the measurement efficiency and accuracy of some specific measurement objects. For example, many coordinate measuring machines are equipped with special measuring software packages for common parts and surfaces, such as gears, cams and camshafts, threads, curves and surfaces. Some measuring machines are also equipped with special measuring software packages for measuring automobile body, engine blades and other parts.

Third, the system debugging software

Used to debug the measuring machine and its control system, generally have the following software.

(1) Self-check and fault analysis software package: used to check system faults and automatically display fault categories;

(2) Error compensation software package: it is used to detect the geometric error of the coordinate measuring machine and correct the error of the coordinate measuring machine according to the detection result when the coordinate measuring machine works;

(3) System parameter identification and control parameter optimization software package: used for overall debugging of CMM control system and generating user running files with optimized parameters;

(4) Accuracy test and acceptance measurement software package: used to measure the fixture according to the acceptance standard.

Fourth, the system work software

The measurement software system must be equipped with some software to coordinate and assist the work, some of which are necessary and some are used to expand the functions.

(1) Probe management software: used for probe calibration, probe rotation control, etc.

(2) CNC running software: used for probe motion control;

(3) System monitoring software: used to monitor the system (such as monitoring power supply, gas supply, etc.). );

(4) Compiling system software: compiling with this program to generate running target code;

(5)DMIS interface software: used to translate DMIS format files;

(6) Data file management software: used for all kinds of file management;

(7) Networking communication software: used to realize two-way or one-way communication with other computers.