Compensation error of measuring ball radius of the

The compensation error of measuring ball radius of CMM probe

after decades of rapid development, the coordinate measurement technology has matured and the measurement accuracy has been greatly improved since the British Ferranti company manufactured the first digital probe mobile CMM in 1950, and the West German opton company completed the design of three-dimensional probe and launched the first coordinate measurement system together with the electronic computer before 1973, The function of the measurement software is more powerful, and the operation interface is becoming more and more perfect. Manufacturers all over the world have developed CMM models suitable for different purposes. The development of decades has fully proved that the modern coordinate measurement system breaks the traditional measurement mode, and has the characteristics of versatility, flexibility and efficiency. It can complete the measurement of various complex parts through computer control, which meets the needs of the development of flexible automation in the mainframe of experimental machines, microcomputers and printers, which are more common in the component configuration of mechanical manufacturing industry. It can meet the high-precision High efficiency requirements

in addition to the measurement of spatial dimensions and form and position errors, the application of coordinate measuring machines to measure complex surfaces with unknown mathematical models, extract the original shape information of complex surfaces, reconstruct the measured surfaces, and realize the digitization of the measured surfaces is not only an important field of the application of coordinate measuring machines, but also one of the key technologies in reverse engineering, which has also developed rapidly in recent years

1 classification of probe

as a measuring sensor, the measuring head is a very important component in the coordinate measurement system. The working efficiency and accuracy of CMM are closely related to the measuring head. Without advanced measuring head, the excellent function of CMM cannot be played. The development of coordinate measuring machine promotes the development of new probe, and the development of new probe further expands the application range of measuring machine. According to the measurement method, the probe can be divided into contact type (trigger type) and non-contact type. Trigger type measuring head is divided into mechanical contact type measuring head and electrical contact type measuring head; Non contact probe includes optical microscope, TV scanning head and laser scanning head. The focus of this paper is the trigger probe

(1) mechanical contact probe

contact probe is also known as "rigid probe" and "hard probe", which are generally used for "static" measurement and are mostly used as contact elements. This kind of probe has no sensing system, no range, no signal, and is just a pure mechanical contact. Mechanical contact probe is mainly used for manual measurement. Due to direct manual operation, the measuring force of the probe is not easy to control and is only suitable for general accuracy measurement. Due to its obvious shortcomings, this kind of probe is rarely used at present

(2) electrical contact probe

electrical contact probe is also known as "soft probe", which is suitable for dynamic measurement. As a measuring sensor, this kind of probe is the only part in contact with the workpiece. Every time a point is measured, the sensing part of the probe always has a process of "contact deflection sending reply". The measuring end of the probe can offset after contacting the measured part, and the sensor outputs a signal of analog displacement. This kind of probe can be used not only for aiming (i.e. zero crossing transmission), but also for micrometering (i.e. measuring the deviation value of a given coordinate value). Therefore, according to its function, electrical contact probe can be divided into switch probe for aiming and three-way probe with micrometer function. Electrical contact probe is the most used probe at present

2 compensation error of measuring ball radius

(1) selection of measuring needle

correct selection and use of measuring head is an important factor affecting the measurement accuracy of coordinate measuring machine. The measuring needle is installed on the measuring head, which is the part of the measuring system that directly contacts the workpiece. The communication channel between it and the measuring head is called trigger signal. How to choose the appropriate type and specification of the measuring needle depends on the characteristics of the measured workpiece, but in any case, the rigidity of the measuring needle and the sphericity of the measuring ball are indispensable

ruby for industrial use is a ceramic material with high hardness. Ruby measuring ball has good sphericity, and the ball head wear of ruby measuring ball can be ignored during measurement. Generally, the non-magnetic stainless steel needle rod or carbon tungsten fiber needle rod is used to ensure the rigidity of the measuring needle. The effective working length (EWL) of the measuring needle enables the precise measuring point position to be obtained when the measuring needle contacts the workpiece. The selection of the ball head size and the effective working length of the measuring needle depends on the measured workpiece. If possible, select the measuring needle with the largest ball head diameter and the shortest measuring rod, so as to ensure the maximum ball head/rod distance and obtain the best effective working length and rigidity of the measuring needle. When necessary, the measuring rod can be lengthened to increase the detection depth, but it is worth noting that the use of the measuring needle extension rod will reduce the rigidity, thus reducing the measurement accuracy

(2) compensation error of measuring ball radius

when the measuring needle contacts the workpiece, the coordinate value received by the coordinate measuring machine should be the coordinate of the center point of the ruby ball head. Obviously, the measuring software will automatically add a measuring ball radius value along the direction that the measuring needle retreats from the contact point as the measured value. However, the measured value is a dynamic value related to the mechanical inertia of the probe. In fact, as a dynamic process, the measured value should take into account the machine space movement distance from the point taken by the probe to the actual transmission of the coordinate value of the point to the system. Although this distance is very small, it has a certain impact on the dynamic size of the system

in the actual measurement, the system can automatically distinguish the compensation direction of the measuring ball radius and calculate the correct compensation radius for each element measured. After the start of sampling, the measuring software will compensate the radius of the measuring ball along the direction of the measuring needle contacting the workpiece. However, the compensated point is not a real contact point, but a point on the extension line of the probe along the direction of the probe contacting the workpiece. In this way, the compensation error is caused. The size of the error generated by the ignition source is related to the radius of the measuring ball and the included angle between the measured surface of the workpiece and the Cartesian coordinate axis. The larger the included angle is, the larger the error is

① influence of measuring ball radius r on compensation error

compensation error δ It is proportional to the radius of the measuring ball R, that is, the smaller the radius of the measuring ball R, the compensation error δ The smaller. Therefore, when using CMM for point position measurement, the measuring ball should be as small as possible

② the included angle between the axis of the measuring needle and the normal of the measured surface α Influence on compensation error

when the included angle between the axis of the measuring needle and the normal of the measured surface α When equal to 0, the radius compensation error of the measuring ball δ Also 0. Therefore, during measurement, the axis of the measuring needle should be perpendicular to the measured surface as much as possible, and the measuring head should move along the normal direction of the measured surface, so as to minimize the compensation error of the measuring ball radius

when measuring point elements with coordinate measuring machine, the measurement software will have compensation error of measuring ball radius in the process of automatic compensation of measuring ball radius. By using the reference coordinate system to align the workpiece or using the CNC mode for measurement, Su Ming made a special report on "Research on new energy industry policy", so that the probe moves the coordinates of the acquisition point along the normal direction of the measured surface, which can minimize the compensation error of the measuring ball radius, correctly compensate the measuring ball radius, and improve the measurement accuracy