转台校准用精密角度分度系统设计外文文献翻译、中英文翻译

Design of precision angular indexing system for calibration of rotary tablesMechatronics & Manufacturing Center, Samsung Electronics Co. LTD., Suwon-city, 443-742, KoreaAbstractThe indexing table was developed for angle measurements on machine tools. The measuring techniques, which have been reviewed in this paper, are currently available in manufacturing engineering to calibrate the angle measurement. The measuring principles of each equipment are outlined and their capabilities are also discussed. The new indexing table using 6 points kinematic concept and employing ball and vee grooves location was designed and manufactured to calibrate angle errors of rotary tables using a laser interferometer. The manufacturing method was evaluated to manufacture the accurate vee grooves. A special cam was designed and manufactured to trans-late rotation movement into lift-up and rotation movement. A CNC machining centre and indexing head were used to manufacture the cam. All parts of the new indexing table were manufactured with a manufacturing tolerance according to drawings, squareness and paral-lelism checked to obey the 6 points kinematic concept. Also these factors controlled the repeatability of the new indexing table. After installing the new indexing table, the performance was evaluated using rotary table operating in both the horizontal and vertical orienta-tions.1. IntroductionMany types of indexing tables appear in industry. These in-dexing tables have recently become an increasing necessity for improving the precision and accuracy of angle measure-ment, and circle dividing in various fields of science and tech-nology. Most indexing tables use a HIRTH coupling, worm and wormwheel, or a grating disc. The manufacturing process together with the requirements for precision is very difficult. Consequently these indexing tables are expensive and usually used for manufacturing purpose of rotating.To calibrate these indexing tables, a polygon and autocolli-mator, or the serrated type indexing table which is usually 1 second accuracy, are usually used and the calibration accuracy of the polygon and autocollimator is about 1 second. Many mathematical models have been developed to calibrate rotary tables using polygon and autocollimator . These mathe-matical models usually use a matrix form and calibration takes a long time.Subsequently a calibration method using a laser interfer-ometer was developed by Lin . To accomplish his calibra-tion method, the indexing table requires good repeatability, so an automatic indexing table using a ball and pin was devel-oped 2. Although this performed very well in a laboratory situation, it was not considered robust enough to reproduce as a commercial design. The problem with this automatic index-ing table is the balls which are located on the circumference of the body could move when the top plate was located, or any force was applied to the top plate. Consequently a new mechanism should be considered for the indexing table. Thus a 6 point contact kinematic design concept of ball and vee groove was used, because the ball and vee groove system can attain 0.1 m repeatability 3 and has good rigidity.The aim of this research is to consider the design and manu-facturing of an automatic indexing table using this kinematic design concept.2. Techniques for the calibration of rotary indexing tablesAll measuring instruments possess finite accuracy. The er-ror which arises from instrument limitations is usually not of a magnitude which could significantly affect the accepted accu-racy of specific measurement. Nevertheless, for assuring the reliability of measurements, it is necessary to determine the operating accuracy of circular dividing instruments by means of a dependable calibration process.Fig. 1. Calibration of rotary table with 1440 indexing table and mirror.Fig. 2. Reading the change of angular optics due to a changing angle.2.1 Calibration of indexing table by polygon and autocolli-matorFor the calibration of an indexing table, the most generally used method is based on the use of a polygon and autocolli-mator. Calibration of the rotary table is readily performed by means of a precision polygon mounted on a table and read by a single fixed autocollimator. Several methods to calibrate a rotary indexing table were developed by N.P.L. 1.The fixed autocollimator is null on the centre of the first face of polygon. Next the rotary table is rotated through the prescribed angle, corresponding to the increments (number of faces) of the polygon. Angular deviation shown by the auto-collimator is recorded when returning to its original setting face. A correction factor (error of each polygon face from nominal) furnished by the manufacturer is applied to the read-ing to find the actual error of the rotary table at the sectors calibrated or measured.2.2 Rotary table calibration with serrated tooth type indexing tableFig. 1 shows a 1440 indexing table which has 1440 serrated tooth and 0.1 second accuracy. It is mounted on a rotary table for calibration. A single shield reflecting mirror and autocol-limator, much the same as that employed for self-calibration of the 1440 indexing table are used. The arrangement is typi-cal of that already supplied by the rotary table manufacturers.Errors of the rotary table are read directly on the autocolli-mator, requiring no closing of the circle, nor use of calibra-tion factors. Errors of the 1440 indexing table in this applica-tion may be considered negligible. The advantages compared to the other method of calibration with the polygon, can be summarized as follows:(1) One self - contained unit is the equivalent of several types of polygons(2) The polygon can only be relied on to 1 or 2 seconds, compared to the 0.1 second accuracy of the 1440 indexing table.Only one mirror is used. The importance of the single mir-ror cannot be over emphasized; a large flat mirror may be perfected for maximum reflectivity. Effort need only be ex-pended to assure that it is absolutely flat without having to consider simultaneously its angular position. The same spot in the mirror is always used, instead of shifting mirror to mirror.2.3 Calibration of indexing table by laser interferometer and ball type automatic indexing tableA researcher 2 developed the laser calibration of the rotary table with an automatic indexing table and a laser interferome-ter. The calibration method usually uses a mathematical model with a matrix and off line system. Also the autocollimator is dependant on air currents therefore the calibration data is reli-able only when the air condition is stable. When the laser in-terferometer is used for calibration, air currents can be com-pensated for by the compensator of the laser interferometer.As this system is used for calibration, the most important thing is that automatic indexing table should give good repeat-ability. To accomplish repeatability the automatic indexing table employs the ball and pin using a kinematic design prin-ciple. The balls are in contact with the circumference of the body and outer ring holding the balls fixed on the circumfer-ence of the body. The pins are fixed in the top disc. The top disc can be moved 6 degrees each step by a camshaft and mo-tor. Also the pins are kinematically located at every target position 2.This system incorporates an indexing table and laser inter-ferometer with angular optics. As mentioned above with this system the table only needs to be repeatable. The principle of angular measurement using the laser interferometer is shown Fig. 2. The range of angular optics is normally restricted to 10 but by using the indexing table to repeatedly reset of the angular optics to a nominal zero position, it becomes feasible to undertake a 360 full scale test. The indexing table is driven by a camshaft and motor assembly. This allows indexing by one single step of 6 during angular measurement. The angu-lar optics is mounted on the automatic indexing table under calibration.This reading is accomplished by stepping the table under test forward one step and then indexing backward through the same nominal angular increments. This measuring technique contains two sets of laser readings which are needed to cali-brate errors for both tables.From the Fig. 3Fig. 3. Mathematic modeling for angle analysis. Fig. 5. Kinematic location of balls in radial vee.Fig. 4. Kinematic couplings.CSin ( + i ) = CSin + RiCSin ( + i ) = CSin + ii = i + i .From the above equations, , i , i ,i are calculated 2. It is so called a self-calibration technique that requires no pre - calibration of the indexing table.3. Design of new indexing table3.1 Concept of kinematic designKinematic design is widely recognized as one of the fore-most design concepts in precision engineering. Most objects in space have three degrees of translators and three degrees of rotational freedom 6, 7. Two different philosophies for me-chanical design exist - kinematic and elastic.While being quite different in approach they can be com-bined in a design. In the kinematic design philosophy, the aim is to locate all parts relative to each other, while allowing a degree of freedom as needed, by connecting points together without significant elastic deformation. Fig. 4 shows three examples of kinematic couplings.A simple theorem that allows calculation of the number of points of contact was enunciated by Strong 4. He defined:“Kinematic design is correct when a body in contact with another has at least 6-n points of contact where n is the number of degrees of freedom existing. If the system has more than 6-n points of contact it has mechanical redundancy” 4.In Fig. 4(a), three rotations are possible for the ball, whereas in Fig. 4(b) and (c) no freedom of movement is provided.3.2 Feature of new indexing tableAs mentioned in previous section, the automatic indexing table which was developed by Lin 2 employs pins and balls. The balls are in contact with the circumference of the body and the outer rings hold the balls fixed on the circumference of the body. The ball is moved by applying high speed rota-tion of camshaft. Consequently the automatic indexing table cannot give good repeatability.3.2.1 Detailed consideration of new indexing tableThe basic principle of this new indexing table employs a vee and balls kinematic location system. Each ball is contact with both side of vee surface so each ball has 2 contact points on the vee. Therefore three balls have 6 kinematics contact points. The chosen incremental indexing angle is 5 degree because 5 degree incremental angle is considered to be the smallest increment used for the practical calibration of 360 indexing tables.The detailed specification of this new table is as follows:（1）An incremental angle indexing device which is small and light weight thus does not influence the machine on which it is being used.（2）The size of the device compatible with the angle reflec-tor optics of the laser interferometer. The size of this angle reflector is 404072.6 (mm).（3） The table should be capable of operating in any attitude. (i.e. vertical, horizontal and up-side down)（4）Automatically indexed from a program command on a P.C.（5）Step angle of 5 degrees.（6）Accuracy of step angle of about 1 second of arc but it is not essential using the calibration technique employed.（7）Repeatability of indexing 0.2 seconds of arc.（8）Table can be easily manufactured and does not require high precision technique.（9）Low cost.Fig. 6. Mechanism of indexing table.3.2.2 Mechanism of new indexing tableGenerally, commercially available indexing tables contain two step operations for indexing (i.e. lift up and rotate), but this new type of indexing table employs one step operation, lifting up and rotating simultaneously by using a camshaft and motor. Fig. 6 shows the mechanism of the indexing table.To hold the top disc on the body and lock the top disc on the vee, the indexing table employs a compression spring be-tween the body and pindisc which is connected to the shaft by screws. A needle bearing is fitted between the spring and pin disc to prevent torsion and minimize friction.The vee and ball type of indexing table is designed to mi-nimize the working space and gives good repeatability be-cause the vee and ball contact with 6 points kinematic concept (i.e. each ball contact with vee surface with 2 contact points). Spur gears are employed to translate motor motion into the camshaft and to increase rotation force (i.e. torque).The indexing table employed a pindisc to transform the ro-tation motion into lift-up and rotation motion of the top disc. The pindisc which is assembled with main shaft and top disc, had 36 equally spaced pins giving a 10 degree incremental angle around the circular disc. Pins are designed to guide, lift-up and rotate during camshaft rotation. The pindisc follows the groove of the camshaft during camshaft rotation. The rota-tion of the camshaft is controlled by an opto-disc which is fixed to the camshaft end. 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