Selection and analysis of servomotor for three-axis transmission system in CNC machine tool

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi:0.408/ 03 rans ech Publications, Switzerland Selection and analysis of servomotor for three-axis transmission system in CNC machine tool Yichen Wang,a, Xiuxia Yu *,b, Li Wu 3,c School of Biological and Agricultural Engineering, Jilin University, Changchun, 3005, China * Department of Computer Science, Changchun University, Changchun, 300, China 3 Institute of Mechanical Science and Engineering, Jilin University, Changchun, 3005, China a yxx6765@sina.com, b @qq.com, c wuli.engineer@gmail.com Keywords: servo motor; load / motor inertia ratio; temperature rise ratio; optimization matching Abstract: Based on the load torque ratio, load / rated speed ratio, the temperature rise ratio and load / motor inertia ratio of servo motor s optimization matching principle, servo motor selection mathematical model of transmission system in CNC machine tool is built, compared to traditional selection algorithm, which is using the actual duty cycle conditions to optimal matching and analyzing. hen motor models available to meet the maximum utilization of the servo system, achieve the purpose of improving motor performance and reduce costs. In this paper, by taking servo motor selection of three-axis transmission system in a CNC machine tools for example, horizontal, inclined and vertical transmission system s parameters selection and calculation are analyzed in detail. With the engineering test and motor control software, measure data is achieved, the reasonableness of the method in the paper is proved. Introduction he present selection technology of synchronous servo motor in CNC transmission system is calculating motor nominal speed, continuous load torque, no-load acceleration/ deceleration torque, cutting load torque, short-time feature and continuous nature and matching inertia ratio, then choosing servo motor and the appropriate control system, which can meet the conditions. As early as in 987, R.Krishran [] published a paper on the servo motor drive selection criteria; the paper explained a detailed explanation of the servo motor drive. By modified, many servo motor companies make it to meet their respective technical requirements; this is also commonly used by the machine tool industry []. However, with the continuous improvement of the technical performance of servo motor, the current selection technology exposes a lot of deficiencies. he technical characteristics of servo motor such as speed range, high-speed stability, accuracy, fast dynamic response are improved. At the same time, it achieves the weakly magnetic high-speed control and adapts to the requirements of high-performance servo drive. his requires the selection calculation of servo motor must closer to the actual production conditions. he result not only meets the needs of the transmission system, but also improves the utilization of the performance indicators, even by optimal matching; thereby reduce the cost of the servo motor. According to servo motor s weak magnetic property and high-performance servo drive technology, use the prototype of a CNC machine tool, based on the technical parameters of the machine tool, the paper is to pre-select and analyze three-axis transmission system with servo motors, then select the most optimized motor model. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of rans ech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/6,9:9:50)

2 Advanced Materials Research Vols Mathematical model he transmission system of the CNC machine tool is substantially horizontal, vertical and incline case. Its structure is mainly composed by the servo motor, reducer, coupling, screw, moving parts (table, headstock, column connected components and work piece) and other connection accessories, simplify the structure as shown in Figure. ()Horizontal transmission syste ()Vertical transmission system (3)Incline transmission system. servo motor. reducer(driving/driven gear)3. coupling 4. screw 5, 6. bearing block and bearings 7. table, work piece and other accessory 8. pulley 9. balance weight Figure. Structure of transmission system In machining, bearing forces of the servo motor comprise the continuous load torque (including gravity, friction, etc.), the acceleration/deceleration torque, and the cutting torque. When select the motor, it is necessary to carry out a comprehensive analysis of each of these forces, take into account the mechanical parts match with the drive motor, the operating speed of the motor, the acceleration/deceleration time of the machine tool, motor stopping distance and other factors, ultimately determine motor models. Specific parameters of selection with servo motor are detailed solved and analyzed in the following paper. Rated speed Motor speed about the rapid traverse should be strictly controlled within the motor rated speed, and should be in the range close to the rated speed of the motor, this method can effective use the servo motor power. he motor allows instantaneous speed greater than the maximum speed; maximum speed is greater than the rated speed. Load / motor inertia ratio Load inertia ratio reflects the motor ability of control load. Motor has appropriate inertia can work effectively for machine tool. Selection technical indicator is load inertia ratio (load inertia / motor inertia). When the work piece s surface quality is superior to other factors, as much as possible make the load inertia ratio smaller. If load inertia ratio is too large, it will cause the motor control instability, the motor will be very difficult to debug [3]. It also reduces the precision and the roughness of the machined surface, extends the positioning time. Formula of solving the load inertia,see formula (): J L M N M N 4 h c c c c Ji J j J l Jl J w di li m j i i j i i j 8 8 p m d m () Where J i is the rotational inertia of the rotating bodies; J j is the rotational inertia of the straight-line motion objects; J l3 and J l4 is the rotational inertia of reducer gear; J w is the rotational inertia of others, including commutation inertia in the motor shaft end about deceleration bodies from motor to screw, ρ is the density of the table etc., d i is the diameter of the rotating body, l i is the length of the rotating body, m j is the mass of the linear motion body, m c is the mass of the reducer driven gear, d c is the diameter of the driven gear, m c is the mass of the reducer driving gear, d c is [4,5] the diameter of the driving gear, Recommended load / motor inertia ratio to meet J L J M ~ 3. d J w

3 50 Optoelectronics Engineering and Information echnologies in Industry No-load torque No-load torque is the torque when the cutting is not performed. the calculation as formula () shown. No-load torque mainly include mechanical friction, the screw pre-load and of gravity in the gravitational axis torque, usually no-load torque should not exceed 30% of servomotor s stall torque. f mg p h P p 0 0 h B () Where P 0 is pre-load of the screw, P h is the screw pitch, B is the torque of the ball screw s bearings and other friction torque convert to motor shaft, μ is slideway friction coefficient, η is the mechanical efficiency.for the vertical axis, the friction torque is more difficult to calculate, it can be set according to the characteristics of the machine tool. With the counterweight, the no-load torque of the vertical axis, as the equation (3) shown: f mg F p P p b h 0 0 h (3) B Acceleration/deceleration torque (acceleration/deceleration characteristics) In the machining, he machine tool not only ensure the promote continuous thrust load, but also consider the short machining factors. he machine tool can reach the maximum thrust of machinery need in the process of acceleration and deceleration. herefore, when selecting the motor, it is necessary to consider whether the maximum torque of the motor and mechanical acceleration/deceleration processing matching. Maximum acceleration/deceleration torque as the formula (4) is shown. AC J M J L n 60t AC L Where J L is the load inertia, JM is inertia of the motor, n is the motor speed, t AC is the acceleration/deceleration time constant. In order to ensure the acceleration/deceleration characteristics of the transmission system,the formula must be meet AC 0.8 Amax.From the formula can see that the maximum torque of the motor is the direct impact on the setting acceleration/deceleration time constant. Cutting load torque When the machine tool in the normal working time, in addition to the no-load torque, the torque increase the cutting torque brought by the cutting force. In normal working condition, the cutting load torque does not exceed 80% of the rated torque of the motor, cutting load torque, as formula (5) is shown. F p c h f (5) Where F c is the cutting force, P h is the screw pitch, f is the no-load torque. If F c has the positive pressure in the direction perpendicular to the slideway, it should be add the friction caused by the cutting force. Continuous aging load torque When the machine tool require frequent start and brake, you must check it rms (root-mean-square) value of the motor torque in one cycle, in order to avoid overheating of the motor, the root-mean-square value of the torque in a processing cycle should be less than 90% of the stall torque of the motor. Continuous effective load torque is determined the average of the squares of each action torque, as formula (6) is shown. (4)

4 Advanced Materials Research Vols rms t t t 0 n t n (6) Example analysis In this paper, three-axis transmission systems of a CNC machine are shown as an example to analyze selection of servomotor. First pre-select servomotor, second detailed calculations of the main parameters, and then optimize motor model after re-matching motor models which all can used. Specific technical parameters of three-axis transmission systems, patameters are required for the calculation. able Mechanical parameters X Y Z as able is shown, hese Shaft drive weight (kg) Workpiece weight (kg) Balance weight (kg) Ball screw pitch (mm) Ball screw diameter (mm) Ball screw length (m) Axis direction (0~90) Machining conditions Precise motor selection must be based on the actual processing conditions. his paper set a universal CNC machining conditions, calculation of motor selection based on this condition. he specific conditions are set as shown in able. able Processing periodic table ime(s) V(m/min) note 0 Pause s Accelerated to rapid speed 3 30 Rapid speed s Decelerated to cutting speed Cutting 30s Cutting force 000N Accelerated to rapid speed 7-30 Rapid back s Decelerated to zero According to technology parameters and conditions, calculate main parameters of servomotor, used Siemens motor as sample, pre-selection motor Model is shown in able 3. ransmission system X-axis Y-axis Z-axis able 3 Pre-selection motor model Motor model FK708-3BF7(Nm, 3000rpm, great inertia) FK708-3BF7(Nm, 3000rpm, great inertia) FK70-AF7(7Nm, 3000rpm, conventional inertia) Confirmation items including load / motor inertia ratio, no-load torque ratio, load torque ratio, speed ratio and the temperature rise ratio. he load / motor inertia ratio should be between -3; the temperature rise ratio of horizontal axis does not exceed 90 %, the temperature rise ratio of vertical axis does not exceed 70%; torque utilization does not exceed 90%. After the calculation, there get a confirmation table,as shown in table 4. able 4 Servomotor selection confirmation table Inertia ratio emperature rise ratio Norque utilization ratio Max I X % 88.4% 3.48A Y % 9.% 4.09A Z % 53.6% 7.75A he characteristic curves can be drawn according to the pre-selected servo motor parameters, as shown in figure, it can be seen that the maximum load in the field weakening region, the motor model meet the requirements.

5 5 Optoelectronics Engineering and Information echnologies in Industry Figure. X, Y, Z-axis servo-motor Characteristic Graph Optimized motor selection Although the selected servomotor has meet all the requirements of the transmission system, but an optimal matching relationship between servomotor confirmation.in order to reach the maximum utilization of motor, lowest cost effect. Using MALAB software, make mathematical model, integrated solving a variety of motor, achieve the optimal matching, get optimized motor model as able 5 is shown, optimized motor confirmation, as is shown in able 6. After optimize servomotor, the highest comprehensive utilization is shown in this paper. ransmission system X-axis Y-axis Z-axis able 6 able 5 he optimized motor model Motor model FK7063-AF7(Nm, 3000rpm, conventional inertia) FK7063-AF7(Nm, 3000rpm, great inertia) FK7063-AF7(Nm, 3000rpm, conventional inertia) he optimized servomotor selection confirmation table Inertia ratio emperature rise ratio Norque utilization ratio Max I X % 6.9%.6A Y % 66.8% 3.5A Z(with balance weight) % 67.6% 3.7A Z(without balance weight) % 78.9% 8.8A est detection and data analysis he Optimized servomotors used in CNC machine tools, and come with software to monitor motor torque when running through the system, as shown in Figure 3.By Figure 3 can monitor the utilization of the motor torque up to 8.794%, the inertia moment ratio is.366, meet the requirements. Figure 3. Monitor of orque utilization ratio Conclusions () If the no-load torque and stall torque is the same as in the calculation of the rms value, will result in the entire process of the average torque value exceed the rated torque of the motor. ()General CNC machine tool, acceleration generally is -4m/s, select motor needs more consider inertia ratio, better is -3; if acceleration of high-speed machine tool over 8m/s, torque need to be first considered, if torque is enough used, better inertia ratio is. (3) ransmisssion system of lightest Load s CNC, use conventional inertia s motor is better.

6 Advanced Materials Research Vols Reference [] Fredrik Roos, Hans Johansson, Jan Wikander, Optimal selection of motor and gearhead in mechatronic applications[j], Mechatronics, vol.6, pp.63 7, 006. [] Herman J. Van de Straete, Pascal Degezelle, Joris De Schutter. Servo Motor Selection Criterion for Mechatronic Applications. IEEE/ASME, vol.3 no., pp , 987. [3] Sabri Cetinkunt, Optimal design issues in high-speed high-precision motion servo systems, Mechatronics, vol., no., pp.87 0, 99. [4] Central Machine ool Institute, Machine tool design handbook[m], ata Mcgraw pubishing company linited, bangalore, 98. [5] SEIMENS. Synchronous Motors FK7[M], SEIMENS, 00

7 Optoelectronics Engineering and Information echnologies in Industry 0.408/ Selection and Analysis of Servomotor for hree-axis ransmission System in CNC Machine ool 0.408/