Position Loop Gain Research Articles

Techniques for measuring and compensating for servo mismatch in machine tools using a laser tracker

Servo mismatch, which affects positioning accuracy in computer numerical control (CNC) machine tools, is an important source of error. Dynamic measurements of simultaneous movements in two axes are essential in determining servo mismatch. A circular test using a double ball-bar (DBB) is used commonly to assess such errors. However, servo-mismatch measurements of some machine tools, such as miniaturized machine tools, remain difficult due to structural restrictions of the measuring devices. In this study, a technique for servo-mismatch measurement via a bi-directional circular test using a laser tracker is described. The laser tracker is easy to set up experimentally and has fewer measuring range restrictions than other measuring devices. In order to ignore the effects of quasi-static errors, a bi-directional circular test is conducted using the laser tracker. Additionally, a compensation procedure, with a proportional equation, is proposed. The validity of the technique described was confirmed by comparison with the DBB result. In a machining test after servo-mismatch compensation, by adjusting the position loop gain, the roundness of a cylinder part was improved by ∼47.9%.

Analytical Determination of the CNC Machines High-Speed Feed Drives Position Loop Gain

One of the most important factors which influence on the dynamical behavior of the high-speed feed drives for CNC machines is position loop gain or Kv factor. It directly influences the contouring accuracy of the CNC machines. Usually position loop gain is experimentally tuned on the already assembled CNC machines. This paper gives one approach towards its analytical calculation. The difference between analytical calculated and experimentally obtained Kv-factor is smaller than 5%, which is completely acceptable.

Improving the HSC Linear Motor Milling Machine Contouring Accuracy

In recent years, instrumentation circular profile tests have been specified to assess the contouring accuracy of CNC machine tools. Such an instrumentation type test is the HEIDENHAIN grid encoder system, which is particularly appropriate for dynamic measurements, especially at high feed rates. In this paper influence of the position loop gain and sampling period on the contouring accuracy are effectively studied.

Parameter Optimization of Servo Drivers and Controller for CNC Machine Tools Based on Grey Relational Approach

This paper aims at finding the optimal parameters of the servo drivers and controller for a machine tool in order to improve machining accuracy and reduce machining time. Firstly, the backlash and pitch error of a transmission device were compensated by means of the controller. The control factors of the drivers include the ratio of load inertia moment to servo motor, speed loop gain, and integral time constant of the speed loop. The control factors of the controller are the acceleration/deceleration constant of feed rate and position loop gain. Each factor was classified into three levels and each experiment was repeated three times. The experimental data were converted into the signal-to-noise (SN) ratio to obtain the response tables of the SN ratio. Finally, the grey relational method was applied to find the optimal parameters. We compared the difference of the experimental results by employing the optimal parameters and default values of the drivers and controller of the CNC machine tool. Experimental results show that machining accuracy and machining time can be improved. Therefore, the results demonstrate the effectiveness of the proposed scheme.

The Application of System Parameters in Eliminating Oscillation and Improving Machining Accuracy of CNC Machine Tool

The measures of eliminating system oscillation by using parameters were put forward in semi-closed loop and closed loop control. The setup procedures of parameters were introduced, which included position loop gain, bi-position feedback, mechanical speed feedback. The setting of the parameters such as acceleration of backlash, compensation for static friction, feed forward and look-ahead feed forward of servo were also proposed, for purpose of improving the position accuracy and interpolation accuracy. The application of the system parameters is significant for improving the machining accuracy and ability of CNC machine tools.

Analytical Calculation of the Position Loop Gain for Linear Motor CNC Machine Tool

One of the most important factors which influence on the dynamical behavior of the linear motor servo drives for CNC machine tools is position loop gain or Kv factor. From the magnitude of the Kv-factor depends tracking or following error. In multi-axis contouring the following errors along the different axes may cause form deviations of the machined contours. Generally position loop gain Kv should be high for faster system response and higher accuracy, but the maximum gains allowable are limited due to undesirable oscillatory responses at high gains and low damping factor. Usually Kv factor is experimentally tuned on the already assembled machine tool. This paper presents a simple method for analytically calculation of the position loop gain Kv. A combined digital-analog model of the 4-th order of the position loop is presented. In order to ease the calculation, the 4-th order system is simplified with a second order model. With this approach it is very easy to calculate the Kv factor for necessary position loop damping. The difference of the replacement of the 4-th order system with second order system is presented with the simulation program MATLAB. Analytically calculated Kv factor is function of the nominal angular frequency  and damping D of the linear motor servo drive electrical parts (motor and regulator), as well as sampling period T. :The influence of nonlinearities was taken with the correction factor. Our investigations have proven that experimentally tuned Kv factor differs from analytically calculated Kv factor less than 10%, which is completely acceptable

Development of a High Precision Machining Center and its Motion Accuracy

In the fabrication of parts and components using dies and molds for optical and/or medical use instruments, there are standing requirements that the accuracy of the form of the final products be maintained within a tolerance of 1.0μm or less and that the roughness of the machined surface be held to 0.2μm Ra or less while high production efficiency is maintained. The required accuracy of form, smoothness of the machined surface, and efficiency of production stated above can be achieved by high precision machine tools, but are beyond the performance properties expected of conventional, general-use machining centers, both of which are readily available on the open market. The authors (hereinafter collectively referred to as “the group”) who did this research agreed to refer to the machining centers that can satisfy both of the above-mentioned performance requirements as “Subject Machining Centers,” or “Subject MC,” while the other machining centers are referred to as “Conventional Machining Centers” or “Conventional MC.” This research includes a structural study of a Subject MC, followed by the actual designing and manufacturing processes of such a Subject MC, It was then subjected to an appraisal of its accuracy of motion from the data obtained through actual measurements, and a verification of the precision attained in the manufacturing process of the lens molds it produced. Through the combined use of a frictionless drive train using a hydrostatic guide and linear motor drive, together with the overall enhancement of the rigidity of the structure of the Subject MC, the following results were achieved.1.Straight line motion accuracy with a deviation of 0.2μm/100 mm or less was attained. Also, the roundness of 0.36μm of the arc interpolation movement was achieved, confirmed with a grid encorder. (“Roundness” in this study is defined as the magnitude of the deviation of either an arc or a circle as measured against their geometrically correct counterparts.)2.As a position loop gain as high asKp= 140/s was attained with the servo mechanism adopted, it was confirmed that the actual use of the micro step feed at uniform separations of 10 nanometers had become possible.3.It became possible to mold a lens with a roundness of 0.53μm and a surface roughness of 0.05μm Ra after machining. (“Ra” denotes the arithmetical average of surface roughness after machining.)

FUZZY NEURON HYBRID CONTROL FOR CONTINUOUS STEEL CASTING

The fuzzy neuron hybrid control method is proposed for mold level control of the continuous steel casting in this paper. The dynamic characteristics of the continuous steel casting process is described. To the plant with big uncertainties and grave nonlinearities, the fuzzy neuron hybrid control system is set up and the new model-free controller is designed. In this control system, the fuzzy neuron hybrid controller is constructed by the fuzzy PI controller and the neuron controller, the gain of the neuron controller is tuned by using a fuzzy algorithm. The simulation tests of mold level control are made under various conditions with different casting speeds, new gate valve and old gate valve, changing in the valve position loop gain. The results demonstrate that the new controller has good performance, very strong robustness and adaptability.

Assistant force compensation for hand movement of patients by using exogenous signals and/or neurophysiological signals

Because functional diseases of the brain can cause disabilities related to human movement control, a compensation method was developed for improving the performance of hand movements. The compensation for human hand movements can be carried out by adding an assistant force that is generated from artificial equipment attached to a human arm. From the experiment on visual target tracking, it was found that the tracking trajectory was adequately represented by a dynamic model of the motion of an articulated industrial robot arm, and the different abilities for movement control among healthy people and patients were classified by different model parameters as position loop gain, velocity loop gain, and response delay. Dynamic force compensation was approached by considering the different control features of the patients. The effectiveness of the proposed compensation method was verified in a simulation study on an actual industrial robot arm. A human-machine interface, e.g., a brain-computer interface (BCI), for realizing the control of artificial equipment to compensate for human hand movements is also presented and discussed.

多関節ロボットアームの輪郭制御誤差の解析と動作条件の決定

Mechanism of locus error for contour following control of industrial articulated robot arms was analyzed and the locus error was derived theoretically. Main causes of locus error were found in the nonlinear transformation between kinematics and the inverse kinematics, and the delay of dynamics. The derived equation of the locus error was assured by simulation and experimental studies. Besides, the relationship between locus error and objective velocity was obtained by using the analyzed equation. As a result, objective velocity can be decided based on the maximum locus error and position loop gain of the servo controller. Moreover, optimal ratio of length of links was obtained by use of the derived relationship.

The Low-Level Rf Beam Control System for the Booster of the Beijing Proton Synchrotron (BPS)

A prototype of a low level RF beam control system for the BPS Booster was designed and built at Fermilab during the past year. Machine parameters are: RF Frequency range 2.76-6.31 MHz, Harmonic number h= 3, Repetition rate 12.5 Hz, Maximum synchrotron frequency 12 KHz No phase transition occurs during acceleration. We have adopted computer controlled digital curve generators for the frequency program and other programs such as the radial position offset, radial position loop gain, etc. for the operational flexibility available with such programs. The frequency mixing method was chosen for the analog beam control loops, including the phase loop and radial position loop, for improving the signal to noise ratio within the wide frequency band and for convenience in signal processing. The system is composed of a series of modules which are to be operated in several modified NIM crates. This will allow maximum flexibility during the development phase of accelerator operation. Here we give a brief description of the LLRF system and some preliminary design and test results. Study and improvement of the system will be continued. A simplified block diagram of the system is shown in Figure 1.