Study On Embedded Control System Of Polishing Free-form Surface Machine Tool

The NC technique is one of the computer integration manufacture system andfactory automation foundations, also is promotes the country machine tool industryand the enhancement manufacturing industry international competitive powerfoundation, its development condition decides a national manufacturing industrylevel. In the NC system, plays the key role is the motion control systemcomputation speed, real time, the digital communication, interpolation computation,position control and so on.This paper bases on TI Company's new TMS320F2812 DSP, develops thetype of "PC+NC" NC embedded multi-axis motion controller, the controllerconforms to the PCI bus standard, may control 1 ~ 6 axis, its output form for ±10Vthe analog quantity or the digital pulse output. The controller is used in thepolishing free-surface machine tool and realized free curve NURBS interpolation.TMS320F2812 is the TI Company newest promoted DSP chip, many kinds ofadvanced Peripheral are integrated in the chip, has provided the good platform forthe electomotor and other motion control. With Motorola Corporation, ADCorporation, AT&T Corporation and so on the many patterns DSP chip compares,TMS320F2812 has the obvious superiority in the electromotor control.This paper hardware electric circuit development including the DSP smallesthardware system development (including DSP system power source, clock electriccircuit, repositions electric circuit, memory expansion and so on), DSP and the PCcommunication connection way, the DSP DAC output unit, the position feedbackpart, the electromotor control connection technology and so on. The motioncontroller take TMS320F2812 as a microprocessor, uses in each kind of orderwhich carries out PC to send out or interpolation computation, and completes theaxise position control. After position instruction colding, PC places the data indual-port RAM through the PCI bus ,carries on the data communication with DSP;DSP reads out these data to carry on interpolation computation, obtains the theoryposition;The position feedback function module receive electromotor encodersignal, calculates the theory position and the physical location error;D/Atransformation function module exports analog control voltage, linkes to the speedcommand input connection of the servo electromotor, controls the servoelectromotor movement;On the motion controller also has the expansion theinput/Output function module, uses for to realize the DSP motion controller toexpand the PLC function, realizes in the machine tool to measure I/O control;Thedecoding circuit may by program the component to be composed, completes theentire controller's decoding. This paper develops motion controller structure asFig1 shows.PCI9054PCI BUSCPLDDual-portRAMTMS320F2812driverelectomotorMemory and I/OextendfeedbackCONTROLSTATEPCFig1 motion controller structureIn the NC processing domain, the present CNC machine tools usually only areprovided with the straight line, the circular arc, and the spiral line and so on theminority several kinds interpolation function, The complex profile, the curvedsurface outline components NC processing must draw support from the CAMoff-line programming system, treats the CAD production as the processingcomponents the curve, the curved surface outline is separated the massive microstraight lines or the circular arc carries on the programming processing, in this kindof situation not only is tedious, but also has many shortcomings. NURBS is oneideal simulated method, it already may express the analysis physique also mayexpress the free curve and the curved surface, makes it in CAD/CAM domainobtain the widespread application. Because its own many fine characteristics,NURBS have the quick development and the widespread application in recentyears. The multitudinous international standard and the outstanding CAD softwareas well as three dimensional manufacture software, provides the support to NURBS.The NURBS curve interpolation may obtain the smooth processing shape,fundamentally solves the straight line interpolation makes up the question whichthe processing brings. Performance for following several aspects:(1) The procedure quantitative is fewer.(2) Does not need to carry on the high speed procedure transmission to NC.(3) Because can obtain the smooth processing shape, therefore may reduce themanual polish process time.(4) With the straight line interpolation compares, the speed change is smooth,May reduce the process time.Based on constant speed NURBS interpolation algorithm ,this paper proposestwo kind of NURBS algorithm: One kind is algorithm for controlling chord error,italong with the curve curvature change, according to the biggest chord error whichassigns, auto-adapted adjusts NURBS interpolation curve speed, thus in theguarantee machining process defines the chord error in the low scope;The otherkind is algorithm for constant MRR, which defines the chord error in a lower scope,and has realized material removing rate constant, thus improves the processingquality, lengthenes the cutting tool service life.The key of constant speed NURBS interpolation algorithm is guaranteed eachinterpolation cycle instruction increase ?s equally, but is not guaranteed eachinterpolation cycle the parameter increase ?u to be equal. By using Taylor'sexpansion of the parameter u with respect to time t to obtain the first orderapproximation interpolation algorithm, theapproximation up to the first derivativeis:'2'2'21()()()()()()xyzuTVududPuuuTVuiiuuiiii=+++=+=+(1)The second order approximation can further reduce for the speed undulation,thus improves the face work quality, its corresponding expression is'2'2'222''''''''''2'2'224222212(()()())(())()()()())(())()(22()()(()())()()xyzTVuxxyyzzxyzdtTVuTdVuududPududPuduVuTdPududPuuuVuTiuuuuiuuiiiiis+??+?+?+++=+??=+??===+(2)Algorithm for controlling chord error will automatically adjust curve speedalong with the radius of curvature change in the interpolation process, thus willlimit the chord error in the permission scope.Since the curve speed is adaptively changed accordingto the radius ofcurvature, the following adaptive feedrate rule is proposed for parametric curves:???????????