adjustment of cnc machine tool controller setting values by an experimental method

8/3/2019 Adjustment of CNC Machine Tool Controller Setting Values by an Experimental Method

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International Journal of Machine Tools & Manufacture 38 (1998) 10451065

Adjustment of CNC machine tool controller setting valuesby an experimental method

H.D. Kwona,*, M. Burdekinb

a Industrial Automation Technology R & D Centre, Korea Institute of Industrial Technology (KITECH), San 17-1

HongChonRi IbJangMyun Chon An-Si, 330-820, South KoreabDepartment of Mechanical Engineering, University of Manchester Institute of Science and Technology (UMIST),

PO Box 88, Manchester, M60 1QD, UK

Received 1 February 1997; in final form 13 August 1997

Abstract

This paper presents an adjustment technique on controller setting values in CNC machine tools bymeasurement of servo induced feed drive errors. At high feed rate operations of CNC machine tools, servoinduced errors are usually dominant and large compared with geometric errors. For measurement of the

servo induced errors, an experimental equipment which incorporates two linear displacement sensors anda steel cube was developed, and servo feed drive errors were evaluated along a square corner test path.Based on evaluations of servo feed drive errors with different combination of parameters in machine controlsystem, optimum setting parameters were found. The measuring equipment and optimisation method aredescribed. 1998 Elsevier Science Ltd. All rights reserved.

Keywords: CNC machine tools; Servo error

1. Introduction

At high feed rate operations of CNC machine tools, servo induced feed drive errors are usuallydominant and large, compared with geometric errors of machines. The feed drive errors resultfrom imperfection of machine control systems.

In order to improve servo controller performance, various control techniques were applied toCNC machine tools in the past years [17].

* Corresponding author. Tel: + 82-2-3705-3721, Fax: + 82-417-5608-400, E-mail: [email protected]

0890-6955/98/$19.00 1998 Elsevier Science Ltd. All rights reserved.PII: S 0 8 9 0 - 6 9 5 5 ( 9 7 ) 0 0 0 5 8 - 8


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1046 H.D. Kwon, M. Burdekin/International Journal of Machine Tools & Manufacture 38 (1998) 10451065

However, due to limitation in evaluating servo induced errors on CNC machines, adjustmentsof machine control parameters were usually carried out using simulation techniques of control

systems, which inevitably involved some idealisations and assumptions, and might not be identicalto real system, and machine controller specifications therefore were not fully utilised.In order to achieve high utilisation of controller specifications, adjustment of controller para-

meters was researched, based on experimental evaluations of servo induced feed drive errors,using a ball link system [8,9].

Although the ball link system provided some results related to geometric errors and dynamicerrors of feed drive system in machine tools, full servo induced error features might not beobserved. Because test path of the system was confined to a circle or an arc, and minimum linklength was approximately 100 mm, which corresponded to length of a standard LVDT transducer.Therefore geometric error components could be included in the test results.

To overcome problems of size and profile of test path, an alternative technique which uses a

steel cube and two linear displacement sensors, was devised in this research. In the developedtest system, a square corner test path was used, and servo induced feed drive errors were high-lighted near corner of the test path.

Controller setting values, such as velocity and positional gains with minimum available timeconstant were changed, and feed drive errors on the machining centre were evaluated. Based on theevaluations, optimum controller setting values were determined. The measurement and adjustmenttechnique on controller setting values in CNC machine tools is detailed in the following order:

servo control system in CNC machine toolsexperimental equipmentsapplication of the cube test system to a CNC machining centre

adjustment of machine setting values in a CNC machine toolconclusions.

2. Servo control system on CNC machine tools

Most CNC machine controllers are organised in the cascade structure, comprising the position,velocity and current control loops as shown schematically in Fig. 1. This type of controller struc-ture provides the following advantages over other types:

cascade control loop can be optimised from in to out,there are only a few parameters to adjust,necessary equipments for this organisation are simple and cost effective.

Important components of the feed drive control system are position controller, velocity control-ler, current controller, electric motor, mechanical parts and measuring units. The feed drive controlsystem can be simplified as shown in Fig. 1.

The important position control system parameters influencing on the tool path behaviour arepositional gain, velocity gain and time constant.

With too high values of positional and velocity gains, an overshoot from desired moving path


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Fig. 1. Block diagram of the positional control loop of an axis. [12]

occurs. With low values, rise time to the desired path is long. It is therefore necessary to selectoptimal values of the parameters.

Basic criteria for determining these controller setting values are system stability and preventionof overshoot, which result in an excessive cutting in the workpiece.

2.1. Position control loop

Requirement of the position control loop, is that the desired programmed position must befollowed with minimum time delay and following error during movement of machine.

Positional gain is defined by

Kv =Vactual

x(1)

where Vactual is actual velocity, x is error to be inputted to the positional control loop.Selection of positional gain value is important to improve the servo control performance. When

selecting the positional gain, K, following error, overshoot, transient error and system stability

must be taken into consideration.In order to investigate the control system performance with different positional gains, the pos-

ition control loops in the control loop were simplified as shown in Fig. 2.With the simplified system as shown in Fig. 2, the uncontrolled system acts, as a second order

system, and can be expressed as:

G(s) =1

TeTms2 + Tms + 1

(2)

or

G(s) =

20

s2 + 2D0s + 20

(3)


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Fig. 2. Simplified position control loops of machine axes.

where natural frequency = 0 = 1/TeTm,; damping coefficient = D =1

2Tm

Te; Te = electrical time

constant; Tm = mechanical time constant.

3. Experimental equipments

In order to assess servo induced feed drive errors on CNC machine tools, a system was devised.The test system incorporated a precision reference steel cube, two linear digital displacementsensors, PC-counterboard, micro-computer.

Two digital displacement sensors were mounted orthogonally with respect to the reference cubeas shown in Fig. 3. A third sensor can also be incorporated so that at the same setting, data canbe obtained for the XY-, YZ- and ZX-planes. These digital sensors are interfaced to a micro-computer via PC-counterboard (Heidenhain IK120).

In order to measure relative movement between the cube and the machine table, defined by anNC program, two linear digital displacement sensors (Heidenhain MT12B), employing a precisionglass scale with incremental grating (grating period 10 um), were used for the measuring system.Important specifications of the sensor are:

measuring range: 12 mmmeasuring accuracy: 0.5 umgauging force: 0.60.85 Nfrictional force: 0.12 N


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Fig. 3. Schematics of the cube test system.

permissible measuring velocity: 0.25 m/s

permissible shock: 1000 m/s2

operating temperature: 050c

During movement of the machine table, relative displacements with respect to the cube, attachedto the machine spindle, are detected by the sensors and recorded on the micro-computer.

The cube test system can also be used to investigate machine performance when moving arounda square corner test path (Fig. 4). Along the pre-defined square path, machine moves from thepoint A via B to C in Fig. 4, and returned via the point B to A. Servo induced feed drive errorfeatures are highlighted around sharp corner (Point B) on the path when machining componentshaving pockets.

Before undertaking above test path, a preliminary linear path (Fig. 5) had to be performed, to

establish the alignment of the faces of the cube with respect to the axes of the machine. Thisalignment is achieved by recording data from the sensors when moving along each of the X, Yaxis in turn. Angular offset of the cube with respect to the axes is determined by computing aleast squares line through the alignment data.

4. Data acquisition

When an NC program for the test path is performed, the sensors displacements are equivalentto the programmed positions. The sensors displacements in the X- and Y- directions have to be


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Fig. 4. Square corner test path.

Fig. 5. Preliminary test path.


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sampled by the computer. A sampling rate of up to 50 kHz can be achieved when two sensorsare used simultaneously.

In the developed measurement system, 20000 real data from two different sensors can berecorded in the RAM memory, i.e. 20000 samplings are possible. For safety reasons, the samplingtime interval is long enough to sample all the data during the test path.

The maximum number of samples is limited by the RAM-memory size. Although some micro-computers can have a large RAM (e.g. 16 Mbyte), the cube test system must be able to beemployed on a general micro-computer which can have just 640 Kbytes of RAM used by DOSand the cube system software.

Idea that the data samplings can be held by using the hard disk memory, is difficult toimplement, because data transfer time from the sensors to the hard memory is not uniform andtoo long, compared with sampling cycle.

Using a micro-computer with a 80 486 processor, up to 20000 samples from two displacement

sensors in the cube test system, could be stored in the RAM memory. Limitation of the RAMsize, and number of data for analysis had to be considered, when selecting number of samples.

After invoking the sensor drive program, sensors start to transmit measured displacement tothe computer.

The computer must know when to start recording the transferred data by the sensors. It maybe possible with some CNC controllers to communicate with the computer at the start and finishpoints of the test path by some form of hardware link. However, it is desired to avoid thisapproach, as there are many types of CNC controllers in use, and may not have such outputfacility.

Hardware synchronisation does not exist between the machine, and the measuring system, andit was therefore necessary to devise a technique to identify the start of the test path, when carryingout the measurement.

Before machine performed the test path, data acquisition program was invoked to sample datafrom the sensors. The first sampled data was held as a reference position, and continuously com-pared to new sampled data from the sensors. If difference between first and new sampled datawas larger than 3 um after starting the test path, computer will then begin to store sampled datain the RAM-memory in integer form.

After given number of data were sampled, data acquisition from the sensors stopped, and cap-tured data in the RAM-memory then converted to the real-type, and stored in the hard memoryin file form.

5. Data analysis

For analysing sampled data under various test conditions, a program module was devised foridentifying initial set-up alignment of the cube and analysing servo induced errors.

In order to analyse effectively sampled data, actual path was divided into four intervals fromstart to finish point, which are termed as interval IF1, IF2, IR2, IR1 respectively, as indicated inFig. 6. Criterion point between intervals on the actual path was either turning point, or middlepoint (Point B in Fig. 6).

Turning point on actual measured path, corresponded to square corner point on desired testpath, and had to be identified.


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Fig. 6. The intervals IF1, IF2, IR1, IR2 on the square corner test path.

Based on idea that turning point locates where local velocity is minimum in the interval offorward movement on the test path, velocities were derived from measured data by computerprogram, but calculated velocity curve was not smooth and continuous, containing noise spikes.By a program algorithm employing low pass filter principle, these pulses were eliminated, andpositions of local minimum velocity from sampled data were identified.

Sampled data from the sensors were absolute positions with respect to reference point, whichwere determined when sensors were activated by sensor drive program. These absolute sampleddata were processed by developed computer program.

In case of sampled square corner test data, distance between desired and actual sampled pos-itions d1, d2 as indicated in Fig. 7 at the interval IF1, IF2 was computed as errors, and theseerrors were plotted on computer screen with some magnification.

6. Application of the cube test system to a CNC machining centre

A 3-axis vertical spindle bed type machining centre with a CNC controller (Fig. 8) was testedusing developed cube test system. Available machining feed rate was in the range 15000 mm/min, and positioning resolution was 1 um. Detailed specifications are given in theAppendix. Measurements were undertaken at various feed rates in the range 5004000 mm/min.

According to test result with feed rate of 500 mm/min as shown in Fig. 9, movement turningpoint on actual path did not coincide with corner point (4, 0) on desired path. From measurementdata, errors were computed in the intervals, IF1, IF2, IR1, IR2, which were shown in Fig. 6.

At low feed rate of operation (500 mm/min), small errors were observed, and the actual pathwas relatively close to the desired path. Error of 160.4 um occurred in the interval IF2, and177.6 um in IR1.


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Fig. 7. Errors during the forward movement.

When operating at a high feed rate (4000 mm/min, 80 per cent of maximum feed rate), errorswere large near start, turning and finish points as shown in Fig. 10. In the interval IF1, initialtransient was dominant (Error band: 39.3 um). It resulted from acceleration of the machine inminimum time.

In the interval IF2 in Fig. 10, identified servo induced feed drive errors were large (1184.5 um),when considering that length of the test path (from point A to B in Fig. 10) is 11.3137 mm. Itimplied that setting values in the machine controller should be re-adjusted.

7. Adjustment of machine setting values in a CNC machine tool

Related to servo performance in the machine control system, adjustable controller parametersand ranges are given in Table 1 [10,11].

Tests were carried out with different combinations of controller parameters. Time constantremained with the smallest value for fast reaction. On the machining centre, time constantremained with minimum available value of 8 msec.

First set of measurements (Test A) were carried out under test conditions as shown in Table 1.Figure 11 shows measurement results of Test A1, which were carried out, with the positional

gain of 2500 [0.01/sec], velocity gain of 2500, at the feed rate of 500 mm/min. Test results inbackward movement, were very similar to that in the forward movement.


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Fig. 8. The tested CNC Machining centre.

With feed rate of 4000 mm/min, tests (Test A2) were carried out under same settings withTest A1.

Figure 12 shows test results and large contouring errors occurred near corner in the forwardmovement, and the error band in the interval IF2 is 658 um.

The test result on backward movement was very similar to that using the forward movement.Therefore forward movement test can represent error features of both moving directions.

At high feed rate of machine operation, servo induced feed drive error was highlighted aroundthe corner of the square test path. Hence machine servo characteristics were evaluated, as theerror band and overshoot in the interval IF2.

Test results of Test A1, A3 and A5 are summarised and plotted in Fig. 13. Abbreviations Maxi

1, Mini 1, Band 1 in Fig. 13 are used and represent maximum and minimum machine errors, anderror band in the interval IF1.

Figure 14 shows measurement results with Test A1, A3 and A5 in the interval IF2 on thesquare corner path. The change of the minimum errors was observed as very small.

The errors in the interval IF2 were very large, compared with that of low feed rate test asshown in Fig. 13.

Measured machine errors at low feed rate were relatively small, compared with high feed ratetest, and servo induced feed drive errors were clearly observed in high feed rate test results.Therefore error characteristics at high feed rates were used for error analysis and adjustment ofcontroller setting values.


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Fig. 9. Test result at 500 mm/min of feed rate.

Band 1, as indicated in Fig. 14, increased from 41.8 to 77.8 um, with increase in positionalgain. This error band corresponded to initial transient of test path.

Increasing in positional gain, Mini 2 (Fig. 14) decreased from 21.3 to 4.4 um, and Maxi 2decreased from 679.3 to 206.4 um. Overshoot did not occur with the positional gain of 3500[0.01/sec], but positional gain of near 4500 [0.01/sec], the overshoot is expected to occur.

With different positional gain with fixed velocity gain (3000), a second set of measurements(Test B) were carried out under conditions as shown in Table 3

Detailed test results are summarised in Fig. 16 and value changes, such as Band 1, Mini 2 andMaxi 2 were observed.

Increasing in positional gain, with fixed velocity gain (3000), Band 1 increased from 46 um to90.2 um, and Mini 2 decreased from 10.1 um to 4 um, and Maxi 2 also decreased from435.8 um to 176.6 um. From positional gain of 3250 [0.01/sec], overshoot started to occur.

Changing positional gain with a fixed velocity gain (3200), a third set of measurements (TestC) were carried out under conditions as shown in Table 4.

Measurement results such as Band 1, Mini 2 and Maxi 2 are summarised in Fig. 17.Increasing in positional gain, with fixed velocity gain (3200), Band 1 increased from 52.1 um

to 108 um, and was slightly larger than that in case of positional gain value of 3000 [0.01/sec].Mini 2 decreased from 9.6 um to 11.8 um, and Maxi 2 also decreased from 393 um to 134.9 um.From position gain value of 3000 [0.01/sec], overshoot started to occur.


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Fig. 10. The test result at 4000 mm/min of feed rate.

Table 1

The adjustable controller parameters and adjustable ranges.

Adjustable range of the parameter Default value

Back-lash compensation factor 0-255 [um] X = 8 um, Y = 8 umVelocity gain 1-9999 1434Positional gain 1-9999 [0.01/sec] 3000Time constant 8-4000 [msec] 8

Changing velocity gain with fixed positional gain (3000 [0.01/sec]), a fourth set of measure-ments (Test D) were carried out under conditions as shown in Table 5.

Measurement results, such as Band 1, Mini 2 and Maxi 2, are summarised in Fig. 18.Increasing in velocity gain, with fixed positional gain (3000 [0.01/sec]), Band 1 increased from

39.3 to 99.6 um, Mini 2 decreased from 72.1 to 3 um, and Maxi 2 also decreased, from 1256.6to 173.3 um. From velocity gain value of 3000, overshoot started to occur.

Increasing in positional gains, with fixed velocity gains (2500, 3000, 3200), Band 1 decreasedand initial transient increased. Higher the positional gain in the stable system, error around turningpoint on actual measured path, was decreasing. However, from a certain positional gain value,overshoot started to occur.

Combination of the positional and velocity gains, which resulted in small following errors andovershoot of less than 0.5 um, were found, and plotted in Fig. 18.


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Fig. 11. Square test result of Test A1.

One of these combinations, resulting in minimum transient error, was determined as the opti-mum values (Fig. 19), i.e. positional gain = 3250 [0.01/sec] and velocity gain = 3000.

8. Conclusions

A cube test system which incorporates two linear displacement sensors and a steel cube wasdeveloped for assessing servo induce feed drive errors on CNC machine tools. In the test system,square corner test path was used for measurement of servo errors, and near square corner, servoerrors were highlighted and effectively evaluated.

By applying the developed cube test system, a controller parameter adjustment technique was

also developed. With different values, such as velocity and positional gains, servo induced feeddrive errors were experimentally evaluated at feed rate in the range 5004000 mm/min. Timeconstant remained with minimum available value for fast movement.

A combination of velocity and positional gains, which resulted in small following error nearsquare corner of the test path and minimum initial transient, was selected as optimum values.

Compared with simulation techniques for adjustment of servo parameters, the developed tech-nique using the cube test system is based on real measurement on a CNC machine tool, whichincludes whole pictures of the servo feed drive system.

The developed system also provide real data to simulation of feed drive system and adjustingcontroller parameters.


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Fig. 12. Square test result of Test A2.

Required equipments for the developed system were relatively simple and adjustment can becarried out within short time through appropriate communication to CNC machine tools. The testsystem and adjustment technique was shown to be practical and efficient for assessing servoerrors and determining servo parameters, and are expected to be widely used by machine buildersand users.

References

[1] Y. Koren, Design of a digital loop for numerical control, IEEE Transactions on industrial electronics and control

instrumentation IECI- 25 (1978) 212217.[2] Y. Koren, Cross-coupled biaxial computer control for manufacturing systems, Journal of Dynamic Systems,Measurement, and Control 102 (1980) 265272.

[3] J. Huan, Bahnregelung zur Bahnerzeugung an numerisch gesteuerten Werkzeugmaschinen. Springer-Verlag,(Berlin) 1982.

[4] K. Kulkarni, Identification and contouring control of Multiaxial Machine Tool Feed drives, Dissertation Ohio(USA), 1987.

[5] O. Masory, Improving contouring accuracy of NC/CNC systems with additional velocity feed forward loop, Jour-nal of Engineering for Industry 108 (1986).

[6] G. Pritschow, H. Rudloff, B. Schnurr, Verminderung von Bahnabweichung bei Nachfolgeregelungen am Beispieleiner Nockenwellenschleifmaschine, wt-z. ind. Fertig. 79(Heft 11) (1989).

[7] W. Wendt, Bahnregelung von Handhabungsgeraeten und Werkzeugmaschinen, (Springer-Verlag, Berlin) (1987).


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Fig. 13. Summarised test results in the interval IF1.

[8] M. Burdekin, W. Jywe, Optimising the contouring accuracy of CNC machines using the CONTISURE system.28-th international MATADOR Conference 1992.

[9] W. Jywe, A computer-aided accuracy testing device for machine tools, PhD-Thesis, UMIST, (1992).[10] Fanuc Ltd. FANUC SYSTEM 6M-MODEL B, Operators manual, (1982).[11] Fanuc Ltd. FANUC SYSTEM 6M-MODEL B, Maintenance manual, (1982).[12] M. Weck, M., Handbook of machine tools, Automation and Controls, (3) John Wiley and Sons, (Chichester, New

York), (1984).

Appendix

Specification of TAKISAWA MAC-V3 machining centre

The tested CNC machining centre is equipped with the following specifications:

Type: TAKISAWA MAC-V3 Machining Center of 3 axis vertical spindle bed typeWorking travel (mm): 510*400*360 (longitudinal * lateral * spindle head)Distance between the spindle nose and the table top: 140500 mmMaximum permissible load: 200 kgRapid traverse: X, Y = 1200 mm/min, Z = 1000 mm/minMachining feed rate: 15000 (mm/min)


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Fig. 14. Analysis of test results in the interval IF2.

Table 2Measurement conditions for Test A.

Test Velocity gain Positioal gain (0.01/sec) Feed rate (mm/min)

Test A1 2500 2500 500Test A2 2500 2500 4000Test A3 2500 3000 500Test A4 2500 3000 4000Test A5 2500 3500 500Test A6 2500 3500 4000Test A7 2500 4000 4000

Control system: FANUC System 6MB Series Simultaneous 3 axis positioning and linearinterpolationResolution: 1 umSpindle: Diameter = =55 mm, Drive motor 7.5 HP, Speed range 6060000 rpm


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Fig. 15. Test result.

Table 3Measurement conditions for Test B.

Test Velocity gain Positional gain (0.01/sec) Feed rate (mm/min)

Test B1 3000 2500 4000Test B2 3000 3000 4000Test B3 3000 3200 4000Test B4 3000 3500 4000


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Fig. 16. Measurement results of Test B.

Table 4Measurement conditions for Test C.


Test C1 3000 2500 4000Test C2 3000 3000 4000Test C3 3000 3200 4000


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Fig. 17. Measurement results of Test C.

Table 5Measurement conditions for Test D.


Test D1 1434 3000 4000

Test D2 2000 3000 4000Test D3 2500 3000 4000Test D4 3000 3000 4000Test D5 3171 3000 4000Test D6 3200 3000 4000Test D7 3500 3000 4000


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Fig. 18. Measurement results of Test D.


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Fig. 19. Summary of test results.

adjustment of cnc machine tool controller setting values by an experimental method

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