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A Noise-Free Servo-Spindle for High-speed NC Gear Grinding Machines Takashi Emura, Lei Wang, Hisashi Nakamura, Masashi Yamanaka and Yasushi Teshigawara Department of Mechatronics and Precision Engineering, Tohoku University Aoba, Aramaki, Aobaks, Sendai, 980-77 Japan Abstract- The authors developed a productive type NC Numerically Controlled gear grinding machine for automobile plants, This NC ma- chine requires high-accuracy synchronization between the grinding spin- dle and workspindle. We must use high-power servomotors and servo- amplifiers to satisfy the specification required for high-speed grinding. However, since the high-power servo-amplifier causes very large ripple of motor current at PWM Pulse-Width Modulation frequency and in- duces strong noise, it becomes difficult to get the stable synchronous rota- tion among spindles. Therefore, the authors tried to use two-phase type PU to achieve noise-free high-speed synchronous spindles and to reduce the noise caused by the current ripple with a kind of current filter. In this paper, an approach for suppressing the noise and experimental results are descrl bed. I. INTRODUCTION Gear grinding takes extremely long grinding time and it pre- vents increasing the productivity in mass production of gears. Thus, the authors developed a productive type NC gear grind- ing machine for automobile plants, which are the most produc- tive plants of gears. This machine requires two large-power servo-spindles for its grinding-spindle and workspindle. hese servo-spindles also have to be synchronously controlled with high precision even rotating at a high speed. The authors used high-power and high-speed servomotors to drive the spindles. For example, the servomotor for grinding-spindle has a rated power of 22 kW, rated output torque of 22 N-m and maximum rotary speed of 10 000 r/min. The servomotor for workspindle has a rated power of 16 kW, rated output torque of 82 N.m and maximum rotary speed of 2000 r/min. These servomotors are driven by PWM Pulse-Width Mod- ulation type servo-amplifiers, and the PWM type amplifier of large power causes a very large ripple in motor current which appears at PWM frequency. Since this large current ripple in- duces a strong noise which causes trouble to control systems, it is difficult to realize synchronous control of high accuracy. Therefore, the authors tried to use two-phase type PLL Phase- Locked Loop to get noise-free high-speed synchronous spin- dles and tried to use a kind of current filter to reduce the ripple of motor current. Experiments were carried out to confirm the ef- fectiveness of the noise-free systems. In this paper, an approach for suppressing the noise and getting the high-speed and high- precision synchronous spindles is described with experimental results. 11. NOISE CAUSED BY PWM Problems of noise of the grinding-spindle is more severe than that of the workspindle because the rated power of the grinding- spindle is larger than that of the workspindle. Thus we mainly 0-7803-3026-9195 4.00 0 1995 IEEE describe about the case of grinding-spindle in the followings. The grinding-spindle is driven by a high-power brushless ser- vomotor. Since the maximum output voltage of the servo- amplifier is limited to 200 V and the rated power of the ser- vomotor is 22 kW, the current of field coil has to be more than 200 A at a peak load. For this large current, a wire diameter of field coil became large, and the number of turns became a few because of the limitation of winding space. Thus inductance of the field coil was constrained to a very small value. The mea- sured inductance of one phase is about 30 pH at l kHz and it is reduced to very small value of less than 1.5 pH at 25 kHz of PWM frequency. Since the coil inductance is extremely small like this, its electrical time constant is very small. This means that we can get a good control characteristics. However, the extremely short time constant causes very large current ripple at PWM frequency, and this current ripple induces the strong noise. The strong noise gives serious problems to control sys- tems. In order to analyze the ripple current, numerical simulation of motor current was carried out. Fig. la is simplified model of synchronous motor for simulation, where coil inductance L is 1.44 pH, coil resistance R is 0.15 SZ and voltage of DC line E is 200 V. PWM output voltage Eu, Ev and Ew of servo-amplifier are supplied on terminal of three phases U, V, W. PWM pattern shown in Fig. lb are generated by compar- ing reference voltage eu, eV and ew with triangle wave eg of which frequency is 25 kHz. Fig. 2a is an example of simulated results, where the out- put frequency of the servo-amplifier is 300 Hz. As shown in Fig. 2a, motor current is overlapped by large noise. Fig. 2b gives the detail situation by coniparing simulated and measured results of coil current, where simulated results agreed with the measured results. Thus the simulated results shows that the mo- tor current has very large current ripple at PWM frequency. This simulation shows clearly that the servomotor and power amplifier generate very strong noises for the sake of smallness of coil inductance. The coil inductance decreases sharply at high switching frequency in such large power motor because of the magnetic characteristic of field core. The noises caused by switching is a very severe problem in this NC machine and has to be overcome in order to achieve the high precision syn- chronous control. Therefore, the following sections describe two different ap- proaches for overcoming switching noise problem. Section HI presents a synchronous control method of two-phase type PLL which enables the system engage a high-precision and high- 692 U E v W EU EV EW b Fig. 1. Vu, V, , V, are back EMF electromotive force Motor model for numerical simulation. I,, I,, I, are line current and n 3 E s U E V . I, I -1001 I . I I I . I I 0 1 2 3 Time ms a Simulated motor current. In I Time 2Opddiv b Comparison of simulated and measured results. Fig. 2. Ripple of motor current. speed synchronous control under strong noise situation. Sec- tion IV gives discussion of using large power filter to suppress switching noise and the experimental results. 111. SYNCHRONOUS SYSTEM BY TWO-PHASE TYPE PLL The developed gear grinding machine requires high-speed syn- chronous control with high accuracy. The high-speed and high- precision synchronous control systems are realized usually by using a high-resolution and high-speed encoder whose output is two-phase sinusoidal wave. However, the voltage of the sinusoidal waves is usually very small, and it is difficult to detect this small voltage without problems of the noise under the strong noise surroundings. This means that we cannot use the conventional control method of high-precision synchroniza- tion. Therefore, the authors used two-phase type PLL in this synchronous system. The two-phase type PLL was first pro- posed by Emura [l] and its noise rejection ability was already verified [2]-[4]. Fig. 3 is blockdiagram of the high-speed synchronous sys- tem between grinding spindle and workspindle of the NC gear grinding machine. Both of the servo-spindles were controlled by two-phase type PLL. A command pulse generator generates two pulse trains whose frequency ratio is the velocity ratio of grinding-spindle to workspindle. Position controllers of two- phase type PLL control the spindles so as for them to follow each command pulse accurately. By using two-phase type PLL, high-speed and high-precision synchronous control was real- ized. Two interpolators of two-phase type PLL interpolate en- coder signal with high resolution and the interpolated output pulses of rotational angle are sent to servo-amplifiers. A. High-resolution detection of rotary angle with interpolator of two-phase type PLL The servo-amplifiers which we used this time require a two- phase rectangular signal for generating the three-phase field current that synchronizes with rotor angle. However, we could not use the conventional interpolators of comparator type be- cause of the noise induced by the ripple current. Therefore, the authors designed special interpolators which use two-phase type PLL as shown in Fig. 4 to detect rotary angles of spindles. Circuits of this interpolator was designed mainly according to the description in [3], and so they are only briefly described in the following. Input signal is a two-phase sinusoidal wave sin , cos e;, and their phase is compared with the two-phase sinusoidal waves obtained from encoder by the following vector phase operation. cos 8i cos 8, - sin 8; sin 0, cos 8d - cos 8; sin 8, sin 8; cos 8, sin 8d } 1 * Hence wecan get a pair of two-phase sinusoidal waves of which argument is phase difference Bd 8; - 8,. Since we can detect the phase difference 8d with the same principle as the interpola- tor mentioned previously, the servo-error detector of two-phase type PLL detects the servo-error with high resolution and high accuracy. Another feature of this loop is that the servo-error detector is able to detect angular velocity error with high accuracy. This is based on the excellent capability of PLL in frequency detection as known. By feedback of the angular velocity error obtained from the phase detector, we could realize the servo-controller whose D gain is high enough and consequently whose stabil- ity is significantly high. The resolution and maximum speed are 11.25“ and 10 000 r/min for grinding spindle and 1.125“ t Step-displacement Command *- -10.6“ I I I I 0 0.2 0.4 Time [SI Fig. 7. Step response of grinding-spindle el 18-bit BRM ROM s U sin/cos tables 1“1 converters t To grinding-spindle To othor axis ROM s sin/cos tables 4 D/A converters To workspindle Fig. 8. Reference signal generator for synchronous control system of two-phase typePLL and 2000 r/min for workspindle respectively. These values are extremely excellent values that have never been realized. Fig. 7a shows a step response of grinding-spindle. From Fig. 7, it is known that the grinding-spindle tracks command pluses precisely even under the existence of strong noise. C. Reference Signal Generator Both of workspindle and grinding-spindle controlled by the above mentioned two-phase type PLL is positioning ser- vomechanism. Their positioning command is given conse- quently two-phase sinusoidal wave of which angular frequency is decided by the specifications of work piece and tool. The structure of reference signal generator is simple as shown in Fig. 8. As shown in Fig. 8, the output pulse frequency of a refer- ence pulse generator is decreased by BRM Binary Rate Mul- tiplier, of which decreasing rate is decided by the rotary speed 695 of each axis. Since bit length of BRM’s is 18, the accuracy of synchronous rotation is 218. The output pulse of each BRM is counted by binary counter and transformed into two-phase sinu- soidal wave by two ROM’s in which look-up tables are written, and after that it is converted to analog two-phase signal by two D/A converters. Iv. REDUCTION OF RIPPLE CURRENT BY LARGE-CURRENT FILTER As mentioned previously, the ripple of motor current generates very strong noise. Even the synchronous control system can work well under such circumstance, other detectors required for servo-controller are interfered by the switching noise and they did not work without misoperation. Therefore, the ripple of motor current is necessary to be reduced to a tolerable level. As the mean current of motor is about 100 A and the frequency of current ripple is 25 kHz, the authors Uied to decrease this current ripple by using a large-current filter whose frequency characteristics is high enough. A. Large-Current Filter This filter consists of 6 choke coils LO and L1, 3 capacitors C and 3 resistors R as illustrated in Fig. 9a. 10 is current of servo-amplifier, 11 is current of motor and 1, is current dissi- pated by RC circuits. Because the purpose of this filter is to reduce the ripple of switching frequency, choke coils of ferrite core areused to obtain good characteristics at PWM frequency. The rated current of the coils is 110 A. B. Frequency-Domain Analysis One phase of the filter can be simplified as Fig. 9b, where winding inductance of motor is omitted because it is very small compared with that of filter. If output current of servo-amplifier is Io and output voltage is E, impedance of one phase Zo E/Io is given by so L s2o L s3co1 20 * 2 1 sRC s2CL1 If field current of motor is 11, we can use impedance 21 E/11 for expressing suppression ability of current ripple of motor. 21 is given by sL0 Ll SRCL L1 scLOL z1 lsRC . 3 Frequency characteristics of 20 and 21 are shown inFig. 10. In Fig. 10, al, a2, a3 and a4 are constants defined by the following equations. a1 LOCL1, 4 Power amplifier CL IL U a Arrangement of filter b Model of one phase of filter Fig. 9. Smcolre of filter used for reducing ripple of motor current where a1 is the overall inductance of the network. As shown in Fig. lOa, we can obtain good ripple suppression ability over- all frequency range by increasing al. However, since large a1 lowers response frequency of motor current, the authors used a1 100pH. We must use large-size inductors because of large motor current and its large ripple. a2 means the undamped res- onant frequency of loop R - C - L1 and it has no effect on 21. a3 is the characteristic frequency of the filter. Fig. 1Oc shows that a smsll u3 gives good suppression ability of ripple, but this requests to increase C, Lo and L1. Thus, for a certain sum of LO and L1, LO L1 was used. As seen in Fig. 10d, a4 is the damping ratio and kept below 0.7. By choosing the param- eters properly, we could effectively reduce the current ripple in high-frequency range more than 25 kHz of PWM frequency. C. Experiments Fig. 11 is the experimental results. The effectiveness can be confirmed by the comparison of motor current with and without the filter. Fig. lla gives a sharp contrast of motor current, where LO LI 50pH, R 1Q and C 20pF. Rotary speed of motor is 3000 rpm, and spectrum of motor current shown in Fig. llb shows that the induced noise nearby of 25 kHz was significantly reduced by the filter. V. CONCLUSIONS The authors used two high-power servomotors for getting a pro- ductive type NC gear grinding machine. Since inductance of 696 q-E-gq a,lOpH 1 Frequency Hz -- Frequency‘m --,o.l El d lo0 1 I Frequency Hz Frequency Hz d ai 100pH, a2 SkHz, a3 7kHz. Fig. 10. Numeral results of impedanceof Zo, Z1. field coil of these motors is extremely small at PWM frequency, the ripple current of field coil became very large and large ripple current induced strong noise. The induced noise caused misop- eration of control systems. Therefore, the authors used two- phase type PLL to realize high-precision control of spindles under the strong noise surroundings. Moreover, a large-current filter was used to reduce ripple of motor current. From simula- tion and experiments, the following conclusions were obtained. 1 From simulation, it became clear that large ripple of mo- tor current is caused by extremely low inductance at PWM frequency of the field coil. Simulated results were coinci- dent with experimental results. 2 The two-phase type PLL is useful for getting the high- speed and high-precision synchronous spindles. It ro- tates the spindles stably without misoperation under strong noise surroundings. 3 The proposed large-current filter is effective for reducing the ripple of motor current. With filter ‘ a Motor current -50 IIIIIIIIIIIIL Time [lms/div] a Motor current “I 4,1 Without filter P v1 0 n 25 50 Frequency kHz a -. i With filter 3000 dmin Frequency kHz b Spectrum of motor current Fig. 11. Experimental results. VI. ACKNOWLEDGEMENTS The authors thank Mr. T. Sakaguchi NEC for his coopera- tion making control circuits of spindles. A part of this work was supported by the Grant-in-Aid for Scientific Reseach of the Ministry of Education of Japan and the Grant of Toyoda Machinery Co. Ltd. REFERENCES [l] T. Emura, “A study of a servomechanism for NC machines using 90 de- grees phase difference method,” hog. Rep. of JSPE, pp. 419-421, 1982. [2] T. Emura, A. Arakawa, and M. I-Iashitani “A study of high precision servo- spindle for hard gear finishing machines,” The Inter. Conf. on Advanced Mechatronics, pp.427-432,1989 [3] T. Emura, L. Wang and A. Arakawa,”A high-resolution interpolator for incremental encoders by twephase type PLL method,” in Pruc. lEEE [4] T. Emura, L. Wang and A. Arakawa, “A Study on A High-speed NC Gear Grinding Machine Using Screw-Shaped CBN Wheel,” ASME Jomal of Mechanical Design, Vo1.116, No.4, pp.1163-1168, Dec. 1994. IECON’93,1993, pp 1540-1545 697