The 33rd Annual Conference of the IEEE Industrial Electronics Society (IECON)Nov. 5-8, 2007, Taipei, Taiwan

Self-Sensing Sinusoidal Drivefor Spindle Motor Systems

C.S. Soh"2, C. Bi",2'Data Storage Institute, 5 Engineering Drive 1, Singapore 117608

2Department ofElectrical & Computer Engineering,National University ofSingapore, 4 Engineering Drive 3, Singapore 117576

Email: SOH Cheng_Sugdsi.a-star.edu.sg

Abstract-Traditionally, spindle motor systems are drivenwith Brushless DC (BLDC) drive. This drive, however,suffers from torque pulsations which is undesirable from theperspective of motion control and acoustics. In this paper, aself-sensing sinusoidal drive for spindle motor systems isproposed. The drive comprises of (1) an optimal anglesinusoidal drive, (2) an innovative lossless current sensingfor self-sensing methodology, (3) a self-sensing sinusoidaldrive. The drive introduced was implemented on FPGA andits results are presented.

I. INTRODUCTION

The introduction of high energy permanent magnetmaterials coupled with the increasing concerns for powerefficiency has opened the gateway for Permanent MagnetSynchronous Motor (PMSM). The benefits of usingPMSM are high torque to inertia ratio [1,2], superiorpower density, high efficiency and low acoustics noise.As such, PMSM has become an attractive option forindustrial applications, such as Hard Disk Drives (HDD).The motor deployed HDD are a sub category belonging toPMSM, commonly known as Brushless DC Motor(BDCM). Compared to PMSMs, BDCM has severalunique features. The rotor of BDCMs has got surface-mounted permanent magnet constructing a smooth-air-gap machine. As such, reluctance torque contributed byinductance variations can be neglected. In addition, therotor utilizes fractional-slots which in turn make thecogging torque negligible. These, together with otherfeatures, such as sinusoidal/trapezoidal back-emfs and asymmetrical three-phase structure, create an uniquePMSM or a BDCM.

BDCMs are typically driven by a three-phase invertercircuit with BLDC drive whereby each gate turns on for1200 and for each phase, there will be two silent periods,each of 600, where the terminals are floating. Such amethodology aims to inject rectangular currents for torqueproduction. Traditionally, BLDC drive is used to driveBDCMs with trapezoidal back-emf so as to achieveminimal torque ripple. In addition, self sensing is adoptedin BDCM instead of hall sensors due to cost and spaceconstraints. However, for BDCMs with sinusoidal back-emfs, sinusoidal drive should be used. Nevertheless, forthese BDCMs, BLDC drive is usually employed. Thereason being, sinusoidal drive is still yet feasible if self-

sensing schemes are to be adopted. Self sensing isadopted in BDCM instead of hall sensors due to cost andspace constraints. Owing to this, despite the drawbacksofBLDC drive, BDLC drive is being used.

In this paper, the deployment of self-sensing sinusoidaldrive is investigated. Firstly, the derivation of an optimalangle sinusoidal drive for minimal losses will bepresented. Secondly, a sinusoidal drive is proposed.Thirdly, based on this, a lossless current sensing methodfor self-sensing is proposed.

II. OPTIMAL SINUSOIDAL DRIVE

Driving a BDCM with sinusoidal back-emf withsinusoidal drive, will result in a smooth torque as well asgenerate smaller acoustic noise. In the analysis of thesedrives, the effects of armature resistance are usuallyneglected. While its effects might be negligible in certainapplications, it might be imperative to conduct ananalytical derivation.

Consider the equivalent circuit for an arbitrary phase ina BDCM,

Lx Rx

Fig. 1. Electrical Equivalent Circuit

Taking the phase current as reference,

lx =I. sins (1)

where 1m is the peak value of the phase current.

The phase voltage V, and its back-emf E, can bedefined as

V, = Vmsin(Ct + a) (2)

1-4244-0783-4/07/$20.00 C 2007 IEEE 998

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Ex=Emsin(CaO+5)= Keosinf(Ca +5)

where V, is the peak value of the phase voltage,a is the power angle,E, is the peak value of the phase back-emf,Ke is the back-emf constant and4 is the angular phase difference between

Ix and Ex.

To simplify analysis, the impedance per phase isrepresented as

Rx = Zx cos acoLX = Z sin a

III. PROPOSED CONTROL METHODOLOGY(3)

From the derivation in the previous section, it can be seenthat for a given load, an optimal current, lopt, as well as anoptimal angle, Ocopt0 exists. In other words, in sinusoidaloperation and under constant load, for example in HDDs,if the angle is not atOtopt, this means the voltage can be reduced to Vopt toachieve , lopt and xopt. Alternatively, it can also beinferred that the rotation frequency, co, can be increased.Thus, the following control topology is proposed.

.AH ALV BH BL CH CL

(4)(5)

where a = tan-'(cLx I RX),Z ==R+(cL )2

From electromagnetic power equation,

TLo=1.5EXIX COs5 (6)

To run the BDCM at best efficiency,

5; =0

that is, the phase currents are in-phase with itsrespective phase voltages. Hence, the optimal currentfrom (6) would be

nopt= TL)/(1.5Em) (7)

Vopti.lofpt XaL

Em lopt R

Fig. 2. Phasor Diagram Under Optimality

Thus,

Vopt = (IoptL)2 + (Em + IoptR)2 (8)

aopt = tan {(I,twcL) (Em +I,tR)} (9)

Fig. 3. Proposed Control Topology

The estimated speed, Wt, can be estimated from theinjected frequency and the power angle, u0, can beestimated from the current zero crossing points (IZCPs).

IV. LOSSLESS I-SENS

In the adoption of the proposed control methodology,only IZCPs are necessary instead of the full currentprofile. Traditional method for zero current crossingdetection is accomplished via sensing resistors connectedin series to the motor windings. The polarity of theresistive voltage will provide the current direction and thezero current crossing is given by the instant of voltagepolarity change. The drawbacks of this method are (1) itincurs additional resistive losses and (2) it requires the useof additional resistive elements, thereby increasing thecosts. The proposed method avoids the short comings ofthe resistive elements by making use of the freewheelingdiodes for detection.

In avoidance of these shortcomings, the followingmethod is proposed. In a BDCM system, the BDCM istypically driven by a three-phase inverter circuit as shownin Figure 4. It consists of six power semiconductortransistors with a protection diode connected in parallel toeach of these transistors.

Hence, it can be seen that for a given load and requiredspeed, the optimum voltage and its optimum angle can befound.

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clm ea

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Fig. 4. Bridge Circuit for BLDC Drive

In a PWM drive, the upper and lower switches aregated simultaneously. For example, prior to the gating ofthe lower switch QAL, if the output current is negative, itwill be freewheeling through the diode DAH. Thisfreewheeling gives rise to a diode voltage across DAH.Conversely, if the output current is positive, prior toturning on the upper switch QAH, the current will befreewheeling through the diode DAL. This freewheelingsimilarly gives rise to a diode voltage drop across DAL.The polarity of the current can thus be determined fromthe occurrence of voltage drop across DAH or DAL. Anegative current will give a voltage drop across DAH and apositive current will give a voltage drop across DAL.Consequently, current zero crossing can be given by thepolarity change crossing points. The rising edge ofvoltage drop across DAH will give the zero crossing ofpositive to negative current and, conversely, the fallingedge of voltage drop across DAL will give the zerocrossing of negative to positive current. Similarly, therising edge of the voltage drop across DAL will give thezero crossing of negative to positive current and,conversely, the falling edge of voltage drop across DALwill give the zero crossing of positive to negative cufrrent.

Thus, lossless current zero crossing can be detectedwith the freewheeling diodes without introducingadditional elements.

V. HAREWARE IMPLEMENTATION &EXPERIMENTAL RESULTS

In recent years, owing to the progress of VLSItechnology, the field programmable gate array (FPGA)has gained world wide acceptance. It has traditionallybeen perceived as a essential platform for ApplicationSpecific Integrated Circuit (ASIC) prototyping. However,in recent years, it has gained significant market share inend-product solutions as fundamentally, FPGA offers fasttime to market, low design/manufacturing cost and risk,extremely high processing performance, andprogrammability [3], [4].

FX12. Connected to the IOs of V4FX12 are comparatoroutputs providing the sign of the voltage across thediodes.

1. IZCP Sensing

In the initial investigation, due to the narrowing pulsewidth of the gating signal as the current magnitudeincreases, the voltage drop across the diode is difficult todetect as the notch goes narrower. The resultant currentzero crossing waveform is distorted. Figure 5 shows thedistorted IZCP waveform for the upper diode for aparticular phase. To rectify this problem, it would beuseful to peek into the information that can be availablefrom the waveform generation. Taking a sinusoidalreference, it is reasonable to assume that the currenttransitions will not occur from 2100 - 3300 as well as thecurrent would be negative in that interval. Hence, bydoing a logic OR, the gap will be set to high. Conversely,in the detection of positive currents, its reasonable toassume that current transistions would not occur 300 -1500. Figure 6 shows the block diagram of the algorithmand Figure 7 gives the experimental waveform of themodified algorithm.

Fig. 5. Current and IZCP Waveforms

Fig. 6. Modified Algorithm for IZCP Detection

In this paper, the proposed solution is implemented onFPGA for the above reasons and on a Xilinx Virtex-4TM

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Fig. 7. Current and IZCP Waveformsbased on Modified Algorithm

2. OptimalAngle Control

In the determination of the power angle, a HDD spindlemotor was used and here are its parameters.

TABLE IMOTOR PARAMETERS FOR EXPERIMENTAL TESTS

MOTOR PARAMETERS VALUESPole Pairs 4Armature Resistance (Q) 2.40Armature Inductance (mH) 0.562BackEmf Constant (V/krpm) 0.753Load Torque (mNm) 2.5[Inertia(gcm2) 33.14[Rated Speed (rpm) 7200

The proposed methodology was implemented and thecurrent waveforms are provided below.

Fig. 9. Current Waveforms For Phase A, B, C

As an indicative comparison, the waterfall acoustic forthe drive is compared to a BLDC drive on the BDCM. Itcan be observed that the acoustics for the sinusoidal drive(solid line) is comparatively lesser to that on a BLDCdrive (dotted line).

Fig. 9. Acoustics Waveform forBLDC vs Sinusoidal Drive

Based on the equation (7)-(9),

Iopt = 0.23A VOpt = 8.39V aopt = 0.47rad

Following the proposed control methodology in Figure3, the a measurement flow chart is provided in the Figure8 below:

tatou Phase CurrentP ~~~~~~~~~~~~eroCrossing Point

Phase Voltage/Zero Crossing Point

PhaseV o Start Count)-hsCurnPhase Current

Zero Crossing Point Zero Crossing Point

VI. CONCLUSIONS

In this paper, a self-sensing sinusoidal drive forspindle motor systems is proposed. This drivecomprises of (1) derivation of an optimal angle sinusoidaldrive (2) an innovative lossless current sensing for self-sensing methodology and (3) a self-sensing sinusoidaldrive. The drive introduced was been successfullyimplemented on FPGA and its results were presented.

ACKNOWLEDGMENT

The author would like to express his thanks to all staffat DSI for their help in this work.

Fig. 8. Flow Chart for (c Measurement

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BLDC vs Sinusoidal Waterfall 7200rpm)dB

30 |- -- -- - - -- - - -- -- --------- -- -- -- -- -- r---- --- -------- ----* - X't-;--------

30 L-

BLDCSinusoidal

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REFERENCES

[1] R. Krishnan and A. J. Beutler, " Performance and design of anaxial field permanent magnet synchronous motor servo drive",Proc. of the IEEE Industry Applications Society Annual Meeting,pp. 634-640, 1985

[2] P. Pillay and R. Krishnan, "Modeling, simulation, and analysis ofpermanent-magnet motor drives. I. The permanent-magnetsynchronous motor drive", IEEE Trans. on Industry Applications,Vol. 25, No. 2, pp. 265 - 273, March-April 1989.

[3] S. L. Jung, M. Y. Chang, and Y. Y. Tzou, "FPGA-based control ICfor an 1-phase PWM inverter used for UPS," IEEE PEDS Conf.Rec., pp. 344-349, May 26-29, 1997.

[4] S. L. Jung, M. Y. Chang, J. Y. Jyang, L. C. Yeh, and Y. Y. Tzou,"Design and implementation of an FPGA-based control IC for AC-voltage regulation," IEEE Trans. Power Electronics, vol. 14, no. 3,May, 1999.

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