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For years, the technological progress in the area of hard disk-spindle systems was focused on cost, rather than performance. Recent introduction of hard disks operating with 10,000 rpm and 15,000 rpm brings the data storage industry to the point where spindle performance substantially influences characteristics of data storage devices. This article will deal with the spindle system of the hard disk and will discuss the limitations of spindle technology, new industry trends in this area, and potential directions of further development.
Figure 1 presents a block diagram of a permanent-magnet motor, variable-speed drive system, the spindle system typically employed in data storage devices. The system contains five distinct elements: the load (platters), motor, modulator, power converter (inverter), and controller. The motor is supplied by the inverter, which consists of a set of power semiconductor devices switching the current according to a specific modulation algorithm. The modulator translates the motor shaft position and linear signal from the controller into the ON and OFF pulses controlling the inverter. The controller gathers information on the current state of the system and makes decisions influencing the system behavior (start-up sequence, breaking) and performance (closed-loop speed control, motor current/torque control).
The motor of choice in the data storage industry is the permanent-magnet synchronous motor (PMSM). Such a motor is often referred to as a "brushless DC" motor, or electronically commutated motor (ECM). Unlike other motors, the PMSM characteristics substantially depend on the employed power converter topology and control concept. The PMSM is a synchronous motor—the mechanical movements of the shaft exactly follow the position of the rotating magnetic field generated by the current in the motor's winding. Generally, depending on the field distribution in the air gap, motors are divided into two groups: trapezoidal and sinusoidal.
The power-section (inverter) transistors are turned ON and OFF according to the modulation algorithm. In the typical case of the voltage-source inverters, the voltage is the modulated parameter. The motor current is a result of the pulse-width-modulated (PWM) voltage applied to the windings. A review of the PWM techniques employed in the motor drives supplying induction motors can be found in Pulsewidth modulation—a survey[1].
The energy consumed by the spindle system can be divided into three groups: motor losses, inverter losses, and energy supplied to the load. The motor losses can be divided into: winding losses (power lost in the resistance of the motor's windings, due to the eddy currents in the winding and to the skin effect) and iron losses, due to the characteristics of the magnetic materials (hysteresis and eddy-current losses in iron). The losses in the inverter can be attributed to the switching losses of the switching devices and diodes, and conduction losses of the switching devices and anti-parallel diodes. More details on this subject may be found in the following publications: Power Electronics, Converters, Applications and Design[2], The Field Orientation Principle in Control of Induction Motors[3], and Control of Electrical Devices[4].
Servomotor drives typically require shaft position and current sensors for both control and protection. The simple six-step commutation commonly used on small brushless DC motors, employs Hall effect sensors activated by the rotor magnet. Other alternatives are high- resolution encoders or resolver feedback, as used in precision brushless servo systems. In data storage applications, the motor drive operating conditions are well predetermined. This greatly simplified set of requirements, combined with certain switching/modulation techniques, allows implementation of the sensorless detection of the shaft position. The shaft-position information is typically derived from the zero crossing of the back-EMF voltage of the undriven phase, with respect to the center point.
For voltage-source inverters there are three basic motor/ drive configurations characterized in terms of the phase voltage applied to the motor: six-step (square wave), trapezoidal, and sinusoidal. Further references to six-step (square wave), trapezoidal, and sinusoidal drives assume voltage-source inverters.
The Six-Step (Square-Wave) Drive System
The six-step (square-wave) drive system sequentially changes the voltage applied to the motor windings and generates the synchronous field required for motor operation. The switching sequence is coupled with the shaft (rotor-field) position. To improve six-step system performance, the motor voltage is added as another degree of freedom. The voltage control is achieved by either control of the inverter supplying DC voltage (double conversion), or by superimposing the PWM pattern on the six-step waveform. The pure six-step requires very low switching frequency and, in effect, inverter efficiency is high. The drawback of this method is that the motor is treated with substantially distorted currents that lead to in-creased power losses. The current harmonics are also responsible for the high level of motor-generated noise. The hardware implementation of the six-step drive is relatively simple; it also permits an easy introduction of the sensorless commutation.
The Trapezoidal Drive System
The trapezoidal drive system, although similar in principle to the six-step drive, generates trapezoidal instead of square-wave waveform. The change of shape is achieved by applying a PWM-modulated-voltage transition instead of the simple switching from one polarity to another. The average voltage value (not instantaneous) forms a trapezoid. The trapezoidal drive system provides a balanced solution, striking a good compromise between performance and system complexity. It performs better than six-step in terms of efficiency and torque pulsations, but does not require sophisticated and costly engineering solutions required by the sinusoidal approach. Lower losses and noise, combined with simplicity of implementation and easy implementation of the sensorless commutation, are the benefits of the trapezoidal approach. For the full optimization of the trapezoidal solution, the motor requires special design.
The Sinusoidal Drive System
The sinusoidal drive system generates PWM-modulated, sinusoidal voltages. The drive controls the voltage amplitude and frequency. This is the most sophisticated drive system, bringing typically high motor efficiency and low noise, but at the expense of inverter efficiency and substantial system complexity. Implementation of the sensorless sinusoidal drive system is not trivial. In this case, the motor design requires special attention, as well.
A comparison of the most important characteristics of the three basic spindle motor/drive configurations is presented in Table 1.
Although seldom associated with drive perform- ance, spindle technology influences (and often limits) performance of the data storage device in direct and indirect ways. The higher rotational speed results in a shorter latency time, improving the access time. Let us examine the implications of increasing the rotational speed from the motor and drive perspective.
Assuming an unchanged supply voltage, to reach the maximum allowable back-EMF value, the number of turns should be reduced in the higher-speed motor. The smaller number of turns reduces the winding inductance. The smaller winding inductance then results in an increase of the current harmonics associated with the switching operation of the inverter and amplifies other harmonics caused by interactions between the inverter and the motor. The harmonics cause parasitic losses, torque ripple, noise, and other undesirable effects. A simple method of reducing the current harmonics is to increase the switching frequency of the inverter. However, it also means an increase of the switching losses, and for finite switching times, distortion of the generated voltage and, in effect, more current harmonics.
The smaller winding inductance and lower number of pole-pairs of high-speed motors, in connection with simple extrapolation of the operating parameters and no fundamental changes in the spindle technology, causes the quick deterioration of the high-speed spindle efficiency. To minimize the impact of the deteriorating spindle performance and still reach the high rotational speed, it has become common practice to reduce the load by decreasing the diameter of the platters. This also reduces the maximum storage capacity, but recent increases of the maximum data density help to compensate for the loss of capacity due to spindle-induced limitations. Platter diameter reduction forced by the spindle performance is one of the most evident limitations imposed by the spindle system on the data storage device. Paradox-ically, the smaller platter diameter reduces the load inertia and magnifies the impact of the parasitic torque components.
Are there other spindle-induced limitations? One can certainly draw a connection between parasitic torque components and the maximum data density or the maximum bandwidth of the voice-coil actuator. In both cases, it is quite difficult to precisely define the correlation, especially having no ideal spindle motor for establishing the reference point.
The parasitic torque components with their wide frequency spectrum are often responsible for exciting mechanical resonances. Those resonances occur at frequencies assumed to be outside the range of frequencies exciting the mechanical system; they are very difficult to model, predict, and eliminate in the design stage. A portion of the losses in the spindle motor is transformed in acoustic energy. The motor-generated noise is in direct proportion to the level of current and current harmonics in the motor windings. The loosely wound motor windings often act as the electro-dynamical transformers of the electrical energy into acoustic energy. The parasitic torque components increase the noise level generated by the high-speed motor.
The high-speed spindle design for data storage devices rotating with 20,000 rpm or more will require high-efficiency, smooth-torque solutions. The fully sinusoidal drive seems to be the most promising. Recently, a number of purely sinusoidal solutions were introduced; however, the majority of them, having their roots in the induction motor drives or servo drives, do not offer benefits clearly outweighing the drawbacks. Unfortunately, for developers involved in spindle systems for data storage applications, not only technology, but also the means of implementing the technology (such as modulators, microcontrollers, or DSP chips) are developed with relatively slow induction-motor drives or servo drives in mind.
Preliminary work by the authors would indicate that by applying a sinusoidal drive system specifically designed for the high-speed spindle motor the power losses can be significantly reduced. Mainly due to the reduction of the parasitic torque components, the efficiency of a 10,000-rpm enterprise-class spindle system employing a four-pole-pair motor with trapezoidal back-EMF was improved by 17 percent (see Figure 2).
It is important to point out that a substantial portion of the eliminated power was originally transformed in the energy exciting other electromechanical components and subsystems of the data storage device, causing other performance limitations. The reduction of the spindle power losses is often perceived only as a relatively small change of one of the components in the thermal budget. However, this view does not reflect the true meaning of this change. The efficiency improvement of the spindle system should rather be viewed as a reduction of the parasitic energy that excites the mechanical components of the hard drive in an uncontrolled way.
Establishing hard data presenting correlation between the spindle efficiency (parasitic torque components) and the hard disk performance has yet to be done; the probability of such dependencies appears to be quite high.
Parasitic torque components may play an even greater role in the case of spindle motors employing low stiffness bearings, such as hydrodynamic, air, or magnetic bearings. Time-variable torque components are created as a result of the employed modulation technique and interaction between the inverter and the motor. They may occur on almost any point of the frequency spectrum, from sub-harmonics up to the 40th harmonics of the fundamental frequency of the motor current. Excitation of mechanical systems employing low stiffness bearing, with so wide a spectrum of time-variable torque components, may lead to more severe mechanical resonances and oscillations than in the case of ball bearings. Ball bearings play an important role in "torque budget" of the spindle motor. A number of motor-design techniques rely on the cancellation of certain torque components, which actually occur in the bearings. Application of the low-stiffness bearings may amplify the need for spindle systems capable of reduction or elimination of the parasitic torque components.
The ultimate solution, offering an unlimited spindle lifetime, is a fully integrated spindle motor and magnetic bearing system. The integrated motor/magnetic bearing system would offer a practically unlimited lifetime of the spindle system. Although there is substantial progress in magnetic bearing technology, fluid bearings may offer an immediate solution to certain drawbacks of ball bearings.
Summary
Although seldom associated with drive performance, spindle technology influences (and often limits) performance of the data storage device in direct and indirect ways. Platter diameter reduction forced by the thermal budget limitation is one of the most obvious.
Interactions between the inverter and motor increase the level of parasitic torque components, resulting in increased power losses and increased noise level. The parasitic torque components are often responsible for exciting resonances that are difficult to predict and avoid during the design stage.
Reduction of spindle power consumption should be viewed as a reduction of the parasitic energy that excites the mechanical components of the hard drive in an uncontrolled way, rather than a small efficiency improvement.
Reduction of parasitic torque components is especially important in the case of spindle motors employing low- stiffness bearings, such as hydrodynamic, air, or magnetic bearings.
The specialized, sinusoidal high-speed spindle controller has been developed. Mainly due to reduction of parasitic torque components, the efficiency of a 10,000-rpm enterprise-class spindle system was improved by 17 percent.
The dedicated sinusoidal-based drive and fully integrated motor and magnetic-bearing system are two of the most promising directions in which the high-speed spindle for data storage devices may evolve in the future.
References
1 J. Holtz, Pulsewidth modulation—a survey. IEEE Transactions on Industrial Electronics, Vol. 39, 1992, pg. 410.
2 N. Mohan, T. Underland and W. Robbins, Power Electronics, Converters, Applications and Design published by John Wiley and Sons, Inc. in 1995.
3 Andrzej M. Trzynadlowski, The Field Orientation Principle in Control of Induction Motors published in 1994 by Kluwer Academic Publishers.
4 Werner Leonhard, Control of Electrical Drives published by Springer-Verlag in 1996.
Iwona Bida is president of FAMA Corporation. Since its inception in 1999, the company has been engaged in technology development and consulting services in the areas of power conversion, high-speed motor drives, and high-performance actuators. FAMA Corporation is presently in the process of introducing a family of specialized integrated circuits for efficient and cost-effective control of high-speed motors. The integrated circuits, based on proprietary technology, generate cost-savings and improved motor efficiency and overall performance of hard disks and other storage devices employing rotating media.
Mrs. Bida holds a master's degree in engineering from the Technical University of Wroclaw, Poland. She may be reached by e-mail at Iwona_Bida@email.msn.com.
Roman Bida is a consultant engaged primarily in the areas of high-speed motor drives, integrated motor-drive systems, and efficient power-modulation strategies. For over fifteen years he has been involved in R&D activities with companies such as Siemens, ABB (Asea Brown Boveri), and Eaton. Mr. Bida has recently applied for a number of patents in the areas of permanent-magnet and high-speed motor drives. He holds a master's degree in electronics engineering and control systems from the Technical University of Wroclaw, Poland.
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Reprinted with permission from INSIGHT May/June 2001, pp 10,11,14,15,17
© Copyright 2001 IDEMA. All rights reserved.
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Figure 1. Block Diagram of a Permanent-Magnet Motor, Variable-Speed Drive System, the Spindle System Typically Employed in Data Storage Devices.
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Table 1. Comparison of the Most Important Characteristics of the Three Basic Spindle Motor/Drive Configurations.
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Figure 2. Comparison of the Power Consumption of a 10,000-rpm Spindle System Employing Trapezoidal and High-Speed, Low Parasitic Torque Spindle Systems.
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