Dual speed motor drive circuit

Details Dual speed motor drive circuit Dual speed motor drive circuit

2000-04-24

2001-07-17

Nappi, Robert E. (Department: 2837)

Electricity: motive power systems

Switched reluctance motor commutation control

C318S434000, C318S132000, C360S073050

Reexamination Certificate

active

06262545

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the spindle motor control in hard disk drives and more particularly to dual mode spindle motor control in hard disk drives to provide a high-efficiency mode and a high-performance mode in portable computing devices.
DESCRIPTION OF THE RELATED ART
Portable computing devices, particularly notebook computers, are gaining popularity in recent years due to their compact size, weight, and mobility. Notebook computers can operate either from AC power or from battery power. However, unlike AC power, battery power will get depleted as the notebook is being operated. The hard disk drive (HDD) is one of the devices in a notebook that uses large amounts of power when in operation. Therefore, the spindle motors in hard disk drives are intentionally operated at low speeds, measured in revolutions per minute (RPM), to minimize power consumption during battery operation. The trade-off for this lower power consumption is a decrease in RPM of the spindle motor which directly leads to a decreased performance in data access time.
When AC power is in use, power consumption is less of a concern as AC power is practically unlimited. High performance becomes more important in a hard disk drive than low power consumption. In this situation, it is desirable to have the spindle motor in the hard disk drive spin at a significantly higher RPM to allow faster data retrieval.
In a conventional spindle motor system, an main processor unit (MPU) is used to control a spindle motor driver, which in turn supplies a current sufficient to achieve a desired RPM to drive the spindle motor. The spindle motor driver sends back spindle motor RPM readings to the MPU and the readings are compared to a desired RPM. Based on this comparison, the MPU sends a signal to the spindle motor driver to either increase or decrease the current supplied to the spindle motor to increase or decrease the motor RPM to the desired RPM.
A dual-speed disk drive has the unique requirement in that the lowest RPM operating mode should have the highest efficiency. The purpose of a dual-speed disk drive is to minimize battery power consumption in the low RPM mode and maximize performance when operating on AC power in the high RPM mode.
Operating a spindle motor at multiple RPMs from a fixed supply voltage can be done in several different ways. The simplest solution is to use a linear motor driver. However, a linear motor driver is the least efficient when operated at low RPMs which runs counter to the goal of minimizing power consumption at low RPM. Referring to
FIG. 1 and 2
, voltage plot
12
illustrates at high RPM, the back emf approaches the supply voltage and therefore voltage drop across the motor driver is small. Energy dissipation in the motor driver is also small (illustrated in power plot
22
) and therefore high efficiency can be achieved. At low RPM, there is a significant voltage drop in the motor driver due to the decrease in back-emf as illustrated in voltage plot
10
, resulting in significant energy dissipation (illustrated in power plot
20
) in the motor driver's output transistors and therefore efficiency is very low.
Referring to
FIGS. 3 and 4
, spindle motors can be optimized in terms of attaining maximum efficiency at a given voltage for a desired RPM. For example, a spindle motor optimized for low RPM (e.g. 4200 RPM) at +5.0 V supply voltage exhibits a voltage plot
32
and a power plot
42
. Voltage drop and energy dissipation in the motor driver is small therefore efficiency is high. However, using a spindle motor that is optimized for low RPM at the standard +5.0 V supply voltage would mean that the spindle motor would not be able to run at a higher RPM at this standard voltage. This is undesirable when high performance is required.
On the other hand, a spindle motor optimized for high RPM (e.g. 5400 RPM) at +5.0 V supply voltage exhibits a voltage plot
38
and a power plot
48
. Again, a small voltage drop and a small energy dissipation makes the spindle motor highly efficient. A linear motor driver can be used to operate the spindle motor at 4200 RPM that exhibits voltage plot
36
and power plot
46
. As illustrated, there is a significant voltage drop and energy dissipation across the motor driver greatly reducing the efficiency.
To achieve better efficiency when using a linear motor driver, voltage conversion techniques such as DC—DC voltage step down converters have been used to lower the effective supply voltage to the linear motor driver output stage. By lowering the supply voltage to the output stage to the lowest necessary voltage to maintain the desired RPM, voltage drop and hence energy dissipation can be minimized, dramatically improving the motor driver efficiency (see
34
in FIG.
3
). However, DC—DC voltage step down converters usually have efficiency rates of only 80-90% thereby introducing a new power loss, which negates some of the gained efficiency. This can be illustrated in power plot
44
. On top of the energy for motor rotation, heat loss of motor, and energy dissipation in the driver, there is a new loss called DC—DC Converter Loss.
Thus, using a DC—DC down converter in the low RPM mode to lower the effective supply voltage which introduces efficiency losses is undesirable.
Typical RPM of a spindle motor supplied by the standard +5.0 V voltage is in the range of 4000-5400 RPM. The high speed mode of dual-speed drives typically require 7200-10,000 RPM, which is often beyond the capability of a conventional +5.0 V spindle motor.
U.S. Pat. No. 4,307,326 describes the use of a sensing resistor and a switch to control the use of a DC—DC down converter at low current (normal load) to achieve high efficiency. The DC—DC down converter is bypassed with the switch at high current (high load) operation. The DC—DC down converter is not necessary in high current (high load) operations because the voltage drop across the motor driver is small and therefore the motor driver is already operating at maximum efficiency.
U.S. Pat. No. 4,359,674 describes the use of an on-time ratio of a switching semiconductor and a voltage controller to control the output voltage of a DC—DC down converter to achieve high efficiency over a range of spindle motor load operations.
U.S. Pat. No. 4,839,754 describes the use of adjusting the duty cycle of a switching regulator to control the DC supply voltage to control the speed of the spindle motor as well as to achieve high efficiency.
None of the prior art mentioned above is capable of producing the highest efficiency at low RPM speeds without introducing new power losses. There is also no mention of any solution to significantly increase the RPM of a spindle motor during high performance modes.
A need therefore exists for providing an spindle motor control having dual performance modes. One mode will be for high efficiency operation of the spindle motor at low RPM (3600 RPM for example) during battery power operation. The other mode will be for high performance operation which is achieved by significantly increasing the RPM (7200-10,000 RPM for example) of the spindle motor, during AC power operation.
SUMMARY OF THE INVENTION
A principle objective of the present invention is to provide a spindle motor control for a dual-speed HDD that has two performance modes. One mode is the high efficiency mode for use when in battery operation where power conservation is paramount. In the high efficiency mode, the spindle motor operates at a low RPM to conserve battery power. The spindle motor is optimized for this low RPM operation. The other mode is the high performance mode used when AC power is in use where high spindle motor RPM is desired to maximize data retrieve performance.
Another objective of the present invention is to provide a spindle motor that is capable of operating at significantly higher RPM utilizing the standard supply voltage of +5.0 V while keeping the same form factor to fit existing space limitations.
A further objective of the present invention is to provide a method that

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