Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2002-03-14
2004-09-07
Sniezek, Andrew L. (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S078050, C360S078060, C318S561000
Reexamination Certificate
active
06788490
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a circuit for estimating the speed of an electromagnetic actuator associated with a reading head of a disk storage unit.
BACKGROUND OF THE INVENTION
A disk storage device such as a hard disk, CD-ROM or DVD system comprises at least one disk on which data is stored or can be stored, a reading or reading/writing head mounted on a movable arm, and control circuits. The control circuits control an electric motor to rotate the disk about its axis and on an electromagnetic actuator of the type known as a voice coil motor (VCM) for moving the arm radially over the disk so as to position the head over predetermined points on the surface of the disk.
During the normal operation of the device, the head moves over the disk without touching it, supported by air-currents generated by the rotating disk. In some conditions, for example, when reading/writing operations are not to be performed, or when the energy supply to the device is removed, the head is brought to a parked position outside the surface of the disk. This position is typically defined by a ramp onto which the head is loaded (ramp loading), or from which the head is unloaded (ramp unloading).
For head loading operations it is important to apply to the actuator a voltage which is just sufficient to bring the head to the parked position, taking account of the power losses due to friction and leakages. This operation is particularly critical because it is often performed during the switching-off of the device when little electrical energy is available.
The unloading operation, that is, the downward movement of the head from the ramp, is perhaps even more important. In this case, the movement of the head has to be as quick as possible to permit prompt reading/writing of the data, but the head must definitely be prevented from coming into contact with the disk.
To perform the loading and unloading operations correctly, particularly the unloading operation, the speed of the head must be completely controlled in a manner such that the energy supplied to the device is that which is necessary and sufficient for the operation to be performed at every moment.
The instantaneous speed of the head can be estimated by analyzing electrical characteristics of the VCM actuator. The actuator is formed substantially by a coil fixed to the movable arm which carries the head, and by one or more fixed magnets which create a magnetic field that extends through the coil. When a current flows through the coil, a corresponding magnetic force is generated and moves the head relative to the fixed magnet in order to position the head over a predetermined point of the disk.
When the coil cuts the field lines during its movement, a back electromotive force (Vbemf) is generated, which varies based upon the speed of the head. The Vbemf can therefore be measured to make an estimate of the speed. To take a measurement of the Vbemf when a current is flowing through the coil, it is possible to measure the current with a resistor Rs placed in series with the coil, and to take account both of the resistance of the measuring resistor and of the internal resistance Rm of the coil.
According to a known method, the Vbemf is estimated by subtracting the voltage drops in the resistors Rm and Rs from the voltage controlling the VCM actuator. The regulation circuit is calibrated in a manner such that, when the actuator is stationary, the output of the circuit is 0.
A known circuit for controlling a VCM electromagnetic actuator is shown in
FIG. 1. A
VCM
10
is represented by an inductor Lm (the inductance of the actuator coil), a resistor Rm (the resistance of the coil) and a generator of a voltage Vbemf (the back electromotive force), which are connected in series. The VCM
10
is supplied by a bridge power amplifier represented by two operational amplifiers
11
and
12
with gains of G and -G, respectively. A resistor Rs is connected in series with the VCM
10
to provide a voltage signal proportional to the current passing through the VCM at the input of a measuring amplifier
13
with a gain Gs.
A control voltage Vp−Vm is applied to the bridge amplifier by an error amplifier
14
having a non-inverting input connected to a reference voltage terminal, represented by the ground symbol. An inverting input is connected to a control circuit which supplies it with an analog voltage Vin through a series input resistor R
1
. The control circuit is formed by a digital controller
15
and by a digital/analog converter
16
. The output and the inverting input of the error amplifier
14
are connected to one another by a resistor Rc in series with a compensation capacitor Cc. The output of the measuring amplifier
13
is connected to the inverting input of the error amplifier
14
through a feedback resistor R
2
.
In operation, a signal containing information relating to the desired position for the head is applied to the error amplifier
14
as a voltage Vin. The bridge amplifier
11
,
12
is controlled by the output voltage of the error amplifier
14
and supplies to the VCM
10
a controlled current I which brings the head precisely to the desired position.
The following equation is used to find the speed of the head:
Vp−Vm=Vbemf+
(
Rs+Rm
)*
I+L*dI/dt
With the VCM
10
in a constant current condition, this equation provides:
Vbemf=Vp−Vm−Rs*I*
(1+
Rm/Rs
)
The Vbemf, and hence the speed of the head, can be estimated precisely if the ratio Rm/Rs is known. The resistance Rs can be determined precisely by selecting a precision resistor which is substantially not sensitive to variations in the operating conditions. However, the resistance Rm, that is, the internal resistance of the VCM, varies with the type of device used at any particular time, and once the actuator has been selected, also varies considerably with the operating temperature (up to 30% by the typical value). The circuit for measuring the Vbemf has to be calibrated accurately to take account of these variations.
A known circuit for estimating the speed of an electromagnetic actuator is shown in
FIG. 2. A
VCM actuator
10
in series with a measuring resistor Rs are connected between the supply terminals P and M of a bridge power amplifier such as that of
FIG. 1
, which is not shown in order to not make the drawing unnecessarily complex.
The circuit is formed substantially by an adder, which is formed by an operational amplifier
20
and by a resistive network. More particularly, a resistor
21
with a resistance of R is connected between the terminal P and the non-inverting terminal of the amplifier
20
. A resistor
22
, also with a resistance of R, is connected in parallel with a resistor
23
of variable resistance Ra to the node S between the resistor Rs and the VCM
10
at one end, and to the inverting terminal of the amplifier
20
at the other end. The output of the amplifier
20
is connected to its inverting input by a resistor
24
with a resistance of Rb.
The non-inverting terminal of the amplifier
20
is also connected, by a variable resistor
25
with a resistance of Ra′ to the terminal M, and by a resistor
26
with a resistance of Rb to a source of a constant reference voltage Vref. The output of the amplifier
20
is connected to the digital controller
15
via an analog/digital converter
27
. If the variable resistors Ra and Ra′ are calibrated correctly, the circuit shown supplies to the controller
15
a signal which depends solely on the Vbemf, and hence on the speed of the head. To calibrate the resistors, the controller
15
measures the voltage between the terminals P and M when the head is stationary, and adjusts the resistors in a manner such that the output of the circuit is at the reference voltage Vref.
The circuit described above can provide precise speed data but has some disadvantages when the circuit has to be integrated in a semiconductor chip with the smallest possible dimensions. The production of the variable resistors Ra and Ra′ is in fact quit
Galbiati Ezio
Nessi Maurizio
Schillaci Luca
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Jorgenson Lisa K.
Sniezek Andrew L.
STMicroelectronics S.r.l.
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