Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates generally to magnetic disk
drives and more particularly to an improved emeryency head
unload system for use therein.
Modern moving head magnetic disk drives generally
employ heads carried by a support structure coupled to a positioner
motor. The positioner motor typically includes a coil mounted within a mag-
netic field for linear movement and oriented relative to the disk to m~ve
the heads radially over the disk surface to thereby enable the
heads to be positioned over any annular tracX on the surface.
The heads are designed to actually fly above the disk recording
surface at heights of less than 100 microinches. If, during
normal operation of the drive, power is lost causing the disk
rotational speed to gradually decrease, the heads cannot con-
lS tinue to fly and would ultimately crash into the disk surface.
In order to protect the data, the heads and the disk, it is neoessary to
remove the heads from the disk surface as fast as possible when
a power fault is detected. The process of removing the heads
from the disk in an emergency situation is referred to as an
emergency unload procedure and requires that the head support
structure be moved radially toward the disk outer track to
axially move the heads away from the disk surface. Although
loss of power is probably the primary reason for initiating
the emergency unload procedure, the procedure is typically
also initiated when the following conditions are encountered:
(1) Disk speed does not remain within tolerance;
(2) Positioner error is detected;
(3) Write circuit faults that could effect stored
data are detected.
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Essentially all modern disk drives incorporate some
system for executing an emergency unload procedure in order to
avoid loss of data and prevent disk and/or head damage. In a
typical prior art emergency unload system, a capacitor is charged by
the drive power supply during normal operation. Upon the
detection of an emergency condition, a relay or equivalent
switching means switches the capacitor across the positioner
coil terminals to provide the electromotive force necessary to
move the head support structure across the disk surface. Vpon
approaching the disk outer edge, the head support structure
encounters a mechanical ramp. The mechanical ramp imparts an
axial force to the support structure thus unloading the head
from the disk.
The capacitor typically supplies a relatively cons-
tant voltage across the positioner coil causing the coil andsupport structure to accelerate as it is moving toward the
disk outer edge. A~ a result, the support structure sometimes
contacts the mechanical ramp at a high velocity. The resulting
impact can cause the head to oscillate and impact against the
disk surface thus causing damage to the disk and head.
SUMMARY OF THE INVENTION
The present invention is directed to an improved
means for use in a magnetic disk drive for unloading the heads
under emergency conditions. In accordance with one aspect of
the invention, a dynamic braking circuit is activated in
response to an emergency unload command to reduce the positioner
coil velocity toward zero.
In accordance with a further aspect of the invention,
a velocity control circuit is activated, a short time delay
~lZ7755
after the emergency unload command, to move the head support structure
at a controlled substantially constant velocity across the disk surface
toward the disk outer track.
Thus, in accordance with one broad aspect of the invention,
there is provided, in a disk drive including a disk having a magnetic
surface and a motor having a positioner coil for radially moving a head
support structure along said disk surface, apparatus responsive to an
emergency condition for causing said coil to rapidly move said support
structure radially outwardly at a controlled velocity, said apparatus
comprising: switch means having first and second input terminals and
including means capable of selectively defining a first state to connect
said first input terminal to said positi.oner coil or a second state to
connect said second input terminal to said positioner coil; brake circuit
means elcctrically connected to said first input terminal and responsive
to a signal indicative of said emergency condition for reducing the
velocity of said positioner coil toward zero; means responsive to said
emergency condition signal for switching said switch means from said
first to said second state after a predetermined time delay; velocity
control circuit means connected to said second input terminal for supply-
ing a substantially constant current through said switch means to said
positioner coil; emergency power supply means; and means for connecting
said emergency power supply means to said brake and velocity control
circuit means to supply power thereto during said emergency condition.
In accordance with another broad aspect of the invention
there is provided an emergency head unload system for a magnetic disk
drive including a positioner coil coupled to a support structure carrying
a head, said support structure including a ramp surface positioned to
engage a fixed cam as said support structure is moved radially outwardly
over a disk surface to move said head axially with respect to said
surface, said system comprising: means for supplying an emergency signal
indicative of an emergency condition; means for producing a very low-
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,,
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resistance shunt path across said positioner coil for a limited time
interval following said emergency signal to brake the velocity of said
coil to substantially zero; velocity control means active after said
limited time interval for initially supplying a substantially constant
current to said positioner coil to move said support structure ramp
surface to said fixed cam at a controlled low velocity; emergency power
supply means; and means fGr supplying electrical power from said emer-
gency power supply means to both said means for producing a shunt path
and said velocity control means during said emergency condition.
In accordance with the disclosed embodiment of the invention,
the velocity control circuit operates first in a low current mode to
move the head support structure at a substantially constant velocity
toward the outer track and then in a high current mode to enable the
head support structure to climb an unload ramp which moves the heads in
an axial direction away from the disk surface.
The novel features of the invention are set forth with parti-
cularity in the appended claims. The invention will best be understood
from the following description when read in conjunction with the accom-
panying drawings.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a block diagram of a portion of the electronic
control system of a typical magnetic disk drive showing in dashed line
the modifications in accordance with the present invention; and
Figure 2 is a circuit diagram of an exemplary embodiment of
the present invention.
DETAILED DESCRIPTION OF THE DISCLOSED
EMBODIMENT OF THE INVENTION
Attention i~ initially directed to Figure 1 which schematically
illustrates a magnetic disk drive 10 which includes a spindle 12 support-
ing one or more magnetic disks 14 and 16. As is well-known in the art,
the disks 14 and 16 may either be fixed to the spindle 12 or user-replace-
able- -4a-
L -
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The spindle 12 is driven by a spindle motor 18 which
rotates the disks 14, 16 about an axis defined by the
spindle 12.
As is well-known in the art, the disks 14, 16
have magnetic material coated on either one or both sur-
faces thereof to enable data to be magnetically recorded
thereon. Data is recorded on the disk surfaces by signals
applied to a magnetic transducer or head. In a typical
disk drive, a head support structure 22 is provided. The
support structure 22 typically includes, for each disk,
first and second head support arms 24 and 26, respectively
carrying heads 28 and 30. The support structure 22 is
physically coupled to a positioner coil 32 of a positioner
motor 33, which, in response to the application of positioning
signals thereto, is capable of linearly moving the support
structure 22, as represented by the arrows 34. The support
structure ~2 is mounted relative to the disks 14, 16 so that
the heads 28, 30 move radially with respect to the disk
recording surface. That is, the support structure 22 can
be moved linearly to enable the heads to be selectively po-
sitioned over any one of a plurality of annular recording
tracks defined on the disk recording surface.
In normal operation, that is when the heads are
writing data onto the disk surface or reading data therefrom,
the heads will be loaded. When the heads are loaded, it
simply means that a spring force is acting upon the heads,
generally through the support arms 24, 26 to force the heads
toward the disk surface. As is well-known in the art, the
heads are configured so as to actually fly immediately
above the disk recording surface so long as the disk is
rotating at a certain speed. Typically, the heads
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may fly on the order of 60 microinches above the disk surface.
If, while the heads are positioned above the disk surface,
the disk speed happens to fall below a certain value, the
heads can no longer fly and will crash into the surface of
the disk. Obviously, this type of catastrophe is to be
avoided and as a consequence, most modern disk drives incor-
porate some type of emergency subsystem which senses a power
fault or disk speed variation and unloads the heads prior to
crashing. The process of unloading the heads generally re-
quires that the support structure 22 be retracted; that is,it should move to the left, as represented in Figure 1, in
order to move the heads 28, 30 toward the outermost annular
track on the disk surface.
The support arms 24, 26 are typically provided with
ramp surfaces 40, 42 which are used to unload the heads; that
is, move them axially away from the disk surface. More
particularly, the ramp surfaces 40, 42 are operated in con-
junction with a fixed cam 44 which engages the ramp surfaces
40, 42 as the support structure 22 is pulled by the positioner
coil 32 to the left (Figure 1). The engagement of the fixed
cam 44 agai~st the ramp surfaces 40, 42~will move the support
arms 24, 26 and particularly the heads 28, 30 axially away
from the recording surfaces of the disk.
Figure 1 illustrates in solid line a block diagram
of a typical emergency head unload system characteristic of
the prior art. ~he dashed lines illustrate modifications to
such a prior art system in accordance with the present inven-
tion, as is further detailed in Figure 2.
Initially considering the typical prior art system
represented in solid line in Figure 1, an emergency unload
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relay 50 is provided which is represented as a single pole
double throw switch. That is, the blade contact 52 can be
placed in a first state (illustrated) to connect first
input terminal 54 to the positioner coil 32. The
emergency unload relay 50 can be switched to a second state
in which the contact 52 connects the second input terminal
56 to the positioner motor coil. The state of the emergency
unload relay 50 is controlled by the emergency unload relay
driver 60. The emergency unload relay driver 60 is responsive
to an emergency command signal appearing on the emergency
unload enable line 62. More particularly, means ~not shown)
are provided in a typical disk drive to detect various
emergency conditions whose occurrence are intended to initiate
an emergency head unload procedure. Such means for detecting
these emergency conditions are well-known in the art and it
has been assumed herein that an emergency command signal will
be applied to the emergency unload enable line 62 upon the
detection of such a condition. The emergency unload relay
driver 60 will respond to that emergency command signal to
switch the state of the emergency unload relay 50 from the
first state (illustrated) to the second state in which the
contact 52 engages the second input terminal 56.
Although the emergency unload relay 50 is depicted
in Figure 1 as merely comprising a single pole double throw
electromechanical switch, it should be recognized that this
showing is for convenience only and that other switching means,
including all electronic switching circuits, could readily be
utilized.
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During normal operating conditions, the emergency
unload relay contact 52 will be in the first state engaged
with input terminal 54. Input terminal 54 is connected to
the output of power amplifier 64 which receives as its input
the output of a summing amplifier (not shown). The summing
amplifier typically provides t~e positioning command informa-
tion to the power amplifier. The summing amplifier (not shown)
will develop its output signal based upon various inputs such
as track position desired, temperature compensation required,
etc. In order to move the heads from their present track to
any other desired track, the power amplifier 64 will of course
supply the appropriate current through the emergency unload
relay terminal 54 and contact 52 to the positioner motor coil
32. Typically, a current sensor means 68 is provided to
sense the current in the positioner coil. The current sensor
is connected back to the power amplifier 64 and provides an
error signal thereto which assures that the positioner coil
current is made equal to the current intended by the power
amplifier 64.
Thus, under normal operating conditions, the contact
52 will be electrically connected to input terminal 54 and the
position information supplied on line 66 to the power amplifier
will enable the power amplifier to develop an appropriate
current in the positioner motor coil 32 to position the heads
28, 30 over the desired annular track on the surface of disks
14, 16.
When an emergency command signal is detected by the
emergency unload relay driver 60 on the enable line 62, the
emergency unload relay 50 is switched to its second state to
connect contact 52 to input terminal 56. This action disconnects
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the power amplifier 64 from the positioner coil 32 and instead
connects the emergency unload system 70 to the positioner coil.
The emergency unload system 70 is powered by an emergency un-
load capacitor 72 which is fully charged during normal operating
conditions. The capacitor 72 must be of a size to supply
sufficient electrical energy through the emergency unload system
70 to retract the head support arms 24, 26 and pull them over
the fixed cam 44 to unload the heads. In typical prior art
systems, the fully charged capacitor 72 is connected, during
an emergency unload procedure, directly across the positioner
motor coil. The capacitor 72 has typically supplied a relatively
constant voltage across the coil of a relatively high value since
it is of course an objective to-retract the heads from the disk
surface as ~uickly as possible. If, at the time the emergency
occurs, the heads are over an inner track (i.e., close to the
spindle 12), the positioner coil, powered by the capacitor 72,
will accelerate over a relatively long stroke meaning that the
ramps 40 and 42 will impact against the fixed cam 44 at a rela-
tively high velocity. Under certain conditions, this has caused
the support arms 24, 26 to oscillate, thereby on occasion
impacting the heads 2~, 30 against the disk surface.
The present invention is directed to a modification
of an emergency head unload system as depicted in solid line in
Figure 1 or the purpose of reducing the risk of data loss and
head or disk damage during the execution of an emergency unload
operation. The details of an embodiment of the present inven-
tion are illustrated in Figure 2 and will be explained in con-
junction therewith. However, Figure 1 represents in dashed
line the modifications to the typical prior art emergency un-
load system, in accordance with the present invention.
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Briefly, in accordance with the present invention,the emergency unload system 70 of Figure 1 includes a dynamic
brake circuit and a velocity control circuit. The output of
the dynamic brake circuit is represented by output line 76
connected to emergency unload relay input terminal 54. The
output of the velocity control circuit is represented by output
- line 78 connected to the emergency unload relay second input
terminal 56. Additionally, the emergency unload system 70 has
an input control terminal 80 which is connected to the emer-
gency unload enable line 62. Briefly, upon the~detectLon of
an emergency command signal on enable line 62, the emexgency
unload system 70, in accordance with the invention, activates
its dynamic~brake circuit to create, via line 76, an electrical
short across the positioner motor coil. The positioner motor
typically comprises a voice coil mounted for linear movement
within a permanent magnet field. The electrical short across
the positioner motor coil produces a back electromotive force
in the coil opposing the motion of the coil. Thus, regardless
of the action of the coil at the instant the emergency command
5ignal is recognized, the effect of the electrical short will
be to reduce the coil velocity to substantially zero. It sh~uld be
recognized that at the instant of the occurrence of the emergency
command signal the heads can be in fixed position over a track,
or can be 5eeking a track moving either radially inwardly or
outwardly. Regardless, the function of the dynamic brake cir-
cuit of the emergency unload system 70 is to reduce the coil
velocity to substantially zero. After a short time delay intro-
duced by the emergency unload relay driver 60, the emergency
unload relay 50 switches to its second state. The emergency
unload system velocit~ control circuit then supplies a current
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to the positioner motor coil 32. Initially, the velocity
control circuit provides a substantially constant current
to the coil to retract the heads at a controlled low
velocity which is insufficient to permit movement of the
ramps 40, 42 past the fixed cam 44. As the coil velocity
slows upon engagement of the cam 44 against the ramps 40,
42, the coil current increases. The coil current is sensed
by the current sensor 68 which, via line 82, switches the
velocity control circuit into a high current mode to supply
sufficient energy to the positioner coil to enable the
ramps 40, 42 to move past the fixed cam 44.
Attention is now directed to Figure 2 which illus-
trates a detailed embodiment of the emergency unload system
70, as well as the emergency unload capacitor 72, the emer-
gency unload relay 50, the positioner coil 32, and the current
sensor 68, all of Figure 1. In Figure 2, the emergency unload
relay 50 is illustrated as comprising a solenoid 100 operating
a double pole double throw switch. The double pole double
throw switch includes blade contacts 102 and 104. During -
normal operating conditions, the blade contacts 102 and 104
are in the solid line position illustrated respectively con-
tacting input terminals-106 and 108. Input terminal 106 is
connected to the output of the power amplifier 64 of Figure 1.
Input terminal 108, as will be seen hereinafter, is connected
to the output of the dynamic brake circuit 110 which comprises
a portion of the emergency unload system 70.
In the second state of the double pole double throw
switch means, the blade contacts 102 and 104 move into contact
with input terminals 112 and 114 respectively. Input terminal
112 is connected to the output of the velocity control circuit
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116 which, as has been previously mentioned, forms part of
the emergency unload system 70 of Figure 1. Input terminal
114 is not utilized in the disclosed embodiment of the
invention.
Figure 2 also illustrates the emergency unload
capacitor 72 previously mentioned in the description of
Figure 1. Zener diode VR2 and resistor R5 operate in
conjunction with the emergency unload capacitor 72 (C3) to
- charge the capacitor to substantially -12 volts during
normal operating conditions. As will be seen hereinafter,
during emergency conditions, the capacitor 72 provides the
power necessary to operate the dynamic braking circuit 110
and the velocity control circuit 116.
During an emergency condition, a high logic level
signal will be applied to the emergency unload enable line
62. This action causes transistor Q5 to turn on. This ac-
! tion in turn provides base drive for transistor Q6 controlled
by resistor R10. When transistor Q6 conducts, the gate
circuit of the triac SCRl will receive a negative bias current
controlled primarily by the value of resistor R12 thereby
turning the triac on. As a consequence, the triac SCRl will
produce a very low resistance (essentially an electrical short)
shunt path across the positioner coil 32 via the switch means
input terminal 108 and blade contact 104.
It has previously been mentioned that the emergency
unload relay driver 60 provides a certain time delay, e.g.
20-30 milliseconds. During this interval and prior to the
contacts 102 and 104 switching to the second state, the
velocity of the positioner coil will be reduced substantially
to zero as a consequence of the short circuit across the
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positioner coil. When the contacts switch to the second
state after the 20-30 millisecond interval, the positioner
coil velocity should be substantially zero and the dynamic
braking circuit 110 will be thereafter disconnected from
the positioner coil. Resistors Rll and R13 provide turn-off
bypass for transistor Q6 and triac SCRl respectively.
Capacitor C4 is provided to suppress false triggering of the
triac SCRl due to noise.
After the 20-30 millisecond delay introduced by
the emergency unload relay driver 60 tFigure 1), the switch
contacts 102 and 104 move to their second state to thus
connect the output of the velocity control circuit 116 to
the positioner coii 32. As will be seen hereinafter, the
velocity control circuit 116 operates in two modes. In the
first low voltage low current mode, a voltage on the order
of 4-5 yolts will be established by transistor Q4 on the
velocity control circuit output terminal 120. During this
mode, the positioner coil will be retracted at a controlled
low velocity to move the previously mentioned ramp surfaces
to the fixed cam 44 (Figure 1). As the cam 44 contacts the
ramp surfaces, the coil velocity slows, thereby increasing
the coil current. As will be seen hereinafter, this action
is sensed and switches the velocity control circuit 116 to
a high current, high voltage mode in which the transistor Q4
supplies approximately an 8-volt potential at the velocity
control circuit output terminal 120 to enable the ramp sur-
faces 40, 42 of the support arms to climb the fixed cam 44.
During an unload sequence, the positioner coil current
is sensed at the positive input of the operational amplifier Ul
by measuring the voltage drop across the coil current sensing
resistor R14. ~ current sensing threshold is established by
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the resistor voltage divider network formed by resistors Rl
and R2. When the positioner coil current is below the
threshold established by resistors Rl and R2, the positive
output of amplifier Ul will drive transistor Ql into satura-
tion by forward biasing its base collector junction. ResistorR4 limits the base current to transistor Ql. The voltage
drop across Zener diode VRl essentially establishes the low
output voltage. The voltage across diodes D2 and D3, plus
- that across resistor R6, is nearly cor.lpensated for by the
YDE drop across the Darlington output transistor Q4 plus the
voltage drop across resistor R8. Resistor R8 acts to sense
the output current of the velocity control circuit and operating
in conjunction with resistor R9 at transistor Q3, provides
current limiting in case of a short circuit to ground.
The purpose of transistor Q2 is to supply dynamic
braking to assure that the velocîty of the positioner coil
does not exceed a certain value while being retracted. More
particularly, if the positioner coil velocity is too high,
the back EMF produced by the coil will become more negative
than the control voltage at output terminal 120. As a conse-
quence, transistor Q2 will conduct, thus presenting a low
impedance path to the coil and causing transistor Q4 to turn
off. The velocity of the coil will thus decrease until the
desired velocity is attained causing transistor Q4 to turn
on and transistor Q2 to come out o conduction. The alternate
conduction of transistors Q2 and Q4 (push-pull) acts to main-
tain the positioner coil velocity substantially constant.
The low output voltage mode of the velocity control
circuit, i.e., when transistor Ql is conducting, is designed
to provide the desired positioner coil retract velocity during
an unload ~ituation. The magnitude of the retract control
voltage is, however, normally not adequate to allow the support
14
l~Z~55 77~421
arm ramp surfaces 40, 42 (Figure 1) to retract over the
fixed cam 44 and thus unload the heads. In order for the
ramp surfaces to climb over the cam 44, a much higher
voltage is normally re~uired. The velocity control cir-
cuit 116 provides for the necessary voltage boost byswitching off transistor Ql. ~he key to switching the
velocity control circuit into the high output mode is the
current in the positioner coil 32.
More particularly, during an unload operation,
the head supporting structure will retract at a controlled
low velocity until the unload~ramps 40, 42 encounter the
fixed cam 44 (Figure 1). The velocity of the head supporting
structure will be reduced to zero if there is insufficient
inertia to,allow the support arm ramps to climb the cam 44.
With the velocity of the positioner coil at or near zero, the
current in the coil will rapidly increase. When the coil
current exceeds the threshold established by resistors Rl
and R2 at the input to operational amplifier Ul, the output
of amplifier Ul will switch negative causing transistor Ql to
come out of conduction. The output of the velocity control
circuit 116 will switch to a hlgh negative value, determined
primarily by resistor R7, as transistor Q4 switches into full
conduction. The final voltage value established in output
terminal 120 by the velocity control circuit 116 depends upon
the charge remaining on the emergency unload capacitor C3 and
the VcE drop across transistor Q4.
From the foregoing, it should now be apparent that
an emergency head unload subsystem has been disclosed herein
for ,use in magnetic disk drives for unloading the heads from
the disk surface. The invention is characterized by the
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utilization of a dynamic brake circuit which, in response
to an emergency condition, initially reduces the velocity
of the positioner coil to substantially zero. Thereafter,
the velocity control circuit supplies a relatively low but
constant voltage to the positioner coil constant voltage
to the positioner coil to retract it at a controlled low
velocity. As the coil velocity slows upon the ramps en-
countering the fixed cam, the voltage provided by the
velocity control circuit to the positioner coil is boosted
to move the head supporting structure ramps over the cam.
Although particular embodiments of the invention
have been described and illustrated herein, it is recognized
that modifications and variations may readily occur to those
skilled in the art, and consequently it is intended that the
claims be interpreted to cover such modifications and equiva-
lents.
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