AU525976B2 - Emergency head unload system for magnetic disk drive - Google Patents

Description

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APPLICATION FOR A STANDARD PAT2ENT

4887 1'79 We, PERTEC COMPUTER CORPORATION, of 12910 Culver Boulevard, LJos Angeles, California 90066, United States of' America, hareby apply f'or the grant of a standard patent for an invention entitlerl: APL kT@ AC TE AN aMEDMN

"Eg.=jC H.EAD, WOIAD SYSTEM F_

which is described the accompanying Complete Specification.

0~Details of basic application:- oat" Number: 927,763

Country: C U.S .A.

00n Date: 25th July, 1978. 0 0

0 01Our address for ser~vice is:- SHESTON WATERS, 00,0

Clarence Street, SYDNEY. N.S.W. 2000 000 D T D t i 0 2 h d y o U Y 9 9

0

PERTEC COMPUTER CORPORATION 0*

Focllow Institte cf Pateti Attorneys of Austrafia ot S HELSTON 'WATERS S To: The Commissioner of Patents, WODEN. A.C 6 File: D._Th 154 gf PT IF

Fee: $64.00

W7~7, -1

PATENTS ACT, 1952-16 7 DECLARATION IN SUPPORT OF A COt'VENTION APPL.ICATION FOR A PATENT in support of the Convention Application made Hae Insert (in full) by(a) Cor~ra ie Naeof Cosnh.any. byxa 4.1881...7 (hereinafter referred in as "Applicant") for a patent for an invention enth~qd: tb) Here insert Title of EMERGENCY HEAD UN'LOAD SYSTEM FOR MAGNETIC DISK DRIVE

and Here ins- l, c) sert Full Name end Vi.0?eAPri... eAn... Gen.exa. uxsel1 Address of Conrnany official euthormsad to mrrke declareall n.

of .Culm n.oileyxard Los Angeles, California 90066, U.S.A. do solemnly and sincerely declare as fol~ows: 1. 1 am authorised by Applicant to nmake tiis declaration on its behalf. 2. The basic Application(s) as defined by section 141 of the Act was/were lot Here Insert Basic C-utyor Co ntries made in on of 19. 7 ~otwd by date or on the dy of d a te o f Basic

Here Insert Full1 Name(s) of Applicant(s) by M na...Ba in Basic Country,

"ae endInsert (in fuill) 3. e~i amj!i Neme Address of actual Inventor or Inventors.

of td,. NlaJr;~~ Qlfana 3744 Mc-ntuso Place, Encino, California, resp,.i/r I r

the actul lnventor.Uof the invention and the f&i-s upon which Applicant entitled to make the Application are as follows, Applicant is the Assignee of the said Inventor(s).

4. The basic Application(s) referred to in paragraph 2 of this Declaration was/were the first Application(s) made in a Convention country in respect of the invention, the subject of the Application.

DECLARED at Lo.j Angeles... Nb Personel Signature th

s day of 1MR 9 79 of Declarant (no ~I I

legaliSation)

(h)

Richard K. Hansen

Vice President-General Counsel

To THE COMMISSIONER OF PATENrfS.

SHELSTON WATERS

PATEnT AOEs_

1 $r

I BACKGROUND OF THE INVENTION

This invention relates generally to magnetic disk drives and more particularly to an improved emergency head

(12) AUSTRALIAN PATENT ABRIDGMENT (19) AU (11) AU-B-48 873/79

(54) EMERGENCY HEAD UNLOAD SYSTEM FOR MAGNETIC DISK DPIVE (71) PERTEC COMPUTER CORPORATION (21) 48 873/79 525 976 (22) 1?-7.79 (24) 2 78 (31) 927763 (32) 25.7.78 (33) US (43) 7.2.30 (44) 9.12.82 (51) 2 HO2H 7/00 G11B 5/40 (72) EVAN LESLIE BARMACIIE AND ABRAHAM BRAND (74) SW (57) Claim 1. An emergency head unload system for a magnetic disk drive including a disk having a magnetic surface and a motor having a positioner coil for radially moving a head support strtcture along said disk surface and 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 sta+e to connect said first input terminal to said positioner coil or a second state to connect said second input terminal to said positionar coil; f:lectricalbrake circuit means connected to said first input terminal and respooisive 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 front said first to said second state after a predetermined time delay;

./2

01 46* 2

*116/70/17

positioner coil. When the contacts switch to the second

state after t 20-30 millisecond interval, the positioner

coil velocity should be substantially zero and the dynamic

'V n

AU-B-48 873/79 2- (11)

circuit means connected to said second velocity control input terminal for supplying a substantially constant current +hrough 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 therto during said emergency condition.

*116/70/18

the resistor voltage divider network formed by resistors R1

and R2. When the positioner coil current is below the

threshold established by resistors R1 and R2, the positive

nur-htift- nff rmnl i f4 i T1 i4l A 'I -Tw

FORM COMMONWEALTH OF AUSTRALIA PATENT$ ACT 1952-6P

COMPLETE SPECIFICATION 4887 3/79 FOR OFFICE USE: tnt. Class Application Number: Lodm~r

Complete Specification Lodged: *0 A%epted: Published: -14 Priority:

'R1elated Art:

0~

Name of Applicant: PEIATEC COMPUTER CORPORATION

.Address of Applicant: 12910 Culver Boulevard Los Angeles, Calif ornia 90066 U.S.A.

Actual Inventor: EVAN LESLIE BARMACHE ~a ABRAHAM BRAn~

Address for Service: Shelston Waters, 163 Clarence Strecit, Sydney

Complete Specificoiot for the Invention entitled: t EMERG-ENCY HEAD UNLOAD SYSTEM FOR MAGNETIC DISK DRIVE' tf

The followft~ st~ernent Is a full description of this Invention, Itipludlng the best method of performing It krnown to nie/u*:-

BACKGROUND OF THE INVENTION

This invention relates generally to magnetic disk

drives and more particularly to an improved emergency 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 iounte within a mag-

netic field for linear novement and oriented relative to the disk to move

the heads radially over the disk surface to thereby enable the

heads to be positioned over any annular track on the surface.

The heads are designed to actually fly above the disk recording

surface at heights of less than 100 microinches. If, during

S normal operation of the drive, power is lost causing the disk

rotational speed to gradually decrease, the heads cannot con-

tinue to fly and would ultimately crash into the disk surface.

o In order to protect the data, the heads and the disk, it is necessary 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 .4

iaxially .O 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:

Disk speed does not remain within tolerance;

Positioner error is detected;

Write circuit faults that .ould effect stored

data are detected.

2

utilization of a dynamic brake circuit which, in response

to an emergency condition, initially reduces the velocity

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. Upon

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-

,°o°iS tant voltage across the positioner coil causing the coil and

support structure to accelerate as it is moving toward the

disk.outer edge. As 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

P 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 acc. dance 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

'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.

In accordance with/fee dile 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 noves the heads in an

axial direction away from the disk surface.

The novel features of the invention are set forth

with particularity in the appended claims. The invention

o will best be understood from the following description when

read in conjunction with the accompanying drawings. C

15 BRIEF DESCRIPTION OF THE DRAWINI

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

A-.tention is initially directed to Figure 1 which

schematically illustrates a magnetic disk drive 10 which

includes a spindle 12 supporting 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-replaceable.

The .spindle 12 is driven by a spindle motor '8 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

structvre 22 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

2. The apparatus of Claim 1 wherein said brake

i circuit means includes means for creating an electrical

3 short across said positioner coil.

12 r f ric ('1m 1 int-11iirii0 meabns fnr 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 h1 i ;ds prior to

crashing. The process of unloading the heac."- .ieerally 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, iyove 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

4 as the support structure 22 is pulled by the ypsitioner

coil 32 to the left (Figure The engagement of the fixed

cam 44 against the ramp surfaces 40, 42 will move +he 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. The dashed lines illustrate modifications to

such a prior art system in accordance with the present inven-

tion, as is furtner detailed in Figure 2.

Initially considering the typical prior art system

represented in solid line in Figure 1, an emergency unload elay 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 5' 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 toe emergency

unload enable line 62. More partiou.av' v mejrs 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

sw tch 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.

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 p,'wer amplifier 64 which receives as its input

the output of a summing amplifier (not shown). The summing

amplifier typically provides the 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 enab.e line 62, the

emergency unload relay 50 is switched to its second state to

connect contact 52 to input terminal 56. This action disconnects

8

Li hT-~ the power amplifier 64 from the positioner coil 32 and insg(Ad 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

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 quickly as possible. If, at the time the emergency

occurs, the heads are over an inner track 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 28, 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 for 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 inven.ion.

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 detection of

an emergency command signal on enable line 62, the emergency

unload system 70, in accordance with the invention, activates

2; 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

2 signal is recognized, the effect of the electrical short will

be to reduce the coil velocity to substantially zero. It slould.be

o 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 seeking 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 velocity control circuit then supplies a current

i; on, 116 wl amic to the positioner motor coil 32. Initially, the velocity the er of control circuit provides a substantially constant current 114 is to the coil to retract the heads at a controlled low invent he velocity which is insufficient to permit movement of the output ramps 40, 42 past the fixed cam 44. As the coil velocity capaci put slows upon engagement of the cam 44 against the ramps 40, Figure 0 has 42, the coil current increases. The coil current is sensed conjun er- by the current sensor 68 which, via line 82, switches the chr rge of velocity control circuit into a high current mode to supply 10 normal ncy sufficient energy to the positioner coil to enable the dur ;l:n ates ramps 40, 42 to move past the fixed cam 44. pow r trical Attention is now directed to Figure 2 which illus- and ti otor trates a detailed embodiment of the emergency unload system ent 70, as well as the emergency unload capacitor 72, the einer- signa.1 ross *01 gency unload relay 50, the positioner coil 32, and the current 62. 11 e OW orce sensor 68, all of Figure 1. In Figure 2, the emergency unload Iion i v dless t relay 50 is illustrated as comprising a solenoid 100 operating by res mmand a double pole double throw switch. The double pole double ircui will throw switch includes blade contacts 102 and 104. During clonto slould. be -0 normal operating conditions, the blade contacts 102 and 104 turnin mergency are in the solid line position illustrated respectively con- track, tacting input terminals 106 and 108. Input terminal 106 is sh,,.vit or connected to the output of the power amplifier 64 of Figure 1. inlput cir- Input terminal 108, as will be seen hereinafter, is connected oil 25 to the output of the dynamic brake circuit 110 which comprises uxloiiI6 intro- a portion of the emergency unload system 70. 20-30 ncy In the second state of the double pole double throw ncy switch means, the blade contacts 102 and 104 moe into contact rrent with.input terminals 112 and 114 respectively. Input terminal 30 to z li 112 is connected to the output of the velocity control circuit

116 which, as has been previ~usly mentioned, forms part of the emergency unload syste. '70 of Figure 1. Input terminal

1.14 is not utilized in the disclosed embodiment of the

invention. Figure 2 also ilustrates the emergency unload I capacitor 72 previously mentioned in the description of~ Figure 1. Zener diode VR2 and resistor R5 operate in red conjunction with the emergency unload capacitor 72 (C3) to

B ~charge the capacitor to substantially -12 volts during ?ly19 normal. operating conditions. As will be seen herni itafter,

dur.ug 7-mergen-l,-, c-!nditions, the capacit~n. 72 p~rovi~as the

pow%'Ic no'es.,ary to cye,,arte the dyn~aynic braking '-ir'cuit 110 Llus-and the velocity colwtroi. circuit 11L6. tem Durina a~t- Qmergevicy condition, a high logic leliel

1 will be applie~d to the emergency unload enable line .irrent 62. This act:ion catses transistor Q5 to turn on. This ac-

inload tion in turn provides base drive for transistor Q6 controlled cating by resistor R10. When trctxsistor Q6 conducts, -the qa.te L~e circuit of the triac SCRI wa.1l rece-r? a negative bias ci=rzent

a' controlled primnarilyj by the value of resisLor R12 thereby L04 turnng the triac on. As a consequettce, the triac SCRI Will ~O'C4a very l~resistance (es%-eiti'iily an eleolhrical shoxt)

is shiat path across %'ke positioner coL. 32 via the stwitch means Lire 1. inpwivc. terminal 108 and blade contact 104. sacted 25it has previously been mentioned that the emerg~ency unloi,,v relay driver 60 provides a certain time delay, e.g.

20-3~0 mdi.seconds. During this inti7)rval and prior to the throw contaO.. r, 102 and 104 switching to the second state, thei o)ntact vliVof' th~e positioner coil will, be reduced stlbstant.ially rminal 30 to zoro ao a consequ~tmce ofl the short circuit acros,,, the

I ircuit

12

487 SI79

*116/70/17

positioner coil. When the contacts switch to the second

state after 4 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 qCR1 respectively.

Capacitor C4 is provided to suppress false triggering of the

triac SCR1 due to noise.

After the 20-30 millisecond delay introduced by

the emergency unload relay driver 60 (Figure 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 coil 32. As will be seen hereinafter, the

velocity control circuit 116 operates in two modes. In the

first low voltage low current mode, a voltat-i on the order

of 4-5 volts 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 elocity to move the previously mentioned ramp surfaces

to Ll fixed cam 44 (Figure 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 volt~ge mode in which the transistor Q4

supplies approximately an 8-volt potential at the velocity

contxol circuit output terminal 120 to enable the ramp sur-

fPces 40, 42 of the support arms to climb the fixed cam 44.

During an unload sequence, the positioner coil current

is -ensed at the positive input of the operational amplifier Ul

by meaurinL the voltage drop across the coil current sensing

resistor R.14 A current sensing threshold is established by

L.

the'resistor voltage divider network formed by resistors Rl

and R2. When the positioner coil current is below the

threshold established by resistors R1 and R2, the positive

output of amplifier Ul will drive transistor Q1 into satura-

tion by forward biasing its base collector junction. Resistor

R4 limits the base current to transistor Ql. The voltage

drop across Zener diode VR1 essentially establishes the low

output voltage. The voltage across diodes D2 and D3, plus

that across resistor R6, is nearly compensated for by the

VDE drop across the Darlington output transistor Q4 plus the

voltage drop across resistor R8. Resistor R8 acts to sense

te output current of the velocity control circuit and operating

in conjunction with resistor R9 at transistor Q3, provides c current limiting in case of a short circuit to ground.

*15 The purpose of transistor Q2 is to suipply dynamic

braking to assure that the velocity of the positioner coil

does not exceed a certain value while being retracted. More particularly, if the positioner coil ,elocity 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. Thf 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 of conduction. The alternate

conduction of transistors Q2 an, (push-pull) acts to main-

tain the positioner coil velocity substantially constant.

The low output voltage mode of the velocity control

circuit, when transistor Q1 is conducting, is designed

to provide the desired positioner coil retract velocity during

an iload situation. The magnitude of the retract control

vctage is, however, normally not adequate to allow the support

SW16

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 required. The velocity control cir-

cuit 116 provides for the necessary voltage boost by

switching off transistor Ql. The key to switching the

velocity control circuit into the high output iode 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 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 th= 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 the input to operational amplifier Ul, the output

of ampli.ier U1 will switch negative causing transistor Q1 to

come out of conduction. The output of the velocity control

circuit 116 will switch to a high 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

k^jj'

T'm

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 .s boosted

to move the head supporting structure ramps over the cam.

Although particular -mbodiments 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

clqims be interpreted to cover such modifications and equiva-

lents.

Claims (2)

1. if m 1

   THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

   i. An emergency head unload system for a magnetic 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 and 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

   i0 state to connect said first input terminal to said positioner

   coil or a second state to connect said second input terminal

   to said positioner coil;

   electrical brake circuit means 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 supplying a substantially constant

   current through said switch means to sid 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.

   17

   k

   -y

   S The apparatus of Claim 1 wherein said brake

   circuit means includes means for creating an electrical

   3 short across said positioner coil.

   3. The apparatus of Claim 1 including means for

   2 sensing the current level in said positioner coil;

   3 said velocity control circuit means including

   4 means responsive to said positioner coil current exceeding

   a certain level for increasing the current supplied by said

   velocity control circuit means to said positioner coil.

   1 4. The apparatus of Claim 1 further including

   2 means responsive to said positioner coil velocity exceeding

   3 a predetermined velocity for reducing the velocity thereof.

   1 5. The apparatus of Claim 1 wherein said velocity

   2 control circuit means further includes means responsive to

   3 said positioner coil velocity exceeding a predetermined

   4 velocity for modifying said current supplied to said coil to

   reduce the velocity thereof.

   1 6. An emergency head unload system for a magnetic

   2 disk drive including a positioner coil coupled to a support

   3 structure carrying a head, said support structure including

   4 a ramp surface positioned to engage a fiyed cam as said

   support structure is moved radially outwardly over a disk

   6 surface to move said head axially with r- ect to said surface,

   7 said system comprising: means for supplying an emergency signal indicative of an emergency condition; means for producing a very low-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 for supplying electrical power from said emergency power supply means to both said means for producing a shunt path and said velocity control means during said emergency condition.

   7. The system of claim 6 wherein said velocity control

   means includes means responsive to the velocity of said

   positioner coil increasing above a certain level for

   modifying the current in said positioner coil to reduce the

   velocity thereof.

   8. The system of claim 6 wherein said velocity control

   means includes means responsive to the velocity of said

   positioner coil decreasing below a certain level for

   modifying the current in said positioner coil to increase

   the velocity thereof.

   9. The system of claim 6 including means for sensing the

   current level in said positioner coil and responsive to said

   level exceeding a predetermined level for substantially increasing the current supplied by said velocity control means to said positioner coil to move said ramp surface pver said fixed cam,

   The apparatus of claim 1 wherein said emergency power

   supply means includes a capacitor and means for charging said

   capacitor during normal operating conditions of said disk

   drive.

   0R 11. The apparatus of claim 6 wherein said emergency power 0e 0

   t0 supply means includes a capacitor and means for charging

   said capacitor during normal operating conditions of said

   disk drive.

   DATED this 21st day of September 1982

   I PERTEC COMPUTER CORPORATION

   ,ucing Atiorney: ROBERET C. S.ELSTON i+itOw Institute. of P.atrtAttorneyo of Austrtdb' of SHELSTON WATERS

   he said

   20

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   -43 LA)

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