CRASH-PREVENTION FOR DISK DRIVE POSITIONING SYSTEM

Abstract: Excessive actuator speed and damaging head crashes are prevented in a disk drive servo system by providing a velocity safety circuit comprising a velocity detector (1), velocity threshold comparator (2), latch (3), power disable unit (4) and current cut-off unit (5), these units controlled from the latch (3).

Description: CRASH-PREVENTION FOR DISK DRIVE POSITIONING SYSTEM The present disclosure relates to a positioning servo mechanism and more particularly one operating to control a carriage advanced across the high density recording disk. BACKGROUND, FEATURES Workers in the art of making and using servo mechanisms for positioning transducer-carriages, such as those used in a high density magnetic disk drive, are aware of positioning problems and particularly the damage that may occur in case of carriage actuator "run-away" ( translation at unduly high speed) . At "run-away speed," the carriage is very liable to suffer a damaging collision with end stops and is apt to damage the servo mechanism or the carriage, etc. Workers are familiar with means for preventing or minimizing "run-away," for instance, by anticipating a related fault condition developing in the controller or servo mechanism. This invention addresses the problem of unsafe actuator speed. As workers know, the recording transducers carried on such carriages are generally quite fragile and are mounted on flimsy flexures. And, the current trend toward higher density storage on disks has obsolesced the use of relatively slow stepping motor positioners in favor of faster magnetic linear or rotary actuators; e.g., inductive types operating at a speed proportional to current to position heads under the control of positional feedback information and externally supplied demand and control signals. The high cost of present-day heads make "run-away acceleration" and crashes too costly to contemplate, epsecially in light of the relatively simple precautionary avoidance techniques that can be taken. This system addresses these problems. In general, a problem with some other approaches t solving this problem is that they involve a too-sudden deceleration of the actuator, with force as high as 100 Gs. This can induce violent vibration which can in turn cause the flying heads to momentarily dip and crash against the disk surface, damaging themselves and the disk. This can lead to a catastrophic failure where the heads do not recover their flying ability and/or where data stored on the disk is lost. One particular approach to solving this problem is described in European patent application 0131469 to Berti (see USSN 512,873 filed December 7, 1983, now US 4553181) . Here, a specially chosen RC network is interposed between the head position control servo and the head actuator so as to automatically limit maximum head velocity preferably to a value which will prevent damaging head crashes, even when a f ult occurs in the system. Berti does use passive circuit means with just few reliable components; however, component limitations can degrade actuator performance and increase access time; also the system fails to report system faults, which induce "run-away." Also, Berti unfortunately requires a rather expensive, albeit reliable, RC network and also requires a higher servo output voltage to maintain prescribed brief track-to-track access times. Further, the Berti approach does not absolutely prevent head crashes; it only limits head velocity during impact (vs endstop) . Of course, a head crash at any velocity is undesirable; even a few "minor" head crashes can eventually cause serious damage, as workers well know. Another approach to solving this problem is disclosed in U. S. Patent No. 4,375 r 711 where a similar actuator power de-rating circuit is invoked in case of excess velocity. Yet another approach provides an external transducer and associated crash prevention means. In this approach (using closed loop detection of speed) , detection sensitivity will vary across the linear actuator stroke, based on expected speeds. Faults are "reported" in a limited sense. This system can't prevent impact (unless an additional "stopping-circuit" is added) ; and the required external transducer increases the number of components and gives alignment problems, as well as degrading reliability. Also it essentially involves only speed detection; so it must be combined with actuator deactivation and carriage-stopping means to fully meet the crash problem. Yet another approach is given in UK Patent application GB2078030 to Greig (published 23 Dec 1981) involving the provision of saturation detector monitoring the output level of an inhibitable power amplifier, while providing an indication to a timer of amplifier saturation. - _ • - Detection of saturation inhibits input to the amplifier if saturation persists longer than a predetermined period. This system has the virtue of reporting (some) fault conditions; also it will detect an "open coil" condition (and could prevent impact if a "stopping circuit" were added) . However, Greig falls short in using an "open loop" servo wherein, if the failure mode doesn't involve 100% saturation, the timers are reset and restarted — hence actuator speed can still accumulate dangerously. And this system is ineffective where the actuator amplifier is itself faulty. Also, while it detects and disables, it does not stop the carriage. —Invention features: As particularized below, the present system provides a "dynamic brake" which can entirely prevent actuator impact, while automatically removing the brake when speed is sufficiently reduced (permitting external bias circuit to move the carriage to non-data zones) . Also, it involves closed-loop detection and will detect an "open coil" condition. A preferred embodiment compris a velocity detector, a fault detector and associated latc means for disabling the power amplifier to the actuator coil, plus means for absolutely stopping the actuator — an unusual advantageous combination of features . In one of its forms the subject system involves detecting "actual" actuator velocity (e.g., as function of back emf from actuator voice coil) , monitoring this to see when it exceeds a prescribed maximum (Vr._._ * , > Vm<ΞLX ) , shutting-down the current source entirely, before the actuator hits an endstop, by shorting-out current to the actuator. In combination with this, one may also shut¬ down or derate, input to the actuator power supply; e.g., to decelerate actuator, and possibly avoid shutdown. Preferably, a delay is also used to ignore "transients" and to allow the power-derating to bring velocity back to a safe level without shutting-down. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a preferred velocity safety circuit embodiment, with FIG. 2 showing the elements thereof in preferred circuit implementation; while FIG. 3 is a block diagram of an overall disk drive servo system apt for using the subject embodiment. DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 schematically illustrates a velocity safety arrangement, constructed according to principles of this system (implemented in the circuit of FIG. 2) . This and other means discussed herein will generally be understood as constructed and operating as presently known in the art except where otherwise specified and, except as otherwise specified, all materials, methods, devices and apparatus herein will be understood as implemented by known expedients according to present good practice. The arrangement in FIG. 1 principally comprises a control system CS including: a Velocity Detector unit 1 receiving input from an actuator coil (e.g., for disk drive servo, as known in the art); a Velocity Threshold Comparator stage 2 receiving signals from unit 1; Latch means 3 receiving input from stage 2 (as well as a controlling microprocessor) ; a Power Amplifier Disable stage 4 and Delay means 6, both receiving signals (aa) from Latch 3 and a "crowbar Activation" stage 5 control via Delay 6. Velocity Detector unit is adapted to receive values of voice-coil back-emf (from voice coil motor VC driving the linear actuator for a disk drive, as known in the art) and, responsively apply "velocity" signals vv to Velocity Threshold Comparator stage 2. Comparator 2 is particularly adapted to compare signals vv with a preset V = , and when vv > V issue a signa ve. Arrival of signal ve at Latch circuit 3 will indicate that present actuator velocity v exceeds v.„,,. Latch 3 is adapted to be activated in response MAX c to this input, along with prescribed microprocessor activation signals ma as indicated and to responsviely emit "fault-indicating" signals aa, applying these to Power Amplifier Disable stage 4; to a Fault Reporting means 8 (preferably part of the controlling microproces and also to a Crowbar (power cut-off) stage 5 via a prescribed delay circuit 6. Disable stage 4 will be understood as adapted t responsively disable a prescribed power amplifier drivi the actuator. Crowbar stage 5 is adapted to responsive cut-off all power to the actuator and force the carriag to an immediate stop (preferably, including a triac or the like adapted to virtually short the current across coil VC — workers will recognize that a "triac" is an active solid state arrangement that operates like back- to-back low-impedance diodes, impressing a very low impedance, bi-directionally, across a device like the v-coil) . The Fault Reporting means 8 will be used as known in the art; for instance, as present means for reporting a major fault that may require operator intervention, and to set a flag, activate a warning signal, etc. One might summarize the operation as follows: If the actual actuator velocity as detected at Detector 1 exceeds the prescribed maximum, Vmax , this will initiate an output from Comparator 2 to set Latch 3 (assumed enabled by the microprocessor input as well) so Latch 3 will emit fault signals (aa) . Signals aa applied to Power Amplifier Disable stage 4 will disable (or derate) the actuators power amplifier 7 (which energizes voice coil VC) . Fault signals aa are also applied to a Delay 6 which will delay application thereof to Crowbar unit 5 for a prescribed time, sufficient to determine if the "excess-velocity condition" persists, and especially to determine whether the "disable" stage 4 has been able to correct the problem by reducing actual velocity J below Vmax Such may not be the case, particularly when amplifier 7 (or associated circuitry) is the culprit, and responsible for run-away velocity — in which case, after the prescribed delay introduced by unit 6, input is provided to activate Crowbar unit 5 to, essentially, short-out all current to coil VC and immediately bring the actuator to a halt. Note that detector 1 detects unsafe velocity by sensing back-emf from coil VC, thus also detecting whether VC is "open" ( — if VC is "open," its voltage will xise quickly and its current go to zero, so this condition may be interpreted as "fault" — e.g., an open connection or the like) . Back-emf times a constant will yield actual velocity. Conveniently, back-emf can be determined as full coil voltage less the IR drop thereacross as indicated in FIG. 1. After derating of amplifier 7 and/or activation of Crowbar unit 5, the microprocessor will issue a signal resetting Latch 3 as indicated in FIG. 1. Workers will note that Power Amplifier Disable can provide a "dynamic brake" which can prevent actuator impact with the brake automatically removed when, and if, speed is sufficiently reduced (e.g., permitting external bias circuit to move the carriage to a non-data zone) . It involves "closed-loop" feedback. The system also involves a crowbar unit which stops the carriage in its tracks if all else fails — something unique in the art — taking care of Amplifier faults and open-coil situations — also something special in such arrangements. Workers will appreciate that this system may be implemented with a few simple passive, inexpensive, high reliability components (e.g., see circuit in FIG. 2) which do not degrade performance or increase access time. It will be understood that the preferred embodiments described herein are only exemplary, and that the invention is capable of many modi ications and variations in construction, arrangement and use without departing from the spirit of the invention. Further modifications of the invention are also possible. For example, the means and methods disclosed herein are also applicable to other high density disk files, as well as to related systems. Also, the present invention is applicable for enhancing other servo systems.

Claims: To What is claimed is: 1. A disk drive servo system for controllably translating servo arm means across disk tracks using actuator means including drive coil means, CL, this system including a safety arrangement adapted to reduce actuator velocity below prescribed target velocity V , this arrangement including means A for detecting actual actuator velocity V . at any given moment during operation; means B for comparing Vact with target velocity V and responsively providing an alarm output signal s^ when V > V , and means C responsive to output s^ for essentially eliminating the voltage drop across coil means CIr. 2. The invention of claim 1 wherein the arrangement also reduces power input to coil means CL, responsive to appearance of output s _H_ to decelerate the actuator means. 3. The invention of claim 2 wherein Latch means L is provided at the output of compare means B, being adapted, responsive to receipt of signal s _τi,, to cause said reduction of power-input to coil means CL as well as virtually shorting-out current to coil means CL afteraprescribed delay. - 1 I - 4. The invention of claim 3 wherein Power Amplifier means PA is included in the system for providing a controlled driving power to coil means CL: and wherein, said reduction of power-input is effected by an Amplifier Disable portion AD, of the arrangement operating responsive to Latch means L. 5. The invention of claim 4 wherein Latch means L is adapted, responsive to receipt of alarm signal S. from Compare means C, to issue a trigger signal tt to initiate said power reduction by portion AD and said coil shorting. 6. The invention of claim 1 wherein means C includes means CR for virtually shorting-out the current across coil means CL. 7. The invention of claim 6 wherein means CR comprises bi-directional low-impedance means Z arranged to be selectively coupled across the coil means, responsive to application of alarm signal S-, after a preset delay. 8. The invention of claim 7 wherein means Z comprises a "TRIAC." 9. The invention of claim 7 wherein means CR also includes delay means DL coupled at the input to means Z so as to introduce said preset delay. 10. The invention of claim 9 wherein the arrangement also reduces power input to coil means CL, responsive to appearance of output s, to decelerate the actuator means. Wherein A Latch means L is provided at the output of compare means B, being adapted, responsive to receipt of signal s A , to cause said reductio of power-input to coil means CL as well as virtually shorting-out current to coil means CL after a prescribed delay. Wherein Power Amplifier means PA is included in the system for providing a controlled driving power to coil means CL; and wherein, said reduction of power-input is effected by an Amplifier Disable portion AD, of the arrangement operatin responsive to Latch means L. Wherein Latch means L is adapted, responsive to receipt of alarm signal s A from Compare means C, to issue a trigger signal tt to initiate said power reduction by portion AD and said coil shorting. 11. A method of providing improved servo control t a disk drive servo system adapted to controlla translate servo arm means across disk tracks using actuator means including drive coil mean CL, this system including a safety arrangement adapted to reduce actuator velocity below prescribed target velocity V , this method including: detecting actual actuator velocity, VclC " C; comparing V with a prescribed target veloci V, and responsive to V > V issuing an alar signal s t - and using alarm signal s to essentially short-out current across the coil means CL. 12. The invention of claim 11 including the step of: reducing power-input to coil means CL responsiv to issuance of alarm signals s , and for a prescribed warning period tw before later shorting out said current.

Inventors: REVELS WILLIAM R, ROSE ANDREW M, REVELS, WILLIAM, R., ROSE, ANDREW, M.
Applicants: UNISYS CORP, UNISYS CORPORATION
Application year: 1988
Main classification: G11B21
IPC codes: G05D3, G11B19, G11B21