Method and apparatus for adaptive feedforward control with reduced noise during track seek operations

An adaptive feedforward control system for reducing noises during a track seek operation. A servo control apparatus of the invention generates a first adaptive feedforward, in which the first adaptive feedforward adaptively follows a current command for servo-controlling the hard disk drive at a servo control decelerating interval for the track seek, and the first adaptive feedforward has a linear slope. Further, a high frequency component is removed from the first adaptive feedforward. Therefore, it may be possible to remove the current control error as well as the noises during the track seek operation.

CLAIM OF PRIORITY 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.119 from an application 
for ADAPTIVE FEEDFORWARD CONTROL METHOD FOR REDUCING NOISE DURING TRACK 
SEEK OPERATION earlier filed in the Korean Industrial Property Office on 
the 2nd day of Sep. 1996 and there duly assigned Serial No. 1996-37941, a 
copy of which application is annexed hereto. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a hard disk drive and, more 
particularly, to a method and apparatus for performing adaptive 
feedforward control during track seek operations with the disk drive. Even 
more particularly, the present invention is directed toward such a method 
and apparatus that perform adaptive feed forward control with reduced 
noise generation. 
2. Description of the Related Art 
The present invention uses an adaptive feedforward device to provide 
improved servo control of a disk drive head actuator during seek 
operations. It applies adaptive feedforward control during the 
deceleration phase of a seek maneuver to reduce the current gain needed to 
carry out the maneuver, thereby improving system performance by reducing 
current control errors. 
It has been found, however, that application of feedforward current control 
signals specifically to the deceleration phase of a seek operation tends 
to induce a certain level of noise in the servo control signals generated 
by the servo control apparatus. This noise occurs as the apparatus shifts 
from acceleration to deceleration and appears as a rapid variation or 
oscillation in the current control commands of the servo apparatus at the 
shift point. 
General servo control technology has followed parallel and occasionally 
intersecting paths for improvements in feedforward control and noise 
compensation, respectively. U.S. Pat. No. 3,940,594, for example, shows a 
process controller that eliminates mode-switching noise by the rather 
direct expedient of freezing the actuator command signal and then 
gradually mixing the target state signal through a settle circuit. 
Alternatively, U.S. Pat. No. 3,961,234 shows a system for industrial 
process control using electrical actuators that adaptively filters 
measurement noise in a high-gain feedback loop. Here the system avoids 
dithering due to noise by limiting the time allowed for control signals 
when the error measurement signal is small. 
The disk drive servo control arts, in particular, have also confronted the 
general problem of noise arising in control systems. For example, U.S. 
Pat. No. 4,642,541 discloses a servo system that uses a filter network in 
track following mode to generate a substantially noise-free position error 
signal. The significance of this improvement does not extend far beyond 
its direct context, however, because it does not consider other noise 
processes such as mode-transition noise. This patent also does not 
consider the use of adaptive control, noise reduction in seek operations, 
or noise arising in control signals themselves. 
U.S. Pat. No. 4,893,068 for a Digital Employing Switch Mode Lead/Lag 
Integrator to D. D. Evans, shows a sophisticated digital servo control 
system employing an integrator located outside the conventional feedback 
control loop. This configuration permits the integrator to be applied to 
both velocity error signals in seek-type operations and position error 
signals in position holding operations. The system mitigates perturbations 
arising from switching between velocity and position error integrator 
inputs (i.e., mode-switching noise) by mixing the output from the previous 
integrator cycle with the output from the current integrator cycle. 
Unfortunately, the rather specialized structure employed in the disclosed 
system does not easily lend itself to application in other servo control 
contexts. It is not clear how one would adapt the system of this patent to 
mitigate current oscillations arising from an adaptive control feedforward 
network while not effectiveness of the feedforward signal. 
The disk drive control arts have explored the uses of adaptive control in 
some contexts, including to minimize transients arising from shifts 
between track seeking and track following modes. U.S. Pat. No. 4,697,127 
shows an adaptive control technique that estimates the forward gain of an 
open loop system to compensate for variations in plant parameters. This 
system minimizes seek-follow transition transients by applying, during 
seeking, a velocity command profile selected to match the normal modes 
(i.e., eigenvectors) of the system in track following mode. Prospective 
selection of seek command profiles enables the system to blend the end of 
the seek operation with the beginning of the track following operation, 
thus preventing the generation of transient components in the system 
response. 
The sophisticated approach taken in the '127 patent also unfortunately 
limits the extent to which the disclosed system can be adapted to other 
contexts. First, its use of forward gain estimation and accordant 
selection of seek command profiles necessarily depends upon a 
sophisticated plant model and substantial computation to achieve its 
desired results. The disclosed system also considers only process noise 
(such as might arise from resonances, unmodeled bearing drag, and so 
forth) and measurement noise. In particular, it does not consider the 
constellation of problems arising from noise induced in the control signal 
itself. 
U.S. Pat. No. 5,128,812 provides a disk drive servo control system that 
eliminates high frequency components from the velocity error signal to be 
amplified in a feedback loop. The previously existing configuration 
included a low pass filter to remove undesired high frequency components 
of the velocity difference signal in a feedback control loop. The 
improvement of the patent consists in a filter with variable cutoff 
frequency to minimize the phase lag the filter introduces into the error 
signal while maintaining stability of the system. 
The system of the '812 patent provides an important improvement directed 
toward a specific problem in servo control, but it also carries several 
limitations. The careful balance it observes, between reducing phase lag 
and preserving system stability, rather clearly acknowledges the 
considerable complications that are known to arise from employing low pass 
filter networks in servo control loops. Simple filtering appears, from 
this patent, to have been abandoned by the art, at least for processing 
servo control signals. This patent also does not consider the mitigation 
of transient components in the context of feedforward loops or through 
digital signal processing techniques. 
The known approaches to eliminating noise from signals in servo control 
systems thus have not offered an obvious solution to the transient noise 
problem I have noticed in using adaptive feedforward control to reduce 
current control errors. Such a solution should be easily and economically 
implemented in the adaptive feedforward device of my previous invention 
and should effectively eliminate the control signal noise that arises 
there. It should not hinder system stability and also should not reduce 
the effectiveness of the control system in the way that low pass filters 
reduce the effectiveness of velocity feedback systems by introducing phase 
lags. Desirably, this noise control solution should be as simple as 
possible so that my adaptive feedforward control system employing this 
solution will be applicable in as many differing contexts as possible. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
process and apparatus for controlling hard disk drives. 
It is another object to provide, in the context of servo control systems 
for hard disk drives, an apparatus and method for adaptive feedforward 
control with reduced noise. 
These and other object may achieved in the practice of the present 
invention provides in a first aspect a servo control apparatus utilizing 
adaptive feedforward control, said apparatus comprising a velocity command 
device adapted to receive a position indicating signal and to generate 
therefrom a velocity command signal; a velocity error signal generator in 
communication with the velocity command device and adapted to receive the 
velocity command signal and a velocity indicating signal and to generate 
therefrom a velocity error signal; a gain function device in communication 
with the velocity error signal generator and adapted to receive the 
velocity error signal and one or more plant state variable signals and to 
generate therefrom a current command signal, with each one of the one or 
more plant state variable signals representative of a value of a plant 
state variable of a plant undergoing servo control by the apparatus; a 
current control signal generator in communication with the gain function 
device and adapted to receive the current command signal and an adaptive 
feedforward signal and to generate therefrom a current control signal for 
control of the plant; and an adaptive feedforward device in communication 
with the velocity command device and the current control signal generator 
and adapted to receive the velocity command signal and to generate 
therefrom the adaptive feedforward signal, with the adaptive feedforward 
device including a feedforward calculator and a low pass filter and with 
the low pass filter adapted to substantially prevent high frequency signal 
components from arising in the current control signal. 
The present invention also provides, in a second aspect, feedforward 
control method for providing an adaptive feedforward signal which reduces 
a search noise and a current control noise during a servo control process, 
the method comprising the steps of: generating a first adaptive 
feedforward signal adaptively following a current control signal for servo 
control of a plant in a deceleration phase, the first adaptive feedforward 
having a linear slope; removing a high frequency component from the first 
adaptive feedforward signal to generate a second adaptive feedforward 
signal; and applying the second adaptive feedforward signal to a current 
command signal to generate the current control signal. 
According to an aspect of the present invention, a feedforward control 
method for providing a feedforward which reduces a search noise and a 
current control noise during a servo control in a hard disk drive includes 
the steps of: generating a first adaptive feedforward adaptively following 
a current command for servo-controlling the hard disk drive at a servo 
control decelerating interval for the track seek, the first adaptive 
feedforward having a linear slope; and removing a high frequency component 
from the first adaptive feedforward, to generate a second adaptive 
feedforward.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is an improvement upon the applicant's invention 
disclosed in the copending U.S. patent application, Ser. No. 08/840,603, 
entitled "Adaptive Feed Forward Device for Reducing Current Control 
Errors" and filed Apr. 30, 1997, the disclosure of which is incorporated 
herein by reference. The invention of that application provides for 
improved servo control of a disk drive head actuator during seek 
operations. It uses an adaptive feedforward device during deceleration of 
the head to reduce current gains and thereby to reduce current control 
error. These features have constituted a significant advance over the 
previously existing servo control technology. 
The referenced invention also has certain weaknesses the elimination of 
which would provide even greater performance improvements over previously 
existing systems. Its principal weakness arises precisely from its 
operation to provide adaptive feedforward control. In particular, the 
adaptive feedforward device adapts its feedforward operation to the 
dynamic state of the plant (for example, the actuator-head combination) 
undergoing servo control. The adaptation action includes applying 
feedforward control signals specifically when the plant is executing a 
deceleration maneuver in a later stage of a seek operation. These 
feedforward signals are subtracted from feedback control signals of the 
servo control apparatus, thereby reducing the current gain required for 
the apparatus to carry out high performance servo control. 
FIG. 1 is a block diagram illustrating one embodiment of the invention of 
the applicant's copending application referenced above. The illustrate 
system is a servo control apparatus for adaptive feedforward control in a 
hard disk drive. The exemplary servo control apparatus shown in this 
figure includes a plant 2 which is an object of servo control, an 
angle-to-position converter 4, a digital signal processor (DSP) 6 for 
providing servo control signals with respect to plant 2, a 
digital-to-analog converter (DAC) 8, and an adder 10. 
Plant 2 has, for example, a transfer function 1/s.sup.2 representing the 
characteristic response of a DC motor in terms of a Laplace transform. 
Plant 2 generates angle information .theta. and provides angle-to-position 
converter 4 with the angle information .theta.. Angle-to-position 
converter 4 converts the angle information .theta. into a position 
information signal X and provides this signal to DSP 6. 
DSP 6 includes, for example, an estimator 12, a velocity command device 14, 
an adder 16, a gain function device 18. DSP 6 also includes an adaptive 
feedforward device 100 for reducing current control error. Estimator 12 
receives the position information signal X, generated by angle-to-position 
converter 4, and a precious current control signal U(n-1). U(n-1) is 
generated by adder 38, which subtracts the output of adaptive feedforward 
device 100 from the output of gain function device 18. Estimator 12 
generates a position estimate signal X1, a velocity estimate signal X2, 
and a disturbance estimate signal X3 in accordance with a predetermined 
estimate calculating function. 
Velocity command device 14 generates a velocity command signal X2.sub.-- 
cmd in response to the position estimate signal X1. Adder 16 receives 
velocity command signal X2.sub.-- cmd subtracts from it the velocity 
estimate signal X2 to generate a velocity error signal Verr. Gain function 
device 18 receives the velocity error signal Verr, the position estimate 
signal X1, and the disturbance estimate signal X3 and from them generates 
a current command signal for servo control of plant 2. 
Adaptive feedforward device 100 operates during the servo control 
deceleration phase of track seek operations. It generates a deceleration 
command signal DCS (an adaptive feedforward) corresponding to the current 
command signal for the deceleration phase. The output of gain function 
device 18 thus need only provide a signal determined by subtracting the 
deceleration command signal DCS from the desired current command signal U. 
Therefore, gain function device 18 can undertake current control with 
respect to plant 2 while using a low gain coefficient. It follows, 
moreover, that the current command signal output from gain function device 
18 will have reduced current control error by virtue of the contribution 
of adaptive feedforward device 100. 
Adaptive feedforward device 100 comprises a delay unit 30, an adder 32, a 
feedforward calculator 34, and a switch 36. The output signal DCS of 
adaptive feedforward device 100 can be represented by the following 
Equation (1): 
##EQU1## 
in which J: actuator inertia, 
K.sub.T : torque coefficient, 
A.sub.rml : arm length, 
V.sub.max : maximum velocity of motor 
I.sub.max : maximum current supplied to motor, 
.DELTA.t: servo sampling interval, and 
.DELTA.V: velocity command (n)-velocity command (n-1). 
FIG. 2 illustrates characteristic curves of the control variables in terms 
of Equation (1). It is noted from the drawing that the adaptive 
feedforward curve 200, representing the time evolution of the deceleration 
command signal DCS, adaptively follows the current command signal curve. 
On the other hand, the adaptive feedforward curve 200 produces a current 
command curve that is not smooth over the entire deceleration interval. In 
particular, the current command curve exhibits a rapid variation, 
corresponding to rapid changes in acceleration of plant 2, in a region A. 
A preferred embodiment of the present invention will be described in detail 
hereinbelow with reference to FIGS., 3 and 4, in which like reference 
numerals used throughout the specification represent like elements. Also, 
it should be clearly understood that many specifics such as the particular 
components of the disclosed device are shown only by way of example to 
provide a better understanding of the present invention. As will be 
readily understood by persons of skill in the art, the present invention 
may be embodied without conforming with the specific details disclosed 
herein. Moreover, it should be noted that a detailed description of 
standard components has been intentionally omitted where it is believed 
that such description is unnecessary to, and may obscure a full 
understanding of and appreciation for, the concepts of the present 
invention. 
FIG. 3 provides a block diagram of a servo control apparatus according to 
the present invention. The apparatus of FIG. 3 is differs from the 
apparatus of FIG. 1 in the structure of the adaptive feedforward device 
150. That is, unlike the earlier apparatus, adaptive feedforward device 
150 according to the present invention further includes a low pass filter 
40 interposed between feedforward calculator 34 and switch 36. 
Notwithstanding the clear indications to the contrary that exist in the 
known art, it has been found that low pass filter 40 effectively removes 
high frequency components from the adaptive feedforward curve 200 shown in 
FIG. 2, thereby smoothing it, without degrading the fidelity with which 
the feedforward curve follows the current command curve. This effect is 
made even more remarkable by the fact that the direct action of low pass 
filter 40 occurs before the output of adaptive feedforward device 150 is 
combined with the current control signal by adder 38. Thus, low pass 
filter cannot be said to merely remove high frequency components generated 
in the signal addition process of adder 38. 
A first adaptive feedforward signal FF.sub.1 generated by the feedforward 
calculator 34 of FIG. 3 can be represented by the expression of the 
following Equation (2): 
##EQU2## 
If the first adaptive feedforward signal FF is low-pass filtered by low 
pass filter 40, then a high frequency component (1-a)/(1-aZ.sup.-1) will 
be removed therefrom. Low pass filter 40 thus generates a second adaptive 
feedforward signal FF.sub.2 that can be represented by the expression of 
the following Equation (3): 
##EQU3## 
where .alpha. represents a low pass filter coefficient. 
FIG. 4 illustrates a set of characteristic curves, similar to those shown 
in FIG. 2, wherein the reference numeral 300 indicates a curve 
representing the time evolution of the second adaptive feedforward signal 
FF.sub.2. It is easily appreciated from FIG. 4 that the second adaptive 
feedforward curve 300 is smooth throughout the entire deceleration 
interval. In particular, the current command signal does not show a rapid 
variation from an acceleration to a deceleration in a region A' 
corresponding to the region A of FIG. 2. Moreover, comparison of FIG. 4 
with FIG. 2 shows that the second adaptive feedforward curve 300 retains 
or even improves upon the fidelity with which the corresponding adaptive 
feedforward curve 200 follows the current command curve. 
Returning to FIG. 3, the second adaptive feedforward signal FF.sub.2 output 
from adaptive feedforward device 150 is applied to adder 38 when switch 36 
is closed during the servo control deceleration phase of track seek 
operation. Switch 38 is closed in response to a feedforward control signal 
FFC. Adder 38 subtracts the second adaptive feedforward signal FF.sub.2 
from the current command signal output by gain function device 18, to 
generate current control signal U. 
As shown in FIG. 4, the current control signal U according to the present 
invention is smooth at the track seek decelerating interval, which thereby 
reduces the noise generated in the hard disk drive. As described in the 
foregoing, a feedforward control system according to the present invention 
is adaptive to the current control signal. Further, high frequency 
components are removed by adaptive feedforward device 150. In accordance 
with the present invention, therefore, it is possible to remove adaptive 
feedforward noise as well as the current control error during the track 
seek operation. 
Although a preferred embodiment of the present invention has been described 
in detail hereinabove, it should be clearly understood that many 
variations on and modifications of the basic inventive concepts herein 
taught, which may appear to those skilled in the art ,will still fall 
within the spirit and scope of the present invention as defined in the 
appended claims.