JPH05166312A - Digital actuator controller using low-pass filter - Google Patents

Abstract

PURPOSE: To provide a digital actuator controller having a margin in the resonance mode management of head suspension. CONSTITUTION: A low-pass filter(LPF) is inserted between a digital-to-analog converter(DAC) 18 and a power amplifier(PA) 12 to lower the gain of a high frequency zone to reduce the effect of return distortion. In addition, a means 16 or 22 digitally compensating the loss of a low frequency zone caused by the insertion of LPF 20 is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital actuator control device for a magnetic recording device, and more particularly to a digital actuator control device for reducing the influence of folding distortion peculiar to digital control.

[0002]

2. Description of the Related Art Magnetic recording devices, especially hard disks
In actuator control of a drive (HDD), folding back strain caused by mechanical resonance of a head suspension becomes a problem. In order to solve this, conventionally, it is generally practiced to insert an analog notch filter at the resonance point. There is no problem if the resonance frequency is always constant, but in reality, the resonance frequency fluctuates to some extent, which may not match the characteristics of the notch filter.
Therefore, in the method using the normal notch filter, it is necessary to strictly control the resonance mode of the head suspension, and there are problems in terms of cost and quality.

IBM Techn, issued in August 1990
ic Disclosure Bulletin, Vol. 33, No. 3A, pp. 222-223, proposes to use an elliptic filter instead of a conventional notch filter. The most important feature of the elliptic filter is to minimize the phase delay, but it cannot be reduced to zero. There are also problems when the resonance frequency fluctuates greatly, and the cost is higher than that of a normal notch filter. It is stated in this document that the use of low pass filters is generally not considered because they cause a very large phase shift at the servo zero dB crossover frequency (ZCF).

In terms of digital control, US Pat.
It has also been proposed to eliminate the influence of mechanical resonance by matching the sampling frequency with the mechanical resonance frequency, as in Japanese Patent No. 98228. Since the sampling frequency is uniquely determined by the rotational speed of the disk and the number of sectors, the resonance frequency is actually adjusted to the sampling frequency, but it is difficult to keep the resonance frequency constant.

[0005]

As described above, in the conventional method, it is necessary to strictly control the resonance mode of the head suspension, and there are problems in terms of cost and quality.

Therefore, it is an object of the present invention to provide a digital actuator control device which has a margin in managing the resonance mode of a head suspension.

Another object of the present invention is to provide a digital actuator controller which can solve the problem of mechanical resonance with a simple structure.

[0008]

In a digital actuator control apparatus according to the present invention, a low pass filter (LPF), which has been heretofore unsuitable, is inserted between a digital / analog converter and a power amplifier. By reducing the gain in the high frequency band, the effect of folding distortion peculiar to digital control is reduced. Since such an LPF has a great influence not only on the high frequency but also on the frequency characteristic of the low frequency region,
In the present invention, means for digitally compensating for the influence in the low frequency region lower than the Nyquist frequency is used. Thereby, the problem of mechanical resonance in a high frequency region higher than the Nyquist frequency can be solved.

[0009]

1 is a block diagram showing the arrangement of a digital actuator control apparatus according to the present invention. This is an actuator 1 including a head suspension mechanism and a DC motor for moving the head (a voice coil motor is typical).
0, a power amplifier (PA) 12 that drives the actuator 10, an analog signal that converts a signal y (t) indicating the head position from the actuator 10 into a digital signal
A digital converter (ADC) 14, a microprocessor 16 for generating a control signal for moving a head to a desired position in response to a digital position signal, and a digital control signal from the microprocessor 16 for converting an analog control signal u
Digital-to-analog converter (DAC) that converts to (t)
18 and an analog low pass filter (LPF) 20 inserted between the DAC 18 and the power amplifier 12. Conventionally, a notch filter was connected to the LPF 20, but the present invention uses the LPF instead of the above because of the problems as described above.
Means are provided for digitally compensating for the gain reduction from the cutoff frequency to the Nyquist frequency.

Next, the operating principle of the digital actuator control device of FIG. 1 will be described with reference to FIGS. 2 to 4 show frequency characteristics of a control target including the actuator 10, in which the vertical axis represents gain and the horizontal axis represents frequency (logarithmic scale). 2 is L
It is a characteristic when the PF 20 is not included, and shows a damping characteristic of -12 dB / oct when the control target is a system of second-order integration. Moreover, the peak of the frequency characteristic due to mechanical resonance occurs in the high frequency range than the Nyquist frequency f _N.

FIG. 3 shows an LPF having a cutoff frequency f _c .
The characteristics when 20 is inserted as shown in FIG. 1 are shown.
In the present embodiment, the LPF 20 is a first-order filter, and therefore the characteristic curve of FIG. 2 (shown by the dotted line in FIG. 3) is
Further, it shows an attenuation characteristic of -6 dB / oct. Summed up,
-1 in the frequency range above the cut-off frequency f _c
The attenuation characteristic is 8 dB / oct. The peak due to mechanical resonance in the high frequency region is also attenuated by this, but the gain in the lower low frequency region is also attenuated by the insertion of the LPF 20, so the present invention digitally represents the loss from the cutoff frequency f _c to the Nyquist frequency f _N. Compensation is performed so that the original characteristic appears in the region below the Nyquist frequency. This state is shown in FIG. As shown in the figure, the characteristic in the frequency region lower than the Nyquist frequency f _N , that is, in the frequency region covered by digital control is substantially the same as the characteristic in FIG. 2, and only the high frequency region higher than the Nyquist frequency is greatly attenuated. ..

The cutoff frequency f _c of the LPF 20 needs to be set so that the peak portion in the high frequency region is sufficiently attenuated. Depending on the frequency at which mechanical resonance occurs, L
When the PF 20 is a primary filter, f _c is preferably about 1/5 to 1/20 of the Nyquist frequency f _N. At lower cutoff frequencies, the loss in the low frequency region may not be compensated digitally, and if the cutoff frequency is made too high, peak attenuation will be insufficient. Although the cutoff frequency can be increased by using a second-order or higher-order filter, the optimum design of digital control becomes complicated. The first-order filter is practically sufficient.

Next, an optimum design method for digitally compensating for the loss in the low frequency region due to the insertion of the LPF 20 will be described. The digital control including this compensation is preferably realized by the microprocessor 16, but it is also possible to provide an independent digital compensation circuit as described later. In the following description, the “low frequency range” means a frequency range that is lower than the Nyquist frequency and that can be digitally controlled.

In the low frequency region, since the actuator is a second-order integral system, the transfer function f (s) of the controlled object including the LPF with the time constant a is

[Equation 1] Becomes In Equation 1, b is an input gain and c is an output gain. In reality, there are several resonant modes, but their frequencies are higher than the Nyquist frequency, so they are ignored here. When Equation 1 is expressed by the state equation at time t, Equation 2 is obtained, and the output equation is Equation 3.

[Equation 2]

[Equation 3]

X ^ (t), x "(t) and the like in the equations 2 and 3 represent the primary differential (velocity) and secondary differential (acceleration) of the head position x (t). U (t) is the input of the control target (output of the DAC 18) at time t, and y (t)
(t) is the output of the control target (input of the ADC 14) at time t. As is clear from Equation 3, y (t) is the time t
Is proportional to the position x (t) of the head at, and if c is 1, then y (t) = x (t). The head position can be detected by sampling the angle of the actuator at a cycle T (= 1 / 2fN). U which is the output of the DAC 18
(t) is an analog conversion of the digital output from the microprocessor 16. However, as shown in FIG. 5, the microprocessor 16 has an operation time delay τ. 3 is the sampling period T
When discretized with, it becomes as follows.

[Equation 4]

[Equation 5]

The coefficient A1 on the right side of the equation 4 is a square matrix of 3 rows and 3 columns, and the coefficients B1 and B2 are both column vectors of 3 elements. Assuming that the coefficient of 3 rows and 3 columns on the right side of Expression 2 is A, A1 can be expressed by the following equation.

[Equation 6] A1 = exp (AT)

Also, the three elements (0 0
If the column vector in b) is B, the coefficients B1 and B2 can be expressed by the following equations.

[Equation 7]

[Equation 8]

The right side of equation 6 is

[Equation 9] The following results are obtained by solving this analytically.

[Equation 10]

The column vector B1 is as follows assuming that its three elements are b11, b21 and b31 from the top.

[Equation 11]

[Equation 12]

[Equation 13]

Similarly, the column vector B2 is as follows, assuming that its three elements are b12, b22 and b32 from the top.

[Equation 14]

[Equation 15]

[Equation 16]

Since the actual servo system includes an integrator, rewriting Eqs. 4 and 5 using the digital integral term v gives the following.

[Equation 17]

[Equation 18]

After that, a known optimum control theory (for example, LQ method) may be applied to this to obtain the feedback gain. At that time, of the state variables x (i), x ^ (i), and x "(i) that respectively represent the position, velocity, and acceleration of the head, only x (i) can be directly observed. x ^ (i) and x ″ (i) will be estimated using a state estimator such as a Kalman filter. FIG. 6 shows Bode diagrams of open loop and closed loop when the present invention is applied to an actual servo system. In this example, the sampling frequency is 46
80 Hz, the calculation time delay τ is 100 μs. 300
A phase margin of about 30 degrees and a gain margin of -6 dB is obtained with an open loop bandwidth of Hz. The closed loop curve shows that the high frequency gain is greatly attenuated in the band higher than the Nyquist frequency (= 2340 Hz). As described above, when the digital control system is designed to optimally control the controlled object based on Equation 1, the peak due to mechanical resonance in the high frequency region is sufficiently attenuated and the loss from the cutoff frequency f _c to the Nyquist frequency f _N is reduced. A compensated system is obtained.

As a designing method, other than the above-mentioned state feedback method, a method of obtaining a transfer function from equations (4) and (5) and stabilizing the system by a classical method is also possible.
In any case, it is preferable that the designed digital control system is realized by a microprocessor, but the control algorithm of the microprocessor remains the same (1 in the equation 1).
/ (S + a) is not included) and the loss due to the insertion of the LPF can be compensated by a separately provided digital filter. An example thereof is shown in FIG.

In FIG. 7, the digital filter 22 connected between the microprocessor 16 and the DAC 18 is designed to compensate for the loss as shown in FIG. If the LPF 20 is a first-order filter, the cutoff frequency f _c to the Nyquist frequency f _N of −
In order to compensate for the 6 dB / oct attenuation, the digital filter 22 has the opposite characteristic, that is, f _c to f.
_It suffices to have a frequency characteristic that increases at a rate of +6 dB / oct over _N. The transfer function is expressed in the form of z-transform suitable for digital control as follows.

F (z) = 1 + az ^-1

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to this, and various modifications are possible. For example, although the digital control design is slightly complicated, the LPF 20 may be a second-order or higher-order filter. If it is desired to increase the peak attenuation in the high frequency region, the present invention may be used in combination with a conventional notch filter. If this is done, there will be more room to manage the resonance mode of the head suspension. If a notch filter is also used, it is desirable to connect it between the DAC 18 and the LPF 20. However, if only LPF is used,
As a practical matter, at most one capacitor can be added, so
If not used in combination, it will be advantageous in terms of cost.

[0026]

According to the present invention, the control of the resonance mode can be made more marginal than the conventional countermeasure against mechanical resonance, and the LPF can be composed of one capacitor.
It is also advantageous in cost compared to notch filters.

[Brief description of drawings]

FIG. 1 is a block diagram showing a preferred embodiment of the present invention.

FIG. 2 is a graph showing an example of frequency characteristics in which a peak due to mechanical resonance occurs in a high frequency region higher than the Nyquist frequency f _N.

FIG. 3 is a graph showing frequency characteristics when only a low pass filter (LPF) having a cutoff frequency of f _c is inserted.

FIG. 4 is a graph showing frequency characteristics after digitally compensating for a loss in a low frequency region due to insertion of an LPF.

FIG. 5 is a graph showing the output of the digital-analog converter when there is a calculation time delay τ.

FIG. 6 is a graph showing an example of frequency response of an actuator control system when the present invention is actually applied.

FIG. 7 is a block diagram showing another embodiment of the present invention.

[Explanation of symbols]

 10 Actuator 12 Power Amplifier (PA) 14 Analog-to-Digital Converter (ADC) 16 Microprocessor 18 Digital-to-Analog Converter (DAC) 20 Low Pass Filter (LPF) 22 Digital Filter for Compensation

Claims (16)

[Claims] 1. A digital actuator controller for generating a control signal to an actuator driven by a power amplifier in response to a position signal from the actuator, the digital actuator converting the control signal into an analog signal. A digital actuator control device comprising: a low-pass filter connected between an analog converter and the power amplifier; and digital control means for digitally compensating for a loss due to insertion of the low-pass filter. 2. The digital actuator controller according to claim 1, wherein a cutoff frequency of the low pass filter is set so as to sufficiently attenuate a peak in a frequency region higher than a Nyquist frequency. 3. The low-pass filter is a first-order filter, and the cutoff frequency is 1 of the Nyquist frequency.
The digital actuator control device according to claim 2, wherein the digital actuator control device is set to / 5 to 1/20. 4. The digital actuator controller according to claim 2, wherein the low-pass filter is a second-order or higher-order filter. 5. The digital control according to claim 2, wherein the digital control means compensates for loss in a frequency region lower than the Nyquist frequency.
Actuator control device. 6. The digital actuator controller according to claim 5, wherein the digital control means is a microprocessor. 7. The digital actuator controller of claim 6, wherein the microprocessor is designed to optimally control the system including the low pass filter. 8. The digital actuator control apparatus according to claim 5, wherein the digital control means is a digital filter having a characteristic opposite to that of the low pass filter. 9. A digital actuator controller for a magnetic disk device, comprising: an actuator, a power amplifier for driving the actuator, and an analog / digital converter for converting a position signal from the actuator into a digital signal. A digital control means for receiving the digital signal and generating a digital control signal to the actuator; a digital-analog converter for converting the control signal into an analog signal; and a digital-analog converter between the power amplifier and the power amplifier. A low pass filter connected to the digital actuator control means, wherein the digital control means includes means for digitally compensating for a loss due to insertion of the low pass filter. 10. The cutoff frequency of the low-pass filter is set so as to sufficiently attenuate a peak in a frequency region higher than the Nyquist frequency.
The digital actuator controller according to 1. 11. The digital actuator controller according to claim 10, wherein the low-pass filter is a first-order filter, and the cutoff frequency is set to ⅕ to 1/20 of the Nyquist frequency. 12. The digital actuator controller according to claim 10, wherein the low-pass filter is a second-order or higher-order filter. 13. A digital actuator control apparatus according to claim 10, wherein the compensating means compensates for a loss in a frequency region lower than the Nyquist frequency. 14. A digital actuator controller according to claim 13, wherein the digital control means is a microprocessor. 15. The digital actuator controller according to claim 14, wherein the microprocessor is designed to optimally control a system including the low pass filter. 16. The compensating means is a digital filter having a characteristic opposite to that of the low pass filter.
The digital actuator control device according to claim 14.