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Loading control by PWM-PAM combined drive Disclosure Number: IPCOM000014327D
Original Publication Date: 2000-Aug-01
Included in the Prior Art Database: 2003-Jun-19
Document File: 2 page(s) / 29K

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Disclosed is a loading control method by a pulse drive method.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 52% of the total text.

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Loading control by PWM-PAM combined drive

Disclosed is a loading control method by a pulse drive method.

First, a general velocity control method by back electromotive
force is shown.

In this document following notations are used:
Vm: voltage applied to VCM coil
Im: VCM current
Rm: VCM coil resistance
Vb: back electromotive force
w : acutuator's radius velocity
Kt: torque constant of VCM

Electrical circuit theory provides following equations (a) and

Vm=Im*Rm+Vb (a)
Vb=Kt*w (b)

  These two equetions induce equation (c).
w = (Vm - Im * Rm)/Kt (c)

Above is shown the electrical theory that is used for velocity
control of the actuator. Generally the term of (Vm - Im * Rm) in
equation (c) is caluculated by velocity amplifier. And the
output of the amplifier is sampled for the input to velocity
servo controller throuth AD converter.

Now I must point out that traditional continuous current control
cannot handle large power application. What we are concerned with
in equation (c) is that Rm value is drifted by temperature:
Rm = Rm(T),(T:temperature)

And VCM current Im makes the coil heat up then Rm becomes larger.
But we don't know Rm value directly. So under continuous current
control where Im is not always zero, there is always calculation
error of velcity w. To neglect the error, traditional continuous
current control should limit the maximum current to about 0.5[A].

To solve the problem shown above, I disclose PWM-PAM combined
control. Both of PWM(Pulse Width Modulation) and PAM (Pulse
Amplitude Modulation) are pulse(on- off) current drive methods,
not continuous current drive ones. The difference of them is
manupulated variable. The former manupulates on-duration time
with on-amplitude fixed and the later does on-amplitude with
on-duration time fixed.

For HDD application, there are two types of latch. One is
mechanical latch and the other is magnetic latch. It needs about
0.1[A] to detach the actuator from mechanical latch. And about


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1[A], from magnetic latch. Therefore continuous current control
cannot detach the actuator from the magnetic latch. On the other
hand, pulse drive control can do because there are no limit to
the c...