Hardware Reference
In-Depth Information
Figure 2.8: Amplifiers for VCM driver: Voltage source (left) and Current
source (right).
by the differential equation
V
O
(t)=R
v
I(t)+L
v
dI(t)
dt
.
(2.3)
In this case, the transfer function between the input (u) and the coil current
(I)is
I(s)
U(s)
=
K
VA
L
v
s + R
v
,
(2.4)
and the overall transfer function is
G
v,v
=
Y (s)
KK
VA
s
2
(L
v
s + R
v
)
.
U(s)
=
(2.5)
On the other hand, if a current amplifier(shown on the right of Figure 2.8) is
used, i.e., I = K
CA
u, then the overall transfer function is
G
v,c
=
Y (s)
U(s)
=
KK
CA
.
(2.6)
s
2
In HDD, the VCM driver is usually implemented as a voltage controlled cur-
rent amplifier. A sensing resistor of very low ohm is connected in series with
the VCM coil. Voltage across the sensing resistor is proportional to the coil
current, which is then used as feedback to control the coil current. Circuit
representation of a typical VCM driver is shown in Figure 2.9.
The current in the VCM driver is proportional to the input voltage as long
as the amplifier operates in the unsaturated mode. If the amplifier is saturated,
the output current can not be increased anymore. So one can model the VCM
driver as a current amplifier with an upper limit bounding the amplitude of
the current. The fact that the amplitude of current is upper bounded must be
taken into consideration while designing the closed loop feedback controller.
Assuming that the output current of the amplifier operating in the linear region
is proportional to the input, setting an upper bound on the input u is equivalent
to setting an upper bound on the current I. An working model of the VCM
actuator plus the driver is
G
v,rigid
(s)=
Y (s)
U(s)
=
a
s
2
;
given
|u| ≤ U
m
.
(2.7)
