Hardware Reference

In-Depth Information

Figure 2.7: Generation of torque in rotary VCM actuator.

geometry is fixed for a given actuator, the torque it generates is function of coil

current only and we can assume this to be K
t
I,whereK
t
and I are the torque

constant of the VCM and the coil current, respectively. In practice, the torque

constant may vary with the position of the coil, i.e., K
t
(θ) is a function of

actuator position θ. However, the change in the magnitude of torque constant

is usually very small and insignificant. We shall assume throughout this topic

a constant value for this parameter of VCM.

The motion of the actuator arm, defined according to the Newton's second

law of motion, can be modeled as

θ(t)=
K
t

J
I(t),

(2.1)

where J is the moment of inertia of the rotating arm, and θ is the angular

acceleration of the actuator's motion. If the distance between the pivot center

and the read head is L inches,
‡
then the linear displacement of the read head

corresponding to an angular displacement (θ)isx = Lθ. It is very common in

the HDD industry to express the displacement of read head in units of track,

i.e., y = D
trk
Lθ,whereD
trk
is the track density in units of Tracks per Inch

(TPI). Taking all these factors into consideration, the rigid body dynamics of

the VCM actuator is given by

y(t)=
D
trk
LK
t

J

I(t)=KI(t).

(2.2)

The corresponding transfer function model is G
v
(s)=
s
2
.

If the VCM is driven by a voltage amplifier, shown on the left of Figure 2.8,

the output (V
O
) of the amplifier is proportional to the input, i.e., V
O
= K
VA
u.

The current in the VCM coil and the applied voltage are related to each other

‡

Imperial units of measurement are widely used in the HDD industry.