Voice coil motor feedback control circuit

A circuit for driving a voice coil motor used to position the heads of a disk drive is disclosed. The circuit consists of a an H-bridge circuit, a controller, and a feedback loop. The feedback loop prevents the BEMF from driving a voltage on the voice coil motor above the supply voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electronic circuits used to control voice coil 
motors (VCM's) used in disk drives and more particularly to feedback 
circuits used to reduce the distortion in the acceleration profile of the 
voice coil motor actuated heads in a disk drive. 
2. Description of the Relevant Art 
The problem addressed by this invention is encountered in the head 
positioning systems in the disk drive industry. Typically, a modern fixed 
disk drive has two or more double sided disks and over a thousand tracks 
per disk. Each track is divided into sectors. Each side of a disk requires 
at least one head to read and write information onto the surface of the 
disk. The information is usually grouped by surface, track, and sector. 
Consequently, an important performance characteristic of a disk drive is 
how quickly the heads of a disk drive can move from one track to another, 
commonly referred to in the industry as head seek time. The faster the 
head seek time, the higher the performance of the disk drive since the 
transfer rate of the information is increased as head seek time is 
decreased. 
It has become common in the disk drive industry to use voice coil motors 
(VCM's) to move the heads in a disk drive. Voice coil motors offer the 
advantages of higher speed and higher track to track resolution than 
stepper motors. 
FIG. 1 shows a typical circuit for controlling a voice coil motor 10 as is 
known in the prior art. The circuit is commonly referred to as an 
H-configuration because the four n-channel transistors 12, 14, 16, and 18 
form an "H" around voice coil motor 10. It is understood in the industry 
that the transistors in the H-configuration can be any common transistor 
such as bipolar transistors and the like. In operation, controller 20 
controls the position of the heads of a disk drive by controlling the 
current in the voice coil motor 10. For example, to accelerate the heads 
in a first direction, the controller would send an enabling signal to 
transistor 22 turning on transistor 12 and thus applying a high voltage to 
node 30. At the same time, controller 20 sends a DAC signal to amplifier 
34 to turn on transistors 28 and 18 proportional to the strength of the 
DAC signal, pulling node 32 low. Since node 30 is at a higher voltage 
potential than node 32, current flows from node 30 to node 32 and the 
heads of a disk drive will respond to the resulting magnetic field by 
accelerating proportionally to the current magnitude. Conversely, the 
heads of a disk drive are accelerated in the opposite direction by 
enabling transistor 26 and transistor 14, thereby raising node 32 to a 
higher voltage potential than node 30. The resulting current flow will 
accelerate the heads in the opposite direction. 
To achieve a fast head seek time, the head is conventionally accelerated 
until it is half way to the desired track, and then decelerated until the 
head reaches its destination, as shown in FIG. 2. This 
acceleration-deceleration profile is ideally accomplished by driving 
current through the voice coil to accelerate the heads and then reversing 
the current through the voice coil to decelerate the heads. However, 
during acceleration, heads store energy in the form of kinetic momentum 
and back electromotive force (BEMF). If the prior art H-bridge circuit is 
used, the BEMF will drive the voltage on node 30 to a voltage equal to Vcc 
plus the BEMF which can exceed the safe operating limits of the circuit 
elements. To protect the circuit elements, the BEMF voltage is typically 
clamped by diodes, such as diode 13, to a safe operating voltage. When the 
BEMF voltage is clamped, it distorts the acceleration profile as shown in 
FIG. 3. This distortion causes excessive wear and drive noise which 
negatively affects the disk drive. 
Therefore, it is an object of the invention to eliminate the distortion in 
the acceleration profile due to the clamping of the BEMF voltage. 
It is further an object of this invention to eliminate the diodes used to 
clamp the BEMF voltage. 
These and other objects, features, and advantages will be apparent to those 
skilled in the art from the following detailed description when read in 
conjunction with the accompanying drawings and appended claims. 
SUMMARY OF THE INVENTION 
The invention can be summarized as a circuit for driving a voice coil motor 
used to position the heads of a disk drive. The circuit consists of a an 
H-bridge circuit, a controller, and a feedback loop. The feedback loop 
prevents the BEMF of the VCM from driving a voltage on the voice coil 
motor above the supply voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A circuit for driving a voice coil motor to position the heads in a disk 
drive according to an embodiment of the invention will be described. 
Referring now to FIG. 4, a voice coil motor 10 has a node 30 and a node 32 
for receiving current. Node 30 is connected to the source of n-channel 
transistor 12, to the drain of n-channel transistor 14, and to the 
non-inverting input of operational transconductance amplifier (OTA) 50. 
Transistor 12 has a drain connected to a voltage supply Vcc and gate 
connected to the source of n-channel transistor 22. The drain of 
transistor 22 is connected to a pumped voltage Vp and the gate of 
transistor 22 is connected to a controller 20. The gate of transistor 14 
is connected to the drain of n-channel transistor 24 and to a Vcc voltage. 
The gate of transistor 24 is connected to the output of an OTA 36 and the 
source of transistor 24 is connected to a voltage reference (ground). The 
source of transistor 14 is connected to a sense resistor 38, to the 
inverting inputs of OTA 36 and 34, and to the source of n-channel 
transistor 18. The non-inverting input of OTA 36 is connected to the 
controller 20. Transistors 12, 14, 16 and 18 form an H-bridge wherein 
transistors 12, 14, 16, and 18 are the first, second, third and fourth 
transistors of the H-bridge, respectively. 
Similarly, the non-inverting input to OTA 34 is connected to controller 20. 
The drain of n-channel transistor 16 is connected to Vcc and the source is 
connected node 32. The drain to n-channel transistor 18 is connected to 
node 32 and its source is connected to resistor 38. The other end of 
resistor 38 is connected to ground. 
Feedback loop 60 senses the voltage on node 30 and controls the 
conductivity of transistors 16 and 18 responsive to the voltage on node 30 
exceeding Vcc. 
In feedback loop 60, the inverting input of OTA 50 is connected to Vcc and 
its output is connected to node 55. The drain of p-channel transistor 52 
is connected to Vcc and the drain of p-channel transistor 54. The gate of 
transistor 52 is connected to the gate and source of transistor 54 and to 
node 55. The source of transistor 52 is connected to drain of p-channel 
transistor 26 whose source is connected to the gate of transistor 16. The 
gate of transistor 26 is connected to controller 20. A current source 56 
has a drain connected to node 55 and a source connected to ground. The 
gate of n-channel transistor 58 is connected to node 55 and has a current 
path from the source of transistor 28 to ground. Transistor 28 has a drain 
connected to the gate of transistor 18 and means for pulling the gate up 
to Vcc. The gate of transistor 28 is connected to the output of OTA 34. 
In operation, the current through voice coil motor 10 is controlled by the 
controller 20 controlling H-bridge transistors 12, 14, 16, and 18. A 
typical control sequence for moving the heads of a disk drive is for the 
controller to send an enabling signal to transistor 22 turning on 
transistor 12 and thus applying a high voltage to node 30. At the same 
time, controller 20 sends a DAC signal to amplifier 34 to turn on 
transistors 28 and 18 proportional to the strength of the DAC signal, 
pulling node 32 low. This raises the voltage at node 30 to just below Vcc 
while lowering the voltage at node 32 to just above ground as shown at 
T.sub.0 in FIG. 5. Consequently, current flows from node 30 to node 32 
thereby causing the heads to accelerate and gain velocity. 
To stop the heads at a desired location, transistors 12 and 18 are turned 
off by controller 20 while transistors 16 and 14 are turned on by 
controller 20 as shown at T.sub.1 in FIG. 5. At this point in the 
sequence, energy is stored in the VCM in the form of momentum and BEMF. 
Since node 32 is being driven to Vcc and the VCM has a stored BEMF 
voltage, the voltage at node 30 is driven to a voltage just above Vcc as 
shown by the voltage spike on node 30 in FIG. 5. The BEMF above Vcc is 
sensed by the feedback loop 60 which actively lowers the voltage of node 
32 until the BEMF is no longer above Vcc by controlling the conductivity 
of transistor 16 and transistor 18. After the BEMF voltage is absorbed by 
transistor 14, the voltage at node 30 is quickly driven to just above 
ground while the voltage on node 32 is driven to just below Vcc which 
reverses the current flow in the VCM thereby decelerating the heads, 
eventually stopping the heads at the desired location as is known in the 
industry. 
The feedback loop 60 is comprised of OTA 50, a current mirror 53 made from 
transistors 52 and 54, a current source 56, and transistor 58. The 
feedback loop operates by sensing the voltage on node 30 and feeding 
current into node 55 proportional to the BEMF voltage. The higher the BEMF 
voltage, the more current that is injected into node 55. As current is 
injected into node 55 the current through the current mirror 53 is reduced 
which effectively reduces the gate voltage of transistor 16 thereby 
allowing transistor 16 to operate linearly allowing node 32 to drop in 
voltage, as shown in FIG. 5. Additionally, the current from OTA 50 is 
injected into the gate of transistor 58 which effectively allows for 
transistor 18 to operate linearly pulling node 32 towards ground to a 
voltage equal to the supply voltage minus the sensed BEMF voltage above 
the supply voltage, as qualitatively shown in FIG. 5. Consequently, 
transistor 16 and 18 are operating in unison to drive node 32 to the lower 
voltage necessary to keep node 30 from rising above Vcc. The BEMF voltage 
can then be dissipated through transistor 14. As discussed above, the 
feedback loop 60 relinquishes control after the BEMF is dissipated. 
Using the feedback circuit offers the advantages of eliminating the diodes 
used to clamp the BEMF voltage and the distortion in the acceleration 
profile due to the clamping of the BEMF voltage. 
Although this invention has been described as having one feedback loop, it 
is understood that a second loop is necessary to handle the BEMF voltage 
caused from accelerating the heads in the opposite direction. The second 
feedback loop would sense node 32 and control transistors 12 and 14 in an 
analogous manner as described above. Even though the invention has been 
described and illustrated with a certain degree of particularity, it is 
also understood that there a numerous methods for implementing the 
feedback loop and/or the VCM control circuit without departing from the 
spirit of the invention, as hereinafter claimed.