This invention relates to a circuit protection device for an actuator
drive circuit. In particular, the invention relates to a current limiting device
which allows large amounts of current to flow for short period of time and acts
as a circuit overload protector.
Present circuit overload protectors set a low limit on the maximum
current fed to an actuator. However, this limits the acceleration achievable by
the actuator, thus making it slow to function. Examples of such actuators are voice
coil motors, linear or rotary motors or any coil of wire in a magnetic field.
To achieve a fast response by the actuator, a large amount of current
must be sent through the actuator for a short period of time. However, if the current
were to persist for a long period of time, it would overheat the actuator and damage
One purpose of the present invention is to provide a current limiting
device to the actuator drive circuit which allows large amounts of current to flow
for a short period time, but before the actuator can overheat.
According to a first aspect, the invention comprises an actuator drive
wherein during current flow, resistence of the current limiting
device rises and current flow to the actuator is lowered.
- an actuator;
- at least one current limiting device; and
- a signal to allow current flow through the actuator drive circuit;
According to a second aspect, the invention comprises a CD or DVD
player including an electro dynamic actuator comprising wire coil means which is
driven by an actuator drive circuit in accordance with the first aspect of the
In arrangements according to the invention, an actuator drive circuit
employs a current limiting device with a strong positive temperature coefficient
of resistance. The current limiting device allows large amounts of current to flow
for a short period of time, prior to the actuator overheating, by having the device's
resistance rise and thus lowering the current flow.
Other features and advantages of the invention will be apparent from
the accompanying drawing and the detailed description that follows.
The invention will be described with reference to the following drawing,
wherein like numerals refer to like objects and wherein:
- FIG. 1 is a schematic illustration of an actuator drive circuit employing a
current limiting device according to the present invention.
Figure 1 is a schematic illustration of an actuator drive circuit
100 for a position tracking servo mechanism The actuator drive circuit 100 includes
an actuator control signal 105, an amplifier 110, a current limiting device 115
and an actuator 120. In operation, the actuator control signal 105 is sent through
the amplifier 110 to the actuator 120. The current limiting device 115 is electrically
connected in series with the actuator 120. In the embodiment shown, the current
limiting device 115 is an incandescent lamp.
A user can choose the characteristics of the filament in the lamp
such that larger short duration currents that are needed to move the actuator 120
pass through the light bulb. A typical incandescent bulb filament has a rather
large change in resistance between a cold state and the illuminated state. Although
the filament warms up a small amount during the flow of the current, it passes
essentially unimpeded by the resistence of the filament. However, if there was
a problem or if the demands were high for some reason due to, for example, the
motion of the actuator device, and the high current persisted too long, the filament
of the current limiting device 115 would heat up more rapidly than the actuator's
120 coil that is being driven by this current. The lamp will absorb the energy
and its resistence would increase and thus prevent the high current from existing
for a longer period of time than would be safe for the actuator 120.
If the current were to persist for an extended period of time, it
overheats the actuator 120 and burns up the coil of wire. However, by placing the
current limiting device 115 or any other device with a strong positive temperature
coefficient of resistence in series with the actuator 120, a larger amount of current
flow is able to be used for a short period of time.
As as example, CD and DVD players contain electro dynamic actuators
comprised of little coils of wire and magnets that move a lens such that a laser
spot is tracked with extreme accuracy on the data to be read or written. In these
systems there is a need to place large amounts of current through very small coils
of wire. However, the current only needs to be placed for a short period of time
since the coil of wire is not very heavy. Therefore, the actuator drive circuit
100 has to move very quickly. In order to move quickly, a short pulse of current
is sent through the coil of wire that causes the necessary motion. In these applications
the current limiting device 115 limits the amount of current sent to the actuator
120 to keep the coils in the actuator 120 from burning up. This is accomplished
by the current limiting device's 115 resistance rising and thus lowering the current
flow to the actuator 120.
The terms and descriptions used herein are set forth by way of illustration
only and are not meant as limitations. In particular, although the invention has
been described with reference to specific data structures, the invention may work
equally well with variations of the data structure having more or less fields.
Further, although the invention has been described with reference to specific ordered
processing steps, the invention is operable with the steps performed in different
orders or simultaneously. Those skilled in the art will recognize that these are
other variations are possible within the scope of the invention as defined in the