Mass data storage devices including compact disk ("CD" ) players or the like allow for optical recording and/or playback of audio signals from rotating media such as compact optical disks.
The typical mechanism used to move a signal pick-up device radially back-and-forth with respect to the surface of the rotating media in the mass data storage device is called an actuator. The actuator may include a baseplate or frame, a carriage, and a pick-up device, such as a recording head or a playback head, which is mounted in the carriage. The actuator also includes a motor assembly for moving the carriage with respect to the frame. The frame maintains the axis of motion of the carriage and the center of the media disc in a fixed relationship. The pick-up head/carriage assembly is supported near the surface of the media. A spindle motor is used to rotate the media disk. The actuator is thus utilized to position the pick-up head close to the desired track on the rotating media surface.
Within the pick-up head assembly is another stage of positioning apparatus which provides vernier control of the optical pick-up beam position and focus. The mechanisms used in playback machines for controlling position and focus within the pick-up head assembly are usually moving coil devices, which respond to feedback signals derived from the permanent data track in the media to apply the necessary corrections. By contrast, the pick-up head for optical recorders must be able to locate a data track upon blank media, with only a datum as a reference, and maintain uniform track spacing to enable accurate, error-free playback.
Further, there are many possible sources of error which must be overcome by the positioning mechanism in the optical data recorder. These sources of error include disk errors such as eccentricity, warping, surface flaws and variations in optical properties. Various mechanical disturbances such as external vibration must also be included in the list of potential tracking errors.
One type of ordinary actuator used in audio CD players includes a rack-and-pinion mechanism which is electrically driven and controlled. The pick-up head assembly or carriage is attached to the rack and travels on two parallel rails. These parallel rails are supported by the frame, in a plane parallel to the plane of the rotating disc. The rack is usually located outside of the rails. This method offers low manufacturing cost and simplicity. However, the rack-and-pinion parts are subject to wear, requiring frequent lubrication and adjustment. Rack-and-pinion actuators do not provide high precision OF motion of the actuator which is desired especially in high quality recording apparatus. Inevitable wear and manufacturing tolerances applied to mass-produced equipment further reduce the precision available from this type of actuator. Other problems with the rack-and-pinion actuator include gear backlash and poor seek times. The latter problem is an unavoidable result of the high-reduction gear ratio required to move a relatively large mass.
Other audio compact disc players employ moving coil actuators to obtain more accurate positioning of the optical playback head. A representative type has a magnetic driving circuit consisting of a movable drive coil and a stationary core. The drive coil is attached to the side of the playback head carriage assembly at a point outside of and adjacent to one of the parallel supporting rails similar to the rack-and-pinion system described above. The drive coil or solenoid, when electrically energized by a predetermined tracking signal, moves along and over the core, which is also parallel to the carriage rails. This moving coil mechanism, often called a voice coil, improves the positioning accuracy, reliability and performance of the playback head actuator by eliminating all of the free play and wear characteristics of the rack and pinion.
There are two inherent sources of error in the actuator designs described above. One is the offset application point of the force used to move the head carriage. When the driving force is applied at any point other than the center of gravity, Cg, and the center of friction resistance, a moment (or torque) is created on the carriage. This results in servo errors and/or unwanted resonances. Also, any slight misalignment of the rails may introduce binding of the head carriage motion or other friction effects with resulting dissipation of heat in the bearings and the solenoid winding. The impeded motion and uneven wear together with the energy losses can result in unacceptable tracking performance. These effects typically degrade even further with length of service.
The second, more serious source of error is the lack of perfect rigidity of the assembly comprised of the head carriage and the arms which support the drive coil or solenoid. Flexing of the solenoid support arms limits the resolution of the system. A number of design trade-offs must be made to solve this problem. For example, the solenoid support arms may be made larger or more massive to increase their stiffness; but the increased mass will slow the response time and require stronger magnets and/or require higher current in the solenoid coil. Stronger magnets add cost. And higher current may require more expensive construction of the solenoid coil or a higher capacity power supply. This approach also effectively reduces system bandwidth. Alternatively, the solenoid support arms may be fabricated of a stiffer, lighter-weight material but at increased cost.
Yet another type of moving coil actuator found in rigid magnetic disc peripherals and compact audio players supports the pick-up on one end of a pivoting arm and the moving coil on the opposite end of the pivoting arm. The arm is usually pivoted at a point closer to the end attached to the moving coil. This moving solenoid coil, which is mounted at right angles to the long axis of the pivoting arm, and energized by a predetermined tracking signal, moves in an arc about the pivot. Likewise, the pick-up head moves in an arc next to the media surface approximately along a radius of the rotating media. These curved paths of motion complicate the actuator and compromise the path of the optical pick-up head, thereby requiring equalization apparatus to compensate for this curved path.
Heretofore unrealized is an actuator assembly for an optical record and playback device which overcomes the above shortcomings in precision, complexity and cost of manufacture.
Accordingly, it is an object of the invention to provide a linear moving coil actuator capable of highly accurate positioning of the optical recording head.
It is further an object of the invention to provide a linear moving coil actuator of relatively simple construction which can be manufactured in a minimum number of steps, in high volume and at low cost.
It is yet a further object of the invention to provide a linear moving coil actuator for an optical recording device employing a baseplate and magnetic structure of unitary design.