1. Field of the Invention
The invention relates disk drives having rotary-type actuator assemblies and, more particularly, to a vibration sensor for measuring the vibration on a flexible circuit that connects the disk drive electronics to the actuator assembly.
2. Description of the Prior Art
Data storage devices employing rotating magnetic or optical media disks are known for high capacity, low cost storage of digital data. Such disks typically contain a multiplicity of concentric data track locations, each capable of storing useful information. The information stored in each track is accessed by a transducer head assembly which is moved among the concentric tracks.
Physical positioning of the transducer head assembly is typically accomplished by a rotary actuator assembly which supports the transducer assembly at one end of the rotary actuator assembly. At an opposing end of the actuator assembly is an actuator motor that causes the actuator assembly to pivot about a centrally located axis and position the transducer head assembly over the disk. Control circuitry, within the disk drive electronics, controls the actuator motor such that the head assembly is accurately positioned amongst the concentric tracks on the disk. Typically, the actuator motor forms a portion of a continuously positionable system (servo system) that uses a closed loop servo circuit to control the position or the transducer assembly relative to the tracks on the disk, i.e., the servo system continuously adjusts the position of the actuator assembly based upon servo information read by the transducer assembly from the disk.
Electrical signals are provided to and from the transducer heads and the actuator motor via a flexible circuit (also referred to as a flex circuit or flex cable). The flex circuit typically contains a plurality of copper conductors sandwiched between two polyester or polymer cover layers (e.g. Mylar film). One end of the flex cable connects to the disk drive electronics and the other end of the cable connects to wire leads that, in turn, connect to the transducer heads and the actuator motor.
As the actuator assembly repositions the transducer assembly over the disk, i.e., seeks, the actuator assembly movement excites resonances in the flex circuit. In high capacity drives having a high number of tracks-per-inch (TPI), e.g., 4000 TPI, these resonances can cause significant off-track motion of the transducer heads. For example, empirical testing in 4000 TPI disk drives has discovered 10 to 20 .mu.inches of off-track motion.
Although there are many flex circuit resonances that contribute to the off-track motion, the first mode (lowest frequency) produces the most significant off-track motion. This mode is typically in the 200-300 Hz range for both 3.5 and 2.5 inch form factor disk drives.
Presently in the art this problem is addressed by selecting an appropriately sized and shaped flex circuit. In general, a thicker circuit increases the resonant frequency of the flex circuit. However, a thicker circuit also requires a larger bias force (a larger VCM) to move the actuator assembly across the disk. On the other hand, decreasing the thickness of the flex circuit decreases the resonant frequency of the circuit. However, such a thin flex circuit can be easily damaged during assembly and may not be capable of accommodating copper that is sufficiently thick for signal transmission to and from the actuator motor. Consequently, the problem is typically solved by selecting a thickness of flex circuit that is between the extremes and increasing the TPI such that the first resonance mode does not impact tracking performance to any significant degree.
Therefore, a need exists in the art for a method and apparatus for sensing the resonance vibration of the flex circuit such that the sensed vibration can be utilized by the existing servo system of the disk drive to reduce the off-track motion that would normally be caused by the vibration.