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Vertical Stiffness Gradient of Arms in a Stacked Actuator

IP.com Disclosure Number: IPCOM000013237D
Original Publication Date: 2000-Aug-01
Included in the Prior Art Database: 2003-Jun-18

Publishing Venue

IBM

Abstract

Disclosed is a design for an actuator comb or E-block that has varying structural stiffness to resist problematic actuator arm vibration modes that contribute to track misregistration (TMR). This is accomplished by stacking components (arms, spacer rings and coil yoke) to form an E-block with these components made from different materials having a desired range of stiffnesses. By selecting the arm material’s stiffness and damping ratio the actuator modes can be tuned to reduce TMR by bringing arm in-plane flexing amplitude in control. Resonant frequencies increase and modal damping decrease for the problematic actuator structural modes according to the following sequence: S-mode, M-mode and end-arm mode. Each mode has specific heads that are driven furthest off track (i.e. high TMR) while neighboring heads are stable allowing them to easily track follow (i.e. low TMR). Arm scissor motion dominates the S-mode. In a six arm actuator the greatest translational motion are located at arms 2 and 5 (heads 1, 2, 7 and 8) which forces them off track radially producing high TMR. Arm scissor motion dominates the M-mode, as well. In this mode arms 3 and 4 (heads 3, 4, 5 and 6) that see the mode’s greatest translational motion which forces them off track. The end-arm mode is dominated by the outer two arms 1 and 6 (heads 0 and 9) moving with high gain, while all the inner arms move as one with a much smaller, out-of-phase amplitude. This mode can have very high off track gain. When a specific mode shape is the primary cause of TMR, then diminishing the vibration amplitude of the problematic arms will drastically reduce the TMR. This can be accomplished by increasing the stiffness or damping of specific arms in the actuator head stack. For example, arms 2 and 5 for S-mode, arms3 and 4 for the M-mode, and arms 1 and 6 for the end-arm mode. Critical arms are made with higher modulus and greater damping ratio material and are stacked along with other arms made of the traditional material, stainless steel. This design places the more expensive material only where it is need to achieve performance objectives and will minimize the cost impact. 1