Comparison of AFC and FACT method for periodic disturbance suppression in optical disk drives

The adaptive frequency control (AFC) and frequency adaptive control technique (FACT) are effective methods in eliminating periodic disturbances. Especially for cancellation of repeatable runout in hard disk drives (HDD), AFC schemes have been applied successfully. This paper presents and compares the applicability from such two methods in optical disk drives (ODD). A surprising result is that a DC content dominating the periodic error signals is necessary for AFC schemes to function properly. This implies that AFC is inapplicable in systems where the DC content is small or intended to be reduced. The property is analytically examined through the well-known amplitude modulation (AM) process and is verified experimentally on the disk wobble compensation in ODD. Experimental results show that FACT has better properties than AFC in terms of DC reduction and harmonic independence. It is also shown that AFC fails if the DC content and harmonics are compensated simultaneously. Regarding the application on ODD, the FACT scheme is practical in disk runout and wobble control.