Robust design of a spindle motor: a case study

Robust design is the process of minimising variability of products and processes in order to improve their quality and reliability. In recent years, it has seen wide spread application in many fields such as mechanical [J Quality Technol 24 (1992) 22], chemical [Quality Engng 9 (1997) 391] and civil [Quality Engng 9 (1997) 441] engineering. As an additional application, this paper investigates the use of robust design techniques during early design process, in order to predict, analyse and improve quality and reliability of spindle motors found within hard disks. The spindle motor is an integral component of the hard disk that rotates the recording media. Constant speed rotation is an absolute necessity for accurate reading and recording of information. However, it is difficult to achieve this due to the effects of cogging torque. In this paper, the robust design of a spindle motor is attempted. The motor was modelled using multiple regression analysis. A dual response approach [J Quality Technol 22 (1990) 38] was then used. This performed a constrained minimisation of the mean of the cogging response, the constraint being that the variance be within a specified limit. The results showed that the current operating point was near optimal. However, the redesign was still able to achieve a 4% and 25% reduction in the mean and the variance of the cogging torque, respectively, before actual prototypes were made.