A microstep controller of a DC servomotor

A microstep controller of a DC servomotor is developed for accurate positioning and smooth movement at low-speed rotation. It consists of digital and analog positioning loops. Dividing one period of quadrature sinusoidal signals generated by an incremental encoder attached to a motor shaft into quarter sections, the digital loop controls the movement between the sections. The analog loop divides each section further into N equiangle segments to control the movement within each section. The effective angle resolution is thus 90°/MN with M being the number of slits etched on an incremental encoder. A prototype controller assembled using monolithic and hybrid integrated components has confirmed the principles of operation. Positioning accuracy was 0.009°±0.002°. A further improvement is possible with this controller by increasing the number of slits on the encoder and reducing the interpolation step size. Positioning accuracy is dominated ultimately by the amplitude fluctuation of quadrature sinusoidal outputs, and thus an automatic amplitude control should be incorporated into the encoder. The controller was first intended for applications such as crystal-pulling apparatus, astronomical observation systems, and tracking radars, which require smoothly rotating mechanisms, but it can also be applied to fine-positioning mechanisms in scanning electron and tunneling microscopes and semiconductor manufacturing equipment