to the base. A locking mechanism may be provided to fix the platform to the base at the desired position. Manual worm drive For more precise position control, a worm drive may be used. A worm wheel is fixed to the rotating platform and meshes with a worm in the base. Rotation of the worm via a manual control knob causes the platform to rotate with respect to the base. Index marks on both the control knob and the platform can be used to locate the platform very precisely and repeatably with respect to the base. Stepper motor with worm drive Replacing the manual control knob in the above manual worm drive scenario a stepper motor allows positioning of the rotary stage to be automated. A stepper motor rotates in fixed increments or steps. The number of steps moved is controlled by the stepper motor controller. In this sense, the stepper motor behaves much like an indexed control knob. DC motor and encoder with worm drive A DC motor may also be used in place of a manual control knob. A DC motor does not move in fixed increments. Therefore, an alternate means is required to determine stage position. An encoder may be attached to the DC motor and used to report the angular position of the motor to the motor controller, allowing a motion controller to reliably and repeatably move the stage to set positions. Linear actuator When precise angular positioning over only a small total angle is required, a linear actuator (either manual, or motorized) may be used. Typically, the range of motion possible is only 10° to 20° of rotation. The linear actuator presses against a contact surface fixed to the stage platform such that extension or retraction of the actuator causes the platform to rotate. The stage platform is sprung against the actuator tip so that the contact surface stays in contact with the actuator tip when the actuator retracts. See also Linear stage Plain bearing Rolling-element bearing References Positioning instruments Scientific equipment Optomechanics