Minimum Incremental Motion and Holding Stability in Beamline Positioning
Stepper versus Servo — A Comparison of Drive Technologies
Many applications, such as X·ray microscopy and Computed Tomography (CT), require positioning of samples, detectors, and optics in order to perform measurements. Microscopy applications often require imaging of the structure of matter at the sub-micrometer and even nanometer level. Good holding stability, both short-term and long-term, is required because movement of the sample, optics, or detector over the time of measurement will cause poor images. Also, the ability to make small mechanical movements on the order of nanometers is often critical for alignment and adjustment of samples or optics.
Stepper and servomotors are two common means of controlling position in mechanical systems used in beamline applications. In this study, the performance of each motor and feedback type is evaluated for minimum incremental motion, short-term stability, and long-term stability. To evaluate and isolate these aspects of performance, Aerotech designed a mechanical positioning stage based on the ANT180·L platform that could be fitted with several different drive technologies including a 12 mm diameter x 1 mm pitch ball-screw driven by a stepper motor, a 12 mm diameter x 1 mm pitch ball-screw driven by a rotary servomotor, and a linear servomotor. Optical encoders were used for position feedback and commutation on the servomotors. These included both rotary encoders (1000 lpr, 1 Vpp output) mounted to the motors and direct-metrology linear encoders (glass scales, 20 μm signal period, 1 Vpp output) in line with the direction of motion.
(A) ANT180-L stage with ball screw and servomotor, (B) ANT180-L stage with ball screw and stepper motor, and (C) ANT180-L stage with linear motor
Our testing results show that servomotors outperform stepper motors in terms of long-term holding stability and minimum incremental motion, and rival the performance of screw-driven stepper-motor stages for shorter-term holding stability (in-position jitter). For all test configurations, the in-position jitter, or short-term stability, was <4 nm pk-pk. Minimum incremental motion was <2 nm for both the linear-motor stage and ball screw/servomotor stage with direct linear encoder feedback. Without the linear encoder, the ball screw/servomotor stage demonstrated a 50 nm minimum incremental motion while the ball screw/stepper-motor stage demonstrated a 250 nm minimum incremental motion. Finally, long-term stability was measured over the duration of one hour after performing a 45 second move routine. The stepper motor ball-screw stage had 23 times higher drift than the linear motor stage and about 7 times higher drift than the servomotor stage with the linear encoder.
The positioning system will play an ever-increasing role in achieving high-quality images. The capability for extremely fine positioning and stability makes both linear and rotary servomotors an excellent choice for applications in which achieving sub-micrometer or even nanometer precision in sample and optics positioning is critical.
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