A Laser Speckle Sensor for Compound Rotary-linear Motion Metrology

In previous work [1], our group implemented a tilted-mirror interferometric sensor that can measure rotation of a hybrid rotary-linear axis as part of a 5-DOF machine tool intended for fabricating centimeter-scale parts. Attached to the shaft of the hybrid axis is a tilted mirror rotary sensor, which uses two laser interferometers to measure the orientation of a tilted 3-inch diameter mirror attached to the shaft. The sensor has a resolution of 1,336,000 counts/rev (4.6 μrad) corresponding to a linear resolution of 0.046μm at a 1cm radius. However, the tilted-mirror’s inertia dominates the rest of the axis, limiting accelerations, and its mass causes a cantilever resonance, limiting the controller bandwidth. A promising alternative sensor that can be used for cylindrical motion metrology is a laser speckle based machine vision sensor. We have implemented such a prototype metrology system for a 4-inch diameter air bearing spindle. Off-line measurements from the experiments performed on our setup show that images obtained from the spindle surface can be used to measure angular displacements of the spindle with sub-pixel resolution. Real-time implementation on our prototype setup is expected to be able to run at update rates of 10 Hz, thus allowing for closed loop control with a bandwidth of the order of 1 Hz. Further, we predict that the sensor can achieve resolutions of 0.1 μm for translation and 5 μrad for rotation. Yamaguchi [2] developed a laser speckle rotary encoder achieving a resolution of 35 μrad for angle of a 115-mm diameter metal cylinder. However, the encoder is based on measuring relative displacement across images. Measurements based on relative registrations are prone to significant drift, which cannot be tolerated for high precision measurements. In our implementation, we intend to eliminate drift by referencing all our image registrations to global images retrieved from a library generated over the range of motion.