Real-time camera-based distance gage can replace position-sensitive detectors

In surface treatments such as depositions, fine machining, and dispensing of a cover or protective layers, online real-time height measurement may be required to control process quality. Typically, the distance between a reference point on the processing head and the surface should be monitored at high frequency (10kHz) to provide effective machine feedback. Other requirements often include non-contact measurement, maximum optical resolution (minimal combined errors due to aberrations, diffraction, defocus, etc.), a sub-millimeter laser spot size, compact and lightweight packaging, and the ability to calculate measurements prior to the surface treatment process. In manufacturing process control, position-sensitive detectors (PSDs) are commonly used for distance measurement. They produce an analog voltage output that is proportional to the location of the laser spot image on the detector.1, 2 However, PSD technology has several drawbacks, such as performance degradation in the presence of multiple reflections, stray light, variations in the beam shape and intensity distribution, and surface tilt3 (see Figure 1). In addition, the single-unit packaging of the PSD is geometrically incompatible with various in-process measurement applications, as evidenced by several product examples.4, 5 One alternative is to use a camera-based solution that offers physical and algorithmic flexibility advantages. However, this type of system is often not capable of real-time processing due to host PC constraints. We developed a high-speed, camera-based distance gage that eliminates some of the challenges associated with PSDs.6 Analysis of geometrical and illumination efficiency constraints suggested that imaging the reflection of a laser spot is the best method, so our system includes a linescan camera with sufficient Figure 1. Measurement error using a position-sensitive detector in the presence of multiple reflections and surface tilt.