Non-Contact Digital Measurement

The Digital ShadowGage from Resec Systems Inc. (Bergenfield, New Jersey) is designed to provide better and faster inspection, measurement and data collection. This system digitizes all dimensions of a piece, so the user can read off any dimensions exactly as they would be read from a print. Normally, the user would be in the same room with the instrument, but the system can be controlled remotely as well. Also, the system is portable, allowing the user to bring measurement directly to where the parts are located.

Non-contact operation makes the system especially useful for parts that deform easily. Precision optics allow simultaneous measurement of multiple dimensions, with a field of view of up to 1.750 inches.

During operation, the user simply selects a part inspection program and places the part on the gage’s pedestal. All measurements are instantly displayed on the screen. The measurements’ outside tolerances are highlighted and color coded for easy detection. Measurements are also available through a serial port for data collection.

Software tools featured in the system are available to perform various types of measurements, including distances between parallel edges and points; angles; and centers and radii of circles, arcs and corners. Using the display, the tools can be positioned anywhere over the outline of the part. Each tool can be programmed with tolerance and warning levels. The measurements are then displayed on the screen and on the color-coded Status Display Panel.

The unit is offered in four standard accuracy grades ranging from ± 0.002 inch on the 1.750 inches maximum field of view configuration to ± 0.000050 inch with a maximum field of view of 0.250 inch. Other custom configurations are also available.

The optical system in the ShadowGage is one of the most important components when providing accurate measurements. The video processor relies on a high-quality, exact image to detect edges with a high degree of accuracy. The optical system provides 0.02 percent maximum distortion, temperature compensation, round edge compensation and constant magnification independent of camera-to-part distance.

For further accuracy, the optical system is designed for ultra-low distortion, providing accurate measurements anywhere within the field of view. By contrast, in systems with higher distortion, the same measurement may appear larger or smaller in different areas of the screen. Systems with higher distortion require the use of a master grid to measure the error and compensate for it.

In many systems, temperature variations can also cause inconsistent measurements. As the ambient temperature varies, slight expansion and contraction of the metal in an optical system affects the distance between its elements. This expansion or contraction can cause changes in magnification, focus and distortion, which ultimately affects measurement accuracy. ShadowGage’s temperature-compensated optics minimize the temperature effects to a negligible level, providing accurate results from 60° F to 90° F.

For magnification consistency, the ShadowGage uses telecentric optics: The image is formed only by rays that are perpendicular to the lens. The size of the image is not dependent on the distance to the object, so parts do not need to be positioned exactly at the lens’ focal plane for exact measurements.

One other situation that can cause difficulty in measurement is imaging round surfaces. Light reflecting at an angle off the rounded surface makes it appear smaller than it really is. The error of this effect varies with part position and surface radius, making it almost impossible to compensate for electronically. The use of collimated light (a light source that produces a light beam of parallel rays) eliminates this effect, but has a tendency to cause diffraction. This diffraction causes interference fringes at the camera, making it almost impossible to determine an exact edge position. The ShadowGage light source produces a beam of light that is collimated in nature but produces limited angle rays that eliminate the diffraction problems created by pure collimated light.