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Quality Scan: Positioning Control Influences Scanning Accuracy







By Jonathan J. O'Hare
Brown & Sharpe Inc.
North Kingstown, RI

By sending an uninterrupted stream of data back to a computer using continuous scanning techniques, it's possible to efficiently gather a large number of data points to better evaluate the surfaces of 2 and 3-D geometric elements.

In the continuous scanning process, as soon as the start/finish points and vectors are determined, the measuring probe travels along the contours of the workpiece, never leaving its surface. Dimensional data are continuously read off the machine scales and probe LVDTs, and sent to the software for analysis. However, the probe's direction is subject to constant change due to the changes in workpiece shape. Data accuracy depends upon the linearity of the probe as it reacts to surface changes (the force it takes to deflect the probe's stylus equals the deflection producing the same accuracy over the full range of the deflection). Thus, the key in designing a flexible scanning CMM is to have very high and repeatable accuracy over the widest probe deflection range. 

The scanning accuracy of a CMM is highly dependent on the system's positioning control. Position control can be influenced in a number of ways, and is application-dependent. For example, the flexibility (rigidity) of the stylus extension is a significant influence on position control. Other influences include the bending moment, the weight of the stylus extension, and speed. In general, all of these influences increase with stylus length, which is why the stylus becomes an issue while scanning deep bores in large machined parts. These parameters influence "lost motion" of the probe tip, or the unseen motion of the tip by the sensor, which is primarily stylus deflection.

One solution involves using sensors designed for fixed probe heads. Fixed-head sensors usually support heavier loads and have higher spring constants. By fixing the probe head to the ram, large motors with high torque are not needed to drive long, heavy stylus clusters. Also, stiffer springs within the sensor are less susceptible to static moments and excessive deflection, namely twist and drag, caused by long, heavy extensions.

Because stylus deflection depends on the amount of force applied to the tip, as well as stiffness, another solution is to utilize the measuring machine's control system to ensure that a consistent gaging force is maintained during the entire scanning process. Forces that act at the point of measurement during scanning are both directly applied and reactionary. Forces directly applied are caused by acceleration, centripetal force, and inertial changes in the bridge of the CMM. Reaction forces, so termed because they respond to direct forces, include forces caused by the sensor springs, the flexing of the stylus, and friction.

Variable High-Speed Scanning (VHSS) is one approach to maintaining consistent gaging force during the scanning process. A firmware enhancement for better path control on surfaces with changing curvature, VHSS is a modification of the original High-Speed Scanning (HSS) defined-path mode. The term variable refers to this algorithm's ability to vary the scanning speed automatically by detecting any changes in the radius of the curve, anticipating when to speed up or slow down. On a CMM, VHSS allows a minimum number of data points to be used to describe a path. Previously, a constant rate of points was needed. A minimum number of points are used to define straighter paths; dense sets are used for areas of greater curvature.

Also, VHSS works equally well with prismatic features such as bores. It generates data points proportional to bore curvature, producing a dimensional description of the feature using the optimum number of data points. This data collection method improves overall inspection throughput while maintaining accuracy.

The speed at which the control system reacts, even to the smallest contour changes, determines the CMM's throughput. To maximize accuracy, the regulation process must take place within the deflection range in which the scanning system is linear. The more extensive this linear range, the better the probe will handle dramatic surface changes while maintaining high speeds.


This article was first published in the June 2004 edition of Manufacturing Engineering magazine. 



Published Date : 6/1/2004

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