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Gary Card

Metrology

Probes Are the Key to Accurate Scanning

Published: Monday, November 3, 2003 - 23:00

Coordinate measuring machines gather data by means of a probe or sensor. Conventional CMMs equipped with touch-trigger probes can use a stitch-scanning method to record point streams from part surfaces. In stitch scanning, the CMM lifts the probe head from the surface of the part, moves it slightly forward and lowers it until contact is made for every data point collected. This single-point procedure is slow and unsuitable for efficient form measurement. Analog probes for continuous scanning are designed to send an uninterrupted stream of data back to a system computer. These probes avoid the time-wasting auxiliary movements required by point-to-point measuring probes.

Analog probes can be designed with solenoid systems, laser sensors or inductive displacement transducers. In the scanning process, the probe stylus is in constant contact with the part surface, and the measuring machine control system ensures that a consistent gaging force is maintained by detecting any deviations and regulating them immediately. This is called active probing, meaning that the force is constant regardless of the probe’s path. This force deflects the three probe axes, and high-resolution electronic transducers record the displacement. The dimensional data is read continuously from machine scales and analog probe transducers and analyzed by appropriate software.

The accuracy of the data depends on the linearity of the probe as it reacts to surface changes (i.e. the force it takes to deflect the probe’s stylus is proportional to the deflection, producing the same accuracy over the full range of the deflection). The more extensive the linear range of the scanning system, the better the probe will handle dramatic surface changes while maintaining high speeds.

In all CMMs, scale readings are referred to the center of the probe tip. Its location relative to the machine coordinate system is defined through a probe calibration cycle executed prior to measuring. The purpose of the calibration cycle is to determine the exact center and radius of the probe tip in use. Once these values are known, the software mathematically offsets the coordinates by the radius of the tip during the measuring operation. The normal vector to the probed surface at the point of contact determines the direction of this offset. With a 3-D probe, the direction of this vector is automatically determined on the basis of the readings of the three probe axes. The bending of the probe stylus can also be calculated and compensated. This permits the use of very long probe tip extensions to allow full accessibility to all workpiece features.

Problem solving with noncontact sensors

When the characteristics of a part surface make direct probing impractical, noncontact data-gathering sensors can solve the problem. These optical sensors integrate a light source with a photoelectric detector and work on the principle of triangulation. The light source emits a precisely focused laser or infrared light beam. When this beam strikes the workpiece surface, it forms the image of a spot. The diffused, scattered light is then focused on a photoelectric array. Any variation on the surface distance from the sensor changes the position of the spot image on the array.

Some noncontact sensors integrate a structured light source that emits a plane of light. When this plane intersects the part, a light line forms along the contour of the part. This line is detected by the image sensor that transforms it into a measurable digital image. Distance measurements can be obtained based on the shape and position of this line on the part surface. The structured light sensors are able to simultaneously triangulate and calculate the X, Y and Z coordinates of hundreds of points along the line.

Noncontact heads scan faster than analog heads but have limited accessibility to part features. Part accessibility limitations often result in the need for frequent orientations of the head during the scanning process, and this sometimes results in a reduction of the effective overall system throughput.

Non-contact heads can be combined with analog scanning heads to extend the application field of the CMMs to include the inspection of small, complex parts such as clock components, plastic parts and micro gears.

The probe plays a key role in accurate and efficient data gathering. The choice of a probe depends on the dynamics of the CMM and the application.

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About The Author

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Gary Card

Gary Card is the marketing manager at Brown & Sharpe, where he has also held positions as product manager and technical sales support manager for coordinate measuring systems. He holds a bachelor’s degree in industrial technology from Roger Williams University and is experienced in both metrology and the machine tool industry.