At the recent HxGN 2013 event Dr. John Ziegert, Professor, Department of Mechanical Engineering Science, University of North Carolina, presented an informative metrology technology session on Self-Aware Machining. "Tight integration of metrological sensors and systems into manufacturing machines will enable new 'self-aware' operational paradigms. Process control loops can be closed around direct and in-situ measurement of metrological specifications of the workpiece that can result in first part correct, improved quality and reduced scrap and rework. This can apply to large scale manufacturing of both high value and low volume production," said Dr. Ziegert.
"Process control implies feedback loops. In the current paradigm we routinely close control loops around 'proxy' variables that are conveniently sensed, rather than metrological quantities shown in the part specification.
"For example, part specification (size, form and location of features) is determined by spatial trajectory of the tool relative to the part coordinate system. As a controlled variable the position of the machine axis is determined by feedback sensors relative to adjacent links in the kinematic chain. Tool position is estimated from a kinematic model of the machine. However, this model is inevitably imperfect and changes over time due to thermal variations and other factors. Another example is part surface finish. Instead of in-situ measurement and feedback of surface finish arising from a machining operation, we control variables such as speeds and feeds. Tool wear plays a pivotal role in part dimensional accuracy and surface finish, and many models and strategies exist to attempt to monitor tool wear and predict when a tool change is required. A common failing of such systems is that the rate of tool wear shows substantial variability from edge to edge and part to part. Production practices for changing tools often select very conservative operating parameters to avoid catastrophic failures; with the result that significant tool life often remains with the resulting increase in tooling costs. In self-aware machining, tool change decisions would be made based on process intermittent surface quality measurements performed on the machine tool.
"Self-aware machining utilizes a tight integration of the sensor systems and measurement tools in the machine. This enables control of the machining process using real time feedback of actual component specification variables, by providing in process measurements of size, location, form and surface texture of the workpiece. Self-aware machines will operate with multiple nested control loops with widely varying time constants. For example, axis position control loops typically have time constants in microseconds, while thermal growth of the machine structure typically has a time constant measured in hours. The required time scale is determined by the physics involved in the part characteristic being controlled.
"Self-Aware Machining requires a sensor magazine analogous to the common tool magazine. Items in the sensor magazine might include: laser interferometers, ADM; structured light systems and fringe projection systems; LIDAR; photogrammetry; stylus profilometers; coherence scanning interferometry; shape from focus; and fiducial target localization. The goal is to enable all metrological specifications of the workpiece to be assessed while the part is on the machine.
"One strategy that can be employed in self-aware machining is the use of fiducial markers placed onto the workpiece to transfer the reference coordinate system from the machine structure to the part itself. Axis motions in traditional CNC machining are controlled relative to adjacent machine axes, while part specifications locate feature surfaces relative to datum surfaces on the part. Fiducial based machining transfers the reference coordinate system directly to the part via externally calibrated fiducial markers placed on the workpiece surface, and exploits the fact that positioning accuracy of machine tools is typically much better in small regions than it is over the entire workspace. Fiducial markers can be any temporary or permanent feature or mark, e.g. tooling balls, photogrammetry targets or cross hairs, whose position can be conveniently sensed by a sensor carried by the machine spindle. The fiducial machining procedure involves, first, placing markers on the workpiece surface and measuring their locations with a Coordinate Measuring System (CMM) in a thermally controlled environment to establish a reference coordinate frame fixed in the part. Subsequently, if the part dimensions change due to thermal changes or clamping forces, the fiducial markers move with the part and still represent specific locations in the part coordinate system when the part is returned to its normal state. The sensor in the machine spindle measures the fiducial locations relative to the machine coordinate system, and a local transformation from machine coordinates to part coordinates is computed. Next, the part program is updated to reflect part coordinates. Now you are ready to machine in the local region around fiducials.
"By knowing where the fiducials are (functional frame), and where they were at a time when the workpiece thermal state was well-known (reference frame), it is possible to correct the part program for the unknown machine errors and the unknown and relatively uncontrolled thermal state of the manufacturing environment. The part program can be transformed based on this correction to alter the original programmed coordinates such that the machined features will be correct regardless of the manufacturing environment or machine tool errors. The correction of the machine tool is only in the region of interest at the time of interest. The algorithm used is not machine specific. This represents a new machining strategy.
"Referencing the machine positioning metrology directly to the current state of a workpiece through the measurement of the fiducials eliminates or minimizes error due to part deformations from thermal fluctuations or clamping forces. It also reduces sensitivity to errors arising from the machine thermal deformations."
The research and educational focus at the Center for Precision Metrology, University of North Carolina, established in 1989, includes: dimensional, coordinate, machine tool, optical and computational metrology; instrumentation design for nano-manufacturing; manufacturing process modeling; machining dynamics and high-speed machining; computer-aided tolerancing; and electro-optics.
The Center has 25 faculty and staff from five academic departments, capital equipment in excess of $20 million, and operates an Industrial Affiliates program with 15 corporate members.
For more information contact:
Dr. John Ziegert, Professor
Department of Mechanical Engineering Science
The University of North Carolina at Charlotte
9201 University City Blvd.
Charlotte, NC 28223-0001
704-687-8603
jziegert@uncc.edu
cpm.uncc.edu